FINAL DRAFT                                  Chapter 3     IPCC WGII Sixth Assessment Report


2                Chapter 3: Oceans and Coastal Ecosystems and their Services


4 Coordinating Lead Authors: Sarah Cooley (USA) and David Schoeman (Australia)


6 Lead Authors: Laurent Bopp (France), Philip Boyd (Australia/United Kingdom), Simon Donner (Canada),
7 Shin-Ichi Ito (Japan), Wolfgang Kiessling (Germany), Paulina Martinetto (Argentina), Elena Ojea (Spain),
8 Marie-Fanny Racault (United Kingdom /France), Björn Rost (Germany), Mette Skern-Mauritzen (Norway),
9 Dawit Yemane Ghebrehiwet (South Africa/Eritrea)


11 Contributing Authors: Johann D. Bell (Australia), Julia Blanchard (Australia), Jessica Bolin (Australia),
12 William W. L. Cheung (Canada), Andrés Cisneros-Montemayor (Canada/Mexico), Sam Dupont
13 (Sweden/Belgium), Stephanie Dutkiewicz (USA), Thomas Frölicher (Switzerland), Juan Diego Gaitán-
14 Espitia (Hong Kong, Special Administrative Region, China/Colombia), Jorge García Molinos (Japan/Spain),
15 Helen Gurney-Smith (Canada), Stephanie Henson (United Kingdom), Manuel Hidalgo (Spain), Elisabeth
16 Holland (Fiji), Robert Kopp (USA), Rebecca Kordas (United Kingdom/USA), Lester Kwiatkowski
17 (France/United Kingdom), Nadine Le Bris (France), Salvador E. Lluch-Cota (Mexico), Cheryl Logan
18 (USA), Felix Mark (Germany), Yunus Mgaya (Tanzania), Coleen Moloney (South Africa), Norma Patricia
19 Muñoz Sevilla (Mexico), Gregoire Randin (Fiji/France/Switzerland), Nussaibah B. Raja
20 (Germany/Mauritius), Anusha Rajkaran (South Africa), Anthony Richardson (Australia), Stephanie Roe
21 (Philippines/USA), Raquel Ruiz Diaz (Spain), Diana Salili (Vanuatu), Jean-Baptiste Sallée (France), Kylie
22 Scales (Australia/United Kingdom), Michelle Scobie (Trinidad and Tobago), Craig T. Simmons (Australia),
23 Olivier Torres (France), Andrew Yool (United Kingdom)


25 Review Editors: Karim Hilmi (Morocco) and Lisa Levin (USA)


27 Chapter Scientist: Jessica Bolin (Australia)


29 Date of Draft: 1 October 2021


31 Notes: TSU Compiled Version



34 Table of Contents


36 Executive Summary..........................................................................................................................................3

37 3.1 Point of Departure ....................................................................................................................................8

38 FAQ3.1: How do we know which changes to marine ecosystems are specifically caused by climate

39  change? ....................................................................................................................................................10

40 3.2 Observed Trends and Projections of Climatic Impact-Drivers in the Global Ocean ......................16

41  3.2.1 Introduction...................................................................................................................................16

42  3.2.2 Physical Changes..........................................................................................................................18

43  3.2.3 Chemical Changes ........................................................................................................................22

44  3.2.4 Global Synthesis on Multiple Climatic Impact Drivers ................................................................25

45 3.3 Linking Biological Responses to Climatic-Impact Drivers.................................................................28

46  3.3.1 Introduction...................................................................................................................................28

47  3.3.2 Responses to Single Drivers..........................................................................................................29

48  3.3.3 Responses to Multiple Drivers ......................................................................................................32

49  3.3.4 Acclimation and Evolutionary Adaptation....................................................................................36

50  3.3.5 Ecological Response to Multiple Drivers .....................................................................................38

51 Box 3.1: Challenges for Multiple-Driver Research in Ecology and Evolution .........................................39

52 3.4 Observed and Projected Impacts of Climate Change on Marine Systems .......................................41

53  3.4.1 Introduction...................................................................................................................................41

54  3.4.2 Coastal Ecosystems and Seas .......................................................................................................42

55  3.4.3 Oceanic Systems and Cross Cutting Changes ..............................................................................68

56 Box 3.2: Marine Birds and Mammals...........................................................................................................82

57 Box 3.3: Deep Sea Ecosystems .......................................................................................................................96

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1   3.4.4 Reversibility and Impacts of Temporary Overshoot of 1.5°C or 2°C Warming ...........................98

2 3.5 Vulnerability, Resilience and Adaptive Capacity in Marine Social-Ecological Systems, including

3   Impacts to Ecosystem Services ..............................................................................................................99

4   3.5.1 Introduction...................................................................................................................................99

5   3.5.2 Biodiversity .................................................................................................................................104

6   3.5.3 Food Provision............................................................................................................................105

7   3.5.4 Other Provisioning Services .......................................................................................................107

8   3.5.5 Supporting and Regulating Services ...........................................................................................108

9 Box 3.4: Blue Carbon Ecosystems...............................................................................................................111

10  3.5.6 Cultural Services.........................................................................................................................114

11 3.6 Planned Adaptation and Governance to Achieve the Sustainable Development Goals (SDGs) ...116

12  3.6.1 Point of Departure ......................................................................................................................116

13  3.6.2 Adaptation Solutions ...................................................................................................................118

14  3.6.3 Implementation and Effectiveness of Adaptation and Mitigation Measures ..............................124

15 Cross-Chapter Box SLR: Sea Level Rise ...................................................................................................125

16  3.6.4 Contribution to the Sustainable Development Goals and Other Relevant Policy Frameworks.139

17  3.6.5 Emerging Best Practices for Ocean and Coastal Climate Adaptation.......................................143

18 FAQ3.2: Are we approaching so-called tipping points in the ocean and what can we do about it?.....144
19 FAQ3.3: How are marine heatwaves affecting marine life and human communities?..........................146
20 FAQ3.4: Which industries and jobs are most vulnerable to the impacts of climate change in the

21  oceans? ...................................................................................................................................................148

22 FAQ3.5: How can nature-based solutions, including marine protected areas, help us to adapt to

23  climate driven changes in the oceans? ................................................................................................150

24 References......................................................................................................................................................152



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 1 Executive Summary


 3 Ocean and coastal ecosystems support life on Earth and many aspects of human well-being. Covering two-
 4 thirds of the planet, the ocean hosts vast biodiversity and modulates the global climate system by regulating
 5 cycles of heat, water, and elements including carbon. Marine systems are central to many cultures, and they
 6 also provide food, minerals, energy and employment to people. Since previous assessments1, new laboratory
 7 studies, field observations and process studies, a wider range of model simulations, Indigenous Knowledge,
 8 and local knowledge provide increasing evidence on the impacts of climate change on ocean and coastal
 9 systems, how human communities are experiencing these impacts, and the potential solutions for ecological
10 and human adaptation.


12 Observations: vulnerabilities and impacts


14 Anthropogenic climate change has exposed ocean and coastal ecosystems to conditions that are
15 unprecedented over millennia (high confidence2), and this has greatly impacted life in the ocean and
16 along its coasts (very high confidence). Fundamental changes in the physical and chemical characteristics
17 of the ocean acting individually and together are changing the timing of seasonal activities (very high
18 confidence), distribution (very high confidence), and abundance (very high confidence) of oceanic and
19 coastal organisms, from microbes to mammals and from individuals to ecosystems, in every region.
20 Evidence of these changes is apparent from multi-decadal observations, laboratory studies and mesocosms,
21 as well as meta-analyses of published data. Geographic range shifts of marine species generally follow the
22 pace and direction of climate warming (high confidence): surface warming since the 1950s has shifted
23 marine taxa and communities poleward at an average (mean ± very likely range) of 59.2 ± 15.5 km per
24 decade (high confidence), with substantial variation in responses among taxa and regions. Seasonal events
25 occur 4.3 ± 1.8 to 7.5 ± 1.5 days earlier per decade among planktonic organisms (very high confidence) and
26 on average 3 ± 2.1 days earlier per decade for fish (very high confidence). Warming, acidification and
27 deoxygenation are altering ecological communities by increasing the spread of physiologically sub-optimal
28 conditions for many marine fish and invertebrates (medium confidence). These and other responses have
29 subsequently driven habitat loss (very high confidence), population declines (high confidence), increased
30 risks of species extirpations and extinctions (medium confidence) and rearrangement of marine food webs
31 (medium to high confidence, depending on ecosystem). {3.2, 3.3, 3.3.2, 3.3.3,,,­
32,,,,, Box 3.2}


34 Marine heatwaves lasting weeks to several months are exposing species and ecosystems to
35 environmental conditions beyond their tolerance and acclimation limits (very high confidence). WGI
36 AR6 concluded that marine heatwaves are more frequent (high confidence), more intense and longer
37 (medium confidence) since the 1980s, and since at least 2006, very likely3 attributable to anthropogenic
38 climate change. Open-ocean, coastal and shelf-sea ecosystems, including coral reefs, rocky shores, kelp
39 forests, seagrasses, mangroves, the Arctic Ocean and semi-enclosed seas, have recently undergone mass
40 mortalities from marine heatwaves (very high confidence). Marine heatwaves, including well-documented
41 events along the west coast of North America (2013­2016) and east coast of Australia (2015­2016, 2016­
42 2017 and 2020), drive abrupt shifts in community composition that may persist for years (very high
43 confidence), with associated biodiversity loss (very high confidence), collapse of regional fisheries and

1 Previous IPCC assessments include the IPCC Fifth Assessment Report (AR5) (IPCC, 2013; IPCC, 2014c; IPCC,

2014b; IPCC, 2014d), the Special Report on Global Warming of 1.5°C (SR1.5) (IPCC, 2018), the Special Report on
Ocean and Cryosphere in a Changing Climate (SROCC) (IPCC, 2019b) and the IPCC Sixth Assessment Report
Working Group I (WGI AR6).
2 In this Report, the following summary terms are used to describe the available evidence: limited, medium, or robust;
and for the degree of agreement: low, medium, or high. A level of confidence is expressed using five qualifiers: very
low, low, medium, high, and very high, and typeset in italics, e.g., medium confidence. For a given evidence and
agreement statement, different confidence levels can be assigned, but increasing levels of evidence and degrees of
agreement are correlated with increasing confidence.
3 In this Report, the following terms have been used to indicate the assessed likelihood of an outcome or a result:
virtually certain 99­100% probability, very likely 90­100%, likely 66­100%, about as likely as not 33­ 66%, unlikely
0­33%, very unlikely 0­10%, exceptionally unlikely 0­1%. Additional terms (extremely likely: 95­ 100%, more likely
than not >50­100%, and extremely unlikely 0­5%) may also be used when appropriate. Assessed likelihood is typeset
in italics, e.g., very likely.

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 1 aquaculture (high confidence) and reduced capacity of habitat-forming species to protect shorelines (high
 2 confidence). {WGI AR6 Chapter 9,,­,,,,, 3.5.3}


 4 At local to regional scales, climate change worsens the impacts on marine life of non-climate
 5 anthropogenic drivers, such as habitat degradation, marine pollution, overfishing and overharvesting,
 6 nutrient enrichment, and introduction of non-indigenous species (very high confidence). Although
 7 impacts of multiple climate and non-climate drivers can be beneficial or neutral to marine life, most are
 8 detrimental (high confidence). Warming exacerbates coastal eutrophication and associated hypoxia, causing
 9 'dead zones' (very high confidence), which drive severe impacts on coastal and shelf-sea ecosystems (very
10 high confidence), including mass mortalities, habitat reduction and fisheries disruptions (medium
11 confidence). Overfishing exacerbates effects of multiple climate-impact drivers on predators at the top of the
12 marine food chain (medium confidence). Urbanization and associated changes in freshwater and sediment
13 dynamics increase the vulnerability of coastal ecosystems like sandy beaches, saltmarshes and mangrove
14 forests to sea-level rise and changes in wave energy (very high confidence). Although these non-climate
15 drivers confound attribution of impacts to climate change, adaptive, inclusive, and evidence-based
16 management reduces the cumulative pressure on ocean and coastal ecosystems, which will decrease their
17 vulnerability to climate change (high confidence). {3.3, 3.3.3,­,, 3.5.3, 3.6.2, Cross-
18 Chapter Box SLR in Chapter 3}


20 Climate-driven impacts on ocean and coastal environments have caused measurable changes in
21 specific industries, economic losses, emotional harm, and altered cultural and recreational activities
22 around the world (high confidence). Climate-driven movement of fish stocks is causing commercial,
23 small-scale, artisanal, and recreational fishing activities to shift poleward and diversify harvests (high
24 confidence). Climate change is increasing the geographic spread and risk of marine-borne pathogens like
25 Vibrio sp. (very high confidence), which endanger human health and decrease provisioning and cultural
26 ecosystem services (high confidence). Interacting climatic impact-drivers and non-climate drivers are
27 enhancing movement and bioaccumulation of toxins and contaminants into marine food webs (medium
28 evidence, high agreement), and increasing salinity of coastal waters, aquifers, and soils (very high
29 confidence), which endangers human health (very high confidence). Combined climatic impact-drivers and
30 non-climate drivers also expose densely populated coastal zones to flooding (high confidence) and decrease
31 physical protection of people, property, and culturally important sites (very high confidence). {,
32 3.5.3, 3.5.5,, 3.5.6, Cross-Chapter Box SLR in Chapter 3}


34 Projections: vulnerabilities, risks, and impacts


36 Ocean conditions are projected to continue diverging from a pre-industrial state (virtually certain),
37 with the magnitude of warming, acidification, deoxygenation, sea-level rise and other climatic impact-
38 drivers depending on the emission scenario (very high confidence), and to increase risk of regional
39 extirpations and global extinctions of marine species (medium confidence). Marine species richness near
40 the equator and in the Arctic is projected to continue declining, even with less than 2°C warming by the end
41 of the century (medium confidence). In the deep ocean, all global warming levels will cause faster
42 movements of temperature niches by 2100 than those that have driven extensive reorganisation of marine
43 biodiversity at the ocean surface over the past 50 years (medium confidence). "At warming levels beyond
44 2°C by 2100, risks of extirpation, extinction and ecosystem collapse escalate rapidly (high confidence)."
45 Paleorecords indicate that at extreme global warming levels (>5.2°C), mass extinction of marine species may
46 occur (medium confidence). {Box 3.2,,,,, Cross-Chapter Box PALEO in
47 Chapter 1}


49 Climate impacts on ocean and coastal ecosystems will be exacerbated by increases in intensity,
50 reoccurrence and duration of marine heatwaves (high confidence), in some cases, leading to species
51 extirpation, habitat collapse or surpassing ecological tipping points (very high confidence). Some
52 habitat-forming coastal ecosystems including many coral reefs, kelp forests and seagrass meadows, will
53 undergo irreversible phase shifts due to marine heatwaves with global warming levels >1.5°C and are at high
54 risk this century even in <1.5°C scenarios that include periods of temperature overshoot beyond 1.5°C (high
55 confidence). Under SSP1-2.6, coral reefs are at risk of widespread decline, loss of structural integrity and
56 transitioning to net erosion by mid-century due to increasing intensity and frequency of marine heatwaves
57 (very high confidence). Due to these impacts, the rate of sea-level rise is very likely to exceed that of reef

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 1 growth by 2050, absent adaptation. Other coastal ecosystems, including kelp forests, mangroves and
 2 seagrasses, are vulnerable to phase shifts towards alternate states as marine heatwaves intensify (high
 3 confidence). Loss of kelp forests are expected to be greatest at the low-latitude warm edge of species' ranges
 4 (high confidence). {,,, 3.4.4}


 6 Escalating impacts of climate change on marine life will further alter biomass of marine animals
 7 (medium confidence), the timing of seasonal ecological events (medium confidence) and the geographic
 8 ranges of coastal and ocean taxa (medium confidence), disrupting life cycles (medium confidence), food
 9 webs (medium confidence) and ecological connectivity throughout the water column (medium
10 confidence). Multiple lines of evidence suggest that climate-change responses are very likely to amplify up
11 marine food webs over large regions of the ocean. Modest projected declines in global phytoplankton
12 biomass translate into larger declines of total animal biomass (by 2080­2099 relative to 1995­2014) ranging
13 from (mean ± very likely range) ­5.7% ± 4.1% to ­15.5% ± 8.5% under SSP1-2.6 and SSP5-8.5, respectively
14 (medium confidence). Projected declines in upper-ocean nutrient concentrations, likely associated with
15 increases in stratification, will reduce carbon export flux to the mesopelagic and deep-sea ecosystems
16 (medium confidence). This will lead to a decline in the biomass of abyssal meio- and macrofauna (by 2081­
17 2100 relative to 1995­2014) by ­9.8% and ­13.0% under SSP1-2.6 and SSP5-8.5, respectively (limited
18 evidence). By 2100, 18.8% ± 19.0% to 38.9% ± 9.4% of the ocean will very likely undergo a change of more
19 than 20 days (advances and delays) in the start of the phytoplankton growth period under SSP1-2.6 and
20 SSP5-8.5, respectively (low confidence). This altered timing increases the risk of temporal mismatches
21 between plankton blooms and fish spawning seasons (medium to high confidence) and increases the risk of
22 fish recruitment failure for species with restricted spawning locations, especially in mid-to-high latitudes of
23 the northern hemisphere (low confidence). Projected range shifts among marine species (medium confidence)
24 suggest extirpations and strongly decreasing tropical biodiversity. At higher latitudes, range expansions will
25 drive increased homogenisation of biodiversity. The projected loss of biodiversity ultimately threatens
26 marine ecosystem resilience (medium to high confidence), with subsequent effects on service provisioning
27 (medium to high confidence). {,,­, 3.5, WGI AR6 Section}


29 Risks from sea-level rise for coastal ecosystems and people are very likely to increase tenfold well
30 before 2100 without adaptation and mitigation action as agreed by Parties to the Paris Agreement
31 (very high confidence). Sea-level rise under emission scenarios that do not limit warming to 1.5°C will
32 increase the risk of coastal erosion and submergence of coastal land (high confidence), loss of coastal habitat
33 and ecosystems (high confidence) and worsen salinisation of groundwater (high confidence), compromising
34 coastal ecosystems and livelihoods (high confidence). Under SSP1-2.6, most coral reefs (very high
35 confidence), mangroves (likely, medium confidence) and saltmarshes (likely, medium confidence) will be
36 unable to keep up with sea-level rise by 2050, with ecological impacts escalating rapidly beyond 2050,
37 especially for scenarios coupling high emissions with aggressive coastal development (very high
38 confidence). Resultant decreases in natural shoreline protection will place increasing numbers of people at
39 risk (very high confidence). The ability to adapt to current coastal impacts, cope with future coastal risks, and
40 prevent further acceleration of sea-level rise beyond 2050 depends on immediate implementation of
41 mitigation and adaptation actions (very high confidence). {,,,,, Cross-
42 Chapter Box SLR in Chapter 3}


44 Climate change will alter many ecosystem services provided by marine systems (high confidence), but
45 impacts to human communities will depend on people's overall vulnerability, which is strongly
46 influenced by local context and development pathways (very high confidence). Catch composition and
47 diversity of regional fisheries will change (high confidence), and fishers who are able to move, diversify, and
48 leverage technology to sustain harvests decrease their own vulnerability (medium confidence). Management
49 that eliminates overfishing facilitates successful future adaptation of fisheries to climate change (very high
50 confidence). Marine-dependent communities, including Indigenous Peoples and local peoples, will be at
51 increased risk of losing cultural heritage and traditional seafood-sourced nutrition (medium confidence).
52 Without adaptation, seafood-dependent people face increased risk of exposure to toxins, pathogens, and
53 contaminants (high confidence), and coastal communities face increasing risk from salinisation of
54 groundwater and soil (high confidence). Early-warning systems and public education about environmental
55 change, developed and implemented within the local and cultural context, can decrease those risks (high
56 confidence). Coastal development and management informed by sea-level rise projections will reduce the
57 number of people and amount of property at risk (high confidence), but historical coastal development and

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 1 policies impede change (high confidence). Current financial flows are globally uneven and overall
 2 insufficient to meet the projected costs of climate impacts on coastal and marine socio-ecological systems
 3 (very high confidence). Inclusive governance that accommodates geographically shifting marine life;
 4 financially supports needed human transformations; provides effective public education; and incorporates
 5 scientific evidence, Indigenous knowledge, and local knowledge to manage resources sustainably shows
 6 greatest promise for decreasing human vulnerability to all of these projected changes in ocean and coastal
 7 ecosystem services (very high confidence). {3.5.3, 3.5.5, 3.5.6, 3.6.3, Box 3.4, Cross-Chapter Box ILLNESS
 8 in Chapter 2, Cross-Chapter Box SLR in Chapter 3}


10 Solutions, trade-offs, residual risk, decisions and governance


12 Humans are already adapting to climate-driven changes in marine systems, and while further
13 adaptations are required even under low-emission scenarios (high confidence), transformative
14 adaptation will be essential under high-emission scenarios (high confidence). Low-emission scenarios
15 permit a wider array of feasible, effective and low-risk nature-based adaptation options (e.g., restoration,
16 revegetation, conservation, early-warning systems for extreme events, and public education) (high
17 confidence). Under high-emission scenarios, adaptation options (e.g., hard infrastructure for coastal
18 protection, assisted migration or evolution, livelihood diversification, migration and relocation of people) are
19 more uncertain and require transformative governance changes (high confidence). Transformative climate
20 adaptation will reinvent institutions to overcome obstacles arising from historical precedents, reducing
21 current barriers to climate adaptation in cultural, financial, and governance sectors (high confidence).
22 Without transformation, global inequities will likely increase between regions (high confidence) and conflicts
23 between jurisdictions may emerge and escalate. {3.5, 3.5.2,, 3.6,,,,,
24,, 3.6.5, Cross-Chapter Box SLR in Chapter 3, Cross-Chapter Box ILLNESS in Chapter 2}


26 Available adaptation options are unable to offset climate-change impacts on marine ecosystems and
27 the services they provide (high confidence). Adaptation solutions implemented at appropriate scales,
28 when combined with ambitious and urgent mitigation measures, can meaningfully reduce impacts
29 (high confidence). Increasing evidence from implemented adaptations indicates that multi-level governance,
30 early-warning systems for climate-associated marine hazards, seasonal and dynamic forecasts, habitat
31 restoration, ecosystem-based management, climate-adaptive management, and sustainable harvesting tend to
32 be both feasible and effective (high confidence). Marine protected areas, as currently implemented, do not
33 confer resilience against warming and heatwaves (medium confidence) and are not expected to provide
34 substantial protection against climate impacts past 2050 (high confidence). However, marine protected areas
35 can contribute substantially to adaptation and mitigation if they are designed to address climate change,
36 strategically implemented, and well governed (high confidence). Habitat restoration limits climate-change
37 related loss of ecosystem services, including biodiversity, coastal protection, recreational use and tourism
38 (medium confidence), provides mitigation benefits on local to regional scales (e.g., via carbon-storing `blue
39 carbon' ecosystems) (high confidence), and may safeguard fish stock production in a warmer climate
40 (limited evidence). Ambitious and swift global mitigation offers more adaptation options and pathways to
41 sustain ecosystems and their services (high confidence). {3.4.2,, 3.5, 3.5.2, 3.5.3,,,
42,,,,,, 3.6.5, Figure 3.24, Figure 3.25}


44 Nature-based solutions for adaptation of ocean and coastal ecosystems can achieve multiple benefits
45 when well-designed and implemented (high confidence), but their effectiveness declines without
46 ambitious and urgent mitigation (high confidence). Nature-based solutions such as ecosystem-based
47 management, climate-smart conservation approaches (i.e., climate-adaptive fisheries and conservation) and
48 coastal habitat restoration can be cost-effective and generate social, economic and cultural co-benefits, while
49 contributing to the conservation of marine biodiversity and reducing cumulative anthropogenic drivers (high
50 confidence). The effectiveness of nature-based solutions declines with warming; conservation and restoration
51 will alone be insufficient to protect coral reefs beyond 2030 (high confidence) and to protect mangroves
52 beyond the 2040s (high confidence). The multi-dimensionality of climate change impacts and their
53 interactions with other anthropogenic stressors calls for integrated approaches that identify trade-offs and
54 synergies across sectors and scales in space and time to build resilience of ocean and coastal ecosystems and
55 the services they deliver (high confidence). {3.4.2, 3.5.2, 3.5.3,,,,,,
56 3.6.5, Figure 3.25, Table SM3.6}


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 1 Ocean-focused adaptations, especially those that employ nature-based solutions, address existing
 2 inequalities, and incorporate just and inclusive decision-making and implementation processes,
 3 support the UN Sustainable Development Goals (SDGs) (high confidence). There are predominantly
 4 positive synergies between adaptation options for Life Below Water (SDG14), Climate Action (SDG13), and
 5 social, economic and governance SDGs (SDG1­12, 16­17) (high confidence), but the ability of ocean
 6 adaptation to contribute to the Sustainable Development Goals is constrained by the degree of mitigation
 7 action (high confidence). Furthermore, existing inequalities and entrenched practices limit effective and just
 8 responses to climate change in coastal communities (high confidence). Momentum is growing towards
 9 transformative international and regional governance that will support comprehensive, equitable ocean and
10 coastal adaptation while also achieving SDG14 (robust evidence), without compromising achievement of
11 other SDGs. {,,, Figure 3.26}.

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 1 3.1 Point of Departure


 3 The ocean contains approximately 97% of Earth's water within a system of interconnected basins that cover
 4 71% of its surface. Coastal systems mostly extend seaward from the high-water mark, or just beyond, to the
 5 edge of the continental shelf and include shores of soft sediments, rocky shores and reefs, embayments,
 6 estuaries, deltas and shelf systems. Oceanic systems comprise waters beyond the shelf edge, from ~200 m to
 7 nearly 11,000 m deep (Stewart and Jamieson, 2019), with an average depth of approximately 3700 m. The
 8 epipelagic zone, or upper 200 m of the ocean, is illuminated by sufficient sunlight to sustain photosynthesis
 9 that supports the rich marine food web. Below the epipelagic zone lies the barely-lit mesopelagic zone (200­
10 1000 m), the perpetually dark bathypelagic zone (depth >1000 m) and the deep seafloor (benthic ecosystems
11 at depths > 200 m), which spans rocky and sedimentary habitats on seamounts, mid-ocean ridges and
12 canyons, abyssal plains and sedimented margins. Semi-enclosed seas (SES) include both coastal and oceanic
13 systems.


15 The ocean sustains life on Earth by providing essential resources and modulating planetary flows of energy
16 and materials. Together, harvests from the ocean and inland waters provide more than 20% of dietary animal
17 protein for more than 3.3 billion people worldwide and livelihoods for about 60 million people (FAO,
18 2020b). The global ocean is centrally involved in sequestering anthropogenic atmospheric CO2 and recycling
19 many elements, and it regulates the global climate system by redistributing heat and water (WGI AR6
20 Chapter 9, Fox-Kemper et al., 2021). The ocean also provides a wealth of aesthetic and cultural resources
21 (Barbier et al., 2011), contains vast biodiversity (Appeltans et al., 2012), supports more animal biomass than
22 on land (Bar-On et al., 2018), and produces at least half the world's photosynthetic oxygen (Field et al.,
23 1998). Ecosystem services (Annex II: Glossary) delivered by ocean and coastal ecosystems support
24 humanity by protecting coastlines, providing nutrition and economic opportunities (Figure 3.1, Selig et al.,
25 2019), and providing many intangible benefits. Even though ecosystem services and biodiversity underpin
26 human well-being and support climate mitigation and adaptation (Pörtner et al., 2021b), there are also ethical
27 arguments for preserving biodiversity and ecosystem functions regardless of the beneficiary (e.g., Taylor et
28 al., 2020). This chapter assesses the impact of climate change on the full spectrum of ocean and coastal
29 ecosystems, on their services and on related human activities, and it assesses marine-related opportunities
30 within both ecological and social systems to adapt to climate change.




34 Figure 3.1: Estimated relative human dependence on marine ecosystems for coastal protection, nutrition, fisheries
35 economic benefits and overall. Each bar represents an index value that semi-quantitatively integrates the magnitude,
36 vulnerability to loss and substitutability of the benefit. Indices synthesize information on people's consumption of
37 marine protein and nutritional status, gross domestic product, fishing revenues, unemployment, education, governance
38 and coastal characteristics. Overall dependence is the mean of the three index values after standardization from 0­1
39 (Details are found in Table 1 and supplementary material of Selig et al. (2019)). This index does not include the

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 1 economic benefits from tourism or other ocean industries, and data limitations prevented including artisanal or
 2 recreational fisheries or the protective impact of saltmarshes (Selig et al., 2019). Values for reference regions
 3 established in the WGI AR6 Atlas (Gutiérrez et al., 2021) were computed as area-weighted means from original
 4 country-level data.



 7 Previous IPCC Assessment Reports (IPCC, 2014c; IPCC, 2014b; IPCC, 2018; IPCC, 2019b) have expressed
 8 growing confidence in the detection of climate-change impacts in the ocean and their attribution to
 9 anthropogenic greenhouse gas emissions. Heat and CO2 taken up by the ocean (high to very high confidence)
10 (IPCC, 2021b) directly affect marine systems, and the resultant "climatic impact-drivers" (CIDs, e.g., ocean
11 temperature and heatwaves, sea level, dissolved oxygen levels, acidification, Annex II: Glossary, WGI
12 Figure SPM.9, IPCC, 2021b) also influence ocean and coastal systems (Section 3.2, Cross-Chapter Box SLR
13 in Chapter 3, Cross-Chapter Box EXTREMES in Chapter 2, Figure SM3.1), from individual biophysical
14 processes to dependent human activities. Several marine outcomes of CIDs are themselves drivers of
15 ecological change (e.g., climate velocities, stratification, sea ice changes). This chapter updates and extends
16 the assessment of SROCC (IPCC, 2019b) and WGI AR6 by assessing the ecosystem effects of the CIDs in
17 WGI AR6 Figure SPM.9 (IPCC, 2021b) and their biologically relevant marine outcomes (detailed in Section
18 3.2), which are referred to collectively hereafter as "climate-impact drivers4".

4 We henceforth use the term "climate-impact drivers" in reference to all drivers of ecological change that are related
directly to climate change (CIDs, IPCC, 2021a) as well as those that emerge in response to CIDs.

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2 Detrimental human impacts on ocean and coastal ecosystems are not only caused by climate. Other
3 anthropogenic activities are increasingly affecting the physical, chemical, and biological conditions of the
4 ocean (Doney, 2010; Halpern et al., 2019), and these "non-climate drivers5" also alter marine ecosystems and
5 their services. Fishing and other extractive activities are major non-climate drivers in many ocean and coastal
6 systems (Steneck and Pauly, 2019). Many activities, such as coastal development, shoreline hardening and

7 habitat destruction, physically alter marine spaces (Suchley and Alvarez-Filip, 2018; Ducrotoy et al., 2019;
8 Leo et al., 2019; Newton et al., 2020; Raw et al., 2020). Other human activities decrease water quality by

 9 overloading coastal water with terrestrial nutrients (eutrophication) and by releasing runoff containing
10 chemical, biological and physical pollutants, toxins, and pathogens (Jambeck et al., 2015; Luek et al., 2017;
11 Breitburg et al., 2018; Froelich and Daines, 2020) Some human activities disturb marine organisms by

12 generating excess noise and light (Davies et al., 2014; Duarte et al., 2021), while others decrease natural light
13 penetration into the ocean (Wollschläger et al., 2021). Several anthropogenic activities alter processes that

14 span the land-sea interface by changing coastal hydrology or causing coastal subsidence (Michael et al.,

15 2017; Phlips et al., 2020; Bagheri-Gavkosh et al., 2021). Atmospheric pollutants can harm marine systems or
16 unbalance natural marine processes (Doney et al., 2007; Hagens et al., 2014; Lamborg et al., 2014; Ito et al.,
17 2016). Organisms frequently experience non-climate drivers simultaneously with climate-impact drivers

18 (Section 3.4), and feedbacks may exist between climate-impact drivers and non-climate drivers that enhance
19 the effects of climate change (Rocha et al., 2015; Ortiz et al., 2018; Wolff et al., 2018; Cabral et al., 2019;
20 Bowler et al., 2020; Gissi et al., 2021). SROCC assessed with high confidence that reduction of pollution and

21 other stressors, along with protection, restoration, and precautionary management, supports ocean and
22 coastal ecosystems and their services (IPCC, 2019b). This chapter examines the combined influence of
23 climate-impact drivers and primary non-climate drivers on many ecosystems assessed.


25 Detecting changes and attributing them to specific drivers have been especially difficult in ocean and coastal

26 ecosystems because drivers, responses and scales (temporal, spatial, organizational) often overlap and
27 interact (IPCC, 2014c; IPCC, 2014b; Abram et al., 2019; Gissi et al., 2021). In addition, some marine

28 systems have short, heterogeneous, or geographically biased observational records, which exacerbate the

29 interpretation challenge (Beaulieu et al., 2013; Christian, 2014; Huggel et al., 2016; Benway et al., 2019). It
30 is even more challenging to detect and attribute climate impacts on marine-dependent human systems, where

31 culture, governance and society also strongly influence observed outcomes. To assess climate-driven change

32 in natural and social systems robustly, IPCC reports rely on multiple lines of evidence, and the available
33 types of evidence differ depending on the system under study (Section, Cross-Working Group Box

34 ATTRIB). Lines of evidence used for ocean and coastal ecosystems for this and previous assessments

35 include observed phenomena, laboratory and field experiments, long-term monitoring, empirical and

36 dynamical model analyses, Indigenous knowledge (IK) and local knowledge (LK), and paleorecords (IPCC,
37 2014c; IPCC, 2014b; IPCC, 2019b). The growing body of climate research for ocean and coastal ecosystems

38 and their services increasingly provides multiple independent lines of evidence whose conclusions support

39 each other, raising the overall confidence in detection and attribution of impacts over time (Section,
40 Cross-Working Group Box ATTRIB in Chapter 3).





45 FAQ3.1: How do we know which changes to marine ecosystems are specifically caused by climate

46  change?


48 To attribute changes in marine ecosystems to human-induced climate change, scientists use paleorecords

49 (reconstructing the links between climate, evolutionary and ecological changes in the geological past),
50 contemporary observations (assessing current climate and ecological responses in the field and through

51 experiments) and models. We refer to these as multiple lines of evidence, meaning that the evidence comes
52 from diverse approaches, as described below.


    5 We henceforth use the term "non-climate drivers" in reference to drivers of ecological change that are not caused by
    climate change.

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 1 Emissions of greenhouse gases like carbon dioxide from human activity cause ocean warming, acidification,
 2 oxygen loss, and other physical and chemical changes that are affecting marine ecosystems around the
 3 world. At the same time, natural climate variability and direct human impacts, such as overfishing and
 4 pollution, also affect marine ecosystems locally, regionally and globally. These climate and non-climate
 5 impact drivers counteract each other, add up or multiply to produce smaller or larger changes than expected
 6 from individual drivers. Attribution of changes in marine ecosystems requires evaluating the often-
 7 interacting roles of natural climate variability, non-climate drivers, and human-induced climate change. To
 8 do this work, scientists use
 9 - paleorecords: reconstructing the links between climate and evolutionary and ecological changes of the past
10 - contemporary observations: assessing current climate and ecological responses,
11 - manipulation experiments: measuring responses of organisms and ecosystems to different climate
12 conditions
13 - models: testing whether we understand how organisms and ecosystems are impacted by different stressors,
14 and quantifying the relative importance of different stressors.


16 Paleorecords can be used to trace the correlation between past changes in climate and marine life.
17 Paleoclimate is reconstructed from the chemical composition of shells and teeth or from sediments and ice
18 cores. Changes to sea life signalled by changing biodiversity, extinction or distributional shifts are
19 reconstructed from fossils. Using large datasets, we can infer the effects of climate change on sea life over
20 relatively long timescales ­ usually hundreds to millions of years. The advantage of paleorecords is that they
21 provide insights into how climate change affects life from organisms to ecosystems, without the
22 complicating influence of direct human impacts. A key drawback is that the paleo and modern worlds do not
23 have fully comparable paleoclimate regimes, dominant marine species, and rates of climate change.
24 Nevertheless, the paleorecord can be used to derive fundamental rules by which organisms, ecosystems,
25 environments and regions are typically most affected by climate change. For example, the paleorecord shows
26 that coral reefs repeatedly underwent declines during past warming events, supporting the inference that
27 corals may not be able to adapt to current climate warming.


29 Contemporary observations over recent decades allow scientists to relate the status of marine species and
30 ecosystems to changes in climate or other factors. For example, scientists compile large datasets to determine
31 whether species usually associated with warm water are appearing in traditionally cool-water areas that are
32 rapidly warming. A similar pattern observed in multiple regions and over several decades (i.e., longer than
33 timescales of natural variability) provides confidence that climate change is altering community structure.
34 This evidence is weighed against findings from other approaches, such as manipulation experiments, to
35 provide a robust picture of climate change impacts in the modern ocean.


37 In manipulation experiments, scientists expose organisms or communities of organisms to multiple stressors,
38 for example, elevated CO2, high temperature, or both, based on values drawn from future climate
39 projections. Such experiments will involve multiple treatments (i.e., different aquarium tanks) in which
40 organisms are exposed to different combinations of the stressors. This approach enables scientists to
41 understand the effects of individual stressors as well as their interactions to explore physiological thresholds
42 of marine organisms and communities. The scale of manipulation experiments can range from small tabletop
43 tanks to large installations or natural ocean experiments involving tens of thousands of litres of water.


45 Ecological effects of climate change are also explored within models developed from fundamental scientific
46 principles and observations. Using these numerical representations of marine ecosystems, scientists can
47 explore how different levels of climate change and non-climate stressors influence species and ecosystems at
48 scales not possible with experiments. Models are commonly used to simulate the ecological response to
49 climate change over recent decades and centuries. Convergence between the model results and the
50 observations suggests that our understanding of the key processes is sufficient to attribute the observed
51 ecological changes to climate change, and to use the models to project future ecological changes. Differences
52 between model results and observations indicate gaps in knowledge to be filled in order to better detect and
53 attribute the impacts of climate change on marine life.


55 Using peer-reviewed research spanning the full range of scientific approaches (paleorecords, observations,
56 experiments and models), we can assess the level of confidence in the impact of climate change on observed
57 modifications in marine ecosystems. We refer to this as multiple lines of evidence, meaning that the evidence

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1 comes from the diverse approaches described above. This allows policy-makers and managers to address the
2 specific actions needed to reduce climate change and other impacts.




 6 Figure FAQ3.1: Examples of well-known impacts of anthropogenic climate change and associated nature-
 7 based adaptation. To attribute changes in marine ecosystems to anthropogenic climate change, scientists use
 8 multiple lines of evidence including paleorecords, contemporary observations, manipulation experiments and
 9 models.


11 [END FAQ3.1 HERE]



14 Natural adaptation to climate change in ocean and coastal systems includes an array of responses taking
15 place at scales from cells to ecosystems. Previous IPCC assessments have established that many marine
16 species "have shifted their geographic ranges, seasonal activities, migration patterns, abundances and species
17 interactions in response to climate change," (high confidence) (IPCC, 2014c; IPCC, 2014b), which has had
18 global impacts on species composition, abundance and biomass, and on ecosystem structure and function
19 (medium confidence) (IPCC, 2019b). Warming and acidification have affected coastal ecosystems in concert
20 with non-climate drivers (high confidence), which have affected habitat area, biodiversity, ecosystem
21 function and services (high confidence) (IPCC, 2019b). Confidence has grown in these assessments over
22 time as observational datasets have lengthened and other lines of evidence have corroborated observations.
23 AR5 and SROCC assessed how physiological sensitivity to climate-impact drivers is the underlying cause of

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 1 most marine organisms' vulnerability to climate (high confidence) (Pörtner et al., 2014; Bindoff et al., 2019).
 2 Since those assessments, more evidence supports the empirical physiological models of tolerance and
 3 plasticity (Sections 3.3.2, 3.3.4) and of interactions among multiple (climate and non-climate) drivers at
 4 individual to ecosystem scales (Sections 3.3.3, 3.4.5). New experimental evidence about evolutionary
 5 adaptation (Section 3.3.4) bolsters previous assessments that adaptation options to climate change are limited
 6 for eukaryotic organisms. Tools such as ecosystem models can now constrain probable ecosystem states
 7 (Sections 3.3.4­3.3.5 and 3.4). Observations have increased understanding of how extreme events affect
 8 individuals, populations and ecosystems, helping refine understanding of both ecological tolerance to climate
 9 impacts and ecological transformations (Section 3.4).


11 Human adaptation to climate impacts on ocean and coastal systems spans a variety of actions that change
12 human activity to maintain marine ecosystem services. After AR5 concluded that coastal adaptation could
13 reduce the effects of climate impacts on coastal human communities (high agreement, limited evidence)
14 (Wong et al., 2014), SROCC confirmed that mostly risk-reducing ocean and coastal adaptation responses
15 were underway (Bindoff et al., 2019). However, overlapping climate-impact and non-climate drivers
16 confound implementation and assessment of the success of marine adaptation, revealing the complexity of
17 attempting to maintain marine ecosystems and services through adaptation. SROCC assessed with high
18 confidence that while the benefits of many locally implemented adaptations exceed their disadvantages,
19 others are marginally effective and have large disadvantages, and overall, adaptation has a limited ability to
20 reduce the probable risks from climate change, being at best a temporary solution (Bindoff et al., 2019).
21 SROCC also concluded that a portfolio of many different types of adaptation actions, effective and inclusive
22 governance, and mitigation must be combined for successful adaptation (Bindoff et al., 2019). The portfolio
23 of adaptation measures has now been defined (Section 3.6.2), and individual and combined adaptation
24 solutions have been implemented in several marine sectors (Section 3.6.3). Delays in marine adaptation have
25 been partly attributed to the complexity of ocean governance (Section 3.6.4, Cross-Chapter Box 3 and Figure
26 CB3.1 in Abram et al., 2019) and to the low priority accorded the ocean in international development goals
27 (Nash et al., 2020), but in recent years the ocean is being increasingly incorporated in international climate
28 policy and multilateral environmental agreements (Section 3.6.4).


30 This chapter assesses the current understanding of climate-impact drivers, ecological vulnerability and
31 adaptability, risks to coastal and ocean ecosystems, and human vulnerability and adaptation to resulting
32 changes in ocean benefits, now and in the future (Figure 3.2). It starts by assessing the biologically relevant
33 outcomes of anthropogenic climate-impact drivers (Section 3.2). Next, it sets out the mechanisms that
34 determine the responses of ocean and coastal organisms to individual and combined drivers from the genetic
35 to the ecosystem level (Section 3.3). This supports a detailed assessment of the observed and projected
36 responses of coastal and ocean ecosystems to these hazards, placing them in context using the paleo-record
37 (Section 3.4). These observed and projected impacts are used to quantify consequent risks to delivery of
38 ecosystem services and the socioeconomic sectors that depend on them, with attention to the vulnerability,
39 resilience and adaptive capacity of social-ecological systems (Section 3.5). The chapter concludes by
40 assessing the state of adaptation and governance actions available to address these emerging threats while
41 also advancing human development (Section 3.6). Abbreviations used repeatedly in the chapter are defined
42 in Table 3.1.



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2 Figure 3.2: WGII AR6 Chapter 3 concept map. This chapter assesses how climate changes both the properties (top of
3 wave, Sections 3.1­3.6) and the mechanisms (below wave, Sections 3.2­3.6) that influence the ocean and coastal
4 social-ecological system. The Sustainable Development Goals (top right) represent ideal outcomes and achievement of
5 equitable, healthy and sustainable ocean and coastal social-ecological systems.



8 Table 3.1: List of abbreviations frequently used in this chapter, with brief definitions for many of the abbreviations
9 used.

   Abbreviation                      Definition

   ABNJ                              Areas beyond national jurisdiction. The water column
                                     beyond the exclusive economic zone called the high seas
                                     and the seabed beyond the limits of the continental shelf
                                     established in conformity with United Nations Convention
                                     on the Law of the Sea.

   AMOC                              Atlantic meridional overturning circulation (WGI AR6
                                     Glossary, IPCC, 2021a).

   AR5                               The IPCC Fifth Assessment Report (IPCC, 2013; IPCC,

                                     2014c; IPCC, 2014b; IPCC, 2014d).

   CBD                               Convention on Biological Diversity. An international legal
                                     instrument that has been ratified by 196 nations to
                                     conserve biological diversity, sustainably use its
                                     components and share its benefits fairly and equitably.

   CE                                Common era.

   CID                               Climatic Impact-Driver (WGI AR6 Glossary, IPCC,


   CMIP5, CMIP6                      The Coupled Model Intercomparison Project, Phase 5 or 6
                                     (WGI AR6 Glossary, IPCC, 2021a).

   EbA                               Ecosystem-based adaptation. The use of ecosystem

                                     management activities to increase the resilience and reduce

                                     the vulnerability of people and ecosystems to climate


   EBUS                              Eastern boundary upwelling system (WGI AR6 Glossary,
                                     IPCC, 2021a).

   EBUE/EUS                          Eastern boundary upwelling systems/equatorial upwelling
                                     systems. EBUEs are marine ecosystems in EBUS. EUSs

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ESM                               are located on the equator, mostly on the eastern side of
                                  major ocean basins, where trade winds drive upwelling.
                                  Exclusive economic zone. The area from the coast to 200
GMSL/GMSLR                        nautical miles (370 km) off the coast, where a nation
HAB                               exercises its sovereign rights and exclusive management
ICZM                              authority.
IK and LK
MHW                               Earth-system model. A coupled atmosphere­ocean general
MPA                               circulation model (AOGCM, WGI AR6 Glossary, IPCC,
NbS                               2021a) in which a representation of the carbon cycle is
                                  included, allowing for interactive calculation of
NDC                               atmospheric CO2 or compatible emissions.
OECM                              The Fisheries and Marine Ecosystem Model
OMZ                               Intercomparison Project. Fish-MIP is a component of the
pCO2                              Inter-Sectoral Impact Model Intercomparison
pH                                Project (ISIMIP) that explores the long-term impacts of
POC                               climate change on fisheries and marine ecosystems using
                                  scenarios from CMIP models.
Do Not Cite, Quote or Distribute
                                  Global mean sea level/global mean sea-level rise (Sea-
                                  level change, WGI AR6 Glossary, IPCC, 2021a).

                                  Harmful algal bloom. A HAB is an algal bloom composed
                                  of phytoplankton known to naturally produce bio-toxins
                                  that are harmful to the resident population, as well as

                                  Integrated coastal zone management. ICZM is a dynamic,
                                  multidisciplinary and iterative process to promote
                                  sustainable management of coastal zones (European
                                  Environmental Agency).

                                  Indigenous knowledge and Local knowledge (SROCC
                                  Glossary, IPCC, 2019a).

                                  Marine heatwaves (WGI AR6 Glossary, IPCC, 2021a).

                                  Marine protected area. MPA is an area-based management
                                  approach, commonly intended to conserve, preserve, or
                                  restore biodiversity and habitat, protect species, or manage
                                  resources (especially fisheries).

                                  Nature-based Solution. Actions to protect, sustainably
                                  manage and restore natural or modified ecosystems that
                                  address societal challenges effectively and adaptively,
                                  simultaneously providing human well-being and
                                  biodiversity benefits (IUCN, 2016).

                                  Nationally determined contribution by Parties to the Paris

                                  Net primary production. The difference between how
                                  much CO2 vegetation takes in during photosynthesis (gross
                                  primary production) minus how much CO2 the plants
                                  release during respiration.

                                  Other effective area-based conservation measures. OECM
                                  is a conservation designation for areas that are achieving
                                  the effective in situ conservation of biodiversity outside of
                                  protected areas.

                                  Oxygen minimum zone (WGI AR6 Glossary, IPCC,

                                  Partial pressure of carbon dioxide. For seawater, pCO2 is
                                  used to measure the amount of carbon dioxide dissolved in

                                  Potential of hydrogen (WGI AR6 Glossary, IPCC, 2021a).

                                  Particulate organic carbon. POC is a fraction of total
                                  organic carbon operationally defined as that which does

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    SDG                                             not pass through a filter pore size that typically ranges in
                                                    size from 0.053­2 mm.
    SIDS                                            Sustainable Development Goals. The 17 global goals for
    SLR/RSLR/RSL                                    development for all countries established by the United
    SR15                                            Nations through a participatory process and elaborated in
                                                    the 2030 Agenda for Sustainable Development.

                                                    Semi-enclosed sea. SES means a gulf, basin or sea
                                                    surrounded by land and connected to another sea by a
                                                    narrow outlet.

                                                    Small Island Developing States (WGI AR6 Glossary,
                                                    IPCC, 2021a).

                                                    Sea-level rise/Relative rea-level rise/Relative Sea Level
                                                    (Sea-level change, WGI AR6 Glossary, IPCC, 2021a).

                                                    The IPCC Special Report on 1.5 Degrees C (IPCC, 2018).

    SROCC                                           The IPCC Special Report on the Ocean and Cryosphere
                                                    (IPCC, 2019b).

    SSP/RCP                                         Shared socio-economic pathways/Representative
                                                    concentration pathways (Pathways, IPCC, 2021a).

    SST                                             Sea-surface temperature (WGI AR6 Glossary, IPCC,


    aragonite                                       Saturation state of seawater with respect to the calcium
                                                    carbonate mineral aragonite, used as a proxy measurement
                                                    for ocean acidification.


3 3.2 Observed Trends and Projections of Climatic Impact-Drivers in the Global Ocean


5 3.2.1 Introduction


 7 Climate change exposes ocean and coastal ecosystems to changing environmental conditions, including
 8 ocean warming, SLR, acidification, deoxygenation and other climatic-impact drivers, which have distinct
 9 regional and temporal characteristics (Gruber, 2011; IPCC, 2018). This section aims to build on the WGI
10 AR6 assessment (Table 3.2) to provide an ecosystem-oriented framing of climatic impact-drivers. Updating
11 SROCC, projected trends assessed here are based on a new range of scenarios (Shared Socio-Economic
12 Pathways, SSPs), as used in the Coupled Model Intercomparison Project Phase 6 (CMIP6, Section 1.2.2).



15 Table 3.2: Overview of the main global ocean Climatic Impact-Drivers and their observed and projected trends from

16 WGI AR6, with corresponding confidence levels and links to WGI chapters, where these trends are assessed in detail.

    Climatic-Impact Observed trends over the WGI Section Projected trends over the 21st century WGI

    Drivers (Hazards) historical period                                                                 Section

    Ocean Temperature

    Ocean Warming      At the ocean surface,, 9.2.1 Ocean warming will continue over     9.2.1 (Fox-
                                                                                                        Kemper et
                       temperature has on average (Fox-Kemper et the 21st century (virtually certain),  al., 2021)

                       increased by 0.88 [0.68­ al., 2021; Gulev and with the rate of global ocean

                       1.01] °C from 1850­1900 to et al., 2021) warming starting to be scenario-

                       2011­2020.                                   dependent from about the mid-21st

                                                                    century (medium confidence).

    Marine Heatwaves   MHW have become more         Box 9.2 (Fox-   MHW will become 4 [2­9, likely Box 9.2
    (MHW)              frequent (high confidence),  Kemper et al.,  range] times more frequent in 2081­ (Fox-
                       more intense, and longer     2021)           2100 compared to 1995­2014 under Kemper et
                       (medium confidence) over                     SSP1-2.6, and 8 [3­15, likely range] al., 2021)
                       the 20th century.                            times more frequent under SSP5-8.5.

    Climate Velocities Not assessed in WGI.                         Not assessed in WGI.

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   Global Mean Sea    Since 1901, GMSL has risen 2.3.3, 9.6.1 There will be continued rise in GMSL,
   Level (GMSL)
                      by 0.20 [0.15­0.25] m, and (Fox-Kemper et through the 21st century under all 9.6.3 (Fox-

                      the rate of rise is                 al., 2021; Gulev assessed SSPs (virtually certain). Kemper et

                      accelerating.                       et al., 2021)                                         al., 2021;

                                                                                                                Lee et al.,


   Extreme Sea Levels Relative sea-level rise is 9.6.4 (Fox-              Rising mean relative sea level will 9.6.4 (Fox-
                            driving a global increase in Kemper et al.,   continue to drive an increase in the Kemper et
                            the frequency of extreme sea 2021)            frequency of extreme sea levels (high al., 2021)
                            levels (high confidence).                     confidence).

   Ocean circulation

   Ocean Stratification The upper ocean has      (Fox-   Upper-ocean stratification will
                                                                          continue to increase throughout the   (Fox-
                      become more stably                  Kemper et al.,  21st century (virtually certain).     Kemper et
                                                                                                                al., 2021)
                      stratified since at least 1970 2021)

                      (virtually certain).

   Eastern Boundary Only the California current 9.2.5 (Fox-               Eastern boundary upwelling systems    9.2.5 (Fox-
   Upwelling Systems system has experienced Kemper et al.,                will change, with a dipole spatial    Kemper et
                                                                          pattern within each system of         al., 2021)
                            some large-scale upwelling- 2021)             reduction at low latitude and
                            favourable wind                               enhancement at high latitude (high
                            intensification since the                     confidence).
                            1980s (medium confidence).

   Atlantic Overturning For the 20th century, there is, 9.2.3 The AMOC will decline over the 21st,

   Circulation        low confidence in                   (Fox-Kemper et century (high confidence, but low 9.2.3 (Fox-

   (AMOC)             reconstructed and modelled al., 2021; Gulev confidence for quantitative                   Kemper et

                      AMOC changes.                       et al., 2021) projections).                           al., 2021;

                                                                                                                Lee et al.,



   Arctic Sea-Ice     Current Arctic sea-ice    , 9.3.1 The Arctic will become practically,
   Changes            coverage levels are the             (Fox-Kemper et ice-free in September by the end of    9.3.1 (Fox-
                      lowest since at least 1850          al., 2021; Gulev the 21st century under SSP2-4.5,     Kemper et
                      for both annual mean and            et al., 2021) SSP3-7.0, and SSP5-8.5 (high            al., 2021;
                      late-summer values (high                                                                  Lee et al.,
                      confidence).                                             confidence)                      2021)

   Antarctic Sea Ice  There is no global        , 9.3.2 There is low confidence in model       9.3.2 (Fox-
   Changes                                                                                                      Kemper et
                      significant trend in Antarctic (Fox-Kemper et simulations of future Antarctic sea         al., 2021)

                      sea-ice area from 1979 to al., 2021; Gulev ice.

                      2020 (high confidence). et al., 2021)

   Ocean Chemistry

   Changes in Salinity The large-scale, near-surface, Fresh ocean regions will continue to

                      salinity contrasts have             (Fox-Kemper et get fresher and salty ocean regions (Fox-

                      intensified since at least al., 2021; Gulev will continue to get saltier in the 21st Kemper et

                      1950 (virtually certain). et al., 2021) century (medium confidence).                      al., 2021)

   Ocean Acidification Ocean surface pH has     , Ocean surface pH will continue to,
                            declined globally over the                                                ,
                            past four decades (virtually  (Canadell et al., decrease through the 21st century,
                            certain).                                                                           (Lee et al.,
                                                          2021; Gulev et except for the lower-emission          2021)
                                                          al., 2021)      scenarios SSP1-1.9 and SSP1-2.6,      et al.,
                                                                          (high confidence).

   Ocean              Deoxygenation has occurred, Subsurface oxygen content is        
                      in most open ocean regions (Canadell et al., projected to transition to historically (Canadell

                      since the mid 20th (high 2021; Gulev et unprecedented condition with decline et al.,

                      confidence).                        al., 2021)      over the 21st century (medium         2021)


   Changes in Nutrient Not assessed in WGI.                               Not assessed in WGI.


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 2 3.2.2 Physical Changes


 4 Ocean Warming, Climate Velocities and Marine Heatwaves


 6 Global mean SST has increased since the beginning of the 20th century by 0.88°C (very likely range: 0.68­
 7 1.01°C), and it is virtually certain that the global ocean has warmed since at least 1971 (WGI AR6 Section
 8 9.2, Fox-Kemper et al., 2021). A key characteristic of ocean temperature change relevant for ecosystems is
 9 climate velocity, a measure of the speed and direction at which isotherms move under climate change
10 (Burrows et al., 2011), which gives the rate at which species must migrate to maintain constant climate
11 conditions. It has been shown to be a useful and simple predictor of species distribution shifts in marine
12 ecosystems (Chen et al., 2011; Pinsky et al., 2013; Lenoir et al., 2020). Median climate velocity in the
13 surface ocean has been 21.7 km per decade since 1960, with higher values in the Arctic/sub-Arctic and
14 within 15° of the Equator (Figure 3.3, Burrows et al., 2011). While climate velocity has been slower in the
15 mesopelagic layer (200­1000 m) than in the epipelagic layer (0­200 m) over the last 50 years, it has been
16 shown to be faster in the bathypelagic (1000­4000 m) and abyssopelagic (>4000 m) layers (Figure 3.4,
17 Brito-Morales et al., 2020), suggesting that deep-ocean species could be as exposed to effects of warming as
18 species in the surface ocean (Brito-Morales et al., 2020).




22 Figure 3.3: Observed surface ocean warming, surface climate velocity and reconstructed changes in marine heatwaves

23 (MHWs) over the last 100 years. (a) Sea-surface temperature trend (°C per century) over 1925­2016 from Hadley

24 Centre Sea Ice and Sea Surface Temperature 1.1 (HadISST1.1), (b) surface climate velocity (km per decade) over

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 1 1925­2016 computed from HadISST1.1, and (c) change in total MHW days for the surface ocean from 1925­1954 to
 2 1987­2016 based on monthly proxies, from Oliver et al. (2018).



 5 Marine heatwaves (MHW) are periods of extreme seawater temperature relative to the long-term mean
 6 seasonal cycle, that persist for days to months, and that may carry severe consequences for marine
 7 ecosystems and their services (WGI AR6 Box 9.2, Hobday et al., 2016a; Smale et al., 2019; Fox-Kemper et
 8 al., 2021). MHW have become more frequent over the 20th century (high confidence), approximately
 9 doubling in frequency (high confidence) and becoming more intense and longer since the 1980s (medium
10 confidence) (WGI AR6 Box 9.2, Fox-Kemper et al., 2021). These trends in MHW are explained by an
11 increase in ocean mean temperatures (Oliver et al., 2018), and human influence has very likely contributed to
12 84­90% of them since at least 2006 (WGI AR6 Box 9.2, Fox-Kemper et al., 2021). The probability of
13 occurrence (as well as duration and intensity) of the largest and most impactful MHWs that have occurred in
14 the past 30 years has increased more than 20-fold due to anthropogenic climate change (Laufkötter et al.,
15 2020).




19 Figure 3.4: Historical and projected climate velocity. Climate velocities (in km per decade) for the (a,d,g) historical
20 period (1965­2014), and for the last 50 years of the 21st century (2051­2100) under (b,e,h) SSP1-2.6 and (c,f,i) SSP5-
21 8.5. Shown are the epipelagic (0­200 m), mesopelagic (200­1000 m) and bathypelagic (1000­4000 m) domains.
22 Updated figure from Brito-Morales et al. (2020), with Coupled Model Intercomparison Project 6 models used in
23 Kwiatkowski et al. (2020).



26 Ocean warming will continue over the 21st century (virtually certain), and with the rate of global ocean
27 warming starting to be scenario-dependent from about the mid-21st century (medium confidence). At the
28 ocean surface, it is virtually certain that SST will continue to increase throughout the 21st century, with
29 increasing hazards to many marine ecosystems (WGI AR6 Box 9.2, Fox-Kemper et al., 2021). The future
30 global mean SST increase projected by CMIP6 models for the period 1995­2014 to 2081­2100 is 0.86°C
31 (very likely range: 0.43­1.47°C) under SSP1-2.6, 1.51 °C (1.02­2.19°C) under SSP2-4.5, 2.19°C (1.56­
32 3.30°C) under SSP3-7.0, and 2.89°C (2.01­4.07°C) under SSP5-8.5 (WGI AR6 Section 9.2.1, Fox-Kemper
33 et al., 2021). Stronger surface warming occurs in parts of the tropics, in the North Pacific, and in the Arctic

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 1 Ocean, where SST increases by >4°C in 2080­2099 under SSP5-8.5 (Kwiatkowski et al., 2020). The CMIP6
 2 climate models also project ocean warming at the seafloor, with the magnitude of projected changes being
 3 less than that of surface waters but having larger uncertainties (Kwiatkowski et al., 2020). The projected end-
 4 of-the-century warming in CMIP6 as reported here is greater than assessed with Coupled Model
 5 Intercomparison Project 5 (CMIP5) models in AR5 and in SROCC for similar radiative forcing scenarios
 6 (Figure 3.5, Kwiatkowski et al., 2020), because of greater climate sensitivity in the CMIP6 model ensemble
 7 than in CMIP5 (WGI AR6 Chapter 4, Forster et al., 2020; Lee et al., 2021).


 9 MHWs will continue to increase in frequency, with a likely global increase of 2­9 times in 2081­2100
10 compared to 1995­2014 under SSP1-2.6, and 3­15 times under SSP5-8.5, with the largest increases in
11 tropical and Arctic oceans (WGI AR6 Box 9.2, Frölicher et al., 2018; Fox-Kemper et al., 2021).


13 Sea-Level Rise and Extreme Sea Levels


15 Global mean sea-level (GMSL, see also Cross-Chapter Box SLR in Chapter 3) has risen by about 0.20 m
16 since 1901 and continues to accelerate (WGI AR6 Section, Church and White, 2011; Jevrejeva et al.,
17 2014; Hay et al., 2015; Kopp et al., 2016; Dangendorf et al., 2017; Cazenave et al., 2018; Kemp et al., 2018;
18 Ablain et al., 2019; Gulev et al., 2021).


20 Most coastal ecosystems (mangroves, sea grasses, saltmarshes, shallow coral reefs, rocky shores and sandy
21 beaches) are affected by changes in relative sea-level (RSL, the change in the mean sea level relative to the
22 land, Section 3.4.2). Regional rates of RSL rise differ from the global mean due to a range of factors,
23 including local subsidence driven by anthropogenic activities such as groundwater and hydrocarbon
24 extraction (WGI AR6 Box 9.1, Fox-Kemper et al., 2021). In many deltaic regions, anthropogenic subsidence
25 is currently the dominant driver of RSL rise (WGI AR6 Section, Tessler et al., 2018; Fox-Kemper et
26 al., 2021). RSL rise is driving a global increase in the frequency of extreme sea levels (high confidence)
27 (WGI AR6 Section, Fox-Kemper et al., 2021).


29 GMSL rise through the middle of the 21st century exhibits limited dependence on emissions scenario;
30 between 1995­2014 and 2050, GMSL is likely to rise by 0.15­0.23 m under SSP1-1.9 and 0.20­0.30 m
31 under SSP5-8.5 (WGI AR6 Section 9.6.3, Fox-Kemper et al., 2021). Beyond 2050, GMSL and RSL
32 projections are increasingly sensitive to the differences among emission scenarios. Considering only
33 processes in which there is at least medium confidence (thermal expansion, land water storage, land ice
34 surface mass balance, and some ice sheet dynamic processes), GMSL between 1995­2014 and 2100 is likely
35 to rise by 0.28­0.55 m under SSP1-1.9, 0.33­0.61 m under SSP1-2.6, 0.44­0.76 m under SSP2-4.5, 0.55­
36 0.90 m under SSP3-7.0, and 0.63­1.02 m under SSP5-8.5 (Figure 3.5). Under high-emission scenarios, ice-
37 sheet processes in which there is low confidence and deep uncertainty might contribute more than one
38 additional metre to GMSL rise by 2100 (WGI AR6 Chapter 9, Fox-Kemper et al., 2021).


40 Rising mean RSL will continue to drive an increase in the frequency of extreme sea levels (high confidence).
41 The expected frequency of the current one-in-100-year extreme sea level is projected to increase by a median
42 of 20­30 times across tide-gauge sites by 2050, regardless of emission scenario (medium confidence). In
43 addition, extreme-sea-level frequency may be affected by changes in tropical cyclone climatology (low
44 confidence), wave climatology (low confidence), and tides (high confidence) associated with climate change
45 and sea-level change (WGI AR6 Section, Fox-Kemper et al., 2021).



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 2 Figure 3.5: Projected trends in climatic-impact drivers for ocean ecosystems. Panels (a,b,c,d) represent Coupled Model
 3 Intercomparison Project 5 (CMIP5) Representative Concentration Pathway (RCP) and CMIP6 Shared Socioeconomic
 4 Pathway (SSP) end-of-century changes in (a) global sea-level rise (SLR), (b) average surface pH, (c) subsurface (100­
 5 600 m) dissolved oxygen concentration and (d) euphotic-zone (0­100 m) nitrate (NO3) concentration against anomalies
 6 in sea surface temperature. All anomalies are model-ensemble averages over 2080­2099 relative to the 1870­1899
 7 baseline period (from Kwiatkowski et al., 2020), except for SLR, which shows model-ensemble median in 2100 relative
 8 to 1901 (from AR6 WGI Chapter 9). Error bars represent very likely ranges, except for SLR where they represent likely
 9 ranges. Very likely ranges for pH changes are too narrow to appear on the figure (see text). Panels (e,f,g,h) show regions
10 where end-of-century projected CMIP6 surface warming exceeds 2°C, where surface ocean pH decline exceeds 0.3,
11 where subsurface dissolved oxygen decline exceeds 30 mmol m-3 and where euphotic-zone (0­100 m) nitrate decline
12 exceeds 1 mmol m-3 in (e) SSP1-2.6, (f) SSP2-4.5, (g) SSP3-7.0 and (h) SSP5-8.5. All anomalies are 2080­2099
13 relative to the 1870­1899 baseline period (modified from Kwiatkowski et al., 2020).



16 Changes in Ocean Circulation, Stratification and Coastal Upwelling


18 Ocean circulation and its variations are key to the evolution of the physical, chemical and biological
19 properties of the ocean. Vertical mixing and upwelling are critical factors affecting the supply of nutrients to
20 the sunlit ocean and hence the magnitude of primary productivity. Ocean currents not only transport heat,
21 salt, carbon, and nutrients, but they also control the dispersion of many organisms and the connectivity
22 between distant populations.


24 Ocean stratification is an important factor controlling biogeochemical cycles and affecting marine
25 ecosystems. WGI AR6 Section (Fox-Kemper et al., 2021) assessed that it is virtually certain that
26 stratification in the upper 200 m of the ocean has been increasing since 1970. Recent evidence has
27 strengthened estimates of the rate of change (Yamaguchi and Suga, 2019; Li et al., 2020a; Sallée et al.,
28 2021), with an estimated increase of 1.0 ± 0.3% (very likely range) per decade over the period 1970­2018
29 (high confidence) (WGI AR6 Section, Fox-Kemper et al., 2021), higher than assessed in SROCC. It
30 is very likely that stratification in the upper few hundred metres of the ocean will increase substantially in the
31 21st century in all ocean basins, driven by intensified surface warming and near-surface freshening at high
32 latitudes (WGI AR6 Section, Capotondi et al., 2012; Fu et al., 2016; Bindoff et al., 2019;
33 Kwiatkowski et al., 2020; Fox-Kemper et al., 2021).


35 Contrasting changes among the major eastern boundary coastal upwelling systems (EBUS) were identified in
36 AR5 (Hoegh-Guldberg et al., 2014). While SROCC assessed with high confidence that three (Benguela,
37 Peru-Humboldt, California) out of the four major EBUS have experienced upwelling-favourable wind
38 intensification in the past 60 years (Sydeman et al., 2014; Bindoff et al., 2019), WGI AR6 revisited this

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 1 assessment based on evidence showing low agreement between studies that have investigated trends over
 2 past decades (Varela et al., 2015). WGI AR6 assessed that only the California Current system has undergone
 3 large-scale upwelling-favourable wind intensification since the 1980s (medium confidence) (WGI AR6
 4 Section, García-Reyes and Largier, 2010; Seo et al., 2012; Fox-Kemper et al., 2021).


 6 While no consistent pattern of contemporary changes in upwelling-favourable winds emerges from
 7 observation-based studies, numerical and theoretical work projects that summertime winds near poleward
 8 boundaries of upwelling zones will intensify, while winds near equatorward boundaries will weaken (high
 9 confidence) (WGI AR6 Section, García-Reyes et al., 2015; Rykaczewski et al., 2015; Wang et al.,
10 2015; Aguirre et al., 2019; Fox-Kemper et al., 2021). Nevertheless, projected future annual cumulative
11 upwelling wind changes at most locations and seasons remain within ±10­20% of present-day values
12 (medium confidence) (WGI AR6 Section, Fox-Kemper et al., 2021).


14 Continuous observation of the Atlantic meridional overturning circulation (AMOC) has improved the
15 understanding of its variability (Frajka-Williams et al., 2019), but there is low confidence in the
16 quantification of AMOC changes in the 20th century because of low agreement in quantitative reconstructed
17 and simulated trends (WGI AR6 Sections 2.3.3,, Fox-Kemper et al., 2021; Gulev et al., 2021). Direct
18 observational records since the mid-2000s remain too short to determine the relative contributions of internal
19 variability, natural forcing and anthropogenic forcing to AMOC change (high confidence) (WGI AR6
20 Sections 2.3.3,, Fox-Kemper et al., 2021; Gulev et al., 2021). Over the 21st century, AMOC will very
21 likely decline for all SSP scenarios, but will not involve an abrupt collapse before 2100 (WGI AR6 Sections
22 4.3.2,, Fox-Kemper et al., 2021; Lee et al., 2021).


24 Sea Ice Changes


26 Sea ice is a key driver of polar marine life, hosting unique ecosystems and affecting diverse marine
27 organisms and food webs through its impact on light penetration and supplies of nutrients and organic matter
28 (Arrigo, 2014). Since the late 1970s, Arctic sea-ice area has decreased for all months, with an estimated
29 decrease of two million km2 (or 25%) for summer sea-ice (averaged for August, September, October) in
30 2010­2019 as compared to 1979­1988 (WGI AR6 Section, Fox-Kemper et al., 2021). For Antarctic
31 sea-ice there is no significant global trend in satellite-observed sea-ice area from 1979 to 2020 in either
32 winter or summer, due to regionally opposing trends and large internal variability (WGI AR6 Section
33, Maksym, 2019; Fox-Kemper et al., 2021).


35 CMIP6 simulations project that the Arctic Ocean will likely become practically sea-ice free (area below 1
36 million km2) for the first time before 2050 and in the seasonal sea-ice minimum in each of the four emission
37 scenarios SSP1-1.9, SSP1-2.6, SSP2-4.5, and SSP5-8.5 (Figure 3.7, WGI AR6 Section, SIMIP
38 Community, 2020; Fox-Kemper et al., 2021). Antarctic sea-ice area is also projected to decrease during the
39 21st century, but due to mismatches between model simulations and observations, combined with a lack of
40 understanding of reasons for substantial inter-model spread, there is low confidence in model projections of
41 future Antarctic sea-ice changes, particularly at the regional level (WGI AR6 Section, Roach et al.,
42 2020; Fox-Kemper et al., 2021).


44 3.2.3 Chemical Changes


46 Ocean Acidification


48 The ocean's uptake of anthropogenic carbon affects its chemistry in a process referred to as ocean
49 acidification, which increases the concentrations of aqueous CO2, bicarbonate and hydrogen ions, and
50 decreases pH, carbonate ion concentrations and calcium carbonate mineral saturation states (Doney et al.,
51 2009). Ocean acidification affects a variety of biological processes with, for example, lower calcium
52 carbonate saturation states reducing net calcification rates for some shell-forming organisms and higher CO2
53 concentrations increasing photosynthesis for some phytoplankton and macroalgal species (Section 3.3.2).


55 Direct measurements of ocean acidity from ocean time series, as well as pH changes determined from other
56 shipboard studies, show consistent decreases in ocean surface pH over the past few decades (virtually

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 1 certain) (WGI AR6 Section, Takahashi et al., 2014; Bindoff et al., 2019; Sutton et al., 2019; Canadell
 2 et al., 2021).


 4 Since the 1980s, surface ocean pH has declined by a very likely rate of 0.016­0.020 per decade in the
 5 subtropics and 0.002­0.026 per decade in the subpolar and polar zones (WGI AR6 Section, Canadell
 6 et al., 2021). Typically, the pH of global surface waters has decreased from 8.2 to 8.1 since the pre-industrial
 7 era (1750 CE), a trend attributable to rising atmospheric CO2 (virtually certain) (Orr et al., 2005; Jiang et al.,
 8 2019).


10 Ocean acidification is also developing in the ocean interior (very high confidence) due to the transport of
11 anthropogenic CO2 to depth by ocean currents and mixing (WGI AR6 Section, Canadell et al., 2021).
12 There, it leads to the shoaling of saturation horizons of aragonite and calcite (high confidence) (WGI AR6
13 Section, Canadell et al., 2021), below which dissolution of these calcium carbonate minerals is
14 thermodynamically favoured. The calcite or aragonite saturation horizons have migrated upwards in the
15 North Pacific (1­2 m yr­1 over 1991­2006, Feely et al., 2012) and in the Irminger Sea (10­15 m yr­1 for
16 the aragonite saturation horizon over 1991­2016, Perez et al., 2018). In some locations of the Western
17 Atlantic Ocean, calcite saturation depth has risen by ~300 m since the pre-industrial due to increasing
18 concentrations of deep-ocean dissolved inorganic carbon (Sulpis et al., 2018). In the Arctic, where some
19 coastal surface waters are already undersaturated with respect to aragonite due to the degradation of
20 terrestrial organic matter (Mathis et al., 2015; Semiletov et al., 2016), the deep aragonite saturation horizon
21 has shoaled on average by 270 ± 60 m during 1765­2005 (Terhaar et al., 2020).


23 Detection and attribution of ocean acidification in coastal environments are more difficult than in the open
24 ocean due to larger spatial and temporal variability of carbonate chemistry (Duarte et al., 2013; Laruelle et
25 al., 2017; Torres et al., 2021), and to the influence of other natural acidification drivers such as freshwater
26 and high-nutrient riverine inputs (Cai et al., 2011; Laurent et al., 2017; Fennel et al., 2019; Cai et al., 2020)
27 or anthropogenic acidification drivers (Section 3.1) like atmospherically deposited nitrogen and sulphur
28 (Doney et al., 2007; Hagens et al., 2014). Since AR5, the observing network in coastal oceans has expanded
29 substantially, improving understanding of both the drivers and amplitude of observed variability (Sutton et
30 al., 2016). Recent studies indicate that two more decades of observations may be required before
31 anthropogenic ocean acidification emerges over natural variability in some coastal sites and regions (WGI
32 AR6 Section, Sutton et al., 2019; Turk et al., 2019; Canadell et al., 2021).


34 Mean open-ocean surface pH is projected to decline by 0.08 ± 0.003 (very likely range), 0.17 ± 0.003, 0.27 ±
35 0.005 and 0.37 ± 0.007 pH units in 2081­2100 relative to 1995­2014, for SSP1-2.6, SSP2-4.5, SSP3-7.0 and
36 SSP5-8.5, respectively (Figure 3.5, WGI AR6 Section 4.3.2, Kwiatkowski et al., 2020; Lee et al., 2021).
37 Projected changes in surface pH are relatively uniform in contrast with those of other surface-ocean
38 variables, but they are largest in the Arctic Ocean (Figure 3.6, WGI AR6 Section, Canadell et al.,
39 2021). Similar declines in the concentration of carbonate ions are projected by Earth System Models (ESMs,
40 Bopp et al., 2013; Gattuso et al., 2015; Kwiatkowski et al., 2020). The North Pacific, the Southern Ocean
41 and Arctic Ocean regions will become undersaturated for calcium carbonate minerals first (Orr et al., 2005;
42 Pörtner et al., 2014). Concurrent impacts on the seasonal amplitude of carbonate chemistry variables are
43 anticipated (i.e., increased amplitude for pCO2 and hydrogen ions, decreased amplitude for carbonate ions,
44 McNeil and Sasse, 2016; Kwiatkowski and Orr, 2018; Kwiatkowski et al., 2020).


46 Future declines in subsurface pH (Figure 3.6) will be modulated by changes in ocean overturning and water-
47 mass subduction (Resplandy et al., 2013), and in organic matter remineralisation (Chen et al., 2017). In
48 particular, decreases in pH will be less consistent at the seafloor than at the surface and will be linked to the
49 transport of surface anomalies to depth. For example, >20% of the North Atlantic seafloor deeper than 500
50 m, including canyons and seamounts designated as marine protected areas (MPAs), will experience pH
51 reductions >0.2 by 2100 under RCP8.5 (Gehlen et al., 2014). Changes in pH in the abyssal ocean (>3000 m
52 deep) are greatest in the Atlantic and Arctic Oceans, with lesser impacts in the Southern and Pacific Oceans
53 by 2100, mainly due to ventilation time scales (Sweetman et al., 2017).


55 Ocean Deoxygenation


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 1 Ocean deoxygenation, the loss of oxygen in the ocean, results from ocean warming, through a reduction in
 2 oxygen saturation, increased oxygen consumption, increased ocean stratification and ventilation changes
 3 (Keeling et al., 2010; IPCC, 2019a). In recent decades, anthropogenic inputs of nutrients and organic matter
 4 (Section 3.1) have increased the extent, duration, and intensity of coastal hypoxia events worldwide (Diaz
 5 and Rosenberg, 2008; Rabalais et al., 2010; Breitburg et al., 2018), while pollution-induced atmospheric
 6 deposition of soluble iron over the ocean has accelerated open-ocean deoxygenation (Ito et al., 2016).
 7 Dexoygenation and acidification often coincide because biological consumption of oxygen produces CO2.
 8 Deoxygenation can have a range of detrimental effects on marine organisms and reduce the extent of marine
 9 habitats (Sections 3.3.2,, Vaquer-Sunyer and Duarte, 2008; Chu and Tunnicliffe, 2015).


11 Changes in ocean oxygen concentrations have been analysed from compilations of in situ data dating back to
12 the 1960s (Helm et al., 2011; Ito et al., 2017; Schmidtko et al., 2017). SROCC concluded that a loss of
13 oxygen had occurred in the upper 1000 m of the ocean (medium confidence), with a global mean decrease of
14 0.5­3.3% (very likely range) over 1970­2010 (Bindoff et al., 2019). Based on new regional assessments
15 (Queste et al., 2018; Bronselaer et al., 2020; Cummins and Ross, 2020; Stramma et al., 2020). WGI AR6
16 assesses that ocean deoxygenation has occurred in most regions of the open ocean since the mid-20th century
17 (high confidence), but is modified by climate variability on interannual and decadal time-scales (medium
18 confidence) (WGI AR6 Sections,, Canadell et al., 2021; Gulev et al., 2021). New findings
19 since SROCC also confirm that the volume of oxygen minimum zones (OMZs) are expanding at many
20 locations (high confidence) (WGI AR6 Section, Canadell et al., 2021).


22 The most recent estimates of future oxygen loss in the subsurface ocean (100­600 m), using CMIP6 models,
23 amount to ­4.1 ± 4.2 (very likely range), ­6.6 ± 5.7, ­10.1 ± 6.7 and ­11.2 ± 7.7% in 2081­2100 relative to
24 1995­2014 for SSP1-2.6, SSP2-4.5, SSP3-7.0 and SSP5-8.5, respectively (Figure 3.5, Kwiatkowski et al.,
25 2020). Based on these CMIP6 projections, WGI AR6 concludes that the oxygen content of the subsurface
26 ocean is projected to decline to historically unprecedented conditions over the 21st century (medium
27 confidence) (WGI AR6 Section, Canadell et al., 2021). These declines are greater (by 31­72%) than
28 simulated by the CMIP5 models in their Representative Concentration Pathway (RCP) analogues, a likely
29 consequence of enhanced surface warming and stratification in CMIP6 models (Figure 3.5, Kwiatkowski et
30 al., 2020). At the regional scale and for subsurface waters, projected changes are not spatially uniform, and
31 there is lower agreement among models than they show for the global mean trend (Bopp et al., 2013;
32 Kwiatkowski et al., 2020). In particular, large uncertainties remain for these future projections of ocean
33 deoxygenation in the subsurface tropical oceans, where the major OMZs are located (Cabré et al., 2015;
34 Bopp et al., 2017).


36 Changes in Nutrient Availability


38 The availability of nutrients in the surface ocean often limits primary productivity, with implications for
39 marine food webs and the biological carbon pump. Nitrogen availability tends to limit phytoplankton
40 productivity throughout most of the low-latitude ocean, whereas dissolved iron availability limits
41 productivity in high-nutrient, low-chlorophyll regions, such as in the main upwelling region of the Southern
42 Ocean and the Eastern Equatorial Pacific (high confidence) (Moore et al., 2013; IPCC, 2019b). Phosphorus,
43 silicon, other micronutrients such as zinc, and vitamins can also co-limit marine phytoplankton productivity
44 in some ocean regions (Moore et al., 2013). Whereas some studies have shown coupling between climate
45 variability and nutrient trends in specific regions, such as in the North Atlantic (Hátún et al., 2016), North
46 Pacific (Di Lorenzo et al., 2009; Yasunaka et al., 2014) and tropical (Stramma and Schmidtko, 2021)
47 Oceans, very few studies have been able to detect long-term changes in ocean nutrient concentrations (but
48 see Yasunaka et al., 2016 ).


50 Future changes in nutrient concentrations have been estimated using ESMs, with future increases in
51 stratification generally leading to decreased nutrient levels in surface waters (IPCC, 2019b). CMIP6 models
52 project a decline in the nitrate concentration of the upper 100 m in 2080­2099 relative to 1995­2014 of ­
53 0.46 ± 0.45 (very likely range), ­0.60 ± 0.58, ­0.80 ± 0.77 and ­1.00 ± 0.78 mmol m­3 under SSP1-2.6,
54 SSP2-4.5 and SSP5-8.5, respectively (Figure 3.5, Kwiatkowski et al., 2020). These declines in nitrate
55 concentration are greater than simulated by the CMIP5 models in their RCP analogues, a likely consequence
56 of enhanced surface warming and stratification in CMIP6 models (Figure 3.5, Kwiatkowski et al., 2020). It is

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 1 concluded that the surface ocean will encounter reduced nitrate concentrations in the 21st century (medium
 2 confidence).


 4 3.2.4 Global Synthesis on Multiple Climatic Impact Drivers


 6 In the 21st century, ocean and coastal ecosystems are projected to face conditions unprecedented over past
 7 centuries to millennia (high confidence) (Section 3.2, WGI AR6 Chapters 4, 9, Fox-Kemper et al., 2021; Lee
 8 et al., 2021), with increased temperatures (virtually certain) and frequency and severity of MHWs (very high
 9 confidence), stronger upper-ocean stratification (high confidence), continued rise in GMSL through the 21st
10 century (high confidence) and increased frequency of extreme sea levels (high confidence), further
11 acidification (virtually certain), oxygen decline (high confidence), and decreased surface nitrate inventories
12 (medium confidence).


14 The rates and magnitudes of these changes largely depend on the extent of future emissions (very high
15 confidence), with surface ocean warming and acidification (very likely range) at +3.47°C ± 1.28°C and ­0.44
16 pH units ± 0.008 in 2080­2099 (relative to 1870­1899) for SSP5-8.5 compared to +1.42°C ± 0.53°C and ­
17 0.16 ± 0.003 for SSP1-2.6 (Figure 3.5, Kwiatkowski et al., 2020).


19 Compound Changes in the 21st Century


21 ESMs project distinct regional evolutions of the different climatic-impact drivers over the 21st century (very
22 high confidence) (Figures 3.5, 3.6, 3.7, Kwiatkowski et al., 2020). Tropical and subtropical oceans are
23 characterized by projected warming and acidification, accompanied by declining nitrate concentrations in
24 equatorial upwelling regions. The North Atlantic is characterized by a high exposure to acidification and
25 declining nitrate concentrations. The North Pacific is characterized by high sensitivity to compound changes,
26 with high rates of warming, acidification, deoxygenation and nutrient depletion. In contrast, the development
27 of compound hazards is limited in the Southern Ocean, where rates of warming and nutrient depletion are
28 lower. The Arctic Ocean is characterized by the highest rates of acidification and warming, strong nutrient
29 depletion, and it will likely become practically sea-ice free in the September mean for the first time before
30 the year 2050 in all SSP scenarios (high confidence) (Figures 3.5, 3.6, 3.7, Sections 3.2.2­3.2.3).


32 In general, the projected changes in climatic-impact drivers are less in absolute terms in the deep-sea
33 (mesopelagic and bathypelagic domains and deep-sea habitats) than in the surface ocean and in shallow-
34 waters habitats (kelp ecosystems, warm-water corals) (very high confidence) (Figures 3.6, 3.7, Mora et al.,
35 2013; Sweetman et al., 2017). The mesopelagic domain will be nevertheless exposed to high rates of
36 deoxygenation (Figure 3.6) and high climate velocities (Figure 3.4, Section, as well as impacted by
37 the shoaling of aragonite or calcite saturation horizon (Section Significant differences in projected
38 trends between the SSPs show that mitigation strategies will limit exposure of deep-sea ecosystems to
39 potential warming, acidification and deoxygenation during the 21st century (very high confidence) (Figure
40 3.6, Kwiatkowski et al., 2020).



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 2 Figure 3.6: Projected trends across open-ocean systems. Projected annual and global (a) average warming, (b)
 3 acidification, (c) changes in dissolved oxygen concentrations and (d) changes in nitrate (NO3) concentrations for four
 4 open-ocean systems, including the epipelagic (0­200 m depth), mesopelagic (200­1000 m), bathypelagic (>1000 m)
 5 domains, and deep benthic waters (>200 m). All projections are based on Coupled Model Intercomparison Project 6
 6 models and for three Shared Socioeconomic Pathways (SSPs), SSP1-2.6, SSP2-4.5 and SSP5-8.5 (Kwiatkowski et al.,
 7 2020). Anomalies in the near-term (2020­2041), mid-term (2041­2060) and long-term (2081­2100) are all relative to
 8 1985­2014. Error bars represent very likely ranges.



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 2 Figure 3.7: Projected trends across coastal-ocean ecosystems. Projected (a) warming, (b) acidification, (c) changes in
 3 dissolved oxygen concentrations, (d) changes in nitrate (NO3) concentrations and (e) changes in summer sea ice cover
 4 fraction (September and north of 66°N for the Northern Polar Oceans, and March and south of 66°S for the Southern
 5 Polar Ocean) for five coastal-ocean ecosystems. All projected trends are for the surface ocean, except oxygen
 6 concentration changes that are computed for the subsurface ocean (100­600 m depth) for the upwelling ecosystems and
 7 the polar seas. All projections are based on Coupled Model Intercomparison Project 6 (CMIP6) models and for three
 8 Shared Socioeconomic Pathways (SSPs): SSP1-2.6, SSP2-4.5 and SSP5-8.5 (Kwiatkowski et al., 2020). Anomalies in
 9 the near-term (2020­2041), mid-term (2041­2060) and long-term (2081­2100) are all relative to 1985­2014. Error bars
10 represent very likely ranges. Coastal seas are defined on a 1° × 1° grid when bathymetry is less than 200 m deep.
11 Distribution of warm-water corals is from UNEP-WCMC et al. (2018). Distribution of kelp ecosystems is from OBIS
12 (2020). Upwelling areas are defined according to Rykaczewski et al. (2015).



15 Time of Emergence


17 Anthropogenic changes in climatic impact-drivers assessed here exhibit vastly distinct times of emergence,
18 which is the time scale over which an anthropogenic signal related to climate change is statistically detected
19 to emerge from the background noise of natural climate for a specific region (Christensen et al., 2007;
20 Hawkins and Sutton, 2012). SROCC concluded that for ocean properties, the time of emergence ranges from
21 under a decade (e.g., surface ocean pH) to over a century (e.g., net primary production, see Section
22 for time of emergence of biological properties, Bindoff et al., 2019).


24 The literature assessed in SROCC mainly focused on surface ocean properties and gradual mean changes.
25 Since then, the time of emergence has also been investigated for subsurface properties, ocean extreme events
26 and particularly vulnerable regions, such as the Arctic Ocean (Hameau et al., 2019; Oliver et al., 2019;
27 Burger et al., 2020; Landrum and Holland, 2020; Schlunegger et al., 2020), but subsequent assessments are
28 low confidence due to limited evidence. Below the surface, changes in temperature typically emerge from
29 internal variability prior to changes in oxygen. However in about a third of the global thermocline,
30 deoxygenation emerges prior to warming (Hameau et al., 2019). Permanent MHW states, defined as when

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 1 SST exceeds the MHW threshold continuously over a full calendar year, will emerge during the 21st century
 2 in many parts of the surface ocean (Oliver et al., 2019). Ocean acidification extremes have already emerged
 3 from background natural internal variability during the 20th century in most of the surface ocean (Burger et
 4 al., 2020). In the Arctic, anthropogenic sea-ice changes have already emerged from the background internal
 5 variability, and anthropogenic alteration of air temperatures will emerge in the early- to mid-21st century
 6 (Landrum and Holland, 2020).


 8 Perspectives from Paleo Data


10 Paleo observations are useful to assess multiple hazards of environmental change while excluding direct
11 anthropogenic impacts (Section Ancient intervals of rapid climate warming that occurred between
12 300 to 50 million years ago (Ma) were triggered by the release of greenhouse gases (high confidence). The
13 sources of greenhouse gases varied but include volcanic degassing from continental flood basalts and
14 methane hydrates stored in marine sediments and soils (Foster et al., 2018). Six extreme ancient
15 hyperthermal events are known from the last 300 Ma, when tropical SSTs reached 1.5°C­10°C warmer than
16 pre-industrial conditions, and with substantial impacts on ancient life (Cross-Chapter Box PALEO in
17 Chapter 1). Warming and deoxygenation in the oceans were closely associated in hyperthermal events (high
18 confidence), with anoxia reaching the photic zone and abyssal depths (Kaiho et al., 2014; Müller et al., 2017;
19 Penn et al., 2018; Weissert, 2019), whereas ocean acidification has not been demonstrated consistently
20 (medium confidence) (Hönisch et al., 2012; Penman et al., 2014; Clarkson et al., 2015; Harper et al., 2020a;
21 Jurikova et al., 2020; Müller et al., 2020).


23 Greenhouse gases also contributed substantially to shaping the longer-term climate trends over the last 50
24 million years, although changes in continental configuration and ocean circulation as well as planetary
25 orbital cycles were equally important (WGI AR6 Cross-Chapter Box 2.1, Westerhold et al., 2020; Gulev et
26 al., 2021). There is little evidence for ocean acidification in the last 2.6 Ma (low confidence) (Hönisch et al.,
27 2012), but ocean ventilation was highly sensitive to even modest warming such as observed in the last
28 10,000 years (medium confidence) (Jaccard and Galbraith, 2012; Lembke-Jene et al., 2018).



31 3.3 Linking Biological Responses to Climatic-Impact Drivers


33 3.3.1 Introduction


35 This section assesses new evidence since AR5 (Pörtner et al., 2014) and SROCC (Bindoff et al., 2019)
36 regarding biotic responses to multiple environmental drivers. It assesses differential sensitivities among life
37 stages within individual organisms, changing responses across scales of biological organisation and the
38 potential for evolutionary adaptation to climate change (e.g., Przeslawski et al., 2015; Boyd et al., 2018;
39 Reddin et al., 2020), providing examples and identifying key gaps and uncertainties that limit our ability to
40 project the ecological impact of multiple climate-impact drivers (Figure 3.8a). The assessment includes
41 physiological responses to single environmental drivers and their underlying mechanisms (Section 3.3.2), the
42 characteristics of multiple drivers and organisms' responses to them (Section 3.3.3), short-term acclimation
43 and longer-term evolutionary adaptation of populations (Section 3.3.4), and it concludes with an assessment
44 of progress in upscaling laboratory findings to ecosystems within in situ settings (Figure 3.8b, Section 3.3.5).



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 2 Figure 3.8: The state of knowledge regarding ecological responses to environmental drivers in experimental settings.
 3 (a) Schematic indicating where themes are discussed within Section 3.3, and how they jointly inform policy; adapted
 4 from Riebesell and Gattuso (2014). (b) The hierarchy of accumulating physiological knowledge (grey layers), from
 5 single (e.g., Pörtner et al., 2012) to multiple drivers, and from simple outcomes (e.g., Sciandra et al., 2003), interactions
 6 among drivers (e.g., Crain et al., 2008) and identification of physiological roles of drivers (e.g., Bach et al., 2015) to
 7 mechanistic understanding of drivers (e.g., Thomas et al., 2017). At present, the upper grey layer has been achieved, in
 8 full, for two drivers e.g., temperature and nutrient concentrations, with validation of dual controls on phytoplankton
 9 growth rate, Thomas et al. (2017). Hatched layers denote major advances since WGII AR5 Chapter 6 (Pörtner et al.,
10 2014). The green layer indicates the level of understanding potentially needed to project the response of marine life
11 subjected to multiple drivers. Red horizontal arrows indicate the influence of confounding factors on our current
12 understanding, including population genetics, fluctuating oceanic conditions, or extreme events.



15 3.3.2 Responses to Single Drivers


17 Anthropogenic CO2 emissions trigger a suite of changes that alter ocean temperature, pH and CO2
18 concentration, oxygen concentration, and nutrient supply at global scales (Section 3.2). The response
19 pathways of these climate-impact drivers have been investigated primarily as single variables.


21 Temperature affects the movement and transport of molecules and, thereby, the rates of all biochemical
22 reactions. Thus, ongoing and projected warming (Section that remains below an organism's
23 physiological optimum will generally raise metabolic rates (very high confidence) (Pörtner et al., 2014).
24 Beyond this optimum (Topt, Figure 3.9), metabolism typically decreases sharply, finally reaching a critical
25 threshold (Tcrit) beyond which enzymes become thermally inactivated and cells undergo oxidative stress.
26 Local and regional adaptation affect the heat tolerance thresholds of organisms. For example, organisms
27 adapted to thermally-stable environments (e.g., tropical, polar, deep-sea) are often more sensitive to warming
28 than those from thermally variable environments (e.g., estuaries) (very high confidence) (Section 3.4, Sunday
29 et al., 2019; Collins et al., 2020). Heat tolerance also decreases with increasing organisational complexity
30 (Storch et al., 2014; Pörtner and Gutt, 2016), and is lower in eggs, embryos and spawning fish than for their
31 larval stages or adults outside the spawning season (high confidence) (Dahlke et al., 2020b). By altering
32 physiological responses, projected changes in ocean warming (Section will modify growth,
33 migration, distribution, competition, survival and reproduction (very high confidence) (Messmer et al., 2017;
34 Dahlke et al., 2018; Andrews et al., 2019; Pinsky et al., 2019; Anton et al., 2020).


36 Altered seawater carbonate chemistry (Section affects specific processes to varying degrees. For
37 example, higher CO2 concentrations can increase photosynthesis and growth in some phytoplankton,
38 macroalgal and seagrass species (high confidence) (Pörtner et al., 2014; Seifert et al., 2020; Zimmerman,
39 2021), while lower pH levels decrease calcification (high confidence) (Pörtner et al., 2014; Falkenberg et al.,
40 2018; Doney et al., 2020; Fox et al., 2020; Reddin et al., 2020) or silicification (low confidence) (Petrou et
41 al., 2019). Organisms' capacity to compensate for or resist acidification of internal fluids depends on their
42 capacity for acid-base regulation, which differs due to organisms' wide-ranging biological complexity and
43 adaptive abilities (low to medium confidence) (Vargas et al., 2017; Melzner et al., 2020). Detrimental
44 impacts of acidification include decreased growth and survival, and altered development, especially in early
45 life stages (high confidence) (Dahlke et al., 2018; Onitsuka et al., 2018; Hancock et al., 2020), along with

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 1 lowered recruitment and altered behaviour in animals (Kroeker et al., 2013a; Wittmann and Pörtner, 2013;
 2 Clements and Hunt, 2015; Cattano et al., 2018; Esbaugh, 2018; Bednarsek et al., 2019; Reddin et al., 2020).
 3 For finfish, laboratory studies of behavioural and sensory consequences of ocean acidification showed mixed
 4 results (Rossi et al., 2018; Nagelkerken et al., 2019; Stiasny et al., 2019; Velez et al., 2019; Clark et al.,
 5 2020; Munday et al., 2020). Calcifiers are generally more sensitive to acidification (e.g., for growth and
 6 survival) than non-calcifying groups (high confidence) (Kroeker et al., 2013a; Wittmann and Pörtner, 2013;
 7 Clements and Hunt, 2015; Cattano et al., 2018; Bednarsek et al., 2019; Reddin et al., 2020; Seifert et al.,
 8 2020). For calcifying primary producers, including phytoplankton and coralline algae, ocean acidification
 9 has different, often opposing effects, for example, decreasing calcification while photosynthetic rates
10 increase (high confidence) (Riebesell et al., 2000; Van de Waal et al., 2013; Bach et al., 2015; Cornwall et
11 al., 2017b; Gafar et al., 2019).




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 2 Figure 3.9: Organismal responses to single and multiple drivers. (a) The generic temperature-response curve describing
 3 physiological process rates as a nonlinear function of a particular driver (e.g., temperature) with maximum rates (Rmax)
 4 and temperature optima (Topt). The driver range that keeps physiological rates above a certain threshold represents the
 5 organism's range of phenotypic plasticity, while below that threshold, the critical temperature (Tcrit), physiological
 6 performance is so low as to constitute stressful conditions. (b) The response curve for one driver can depend on other
 7 drivers, here exemplified for temperature and pH in the central panel. This interaction causes rates as well as optima to
 8 change with (left) pH and (right) temperature, indicated by the coloured lines. (c) Impacts of multiple drivers on
 9 processes can be (blue) additive, (red) synergistic or (green) antagonistic, that is, the cumulative effects of two (or
10 more) drivers are equal to, larger than or smaller than the sum of their individual effects, respectively. Potential
11 experimental outcomes affected by additive, synergistic, and antagonistic interactions are shown for scenarios where
12 drivers (left) increase rates, (centre) decrease rates, or (right) cause opposite responses, showing how experimental
13 outcomes can mask these mechanistic interactions. Adapted from Crain et al. (2008) and Piggott et al. (2015). For a
14 quantitative analysis of effects of driver pairs on animals, see Figure SM3.2.



17 Oxygen concentrations affect aerobic and anaerobic processes, including energy metabolism and
18 denitrification. Projected decreases in dissolved oxygen concentration (Section will thus impact
19 organisms and their biogeography in ways dependent upon their oxygen requirements (Deutsch et al., 2020),
20 which are highest for large, multicellular organisms (Pörtner et al., 2014). The upper ocean generally
21 contains high dissolved-oxygen concentrations due to air-sea exchange and photosynthesis, but in subsurface
22 waters, deoxygenation may impair aerobic organisms in multiple ways (Oschlies et al., 2018; Galic et al.,
23 2019; Thomas et al., 2019; Sampaio et al., 2021). Many processes contribute to lowered oxygen levels:
24 altered ventilation and stratification; microbial respiration enhanced by nearshore eutrophication; and less
25 oxygen solubility in warmer waters. For example, deoxygenation in highly eutrophic estuarine and coastal
26 marine ecosystems (Section 3.4.2) can result from accelerated microbial activity, leading to acute organismal
27 responses. Under hypoxia (oxygen concentrations 2 mg L­1, Limburg et al., 2020), physiological and
28 ecological processes are impaired and communities undergo species migration, replacement and loss,
29 transforming community composition (very high confidence) (Chu and Tunnicliffe, 2015; Gobler and
30 Baumann, 2016; Sampaio et al., 2021). Hypoxia can lead to expanding OMZs which will favour specialised
31 microbes and hypoxia-tolerant organisms (medium confidence) (Breitburg et al., 2018; Ramírez-Flandes et
32 al., 2019). As respiration consumes oxygen and produces CO2, lowered oxygen levels are often interlinked
33 with acidification in coastal and tropical habitats (Rosa et al., 2013; Gobler and Baumann, 2016; Feely et al.,
34 2018) and is an example of a compound hazard (Sections,


36 Increased density stratification and mixed-layer shallowing, caused by warming, freshening and sea-ice
37 decline, can alter light climate and nutrient availability within the surface mixed layer (high confidence)
38 (Section As light and nutrient levels drive photosynthesis, changes in these drivers directly affect
39 primary producers, often in different directions (Matsumoto et al., 2014; Deppeler and Davidson, 2017).
40 Decreased upward nutrient supply is expected to decrease primary production in the low-latitude ocean
41 (medium confidence) (Section, Moore et al., 2018a; Kwiatkowski et al., 2019). Alternatively, higher
42 mean underwater light levels resulting from changes in sea ice and/or mixed layer shallowing can increase

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 1 primary production in high-latitude offshore regions, provided nutrient levels remain sufficiently high
 2 (medium confidence) (Section, Cross-Chapter Paper 6, Vancoppenolle et al., 2013; Deppeler and
 3 Davidson, 2017; Tedesco et al., 2019; Ardyna and Arrigo, 2020; Lannuzel et al., 2020). In some parts of the
 4 open Southern Ocean, where iron limitation largely controls primary productivity (Tagliabue et al., 2017),
 5 changes in wind fields will deepen the summer mixed-layer depth (Panassa et al., 2018), entrain more
 6 nutrients, and raise primary productivity in the future (medium confidence) (Cross-Chapter Paper 6, Hauck et
 7 al., 2015; Leung et al., 2015; Moore et al., 2018a; Kwiatkowski et al., 2020).


 9 Climate-impact drivers fluctuate on time scales ranging from diurnal to annual, with potential consequences
10 for organismal responses (Figure 3.10), but these fluctuations are commonly not incorporated
11 experimentally. Experiments that simulate natural fluctuations in drivers, especially beyond tidal or diel
12 cycles, can result in more detrimental impacts than those based on quasi-constant conditions (Eriander et al.,
13 2015; Sunday et al., 2019), but can also ameliorate effects (Comeau et al., 2014; Laubenstein et al., 2020;
14 Cabrerizo et al., 2021), confirming that the influence of environmental variability requires evaluation (Dowd
15 et al., 2015). MHWs exacerbate the impacts of rising mean temperatures, with major ecological
16 consequences (very high confidence) (Frölicher et al., 2018; IPCC, 2018; Arafeh-Dalmau et al., 2020;
17 Laufkötter et al., 2020). Higher temperature variability decreased phytoplankton growth and calcification in
18 Emiliania huxleyi relative to a stable warming regime (Wang et al., 2019b). Diel fluctuations (i.e., over 24 h)
19 in carbonate chemistry superimposed on current and future pCO2 levels influenced diatom species
20 differently, depending on their habitat (Li et al., 2016). CO2 fluctuations overlaid on changing mean values
21 also altered phenotypic evolutionary outcomes of picoeukaryotic algae (Schaum et al., 2016). In the bivalve
22 Mytilus edulis, fluctuating pH regimes exerted higher metabolic costs (Mangan et al., 2017), while salinity
23 fluctuations might be more influential than pH fluctuations in other bivalves (Velez et al., 2016). The
24 amplitude of diel and seasonal pH and CO2 changes are projected to increase in the future due to lowered
25 CO2 seawater buffering capacity (very high confidence) (Section, Burger et al., 2020), which can
26 impose additional stress on organisms.


28 3.3.3 Responses to Multiple Drivers


30 Each organism encounters a unique combination of local and climate-impact drivers, which vary in space
31 and time. The contribution of these drivers to an organism's overall biological response, and thereby also
32 potential risks for the organism, depends on the intensity and duration of its exposure to these drivers and
33 associated sensitivities. Both geographical location (e.g., polar, tropical) and marine habitat (e.g., benthic,
34 pelagic) strongly affect the combination of climate and non-climate drivers that organisms are exposed to.
35 Non-climate drivers (Section 3.1) can dominate outcomes or amplify vulnerability to climate-impact drivers,
36 with mostly detrimental effects such as extirpation (very high confidence) (Section 3.4, Boyd et al., 2018;
37 Gissi et al., 2021), and unique feedbacks may exist between climate change and drivers like habitat loss or
38 invasive species that further confound climate change effects (Ortiz et al., 2018; Wolff et al., 2018; Gissi et
39 al., 2021). Individual responses are further influenced by an organism's behaviour, trophic level and life-
40 history strategy (Figure 3.10, Przeslawski et al., 2015; Boyd et al., 2018). Evidence is increasing that some
41 life-history stages are more sensitive to specific drivers than others (Dahlke et al., 2020b). To identify the
42 most influential drivers for an organism requires targeting key traits (e.g., calcification, reproduction). The
43 trophic level of the organism must also be considered, because autotrophs directly depend on light and
44 nutrients while invertebrates are often more sensitive to changes in oxygen or altered prey, but temperature
45 plays a key role for both groups (Figure 3.10b).


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 2 Figure 3.10: The effect of environmental drivers differs depending upon organisms' life history, and trophic strategy or
 3 habitat. (a) pH variability differs for benthic invertebrates, such as sea urchins (in blue), and their pelagic larvae (in
 4 green); pH fluctuations over the annual cycle can be much larger in the water column (due to primary production)
 5 relative to the seafloor. Variability associated with behaviour and life stage strongly defines organisms' niches and
 6 sensitivities to present and future conditions. (b) Examples of organisms that are influenced by different suites of
 7 drivers that are set jointly by their habitat (e.g., benthic versus epipelagic settings) and trophic strategy (e.g., nutrients
 8 for phytoplankton, prey characteristics for grazers).



11 Co-occurring environmental drivers often cause complex organismal responses (high confidence) (Pörtner et
12 al., 2014). Individual drivers can have detrimental, neutral or beneficial effects, depending on the
13 relationship between driver and physiological process (Section 3.3.2, Figure 3.9a). Multiple drivers can have
14 interactive effects, where the response to one driver alters the sensitivity to another, and outcomes cannot be
15 deduced from individual drivers' effects (Figure 3.9b). Impacts of multiple drivers can be additive,
16 synergistic or antagonistic (Figure 3.9c, Crain et al., 2008; Piggott et al., 2015; Boyd et al., 2018; Bindoff et
17 al., 2019). Well-controlled laboratory studies on multiple-driver effects have revealed insights into the mode
18 of action of individual drivers and their interdependence (Kroeker et al., 2017; Gao et al., 2019; Reddin et
19 al., 2020; Seifert et al., 2020; Green et al., 2021b; Sampaio et al., 2021). Understanding the outcomes of
20 interactive drivers is important for robustly assessing risks to organisms under different climate-change
21 scenarios.


23 Effects of Multiple Drivers on Primary Producers


25 Warming and rising CO2 concentrations enhance growth and/or photosynthetic rates in many species of
26 cyanobacteria, picoeukaryotes, coccolithophores, dinoflagellates and diatoms (high confidence) (Fu et al.,
27 2007; Sett et al., 2014; Hoppe et al., 2018a; Wolf et al., 2018; Brandenburg et al., 2019), and the optimum
28 pCO2 for growth and/or primary production shifts upward under warming (medium confidence) (Sett et al.,
29 2014; Hoppe et al., 2018a). Warming and ocean acidification appear to jointly favour the proliferation and
30 toxicity of harmful algal bloom (HAB) species (limited evidence, high agreement) (Section, Bindoff
31 et al., 2019; Brandenburg et al., 2019; Griffith et al., 2019a; Wells et al., 2020), but a 2021 analysis found no
32 uniform global trend in HABs or their distribution over 1985­2018, once field data were adjusted for
33 regional variations in monitoring effort (Hallegraeff et al., 2021). The predominantly detrimental impacts of
34 ocean acidification on coccolithophores can partly be offset by warming (Seifert et al., 2020), but also be
35 exacerbated, depending on the magnitudes of drivers (D'Amario et al., 2020). For non-calcifying
36 macroalgae, responses are highly species-specific and often indicate synergistic interactions between
37 warming and acidification (Kram et al., 2016; Falkenberg et al., 2018). Ocean acidification poses a large risk
38 for coralline algae that is further amplified by warming (medium confidence) (Section, Cornwall et
39 al., 2019). However, temperatures up to 5°C above ambient do not decrease calcification (Cornwall et al.,
40 2019), and there is limited evidence that some species have the physiological capacity to resist acidification
41 via pH upregulation at the calcification site (Cornwall et al., 2017a). For seagrass, warming beyond a
42 species' thermal tolerance will limit growth and impact germination, but ocean acidification appears to

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 1 increase thermal tolerance of some eelgrass species by increasing the photosynthesis-to-respiration ratio
 2 (medium confidence) (Egea et al., 2018; Scalpone et al., 2020; Zimmerman, 2021).


 4 Thermal sensitivity of pelagic primary producers changes with nutrient supply (high confidence) (Thomas et
 5 al., 2017; Marañón et al., 2018; Fernández et al., 2020). Phosphorus limitation lowers the temperature
 6 optimum for growth of phytoplankton, making these organisms more prone to heat stress (Thomas et al.,
 7 2017; Bestion et al., 2018). This trend may hold for open-ocean phytoplankton, which are often iron-limited
 8 (medium confidence) (Boyd, 2019). Such temperature-nutrient interactions might be especially relevant
 9 during summer MHWs (Section, Cross-Chapter Box EXTREMES in Chapter 2, IPCC, 2018;
10 Holbrook et al., 2019; DeCarlo et al., 2020; Hayashida et al., 2020), when primary producers are often
11 nutrient-limited and near their thermal limits. Increasingly frequent and intense MHWs along with projected
12 decreases in nutrient availability (Section may push some primary producers beyond tolerance
13 thresholds. Temperature-nutrient interactions can also alter the photosynthesis-to-respiration ratio in
14 phytoplankton (Marañón et al., 2018). Overall, rising metabolic rates due to warming will be restricted to
15 primary producers in high-nutrient regions (medium confidence) (Thomas et al., 2017; Marañón et al., 2018).
16 For zooxanthellae-containing corals, nutrient supply from upwelling or from runoff can increase coral
17 susceptibility to bleaching during warm-season MHWs (DeCarlo et al., 2020; Wooldridge, 2020).


19 The effects of ocean acidification on growth, metabolic rates or elemental composition of primary producers
20 changes with nutrient availability and light conditions (high confidence) (Gao et al., 2019; Seifert et al.,
21 2020). While interactions with nutrients are often additive in phytoplankton, diatoms revealed predominantly
22 synergistic interactions (Seifert et al., 2020). Growth or photosynthesis of some diatom and HAB species, for
23 instance, are stimulated by ocean acidification only if nutrients are replete (Hoppe et al., 2013; Boyd et al.,
24 2015b; Eberlein et al., 2016; Griffith et al., 2019a). Interactions with light are more complex because relative
25 effects of ocean acidification are larger under limiting irradiances, while saturating light levels decrease
26 beneficial or detrimental effects on these processes (Kranz et al., 2010; Garcia et al., 2011; Rokitta and Rost,
27 2012; Heiden et al., 2016). For the coccolithophore Emiliania huxleyi, for example, the impacts of ocean
28 acidification are less detrimental under high light availability, which could partly explain why this species is
29 moving poleward (Winter et al., 2013; Kondrik et al., 2017; Neukermans et al., 2018), although acidification
30 is more pronounced in polar waters (Section, Cross-Chapter Paper 6). Under excess light, however,
31 the detrimental impacts of ocean acidification are amplified for many species (high confidence) (Gao et al.,
32 2012; Li and Campbell, 2013; Zhang et al., 2015; Kottmeier et al., 2016; Gafar et al., 2019). Lowered photo-
33 physiological capacity to cope with high-light stress and avoid photodamage (Gao et al., 2012; Li and
34 Campbell, 2013; Hoppe et al., 2015; Kvernvik et al., 2020) is also consistent with observations that dynamic
35 light regimes can become more stressful under ocean acidification (Jin et al., 2013; Hoppe et al., 2015).
36 Given the expected mixed-layer shallowing in some regions (Section, the exposure to overall higher
37 mean irradiances could shift the effects of acidification from beneficial to detrimental for some primary
38 producers, depending on species and organismal traits (medium confidence) (Gao et al., 2019; Seifert et al.,
39 2020).


41 Studies investigating two drivers provide most of the information on the wide range of interactive effects of
42 drivers on phytoplankton (Gao et al., 2019; Seifert et al., 2020), although climate change alters several
43 oceanic drivers concurrently (Section 3.2). The few experimental studies that have addressed three or more
44 drivers (Xu et al., 2014; Boyd et al., 2015b; Brennan and Collins, 2015; Brennan et al., 2017; Hoppe et al.,
45 2018b; Moreno-Marín et al., 2018) indicate that one or two drivers generally dominate the cumulative
46 outcome, with others playing a subordinate role (medium confidence). In these studies, temperature had a
47 disproportionately large influence, while other drivers differed in importance, depending on the type of
48 primary producer, ecosystem characteristics and selected driver values.


50 Effects of Multiple Drivers on Animals


52 When changing CO2 concentrations affect marine ectotherms, they typically combine additively or
53 synergistically with warming (medium confidence) (e.g., Lefevre, 2016; Reddin et al., 2020; Sampaio et al.,
54 2021), and their cumulative effects can lead to detrimental, neutral or beneficial effects (high confidence)
55 (Figure 3.9a, Bennett et al., 2017; Büscher et al., 2017; Dahlke et al., 2017; Foo and Byrne, 2017; Johnson et
56 al., 2017b; Cominassi et al., 2019). Higher ocean CO2 influences the thermal tolerance of species adapted to
57 extreme but stable habitats in tropical and polar regions, more than that of thermally tolerant generalists

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 1 (high confidence) (Byrne et al., 2013; Schiffer et al., 2014; Flynn et al., 2015; Kunz et al., 2016; Pörtner et
 2 al., 2017; Kunz et al., 2018; Bindoff et al., 2019, but see) (Ern et al., 2017), especially in early life stages
 3 (Dahlke et al., 2020a). In thermal generalists from temperate and subtropical species, warming and ocean
 4 acidification generally have detrimental effects on growth and survival (e.g., Gao et al., 2020), but warming
 5 can also alleviate the detrimental effects of ocean acidification by increasing metabolic rate and/or growth
 6 (Garzke et al., 2020), provided that other conditions (e.g., thermal niche, food availability) are beneficial. For
 7 example, larval growth and survival of Australasian snapper (Pagrus auratus) appear to benefit from
 8 combined acidification and warming (but see Watson et al., 2018; McMahon et al., 2020), introducing major
 9 uncertainties to population modelling (Section 3.3.4, Parsons et al., 2020).


11 As with ocean acidification, reduced oxygen availability further alters the influence of warming on metabolic
12 rates (high confidence). Acidification and hypoxia can contribute to a decrease or shift in thermal tolerance,
13 while the magnitude of this effect depends on the duration of exposure (Tripp-Valdez et al., 2017; Cattano et
14 al., 2018; Calderón-Liévanos et al., 2019; Schwieterman et al., 2019). Warming and hypoxia are mostly
15 positively correlated and tolerance to both phenomena are often linked after long-term acclimation (e.g.,
16 Bouyoucos et al., 2020). Acute short-term heat shocks can impair hypoxia tolerance, for instance in intertidal
17 fish (McArley et al., 2020). This is relevant for shallow waters, specifically for MHWs (Section,
18 Hobday et al., 2016a; IPCC, 2018; Collins et al., 2019a). Ocean acidification can increase hypoxia tolerance
19 in some cases, possibly by downregulating activity (Faleiro et al., 2015) and/or changing blood oxygenation
20 (Montgomery et al., 2019). Other studies, however, reported additive negative effects of acidification and
21 warming on hypoxia tolerance (Schwieterman et al., 2019; Götze et al., 2020), in line with the oxygen- and
22 capacity-limited thermal tolerance (OCLTT) hypothesis presented in AR5 (Pörtner et al., 2014): warming
23 causes increased metabolic rates and oxygen demand in ectotherms, which at some point exceed supply
24 capacities (that also depend on environmental oxygen availability) and reduce aerobic scope. In
25 consequence, expansion of OMZs and other regions where warming, hypoxia and acidification combine will
26 further reduce habitat for many fish and invertebrates (high confidence) (Sections­


28 Food availability modulates, and may be more influential than, other driver responses by affecting the
29 energetic and nutritional status of animals (Cole et al., 2016; Stiasny et al., 2019; Cominassi et al., 2020).
30 Laboratory studies conducted under an excess of food risk underestimating the ecological effects of climate-
31 impact drivers, because increased feeding rates may help mitigate adverse effects (Nowicki et al., 2012;
32 Towle et al., 2015; Cominassi et al., 2020). Lowered food availability from reduced open-ocean primary
33 production (Sections, will act as an additional driver, amplifying the detrimental effects of
34 other drivers. However, warming and higher CO2 availability may increase primary productivity in some
35 coastal areas (Section, ameliorating the adverse direct effects on animals (e.g., Sswat et al., 2018).
36 Due to the few studies addressing food availability under multiple-driver scenarios (Thomsen et al., 2013;
37 Pistevos et al., 2015; Towle et al., 2015; Ramajo et al., 2016; Brown et al., 2018a; Cominassi et al., 2020),
38 there is medium confidence in its modulating effect on climate-impact driver responses.


40 Animal behaviour can be affected by ocean acidification, warming and hypoxia. While warming and hypoxia
41 mostly induce avoidance behaviour, potentially leading to migration and habitat compression (Section 3.4,
42 McCormick and Levin, 2017; Limburg et al., 2020), the effects of acidification appear more complex. Some
43 studies reported that acidification dominates behavioural effects (Schmidt et al., 2017), although outcomes
44 vary with experimental design and duration of exposure (low confidence, low agreement) (Maximino and de
45 Brito, 2010; Munday et al., 2016; Laubenstein et al., 2018; Munday et al., 2019; Sundin et al., 2019; Clark et
46 al., 2020; Munday et al., 2020; Williamson et al., 2021). Behaviour represents an integrated phenomenon
47 that can be influenced both directly and indirectly by multiple drivers. For instance, increased pCO2 can
48 directly act on neuronal signalling pathways (e.g., Gamma-aminobutyric acid hypothesis, Nilsson et al.,
49 2012; Thomas et al., 2020) and influence learning (Chivers et al., 2014), vision (Chung et al., 2014), and
50 choice and escape behaviour (Watson et al., 2014; Wang et al., 2017b). There is further evidence that
51 observed alterations in fish olfactory behaviour under ocean acidification may result from physiological and
52 molecular changes of the olfactory epithelium, influencing olfactory receptors (Roggatz et al., 2016; Porteus
53 et al., 2018; Velez et al., 2019; Mazurais et al., 2020). Temperature mainly drives metabolic processes and
54 thus energetic requirements, which can indirectly influence behaviour, including increased risk-taking during
55 feeding (Marangon et al., 2020). Ocean warming also accelerates the biochemical reactions and metabolic
56 processes that are primarily influenced by acidification. It is therefore difficult to generalise to what extent
57 co-occurring ocean warming ameliorates or exacerbates effects of acidification on behaviour (Laubenstein et

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 1 al., 2019); outcomes depend upon species and life stage (Faleiro et al., 2015; Chan et al., 2016; Tills et al.,
 2 2016; Wang et al., 2018b; Jarrold et al., 2020), interactions between species (e.g., Paula et al., 2019) along
 3 with confounding factors including food availability and salinity (medium confidence) (Ferrari et al., 2015;
 4 Pistevos et al., 2015; Pimentel et al., 2016; Pistevos et al., 2017; Horwitz et al., 2020).


 6 While hypoxia can dominate multiple-driver responses locally (Sampaio et al., 2021), warming is the
 7 fundamental physiological driver for most marine ectotherms, globally, as it directly affects their entire
 8 biochemistry and energy metabolism. Other influential drivers include ocean acidification, salinity (high
 9 confidence) (Lefevre, 2016; Whiteley et al., 2018; Reddin et al., 2020) or food availability/quality (medium
10 confidence) (Nagelkerken and Munday, 2016; Gao et al., 2020). Fluctuating and decreasing salinity may
11 aggravate the detrimental effects of warming and elevated CO2, because dilution with freshwater lowers
12 acid-base buffering capacity, resulting in lower pH and calcium carbonate saturation state (Dickinson et al.,
13 2012; Shrivastava et al., 2019; Melzner et al., 2020).


15 3.3.4 Acclimation and Evolutionary Adaptation


17 Climate change is and will continue to be a major driver of natural selection, causing important changes in
18 fitness-related (e.g., growth, reproduction, survival) and functional (e.g., body/cell size, morphology,
19 physiology) traits, and in the genetic diversity of natural populations (medium confidence) (Pauls et al., 2013;
20 Merilä and Hendry, 2014). Climate-change impacts will continue to be exacerbated by interactions with non-
21 climate drivers such as habitat fragmentation or loss, pollution, or resource overexploitation, which limit the
22 adaptive potential of populations to future conditions (Trathan et al., 2015; Gaitán-Espitia and Hobday,
23 2021). However the ultimate responses to complex change are conditioned by the rate and magnitude of
24 environmental change, organisms' capacity for acclimation, the degree of local adaptation of natural
25 populations, and populations' potential for adaptive evolution (Figure 3.11, Pespeni et al., 2013; Calosi et al.,
26 2017; Vargas et al., 2017). These controlling factors are mainly determined by local environmental
27 conditions encountered by populations across their geographical distribution (Boyd et al., 2016). In highly
28 fluctuating environments (e.g., upwelling regions, coastal zones), multiple drivers can change and interact
29 across temporal and spatial scales, generating geographical mosaics of tolerances and sensitivities to
30 environmental and climate change in marine organisms (medium confidence) (Pespeni et al., 2013; Boyd et
31 al., 2016; Vargas et al., 2017; Li et al., 2018a). A further challenge for marine life lies in its ability to cope
32 with extreme events such as MHWs (Cross-Chapter Box EXTREMES in Chapter 2). The interplay between
33 the abruptness, intensity, duration, magnitude and reoccurrence of extreme events may alter or prevent
34 evolutionary responses (e.g., adaptation) to climate change and the potential for acclimation to extreme
35 conditions such as MHWs (Cheung and Frölicher, 2020; Coleman et al., 2020a; Gurgel et al., 2020; Gruber
36 et al., 2021).


38 Some studies have documented higher phenotypic plasticity and tolerance to ocean warming and
39 acidification in marine invertebrates (Dam, 2013; Kelly et al., 2013; Pespeni et al., 2013; Gaitán-Espitia et
40 al., 2017a; Vargas et al., 2017; Li et al., 2018a), seaweeds (Noisette et al., 2013; Padilla-Gamiño et al., 2016;
41 Machado Monteiro et al., 2019), and fish (medium confidence) (Sandoval-Castillo et al., 2020; Enbody et al.,
42 2021) living in coastal zones characterised by strong temporal fluctuations in temperature, pH, pCO2, light
43 and nutrients. For these populations, strong directional selection with intense and highly fluctuating
44 conditions may have favoured local adaptation and increased tolerance to environmental stress (low
45 confidence, low evidence) (Hong and Shurin, 2015; Gaitán-Espitia et al., 2017b; Li et al., 2018a).


47 Other mechanisms acting within and across generations can influence selection and inter-population
48 tolerances to environmental and climate-impact drivers. For instance, transgenerational effects and/or
49 developmental acclimation, both so-called "carry-over effects" (where the early-life environment affects the
50 expression of traits in later life stages or generations), can influence within- and cross-generational changes
51 in the tolerances of marine organisms (medium confidence) to ocean warming (Balogh and Byrne, 2020) and
52 acidification (Parker et al., 2012). Over longer time scales, increasing tolerance to these drivers may be
53 mediated by mechanisms such as transgenerational plasticity (Murray et al., 2014), leading to locally-
54 adapted genotypes, as seen in bivalves (Thomsen et al., 2017), annelids (Rodríguez-Romero et al., 2016;
55 Thibault et al., 2020), corals (Putnam et al., 2020), and coralline algae (Cornwall et al., 2020). However,
56 transgenerational plasticity is species-specific (Byrne et al., 2020; Thibault et al., 2020) and, depending on
57 the rate and magnitude of environmental change, it may either be insufficient for evolutionary rescue

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 1 (Morgan et al., 2020) or could induce maladaptive responses (i.e., reduced fitness) in marine organisms
 2 exposed to multiple drivers (medium confidence, low evidence) (Figure 3.11, Griffith and Gobler, 2017;
 3 Parker et al., 2017; Byrne et al., 2020).


 5 Acclimation to environmental pressures and climate change via phenotypic plasticity (Section 3.3.3, Collins
 6 et al., 2020) enables species to undergo niche shifts, such that their present-day climatic niche is altered to
 7 incorporate new or shifted conditions (Fox et al., 2019). Although plasticity provides an adaptive
 8 mechanism, it is unlikely to provide a long-term solution for species undergoing sustained directional
 9 environmental change (e.g., global warming) (medium confidence) (Fox et al., 2019; Gaitán-Espitia and
10 Hobday, 2021). Beyond the limits for plastic responses (Figure 3.9, DeWitt et al., 1998; Valladares et al.,
11 2007), genetic adjustments are required to persist in a changing world (Figure 3.11, Fox et al., 2019). The
12 ability of species and populations to undergo these adjustments (i.e., adaptive evolution) depends on
13 extrinsic factors including the rate and magnitude of environmental change (important determinants of the
14 strength and form of selection, Hoffmann and Sgrò, 2011; Munday et al., 2013), along with intrinsic factors
15 such as generation times and standing genetic variation (Mitchell-Olds et al., 2007; Lohbeck et al., 2012).
16 Accurately assessing the degree of acclimation and/or adaptation across space and time is difficult and
17 constrains studying adaptive evolution in natural populations. There is a major gap in climate-change
18 biology related to the study of evolutionary responses in complex and long-lived multicellular organisms.
19 Insights on organismal acclimation, adaptation, and evolution rely on studies of small, short-lived marine
20 organisms such as phytoplankton that divide rapidly and contain high genetic variation in large populations.
21 (Schaum et al., 2016; Cavicchioli et al., 2019; Collins et al., 2020).


23 Experimental evolution suggests that microbial populations can rapidly adapt (i.e., over 1­2 years) to
24 environmental changes mimicking projected effects of climate change (medium confidence). Phytoplankton
25 adaptive mechanisms include intraspecific strain sorting and genetic changes (Bach et al., 2018; Hoppe et al.,
26 2018b; Wolf et al., 2019). The evolutionary responses of microbes are conditioned by the number and
27 characteristics of interacting drivers (low confidence) (Brennan et al., 2017). For example, in a high-salinity
28 adapted strain of the phytoplankton Chlamydomonas reinhardtii, the selection intensity and the adaptation
29 rate increased with the number of environmental drivers, accelerating the adaptive evolutionary response
30 (Brennan et al., 2017). For this and other phytoplankton species, a few dominant drivers explain most of the
31 phenotypic and evolutionary changes observed (Boyd et al., 2015a; Brennan and Collins, 2015; Brennan et
32 al., 2017).


34 Adaptation can be impeded, delayed or constrained in eukaryotic microbial populations as a result of reduced
35 genetic diversity, and/or the presence of functional and evolutionary trade-offs (Aranguren-Gassis et al.,
36 2019; Lindberg and Collins, 2020; Walworth et al., 2020). In the marine diatom Chaetoceros simplex, a
37 functional trade-off between high-temperature tolerance and increased nitrogen requirements underlies
38 inhibited thermal adaptation under nitrogen-limited conditions (low confidence) (Aranguren-Gassis et al.,
39 2019). When selection is strong due to unfavourable environmental conditions, microbial populations can
40 encounter functional and evolutionary trade-offs evidenced by reducing growth rates while increasing
41 tolerance and metabolism of reactive oxygen species (Lindberg and Collins, 2020). Other trade-offs can be
42 observed in offspring quality and number (Lindberg and Collins, 2020). These findings contribute towards a
43 mechanistic framework describing the range of evolutionary strategies in response to multiple drivers
44 (Collins et al., 2020), but other hazards such as extreme events (e.g., MHWs) still need to be included
45 because their characteristics may alter the potential for adaptation of species and populations to climate
46 change (Gruber et al., 2021).



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 2 Figure 3.11: Micro-evolutionary dynamics in response to environmental change. Simplified conceptual framework
 3 shows two main eco-evolutionary trajectories for natural populations over time (vertical axis from top to bottom). If
 4 environmental stress is low, rapid responses (within a generation) through plastic phenotypic adjustments and selection
 5 (across generations) sustain fitness, enhancing maintenance of viable populations across generations. In contrast, if
 6 environmental stress is high, ongoing phenotypic plasticity and acclimation may be insufficient to buffer the negative
 7 effects, exacerbating the loss of fitness (change of colour to orange/yellow/red). Ultimately, very high stress conditions
 8 accelerate population decline, enhancing the risk of species extinction.



11 3.3.5 Ecological Response to Multiple Drivers


13 Assessing ecological responses to multiple climate-impact drivers requires a combination of approaches,
14 including laboratory- and field-based experiments, field observations (e.g., natural gradients, climate
15 analogues), study of paleo-analogues and the development of mechanistic and empirical models (Clapham,
16 2019; Gissi et al., 2021). Experimental studies of food-web responses are often limited to an individual
17 driver, although recent manipulations have used a matrix of >1000-L mesocosms to explore ecological
18 responses to both warming and acidification (Box 3.1, Nagelkerken et al., 2020). Hence, complementary
19 approaches are needed to indirectly explore the mechanisms underlying ecosystem responses to global
20 climate change (Parmesan et al., 2013). Observations from time series longer than modes of natural
21 variability (i.e., decades) are essential for revealing and attributing ecological responses to climate change
22 (e.g., Section 3.4, Barton et al., 2015b; Brun et al., 2019). Also, paleo records provide insights into the
23 influence of multiple drivers on marine biota (Cross-Chapter Box PALEO in Chapter 1, Reddin et al., 2020).
24 Specifically, associations between vulnerabilities and traits of marine ectotherms in laboratory experiments
25 correspond with organismal responses to ancient hyperthermal events (medium confidence) (Reddin et al.,
26 2020). This corroboration suggests that responses to multiple drivers inferred from the fossil record can help
27 provide insights into the future status of functional groups and hence food webs under rapid climate change.


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 1 Multi-species and integrated end-to-end ecosystem models are powerful tools to explore and project
 2 outcomes to the often-interacting cumulative effects of climate change and other anthropogenic drivers
 3 (Section 3.1, Kaplan and Marshall, 2016; Koenigstein et al., 2016; Peck and Pinnegar, 2018; Tittensor et al.,
 4 2018; Gissi et al., 2021). These models can integrate some aspects of the knowledge accrued from
 5 manipulation experiments, paleo and contemporary observations, help test the relative importance of specific
 6 drivers and driver combinations, and identify synergistic or antagonistic responses (Koenigstein et al., 2016;
 7 Payne et al., 2016; Skogen et al., 2018; Tittensor et al., 2018). As these models are associated with wide-
 8 ranging uncertainties (SM3.2.2, Payne et al., 2016; Trolle et al., 2019; Heneghan et al., 2021) they cannot be
 9 expected to accurately project the trajectories of complex marine ecosystems under climate change. Hence,
10 they are most useful for assessing overall trends and in particular for providing a plausible envelope of
11 trajectories across a range of assumptions (Fulton et al., 2018; Peck et al., 2018; Tittensor et al., 2018). On a
12 global scale, ecosystem models project a ­5.7% ± 4.1% (very likely range) to ­15.5% ± 8.5% decline in
13 marine animal biomass with warming under SSP1-2.6 and SSP5-8.5, respectively, by 2080­2099 relative to
14 1995­2014, albeit with significant regional variation in both trends and uncertainties (medium confidence)
15 (Section 3.4.34, Tittensor et al., 2021). Biological interactions may exacerbate or buffer the projected
16 impacts. For instance, trophic amplification (strengthening of responses to climate-impact drivers at higher
17 trophic levels), may result from combined direct and indirect food-web mediated effects (medium
18 confidence) (Section, Lotze et al., 2019). Alternatively, compensatory species interactions can
19 dampen strong impacts on species from ocean acidification, resulting in weaker responses at functional-
20 group or community level than at species level (medium confidence) (Marshall et al., 2017; Hoppe et al.,
21 2018b; Olsen et al., 2018; Gissi et al., 2021). Globally, the projected reduction of biomass due to climate-
22 impact drivers is relatively unaffected by fishing pressure, indicating additive responses of fisheries and
23 climate change (low confidence) (Lotze et al., 2019). Regionally, projected interactions of climate-impact
24 drivers, fisheries and other regional non-climate drivers can be both synergistic and antagonistic, varying
25 across regions, functional groups and species, and can cause non-linear dynamics with counterintuitive
26 outcomes, underlining the importance of adaptations and associated trade-offs (high confidence) (Sections
27 3.5.3,, 4.5, 4.6, Weijerman et al., 2015; Fulton et al., 2018; Hansen et al., 2019; Trolle et al., 2019;
28 Zeng et al., 2019; Holsman et al., 2020; Pethybridge et al., 2020; Gissi et al., 2021).


30 Given the limitations of individual ecological models discussed above, model intercomparisons, such as the
31 Fisheries and Marine Ecosystem Model Intercomparison Project (Fish-MIP, Tittensor et al., 2018) show
32 promise in increasing the robustness of projected ecological outcomes (Tittensor et al., 2018). Model
33 ensembles include a greater number of relevant processes and functional groups than any single model and
34 thus capture a wider range of plausible responses. Among the global Fish-MIP models, there is high
35 (temperate and tropical areas) to medium agreement (coastal and polar regions) on the direction of change,
36 but medium (temperate and tropical regions) to low agreement (coastal and polar regions) on magnitude of
37 change (Lotze et al., 2019; Heneghan et al., 2021). Although model outputs are validated relative to
38 observations to assess model skills (Payne et al., 2016; Tittensor et al., 2018), the Fish-MIP models under-
39 represent some sources of uncertainty, as they often do not include parameter uncertainties, and do not
40 usually include impacts of ocean acidification, oxygen loss, or evolutionary responses because there remains
41 high uncertainty regarding the influences of these processes across functional groups. Ensemble model
42 investigations like Fish-MIP have also identified gaps in our mechanistic understanding of ecosystems and
43 their responses to anthropogenic forcing, leading to model improvement and more rigorous benchmarking.
44 These investigations could inspire future targeted observational and experimental research to test the validity
45 of model assumptions (Payne et al., 2016; Lotze et al., 2019; Heneghan et al., 2021). The state-of-the-art in
46 such experimental research is presented in Box 3.1.





51 Box 3.1: Challenges for Multiple-Driver Research in Ecology and Evolution


53 The majority of the examples in Section 3.3 are from studies mimicking projected conditions in the year
54 2100 that report the responses of an individual species or strain to multiple drivers. This powerful generic
55 experimental approach has largely been restricted to single species because it is logistically complex to
56 conduct experiments that straddle multiple trophic levels, and that also include more than two drivers (Figure
57 Box3.1.1b); the need for multiple replicates, drivers, and treatment levels greatly increase the work required

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 1 (Parmesan et al., 2013; Boyd et al., 2018). It is challenging to apply this experimental approach to
 2 communities or ecosystems (Figure Box 3.1.1). To date, most research on community or ecosystem response
 3 to climate-impact drivers has been in large-volume (>10,000 L) mesocosms (Riebesell and Gattuso, 2014),
 4 or at natural analogues such as CO2 seeps, in which only one driver (ocean acidification) is altered (see (4) in
 5 Figure Box 3.1.1). Only very recently have two drivers been incorporated into climate-change manipulation
 6 studies examining responses of primary producers to secondary consumers ((5) in Figure Box3.1.1a,
 7 Nagelkerken et al., 2020). Therefore, `natural experiments' from the geological past (Reddin et al., 2020)
 8 provide insights into how food webs and their constituents respond to complex change involving multiple
 9 drivers. Contemporary observations are occasionally long enough (>5 decades) to capture community
10 responses to complex climate change. For example, Brun et al. (2019) reported a shift in zooplankton
11 community structure in the North Atlantic (1960­2014), with major biogeochemical ramifications.


13 Conducting sufficiently long manipulation experiments to study the effect of adaptation on organisms is
14 equally difficult (Figure Box3.1.1b), with much research restricted to multi-year studies of the
15 microevolution of fast-growing (>one division day­1) phytoplankton species responding to single drivers
16 (Lohbeck et al., 2012; Schaum et al., 2016). In a few experimental evolution studies, ((7) in Figure
17 Box3.1.1a, Brennan et al., 2017), multiple drivers have been used, but none have used communities or
18 ecosystems (Figure Box3.1.1b). Nevertheless, the fossil record provides limited evidence of adaptations to
19 less rapid (relative to present-day) climate change (Jackson et al., 2018). Despite the need to explore
20 ecological or biogeochemical responses to projected future ocean conditions, logistical challenges require
21 that assessments of climate-change impacts at scales larger than mesocosms use large-scale, long-term in situ
22 observational studies (as documented within Section 3.4).




26 Figure Box 3.1.1: Knowledge gaps between current scientific understanding and that needed to inform policy. The
27 conceptual space relating driver number, (Driver axis), ecological organisation (Space axis) and evolutionary
28 acclimation state (Time axis), modified from Riebesell and Gattuso (2014). (a) Spheres indicate suites of studies that
29 illustrate the progress of research, including multiple drivers (Sphere (1) one species and one driver, Hutchins et al.
30 (2013) and (2) (one species and multiple drivers five, Boyd et al. (2015a)); ecology (Sphere (1) (one driver, one
31 species), (3) (one driver, planktonic community, Moustaka-Gouni et al., 2016), (4) one driver (high-CO2 seep) and
32 (benthic) ecosystem, Fabricius et al. (2014), and (5) two drivers and nearshore ecosystem, Nagelkerken et al. (2020));
33 and evolution (Sphere (1) (acclimated organism and one driver), (6) adapted organisms and one driver, Listmann et al.
34 (2016) and (7) adapted organism and multiple drivers Brennan et al. (2017)). (b) Trends in research trajectories since
35 2000 from a survey of 171 studies, Boyd et al. (2018). Note the dominance of multiple-driver experiments at the species
36 level (lower left cluster); the focus on acclimation (red triangle) rather than adaptation (blue dot); the focus of
37 investigation on 3 drivers. Redrawn from Boyd et al. (2018).



40 [END BOX 3.1 HERE]



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 1 3.4 Observed and Projected Impacts of Climate Change on Marine Systems


 3 3.4.1 Introduction


 5 Ocean and coastal ecosystems and their resident species are under increasing pressure from a multitude of
 6 climate-impact drivers and non-climate drivers (Section 3.1, Figure 3.12, Bindoff et al., 2019). This section
 7 builds from the assessment of biological responses to climate-impact drivers (Section 3.3) to examine the
 8 new evidence about climate-change impacts at the level of marine ecosystems. It focuses on detection and
 9 attribution of observed changes to marine ecosystems and the projected changes under different future
10 climate scenarios. This assessment considers emerging evidence on the effects of multiple non-climate
11 drivers and physiological acclimation and/or evolutionary adaptation on these observations and projections.


13 The section focuses first on coastal ecosystems and seas (Section 3.4.2), which have high spatial variability
14 in physical and chemical characteristics, are affected by many non-climate drivers (Section 3.1, Figure 3.12)
15 and support rich fisheries, high biodiversity and high levels of species endemism. The assessment begins
16 with warm-water coral reefs (Section because these highly threatened systems are at the vanguard of
17 research on acclimation and evolutionary adaptation among coastal ecosystems. It follows with the other
18 shallow, nearshore ecosystems dominated by habitat-forming species (rocky shores, kelp systems) and then
19 nearshore sedimentary systems (estuaries, deltas, coastal wetlands, and sandy beaches), before moving on to
20 semi-enclosed seas, shelf seas, upwelling zones, and polar seas.


22 The section continues on to oceanic and cross-cutting changes (Section 3.4.3), which influence large areas of
23 the epipelagic zone (<200 m depth), while also affecting the mesopelagic (200­1000 m), the perpetually dark
24 bathypelagic (depth >1000 m) and the deep seafloor (benthic ecosystems at depths >200 m) zones. Assessed
25 in this section are species range shifts (Section, phenological shifts and trophic mismatches (Section
26, changes in communities and biodiversity (Section, time of emergence of climate-impact
27 signals in ecological systems from background natural variability (Section, and changes in
28 biomass, primary productivity, and carbon export (Sections­




32 Figure 3.12: Summary assessment of observed hazards to coastal ecosystems and seas as assessed in Section 3.4.2.

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3 3.4.2 Coastal Ecosystems and Seas


5 Warm-Water Coral Reefs


 7 Warm-water coral reef ecosystems house one-quarter of the marine biodiversity and provide services in the
 8 form of food, income and shoreline protection to coastal communities around the world. These ecosystems
 9 are threatened by climate and non-climate drivers, especially ocean warming, MHWs, ocean acidification,
10 SLR, tropical cyclones, fisheries/overharvesting, land-based pollution, disease spread and destructive
11 shoreline practices (Hoegh-Guldberg et al., 2018a; Bindoff et al., 2019; Hughes et al., 2020). Warm-water
12 coral reefs face near-term threats to their survival (Table 3.3), but research on observed and projected
13 impacts is very advanced.



16 Table 3.3: Summary of previous IPCC assessments of coral reefs.

    Observations                                                 Projections

    AR5 (Hoegh-Guldberg et al., 2014; Wong et al., 2014)

    Coral reefs are one of the most vulnerable marine            Coral bleaching and mortality will increase in frequency
    ecosystems (high confidence), and more than half of the      and magnitude over the next decades (very high
    world's reefs are under medium or high risk of               confidence). Analysis of the Coupled Model
    degradation.                                                 Intercomparison Project 5 ensemble projects the loss of
                                                                 coral reefs from most sites globally by 2050 under mid to
    Mass coral bleaching and mortality, triggered by positive    high rates of warming (very likely).
    temperature anomalies (high confidence), is the most
    widespread and conspicuous impact of climate change.         Under the A1B scenario, 99% of the reef locations will
    Ocean acidification reduces biodiversity and the             experience at least one severe bleaching event between
    calcification rate of corals (high confidence) while at the  2090 and 2099, with limited evidence and low agreement
    same time increasing the rate of dissolution of the reef     that coral acclimation and/or adaptation will limit this
    framework (medium confidence).                               trend.

    In summary, ocean warming is the primary cause of mass       The onset of global dissolution of coral reefs is at an
    coral bleaching and mortality (very high confidence),        atmospheric CO2 of 560 ppm (medium confidence) and
    which, together with ocean acidification, deteriorates the   dissolution will be widespread in 2100 (Representative
    balance between coral reef construction and erosion (high    Concentration Pathway (RCP)8.5, medium confidence).
                                                                 A number of coral reefs could keep up with the maximum
                                                                 rate of sea-level rise (SLR) of 15.1 mm yr­1 projected for
                                                                 the end of the century, but lower net accretion and
                                                                 increased turbidity will weaken this capability (very high

    SR15 (Hoegh-Guldberg et al., 2018a; IPCC, 2019c)

    Climate change has emerged as the greatest threat to coral Multiple lines of evidence indicate that the majority (70­
    reefs, with temperatures of just 1°C above the 1985­1993 90%) of warm water (tropical) coral reefs that exist today
    long-term summer maximum for an area over 4­6 weeks will disappear even if global warming is constrained to
    being enough to cause mass coral bleaching and mortality 1.5°C (very high confidence).
    (very high confidence).

                                                                            Coral reefs, for example, are projected to decline by a
    Predictions of back-to-back bleaching events have become further 70­90% at 1.5°C (high confidence) with larger
    reality over 2015-2017 as have projections of declining losses (>99%) at 2ºC (very high confidence).
    coral abundance (high confidence).

    SROCC (Bindoff et al., 2019)

    New evidence since AR5 and SR15 confirms the impacts Coral reefs will face very high risk at temperatures 1.5ºC
    of ocean warming and acidification on coral reefs (high of global sea surface warming (very high confidence).
    confidence), enhancing reef dissolution and bioerosion Almost all coral reefs will degrade from their current state,
    (high confidence), affecting coral species distribution, and even if global warming remains below 2ºC (very high
    leading to community changes (high confidence). The rate confidence), and the remaining shallow coral reef

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    of SLR (primarily noticed in small reef islands) may      communities will differ in species composition and

    outpace the growth of reefs to keep up, although there is diversity from present reefs (very high confidence). This

    low agreement in the literature (low confidence).         will greatly diminish the services they provide to society,

                                                              such as food provision (high confidence), coastal

    Reefs are further exposed to other increased impacts, such protection (high confidence) and tourism (medium

    as enhanced storm intensity, turbidity and increased runoff confidence).

    from the land (high confidence). Recovery of coral reefs

    resulting from repeated disturbance events is slow (high The very high vulnerability of coral reefs to warming,

    confidence). Only few coral reef areas show some          ocean acidification, increasing storm intensity and SLR

    resilience to global change drivers (low confidence).     under climate, including enhanced bioerosion (high

                                                              confidence), points to the importance of considering both

                                                              mitigation and adaptation.



 3 Global analyses published since AR5 show that mass coral bleaching events and disease outbreaks have
 4 increased due to more frequent and severe heat stress associated with ocean warming (very high confidence,
 5 virtually certain) (Donner et al., 2017; Hughes et al., 2018a; DeCarlo et al., 2019; Sully et al., 2019; Tracy et
 6 al., 2019). The mass coral bleaching, which occurred continuously across different parts of the tropics from
 7 2014­2016, is considered the longest and most severe global coral bleaching event on record (Section 10.4.3,
 8 Box 15.2, Eakin et al., 2019). The Great Barrier Reef underwent mass bleaching three times between 2016­
 9 2020 (Box 11.2, Pratchett et al., 2021), validating past model projections that some warm-water coral reefs
10 would encounter bleaching-level heat stress multiple times per decade by the 2020s (Hoegh-Guldberg, 1999;
11 Donner, 2009).


13 Heat stress and mass bleaching events caused decreases in live coral cover (virtually certain) (Graham et al.,
14 2014; Hughes et al., 2018b), loss of sensitive species (extremely likely) (Donner and Carilli, 2019; Lange and
15 Perry, 2019; Toth et al., 2019; Courtney et al., 2020), vulnerability to disease (extremely likely) (van Woesik
16 and Randall, 2017; Hadaidi et al., 2018; Brodnicke et al., 2019; Howells et al., 2020) and declines in coral
17 recruitment in the tropics (medium confidence) (Hughes et al., 2019; Price et al., 2019). Recent observations
18 also suggest that excess nutrients can increase the susceptibility of corals to heat stress (DeCarlo et al.,
19 2020). Changes in coral community structure due to bleaching have caused declines in reef carbonate
20 production (high confidence) (Perry and Morgan, 2017; Lange and Perry, 2019; Perry and Alvarez-Filip,
21 2019; Courtney et al., 2020; van Woesik and Cacciapaglia, 2021) and in reef structural complexity (high
22 confidence, very likely) (Couch et al., 2017; Leggat et al., 2019; Magel et al., 2019), which increases water
23 depth, reduces wave attenuation and increases coastal flood risk (Yates et al., 2017; Beck et al., 2018).
24 Corals may also lose reproductive synchrony through climate change (Shlesinger and Loya, 2019), adding to
25 their vulnerability. Bleaching and other drivers promote phase shifts to ecosystems dominated by macroalgae
26 or other stress-tolerant species (very high confidence) (Graham et al., 2015; Stuart-Smith et al., 2018),
27 leading to changes in reef-fish species assemblages (high confidence) (Richardson et al., 2018; Robinson et
28 al., 2019a; Stuart-Smith et al., 2021).


30 Ocean acidification and associated declines in aragonite saturation state (aragonite) decrease rates of
31 calcification by corals and other calcifying reef organisms (very high confidence), reduce coral settlement
32 (medium confidence) and increase bioerosion and dissolution of reef substrates (high confidence) (Hoegh-
33 Guldberg et al., 2018a; Bindoff et al., 2019; Kline et al., 2019; Pitts et al., 2020). Warming can exacerbate
34 the coral response to ocean acidification (Kornder et al., 2018) and accelerate the decrease in coral skeletal
35 density (Guo et al., 2020). In addition, reefs with lower coral cover and a higher proportion of slow-growing
36 species, because of bleaching, are more sensitive to acidification (net dissolution occurs aragonite = 2.3 for
37 100% coral cover, and aragonite >3.5 for 30% coral cover, (Kline et al., 2019)). However, experimental
38 evidence suggests that coral responses to ocean acidification are species-specific (medium confidence)
39 (Fabricius et al., 2011; DeCarlo et al., 2018; Comeau et al., 2019). Evidence from experiments suggests that
40 crustose coralline algae, which contribute to reef structure and integrity and may be resistant to warming at
41 the RCP8.5 level by 2100 (Cornwall et al., 2019), are also sensitive to declines in aragonite (high confidence)
42 (Section, Fabricius et al., 2015; Smith et al., 2020). The integrated effect of acidification, bleaching,
43 storms and other non-climate drivers on corals, coralline algae and other calcifiers can further compromise
44 reef integrity and ecosystem services (Rivest et al., 2017; Cornwall et al., 2018; Perry and Alvarez-Filip,
45 2019).


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 1 Since SROCC, there have been advances in experimental, field and modelling research on the projected
 2 response of coral cover and reef growth to bleaching and ocean acidification (Cziesielski et al., 2019;
 3 Morikawa and Palumbi, 2019; Cornwall et al., 2021; Klein et al., 2021; Logan et al., 2021; McManus et al.,
 4 2021), and on the effect of possible human interventions like assisted evolution on coral resilience (Section
 5, Condie et al., 2021; Hafezi et al., 2021; Kleypas et al., 2021). New model projections incorporating
 6 physiological acclimation, larval dispersal, and evolutionary processes find limited ability to adapt this
 7 century at rates of warming at or exceeding that in RCP4.5 (high confidence, very likely) (Bay et al., 2017;
 8 Kubicek et al., 2019; Matz et al., 2020; McManus et al., 2020; Logan et al., 2021; McManus et al., 2021).
 9 For example, a global analysis (Logan et al., 2021) finds that increased thermal tolerance via evolution or
10 switching to more stress-tolerant algal symbionts enable most (73­81%) coral to survive through 2100 under
11 RCP2.6, but coral-dominated communities with a historical mix of coral taxa still disappear (0­8% coral
12 survival) under RCP6.0 in simulations with adaptive mechanisms (Figure 3.13). Due to the impacts of
13 warming, and to a lesser extent ocean acidification, global reef carbonate production is estimated to decline
14 71% by 2050 in SSP1-2.6, and the rate of SLR is estimated to exceed that of reef growth for 97% of reefs
15 assessed, absent adaptation by corals and their symbionts (WGI AR6 Table 9.9, Cornwall et al., 2021; Fox-
16 Kemper et al., 2021). The increased water depth due to coral loss and reef erosion, as well as reduced
17 structural complexity, will limit wave attenuation and exacerbate the risk of flooding from SLR on reef-
18 fringed shorelines and reef islands (Yates et al., 2017; Beck et al., 2018; Harris et al., 2018). Local coral reef
19 fish species richness is projected to decline due to the impacts of warming on coral cover and diversity (high
20 confidence), with declines up to 40% by 2060 in SSP5-8.5 (Strona et al., 2021).


22 These observed and projected impacts are supported by geological and paleo-ecological evidence showing a
23 decline in coral reef extent and species richness under previous episodes of climate change and ocean
24 acidification (Kiessling and Simpson, 2011; Pandolfi et al., 2011; Kiessling et al., 2012; Pandolfi and
25 Kiessling, 2014; Kiessling and Kocsis, 2015). Major reef crises in the past 300 million years were governed
26 by hyperthermal events (medium confidence) (Section, Cross-Chapter Box PALEO in Chapter 1)
27 longer in timescale than anthropogenic climate change, during which net coral reef accretion was more
28 strongly affected than biodiversity (medium confidence).


30 In response to the global-scale decline in coral reefs and high future risk, recent literature focuses on finding
31 thermal refuges and identifying uniquely resilient species, populations or reefs for targeted restoration and
32 management (Hoegh-Guldberg et al., 2018b). Reefs exposed to internal waves (Storlazzi et al., 2020),
33 turbidity (Sully and van Woesik, 2020) or warm-season cloudiness (Gonzalez-Espinosa and Donner, 2021)
34 are expected to be less sensitive to thermal stress. Mesophotic reefs (30­150 m) have also been proposed as
35 thermal refugia (Bongaerts et al., 2010), although evidence from recent bleaching events, subsurface
36 temperature records, and species overlap is mixed (Frade et al., 2018; Rocha et al., 2018b; Eakin et al., 2019;
37 Venegas et al., 2019; Wyatt et al., 2020). A study of 2584 reef sites across the Indian and Pacific Oceans
38 estimated that 17% had sufficient cover of framework-building corals to warrant protection, 54% required
39 recovery efforts, and 28% were on a path to net erosion (Darling et al., 2019). There is medium evidence for
40 greater bleaching resistance among reefs subject to temperature variability or frequent heat stress (Barkley et
41 al., 2018; Gintert et al., 2018; Hughes et al., 2018a; Morikawa and Palumbi, 2019), but with trade-offs in
42 terms of diversity and structural complexity (Donner and Carilli, 2019; Magel et al., 2019). There is limited
43 agreement about the persistence of thermal tolerance in response to severe heat stress (Le Nohaïc et al.,
44 2017; DeCarlo et al., 2019; Fordyce et al., 2019; Leggat et al., 2019; Schoepf et al., 2020). Recovery and
45 restoration efforts that target heat-resistant coral populations and culture heat-tolerant algal symbionts have
46 the greatest potential of effectiveness under future warming (high confidence) (Box 5.5 in SROCC Chapter
47 5, Bay et al., 2017; Darling and Côté, 2018; Baums et al., 2019; Bindoff et al., 2019; Howells et al., 2021);
48 however, there is low confidence that enhanced thermal tolerance can be sustained over time (Section
49, Buerger et al., 2020). The effectiveness of active restoration and other specific interventions (e.g.,
50 reef shading) are further assessed in Section


52 In sum, additional evidence since SROCC and SR15 (Table 3.3) finds that living coral and reef growth are
53 declining due to warming and MHWs (very high confidence). Coral reefs are under threat of transitioning to
54 net erosion with >1.5ºC of global warming (high confidence), with impacts expected to occur fastest in the
55 Atlantic Ocean. The effectiveness of conservation efforts to sustain living coral area, coral diversity, and reef
56 growth is limited for the majority of the world's reefs with >1.5ºC of global warming (high confidence)
57 (Section, Hoegh-Guldberg et al., 2018b; Bruno et al., 2019; Darling et al., 2019).

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 4 Figure 3.13: Coral reef futures, with and without adaptation. Graphs are based on a model of coral-symbiont
 5 evolutionary dynamics from (Logan et al., 2021), which simulates two coral types and symbiont populations for 1925
 6 reef cells worldwide, from 1950­2100 drawn from simulations with National Oceanic and Atmospheric
 7 Administration-Geophysical Fluid Dynamics Laboratory Earth System Model (ESM2M) under four RCPs. Top panels
 8 show the simulated fraction of cells with healthy reefs, when both coral types are not in a state of severe bleaching or
 9 mortality, (i) without adaptive responses and (ii) with adaptive responses (symbiont evolution). Colours indicate
10 maximum monthly sea-surface temperature increase across all reef cells, versus a 1861­2010 baseline. Panels (a,b,c)
11 depict snapshots of coral reef conditions at time-points in the future each with different levels of warming, drawn from
12 the model-projected cover of the two coral types, and from a literature assessment (Section, Hughes et al.,
13 2018b; Bindoff et al., 2019; Darling et al., 2019; Leggat et al., 2019; Cornwall et al., 2021).



16 Rocky Shores


18 Rocky shore ecosystems refer to a range of temperate intertidal and shallow coastal ecosystems that are
19 dominated by different foundational organisms, including mussels, oysters, fleshy macroalgae, hard and soft
20 corals, coralline algae, bryozoans and sponges, which create habitat for species-rich assemblages of
21 invertebrates, fish, marine mammals and other organisms. Rocky shores provide services including wave
22 attenuation, habitat provision and food resources, and these support commercial, recreational, and
23 Indigenous fisheries and shellfish aquaculture.


25 Observations since AR5 and SROCC (Table 3.4) find increased impacts of ocean warming on rocky shores.
26 This includes extirpation of species at the warm edge of their ranges (Yeruham et al., 2015; Martínez et al.,
27 2018), extension of poleward range boundaries (Sanford et al., 2019), mortality from climate extremes
28 (Seuront et al., 2019), reduction in survival at shallower depths (Sorte et al., 2019; Wallingford and Sorte,
29 2019) and reorganisation of communities (Wilson et al., 2019; Mulders and Wernberg, 2020; Albano et al.,
30 2021). Data collected after MHWs find ecological phase shifts (moderate evidence, high agreement) (e.g.,
31 California (Rogers-Bennett and Catton, 2019; McPherson et al., 2021)) and homogenisation of communities
32 (limited evidence) (e.g., Alaska, (Weitzman et al., 2021)). For example, the collapse of sea star populations
33 in the Northeast Pacific due to a MHW-related disease outbreak (Hewson et al., 2014; Menge et al., 2016;
34 Miner et al., 2018; Schiebelhut et al., 2018), including 80­100% loss of the common predatory sunflower

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1 star, Pycnopodia helianthoides (very high confidence) (Harvell et al., 2019), triggered shifts from kelp- to
2 urchin-dominated ecosystems (Schultz et al., 2016; Gravem and Morgan, 2017; McPherson et al., 2021).



5 Table 3.4: Summary of previous IPCC assessments of rocky shores.

   Observations                                                 Projections

   AR5: (Wong et al., 2014)                                     The abundance and distribution of rocky shore species
   Rocky shores are among the better-understood coastal         will continue to change in a warming world (high
   ecosystems in terms of potential impacts of climate          confidence). For example, the long-term consequences of
   variability and change. The most prominent effects are       ocean warming on mussel beds of the northeast Pacific
   range shifts of species in response to ocean warming         are both positive (increased growth) and negative
   (high confidence) and changes in species distribution and    (increased susceptibility to stress and of exposure to
   abundance (high confidence) mostly in relation to ocean      predation) (medium confidence).
   warming and acidification.

   The dramatic decline of biodiversity in mussel beds of       Observations performed near natural CO2 vents in the
   the Californian coast has been attributed to large-scale     Mediterranean Sea show that diversity, biomass, and
   processes associated with climate-related drivers (high      trophic complexity of rocky shore communities will
   confidence).                                                 decrease at future pH levels (high confidence).

   SR15 (Hoegh-Guldberg et al., 2018a)                          In the transition to 1.5°C, changes to water temperatures
   Changes in ocean circulation can have profound impacts       will drive some species (e.g., plankton, fish) to relocate
   on temperate marine ecosystems by connecting regions         to higher latitudes and for novel ecosystems to appear
   and facilitating the entry and establishment of species in   (high confidence). Other ecosystems (e.g., kelp forests,
   areas where they were unknown before                         coral reefs) are relatively less able to move, however,
   (`tropicalization') as well as the arrival of novel disease  and are projected to experience high rates of mortality
   agents (medium agreement, limited evidence).                 and loss (very high confidence).

                                                                In the case of `less mobile' ecosystems (e.g., coral reefs,
                                                                kelp forests, intertidal communities), shifts in
                                                                biogeographical ranges may be limited, with mass
                                                                mortalities and disease outbreaks increasing in frequency
                                                                as the exposure to extreme temperatures have increased
                                                                (high agreement, robust evidence)

   SROCC (Bindoff et al., 2019)

   Intertidal rocky shores ecosystems are highly sensitive to Intertidal rocky shores are also expected to be at very

   ocean warming, acidification and extreme heat exposure high risk (transition above 3ºC) under the RCP8.5

   during low tide emersion (high confidence).                  scenario (medium confidence). These ecosystems have

                                                                low to moderate adaptive capacity, as they are highly

   Sessile calcified organisms (e.g., barnacles and mussels) sensitive to ocean temperatures and acidification.

   in intertidal rocky shores are highly sensitive to extreme

   temperature events and acidification (high confidence), a Benthic species will continue to relocate in the intertidal

   reduction in their biodiversity and abundance have been zones and experience mass mortality events due to

   observed in naturally acidified rocky reef ecosystems        warming (high confidence). Interactive effects between

   (medium confidence).                                         acidification and warming will exacerbate the negative

                                                                impacts on rocky shore communities, causing a shift

                                                                towards a less diverse ecosystem in terms of species

                                                                richness and complexity, increasingly dominated by

                                                                macroalgae (high confidence).



8 Multiple lines of evidence find that foundational calcifying organisms such as mussels are at high risk of

9 decline due to both the individual and synergistic effects of warming, acidification and hypoxia (high

10 confidence) (Sunday et al., 2016; Sorte et al., 2017; Sorte et al., 2019; Newcomb et al., 2020). Warmer

11 temperatures reduce mussel and barnacle recruitment (e.g., NW Atlantic (Petraitis and Dudgeon, 2020)) and

12 the upper vertical limit of mussels (e.g., NE Pacific (Harley, 2011)) and (SW Pacific (Sorte et al., 2019)).

13 Experiments show that ocean acidification negatively impacts mussel physiology (very high confidence),

14 with evidence of reduced growth (Gazeau et al., 2010), attachment (Newcomb et al., 2020),

15 biomineralisation (Fitzer et al., 2014) and shell thickness (Pfister et al., 2016; McCoy et al., 2018). Net

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 1 calcification and abundance of mussels and other foundational species including oysters are expected to
 2 decline due to ocean acidification (very high confidence) (Kwiatkowski et al., 2016; Sunday et al., 2016;
 3 McCoy et al., 2018; Meng et al., 2018), causing the reorganisation of communities (high confidence)
 4 (Kroeker et al., 2013b; Linares et al., 2015; Brown et al., 2016; Sunday et al., 2016; Agostini et al., 2018;
 5 Teixidó et al., 2018). Experiments indicate that acidification can interact with warming and hypoxia to
 6 increase the detrimental effects on mussels (Gu et al., 2019; Newcomb et al., 2020). In regions where food is
 7 readily available to mussels, detrimental effects of ocean acidification may be dampened (Kroeker et al.,
 8 2016); however, recent findings are inconclusive (Brown et al., 2018a).


10 Coralline algae, foundational taxa that create habitat for sea urchins and abalone, form rhodolith beds in
11 temperate to Arctic habitats, and bind together substrates, are expected to be highly susceptible to ocean
12 acidification because they precipitate soluble magnesium calcite (Kuffner et al., 2008; Williams et al., 2021).
13 Damage from acidification varies among species and regions, and can be due to direct physiological stress
14 (Marchini et al., 2019) or interactions with non-calcifying competitors such as fleshy macroalgae (Smith et
15 al., 2020). Experiments indicate that warming reduces calcification by coralline algae (high confidence)
16 (Cornwall et al., 2019) and exacerbates the effect of acidification (Kim et al., 2020; Williams et al., 2021).


18 In contrast to warm-water coral reefs, there are no regional or global numerical models of rocky shore
19 ecosystem response to projected climate change and acidification. Experiments suggest that existing genetic
20 variation could be sufficient for some mussels (Bitter et al., 2019) and coralline algae (Cornwall et al., 2020)
21 to adapt over generations to ocean acidification. Populations exposed to variable environments often have a
22 greater capacity for phenotypic plasticity and resilience to environmental change (e.g., urchins (Gaitán-
23 Espitia et al., 2017b) and coralline algae (Section 3.3.2, Rivest et al., 2017; Cornwall et al., 2018)). Although
24 parental conditioning within and across generations is an acclimatisation mechanism to global change, there
25 is limited evidence from experimental studies that this is applicable for marine invertebrates on rocky shores
26 (Byrne et al., 2020).


28 This assessment concludes that MHWs, attributable to climate change (Section, can cause fatal
29 disease outbreaks or mass mortality among some key foundational species (high confidence) and contribute
30 to ecological phase shifts (medium confidence). The upper vertical limits of some species will also be
31 constrained by climate change (high confidence). Experimental evidence since previous assessments further
32 indicates that acidification decreases abundance and richness of calcifying species (high confidence),
33 although there is limited evidence for acclimation in some species. Synergistic effects of warming and
34 acidification will promote shifts toward macroalgal dominance in some ecosystems (medium confidence) and
35 lead to reorganisation of communities (medium confidence).


37 Kelp Ecosystems


39 Kelp are temperate, habitat-forming marine macroalgae or seaweeds, mostly of the order Laminariales,
40 which extend across one quarter of the world's coastlines (Assis et al., 2020; Jayathilake and Costello, 2020).
41 The perennial species form dense underwater forest canopies and three-dimensional habitat that provides
42 refuge for fish, crustaceans, invertebrates and marine mammals (Filbee-Dexter et al., 2016; Wernberg et al.,
43 2019). Kelp ecosystems support fisheries, aquaculture, fertiliser, and food provision, including for local and
44 Indigenous Peoples, along with regulating services in the form of wave attenuation and habitat provision.
45 Kelp aquaculture can also buffer against local acidification (Xiao et al., 2021) and contribute to carbon
46 storage (Froehlich et al., 2019).


48 Recent research (Straub et al., 2019; Butler et al., 2020; Filbee-Dexter et al., 2020b; Tait et al., 2021)
49 supports the findings of previous assessments (Table 3.5) that kelp and other seaweeds in most regions are
50 undergoing mass mortalities from high temperature extremes and range shifts from warming (very high
51 confidence). Kelp are highly sensitive to the direct effect of high temperature on survival (Nepper-Davidsen
52 et al., 2019) and indirect impact of temperature on herbivorous species (Ling, 2008; Vergés et al., 2016),
53 upwelling and nutrient availability (Carr and Reed, 2015; Schiel and Foster, 2015). Synergies between
54 warming, storms, pollution and intensified herbivory (due to removal or loss of predators including sea stars
55 and otters that constrain herbivory by fish and urchin populations) can also cause physiological stress and
56 physical damage in kelp, reducing productivity and reproduction (Rogers-Bennett and Catton, 2019; Beas-
57 Luna et al., 2020; McPherson et al., 2021).

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3 Table 3.5: Summary of previous IPCC assessments of kelp ecosystems.

    Observations                                                 Projections

    AR5: (Wong et al., 2014)

    Kelp forests have been reported to decline in temperate Kelp ecosystems will decline with the increased frequency
    areas in both hemispheres, a loss involving climate change of heatwaves and sea temperature extremes as well as
    (high confidence). Decline in kelp populations attributed to through the impact of invasive subtropical species (high
    ocean warming has been reported in southern Australia and confidence).
    the North Coast of Spain.

                                                                            Climate change will contribute to the continued decline in
                                                                            the extent of kelps in the temperate zone (medium
                                                                            confidence) and the range of kelp in the Northern
                                                                            Hemisphere will expand poleward (high confidence).

    SR15 (Hoegh-Guldberg et al., 2018a)                          In the transition to 1.5°C, changes to water temperatures
    Observed movement of kelp ecosystems not assessed.           will drive some species (e.g., plankton, fish) to relocate to
                                                                 higher latitudes and for novel ecosystems to appear (high
                                                                 confidence). Other ecosystems (e.g., kelp forests, coral
                                                                 reefs) are relatively less able to move, however, and will
                                                                 experience high rates of mortality and loss (very high

    SROCC (Bindoff et al., 2019)

    Kelp forests have experienced large-scale habitat loss and Kelp forests will face moderate to high risk at temperatures

    degradation of ecosystem structure and functioning over above 1.5ºC global sea surface warming (high confidence).

    the past half century, implying a moderate to high level of

    risk at present conditions of global warming (high           Due to their low capacity to relocate and high sensitivity to

    confidence).                                                 warming, kelp forests are projected to experience higher

                                                                 frequency of mass mortality events as the exposure to

    The abundance of kelp forests has decreased at a rate of extreme temperature rises (very high confidence).

    ~2% yr­1 over the past half century, mainly due to ocean

    warming and marine heatwaves, as well as from other Changes in ocean currents have facilitated the entry of

    human stressors (high confidence).                           tropical herbivorous fish into temperate kelp forests

                                                                 decreasing their distribution and abundance (medium

    Changes in ocean currents have facilitated the entry of confidence).

    tropical herbivorous fish into temperate kelp forests

    decreasing their distribution and abundance (medium          Kelp forests at low latitudes will continue to retreat due to

    confidence).                                                 intensified extreme temperatures, and their low dispersal

                                                                 ability will elevate the risk of local extinction under

    The loss of kelp forests is followed by the colonisation of RCP8.5 (high confidence).

    turfs, which contributes to the reduction in habitat

    complexity, carbon storage and diversity (high




 6 Trends in kelp abundance since 1950 are uneven globally (Krumhansl et al., 2016; Wernberg et al., 2019),
 7 with population declines (e.g., giant kelp Macrocystis pyrifera in Tasmania, (Butler et al., 2020), sugar kelp
 8 Saccharina latissima in the North Atlantic (Filbee-Dexter et al., 2020b)), more common than increases or no
 9 change (e.g., giant kelp Macrocystis pyrifera in southern Chile (Friedlander et al., 2020). Warming is driving
10 range contraction and extirpation at the warm edge of species' ranges, and expansions at the cold range edge
11 (very high confidence) (Smale, 2019; Filbee-Dexter et al., 2020b). Local declines in populations of kelp and
12 other canopy-forming seaweeds driven by MHWs and other stressors have caused irreversible shifts to turf-
13 or urchin-dominated ecosystems, with lower productivity and biodiversity (high confidence) (Filbee-Dexter
14 and Scheibling, 2014; Filbee-Dexter and Wernberg, 2018; Rogers-Bennett and Catton, 2019; Beas-Luna et
15 al., 2020; Stuart-Smith et al., 2021), ecosystems dominated by warm-affinity seaweeds or coral (high
16 confidence) (Vergés et al., 2019), and loss of genetic diversity (Coleman et al., 2020a; Gurgel et al., 2020).


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 1 Species distribution models of kelp project range shifts and local extirpations with increasing levels of
 2 warming (Japan (Takao et al., 2015; Sudo et al., 2020)), (Australia (Table 11.6, Assis et al., 2018; Martínez
 3 et al., 2018; Castro et al., 2020)), (Europe (de la Hoz et al., 2019)), (North America (Wilson et al., 2019)),
 4 (South America (Figure 12.3)). There is high agreement on the direction but not the magnitude of change
 5 (Martínez et al., 2018; Castro et al., 2020), but effects of MHWs are not simulated. Where the length of
 6 higher-latitude coastlines is limited, range contractions are projected to occur, even with 2°C of global
 7 warming (i.e., SSP1-2.6) due to loss of habitat at the warm edge of species' ranges (Martínez et al., 2018).
 8 Poleward expansion of warm-affinity herbivores including urchins could further reduce warm-edge kelp
 9 populations (Castro et al., 2020; Mulders and Wernberg, 2020). Evidence from natural temperate CO2 seeps
10 suggests that ocean acidification at levels above those in RCP4.5 in 2100 could offset the increase in urchin
11 abundance (Coni et al., 2021). Genetic analyses suggest that kelp populations at the midpoint of species'
12 ranges will have lower tolerance of warming than implied by species distribution models, without assisted
13 gene flow from warm-edge populations (King et al., 2019; Wood et al., 2021).


15 While reducing non-climate drivers can help prevent kelp loss from warming and MHWs, there is limited
16 potential for restoration of kelp ecosystems after transition to urchin-dominant ecosystems (high confidence).
17 Current restoration efforts are generally small-scale (<0.1 km2) and less advanced than those in ecosystems
18 like coral reefs (Coleman et al., 2020b; Eger et al., 2020; Layton et al., 2020). Although abundance of
19 herbivores limits kelp populations, there is limited evidence that restoring predators of herbivores by creating
20 marine reserves, or directly removing grazing species, will increase kelp forest resilience to warming and
21 extremes (Vergés et al., 2019; Wernberg et al., 2019). Active reseeding of wild kelp populations through
22 transplantation and propagation of warm-tolerant genotypes (Coleman et al., 2020b; Alsuwaiyan et al., 2021)
23 can overcome low dispersal rates of many kelp species and facilitate effective restoration (medium
24 confidence) (Morris et al., 2020c).


26 Building on the conclusions of SROCC, this assessment finds that kelp ecosystems are expected to decline
27 and undergo changes in community structure in the future due to warming and increasing frequency and
28 intensity of MHWs (high confidence). Risk of loss of kelp ecosystems and shifts to turf- or urchin-dominated
29 ecosystems are highest at the warm edge of species' ranges (high confidence) and risks increase under
30 RCP6.0 and RCP8.5 by the end of the century (high confidence).


32 Estuaries, Deltas and Coastal Lagoons


34 Estuaries, deltas and lagoons encounter environmental gradients over small spatial scales, generating diverse
35 habitats that support myriad ecosystem services, including food provision, regulation of erosion, nutrient
36 recycling, carbon sequestration, recreation and tourism, and cultural significance (D'Alelio et al., 2021;
37 Keyes et al., 2021). Although these coastal ecosystems have historically been sensitive to erosion-accretion
38 cycles driven by sea level, drought and storms (high confidence) (Peteet et al., 2018; Wang et al., 2018c;
39 Jones et al., 2019b; Urrego et al., 2019; Hapsari et al., 2020; Zhao et al., 2020b), they were impacted for
40 much of the 20th century primarily by non-climate drivers (very high confidence) (Brown et al., 2018b;
41 Ducrotoy et al., 2019; Elliott et al., 2019; He and Silliman, 2019; Andersen et al., 2020; Newton et al., 2020;
42 Stein et al., 2020). Nevertheless, the influence of climate-impact drivers has become more apparent over
43 recent decades (medium confidence) (Table 3.6).



46 Table 3.6: Summary of previous IPCC assessments of estuaries, deltas and coastal lagoons.

    Observations                                           Projections

    AR5: (Wong et al., 2014)

    Humans have impacted lagoons, estuaries and deltas (high Future changes in climate-impact drivers such as warming,

    to very high confidence), but non-climate drivers have acidification, waves, storms, sea-level rise (SLR), and

    been the primary agents of change (very high confidence). runoff will have consequences for ecosystem function and

                                                           services in lagoons and estuaries (high confidence), but

    In estuaries and lagoons, nutrient inputs have driven  with regional differences in magnitude of change in impact

    eutrophication, which has modified food-web structures drivers and ecosystem response.

    (high confidence) and caused more-intense and longer-

    lasting hypoxia, more-frequent occurrence of harmful algal Warming, changes in precipitation and changes in wind

    blooms, and enhanced emissions of nitrous oxide (high strength can interact to alter water-column salinity and


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                                                                stratification (medium confidence), which could impact

   In deltas, land-use changes and associated disruption of water column oxygen content (medium confidence).

   sediment dynamics and land subsidence have driven

   changes that have been exacerbated by relative sea-level Land-use change, SLR and intensifying storms will alter

   rise and episodic events including river floods and oceanic deposition-erosion dynamics, impacting shoreline

   storm surges (very high confidence).                         vegetation and altering turbidity (medium confidence).

                                                                Together with warming, these drivers will alter the

   Increased coastal flooding, erosion and saltwater intrusions seasonal pattern of primary production and the distribution

   have led to degradation of ecosystems (very high             of biota throughout the ecosystems (medium to high

   confidence).                                                 confidence), impacting associated ecosystem services.

                                                                The projected impacts of climate change on deltas are
                                                                associated mainly with pluvial floods and SLR, which will
                                                                amplify observed impacts of interacting climate and non-
                                                                climate drivers (high confidence).

   SR15 (Hoegh-Guldberg et al., 2018a)                          Under both a 1.5°C and 2°C of warming, relative to pre-
                                                                industrial, deltas are expected to be highly threatened by
   Estuaries, deltas, and lagoons were not assessed in this     SLR and localised subsidence (high confidence). The
   report.                                                      slower rate of SLR associated with 1.5°C of warming
                                                                poses smaller risks of flooding and salinisation (high
                                                                confidence), and facilitates greater opportunities for
                                                                adaptation, including managing and restoring natural
                                                                coastal ecosystems and infrastructure reinforcement
                                                                (medium confidence).

                                                                Intact coastal ecosystems may be effective in reducing the
                                                                adverse impacts of rising sea levels and intensifying storms
                                                                by protecting coastal and deltaic regions (medium

                                                                Natural sedimentation rates are expected to be able to
                                                                offset the effect of rising sea levels, given the slower rates
                                                                of SLR associated with 1.5°C of warming (medium
                                                                confidence). Other feedbacks, such as landward migration
                                                                and the adaptation of infrastructure, remain important
                                                                (medium confidence).

   SROCC (Bindoff et al., 2019)

   Increased seawater intrusion caused by SLR has driven Salinisation and expansion of hypoxic conditions will

   upstream redistribution of marine biotic communities in intensify in eutrophic estuaries, especially in mid and high

   estuaries (medium confidence) where physical barriers latitudes with microtidal regimes (high confidence).

   such as the availability of benthic substrates do not limit

   availability of suitable habitats (medium confidence).       The effects of warming will be more pronounced in high-

                                                                latitude and temperate shallow estuaries characterised by

   Warming has driven poleward range shifts in species' limited exchange with the open ocean and strong

   distributions among estuaries (medium confidence).           seasonality that already leads to development of dead

                                                                zones (medium confidence).

   Interactions between warming, eutrophication and hypoxia

   have increased the incidence of harmful algal blooms (high Interaction between SLR and changes in precipitation will

   confidence), pathogenic bacteria such as Vibrio species have greater impacts on shallow than deep estuaries

   (low confidence), and mortalities of invertebrates and fish (medium confidence).

   communities (medium confidence).

                                                                Estuaries characterised by large tidal exchanges and

                                                                associated well-developed sediments will be more resilient

                                                                to projected SLR and changes in river flow (medium

                                                                confidence). Human activities that inhibit sediment

                                                                dynamics in coastal deltas increase their vulnerability to

                                                                SLR (medium confidence).


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 2 Estuarine biota are sensitive to warming (high confidence), with recent responses including changes in
 3 abundance of some fish stocks (Erickson et al., 2021; Woodland et al., 2021), poleward shifts in distributions
 4 of fish species, communities and associated biogeographic transition zones (Table 12.3, Franco et al., 2020;
 5 Troast et al., 2020), recruits of warm-affinity species persisting into winter (Kimball et al., 2020), and
 6 changes in seasonal timing of peaks in species abundance (Kimball et al., 2020). MHWs can be more severe
 7 in estuaries than in adjacent coastal seas (Lonhart et al., 2019), causing conspicuous impacts (very high
 8 confidence), including mass mortality of intertidal vegetation (Section, range shifts in algae and
 9 animals (Lonhart et al., 2019), and reduced spawning success among invertebrates (Shanks et al., 2020).


11 RSLR extends the upstream limit of saline waters (high confidence) (Harvey et al., 2020; Jiang et al., 2020)
12 and alters tidal ranges (high confidence) (Idier et al., 2019; Talke et al., 2020). Elevated water levels also
13 alter submergence patterns for intertidal habitat (high confidence) (Andres et al., 2019), moving high-water
14 levels inland (high confidence) (Peteet et al., 2018; Appeaning Addo et al., 2020; Liu et al., 2020e) and
15 increasing the salinity of coastal water tables and soils (high confidence) (Eswar et al., 2021). These
16 processes favour inland and/or upstream migration of intertidal habitat, where it is unconstrained by
17 infrastructure, topography or other environmental features (high confidence) (Kirwan and Gedan, 2019;
18 Parker and Boyer, 2019; Langston et al., 2020; Magolan and Halls, 2020; Saintilan et al., 2020). The spread
19 of "ghost forests" along the North American east coast (Kirwan and Gedan, 2019) and elsewhere (Grieger et
20 al., 2020) illustrates this phenomenon. Along estuarine shorelines, changing submergence patterns and
21 upstream penetration of saline waters interact synergistically to stress intertidal plants, changing species
22 composition and reducing above-ground biomass, in some cases favouring invasive species (Xue et al.,
23 2018; Buffington et al., 2020; Gallego-Tévar et al., 2020). Overall, changing salinity and submergence
24 patterns decrease the ability of shoreline vegetation to trap sediment (Xue et al., 2018), reducing accretion
25 rates and increasing the vulnerability of estuarine shorelines to submergence by SLR and erosion by wave
26 action (medium confidence) (Zhu et al., 2020b).


28 Drought and freshwater abstraction can reduce freshwater inflows to estuaries and lagoons, increasing
29 salinity, reducing water quality (Brooker and Scharler, 2020) and depleting resident macrophyte
30 communities (Scanes et al., 2020b). Changes in freshwater input and SLR, combined with land-use change,
31 can alter inputs of land-based sediments, causing expansion (Suyadi et al., 2019; Magolan and Halls, 2020)
32 or contraction (Andres et al., 2019; Appeaning Addo et al., 2020; Li et al., 2020b) of intertidal habitats. The
33 same phenomena alter salinity gradients, which are the primary drivers of estuarine species distributions
34 (high confidence) (Douglass et al., 2020; Lauchlan and Nagelkerken, 2020). Extreme reduction of freshwater
35 input can extend residence time of estuarine water, leading to persistent HABs (Lehman et al., 2020) and
36 converting estuaries to lagoons if the mouth clogs with sediment (Thom et al., 2020).


38 Acidification of estuarine water is a growing hazard (medium confidence) (Doney et al., 2020; Scanes et al.,
39 2020a; Cai et al., 2021), and resident organisms display sensitivity to altered pH in laboratory settings
40 (medium confidence) (Young et al., 2019a; Morrell and Gobler, 2020; Pardo and Costa, 2021). However,
41 attribution of the biological effects of acidification is difficult because many biogeochemical processes affect
42 estuarine carbon chemistry (Sections, 3.3.2). Warming can exacerbate the impacts of both
43 acidification and hypoxia on estuarine organisms (Baumann and Smith, 2018; Collins et al., 2019b; Ni et al.,
44 2020). These effects are further complicated by eutrophication, with high nitrogen loads associated with
45 lower pH (Rheuban et al., 2019). Warming (including MHWs) and eutrophication interact to decrease
46 estuarine oxygen content and pH, increasing the vulnerability of animals to MHWs (Brauko et al., 2020), and
47 exacerbating the incidence and impact of dead zones (medium confidence) (Altieri and Gedan, 2015). The
48 impacts of storms on estuaries are variable and are described in SM3.3.1.


50 All these impacts are projected to escalate under future climate change, but their magnitude depends on the
51 amount of warming, the socio-economic development pathway, and implementation of adaptation strategies
52 (medium confidence). Modelling studies (Lopes et al., 2019; Rodrigues et al., 2019; White et al., 2019;
53 Zhang and Li, 2019; Hong et al., 2020; Krvavica and Ruzi, 2020; Liu et al., 2020e; Shalby et al., 2020)
54 suggest that responses of estuaries to SLR will be complex and context dependent (Khojasteh et al., 2021),
55 but project that salinity, tidal range, storm-surge amplitude, depth and stratification will increase with SLR
56 (medium confidence), and that marine-dominated waters will penetrate farther upstream (high confidence).
57 Without careful management of freshwater inputs, sediment augmentation and/or the restoration of

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1 shorelines to more natural states, transformation and loss of intertidal areas and wetland vegetation will
2 increase with SLR (high confidence) (Doughty et al., 2019; Leuven et al., 2019; Yu et al., 2019; Raw et al.,
3 2020; Shih, 2020; Stein et al., 2020), with small, shallow microtidal estuaries being more vulnerable to
4 impacts than deeper estuaries with well-developed sediments (medium confidence) (Leuven et al., 2019;
5 Williamson and Guinder, 2021). Warming and MHWs will enhance stratification and deoxygenation in
6 shallow lagoons (medium confidence) (Derolez et al., 2020) and will continue to drive range shifts among
7 estuarine biota (medium confidence) (Veldkornet and Rajkaran, 2019; Zhang et al., 2020c), resulting in
8 extirpations where thermal habitat is lost and potentially generating new habitat for warm-affinity species
9 (limited evidence, medium agreement) (Veldkornet and Rajkaran, 2019).


11 Vegetated Blue Carbon Ecosystems


13 Mangroves, saltmarshes and seagrass beds (wetland ecosystems) are considered "blue carbon" ecosystems
14 due to their capacity to accumulate and store organic-carbon rich sediments (Box 3.4, Macreadie et al., 2019;
15 Rogers et al., 2019) and provide an extensive range of other ecosystem services (Box 3.4). Because these
16 ecosystems are often found within estuaries and along sheltered coastlines, they share vulnerabilities,
17 climate-impact (Table 3.7) and non-climate drivers with estuaries and coastal lagoons (Section



20 Table 3.7: Summary of previous IPCC assessments of mangroves, saltmarshes and seagrass beds.

    Observations                                             Projections

    AR5: (Wong et al., 2014)

    Seagrasses occurring close to their upper thermal limits are Climate change will drive ongoing declines in the extent of

    already stressed by climate change (high confidence). seagrasses in temperate waters (medium confidence) as

                                                             well as poleward range expansions of seagrasses and

    Increased CO2 concentrations have increased seagrass mangroves, especially in the Northern Hemisphere (high

    photosynthetic rates by 20% (limited evidence, high      confidence).


                                                             Beneficial effects of elevated CO2 will increase seagrass

                                                             productivity and carbon burial rates in salt marshes during

                                                             the first half of the 21st century, but there is limited

                                                             evidence that this will improve their survival or resistance

                                                             to warming.

                                                             As a result, interactions between climate change and non-
                                                             climate drivers will continue to cause declines in estuarine
                                                             vegetated systems (very high confidence).

    SR15 (Hoegh-Guldberg et al., 2018a)                      Intact wetland ecosystems can reduce the adverse impacts
                                                             of rising sea levels and intensifying storms by protecting
    Vegetated blue carbon systems were not assessed in this  shorelines (medium confidence) and their degradation
    report.                                                  could reduce remaining carbon budgets by up to 100

                                                             Under 1.5°C of warming, natural sedimentation rates are
                                                             projected to outpace SLR (medium confidence), but other
                                                             feedbacks, such as landward migration of wetlands and the
                                                             adaptation of infrastructure, remain important (medium

    SROCC (Bindoff et al., 2019; Oppenheimer et al., 2019)

    Coastal ecosystems, including saltmarshes, mangroves, Seagrass meadows (high confidence) will face moderate to

    vegetated dunes and sandy beaches, can build vertically high risk at temperature above 1.5°C global sea surface

    and expand laterally in response to SLR, though this     warming.

    capacity varies across sites (high confidence). These

    ecosystems provide important services that include coastal The transition from undetectable to moderate risk in salt

    protection and habitat for diverse biota. However, because marshes takes place between 0.7°C­1.2°C of global sea

    of human actions that fragment wetland habitats and      surface warming (medium to high confidence), and

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   restrict landward migration, coastal ecosystems           between 0.9°C­1.8°C (medium confidence) in sandy

   progressively lose their ability to adapt to climate-induced beaches, estuaries and mangrove forests.

   changes and provide ecosystem services, including acting

   as protective barriers (high confidence).                 The ecosystems at moderate to high risk under future

                                                             emission scenarios are mangrove forests (transition from

   Warming and SLR-driven salinisation of wetlands are moderate to high risk at 2.5°C­2.7°C of global sea surface

   causing shifts in the distribution of plant species inland and warming), estuaries and sandy beaches (2.3°C­3.0°C) and

   poleward. Examples include mangrove encroachment into salt marshes (transition from moderate to high risk at

   subtropical salt marshes (high confidence) and contraction 1.8°C­2.7°C and from high to very high risk at 3.0°C­

   in extent of low-latitude seagrass meadows (high          3.4°C) (medium confidence).


                                                             Global coastal wetlands will lose between 20­90% of their

   Plants with low tolerance to flooding and extreme         area depending on emissions scenario with impacts on

   temperatures are particularly vulnerable, increasing the their contributions to carbon sequestration and coastal

   risk of extirpation (medium confidence).                  protection (high confidence).

   Extreme weather events, including heatwaves, droughts     Estuarine wetlands will remain resilient to modest rates of
   and storms, are causing mass mortalities and changes in   SLR where their sediment dynamics are unconstrained.
   community composition in coastal wetlands (high           But SLR and warming are projected to drive global loss of
   confidence).                                              up to 90% of vegetated wetlands by the end of the century
                                                             under the RCP8.5 (medium confidence), especially if
   Severe disturbance of wetlands or transitions among       landward migration and sediment supply are limited by
   wetland community types can favour invasive species       human modification of shorelines and river flows (medium
   (medium confidence).                                      confidence).

   The degradation or loss of vegetated coastal ecosystems   Moreover, pervasive coastal squeeze and human-driven
   reduces carbon storage, with positive feedbacks to the    habitat deterioration will reduce the natural capacity of
   climate system (high confidence).                         these ecosystems to adapt to climate impacts (high



 3 Since AR5 and SROCC, syntheses have emphasised that the vulnerability of rooted wetland ecosystems to
 4 climate-impact drivers is exacerbated by non-climate drivers (high confidence) (Elliott et al., 2019;
 5 Ostrowski et al., 2021; Williamson and Guinder, 2021) and climate variability (high confidence) (Day and
 6 Rybczyk, 2019; Kendrick et al., 2019; Shields et al., 2019). Global rates of mangrove loss have been
 7 extensive but are slowing (high confidence) at least partially due to management interventions (Friess et al.,
 8 2020b; Goldberg et al., 2020). From 2000 to 2010 mangrove loss averaged 0.16% yr­1, globally, but with
 9 greatest loss in Southeast Asia (high confidence) (Hamilton and Casey, 2016; Friess et al., 2019; Goldberg et
10 al., 2020), and ubiquitous fragmentation leaving few mangroves intact (Bryan-Brown et al., 2020). Saltmarsh
11 ecosystems have also suffered extensive losses (up to 60% in places, since the 1980s), especially in
12 developed and rapidly developing countries (medium confidence) (Table 12.3, Gu et al., 2018; Stein et al.,
13 2020). Similarly, 29% of seagrass meadows were lost from 1879­2006 due primarily to coastal development
14 and degradation of water quality, with climate-change impacts escalating since 1990 (medium confidence)
15 (Waycott et al., 2009; Sousa et al., 2019; Derolez et al., 2020; Green et al., 2021a). Local examples of habitat
16 stability or growth (e.g., de los Santos et al., 2019; Laengner et al., 2019; Sousa et al., 2019; Suyadi et al.,
17 2019; Derolez et al., 2020; Goldberg et al., 2020; McKenzie and Yoshida, 2020) indicate some resilience to
18 climate change in the absence of non-climate drivers (high confidence). Nevertheless, previous declines have
19 left wetland ecosystems more vulnerable to impacts from climate-impact and non-climate drivers (high
20 confidence) (Friess et al., 2019; Williamson and Guinder, 2021).


22 Warming and MHWs have affected the range, species composition and survival of some wetland
23 ecosystems. Warming is allowing some, but not all (Rogers and Krauss, 2018; Saintilan et al., 2018),
24 mangrove species to expand their ranges poleward (high confidence) (Friess et al., 2019; Whitt et al., 2020).
25 This expansion can affect species interactions (Guo et al., 2017; Friess et al., 2019), and enhance sediment
26 accretion and carbon storage rates in some instances (medium confidence) (Guo et al., 2017; Kelleway et al.,
27 2017; Chen et al., 2018b; Coldren et al., 2019; Raw et al., 2019). Drought, low sea levels and MHWs can
28 cause significant die-offs among mangroves (medium confidence) (Lovelock et al., 2017b; Duke et al.,
29 2021). Seagrasses are similarly vulnerable to warming (high confidence) (Repolho et al., 2017; Duarte et al.,
30 2018; Jayathilake and Costello, 2018; Savva et al., 2018), which has been attributed as one cause of observed

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1 changes in distribution and community structure (medium confidence) (Hyndes et al., 2016; Nowicki et al.,
2 2017). MHWs, together with storm-driven turbidity and structural damage, can cause seagrass die-offs (high
3 confidence) (Arias-Ortiz et al., 2018; Kendrick et al., 2019; Smale et al., 2019; Strydom et al., 2020), shifts

4 to small, fast-growing species (high confidence) (Kendrick et al., 2019; Shields et al., 2019; Strydom et al.,

5 2020), and ecosystem collapse (Serrano et al., 2021).


7 The sensitivity of saltmarshes and mangroves to RSLR depends on whether they accrete inorganic sediment

8 and/or organic material at rates equivalent to rising water levels (very high confidence) (Peteet et al., 2018;

 9 FitzGerald and Hughes, 2019; Friess et al., 2019; Gonneea et al., 2019; Leo et al., 2019; Marx et al., 2020;
10 Saintilan et al., 2020). Otherwise, wetland ecosystems must migrate either inland or upstream, or face
11 gradual submergence in deeper, increasingly saline water (very high confidence) (Section, Andres et
12 al., 2019; Jones et al., 2019b; Cohen et al., 2020; Mafi-Gholami et al., 2020; Magolan and Halls, 2020; Sklar
13 et al., 2021). Ability to migrate depends on local topography, the positioning of anthropogenic infrastructure,
14 and structures placed to defend such infrastructure (Schuerch et al., 2018; Fagherazzi et al., 2020; Cahoon et
15 al., 2021). Submergence drives changes in community structure (high confidence) (Jones et al., 2019b; Yu et
16 al., 2019; Douglass et al., 2020; Langston et al., 2020) and functioning (high confidence) (Charles et al.,
17 2019; Buffington et al., 2020; Stein et al., 2020), and will eventually lead to extirpation of the most sensitive

18 vegetation (medium confidence) (Schepers et al., 2017; Scalpone et al., 2020) and associated animals (low
19 confidence) (Rosencranz et al., 2018). The impacts of storms on wetlands are variable and are described in
20 SM3.3.1.


22 As noted in SROCC, given the diversity of coastal wetlands as well as the dependence of their future
23 vulnerability to climate change on adaptation pathways (Krauss, 2021; Rogers, 2021), projections of future
24 impacts based on shoreline elevation estimated from satellite data and CMIP5 projections (Spencer et al.,

25 2016; Schuerch et al., 2018) vary greatly. Although all approaches have individual strengths and weaknesses

26 (Törnqvist et al., 2021), paleo-records provide some clarity because they yield estimates of wetland
27 responses to changes in climate in the absence of other anthropogenic drivers and are therefore inherently
28 conservative. On the basis of paleorecords (Table 3.8), we assess that mangroves and saltmarshes are likely
29 at high risk from future SLR, even under SSP1-1.9, with impacts manifesting in the mid-term (medium
30 confidence). Under SSP5-8.5, wetlands are very likely at high risk from SLR, with larger impacts
31 manifesting before 2040 (medium confidence). By 2100, these ecosystems are at high risk of impacts under
32 all scenarios except SSP1-1.9 (high confidence), with impacts most severe along coastlines with gently-
33 sloping shorelines, limited sediment inputs, small tidal ranges and limited space for inland migration (very

34 high confidence) (Cross-Chapter Box SLR in Chapter 3, Schuerch et al., 2018; FitzGerald and Hughes, 2019;
35 Leo et al., 2019; Schuerch et al., 2019; Raw et al., 2020; Saintilan et al., 2020).



38 Table 3.8: Estimates of vulnerability of coastal wetlands to sea-level rise (SLR) on the basis of sediment cores.

    Region  Habitat Reference         Rates of SLR at which WGI AR6 Table 9.9 estimate of SLR (Fox-

                                      habitat loss is          Kemper et al., 2021)

                                      Likely Very likely       2040­2060             2090­2100

    Global  Mangrove (Saintilan et al., 4.2* 6.1               SSP1-1.9:
                           2020)                               4.2 (2.9­6.1) mm yr­1 4.3 (2.5­6.6) mm yr­1

    Southeastern Saltmarsh (Törnqvist et al., 3.5# 4.2#        SSP5-8.5:
                                                               7.3 (5.7­9.8) mm yr­1 12.2 (8.8­17.7) mm
    USA          2020)

    UK      Saltmarsh (Horton et al., 4.6* 7.1*


    * Estimate digitised from published figure
    # Published figure digitised and remodelled as binomial generalised linear model (number drowned vs. not).



41 For seagrasses, recent projections for climate-change impacts vary by species and region. Warming is
42 projected to increase the habitat available to Zostera marina on the east coast of the USA by 2100, but
43 contract its southern range edge by 150­650 km under RCP2.6 and RCP8.5, respectively (Wilson and Lotze,

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1 2019). Other species, such as Posidonia oceanica in the Mediterranean, might lose as much as 75% of their
2 habitat by 2050 under RCP8.5 and become functionally extinct (low confidence) by 2100 (Chefaoui et al.,
3 2018). Observed impacts of MHWs (Kendrick et al., 2019; Strydom et al., 2020; Serrano et al., 2021)

4 indicate that increasing intensity and frequency of MHWs (Section will have escalating impacts on

5 seagrass ecosystems (high confidence). Habitat suitability can also be reduced by moderate RSLR, due to its
6 impact on light attenuation (medium confidence) (Aoki et al., 2020; Ondiviela et al., 2020; Scalpone et al.,
7 2020).


 9 Overall, warming will drive range shifts in wetland species (medium to high confidence), but SLR poses
10 greatest risk for mangroves and saltmarshes, with significant losses projected under all future scenarios by
11 mid-century (medium confidence) and substantially greater losses by 2100 under all scenarios except SSP1-
12 1.9 (high confidence). MHWs pose the greatest risk to seagrasses (high confidence). In all cases, losses will
13 be greatest where accommodation space is constrained or where other non-climate drivers exacerbate risk
14 from climate-impact drivers (very high confidence).


16 Sandy Beaches


18 Sandy beaches comprise unvegetated, fine- to medium-grained sediments in the intertidal zones that line
19 roughly one-third of the length of the world's ice-free coastlines (Luijendijk et al., 2018). The amenity value
20 of beaches as cultural, recreational and residential destinations has driven extensive urbanisation of beach-

21 associated coastlines (Todd et al., 2019). Beaches also provide habitat for many resident species, nesting

22 habitat for marine vertebrates, filtration of coastal waters and protection of the coastline from erosion
23 (McLachlan and Defeo, 2018). These soft-sediment coastal ecosystems are particularly vulnerable to habitat
24 loss caused by erosion, especially where landward transgression is inhibited by infrastructure (Table 3.9).



27 Table 3.9: Summary of previous IPCC assessments of sandy beaches.

    Observations                                                Projections

    AR5: (Wong et al., 2014)                                    In the absence of adaptation, beaches, sand dunes, and
    Globally, beaches and dunes have in general undergone       cliffs currently eroding will continue to do so under
    net erosion over the past century or longer.                increasing sea level (high confidence).

    Attributing shoreline changes to climate change is still    Coastal squeeze is expected to accelerate with sea-level
    difficult owing to the multiple natural and anthropogenic   rise (SLR). In many locations, finding sufficient sand to
    drivers contributing to coastal erosion.                    rebuild beaches and dunes artificially will become
                                                                increasingly difficult and expensive as present supplies
                                                                near project sites are depleted (high confidence).

                                                                In the absence of adaptation measures, beaches and sand
                                                                dunes currently affected by erosion will continue to be
                                                                affected under SLR (high confidence).

    SROCC (Bindoff et al., 2019)                                Sandy beach ecosystems will increasingly be at risk of
    Coastal ecosystems are already impacted by the              eroding, reducing the habitable area for dependent
    combination of SLR, other climate-related ocean             organisms (high confidence).
    changes, and adverse effects from human activities on
    ocean and land (high confidence). Attributing such          Sandy shorelines are expected to continue to reduce their
    impacts to SLR, however, remains challenging due to the     area and change their topography due to SLR and
    influence of other climate-related and non-climate          increased extreme climatic erosive events. This will be
    drivers such as infrastructure development and human-       especially important in low-lying coastal areas with high
    induced habitat degradation (high confidence). Coastal      population and building densities (medium confidence).
    ecosystems, including saltmarshes, mangroves, vegetated
    dunes and sandy beaches, can build vertically and           Assuming that the physiological underpinning of the
    expand laterally in response to SLR, though this capacity   relationship between body size and temperature can be
    varies across sites (high confidence) as a consequence of   applied to warming (medium confidence), the body size
    human actions that fragment wetland habitats and restrict   of sandy beach crustaceans is expected to decrease under
    landward migration, coastal ecosystems progressively        warming (low evidence, medium agreement).
    lose their ability to adapt to climate-induced changes and

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provide ecosystem services, including acting as              Sandy beaches transition from undetectable to moderate
protective barriers (high confidence).                       risk between 0.9ºC­1.8ºC (medium confidence) of global
                                                             sea surface warming and from moderate to high risk at
Loss of breeding substrate, including mostly coastal         2.3ºC­3.0ºC of global sea surface warming (medium
habitats such as sandy beaches, can reduce the available     confidence).
nesting or pupping habitat for land breeding marine
turtles, lizards, seabirds and pinnipeds (high confidence).  Projected changes in mean and extreme sea levels and
                                                             warming under RCP8.5 are expected to result in high
Overall, changes in sandy beach morphology have been         risk of impacts on sandy beach ecosystems by the end of
observed from climate related events, such as storm          the 21st century (medium confidence), taking account of
surges, intensified offshore winds, and from coastal         the slow recovery rate of sandy beach vegetation, the
degradation caused by humans (high confidence), with         direct loss of habitats and the high climatic sensitivity of
impacts on beach habitats (e.g., benthic megafauna)          some fauna.
(medium confidence).
                                                             Under RCP2.6, the risk of impacts on sandy beaches is
                                                             expected to be only slightly higher than the present-day
                                                             level (low confidence). Coastal urbanisation lowers the
                                                             buffering capacity and recovery potential of sandy beach
                                                             ecosystems to impacts from SLR and warming and thus
                                                             is expected to limit their resilience to climate change
                                                             (high confidence).

                                                                                 Coastal squeeze and human-driven habitat deterioration
                                                                                 will reduce the natural capacity of these ecosystems to
                                                                                 adapt to climate impacts (high confidence).



 3 Since SROCC, observed trends in coastal erosion continue to be obscured by beach nourishment that
 4 replaces eroded sediment or by coastal protection of areas at risk of erosion (Section, Cross-Chapter
 5 Box SLR in Chapter 3). Nevertheless, RSLR, increases in wave energy and/or changes in wave direction,
 6 disruptions to sediment supplies (including sand mining), and other anthropogenic modifications of the coast
 7 have driven localised beach erosion (very high confidence) at rates up to 0.5­3 m yr­1 (Vitousek et al., 2017a;
 8 Vitousek et al., 2017b; Cambers and Wynne, 2019; Enríquez-de-Salamanca, 2020; Sharples et al., 2020).
 9 Corresponding analyses of coarse-scale (30-m resolution) global data estimate that 15% of tidal flats
10 (including beaches) have been lost since 1984 (medium confidence) (Mentaschi et al., 2018; Murray et al.,
11 2019) but with a corresponding number of the world's beaches accreting (28%) as eroding (24%) (medium
12 confidence) (Luijendijk et al., 2018).


14 Progress is being made toward models that can project beach erosion under future scenarios despite inherent
15 uncertainties and the presence of multiple confounding drivers in the coastal zone (Vitousek et al., 2017b; Le
16 Cozannet et al., 2019; Cooper et al., 2020a; Vousdoukas et al., 2020b; Vousdoukas et al., 2020a). In the
17 interim, models with varying levels of complexity estimate local loss of beach area to SLR by 2100 under
18 RCP8.5-like scenarios, assuming minimal human intervention, ranging 30­70% (low confidence) (Vitousek
19 et al., 2017b; Mori et al., 2018; Ritphring et al., 2018; Hallin et al., 2019; Kasmi et al., 2020). Within
20 regions, projected impacts scale negatively with beach width and positively with the magnitude of projected
21 SLR. None of these local studies, however, considered high-energy storm events, which are known to also
22 impact sandy coasts (high confidence) (e.g., Burvingt et al., 2018; Garrote et al., 2018; Duvat et al., 2019;
23 Sharples et al., 2020), and model structure often had more influence on projected shoreline responses than
24 did physical drivers (Le Cozannet et al., 2019). Nevertheless, the most-advanced available models, which
25 incorporate multiple coastal processes, including SLR, project that without anthropogenic barriers to erosion,
26 13.6­15.2% and 35.7­49.5% of the world's beaches likely risk undergoing at least 100 m of shoreline retreat
27 (relative to 2010) by 2050 and 2100, respectively (low confidence) (Vousdoukas et al., 2020b). Aggregating
28 these trends regionally suggests that relative rates of shoreline change under RCP4.5 and RCP8.5 diverge
29 strongly after mid-century, with trends towards erosion escalating under RCP8.5 by 2100 (medium
30 confidence) (Figure 3.14, Vousdoukas et al., 2020b). This trend supports the WGI AR6 assessment that
31 projected SLR will contribute to erosion of sandy beaches, especially under high-emissions futures (high
32 confidence) (WGI AR6 Technical Summary, Arias et al., 2021).



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 2 Figure 3.14: Relative trends in projected regional shoreline change (advance/retreat relative to 2010). Frequency
 3 distributions of median projected change by (a,c) 2050 and (b,d) 2100 under (a,b) RCP4.5 and (c,d) RCP8.5.
 4 Projections account for both long-term shoreline dynamics and sea-level rise and assume no impediment to inland
 5 transgression of sandy beaches. Data for small island states are aggregated and plotted in the Caribbean. Data from
 6 Vousdoukas et al. (2020b). Values for reference regions established in the WGI AR6 Atlas (Gutiérrez et al., 2021) were
 7 computed as area-weighted means from original country-level data. For model assumptions and associated debate, see
 8 Vousdoukas et al. (2020a) and Cooper et al. (2020a).



11 For beach fauna, emerging evidence links range shifts, increasing representation by warm-affinity species
12 and mass mortalities to ocean warming (limited evidence, high agreement) (McLachlan and Defeo, 2018;
13 Martin et al., 2019). But even amongst the best-studied taxa, such as turtles, vulnerability to warming (high
14 confidence) and SLR (medium confidence) anticipated on the basis of theory (Poloczanska et al., 2009; Saba
15 et al., 2012; Pike, 2013; Laloë et al., 2017; Tilley et al., 2019) yields only a few detected impacts in the field
16 associated mainly with feminisation (female-skewed sex ratios driven by warmer nest temperatures) (Jensen
17 et al., 2018; Colman et al., 2019; Tilley et al., 2019), phenology (Monsinjon et al., 2019), reproductive
18 success (Bladow and Milton, 2019) and inter-nesting period (Valverde-Cantillo et al., 2019). Moreover,
19 although established vulnerabilities imply high projected future risk for turtles (high confidence) (e.g.,
20 Almpanidou et al., 2019; Monsinjon et al., 2019; Patrício et al., 2019; Varela et al., 2019; Santidrián Tomillo
21 et al., 2020), many populations remain resilient to change (Fuentes et al., 2019; Valverde-Cantillo et al.,
22 2019; Laloë et al., 2020; Lamont et al., 2020), perhaps because variation in sand temperatures at nesting
23 depth among beaches very likely exceeds the magnitude of warming anticipated by 2100, even under RCP8.5
24 (medium confidence) (Bentley et al., 2020a). As expected for a taxon with a long evolutionary history, turtles
25 display natural adaptation, not only by virtue of broad geographic distributions that include natural climate-
26 change refugia (Boissin et al., 2019; Jensen et al., 2019), but also because some initial responses to warming
27 might counteract anticipated impacts. For example, although feminisation poses a significant long-term risk
28 to turtle populations (high confidence), it might contribute to population growth in the near- to mid-term
29 (medium confidence) (Patrício et al., 2019). Resilience to climate change might be further enhanced by range
30 extensions, alterations in nesting phenology, and fine-scale nest-site selection (medium confidence) (Abella
31 Perez et al., 2016; Santos et al., 2017; Almpanidou et al., 2018; Rivas et al., 2019; Laloë et al., 2020).


33 New literature since SROCC on climate impacts and risks has been scarce for most beach taxa besides turtles
34 (the impacts of storms on beach fauna are variable and are described in SM3.3.1). Nevertheless, theoretical
35 sensitivity to warming (Section 3.3.2), together with the projected loss of habitat under future climate
36 scenarios, suggest substantial impacts for populations and communities of beach fauna into the future (high
37 confidence). These impacts will be exacerbated by coastal squeeze along urbanised coastlines (high

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1 confidence), albeit with magnitudes that cannot yet be accurately projected (McLachlan and Defeo, 2018; Le
2 Cozannet et al., 2019; Leo et al., 2019).


4 Semi-Enclosed Seas


 6 This section assesses impacts on five SES, or seas larger than 200,000 km2 with single entrances <120 km
 7 wide, including the Persian Gulf, the Red Sea, the Black Sea, the Baltic Sea and the Mediterranean Sea.
 8 These SES are largely landlocked and are thus heavily influenced by surrounding landscapes, local and
 9 global climate-impact drivers, as well as non-climate drivers (Section 3.1), making them highly vulnerable to
10 cumulative threats. Key climate-impact drivers in SES are warming, increasing frequency and duration of
11 MHWs, acidification, and the increasing in size and number of OMZs (Figure 3.12, Hoegh-Guldberg et al.,
12 2014). In AR5, SES were recognised as regionally significant for fisheries and tourism, but highly exposed
13 to both local and global stressors, offering limited options for organisms to migrate in response to climate
14 change (Table 3.10).



17 Table 3.10: Summary of past IPCC assessments of semi-enclosed seas (SES).

    Observations                                             Projections

    AR5 (Hoegh-Guldberg et al., 2014)

    The surface waters of the SES exhibit significant warming Projected warming increases the risk of greater thermal

    from 1982 and most Coastal Boundary Systems show stratification in some regions, which can lead to reduced

    significant warming since 1950. Warming of the           O2 ventilation of underlying waters and the formation of

    Mediterranean has led to the recent spread of tropical   additional hypoxic zones, especially in the Baltic and

    species invading from the Atlantic and Indian Oceans. Black Seas (medium confidence).

    SES are highly vulnerable to changes in global           Changing rainfall intensity can exert a strong influence on

    temperature on account of their small seawater volume and the physical and chemical conditions within SES, and in

    landlocked nature. Consequently, SES will respond faster some cases will combine with other climatic changes to

    than most other parts of the Ocean (high confidence).    transform these areas. These changes are likely to increase

                                                             the risk of reduced bottom-water O2 levels for Baltic and

    The impact of rising temperatures on SES is exacerbated Black Sea ecosystems (due to reduced solubility, increased

    by their vulnerability to other human influences such as stratification, and microbial respiration), which is very

    over-exploitation, pollution, and enhanced runoff from likely to affect fisheries.

    modified coastlines. Due to a combination of global and

    local human stressors, key fisheries have undergone      Persian Gulf, Red Sea: Extreme temperature events such as

    fundamental changes in their abundance and distribution heatwaves are projected to increase (high confidence) and

    over the past 50 years (medium confidence).              temperatures are very likely to increase above established

                                                             thresholds for mass coral bleaching and mortality (very

                                                             high confidence).

        SROCC (Bindoff et al., 2019)
        Semi-enclosed seas were not assessed in this report.

                                                                                Projections from distribution models for multiple fish
                                                                                species show hotspots of decreased species richness in the
                                                                                Indo-Pacific region, and semi-enclosed seas such as the
                                                                                Red Sea and Persian Gulf (medium evidence, high
                                                                                In addition, geographic barriers such as land boundaries or
                                                                                lower oxygen water in deeper waters are projected to limit
                                                                                species range shifts in semi-enclosed seas, resulting in
                                                                                larger relative decrease in species richness (medium



20 Since AR5, there is evidence for increasing frequency and duration of MHWs, extreme weather events and a
21 diversity of threats across depth strata causing mass mortality events, local extirpations and coral reef decline
22 (high confidence) (Section, SM3.3.2, Buchanan et al., 2016a; Shlesinger et al., 2018; Wabnitz et al.,
23 2018b; Garrabou et al., 2019). In most SES, non-climate drivers, including pollution, habitat destruction, and
24 especially overfishing are decreasing the local adaptive capacity of organisms and the ability of ecosystems

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 1 to cope with climate change impacts (high confidence) (Cramer et al., 2018; Hidalgo et al., 2018; Ben-Hasan
 2 and Christensen, 2019). SLR is accelerating faster than expected (high confidence) (Kulp and Strauss, 2019),
 3 posing a key risk to SES' coastal ecosystems and the services they provide in urban areas, including drinking
 4 water provision, housing, recreational activities, among others (Hérivaux et al., 2018; Reimann et al., 2018).


 6 The size and number of OMZs are increasing worldwide and in most SES (high confidence) (Global Ocean
 7 Oxygen Network, 2018), with growing impacts on fish species diversity and ecosystem functioning. In the
 8 Persian Gulf and Red Sea, increasing nutrient loads associated with coastal activities and warming has
 9 increased the size of OMZs (high confidence) (Al-Said et al., 2018; Lachkar et al., 2019). OMZs represent an
10 even greater problem in the Black and Baltic Seas, with broad implications for ecosystem function and
11 services (Levin et al., 2009), especially where actions to reduce nutrient loading from land have been unable
12 to reduce the OMZ coverage (high confidence) (Carstensen et al., 2014; Miladinova et al., 2017; Global
13 Ocean Oxygen Network, 2018). In the Baltic Sea, OMZs are affecting the extent of suitable spawning areas
14 of cod, Gadus morhua (high confidence) (Hinrichsen et al., 2016), while in the Black Sea, the combined
15 effect of OMZs and warming is influencing the distribution and physiology of fish species, and their
16 migration and schooling behavior in their overwintering grounds (medium confidence) (Güraslan et al.,
17 2017). Cascading effects on food webs have been reported in the Baltic, where detrimental effects of
18 changing oxygen levels on zooplankton production, pelagic and piscivorous fish are influencing seasonal
19 succession and species composition of phytoplankton (high confidence) (Viitasalo et al., 2015).


21 In the Mediterranean Sea (Cross-Chapter Paper 4), the increase in climate extremes and mass mortality
22 events reported in AR5 has continued (very high confidence) (Gómez-Gras et al., 2021). Extreme weather
23 events (including deep convection, González-Alemán et al., 2019) and MHWs have become more frequent
24 (Darmaraki et al., 2019) and are associated with mass mortality of benthic sessile species across the basin
25 (high confidence) (Garrabou et al., 2019; Gómez-Gras et al., 2021). Since AR5, in the Persian Gulf and Red
26 Sea, extreme temperatures, together with disease and predation, have continued to cause bleaching-induced
27 mortality of corals, along with declines in the average coral colony size (high confidence) (Burt et al., 2019).
28 Poleward migration and tropicalisation of species (Section has also continued in the Mediterranean,
29 and these phenomena have also become an issue in the Black Sea (high confidence) (Boltachev and Karpova,
30 2014; Hidalgo et al., 2018). Climate impacts on phytoplankton production and phenology show high spatial
31 heterogeneity across the Mediterranean Sea (medium evidence) (Marbà et al., 2015b; Salgado-Hernanz et al.,
32 2019), with consequent effects on the diversity and abundance of zooplankton and fish species (medium
33 confidence) (Peristeraki et al., 2019). Changes in primary production and a decrease in river runoff have also
34 altered the optimum habitats for small pelagic fish in the Mediterranean, from local to the basin scale
35 (Piroddi et al., 2017). Evidence of impacts from ocean acidification is increasing, with the rates of coral
36 calcification showing major decline in the Red Sea (medium confidence) (Section, Steiner et al.,
37 2018; Bindoff et al., 2019). In the Mediterranean Sea, evidence of acidification events have been reported at
38 local scale (Hassoun et al., 2015), with impacts on bivalves and coralligenous species (medium confidence)
39 (Lacoue-Labarthe et al., 2016).


41 Climate models project increasing frequency and intensity of MHWs (high confidence) (Section,
42 which will exacerbate warming-driven impacts in the Red Sea and Persian Gulf regions, and erode the
43 resilience of Red Sea coral reefs (high confidence) (Osman et al., 2018; Genevier et al., 2019; Kleinhaus et
44 al., 2020). In the Persian Gulf region, extreme temperatures, >35ºC (Pal and Eltahir, 2016), have been linked
45 with high rates of extirpation and a decrease in fisheries catch potential (medium confidence) (Wabnitz et al.,
46 2018b). In the Mediterranean Sea, east-west gradients in rates of warming are projected to trigger spatially
47 different changes in primary production, which combined with the increasing arrival of non-indigenous
48 species, may trigger biogeographical changes in fish diversity, increasing in the eastern and decreasing in the
49 western Mediterranean (medium to high confidence) (Albouy et al., 2013; Macias et al., 2015). Projections
50 also show greater impacts from SLR than originally expected in the Mediterranean and Baltic (e.g., Dieterich
51 et al., 2019; Thiéblemont et al., 2019). In the Baltic Sea, under high nutrient load and warming climate
52 scenarios, eutrophication is projected to increase in the future (2069­2098) compared to historical (1976­
53 2005) periods. In contrast, under continued nutrient load reductions following present management
54 regulations, environmental conditions and ecological state will continue to improve independently of the
55 climate warming scenarios (low to medium confidence) (Saraiva et al., 2019).


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1 Shelf Seas


 3 Shelf seas overlie the continental margin, often with maximum depths of <200 m, and represent 7% of the
 4 global ocean by area (Simpson and Sharples, 2012). These ecosystems are found offshore of every continent,
 5 generate 10­30% (Mackenzie et al., 2000; Andersson and Mackenzie, 2004) of global marine net primary
 6 production and play a key role in global biogeochemical cycling, including the export of land-borne carbon
 7 and nutrients (Johnson et al., 1999; Nishioka et al., 2011; Li et al., 2019) to the deep ocean and recycling of
 8 fixed nitrogen back to the atmosphere via denitrification (Devol, 2015). The shelf seas are home to several of
 9 the world's major industrial capture fisheries, such as those of the mid-Atlantic Bight, Scotian Shelf, Eastern
10 Bering Sea Shelf and North Brazil Shelf (Barange et al., 2018), and support other marine industries,
11 including aquaculture, extractive industries (oil, gas, and mining), shipping, and renewable-energy
12 installations.


14 Similar to other coastal ecosystems, evidence since SROCC (Table 3.11) suggests shelf-sea ecosystems and
15 the fisheries and aquaculture they support are sensitive to the interactive effects of climate-impact drivers, as
16 well as non-climate drivers, including nutrient pollution, sedimentation, fishing pressure and resource
17 extraction (Table 3.12, Figure 3.12). Changes in freshwater, nutrient and sediment inputs from rivers due to
18 both climate and non-climate drivers can influence productivity and nutrient limitation, ecosystem structure,
19 carbon export and species diversity and abundance (Balch et al., 2012; Picado et al., 2014), and can result in
20 reduced water clarity and light penetration (Dupont and Aksnes, 2013; McGovern et al., 2019). Seasonal
21 bottom-water hypoxia occurs in some shelf seas (e.g., northern Gulf of Mexico, Bohai Sea, East China Sea)
22 due to riverine inputs of freshwater and nutrients, promoting stratification, enhanced primary production, and
23 organic carbon export to bottom waters (high confidence) (Zhao et al., 2017; Wei et al., 2019; Del Giudice et
24 al., 2020; Große et al., 2020; Jarvis et al., 2020; Rabalais and Baustian, 2020; Song et al., 2020a; Xiong et
25 al., 2020; Zhang et al., 2020a).



28 Table 3.11: Summary of past IPCC assessments of shelf seas.

    Observations                                                Projections

    AR5 (Hoegh-Guldberg et al., 2014)

    Primary productivity, biomass yields, and fish capture Global warming will result in more frequent extreme

    rates have undergone large changes within the East China events and greater associated risks to ocean ecosystems

    Sea over the past decades (limited evidence, medium         (high confidence). In some cases, projected increases will

    agreement, low confidence).                                 eliminate ecosystems, and increase the risks and

                                                                vulnerabilities to coastal livelihoods and vulnerabilities for

    Changing sea temperatures have influenced the abundance food security including Southeast Asia (medium to high

    of phytoplankton, benthic biomass, cephalopod fisheries, confidence). Reducing stressors not related to climate

    and the size of demersal trawl catches in the northern      change represents an opportunity to strengthen the

    South China Sea observed over the period 1976­2004 ecological resilience within these regions, which may help

    (limited evidence, medium agreement).                       biota survive some projected changes in ocean temperature

                                                                and chemistry.

    Concurrent with the retreat of the "cold pool" on the

    northern Bering Sea shelf, bottom trawl surveys of fish and Changes in eutrophication and hypoxia are likely to

    invertebrates show a significant community-wide             influence shelf seas, but there is low confidence in the

    northward distributional shift and a colonisation of the understanding of the magnitude of potential changes and

    former cold pool areas by sub-Arctic fauna (high            impacts on ecosystem functioning, fisheries and other

    confidence).                                                industries.

    Observed changes in the phenology of plankton groups in
    the North Sea over the past 50 years are driven by climate
    forcing, in particular regional warming (high confidence).

    SROCC (Bindoff et al., 2019)

    Species composition of fisheries catches since the 1970s in Direct anthropogenic impacts include coastal land-use
    many shelf seas ecosystems of the world is increasingly change; indirect effects include increased nutrient delivery
    dominated by warm water species (medium confidence). and other changes in river catchments, and marine resource

                                                                            exploitation in shelf seas. There is high confidence that
    Estuaries, shelf seas and a wide range of other intertidal these human-driven changes will continue, reflecting
    and shallow-water habitats play an important role in the coastal settlement trends and global population growth.

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   global carbon cycle through their primary production by
   rooted plants, seaweeds (macroalgae) and phytoplankton,
   and also by processing riverine organic carbon. However,
   the natural carbon dynamics of these systems have been
   greatly changed by human activities (high confidence).



3 Table 3.12: Synthesis of interactive effects and their influence on shelf-sea ecosystems and the fisheries and

4 aquaculture they support.

   Factor                            Example of effect                   Example references

   Temperature                       Altered habitats for species,       (Liang et al., 2018; Maharaj et al.,
                                     change in plankton, fish and        2018; Ma et al., 2019; Meyer and
                                     macrofauna community structure,     Kröncke, 2019; Yan et al., 2019;
                                     influence on species growth,        Bargahi et al., 2020; Bedford et al.,
                                     thermal stress,                     2020; Denechaud et al., 2020;
                                     altered diversity,                  Friedland et al., 2020b; Mérillet et al.,
                                     altered productivity,               2020; Nohe et al., 2020)
                                     altered phenology.

   pH                                Acidification with hypoxia.         (Zhang and Wang, 2019)

   Salinity                          Change in species distribution due to (Liu et al., 2020c)
                                     altered salinity front distribution.

   Oxygen concentration              Deoxygenation.                      (Wei et al., 2019; Del Giudice et al.,

   River discharge                   Change in plankton community        (Shi et al., 2020)

   Nutrient pollution                Enhanced primary production,        (Kong et al., 2019; Nohe et al., 2020)
                                     change in plankton community

   Sedimentation                     Modified ocean chemistry.           (Hallett et al., 2018)

   Fishing pressure                  Increased vulnerability leading to  (Maharaj et al., 2018; Wang et al.,
                                     changes in community structure.     2019c; Hernvann and Gascuel, 2020)

   Resource extraction               Contamination,                      (Hall, 2002)
                                     change in benthic community



 7 Key risks to shelf seas include shifts or declines in marine micro- and macro-organism abundance and
 8 diversity driven by eutrophication, HABs and extreme events (storms and MHWs), and consequent effects
 9 on fisheries, resource extraction, transportation, tourism and marine renewable energy (Figure 3.12). The
10 combined effects of deoxygenation and warming can affect the metabolism, growth, feeding behaviour and
11 mobility of fish species (Section 3.3.3). The increasing availability of observations mean that ecosystem
12 changes in shelf seas can be increasingly attributed to climate change (high confidence) (Liang et al., 2018;
13 Maharaj et al., 2018; Ma et al., 2019; Meyer and Kröncke, 2019; Bargahi et al., 2020; Bedford et al., 2020;
14 Friedland et al., 2020b; Mérillet et al., 2020). Eutrophication and seasonal bottom-water hypoxia in some
15 shelf seas have been linked to warming (high confidence) (Wei et al., 2019; Del Giudice et al., 2020) and
16 increased riverine nutrient loading (high confidence) (Wei et al., 2019; Del Giudice et al., 2020). Since
17 SROCC, some severe HABs have been attributed to extreme events, such as MHWs (Section 14.4.2, Roberts
18 et al., 2019; Trainer et al., 2019). However, a recent worldwide assessment of HABs attributed the increase
19 in observed HABs to intensified monitoring associated with increased aquaculture production (high
20 confidence) (Hallegraeff et al., 2021).


22 Since SROCC, changes in the community structure and diversity of plankton, macrofauna and infauna have
23 been detected in some shelf seas, although attribution has been regionally specific (e.g., bottom-water
24 warming or hypoxia) (Meyer and Kröncke, 2019; Rabalais and Baustian, 2020). Detection of the
25 picoplankton Synechococcus spp. in the North Sea is potentially linked to a summer decrease in copepod
26 stocks and declining food-web efficiency (low confidence) (Schmidt et al., 2020). The seasonally distinct
27 phytoplankton assemblages in the North Sea have begun to appear concurrently and homogenise (Nohe et

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 1 al., 2020). Changes in abundance, species composition, and size of zooplankton have been detected in some
 2 shelf seas (Yellow Sea, North Sea, Celtic Sea, and Tasman Sea), including a decline in stocks of larger
 3 copepods, increased abundances of gelatinous and meroplankton, and a shift to smaller species due to
 4 warming, increased river discharge, circulation change, and/or extreme events (high confidence) (Wang et
 5 al., 2018a; Bedford et al., 2020; Evans et al., 2020; Shi et al., 2020; Edwards et al., 2021).


 7 Ocean warming has shifted distributions of fish (Free et al., 2019; Franco et al., 2020; Pinsky et al., 2020b;
 8 Fredston et al., 2021) and marine mammal species (Salvadeo et al., 2010; García-Aguilar et al., 2018; Davis
 9 et al., 2020) poleward (high confidence) or deeper (low to medium confidence) (Section, Nye et al.,
10 2009; Pinsky et al., 2013; Pinsky et al., 2020b). Warming has also tropicalised the pelagic and demersal fish
11 assemblages of mid- and high-latitude shelves (high confidence) (Montero-Serra et al., 2015; Liang et al.,
12 2018; Maharaj et al., 2018; Ma et al., 2019; Friedland et al., 2020a; Kakehi et al., 2021; Punzón et al., 2021).
13 Fisheries catch composition in many shelf-sea ecosystems has become increasingly dominated by warm-
14 water species since the 1970s (high confidence) (Cheung et al., 2013; Leitão et al., 2018; Maharaj et al.,
15 2018; McLean et al., 2019). Warming has taxonomically diversified fish communities along a latitudinal
16 gradient in the North Sea, but homogenised functional diversity (McLean et al., 2019). However, in some
17 regions, changing predator or prey distributions, temperature-dependent hypoxia, population changes,
18 evolutionary adaptation, and other biotic or abiotic processes, including species' exploitation, confound
19 responses to climate-impact drivers, which must therefore be interpreted with caution (Frank et al., 2018).
20 For example, although, most species' range edges are tracking temperature change on the northeast shelf of
21 the USA (medium confidence) (Fredston-Hermann et al., 2020; Fredston et al., 2021), range edges of others
22 are not.


24 A wide range of responses by fish and invertebrate populations to warming have been observed. The
25 majority of responses have been detrimental, with the direction and magnitude of the response depending on
26 ecoregion, taxonomy, life history, and exploitation history (Free et al., 2019; Yati et al., 2020). For example,
27 fisheries productivity has strongly decreased in the North Sea (Free et al., 2019), and fisheries yields have
28 also decreased in the Celtic Sea, attributed primarily to warming and secondarily to long-term exploitation
29 (Hernvann and Gascuel, 2020; Mérillet et al., 2020). Conversely, fish species diversity and overall
30 productivity have increased in the Gulf of Maine, even with warming (Le Bris et al., 2018; Friedland et al.,
31 2020a; Friedland et al., 2020b). Fisheries yields have decreased in the Yellow Sea, East China Sea and South
32 China Sea due primarily to overexploitation (Ma et al., 2019; Wang et al., 2019c), with warming exerting
33 more influence on the yield of cold-water species than on temperate- and warm-water groups (Ma et al.,
34 2019). The combined effects of exploitation and multi-decadal climate fluctuations make it difficult to assess
35 global climate-change impacts on fisheries yields (Chapter 5, Ma et al., 2019; Bentley et al., 2020b; Johnson
36 et al., 2020).


38 Since AR5, increasing spatial and temporal extent of hypoxia has been projected due to enhanced benthic
39 respiration and reduced oxygen solubility from warming (Del Giudice et al., 2020). Similar to the open
40 ocean, large shifts in the phenology of phytoplankton blooms have been projected for shelf seas throughout
41 subpolar and polar waters (medium confidence) (Henson et al., 2018a; Asch et al., 2019). Zooplankton,
42 which are important prey for many fish species and sea birds, are expected to decrease in abundance on the
43 northeast shelf of the USA (Grieve et al., 2017). However, responses vary by shelf ecosystem (Chust et al.,
44 2014b). Trends towards tropicalisation will continue in the future (high confidence) (Cheung et al., 2015;
45 Stortini et al., 2015; Allyn et al., 2020; Maltby et al., 2020; Costa et al., 2021), but uncertainty of future
46 projections of fisheries production increases substantially beyond 2040 (Maltby et al., 2020). Nevertheless,
47 shelf-sea fisheries at lower latitudes are most vulnerable to climate change (Monnereau et al., 2017). Under
48 future climate change marked by more frequent and intense extreme events and the influences of multiple
49 drivers, more flexible and adaptive management approaches could reduce climate impacts on species while
50 also supporting industry adaptation (high confidence) (Section, Shackell et al., 2014; Stortini et al.,
51 2015; Hare et al., 2016; Stortini et al., 2017; Greenan et al., 2019; Ocaña et al., 2019; Maltby et al., 2020).


53 Upwelling Zones


55 EBUS comprise four important social-ecological systems in the Pacific (California and Peru-Humboldt) and
56 Atlantic (Canary and Benguela) ocean basins. Each is characterised by high primary production, sustained by
57 wind-driven upwelling that draws cold, nutrient-rich, generally low-pH and low-oxygen water to the surface

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1 (Bindoff et al., 2019). Despite their small relative size, the primary productivity in EBUS supports a vast
2 biomass of marine consumers, including some of the world's most productive fisheries (Pauly and Zeller,
3 2016), along with many species of conservation significance (Bakun et al., 2015).


 5 Although upwelling is important in many other oceanic regions, we focus here on the most documented
 6 examples provided by the EBUS. Yet even here, observed changes in upwelling, temperature, acidification
 7 and loss of oxygen (Seabra et al., 2019; Abrahams et al., 2021; Gallego et al., 2021; Varela et al., 2021)
 8 cannot be robustly attributed to anthropogenic climate change, and projected future changes in upwelling are
 9 expected to be relatively small and variable among and within EBUS (Section, WGI AR6 Chapter 9,
10 Fox-Kemper et al., 2021). We therefore have few updates to assessments provided by AR5 and SROCC
11 (Table 3.13) and restrict our brief assessment to the limited amount of new evidence (Figure 3.12).



14 Table 3.13: Summary of previous IPCC assessments of eastern boundary upwelling systems (EBUS).

    Observations                                                    Projections

    AR5 (Hoegh-Guldberg et al., 2014; Lluch-Cota et al.,

    EBUS are vulnerable to changes that influence the               Like other ocean sub-regions, EBUS are projected to warm
    intensity of currents, upwelling, and mixing (and hence         under climate change, with increased stratification and
    changes in sea-surface temperature, wind strength and           intensified winds as westerly winds shift poleward (likely).
    direction), as well as O2 content, carbonate chemistry,         However, cooling has also been predicted for some EBUS,
    nutrient content, and the supply of organic carbon to deep      resulting from the intensification of wind-driven
    offshore locations (high confidence).                           upwelling.

    Climate-change-induced intensification of ocean upwelling There is medium agreement, despite limited evidence, that

    in some EBUS, as observed in the last decades, may lead upwelling intensity and associated variables (e.g.,

    to regional cooling rather than warming of surface waters temperature, nutrient, and O2 concentrations) from the

    and cause enhanced productivity (medium confidence), but Benguela system will change as a result of climate change.

    also enhanced hypoxia, acidification, and associated

    biomass reduction in fish and invertebrate stocks. Owing Any projected increase in upwelling intensity has potential

    to contradictory observations there is currently uncertainty disadvantages. Elevated primary productivity may lead to

    about the future trends of major upwelling systems and decreasing trophic transfer efficiency, thus increasing the

    how their drivers will shape ecosystem characteristics (low amount of organic carbon exported to the seabed, where it

    confidence).                                                    is virtually certain to increase microbial respiration and

                                                                    hence increase low O2 stress.

    Declining O2 and shoaling of the aragonite saturation

    horizon through ocean acidification increase the risk of

    upwelling water being low in pH and O2, with impacts on

    coastal ecosystems and fisheries. These risks and

    uncertainties are likely to involve significant challenges for

    fisheries and associated livelihoods along the west coasts

    of South America, Africa, and North America (low to

    medium confidence).

    There is robust evidence and medium agreement that the
    California Current has experienced an increase of the
    overall magnitude of upwelling events from 1967 to 2010
    (high confidence). This is consistent with changes
    expected under climate change yet remains complicated by
    the influence of decadal-scale variability (low confidence).
    Declining oxygen concentrations and shoaling of the
    hypoxic boundary layer likely reduced the available habitat
    for key benthic communities as well as fish and other
    mobile species. Together with the shoaling of the
    saturation horizon, these changes have increased the
    incidence of low O2 and low pH water flowing onto the
    continental shelf (high confidence; 40 to 120 m), causing
    problems for industries such as the shellfish aquaculture

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   Despite its apparent sensitivity to environmental
   variability, there is limited evidence of ecological changes
   in the Benguela Current EBUS due to climate change.

   SROCC (Bindoff et al., 2019; IPCC, 2019c; IPCC, 2019d)

   Increasing ocean acidification and oxygen loss are            Anthropogenic changes in EBUS will emerge primarily in

   negatively impacting two of the four major upwelling the second half of the 21st century (medium confidence).

   systems: the California Current and Humboldt Current EBUS will be impacted by climate change in different

   (high confidence). Ocean acidification and decrease in ways, with strong regional variability with consequences

   oxygen level in the California Current upwelling system for fisheries, recreation, and climate regulation (medium

   have altered ecosystem structure, with direct negative confidence). The Pacific EBUS are projected to have

   impacts on biomass production and species composition calcium carbonate undersaturation in surface waters within

   (medium confidence).                                          a few decades under Representative Concentration

                                                                 Pathway (RCP)8.5 (high confidence); combined with

   Three out of the four major Eastern Boundary Upwelling warming and decreasing oxygen levels, this will increase

   Systems (EBUS) have shown large-scale wind                    the impacts on shellfish larvae, benthic invertebrates, and

   intensification in the past 60 years (high confidence).       demersal fishes (high confidence) and related fisheries and

   However, the interaction of coastal warming and local aquaculture (medium confidence).

   winds may have affected upwelling strength, with the

   direction of changes varies between and within EBUS (low The inherent natural variability of EBUS, together with

   confidence). Increasing trends in ocean acidification in the uncertainties in present and future trends in the intensity

   California Current EBUS and deoxygenation in California and seasonality of upwelling, coastal warming and

   Current and Humboldt Current EBUS are observed in the stratification, primary production and biogeochemistry of

   last few decades (high confidence), although there is low source waters poses large challenges in projecting the

   confidence to distinguish anthropogenic forcing from          response of EBUS to climate change and to the adaptation

   internal climate variability. The expanding California        of governance of biodiversity conservation and living

   EBUS oxygen minimum zone has altered ecosystem                marine resources in EBUS (high confidence).

   structure and fisheries catches (medium confidence).

                                                                 Given the high sensitivity of the coupled human-natural

   Overall, EBUS have been changing with intensification of EBUS to oceanographic changes, the future sustainable

   winds that drives the upwelling, leading to changes in delivery of key ecosystem services from EBUS is at risk

   water temperature and other ocean biogeochemistry             under climate change; those that are most at risk in the 21st

   (medium confidence).                                          century include fisheries (high confidence), aquaculture

                                                                 (medium confidence), coastal tourism (low confidence) and

   The direction and magnitude of observed changes vary climate regulation (low confidence).

   among and within EBUS, with uncertainties regarding the

   driving mechanisms behind this variability. Moreover, the For vulnerable human communities with a strong

   high natural variability of EBUS and their insufficient dependence on EBUS services and low adaptive capacity,

   representation by global Earth System Models gives low such as those along the Canary Current system,

   confidence that these observed changes can be attributed to unmitigated climate change effects on EBUS (complicated

   anthropogenic causes.                                         by other non-climatic stresses such as social unrest) have a

                                                                 high risk of altering their development pathways (high




 3 The California EBUS is arguably the best-studied of the four ecosystems in terms of robust projections of
 4 climate change, although even here, there is limited evidence and low agreement among projections. For
 5 example, trends in outputs from high-resolution, downscaled models in the California EBUS generally
 6 reflect those from underlying coarser-scale ESMs, but projections for physical variables are more convergent
 7 among modelling approaches than are those for biogeochemical variables (high confidence) (Howard et al.,
 8 2020a; Pozo Buil et al., 2021). Models agree on general warming in the California EBUS, with concomitant
 9 declines in oxygen content (medium confidence) (Howard et al., 2020b; Fiechter et al., 2021; Pozo Buil et
10 al., 2021). But implications for the future spatial distribution of species, including for some fisheries
11 resources (Howard et al., 2020b; Fiechter et al., 2021), are confounded by local-scale oceanographic process
12 (Siedlecki et al., 2021) and by lateral input of anthropogenic land-based nutrients (Kessouri et al., 2021),
13 suggesting that such projections should be accorded low confidence.


15 More generally, changes in upwelling intensity are observed to affect organismal metabolism, population
16 productivity and recruitment, and food-web structure (medium confidence) (van der Sleen et al., 2018;
17 Brodeur et al., 2019; Ramajo et al., 2020). But low confidence in projected trends in upwelling make it

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1 difficult to extrapolate these results to understand potential changes in the ecology of EBUS. Projected
2 changes in fish biomass within EBUS (Carozza et al., 2019) are therefore accorded low confidence. Finally,
3 although MHWs are an important emerging hazard in the global ocean, with intensity, frequency and
4 duration increasing strongly (Section, the number of MHW days yr­1 within EBUS has been
5 increasing more slowly (or decreasing faster, in the case of the Peru-Humboldt system) than in surrounding
6 waters (Varela et al., 2021). Notwithstanding these trends, EBUS remain vulnerable both to MHWs (high
7 confidence) (Sen Gupta et al., 2020) and to their long-lasting impacts (high confidence) (Arafeh-Dalmau et
8 al., 2019; Harvell et al., 2019; McPherson et al., 2021). On this basis, the suggestion that EBUS may
9 represent refugia from MHWs is accorded low confidence.


11 Despite low confidence in detailed projections for ecological changes in EBUS, the WGI assessment (WGI
12 AR6 Chapter 9, Fox-Kemper et al., 2021) that upwelling-favourable winds will weaken (or be present for
13 shorter durations) at low latitude but intensify at high latitude (high confidence), albeit by no more than 20%
14 in either case (medium confidence), presents some key risks to associated EBUS ecosystems. These include
15 potential decreases in provisioning services, including fisheries and marine aquaculture (Bertrand et al.,
16 2018; Kifani et al., 2018; Lluch-Cota et al., 2018; van der Lingen and Hampton, 2018), and cultural services
17 such as nature-based tourism (Section 3.5).


19 Polar Seas


21 The polar seas cover ~20% of the global ocean and include the deep Arctic Ocean and surrounding shelf seas
22 as well as the Southern Ocean south of the polar front. They play a significant role in ocean circulation and
23 absorption of anthropogenic CO2 (Meredith et al., 2019). The Arctic is characterised by polar seas
24 surrounded by land, while the Antarctic comprises continental Antarctica surrounded by the Southern Ocean.
25 These high-latitude ecosystems share key properties, including strong seasonality in solar radiation and sea-
26 ice coverage. Sea ice regulates water-column physics, chemistry and biology, air-sea exchange, and is a
27 critical habitat for many species. In spring, when solar radiation returns and sea ice melts, intense
28 phytoplankton blooms fuel food webs that include rich communities of both resident and summer-migrant
29 species, with typically high dependency on a few key species for trophic transfer (Meredith et al., 2019;
30 Rogers et al., 2020). Over the last two decades, Arctic Ocean surface temperature has increased in line with
31 the global average, while there has been no uniform warming across the Antarctic (high confidence) (WGI
32 AR6 Chapter 9, Fox-Kemper et al., 2021). Thus, the rate of change due to warming, and associated sea-ice
33 loss, is greater in the Arctic than in the Antarctic (high confidence) (Section 3.2, Table 3.14, WGI AR6
34 Chapter 9, Fox-Kemper et al., 2021). Both Arctic and Antarctic regions have a long history of living
35 resource extraction, including some of the largest fisheries on the globe in terms of catches. However, only
36 the Arctic hosts human populations, holding a rich Indigenous knowledge and Local knowledge on these
37 socio-ecological systems (Cross-Chapter Paper 6, Meredith et al., 2019).


39 Previous assessments of polar seas (Table 3.14) concluded that climate change has already profoundly
40 influenced polar ecosystems, through changing species distributions and abundances from primary producers
41 to top predators, including both ecologically and economically important species (high confidence) and that
42 it will continue to do so (Table 3.14).



45 Table 3.14: Summary of previous IPCC assessments for polar seas.

    Observations                                           Projections

    AR5 (Wong et al., 2014)

    Poleward species distributional shifts are due to climate Some marine species will shift their ranges in response to

    warming (medium to high confidence).                   changing ocean and sea ice conditions in the polar regions

                                                           (medium confidence).

    Impacts of shifts in ocean conditions affect fish and

    shellfish abundances in the Arctic (high confidence).  Loss of sea ice in summer and increased ocean

                                                           temperatures are expected to impact secondary pelagic

    Changes in sea ice and the physical environment to the production in some regions of the Arctic Ocean, with

    west of the Antarctic Peninsula are altering phytoplankton associated changes in the energy pathways within the

    stocks and productivity, and krill (high confidence).  marine ecosystem (medium confidence).

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                                                                Ocean acidification has the potential to inhibit embryo
                                                                development and shell formation of some zooplankton and
                                                                krill in the polar regions, with potentially far-reaching
                                                                consequences to food webs in these regions (medium

                                                                Shifts in the timing and magnitude of seasonal biomass
                                                                production could disrupt coupled phenologies in the food
                                                                webs, leading to decreased survival of dependent species
                                                                (medium confidence).

    SR15 (Hoegh-Guldberg et al., 2018a)

    A fundamental transformation is occurring in polar          The losses in sea ice at 1.5°C and 2°C of warming will

    organisms and ecosystems, driven by climate change (high result in habitat losses for organisms such as seals, polar

    confidence).                                                bears, whales and seabirds. There is high agreement and

                                                                robust evidence that phytoplankton species will change

                                                                because of sea ice retreat and related changes in

                                                                temperature and light penetration, and this is very likely to

                                                                benefit fisheries productivity in the Arctic spring bloom


                                                                `Unique and threatened systems' (RFC1), including Arctic
                                                                and coral reefs, display a transition from high to very high
                                                                risk of transition at temperatures between 1.5°C and 2°C of
                                                                global warming, as opposed to at 2.6°C of global warming
                                                                in AR5 (high confidence).

    SROCC (Bindoff et al., 2019)

    Climate-induced changes in seasonal sea ice extent and Future climate-induced changes in the polar oceans, sea

    thickness and ocean stratification are altering marine      ice, snow and permafrost will drive habitat and biome

    primary production (high confidence), with impacts on shifts, with associated changes in the ranges and

    ecosystems (medium confidence).                             abundance of ecologically important species (medium


    Changes in the timing, duration and magnitude of primary

    production have occurred in both polar oceans, with         Projected range expansion of subarctic marine species will

    marked regional or local variability (high confidence). increase pressure for high-Arctic species (medium

                                                                confidence), with regionally variable impacts.

    In both polar regions, climate-induced changes in ocean Both polar oceans will be increasingly affected by CO2

    and sea ice conditions have expanded the range of           uptake, causing corrosive conditions for calcium carbonate

    temperate species and contracted the range of polar fish shell-producing organisms (high confidence), with

    and ice-associated species (high confidence).               associated impacts on marine organisms and ecosystems

                                                                (medium confidence).

    Ocean acidification will affect several key Arctic species

    (medium confidence).                                        The projected effects of climate-induced stressors on polar

                                                                marine ecosystems present risks for commercial and

                                                                subsistence fisheries with implications for regional

                                                                economies, cultures and the global supply of fish, shellfish,

                                                                and Antarctic krill (high confidence).



 3 Since SROCC, evidence demonstrates that warmer oceans, less sea ice and increased advection results in
 4 increasing primary production in the Arctic, albeit with regional variation (high confidence), while trends
 5 remain spatially heterogeneous and less clear in the Antarctic (medium confidence) (Cross-Chapter Paper 6,
 6 Del Castillo et al., 2019; Lewis et al., 2020; Pinkerton et al., 2021; Song et al., 2021a). Furthermore, climate
 7 warming influences key mechanisms determining energy transfer between trophic levels including: (1)
 8 altered size spectra; (2) shifts in trophic pathways; (3) phenological mismatches; and (4) increased top-down
 9 trophic regulation (Table 3.15). However, the scale of impacts from changes in these mechanisms on
10 ecosystem productivity in warming polar oceans remains unresolved and is hence assigned low confidence.


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1 Table 3.15: Examples of mechanisms influencing transfer of energy between lower trophic levels in warmer polar

2 oceans.

    Mechanism                         Examples                                 References

    Altered size spectra              Shifts towards smaller algal cells and   (Aarflot et al., 2018; Kimmel et al.,
                                      zooplankton in warmer and more           2018; Weydmann et al., 2018; Hop et
                                      stratified oceans results in longer and  al., 2019; Møller and Nielsen, 2020;
                                      less-efficient food chains, with lower   Spear et al., 2020), but see Dong et al.
                                      lipid content.                           (2021) and Vernet et al. (2017) for
                                                                               opposite trends.

    Shifts in trophic pathways        Changes in microbial food-web            (Cross-Chapter Paper 6, Fujiwara et

                                      interactions, including strengthening of al., 2016; Onda et al., 2017; Vernet et

                                      microbial loop, may reduce overall al., 2017; Grebmeier et al., 2018;

                                      productivity. Transitions from sea-ice Moore et al., 2018b; Cavan et al.,

                                      algae to open-water phytoplankton 2019; Vaqué et al., 2019; Yurkowski

                                      production may reduce benthic-pelagic et al., 2020)Braekcman et al 2021

                                      coupling and benthic production;

                                      transition from autotroph to

                                      heterotroph benthic production with

                                      increased water turbidity; shifts from

                                      krill-dominated to salp-dominated

                                      ecosystems in the Antarctic may have

                                      negative impacts on higher trophic


    Phenological mismatches           Mismatches in timing arise between       (Søreide et al., 2010; Renaud et al.,
                                      spring phytoplankton blooms and          2018; Dezutter et al., 2019)
                                      zooplankton recruits.

    Increased top-down trophic regulation Increased predation efficiency and top- (Langbehn and Varpe, 2017; Kaartvedt
                                                    down regulation of zooplankton by and Titelman, 2018; Hobbs et al.,
                                                    zooplanktivorous fish (due to more 2021)
                                                    light with less sea ice) disconnects
                                                    zooplankton and phytoplankton



 5 Major community shifts, both gradual and abrupt, are observed in polar oceans in response to warming
 6 trends and MHWs (Arctic only) (high confidence) (Cross-Chapter Paper 6, Figure 3.12, Beaugrand et al.,
 7 2019; Meredith et al., 2019; Huntington et al., 2020). In general, abundances and ranges of Arctic fish
 8 species are declining and contracting, while ranges of boreal fish species are expanding, both geographically
 9 and in terms of feeding interactions and ecological roles (high confidence) (Huserbråten et al., 2019;
10 Meredith et al., 2019; Huntington et al., 2020; Pecuchet et al., 2020a), with variable outcomes for large
11 commercial fish stocks (Cross-Chapter Paper 6, Kjesbu et al., 2014; Holsman et al., 2018; Free et al., 2019).
12 The extreme seasonal solar radiation cycles of these high latitudes may both act as a barrier for species
13 immigration and change predator-prey dynamics in previously ice-covered areas, factors not currently
14 considered in projections (limited evidence) (Kaartvedt and Titelman, 2018; Ljungström et al., 2021).
15 Responses by marine mammals and birds to the ongoing changes in polar ecosystems are both positive and
16 negative (Meredith et al., 2019; Bestley et al., 2020). Phenological, behavioural, physiological, and
17 distributional changes are observed in marine mammals and birds in response to altered ecological
18 interactions and habitat degradation, especially to loss of sea ice (high confidence) (Box 3.2, Cross-Chapter
19 Paper 6, Beltran et al., 2019; Cusset et al., 2019; Descamps et al., 2019; Meredith et al., 2019; Huntington et
20 al., 2020). Reproductive failures and declining abundances attributed to warmer polar oceans and less sea-ice
21 cover are observed in populations of polar bears, Ursus maritimus, seals, whales and marine birds (high
22 confidence) (Box 3.2, Duffy-Anderson et al., 2019; Ropert-Coudert et al., 2019; Bestley et al., 2020;
23 Chambault et al., 2020; Molnár et al., 2020; Stenson et al., 2020). The ongoing changes in polar marine
24 ecosystems can lead to temporary increases in biodiversity and functional diversity (e.g., due to immigration
25 of boreal species in the Arctic, high confidence), but reduced trophic-transfer efficiencies and functional
26 redundancy, with uncertain consequences for ecosystem resilience and vulnerability (limited evidence, low
27 agreement) (Griffith et al., 2019b; Alabia et al., 2020; du Pontavice et al., 2020; Alabia et al., 2021; Frainer
28 et al., 2021).


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 1 Calcareous polar organisms are among the groups most sensitive to ocean acidification (high confidence)
 2 (Section 3.3.2). Niemi et al. (2021) reports that >80% of sampled sea snail, Limacina helicina, a key species
 3 in pelagic food webs, displayed signs of shell dissolution in the Amundsen Gulf. However, bacteria,
 4 phytoplankton, zooplankton and benthic communities are found to be detrimentally impacted, resilient, or
 5 even positively influenced by ocean acidification in observational and experimental studies (Section 3.3,
 6 Hildebrandt et al., 2016; Thor et al., 2018; Ericson et al., 2019; McLaskey et al., 2019; Meredith et al., 2019;
 7 Petrou et al., 2019; Renaud et al., 2019; Brown et al., 2020; Hancock et al., 2020; Henley et al., 2020;
 8 Johnson and Hofmann, 2020; Torstensson et al., 2021).While fish larval stages may be sensitive, adult fish
 9 are expected to have low vulnerability to projected acidification levels (Section 3.3.3, Hancock et al., 2020),
10 although reduced swimming capacity in polar cod in an ocean acidification experiment has been observed
11 (Kunz et al., 2018). Polar organisms' sensitivity to ocean acidification may increase with increasing light
12 levels due to the loss of sea ice (algae, Donahue et al., 2019; Kvernvik et al., 2020), temperature stress
13 (pteropods, Johnson and Hofmann, 2020), or indirectly via increased heterotrophic bacterial productivity
14 (limited evidence) (Vaqué et al., 2019). Due to limited mechanistic understanding of observed effects, and
15 mixed responses among Arctic species, future impacts of ocean acidification are assigned medium
16 confidence for polar species, and low confidence for outcomes for polar ecosystems (Meredith et al., 2019;
17 Green et al., 2021b).


19 While levels of pollutants in biota (e.g., persistent organic pollutants, mercury) have generally declined over
20 the past decades, recent increasing levels are associated with release from reservoirs in ice, snow and
21 permafrost, and through changing food webs and pathways for trophic amplification (medium confidence)
22 (Box 3.2, Ma et al., 2016; Amélineau et al., 2019; Foster et al., 2019; Bourque et al., 2020; Kobusiska et al.,
23 2020). Also, a warmer climate, altered ocean currents and increased human activities elevate the risk of
24 invasive species in the Arctic (medium confidence), potentially changing ecosystems in this region (high
25 confidence) (Chan et al., 2019; Goldsmit et al., 2020). In the remote Antarctic, there is a lower risk of
26 invasive species (limited evidence) (McCarthy et al., 2019; Holland et al., 2021).


28 Fisheries are largely sustainably managed, yet are expanding polewards following sea-ice melt in the Arctic
29 (high confidence) (Fauchald et al., 2021) and possibly in the Antarctic (limited evidence) (Santa Cruz et al.,
30 2018). Tourism is increasing and expanding in both polar regions, while shipping and hydrocarbon
31 exploration are growing in the Arctic, increasing the risks of compound effects on vulnerable and already
32 stressed populations and ecosystems (high confidence) (Sections,, Cross-Chapter Paper 6,
33 Hauser et al., 2018; Meredith et al., 2019; Helle et al., 2020; Rogers et al., 2020; Cavanagh et al., 2021).


35 Ensemble global model projections indicate future increases in primary production and total animal biomass
36 towards 2100 under RCP2.6 (~ 5% and 50%, respectively) and RCP8.5 (~10% and 70%, respectively), in the
37 Arctic (Bryndum-Buchholz et al., 2019; Lotze et al., 2019; Nakamura and Oka, 2019), highlighting
38 opportunities for, and possibly conflicts over, new ecosystem services (Section 3.5). For the Southern Ocean,
39 no overall trends are apparent, but greater variability in both primary production and total animal biomass
40 are projected under RCP2.6, with a ~5% and 15% increase in primary production and total animal biomass
41 under RCP8.5, respectively (Bryndum-Buchholz et al., 2019; Lotze et al., 2019; Nakamura and Oka, 2019).
42 All projections presented exhibit high inter-model variability and hence uncertainty (Heneghan et al., 2021).
43 Furthermore, regional models project significant distributional shifts and wide-ranging trends (i.e., relatively
44 stable, increasing and declining) in productivity for key ecological and commercial species, and functional
45 groups, with weak to strong dependence on emission scenarios, indicating low confidence in future outcomes
46 for polar marine ecosystems and associated ecosystem services (Section 3.5, Piñones and Fedorov, 2016;
47 Griffiths et al., 2017; Klein et al., 2018; Hansen et al., 2019; Meredith et al., 2019; Steiner et al., 2019; Tai et
48 al., 2019; Alabia et al., 2020; Holsman et al., 2020; Reum et al., 2020; Veytia et al., 2020; Sandø et al.,
49 2021). Potentially highly influential tipping points associated with Arctic sea-ice melt and Antarctic ocean
50 circulation change adds to this uncertainty (Cross-Chapter Paper 6, Heinze et al., 2021). Nevertheless,
51 increasing evidence supports that sustainable and adaptive ecosystem-based fisheries practices can reduce
52 detrimental impacts of climate change on harvested populations (medium confidence) (Section,
53 Klein et al., 2018; Free et al., 2019; Hansen et al., 2019; Holsman et al., 2020).


55 3.4.3 Oceanic Systems and Cross Cutting Changes


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1 The oceanic zone, comprising >99% of the ocean's volume, is highly exposed to climate-impact drivers
2 because of its proximity to the atmosphere (Section 3.2, Pörtner et al., 2014; Bindoff et al., 2019), while its
3 relative distance from human settlements and coastal ecosystems decreases variability and interactions, and
4 permits many phenomena to be detected clearly and attributed to climate change. This section assesses how
5 climate-driven changes influence oceanic biological systems over very large spatial scales and notes how
6 impacts on the epipelagic zone affect the mesopelagic, bathypelagic, and deep seafloor ecosystems.


8 Biogeography and Species Range Shifts


10 Observed species range shifts
11 Since previous assessments (Table 3.16), poleward range-shifts have remained a ubiquitous response to
12 climate change (high confidence), moving species from warmer regions into higher-latitude ecosystems
13 (Fossheim et al., 2015; Kumagai et al., 2018; Burrows et al., 2019; Lenoir et al., 2020).



16 Table 3.16: Summary of previous IPCC assessments of biogeography and species range shifts.

    Observations                                               Projections

    AR5: (Hoegh-Guldberg et al., 2014; Pörtner et al., 2014)

    The distribution and abundance of many fishes and          Spatial shifts of marine species due to projected warming
    invertebrates have shifted poleward and/or to deeper,      will cause high-latitude invasions and high local-extinction
    cooler waters (high confidence).                           rates in the tropics and semi-enclosed seas (medium
    On average, species' distributions have shifted poleward
    by 72.0 ± 0.35 km per decade (high confidence).

    SROCC (Bindoff et al., 2019)

    Ocean warming has contributed to observed changes in Recent model projections since AR5 and SR15 continue to

    biogeography of organisms ranging from phytoplankton to support global-scale range shifts of marine fishes at rates

    marine mammals (high confidence).                          of tens to hundreds of km per decade in the 21st century,

                                                               with rate of shifts being substantially higher under RCP8.5

    The direction of the majority of the shifts of epipelagic than RCP2.6.

    organisms are consistent with a response to warming (high

    confidence) but are also shaped by oxygen concentrations

    and ocean currents across depth, latitudinal and

    longitudinal gradients (high confidence).

    Geographic ranges have shifted since the 1950s by 51.5 ±
    33.3 km per decade (mean and very likely range) and 29.0
    ± 15.5 km per decade for organisms in the epipelagic and
    seafloor ecosystems, respectively.



19 Thermal tolerances of epipelagic populations drive biogeographic change (Figures 3.10, 3.15), but the
20 strength and direction of range shifts tend to be modulated by both climate and non-climate drivers (Pinsky
21 et al., 2020b), including: interactive effects of hypoxia and ocean acidification (Sampaio et al., 2021);
22 oceanic dispersal barriers (Choo et al., 2021), food and critical habitat availability (Alabia et al., 2020;
23 Tanaka et al., 2021), geographic position (including depth, Mardones et al., 2021), and ocean currents
24 (Sunday et al., 2015; Chapman et al., 2020; Fuchs et al., 2020). The difference between physiological
25 thermal tolerances (Section 3.3.2) and local environmental conditions determines safety margins against
26 future climate warming in ectotherms (Pinsky et al., 2019). Acclimation and evolution (Section 3.3.4) and
27 life-history stage (Section 3.3.3) also alter species' thermal tolerances. Biogeographic responses are further
28 modulated by other interacting factors (Table 3.17).


30 A large global meta-analysis of range shifts across multiple levels of the marine food web (Lenoir et al.,
31 2020) estimates that marine species are moving poleward at a rate of 59.2 km per decade (very likely range:
32 43.7­74.7 km per decade), closely matching the local climate velocity (high confidence). In some cases,
33 warming-related distribution shifts were followed by density-dependent use of these areas, influencing

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1 associated fisheries (Baudron et al., 2020), and in others, warming influenced competitive interactions: in the
2 Arctic-Boreal Barents Sea, warming-induced increases in cod (Gadus morhua) abundance reduces haddock
3 (Melanogrammus aeglefinus) abundance (Durant et al., 2020).




 7 Figure 3.15: Schematic of range-shift dynamics in marine ectotherms in response to climate warming. As the ocean
 8 warms, conditions at the edge of the species' distribution may become warmer than the maximum thermal tolerance of
 9 the species (such as with T2, see Figure 3.9), causing local populations to undergo a gradual decline in performance, a
10 decreasing population size and ultimately their extirpation, resulting in a range contraction. Conversely, at the cool
11 extreme of the distribution (such as with T1), habitats beyond the current range of the species will become thermally
12 suitable in the future (i.e., within the species' thermal tolerance range) and, providing the species can disperse to those
13 locations, allow for the colonisation and consolidation of new populations and subsequent range expansion. These are
14 processes conditioned by multiple drivers that interact with warming to ultimately define range shift responses; some of
15 which are described in Table 3.17. Note that physiological thermal tolerances relate to body temperatures of the
16 organism rather than ambient temperatures.



19 Table 3.17: Synthesis of selected processes conditioned by multiple environmental drivers that interact with warming
20 to ultimately define range-shift responses.

    Factor                            Effect                                    Example references

    Evolution and acclimation         Evolution of thermal tolerances and (Palumbi et al., 2014; Miller et al.,

                                      acclimation under local climatic          2020a)

                                      conditions can increase resilience to

                                      future climate warming, slowing the

                                      loss of species at trailing (warm) range


    Marine heatwaves (MHWs)           Influence the evolution of thermal        (Buckley and Huey, 2016; Sunday et
                                      tolerances by eliminating genotypes       al., 2019)
                                      that are intolerant of elevated

                                      MHWs can produce widespread die-          (Smale and Wernberg, 2013)
                                      offs of shallow-water benthic
                                      organisms triggering extensive
                                      contractions of their ranges.

                                      MHWs can facilitate range expansions (Leriorato and Nakamura, 2019;
                                      by opening niches and/or enhancing Thomsen et al., 2019; Monaco et al.,
                                      recruitment of warm-affiliated species. 2021)

                                      Cold-waves can halt or even reverse (Leriorato and Nakamura, 2019)
                                      range expansions at leading edges.

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FINAL DRAFT                                          Chapter 3                IPCC WGII Sixth Assessment Report
Ocean currents
                                     Influence range dynamics through (Hunt et al., 2016; Kumagai et al.,
Climatic refugia                     their effect on dispersal, depending on 2018; Fuchs et al., 2020)
                                     their magnitude, direction and seasonal
Oxygen availability                  patterns.

Habitat availability and quality     Where currents align with spatial        (García Molinos et al., 2017)
Biotic interactions, including food
availability                         gradients of warming, range
Do Not Cite, Quote or Distribute
                                     expansions track thermal changes

                                     more closely. Conversely, directional

                                     mismatches result in consistently

                                     slower expansion rates and larger

                                     response lags; an effect more acute for

                                     benthic organisms relying on passive

                                     dispersion of larvae and propagules.

                                     Rates of range contraction across taxa (García Molinos et al., 2017)
                                     decreased (increased) under directional
                                     agreement (mismatch) with ocean
                                     currents, possibly associated with
                                     enhanced (reduced) flows of adaptive
                                     genes to warming in downstream
                                     (upstream) populations within the
                                     distributional range.

                                     Areas of locally stable climatic         (Smith et al., 2014; Assis et al., 2016;

                                     conditions, such as deeper waters or Lourenço et al., 2016; Wyatt et al.,

                                     regions with internal tides or localised 2020)

                                     upwelling, can buffer the effects of

                                     regional warming, facilitating species

                                     persistence and conserving genetic

                                     diversity at rear edge populations.

                                     Distributional shifts into deeper, cooler (Smith et al., 2014; Assis et al., 2016;

                                     habitats can offer an effective          Lourenço et al., 2016)

                                     alternative response to latitudinal

                                     shifts, because sharper thermal

                                     gradients mean vertical displacements,

                                     needed to compensate for the same

                                     amount of warming, are several orders

                                     of magnitude smaller than planar


                                     Oxygen supersaturation may extend        (Giomi et al., 2019)
                                     ectotherm survival to extreme
                                     temperatures and increase thermal
                                     tolerances by compensating for the
                                     increasing metabolic demand at high

                                     Oxygen deprivation increases             (Brown and Thatje, 2015; Roman et
                                     metabolic demand and respiration         al., 2019; Hughes et al., 2020)
                                     rates. Shallowing of oxygen dead
                                     zones and subsequent hypoxic
                                     avoidance can render deep thermal
                                     refuges unsuitable for organisms.

                                     The availability and quality of habitat (Krause-Jensen et al., 2019; Tamir et
                                     (underwater light conditions, adequate al., 2019)
                                     substrate, nutrient and food supply) set
                                     limits to the distribution of organisms
                                     and range shift dynamics (e.g.,
                                     resilience of populations to climate
                                     warming and the consolidation of
                                     range expansions).

                                     Species interactions can confer          (Falkenberg et al., 2015; Giomi et al.,
                                     resilience to warming by retarding       2019)
                                     habitat degradation and buffering the
                                     impacts of warming on organisms.

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   FINAL DRAFT                       Chapter 3                              IPCC WGII Sixth Assessment Report

                                     Changes in biotic interactions (e.g.,  (Selden et al., 2018; Westerbom et al.,
                                     altered predation rates, food          2018; Figueira et al., 2019; Pinsky et
                                     availability, competition or trophic   al., 2020b; Monaco et al., 2021)
                                     mismatches) induced by climate
                                     warming can modify range-shift



3 Biogeographic shifts lead to novel communities and biotic interactions (high confidence) (Zarco-Perello et
4 al., 2017; Pecuchet et al., 2020b), with concomitant changes in ecosystem functioning and servicing (high
5 confidence) (Vergés et al., 2019; Nagelkerken et al., 2020; Peleg et al., 2020). For instance, temperature-
6 driven changes in distribution and abundance of copepods, the dominant zooplankton, were observed
7 between 1960­2014 in the North Atlantic. These changes subsequently affect biogenic carbon cycling
8 through alteration of microbial remineralisation and carbon sequestration in deep water (medium confidence)
9 (Section, Pitois and Fox, 2006; Brun et al., 2019).


11 Observed vertical redistributions
12 Epipelagic isotherms have recently (1980­2015) deepened at an average of 6.6 ± 18.8 m per decade (Pinsky
13 et al., 2019) but, there is low agreement on whether species move deeper in pursuit of thermal refuge. Prior
14 studies suggested range shifts to depth (Dulvy et al., 2008; Pinsky et al., 2013; Yemane et al., 2014), but
15 increasing evidence suggests that fish and planktonic communities across large parts of the North Atlantic,
16 sub-Arctic and northeast Pacific Ocean redistribute horizontally with horizontal climate velocity, except
17 where vertical temperature gradients are particularly steep. There is low confidence for temperature-driven
18 depth shifts in the epipelagic zone (Burrows et al., 2019; Campana et al., 2020; Caves and Johnsen, 2021).
19 At the same time, decreasing oxygen concentrations and the vertical expansion of OMZs have already
20 decreased suitable habitat of pelagic fishes, including tuna and billfishes, by ~15% primarily due to vertical
21 compression of environmental niches (Stramma et al., 2012; Deutsch et al., 2015).


23 Projected changes in species range shifts
24 Continued changes in the biogeography of marine predators and prey are anticipated under future climate
25 change, with climate velocity in the epipelagic zone during 2050­2100 under RCP8.5 projected to be
26 sevenfold faster than that during 1955­2005 (medium confidence) (Figure 3.4, Brito-Morales et al., 2020).
27 This have substantial ecological implications, as projections suggest near-elimination of overlaps between
28 the distributions of certain predator-prey pairs in the northeast Atlantic Ocean when their current joint
29 distributions (1989­2014) are compared with those projected (2037­2062) under RCP8.5 (Sadykova et al.,
30 2020).


32 Deepening of epipelagic isotherms is projected to accelerate over 2006­2100 to rates of 8.5 m per decade
33 under RCP4.5 and 32 m per decade under RCP8.5 (Jorda et al., 2020). Although vertical redistribution of
34 thermal niches is three to four orders of magnitude slower than horizontal displacement, maximum depth
35 limits imposed by the seafloor and photic layer (both of which are projected to be reached in this century)
36 will likely vertically compress suitable habitat for most marine organisms (medium confidence) (Dueri et al.,
37 2014; Jorda et al., 2020).


39 Projections from coupled biogeochemical and ecosystem models suggest a general decline in mesopelagic
40 biomass (Lefort et al., 2015), although this may vary among ocean basins. The volume of OMZs have been
41 expanding at many locations (high confidence), and the oxygen content of the subsurface ocean is projected
42 to decline to historically unprecedented conditions over the 21st century (medium confidence) (Section
43, WGI AR6 Section, Canadell et al., 2021) at a rate of 10­15 µM per decade in OMZs (Section
44, Breitburg et al., 2018). Oxygen availability and the effects of ocean acidification (Sections 3.3,
45 3.4.2) on zooplankton might become a dominant constraint in the upper ocean's metabolic index, which is
46 projected to decrease globally by 20% by 2100 (Deutsch et al., 2015; Steinberg and Landry, 2017). In
47 addition, extremely rapid acceleration of climate velocities projected in the mesopelagic under all emissions
48 scenarios suggest that species in this ocean stratum will be even more exposed to future warming than
49 species in the epipelagic (Figure 3.4, Brito-Morales et al., 2020). But projections also suggest that warming-
50 related increases in trophic efficiency lead to a 17% increase in the biomass of the deep scattering layer
51 (zooplankton and fish in the mesopelagic) by 2100 (low confidence) (Bindoff et al., 2019); observational

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1 studies appears to show that mesopelagic fishes adapted to warm water increased in abundance and
2 distribution in the California Current associated with warming and the expansion of OMZ (Koslow et al.,
3 2019), suggesting that some mesopelagic fish stocks might be resilient to a changing climate (medium
4 confidence).


6 Phenological shifts & trophic mismatches


 8 Observed changes
 9 SROCC reported high confidence in phenological shifts towards earlier onset of biological events (Table
10 3.18), with phenological shifts among epipelagic species attributed to ocean warming (high confidence).



13 Table 3.18: Summary of previous IPCC assessments of phenological shifts & trophic mismatches

    Observations                                                   Projections

    AR5 WGII (Hoegh-Guldberg et al., 2014; Larsen et al., Projections of phenological shifts and trophic mismatches

    2014)                                                          were not assessed in this report.

    Changes to sea temperature have altered the phenology, or
    timing of key life-history events such as plankton blooms,
    and migratory patterns and spawning in fish and
    invertebrates, over recent decades (medium confidence).
    There is medium to high agreement that these changes
    pose significant uncertainties and risks to fisheries,
    aquaculture, and other coastal activities.

    The highly productive high-latitude spring bloom systems
    in the northeastern Atlantic are responding to warming
    (medium evidence, high agreement), with the greatest
    changes being observed since the late 1970s in the
    phenology, distribution, and abundance of plankton
    assemblages, and the reorganisation of fish assemblages,
    with a range of consequences for fisheries (high

    Observed changes in the phenology of plankton groups in
    the North Sea over the past 50 years are driven by climate
    forcing, in particular regional warming (high confidence).

    On average, spring events in the ocean have advanced by
    4.4 ± 0.7 days per decade (mean ± SE).

    Shifts in the timing and magnitude of seasonal biomass
    production could disrupt matched phenologies in the food
    webs, leading to decreased survival of dependent species
    (medium confidence). If the timing of primary and
    secondary production is no longer matched to the timing of
    spawning or egg release, survival could be impacted, with
    cascading implications to higher trophic levels. This
    impact would be exacerbated if shifts in timing occur
    rapidly (medium confidence).

    There is medium to high confidence that climate- induced
    disruptions in the synchrony between timing of spawning
    and hatching of some fish and shellfish and the seasonal
    increases in prey availability can result in increased larval
    or juvenile mortality or changes in the condition factor of
    fish and shellfish species in the Arctic marine ecosystems.

    SROCC (Bindoff et al., 2019)

    Phenology of marine ectotherms in the epipelagic systems Projections of phenological shifts and trophic mismatches
    are related to ocean warming (high confidence) and that were not assessed in this report.

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the timing of biological events has shifted earlier (high

Timing of spring phenology of marine organisms is
shifting to earlier in the year under warming, at an average
rate of 4.4 ± 1.1 days per decade, although it is variable
among taxonomic groups and among ocean regions.

WGI AR6 Chapter 2 (Gulev et al., 2021)

Phenological metrics for many species of marine               Projections of phenological shifts and trophic mismatches

organisms have changed in the last half-century (high were not assessed in this report.

confidence), though many regions and many species of

marine organisms remain under-sampled or even

unsampled. The changes vary with location and with

species (high confidence). There is a strong dependence of

survival in higher trophic-level organisms (fish, exploited

invertebrates, birds) on the availability of food at various

stages in their life cycle, which in turn depends on

phenologies of both (high confidence). There is a gap in

our understanding of how the varied responses of marine

organisms to climate change, from a phenological

perspective, might threaten the stability and integrity of

entire ecosystems.



 3 Since SROCC, field data have continued to show that the phenology of biological events in the ocean is very
 4 likely (high to very high confidence) advancing in response to climate change, with 71.9% of published
 5 observations consistent with these anticipated effects (Figure 3.16a,b; Table 3.19), although most reports
 6 (95.6%) were from the Northern Hemisphere (Figure 3.16a). Biological events that are shifting earlier in
 7 response to climate change include phytoplankton blooms (Scharfe and Wiltshire, 2019; Chivers et al., 2020)
 8 such as those of HAB species (Forsblom et al., 2019; Bucci et al., 2020); peaks in zooplankton abundance
 9 (Chevillot et al., 2017; Forsblom et al., 2019); the migration (Otero et al., 2014; Kovach et al., 2015; Chust et
10 al., 2019) and spawning of commercial fish (McQueen and Marshall, 2017; Kanamori et al., 2019) including
11 crabs and squid (Henderson et al., 2017); and breeding of marine reptiles (Mazaris et al., 2008; Cherkiss et
12 al., 2020). Moreover, different trophic levels within epipelagic food webs are responding at different rates
13 (very high confidence) (Table 3.19, Figure 3.16b,c), with greater and more consistent responses by lower
14 trophic levels (phytoplankton, holozooplankton and meroplankton), but less consistent, weaker and more
15 varied responses by higher trophic levels. There were too few independent time series to make robust
16 estimates for benthic invertebrates, plants, marine reptiles and mammals. This differential response across
17 trophic levels could lead to trophic mismatches (Neuheimer et al., 2018), where predators and their prey
18 respond asynchronously to climate change (Edwards and Richardson, 2004; Rogers and Dougherty, 2019;
19 Rubenstein et al., 2019; Émond et al., 2020), with potential population-level consequences, including
20 declines in fish recruitment (Burthe et al., 2012; Chevillot et al., 2017; McQueen and Marshall, 2017; Asch
21 et al., 2019; Durant et al., 2019; Régnier et al., 2019). Available evidence also suggests that feeding
22 relationships could modulate species responses to climate change, as seen in breeding of surface-feeding and
23 deeper-diving seabirds (Descamps et al., 2019). These differential responses could determine `winners' and
24 `losers' under future climate change (Lindén, 2018).



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 2 Figure 3.16: Observed responses to climate change based on a systematic Web of Science review of marine phenology
 3 studies exceeding 19 years in length to update the assessment in WGII AR5 Chapter 30 (Hoegh-Guldberg et al., 2014).
 4 Error bars indicate 95% confidence limits (i.e., the extremely likely range). (a) Global data showing changes in seasonal
 5 cycles of biota that are attributed (at least partly) to climate change (blue, n=297 observations), and changes that are
 6 inconsistent with climate change (white, n=116 observations). Each circle represents the centre of a study area. (b) The
 7 proportion of phenological observations (showing means and extremely likely ranges) that are attributed to climate
 8 change (i.e., generally showing earlier timing) by taxonomic group. (c) Observed shift in timing (days per decade,
 9 showing means and extremely likely ranges), by taxonomic group, that are attributed to climate change. Negative shifts
10 are earlier, positive shifts are later. Details and additional plots are presented in SM3.3.3, Figure SM3.3 and Table
11 SM3.1.



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1 Table 3.19: Assessment of phenological shifts by taxon based on time series from field observations spanning at least
2 19 years published over the past 25 years.

Taxon               Rate of consistency of Estimated mean rate of Confidence    Notes

                    observations with change in seasonal

                    climate change  timing

Phytoplankton       78.41%          ­7.5 days per decade  Very high confidence  Evidence most robust
                    (n=85)          (n=83)                                      for changes in timing
                                                                                of blooms in the North
                                                                                Atlantic (e.g., Chivers
                                                                                et al., 2020) and Baltic
                                                                                (e.g., Scharfe and
                                                                                Wiltshire, 2019;
                                                                                Wasmund et al., 2019),
                                                                                with limited evidence
                                                                                from the Southern

Holozooplankton     79.74%          ­4.27 days per decade Very high confidence  Evidence most robust
                    (n=77)          (n=58)                                      in the northeast
                                                                                Atlantic (e.g.,
                                                                                Chevillot et al., 2017),
                                                                                but sparse elsewhere.

Meroplankton (taxa  81.06%          ­4.34 days per decade Very high confidence  Includes earlier peak
that are only       (n=72)          (n=64)                                      abundance of fish
temporarily in the                                                              larvae in upwelling
plankton)                                                                       systems (e.g., Asch,

Benthic invertebrates 72.34%        ­8.5 days per decade Low confidence         Evidence is limited,
                            (n=5)                                               uncertainty levels are
                                    (n=5)                 (limited evidence,    high. Rate of
                                                                                consistency of
                                                          medium agreement)     responses with climate
                                                                                change is not
                                                                                significantly different
                                                                                from random chance.

Plants              100%            No estimate available Very low confidence   Just a single study for
                    (n=1)                                                       seagrasses, and only
                                                                                for consistency (Diaz-
                                                                                Almela et al., 2007).

Fish                65.48%          ­3.02 days per decade Very high confidence Includes earlier

                    (n=109)         (n=43)                                      appearance of

                                                                                migratory fish in

                                                                                estuaries (e.g.,

                                                                                Chevillot et al., 2017),

                                                                                earlier spawning

                                                                                migrations for

                                                                                anadromous fish such

                                                                                as salmon (e.g.,

                                                                                Rubenstein et al.,

                                                                                2019), earlier

                                                                                migrations for sole

                                                                                (e.g., Fincham et al.,

                                                                                2013) and tuna (e.g.,

                                                                                Dufour et al., 2010),

                                                                                and earlier spawning

                                                                                of key commercial

                                                                                demersal (bottom-

                                                                                dwelling) species such

                                                                                as cod (e.g., McQueen

                                                                                and Marshall, 2017).

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FINAL DRAFT                               Chapter 3  IPCC WGII Sixth Assessment Report
Marine reptiles
                 100.0%           ­2.89 days per decade Low confidence  Evidence is limited,
                                  (n=4)              (limited evidence, low uncertainty levels are

                                                     agreement)         high. Mean

                                                                        phenological shift is

                                                                        not significantly

                                                                        different from zero.

Seabirds         42.36%           +0.77 days per decade Very low confidence Neither the rate of
                                  (n=51)             (limited evidence, low consistency with

                                                     agreement)         climate change nor the

                                                                        phenological shift

                                                                        differ significantly

                                                                        from null expectations

                                                                        (50% consistency and

                                                                        no shift). Many

                                                                        seabirds are breeding

                                                                        earlier (Byrd et al.,

                                                                        2008; Sydeman et al.,

                                                                        2009), while breeding

                                                                        among others in

                                                                        temperate and polar

                                                                        regions has been

                                                                        delayed, which has

                                                                        been linked to later

                                                                        sea-ice breakup or

                                                                        limited prey resources

                                                                        (Barbraud and

                                                                        Weimerskirch, 2006;

                                                                        Wanless et al., 2009;

                                                                        Chambers et al., 2014).

                                                                        Although the response

                                                                        of lifecycle events for

                                                                        many seabird species

                                                                        is variable in direction,

                                                                        there has usually been

                                                                        a more complex driver

                                                                        associated with climate

                                                                        that has been

                                                                        considered to be

                                                                        responsible (Sydeman

                                                                        et al., 2015). For many

                                                                        species, seasonal

                                                                        timing is moving

                                                                        earlier, especially in

                                                                        the Arctic (e.g., Byrd

                                                                        et al., 2008; Descamps

                                                                        et al., 2019), but for

                                                                        many species in the

                                                                        Southern Ocean, it is

                                                                        not (Barbraud and

                                                                        Weimerskirch, 2006;

                                                                        Chambers et al., 2014).

                                                                        This could be because

                                                                        of a much slower rate

                                                                        of warming in most of

                                                                        the Southern Ocean

                                                                        than in the Arctic.

Marine mammals   100.0%           ­0.34 days per decade Very low confidence All studies of
                                  (n=4)              (limited evidence, low phenological changes

                                                     agreement)         for marine mammals

                                                                        have focused on

                                                                        whales (e.g., Ramp et

                                                                        al., 2015; Hauser et al.,

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                                                                                                                           2017; Loseto et al.,
                                                                                                                           2018) or polar bears
                                                                                                                           (e.g., Cherry et al.,
                                                                                                                           2013; Atwood et al.,
                                                                                                                           2016; Escajeda et al.,
                                                                                                                           2018) and have related
                                                                                                                           timing to aspects of
                                                                                                                           sea ice dynamics,
                                                                                                                           highlighting the
                                                                                                                           complexity of such
                                                                                                                           processes. Mean
                                                                                                                           phenological shift is
                                                                                                                           not significantly
                                                                                                                           different from zero at
                                                                                                                           the global scale.



 3 Projected changes
 4 The CMIP6 ESM ensembles project that, by 2100, 18.8% ± 19.0% (mean ± very likely range) and 38.9% ±
 5 9.4% of the ocean surface will very likely undergo a change of 20 days or more (advance or delay) in the
 6 start of the phytoplankton growth period under SSP1-2.6 and SSP5-8.5, respectively (Figure 3.17a,b) (low
 7 confidence due to the dependence with the projected changes in phytoplankton biomass which trends are
 8 reported with low confidence) (Section and SROCC Section 5.2.3, Bindoff et al., 2019).
 9 Phytoplankton growth is projected to begin later in the Northern Hemisphere subtropics, and earlier at high
10 latitudes in some regions around the Antarctic Peninsula, and over large areas in the Northern Hemisphere
11 (low to medium confidence as there are improved constraints from historical variability in this region and
12 consistency with CMIP5-based studies results) (Henson et al., 2018b; Asch et al., 2019). There is high
13 agreement in model projections that the start of the phytoplankton growth period will very likely advance in
14 the Arctic Ocean under a high-emission scenario for CMIP5 and CMIP6 (Figure 3.17b, Henson et al., 2018b;
15 Asch et al., 2019; Tedesco et al., 2019; Lannuzel et al., 2020). The CMIP6 ensemble projections further
16 show limited changes in phenology across most of the Southern Ocean, but large regional variations in the
17 tropics (Figure 3.17). Overall, the regional patterns are qualitatively similar under SSP1-2.6 and SSP5-8.5,
18 but with greater magnitude and larger areas under SSP5-8.5 (low confidence).


20 At latitudes >40ºN, temperature-linked phenology of fish reproduction with high geographic fidelity to
21 spawning grounds (geographic spawners) is projected to change at double the rate of that for phytoplankton,
22 which will likely cause phenological mismatches resulting in increased risk of starvation for fish larvae
23 (medium to high confidence) (WGI AR6 Section, Asch et al., 2019; Durant et al., 2019; Régnier et
24 al., 2019; Gulev et al., 2021; Laurel et al., 2021). Furthermore, under RCP8.5, trophic mismatch events
25 exceeding ±30 days (Asch et al., 2019) leading to fish recruitment failure are expected to increase 10-fold for
26 geographic spawners across much of the North Atlantic, North Pacific and Arctic Ocean basins (low
27 confidence) (Neuheimer et al., 2018). In contrast, temporal mismatches between fish that relocate spawning
28 grounds in response to environmental variations (environmental spawners) and phytoplankton blooms are
29 projected to remain shorter and less varied, suggesting that across ocean basins, range shifts by
30 environmental spawners may increase their resilience. Nevertheless, this compensation mechanism might
31 fail at locations where phytoplankton bloom phenology is not controlled by temperature-driven water-
32 column stratification, leading to a possible six-fold local increase in extreme mismatches under climate
33 change (Asch et al., 2019).


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 2 Figure 3.17: Projected phytoplankton phenology. (a,c) Spatial patterns and (b,d) density distributions of projected
 3 change in phytoplankton phenology by 2100 under Shared Socioeconomic Pathway (SSP)1-2.6 and SSP5-8.5,
 4 respectively. Difference in the start of the phytoplankton growth period is calculated as 2090­2099 minus 1996­2013.
 5 Negative (positive) values indicate earlier (later) start of the phytoplankton growth period by 2100. The ensemble
 6 projections of global changes in phytoplankton phenology include, under SSP1-2.6 and SSP5-8.5, respectively, a total
 7 of five Coupled Model Intercomparison Project 6 Earth System Models containing coupled ocean biogeochemical
 8 models that cover a wide range of complexity (Kwiatkowski et al., 2019). The phenology calculations are based on
 9 Racault et al. (2017) using updated data.



12 Changes in Community Composition and Biodiversity


14 Evidence of natural adaptive capacity based on species' responses to past climate variability


16 Responses to abrupt climate change in the geological past suggest that adaptive capacity is limited for marine
17 animals (Cross-Chapter Box PALEO in Chapter 1). Temperatures during the last Interglacial (~125 ka),
18 which were warmer than today, led to poleward range shifts of reef corals (medium confidence) (Kiessling et
19 al., 2012; Jones et al., 2019a). Temperature has also driven marine range shifts over multi-million-year
20 timescales (medium confidence) (Gibbs et al., 2016; Reddin et al., 2018). Warming climates, even with low
21 ocean warming rates, inevitably decreased tropical marine biodiversity compared with mid-latitudes (high
22 confidence) (Mannion et al., 2014; Crame, 2020; Yasuhara et al., 2020; Raja and Kiessling, 2021).


24 The paleo record confirms that marine biodiversity has been vulnerable to climate warming both globally
25 and regionally (very high confidence) (Cross-Chapter Box PALEO in Chapter 1, Stanley, 2016). In extreme
26 cases of warming (e.g., >5.2°C), marine mass extinctions occurred in the geological past, and there may be a
27 relationship between warming magnitude and extinction toll (medium confidence) (Song et al., 2021b). A
28 combination of warming and spreading anoxia caused marine extinctions in ancient episodes of rapid climate
29 warming (high confidence) (Bond and Grasby, 2017; Benton, 2018; Penn et al., 2018; Them et al., 2018;
30 Chen and Xu, 2019). The role of ocean acidification in ancient extinctions is yet to be resolved (low
31 confidence) (Clapham and Payne, 2011; Clarkson et al., 2015; Jurikova et al., 2020; Müller et al., 2020).


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1 Community Structure and Biodiversity
2 Observed contemporary changes
3 Ocean temperature is a major driver of species richness in the global ocean at evolutionary timescales

4 (Tittensor et al., 2010; Chaudhary et al., 2021). This, together with temperature-driven range and phenology

5 shifts evident across taxa and ocean ecosystems (Sections,, suggests that recent ocean
6 warming (Section should alter biodiversity at regional to global scales. Since previous assessments
7 (Table 3.20), the most common evidence supporting these expected changes is replacement of cold-adapted

8 species by warm-adapted species within an ecosystem as waters warm (Worm and Lotze, 2021). Known as

 9 tropicalisation (Section, this phenomenon has been attributed to ocean warming (medium to high
10 confidence) in communities as diverse as kelp, invertebrates, plankton and fish (Burrows et al., 2019;
11 Flanagan et al., 2019; Ajani et al., 2020; Villarino et al., 2020; Punzón et al., 2021; Smith et al., 2021).



14 Table 3.20: Summary of previous IPCC assessments of community composition and biodiversity.

    Observations                                                Projections

    AR5: (Hoegh-Guldberg et al., 2014; Pörtner et al.,          Spatial shifts of marine species due to projected warming
    2014)                                                       will cause high-latitude invasions and high local-
    The paleoecological record shows that global climate        extinction rates in the tropics and semi-enclosed seas
    changes comparable in magnitudes to those projected for     (medium confidence).
    the 21st century under all scenarios resulted in large-
    scale biome shifts and changes in community                 Species richness and fisheries catch potential are
    composition; and that for rates projected under RCP6        projected to increase, on average, at mid and high
    and 8.5 were associated with species extinctions in some    latitudes (high confidence) and decrease at tropical
    groups (high confidence).                                   latitudes (medium confidence).

    Loss of corals due to bleaching has a potentially critical  Shifts in the geographical distributions of marine species
    influence on the maintenance of marine biodiversity in      cause changes in community composition and
    the tropics (high confidence).                              interactions. Thereby, climate change will reassemble
                                                                communities and affect biodiversity, with differences
                                                                over time and between biomes and latitudes (high

                                                                Models are currently useful for developing scenarios of
                                                                directional changes in net primary productivity, species
                                                                distributions, community structure, and trophic dynamics
                                                                of marine ecosystems, as well as their implications for
                                                                ecosystem goods and services under climate change.
                                                                However, specific quantitative projections by these
                                                                models remain imprecise (low confidence).

    SROCC (Bindoff et al., 2019)                                Poleward range shifts are projected to decrease species
    Ocean warming has contributed to observed changes in        richness in tropical oceans, counterbalanced by increases
    biogeography of organisms ranging from phytoplankton        in mid to high-latitude regions, leading to global-scale
    to marine mammals (high confidence), consequently           species turnover (medium confidence on trends, low
    changing community composition (high confidence), and       confidence on magnitude because of model uncertainties
    in some cases, altering interactions between organisms      and limited number of published model simulations).
    and ecosystem function (medium confidence).                 The projected intensity of species turnover is lower
                                                                under low-emission scenarios (high confidence).

                                                                Projections from multiple fish species distribution
                                                                models show hotspots of decrease in species richness in
                                                                the Indo-Pacific region, and semi-enclosed seas such as
                                                                the Red Sea and Persian Gulf (medium evidence, high
                                                                agreement). In addition, geographic barriers, such as
                                                                land, bounding the poleward species range edge in semi-
                                                                enclosed seas or low-oxygen water in deeper waters are
                                                                projected to limit range shifts, resulting in larger relative
                                                                decrease in species richness (medium confidence).

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                                                                                 The large variation in sensitivity of different zooplankton
                                                                                 taxa to future conditions of warming and ocean
                                                                                 acidification suggests elevated risk to community
                                                                                 structure and inter-specific interactions of zooplankton in
                                                                                 the 21st century (medium confidence).



 3 At local to regional scales, tropicalisation often increases species richness where warm-water species extend
 4 their ranges to overlap with existing communities, and decreases species richness where warming waters
 5 extirpate species (medium to high confidence) (Friedland et al., 2020a; Chaudhary et al., 2021; Worm and
 6 Lotze, 2021). Latitudinal estimates from catalogued observations show declining species richness in
 7 equatorial waters over the past 50 years, with concomitant increases in species richness at mid-latitudes; the
 8 pattern is especially prominent in free-swimming pelagic species (Figure 3.18, Chaudhary et al., 2021).
 9 Similar patterns among marine animals have been described previously for historical warming events (Song
10 et al., 2020b). Tropicalisation is associated with increased representation of herbivorous species (Vergés et
11 al., 2016; Zarco-Perello et al., 2020; Smith et al., 2021), although observations and theory suggest that
12 dietary generalism can also favour range-shifting species (Monaco et al., 2020; Wallingford et al., 2020).


14 Projected changes
15 At the community level, the magnitude and shape of biodiversity changes differ, depending on what groups
16 are considered (medium confidence) (Chaudhary et al., 2021). Molecular-based richness measures indicate
17 that the most dramatic increases in diversity relative to current conditions are expected for photosynthetic
18 eukaryotes and copepods in the Arctic Ocean (Ibarbalz et al., 2019). However, component eukaryotic taxa,
19 for example diatoms Busseni et al. (2020), are projected to lose diversity by 2100 under RCP8.5. Ecosystem
20 models project a decline in nutrient supply that drives the disappearance of less-competitive and larger
21 phytoplankton types, leading to extinction of up to 30% of diatom types, particularly in the northern
22 hemisphere, by 2100 under RCP8.5 (Henson et al., 2021). Models further suggest that high latitudes are
23 likely to encounter entirely novel phytoplankton communities by 2100 under RCP8.5 (100% change in
24 community composition, Dutkiewicz et al., 2019; Reygondeau et al., 2020). At the polar edges, the increased
25 richness is projected to coincide with high species turnover and increasing dominance of smaller
26 phytoplankton types (Henson et al., 2021). These imply pronounced changes to both the oceans' ecological
27 and biogeochemical function, as regions dominated by small phytoplankton typically support less-productive
28 food webs (Section, Stock et al., 2017; Armengol et al., 2019) and sequester less particulate organic
29 carbon in the deep ocean (Section, Mouw et al., 2016; Cram et al., 2018) than areas dominated by
30 larger size classes (high confidence).


32 The profound climatic and environmental changes projected for the Arctic region by 2100 (Cross-Chapter
33 Paper 6) are also anticipated to alter the composition of apex assemblages like marine mammals (Albouy et
34 al., 2020, Box 3.2). Under both RCP2.6 and 8.5 scenarios the most vulnerable marine mammal species will
35 be the North Pacific right whale (Eubalaena japonica, listed as an endangered species (IUCN, 2020)) and the
36 gray whale (Eschrichtius robustus, which has critically endangered subpopulations (IUCN, 2020)). The
37 extinction of the most-vulnerable species will disproportionately eliminate unique and important
38 evolutionary lineages as well as functional diversity, with consequent impacts throughout the entire marine
39 ecosystem (section 3.3.4). More generally, future warming and acidification simulated in mesocosm
40 experiments support projections of a substantial increase in biomass and productivity of primary producers
41 and secondary consumers, but a decrease by >40% of primary consumers (Nagelkerken et al., 2020). On
42 longer time scales, alteration of energy flow through marine food webs may lead to ecological tipping points
43 (Wernberg et al., 2016; Harley et al., 2017) after which the food web collapses into shorter, bottom-heavy
44 trophic pyramids (medium confidence).


46 Global projections anticipate a likely future reorganisation of marine life of variable magnitude, contingent
47 on emission scenario (Beaugrand et al., 2015; Jones and Cheung, 2015; Barton et al., 2016; García Molinos
48 et al., 2016; Nagelkerken et al., 2020; Henson et al., 2021). Marine organism redistributions projected under
49 RCP4.5 and RCP8.5 include extirpations and range contractions in the tropics, strongly decreasing tropical
50 biodiversity, and range expansions at higher latitudes, associated with increased diversity and
51 homogenisation of marine communities (Figure 3.18b). Under continuing climate change, the projected loss
52 of biodiversity may ultimately threaten marine ecosystem stability (medium confidence) (Albouy et al., 2020;

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 1 Nagelkerken et al., 2020; Henson et al., 2021), altering both the functioning and structure of marine
 2 ecosystems and thus affecting service provisioning (medium confidence) (Section 3.5, Ibarbalz et al., 2019;
 3 Righetti et al., 2019).


 5 However, biodiversity observations remain sparse, and statistical and modelling tools can provide conflicting
 6 diversity information (e.g., Righetti et al., 2019; Dutkiewicz et al., 2020) because correlative approaches
 7 assume that the modern-day relationship between marine species distribution and environmental conditions
 8 remains the same into the future, whereas mechanistic models permit marine species to respond dynamically
 9 to changing environmental forcing. Moreover, existing global projections of future biodiversity
10 disproportionately focus on the effects sea surface temperature (Thomas et al., 2012), typically overlooking
11 other factors such as ocean acidification, deoxygenation and nutrient availability (Section 3.2.3), and often
12 failing to account for natural adaptation (e.g., Section 3.3.4, Box 3.1, Barton et al., 2016; Henson et al.,
13 2021).




17 Figure 3.18: Changes in latitudinal marine species richness latitudinal distribution. (a) Observed species richness for
18 three historical periods. The observed latitudinal patterns in species richness, are for a suite of taxonomic groups based
19 on 48,661 marine species (Chaudhary et al., 2021). (b) Projected changes in species richness under RCP4.5 and RCP8.5
20 calculated as differences by grid cell by 2100 relative to 2006. Latitudinal global median (5° moving average) (based on
21 Figure 1b,c in García Molinos et al., 2016). The projected latitudinal patterns in changes in species richness under climate
22 change are based on a numerical model that includes species-specific information across a suite of taxonomic groups,
23 based on 12,796 marine species (García Molinos et al., 2016).





28 Box 3.2: Marine Birds and Mammals


30 Marine birds (seabirds and shorebirds) and mammals include charismatic species and species that are
31 economically, culturally and ecologically important (Sydeman et al., 2015; Albouy et al., 2020; Pimiento et
32 al., 2020). Their long generation times and slow population growth suggests limited evolutionary resilience
33 to rapid climate change (Section 3.3.4, Sydeman et al., 2015; Miller et al., 2018). According to the Red List
34 Species Assessments of the International Union for Conservation of Nature (IUCN, 2020), the greatest
35 current hazards to these groups include human use of biological resources and areas, invasive species and
36 pollution (Figure Box3.2.1, Dias et al., 2019; Lusseau et al., 2021). Impacts of climate change and severe
37 weather are ranked among the five most-important hazards, influencing 131 and 45 bird and mammal
38 species, respectively (see Figure Box 3.2.1 for selection of species), including 24 bird and seven mammal
39 species that are currently listed as endangered, critically endangered or threatened. Furthermore, according to

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 1 these IUCN assessments, climate change and severe weather are expected to impact an additional 122 and 18
 2 marine bird and mammal species over the next 50­100 years, respectively (Figure Box 3.2.1 Dias et al.,
 3 2019).


 5 Marine birds and mammals are vulnerable to climate-induced loss of breeding and foraging habitats such as
 6 sea ice (Section, sandy beaches (Section, salt marshes (Section and seagrass beds
 7 (high confidence) (Section, Sydeman et al., 2015; Bindoff et al., 2019; Ropert-Coudert et al., 2019;
 8 Von Holle et al., 2019; Albouy et al., 2020; Amano et al., 2020; Bestley et al., 2020; Grose et al., 2020).
 9 With warming, shorebird population abundances decline in the tropics, likely due to heat stress and habitat
10 loss, and increase at higher latitudes (Amano et al., 2020). Marine mammals dependent on sea-ice habitat are
11 particularly vulnerable to warming (medium confidence) (Albouy et al., 2020; Bestley et al., 2020; Lefort et
12 al., 2020), yet vulnerability can differ between populations. Ongoing sea-ice loss is decreasing some polar
13 bear populations while others remain stable, likely related to past harvesting history, regional differences in
14 sea-ice phenology and ecosystem productivity (Hamilton and Derocher, 2019; Molnár et al., 2020).
15 Nevertheless, even under an intermediate emission scenario RCP4.5, increasing ice-free periods will likely
16 reduce both recruitment and adult survival across most polar bear populations over the next four decades,
17 threatening their existence (medium confidence) (Figure Box3.2.2, Molnár et al., 2020).


19 Climate change is affecting marine food-web dynamics (high confidence) (Sections 3.4.2, 3.4.3), and the
20 vulnerability and adaptive capacity of marine birds and mammals to such changes is linked to the species'
21 breeding and feeding ecology. Higher-vulnerability species include central-place foragers (confined to, for
22 example, breeding colonies fixed in space), diet and habitat specialists, and species with restricted
23 distributions such as marine mammal populations in SES (medium confidence) (McMahon et al., 2019;
24 Ropert-Coudert et al., 2019; Albouy et al., 2020; Grose et al., 2020; Sydeman et al., 2021). Surface-feeding
25 and piscivorous marine birds appear to be more vulnerable to food-web changes than diving seabirds and
26 planktivorous seabirds (medium confidence) (Sydeman et al., 2021). During the 2014­2015 Pacific
27 heatwave, around one million piscivorous common murres died along a 1500 km coastal stretch in the
28 Pacific USA due to reduced prey availability (Jones et al., 2018b; Piatt et al., 2020). Marine birds are
29 vulnerable to phenological shifts in food-web dynamics, as they have limited phenotypic plasticity of
30 reproductive timing, with potentially little scope for evolutionary adaptation (medium confidence) (Keogan
31 et al., 2018), although changes in reproduction timing are observed in several species (Section,
32 Sydeman et al., 2015; Descamps et al., 2019; Sauve et al., 2019). There is limited evidence of marine
33 mammals' capacity to adapt to shifting phenologies, but observed responses include changes in the onset of
34 migrations, moulting and breeding (Section, Ramp et al., 2015; Hauser et al., 2017; Beltran et al.,
35 2019; Bowen et al., 2020; Szesciorka et al., 2020).


37 Increased emergence of infectious disease in mammals and birds is expected with ocean warming, due to
38 new transmission pathways from changing species distributions, higher species densities caused by habitat
39 loss, and increased vulnerability due to environmental stress on individuals (limited evidence) (Sydeman et
40 al., 2015; VanWormer et al., 2019; Sanderson and Alexander, 2020). Marine birds and mammals are likely to
41 suffer from increased mortalities due to increasing frequencies of HABs, and of extreme weather, at sea, on
42 sea ice, and in terrestrial breeding habitats (Broadwater et al., 2018; Gibble and Hoover, 2018; Ropert-
43 Coudert et al., 2019; Grose et al., 2020). Also, climate-change driven distributional shifts have strengthened
44 interactions with other anthropogenic impacts, through, for example, increasing risks of ship strikes and
45 bycatch (medium confidence) (e.g., Hauser et al., 2018; Krüger et al., 2018; Record et al., 2019; Santora et
46 al., 2020).



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 2 Figure Box 3.2.1: Hazard assessment for marine birds and mammals. Number of (a) marine birds and (b) mammals
 3 currently impacted by different hazards (blue), and numbers of additional species expected to be exposed to these
 4 threats over the next 50­100 years (red), as assessed in the International Union for Conservation of Nature Red List
 5 (IUCN, 2020). Seabird species include species in the key orders Sphenisciformes, Pelecaniformes, Suliformes,
 6 Anseriformes, Procellariiformes and Charadriiformes categorised as inhabitants of marine ecosystems (n = 483 species,
 7 assessed in the period 2016­2019). Marine mammal species include the species reviewed by Lusseau et al. (2021) (n =
 8 136 species, assessed in the period 2008­2019).



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2 Figure Box 3.2.2: Modelled risk timelines for demographic impacts on circumpolar polar bear subpopulations, and
3 associated confidence assessments, due to extended fasting periods with loss of sea ice. Years of first impact on cub
4 recruitment (yellow), adult male survival (blue) and adult female survival (red) are shown for the (a) RCP4.5 and (b)
5 RCP8.5. Data from Molnár et al. (2020).



8 [END BOX 3.2 HERE]



11 Abrupt ecosystem shifts and extreme events


13 Climate-change driven changes in ocean characteristics and the frequency and intensity of extreme events
14 (Section 3.2) increase the risk of persistent, rapid and abrupt ecosystem change (very high confidence), often
15 referred to as ecosystem collapses or regime shifts (AR6 WGI Chapter 9, Collins et al., 2019a; Canadell and
16 Jackson, 2021; Ma et al., 2021). Such abrupt changes include altering ecosystem structure, function and
17 biodiversity outside the range of natural fluctuations (Collins et al., 2019a; Canadell and Jackson, 2021).
18 They can involve mass mortality events and `tipping points' or `critical transitions,' where strong positive
19 feedbacks within an ecosystem lead to self-sustaining change (Figure 3.19a, Scheffer et al., 2012; Möllmann
20 et al., 2015; Biggs et al., 2018). Abrupt ecosystem shifts have been observed in both large open-ocean
21 ecosystems and coastal ecosystems (Section 3.4.2) with dramatic social consequences through significant
22 loss of diverse ecosystem services (high confidence) (Section 3.5, Biggs et al., 2018; Pinsky et al., 2018;
23 Beaugrand et al., 2019; Collins et al., 2019a; Filbee-Dexter et al., 2020b; Huntington et al., 2020; Trisos et
24 al., 2020; Turner et al., 2020b; Canadell and Jackson, 2021; Ma et al., 2021; Ruthrof et al., 2021). A
25 summary of previous assessments of abrupt ecosystem shifts and extreme events are provided in Table 3.21.



28 Table 3.21: Summary of previous IPCC assessments of observed and projected abrupt ecosystem shifts and extreme
29 events.

    Observations                                         Projections

    AR5 (Wong et al., 2014)

    Observations of abrupt ecosystem shifts and extreme  Warming and acidification will lead to coral bleaching,
    events were not assessed in this report.             mortality, and decreased constructional ability (high
                                                         confidence), making coral reefs the most vulnerable marine
                                                         ecosystem with little scope for adaptation. Temperate
                                                         seagrass and kelp ecosystems will decline with the
                                                         increased frequency of heatwaves and sea temperature
                                                         extremes as well as through the impact of invasive
                                                         subtropical species (high confidence).

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SROCC (Collins et al., 2019a)

Marine heatwaves (MHWs), periods of extremely high Marine heatwaves are projected to further increase in

ocean temperatures, have negatively impacted marine frequency, duration, spatial extent and intensity (maximum

organisms and ecosystems in all ocean basins over the last temperature) (very high confidence). Climate models

two decades, including critical foundation species such as project increases in the frequency of marine heatwaves by

corals, seagrasses and kelps (very high confidence)  2081­2100, relative to 1850­1900, by approximately 50

                                                     times under RCP8.5 and 20 times under RCP2.6 (medium


                                                     Extreme El Niño and La Niña events are projected to likely
                                                     increase in frequency in the 21st century and to likely
                                                     intensify existing hazards, with drier or wetter responses in
                                                     several regions across the globe. Extreme El Niño events
                                                     are projected to occur about as twice as often under both
                                                     RCP2.6 and RCP8.5 in the 21st century when compared to
                                                     the 20th century (medium confidence).

                                                                                Limiting global warming would reduce the risk of impacts
                                                                                of MHWs, but critical thresholds for some ecosystems
                                                                                (e.g., kelp forests, coral reefs) will be reached at relatively
                                                                                low levels of future global warming (high confidence)



 3 Abrupt ecosystem shifts are associated with large-scale patterns of climate variability (Alheit et al., 2019;
 4 Beaugrand et al., 2019; Lehodey et al., 2020), some of which are projected to intensify with climate change
 5 (medium confidence) (WGI AR6 Chapter 1, Wang et al., 2017a; Collins et al., 2019a; Chen et al., 2021).
 6 Over the past 60 years, abrupt ecosystem shifts have generally followed El Niño/Southern Oscillation events
 7 of any strength, but some periods had geographically limited ecological shifts (~0.25% of the global ocean in
 8 1984­1987) and others more extensive shifts (14% of the global ocean in 2012­2015) (medium confidence)
 9 (Figure 3.19b, Beaugrand et al., 2019). Typically, interacting drivers, such as eutrophication and overharvest,
10 reduce ecosystem resilience to climate extremes (e.g., MHW, cyclones) or gradual warming, and hence
11 promote ecosystem shifts (high confidence) (Figure 3.19a, Rocha et al., 2015; Biggs et al., 2018; Babcock et
12 al., 2019; Turner et al., 2020b; Bergstrom et al., 2021; Canadell and Jackson, 2021; Tait et al., 2021). Also,
13 shifts in different ecosystems may be connected through common drivers or through cascading effects
14 (medium confidence) (Rocha et al., 2018a).


16 Recent MHWs (Section have caused major ecosystem shifts and mass mortality in oceanic and
17 coastal ecosystems, including corals, kelp forests and seagrass meadows (Sections,,,
18,, Cross-Chapter Box MOVING SPECIES in Chapter 5 and Cross-Chapter Box EXTREMES
19 in Chapter 2), with dramatic declines in species foundational for habitat formation or trophic flow,
20 biodiversity declines, and biogeographic shifts in fish stocks (very high confidence) (Table 3.15, Cross-
21 Chapter Box MOVING SPECIES in Chapter 5, Canadell and Jackson, 2021). Three major bleaching
22 episodes on Australia's Great Barrier Reef in 5 years corresponded with extreme temperatures in 2016, 2017
23 and 2020 (Pratchett et al., 2021). Between 1981 and 2017, marine heatwaves have increased more than 20-
24 fold due to anthropogenic climate change (Section, WGI AR6 Chapter 9, Laufkötter et al., 2020;
25 Fox-Kemper et al., 2021), increasing the risk of abrupt ecosystem shifts (high confidence) (Figure 3.19a,
26 Cross-Chapter Box EXTREMES in Chapter 2, van der Bolt et al., 2018; Garrabou et al., 2021; Wernberg,
27 2021).


29 Ecosystems can recover from abrupt shifts (e.g., Babcock et al., 2019; Christie et al., 2019; Medrano et al.,
30 2020). However, where climate change is a dominant driver, ecosystem collapses increasingly cause
31 permanent transitions (high confidence), although the extents of such transitions depend on emission
32 scenario (Trisos et al., 2020; Garrabou et al., 2021; Klein et al., 2021; Pratchett et al., 2021; Wernberg,
33 2021). Over the coming decades, MHW are projected to very likely become more frequent under all emission
34 scenarios (Section 3.2, WGI AR6 Chapter 9, Fox-Kemper et al., 2021), with intensities and rates too high for
35 recovery of degraded foundational species, habitats, or biodiversity (medium confidence) (Babcock et al.,
36 2019; Garrabou et al., 2021; Klein et al., 2021; Serrano et al., 2021; Wernberg, 2021). Emission pathways
37 that result in temperature overshoot above 1.5oC will increase the risks of abrupt and irreversible shifts in
38 coral reefs and other vulnerable ecosystems (Section 3.4.4).

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 3 Figure 3.19: Observed ecological regime shifts and their drivers in the oceans. (a) A conceptual representation of
 4 ecosystem resilience and regime shifts. Shift from Regime 1 to Regime 2 can be triggered by either a large shock (i.e.,
 5 an abrupt environmental transition) or gradual internal or external change that erodes the dominant balancing feedbacks,
 6 reducing ecosystem resilience (indicated by the shallower dotted line, relative to the deeper `valley' reflecting higher
 7 resilience). Figure based on Biggs et al. (2018). (b) The sum of the magnitude and extent of the abrupt community shifts
 8 that has been estimated at each geographic cell in the global ocean during 1960­2014, calculated as the ratio of the
 9 amplitude of the change in a particular year to the average magnitude of the change over the entire time series (thus is
10 dimensionless). Figure based on Beaugrand et al. (2019).



13 Time of emergence ­ species exposure to altered environments
14 Since SROCC, more studies have assessed the time of emergence for climate-impact drivers (Section 3.2.3),
15 and the ecosystem attributes through which the impacts manifest. However, as in previous assessments
16 (Table 3.22), the time of emergence for a given driver or ecosystem attribute depends on the reference
17 period, the definition of the signal emergence threshold and the spatial and temporal scales considered (Box
18 5.1 in SROCC, Kirtman et al., 2013; Bindoff et al., 2019).



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1 Table 3.22: Summary of previous IPCC assessments of projected time of emergence.

System                            Projections

Coastal                           Multiple climate-impact drivers will emerge in the 21st
Epipelagic                        century under RCP8.5, while the time of emergence will
Open Ocean                        be later and with few climatic hazards under RCP2.6. Non-
                                  climate impacts such as eutrophication add to, and in some
Deep sea                          cases, exacerbate these large-scale slow climate drivers
                                  beyond biological thresholds at local scale (e.g.,

                                  Observed range shifts in response to climate change in
                                  some regions such as the north Atlantic are strongly
                                  influenced by warming due to both multi-decadal climate
                                  change and variability, suggesting that there is a longer
                                  time-of-emergence of range shifts from natural variability
                                  and a need for longer biological time series for robust

                                  The timing for five primary drivers of marine ecosystem
                                  change (surface warming and acidification, oxygen loss,
                                  nitrate concentration and net primary production change)
                                  are all prior to 2100 for >60% of the ocean area under
                                  RCP8.5 and over 30% under RCP2.6 (very likely).
                                  Anthropogenic signals are expected to remain detectable
                                  over large parts of the ocean even under the RCP2.6
                                  scenario for pH and SST but are likely to be less
                                  conspicuous for nutrients and NPP in the 21st century. For
                                  example, for the open ocean, the anthropogenic pH signal
                                  in Earth System Models (ESM) historical simulations is
                                  very likely to have emerged for three-quarters of the ocean
                                  prior to 1950 and it is very likely over 95% of the ocean
                                  has already been affected, with little discernible difference
                                  between scenarios. The climate signal of oxygen loss will
                                  very likely emerge by 2050 with a very likely range of 59­
                                  80% by 2031­2050 and increasing with a very likely range
                                  of 79­91% of the ocean area by 2081­2100 (RCP8.5
                                  emissions scenario). The emergence of oxygen loss is
                                  smaller in area under RCP2.6 scenario in the 21st century
                                  and by 2090 the area where emergence is evident is
                                  declining. It has also been shown that signatures of altered
                                  oxygen solubility or utilisation may emerge earlier than for
                                  oxygen levels.

                                  Emergence of risk is expected to occur later at around the
                                  mid-21st century under RCP8.5 for abyssal plain and
                                  chemosynthetic ecosystems (vents and seeps). All deep
                                  seafloor ecosystems are expected to be subject to at least
                                  moderate risk under RCP8.5 by the end of the 21st century,
                                  with cold water corals undergoing a transition from
                                  moderate to high risk below 3ºC.



 4 Anthropogenically driven changes in chlorophyll-a concentrations across an ensemble of 30 ESMs are
 5 expected to exceed natural variability under RCP8.5 by 2100 in ~65­80% of the global oceans, when the
 6 natural variability is calculated using the ensemble's standard deviation (Schlunegger et al., 2020). However,
 7 if two standard deviations are used, then significant trends in chlorophyll-a concentration are expected under
 8 RCP8.5 across ~31% of the global oceans by 2100 (Dutkiewicz et al., 2019). In contrast, the anthropogenic
 9 signal in phytoplankton community structure, which has a lower natural variability, will emerge under
10 RCP8.5 across 63% of the ocean by 2100 when two standard deviations are used (limited evidence)
11 (Dutkiewicz et al., 2019).


13 The time of emergence of climate impacts on ecosystems will be modulated jointly by species-specific
14 adaptation potential (Section 3.3.4, Jones and Cheung, 2018; Collins et al., 2020; Gamliel et al., 2020; Miller

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 1 et al., 2020a), speed of range shifts and spatial reorganisation (high confidence) (Sections 3.3, 3.4.2,3.4.3).
 2 These ecosystem responses complicate projections of the time of emergence of environmental properties that
 3 impact biogeochemical cycling (Schlunegger et al., 2019; Schlunegger et al., 2020; Wrightson and
 4 Tagliabue, 2020), ecosystem structure and biodiversity (Figure 3.20a,c, Dutkiewicz et al., 2019; Trisos et al.,
 5 2020), and higher trophic levels, including fisheries targets (Cheung and Frölicher, 2020). Better accounting
 6 for multiple interacting factors in ESMs (Box 3.1), which will provide insight into how marine ecosystems
 7 will respond to future climate (high confidence).


 9 The time of emergence of ecosystem responses supports planning for specific time-bound actions to reduce
10 risks to ecosystems (Sections Bruno et al., 2018; Trisos et al., 2020). Although under RCP 8.5,
11 climate refugia from SST after 2050 are primarily in the Southern Ocean in tropical waters, these refugia are
12 mainly from deoxygenation (Bruno et al., 2018). Marine assemblages in these places will be exposed to
13 unprecedented temperatures after 2050, peaking in 2075 (Figure 3.20a,b, Trisos et al., 2020)(). In contrast,
14 changes in phytoplankton community structure will emerge earlier, and primarily in the Pacific Ocean
15 subtropics, and through much of the North Atlantic Ocean (Figure 3.20c,d, Dutkiewicz et al., 2019). Under
16 RCP8.5, changes in phytoplankton community structure and, to a lesser extent, exposure of marine species to
17 unprecedented temperatures, will emerge earlier in MPAs, covering ~7.7% of the global oceans, (Section
18, UNEP-WCMC and IUCN, 2020; UNEP-WCMC and IUCN, 2021) compared to non-MPAs
19 (Figure 3.20b,d). Such assessment can support planning for future MPA placement and extent. Because
20 MPAs can serve as refugia from non-climate drivers (Sections,, they facilitate opportunities
21 for adaptation among marine species and communities in coastal oceans (Section 3.4.2).




25 Figure 3.20: Time of exposure to altered environments. (a) Simulated spatial variation in the time of exposure of
26 marine species to unprecedented temperatures under RCP8.5. Time of exposure is quantified as the median year after
27 which local species are projected to encounter temperatures warmer than the historical maximum within their full
28 geographic range for a period of at least five years. This estimate is based on 22 Coupled Model Intercomparison
29 Project 5 (CMIP5) models, and is drawn from data presented by Trisos et al. (2020). Only regions that have times of
30 emergence by 2100 are shown. (b) The distribution in the time of exposure to unprecedented temperatures within
31 marine assemblages (Trisos et al., 2020) under RCP8.5 in marine protected areas (in turquoise) and in non-marine
32 protected areas (in purple). Values were calculated after regridding to equal-area 0.5° hexagons. (c) Time of emergence
33 for phytoplankton community structure changes (based on a proxy ­ ecosystem-model reflectance at 500 nm) under
34 RCP8.5. Only regions with statistically significant (p <0.05) trends, that are presently largely ice-free and that have
35 times of emergence by 2100 are shown. Figure based on the results of one model numerical model from Dutkiewicz et
36 al. (2019). (d) The distribution in the time of emergence for changes in phytoplankton community structure (same proxy
37 as in Panel c) (Dutkiewicz et al., 2019) under RCP8.5 in marine protected areas (in turquoise) and in non-marine
38 protected areas (in purple). Values were calculated after regridding to equal-area 0.5° hexagons.



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1 Biomass


3 Observed changes
4 Observed changes in biomass in the global ocean, beyond those for phytoplankton (Table 3.23), have not
5 routinely been attributed to climate-impact drivers, but rather to the compound effects of multiple drivers,
6 especially fishing (Christensen et al., 2014; Palomares et al., 2020). We therefore do not assess observed
7 changes in ocean biomass here.


 9 Projected changes
10 Zooplankton biomass
11 Based on an ensemble of CMIP5 ESMs, SROCC projected declines in global zooplankton biomass by 2100
12 dependent on emission scenario (low confidence) (Table 3.23). The new CMIP6 ESM ensemble projects a
13 decline in global zooplankton biomass by ­3.9% ± 8.2%, (very likely range) and ­9.0% ± 8.9% in the period
14 2081­2100 relative to 1995­2014 under SSP1-2.6 and SSP5-8.5, respectively (Figure 3.21d, Kwiatkowski et
15 al., 2020), thus reinforcing the SROCC assessment albeit with greater inter-model uncertainties.



18 Table 3.23: Summary of previous IPCC assessments of changes in open ocean and deep sea biomass.

    Measure                            Observations                               Projections

    AR5 WGII: (Hoegh-Guldberg et al.,
    2014; Pörtner et al., 2014)

    Chlorophyll-a/phytoplankton biomass Phytoplankton biomass: The                Owing to contradictory observations

                                       approximately 15-year archived time there is currently uncertainty about the

                                       series of satellite-chlorophyll (as a future trends of major upwelling

                                       proxy of phytoplankton biomass) is too systems and how their drivers

                                       short to reveal trends over time and (enhanced productivity, acidification,

                                       their causes (WGII AR5 Section 6.1.2, and hypoxia) will shape ecosystem

                                       Pörtner et al., 2014).                     characteristics (low confidence) (WGII

                                                                                  AR5 Chapter 6 Executive Summary,

                                       Chlorophyll concentrations measured Pörtner et al., 2014).

                                       by satellites have decreased in the

                                       subtropical gyres of the North Pacific,

                                       Indian, and North Atlantic Oceans by

                                       9%, 12%, and 11%, respectively, over

                                       and above the inherent seasonal and

                                       interannual variability from 1998 to

                                       2010 (high confidence; p-value 

                                       0.05). Significant warming over this

                                       period has resulted in increased water

                                       column stratification, reduced mixed

                                       layer depth, and possibly decreases in

                                       nutrient availability and ecosystem

                                       productivity (limited evidence, medium

                                       agreement). The short time frame of

                                       these studies against well-established

                                       patterns of long-term variability leads

                                       to the conclusion that these changes

                                       are about as likely as not due to climate

                                       change (WGII AR5 Chapter 30

                                       Hoegh-Guldberg et al., 2014).

    Animal biomass                     The climate-change-induced
                                       intensification of ocean upwelling in
                                       some eastern boundary systems, as
                                       observed in the last decades, may lead
                                       to regional cooling rather than
                                       warming of surface waters and cause
                                       enhanced productivity (medium
                                       confidence), but also enhanced
                                       hypoxia, acidification, and associated

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                                                biomass reduction in fish and
                                                invertebrate stocks.

SROCC (Bindoff et al., 2019)

Chlorophyll-a/phytoplankton biomass Changes reported in overall open
                                                ocean chlorophyll levels (a proxy of
                                                phytoplankton biomass) of <±1% yr­1
                                                for individual time periods.
                                                Regionally, trends of ±4% between
                                                2002­2015 for different regions are
                                                found when different satellite products
                                                are merged, with increases at high
                                                latitudes and moderate decreases at
                                                low latitudes (SROCC Section,
                                                Bindoff et al., 2019).

Animal biomass                    Observed changes in open ocean and There is high agreement in model

WGI AR6 Chapter 2 (Gulev et al.,  deep sea biomass were not assessed in projections that global zooplankton
                                  this report.                                           biomass will very likely reduce in the

                                                                                         21st century, with projected decline

                                                                                         under RCP8.5 almost doubled that of

                                                                                         RCP2.6 (very likely). However, the

                                                                                         strong dependence of the projected

                                                                                         declines on phytoplankton production

                                                                                         (low confidence) and simplification in

                                                                                         representation of the zooplankton

                                                                                         communities and foodweb render their

                                                                                         projections having low confidence.

                                                                                         The global biomass of marine animals,
                                                                                         including those that contribute to
                                                                                         fisheries, is projected to decrease by
                                                                                         4.3 ± 2.0% (95% confidence intervals)
                                                                                         and 15.0 ± 5.9% under RCP2.6 and
                                                                                         RCP8.5, respectively, by 2080­2099
                                                                                         relative to 1986­2005, while the
                                                                                         decrease is around 4.9% by 2031­2050
                                                                                         across all RCP2.6 and RCP8.5 (very
                                                                                         likely). Regionally, total animal
                                                                                         biomass decreases largely in tropical
                                                                                         and mid-latitude oceans (very likely).

                                                                                         Projected decrease in upper ocean
                                                                                         export of organic carbon to the deep
                                                                                         seafloor is expected to result in a loss
                                                                                         of animal biomass on the deep seafloor
                                                                                         by 5.2­17.6% by 2090­2100
                                                                                         compared to the present (2006­2015)
                                                                                         under RCP8.5 with regional variations
                                                                                         (medium confidence). Some increases
                                                                                         are projected in the polar regions, due
                                                                                         to enhanced stratification in the surface
                                                                                         ocean, reduced primary production and
                                                                                         shifts towards small phytoplankton
                                                                                         (medium confidence). The projected
                                                                                         impacts on biomass in the abyssal
                                                                                         seafloor are larger under RCP8.5 than
                                                                                         RCP4.5 (very likely).

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Chlorophyll-a/phytoplankton biomass The multi-sensor time series of                   Projected changes in open ocean and
                                                chlorophyll-a concentration has been  deep sea biomass were not assessed in
                                                updated to cover two decades (1998­   this report.

                                  Global trends in chlorophyll-a for the
                                  last two decades are insignificant over
                                  large areas of the global oceans, but
                                  some regions exhibit significant trends,
                                  with positive trends in parts of the
                                  Arctic and the Antarctic waters (>3%
                                  yr­1), and both negative and positive
                                  trends (within ±3% yr­1), in parts of
                                  the tropics, subtropics and temperate

                                                        In the last two decades, the
                                                        concentration of phytoplankton at the
                                                        base of the marine food web, as
                                                        indexed by chlorophyll concentration,
                                                        has shown weak and variable trends in
                                                        low and mid-latitudes and an increase
                                                        in high latitudes (medium confidence).



 3 Marine animal biomass
 4 Using an ensemble of global-scale marine ecosystem and fisheries models (Fish-MIP, Tittensor et al., 2018)
 5 with the CMIP5 ESM ensemble, SROCC concludes that projected ocean warming and decreased
 6 phytoplankton production and biomass will reduce global marine animal biomass during the 21st century
 7 (medium confidence). The simulated declines (with very likely range) are ­3.5 ± 4.8% and ­14.0 ± 14.6%
 8 under RCP2.6 and RCP8.5, respectively, by 2080­2099 relative to 1995­2014 (SROCC Section 5.2.3,
 9 Bindoff et al., 2019; Lotze et al., 2019)1. Updated Fish-MIP simulations with CMIP6 (Figure 3.21g,h,i)
10 confirm the projected decline in total marine animal biomass in the 21st century (Tittensor et al., 2021). The
11 simulated declines (with very likely range) are ­5.7% ± 4.1% and ­15.5% ± 8.5% under SSP1-2.6 and SSP5-
12 8.5, respectively, by 2080­2099 relative to 1995­2014 (Figure 3.21g), showing greater declines and lower
13 inter-model uncertainties (Tittensor et al., 2021). These declines result from combined warming and
14 decreased primary production (with low confidence in future changes in primary production, Section
15 and are amplified at each trophic level within all ESM and marine ecosystem model projections across all
16 scenarios (medium confidence) (Kwiatkowski et al., 2019; Lotze et al., 2019; Tittensor et al., 2021).
17 However, there is limited evidence about how underlying food-web mechanisms amplify the climate signal
18 from primary producers to higher trophic levels, and several putative mechanisms have been proposed
19 (Section, Chust et al., 2014a; Stock et al., 2014; Kwiatkowski et al., 2019; Lotze et al., 2019;
20 Heneghan et al., 2021). As assessed in SROCC, the biomass projections contain considerable regional
21 variation, with declines in tropical to temperate regions and strong increases in total animal biomass are
22 projected in polar regions under high-emission scenarios, with climate-change effects that are spatially
23 similar but less pronounced under lower-emission scenarios (Figure 3.21b,c,e,f,h,i; Tai et al., 2019; Tittensor
24 et al., 2021).


26 Benthic biomass
27 SROCC assessed that reduced food supply to the deep sea will drive a reduction in abyssal seafloor biota by
28 2100 for RCP8.5 (Table 3.23). Simulations from one size-resolved benthic biomass model coupled to an
29 ocean-biogeochemistry model forced with the CMIP5 ESM HadGEM2-ES (Yool et al., 2017) project a
30 decline in the globally integrated total seafloor biomass of ­1.1% and ­17.6% by 2100 under RCP2.6 and
31 RCP8.5, respectively (limited evidence, high agreement). In waters shallower than 100 m, total benthic
32 biomass is projected to increase by 3.2% on average by 2100 under RCP8.5, primarily driven warming-

1 SROCC reported declines in total marine animal biomass have been recomputed using 1995­2014 as the baseline
period and the very likely ranges (5­95%) are now computed from the model ensemble ranges assuming a normal

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 1 increased growth rates (Yool et al., 2013), while at depths >2000 m (representing 83% of the ocean
 2 seafloor), declines of ­32% arise from climate-driven decreases in surface primary production and
 3 particulate organic carbon (POC) flux to the seafloor (Yool et al., 2013; Kelly-Gerreyn et al., 2014; Yool et
 4 al., 2015; Yool et al., 2017). These patterns are qualitatively similar under RCP2.6, except in the Pacific and
 5 Indian Ocean basins, where some increased total seafloor biomass is projected (Yool et al., 2013). Updated
 6 simulations with the same benthic biomass model (Kelly-Gerreyn et al., 2014) forced with the CMIP6 ESM
 7 UKESM-1 project declines in total seafloor biomass of ­9.8% and ­13.0% by 2081­2100 relative to 1995­
 8 2014 for SSP1-2.6 and SSP5-8.5, respectively (Figure 3.21j,k,l, updated from Yool et al., 2017). These
 9 projected changes in benthic biomass are based on limited evidence. Development of ensemble projections
10 forced with a range of ESMs and a benthic model that considers the ecological roles of temperature (Hunt
11 and Roy, 2006; Reuman et al., 2014), oxygen (Mosch et al., 2012) and ocean acidification (Andersson et al.,
12 2011) will provide opportunities to better quantify uncertainty in projected declines in total seafloor biomass
13 under climate change.


15 Conclusions
16 Overall, ocean warming and decreased phytoplankton production and biomass will drive a global decline in
17 biomass for zooplankton (low confidence), marine animals (medium confidence) and seafloor benthos (low
18 confidence), with regional differences in the direction and magnitude of changes (high confidence). There is
19 increasing evidence that responses will amplify throughout the food web and at ocean depths, with relatively
20 modest changes in surface primary producers translating into substantial changes at higher trophic levels and
21 for deep-water benthic communities (medium confidence).




25 Figure 3.21: Projected change in marine biomass. Simulated global biomass changes of (a,b,c) surface phytoplankton,
26 (d,e,f) zooplankton, (g,h,i) animals and (j,k,l) seafloor benthos. In (a,d,g,j), the multi-model mean (solid lines) and very
27 likely range (envelope) over 2000­2100 relative to 1995­2014, for SSP1-2.6 and SSP5-8.5. Spatial patterns of
28 simulated change by 2090­2099 are calculated relative to 1995­2014 for (b,e,h,k) SSP1-2.6 and (c,f,i,l) SSP5-8.5.
29 Confidence intervals can be affected by the number of models available for the Coupled Model Intercomparison Project
30 6 (CMIP6) scenarios and for different variables. Only one model was available for panel (j), so no confidence interval is

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1 calculated. For panels (a­f), the ensemble projections of global changes in phytoplankton and zooplankton biomasses
2 updated based on Kwiatkowski et al. (2019) include, under SSP1-2.6 and SSP5-8.5, respectively, a total of nine and 10
3 CMIP6 Earth System Models (ESMs). For panels (g,h,i), the ensemble projections of global changes in total animal
4 biomass updated based on Tittensor et al. (2021) include 6­9 published global fisheries and marine ecosystem models
5 from the Fisheries and Marine Ecosystem Model Intercomparison Project (Fish-MIP, Tittensor et al., 2018; Tittensor et
6 al., 2021), forced with standardised outputs from two CMIP6 ESMs. For panels (j,k,l), globally integrated changes in
7 total seafloor biomass have been updated based on Yool et al. (2017) with one benthic model (Kelly-Gerreyn et al.,
8 2014) forced with the CMIP6 ESM HadGEM2-ES.



11 Changes in Primary Production and Biological Carbon Export Flux


13 Observed changes in primary production
14 Analyses of satellite-derived primary production over the past two decades (1998­2018) showed generally
15 weak and negative trends (up to -3.0%) at low and mid latitudes (Kulk et al., 2020). In contrast, positive
16 trends occurred in large areas of the South Atlantic and South Pacific Oceans, as well as in polar and coastal
17 (upwelling) regions (up to +4.5%, Cross-Chapter Paper 6, Kulk et al., 2020). Data-assimilating ocean
18 biogeochemical models estimate a global decline in primary production of 2.1% per decade in the period
19 1998­2015, driven by the shoaling mixed layer and decreasing nitrate concentrations (Gregg and Rousseaux,
20 2019). This is consistent with previous assessments that identified ocean warming and increased
21 stratification as the main drivers (high confidence) affecting the regional variability in primary production
22 Bindoff et al. (2019). However, as noted in SROCC and WGI AR6 Chapter 2 (Table 3.24, Gulev et al.,
23 2021), observed inter-annual changes in primary production on global and regional scales are nonlinear and
24 largely influenced by natural temporal variability, providing low confidence in the trends.



27 Table 3.24: Summary of previous IPCC assessments of ocean primary production and carbon export flux.

    Process                           Observed Impacts                    Projected Impacts

    SROCC (Bindoff et al., 2019)

    Open ocean                        Past open ocean productivity trends, Net primary productivity is very likely
    primary production
                                      including those determined by       to decline by 4­11% by 2081­2100,

                                      satellites, are appraised with low  relative to 2006­2015, across CMIP5

                                      confidence, due to newly identified models for RCP8.5, but there is low

                                      region-specific drivers of microbial confidence for this estimate due to the

                                      growth and the lack of corroborating in medium agreement among models and

                                      situ time series datasets.          the limited evidence from observations.

                                                                          The tropical ocean net primary

                                                                          productivity is very likely to decline by

                                                                          7­16% for RCP8.5, with medium

                                                                          confidence as there are improved

                                                                          constraints from historical variability

                                                                          in this region.

    Open ocean carbon export          Analyses of long-term trends in     The projected changes in export

                                      primary production and particle export production can be larger than global

                                      production, as well as model        primary production because they are

                                      simulations, reveal that increasing affected by both the net primary

                                      temperatures, leading to enhanced production changes, but also how

                                      stratification and nutrient limitation, shifts in food web structure modulates

                                      will have the greatest influence on the `transfer efficiency' of particulate

                                      decreasing the flux of particulate  organic material, which then affects

                                      organic carbon (POC) to the deep the sinking speed and lability of

                                      ocean. However, different lines of exported particles through the ocean

                                      evidence (including observation,    interior to the sea floor.

                                      modeling and experimental studies)

                                      provide low confidence on the       As export production is a much better

                                      mechanistic understanding of how understood net integral of changing net

                                      climate-impact drivers affect different nutrient supply and can be constrained

                                      components of the biological pump in by interior ocean nutrient and oxygen

                                      the epipelagic ocean, as well as    levels, there is medium confidence in

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                                       changes in the efficiency and   projections for global export

                                       magnitude of carbon export in the deep production changes based on CMIP5

                                       ocean.                          model runs.

   WGI AR6 Chapters 2 and 5 (Canadell
   et al., 2021; Gulev et al., 2021)

   Open ocean                          Global ocean marine primary     In CMIP5 models run under RCP8.5,
   primary production
                                       production is estimated to be 47 ± 7.8 particulate organic carbon (POC)

                                       PgC yr­1 with low confidence because export flux is projected to decline by

                                       of the small number of recent studies 1­12% by 2100 (Taucher and

                                       and the insufficient length of the time Oschlies, 2011; Laufkötter et al.,

                                       series analysed. A small decrease in 2015). Similar values are predicted in

                                       productivity is evident globally for the 18 CMIP6 models, with declines of

                                       period 1998­2015, but regional  2.5­21.5% (median ­14%) between

                                       changes are larger and of opposing 1900 and 2100 under the SSP5-8.5

                                       signs (low confidence) (WGI AR6 scenario. The mechanisms driving

                                       Section, Gulev et al., 2021). these changes vary widely between

                                                                       models due to differences in

                                                                       parameterisation of particle formation,

                                                                       remineralisation and plankton

                                                                       community structure (WGI AR6

                                                                       Section, Canadell et al., 2021).



 3 Projected changes in primary production
 4 Across 10 CMIP5 and 13 CMIP6 ESM ensembles, global mean net primary production is projected to
 5 decline by 2080­2099 relative to 2006­2015, under all RCPs and SSPs (Kwiatkowski et al., 2020).
 6 However, under comparable radiative forcing, the CMIP6 multi-model mean projections of primary
 7 production declines (mean ± SD: ­0.56 ± 4.12% under SSP1-2.6, and ­3.00 ± 9.10% under SSP5-8.5) are
 8 less than those of previous CMIP5 models (3.42 ± 2.47% under RCP2.6, and 8.54 ± 5.88% under RCP8.5)
 9 (WGI AR6 Section, Kwiatkowski et al., 2020; Canadell et al., 2021). The inter-model uncertainty
10 associated with CMIP6 net primary production projections is larger than in CMIP5, and it is consistently
11 larger than the scenario uncertainty. For each SSP across the CMIP6 ensemble, individual models project
12 both increases and decreases in global primary production, reflecting a diverse suite of bottom-up and top-
13 down ecological processes, which are variously parameterised across models (Laufkötter et al., 2015;
14 Bindoff et al., 2019). Further, accurate simulation of many of the biogeochemical tracers upon which net
15 primary production depends (e.g., the distribution of iron, Tagliabue et al., 2016; Bindoff et al., 2019)
16 remains a significant and ongoing challenge to ESMs (high confidence) (Séférian et al., 2020).


18 Regionally, multi-model mean changes in primary production show generally similar patterns of large
19 declines in the North Atlantic and the western equatorial Pacific, while in the high latitudes, primary
20 production consistently increases in CMIP5 and CMIP6 by 2100 (Kwiatkowski et al., 2020, Cross-Chapter
21 Paper 6). In the Indian Ocean and sub-tropical North Pacific, which were regions of consistent net primary
22 production decline in CMIP5 projections (Bopp et al., 2013), the regional declines are reduced in magnitude,
23 less spatially extensive, and are typically less robust in CMIP6. Further assessment of simultaneous changes
24 in processes such as nutrient advection, nitrogen fixation, the microbial loop, and top-down grazing pressure
25 (WGI AR6 Section, Laufkötter et al., 2015; Bindoff et al., 2019; Canadell et al., 2021) are required to
26 fully understand the regional primary production response in CMIP6 (Kwiatkowski et al., 2020). Given the
27 regional variations in the estimates of primary production changes and the uncertainty in the representation
28 of the dominant drivers, there remains low confidence in the projected global decline in net primary
29 production.


31 Observed processes driving changes in global export flux
32 The SROCC medium confidence assessment that warming, stratification, declines in productivity and
33 changes in plankton community in the epipelagic zone result in reduced export of primary production to
34 deeper layers (Table 3.24) is supported by subsequent literature (Bach et al., 2019; Leung et al., 2021). POC
35 export efficiency is constrained by altered mixing and nutrient availability (Boyd et al., 2019; Lundgreen et
36 al., 2019), particle fragmentation (Briggs et al., 2020), as well as viral, microbial, and planktonic community
37 structure (Fu et al., 2016; Guidi et al., 2016; Flombaum et al., 2020; Kaneko et al., 2021) and metabolic rates

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 1 (Cavan et al., 2019). These processes are strongly interlinked and their net effect on primary production
 2 export from the upper ocean remain difficult to quantify observationally (Boyd et al., 2019). Since SROCC,
 3 there is increasing evidence that ocean deoxygenation can alter zooplankton community structure (Wishner
 4 et al., 2018), zooplankton respiration rates (Cass and Daly, 2014; Cavan et al., 2017) and patterns of diel
 5 vertical migration (Aumont et al., 2018), which may focus remineralisation of organic carbon at the upper
 6 margins of OMZs (Section on depth shifts due to OMZ, Bianchi et al., 2013; Archibald et al., 2019).


 8 Data on export flux from the upper ocean are limited either in coverage and consistency (ship-board
 9 sampling) or duration (sediment traps) and are subject to considerable spatial variability (as shown in
10 satellite observations, Boyd et al., 2019). As a result, trends are weak, inconsistent and often not statistically
11 significant (Lomas et al., 2010; Cael et al., 2017; Muller-Karger et al., 2019; Xie et al., 2019). Deep-ocean
12 fluxes are similarly equivocal (Smith et al., 2018; Fischer et al., 2019; Fischer et al., 2020). In coming years,
13 an increasing number of Argo floats equipped with bio-optical sensors should help improve estimates of
14 particle flux spatial and temporal variability (e.g., Dall'Olmo et al., 2016).


16 Projected changes
17 SROCC and WGI AR6 reported global declines in POC export flux, between ­8.9 to ­15.8% by 2100
18 relative to 2000 under RCP8.5 in CMIP5 models, and ­2.5 to ­21.5% (median value ­14%) between 1900
19 and 2100 under SSP5-8.5 in CMIP6 models (WGI AR6, Table 3.24, Bindoff et al., 2019; Canadell et
20 al., 2021). In CMIP5 model runs, the decrease in the sinking flux of organic matter from the upper ocean into
21 the ocean interior was strongly related to the changes in stratification that reduce net nutrient supply (Fu et
22 al., 2016; Bindoff et al., 2019), especially in tropical regions, and the projections for global export
23 production changes are reported with medium confidence. Increasing model complexity with more
24 widespread representation of ocean biogeochemical processes between CMIP5 and CMIP6, and inclusion of
25 more than one or two classes of phyto- and zooplankton will provide opportunities to improve assessments
26 of how climate-impact drivers affect different components of biological carbon pump in the epipelagic
27 ocean, as well as changes in the efficiency and magnitude of carbon export in the deep ocean (high
28 confidence) (Box 3.3, Le Quéré et al., 2016; Séférian et al., 2020; Wright et al., 2021).





33 Box 3.3: Deep Sea Ecosystems


35 Deep-sea ecosystems include all waters below the 200 m isobath as well as the underlying benthos, and they
36 provide habitats for highly diversified and specialised biota, which play a key role in the cycling of carbon
37 and other nutrients (Figure Box3.3.1, Thurber et al., 2014; Middelburg, 2018; Snelgrove et al., 2018). The
38 deep sea covers >63% of Earth's surface (Costello and Cheung, 2010) and is exposed to climate-driven
39 changes in abyssal, intermediate, and surface waters that influence sinking fluxes of particulate organic
40 matter (high confidence) (Figure Box3.3.1, Sections 3.1, 3.2.1, 3.2.2,, WGII AR5 Section 30.5.7,
41 SROCC Sections 5.2.3, 5.2.4, Hoegh-Guldberg et al., 2014; Bindoff et al., 2019). These ecosystems are also
42 influenced by non-climate drivers, especially fisheries, oil and gas extraction (Thurber et al., 2014; Cordes et
43 al., 2016; Zhang et al., 2019a); cable laying (United Nations, 2021); and mineral resource exploration (Hein
44 et al., 2021); with proposed large-scale deep-sea mining a potential future source of impacts (Danovaro,
45 2018; Levin et al., 2020).


47 Ocean warming alters biological processes in deep-sea ecosystems in ways that affect deep-sea habitat,
48 biodiversity, and material processing. Enhancement of microbial respiration by warming attenuates sinking
49 POC, which has been associated with the globally projected declines in total seafloor biomass of ­9.8% and
50 ­13.0% by 2081­2100 relative to 1995­2014 under SSP1-2.6 and SSP5-8.5, respectively (limited evidence)
51 (Section Additionally, climate-change-driven oxygen loss (Section, Luna et al., 2012; Belley
52 et al., 2016), and geographic shifts in predator distributions (Section are anticipated to affect deep-
53 sea biodiversity (limited evidence, high agreement) (Smith et al., 2012; Morato et al., 2020). Complex
54 responses of some bathyal crustacean assemblages to environmental change suggest an increase in
55 phylogenetic diversity but limited decreases in abundances with temperature (Ashford et al., 2019). Acute
56 mortality of some reef-forming cold-water corals to laboratory-simulated warming (Lunden et al., 2014)
57 suggests that both long-term warming and the increase of MHWs in intermediate and deep waters (Elzahaby

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 1 and Schaeffer, 2019) could pose significant risk to associated ecosystems (high confidence). Thermal
 2 tolerance thresholds (lethal and sub-lethal) of scleractinians in laboratory settings depend on their geographic
 3 position and capacity for thermal adaptation, as well as other factors including food, oxygen and pH (medium
 4 to high confidence) (Naumann et al., 2013; Hennige et al., 2014; Lunden et al., 2014; Naumann et al., 2014;
 5 Georgian et al., 2016; Gori et al., 2016; Maier et al., 2016; Büscher et al., 2017).


 7 The extension and intensification of deep-water acidification (Section has been identified as a
 8 further key risk to deep-water coral ecosystems (medium confidence) (Bindoff et al., 2019). Literature since
 9 SROCC supports this assessment (Morato et al., 2020; Puerta et al., 2020), although scleractinians and
10 gorgonians are found in regions undersaturated with respect to aragonite (Thresher et al., 2011; Fillinger and
11 Richter, 2013; Baco et al., 2017). Laboratory experiments on reef-forming scleractinians show variable
12 results, with regional acclimation potential and population-genetic adaptations (Georgian et al., 2016;
13 Kurman et al., 2017). Desmophyllum pertusum6 and M. oculata maintain calcification in moderately low pH
14 (7.75) and near-saturation of aragonite (Hennige et al., 2014; Maier et al., 2016; Büscher et al., 2017), but
15 lower pH (7.6) and corrosive conditions lead to net dissolution of D. pertusum skeletons (high confidence)
16 (Lunden et al., 2014; Kurman et al., 2017; Gómez et al., 2018). Experiments suggest that D. dianthus is more
17 sensitive to warming than acidification and when both are high, as projected under climate change. Warming
18 appears to compensate for declines in calcification, with fitness also sensitive to food availability (Bramanti
19 et al., 2013; Movilla et al., 2014; Gori et al., 2016; Baussant et al., 2017; Büscher et al., 2017; Schönberg et
20 al., 2017; Höfer et al., 2018; Maier et al., 2019).


22 In OMZ regions (Section, benthic species distributions (Sperling et al., 2016; Levin, 2018; Gallo et
23 al., 2020), abundance and composition of demersal fishes in canyons (De Leo et al., 2012) and deep-pelagic
24 zooplankton (Wishner et al., 2018) follow oxygen gradients, indicating that deep-sea biodiversity and
25 ecosystem structure will be impacted by extension of hypoxic areas (medium confidence). Fossil records
26 show benthic population collapse and turnover when oxygen ranged from oxic to mildly or severely hypoxic
27 (Cross-Chapter Box PALEO in Chapter 1, Moffitt et al., 2015). Regional extirpations among cold-water
28 corals in the paleorecord were associated with substantial declines in oxygen, coincident with abrupt
29 warming and altered intermediate water masses properties (Wienberg et al., 2018; Hebbeln et al., 2019).
30 Despite mortality and functional impacts from low oxygen concentrations observed in aquaria (Lunden et al.,
31 2014), recent observations of the deep-water coral D. pertusum suggest adaptive capacity to hypoxia among
32 specimens from OMZ regions that are highly productive (low confidence) (Hanz et al., 2019; Hebbeln et al.,
33 2020).


35 Chemosynthetic ecosystems could be particularly prone to oxygen decline (low to medium confidence).
36 Projected OMZ expansion in the vicinity of seep communities could favour sulphide-tolerant species, as
37 suggested from fossil records (Moffitt et al., 2015), but this will exclude large symbiont-bearing foundation
38 species of methane seep ecosystems (Fischer et al., 2012; Sweetman et al., 2017). Projected warming, or
39 shifts in warm-current circulation along continental margins, could enhance dissociation of buried methane
40 hydrates (Phrampus and Hornbach, 2012; Phrampus et al., 2014), either increasing anaerobic methane
41 oxidation (Boetius and Wenzhöfer, 2013), which benefit seep communities, or increasing gas fluxes, which
42 would decrease anaerobic methane oxidation rates and exclude chemosynthetic fauna.


44 Environmental niche models (FAO, 2019; Morato et al., 2020; Puerta et al., 2020) project that under
45 RCP8.5, >50% of present-day scleractinian habitats in the North Atlantic Ocean, will become unsuitable by
46 2100, with greater impacts on D. pertusum than on D. dianthus or Madrepora oculata. For gorgonians,
47 corresponding habitat loss is likely >80%. Much less is known about the environmental niches of deep-sea
48 sponges, preventing a similar assessment (Kazanidis et al., 2019; Puerta et al., 2020).


50 Climate-driven impacts further limit the resilience of deep-sea ecosystems to impacts from human activities
51 (high confidence) (Levin and Le Bris, 2015; Rogers, 2015; Sweetman et al., 2017). However, assessing
52 cumulative climatic and non-climatic impacts is challenging for these data-poor environments (Ashford et
53 al., 2018; Levin, 2018; Armstrong et al., 2019; Heffernan, 2019; Kazanidis et al., 2020; Orejas et al., 2020),
54 where lack of knowledge increases the possibility of overlooking ecosystem vulnerabilities and risks (Levin,
55 2021). A paucity of information about the natural variability and historical trends of these habitats prevents

6 Previously named Lophelia pertusa

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 1 robust assessment of adaptive capacities and potential vulnerabilities to extreme events (Aguzzi et al., 2019;
 2 Levin et al., 2019; Chapron et al., 2020; Danovaro et al., 2020; Le Bris and Levin, 2020; Levin, 2021). The
 3 spatial resolution of CMIP5 models is too coarse to robustly project changes in mesoscale circulation at the
 4 seafloor (Sulpis et al., 2019), on which deep-sea ecosystems depend for organic material supplies and
 5 dispersal of planktonic and planktotrophic larvae (high confidence) (Fox et al., 2016; Mitarai et al., 2016;
 6 Dunn et al., 2018). Higher-resolution modelling from CMIP6 (Orr et al., 2017), multiannual and high-
 7 frequency records of ocean bottom-water properties (Meinen et al., 2020), and better understanding and
 8 accounting of biogeochemical mechanisms of organic carbon transport to the ocean interior is expected to
 9 improve this capacity (Boyd et al., 2019; Séférian et al., 2020).




13 Figure Box3.3.1: Schematic of the combination of climate-impact drivers in different deep-ocean ecosystems. Key
14 physical and biological drivers of change in the deep-sea and benthic habitats with specific vulnerabilities are discussed
15 in Section



18 [END BOX 3.3 HERE]



21 3.4.4 Reversibility and Impacts of Temporary Overshoot of 1.5°C or 2°C Warming


23 Scenarios limiting warming to the 1.5°C and 2°C limits in the Paris Agreement can involve temporarily
24 exceeding those warming levels before declining again (WGI AR6 Section, Lee et al., 2021). The
25 effect of such "overshoot" on marine and coastal ecosystems depends on the reversibility of both the
26 response of climate-impact drivers, and the response of organisms and ecosystems to the climate-impact
27 drivers, during the overshoot period. WGI AR6 assessed that temporary overshoot of a 2°C warming
28 threshold has irreversible effects on global mean sea-level and also effects on ocean heat content that persist
29 beyond 2100 (WGI AR6 Section, Lee et al., 2021). Model results indicate that sea surface
30 temperatures (high confidence), Arctic sea ice (high confidence), surface ocean acidification (very high
31 confidence) and surface ocean deoxygenation (very high confidence) are reversible within years to decades if
32 net emissions reach zero or below (WGI AR6 Table 4.10, Lee et al., 2021). Although changes in these

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 1 surface ocean variables are reversible, habitat-forming ecosystems including coral reefs and kelp forests may
 2 undergo irreversible phase shifts with >1.5°C warming (Section,, and are thus at high risk
 3 this century in 1.5°C or 2°C scenarios involving overshoot (Tachiiri et al., 2019). In an overshoot scenario in
 4 which CO2 returns to 2040 levels by 2100 (SSP5-3.4-OS, O'Neill et al., 2016), SST and Arctic sea ice do not
 5 fully return by 2100 to levels prior to the CO2 peak (medium confidence) (WGI AR6 Section, Lee et
 6 al., 2021), suggesting that reversal of marine ecological impacts from 21st century climate impacts would
 7 extend into the 22nd century or beyond (McManus et al., 2021). Models also indicate that global sea level
 8 rise, as well as warming, ocean acidification and deoxygenation at depth, are irreversible for centuries or
 9 longer (very high confidence) (WGI AR6 Section and Table 4.10, Palter et al., 2018; Li et al., 2020c;
10 Lee et al., 2021).



13 3.5 Vulnerability, Resilience and Adaptive Capacity in Marine Social-Ecological Systems, including

14  Impacts to Ecosystem Services


16 3.5.1 Introduction


18 This Section assesses the impacts of climate change on ecosystem services (Table 3.25, Chapter 1) and the
19 outcomes on social-ecological systems, building on previous assessments (Table 3.26). Section 3.5.2
20 assesses how changes in biodiversity influence ecosystem services. Then Sections 3.5.3 and 3.5.4 assess
21 provisioning services (food and non-food), Section 3.5.5 assesses supporting and regulating services, and
22 Section 3.5.6, cultural services. Where evidence exists, the section evaluates how the vulnerability and
23 adaptive capacity of social-ecological systems govern the manifestation of impacts on each ecosystem
24 service.



27 Table 3.25: Ocean and coastal ecosystem services. Adapted from IPBES (2017), with examples made specific to ocean
28 and coastal ecosystems by Chapter 3 authors.

    Ecosystem Service Category        Components                          Ocean and Coastal Examples

    Provisioning                      Food and feed                       Status of harvested marine fish,
                                                                          invertebrates, mammals, and plants.

                                      Medicinal, biochemical and genetic  Existence of and access to biological
                                      resources                           resources that could offer future
                                                                          prospects for development, including
                                                                          marine fish, invertebrates, mammals,
                                                                          plants, microbes, viruses.

                                      Materials and assistance            Existence of and access to minerals,
                                                                          shells, stones, coral branches, dyes
                                                                          used to create other goods; availability
                                                                          of marine organisms to exhibit in zoos,
                                                                          aquariums, and as pets.

                                      Energy                              Existence of and access to sources of
                                                                          energy, including oil and gas reserves;
                                                                          solar, tidal, and thermal ocean energy;
                                                                          and biofuels from marine plants.

    Supporting and Regulating         Habitat creation and maintenance    Status of nesting, feeding, nursery, and
                                                                          mating sites for birds, mammals, and
                                                                          other marine life, and of resting and
                                                                          overwintering places for migratory
                                                                          marine life or insects. Connectivity of
                                                                          ocean habitats.

                                      Dispersal and other propagules      Ability of marine life to spread
                                                                          gametes and larvae successfully by
                                                                          broadcast spawning reproduction, and
                                                                          ability of adults to disperse widely.

                                      Regulation of climate               Status of carbon storage and
                                                                          sequestration, methane cycling in

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       Cultural                                                         wetlands, and dimethyl sulfide creation
                                                                        and destruction.
2                                 Regulation of air quality             Status of aquatic processes that
3                                                                       maintain and balance CO2, oxygen,
                                                                        nitrogen oxides, sulfur oxides, volatile
                                                                        organic compounds, particulates, and

                                  Regulation of ocean acidification     Status of chemical and biological
                                  (Section                     aquatic processes that maintain and
                                                                        balance CO2 and other acids/bases.

                                  Regulation of freshwater quantity,    Status of water storage by coastal
                                  location and timing                   systems, including groundwater flow;
                                                                        aquifer recharge; and flooding
                                                                        responses of wetlands, coastal water
                                                                        bodies, and developed spaces.

                                  Regulation of freshwater and coastal  Status of chemical and biological
                                  water quality                         aquatic processes that retain and filter
                                                                        coastal waters, capture pollutants and
                                                                        particles, and oxygenate water (e.g.,
                                                                        natural filtration by sediments
                                                                        including adsorbent minerals and

                                  Regulation of organisms detrimental to Status of grazing that controls harmful

                                  humans and marine life                algal blooms and algal overgrowth of

                                                                        key ecosystems. Environmental

                                                                        conditions that suppress marine


                                  Formation, protection and             Status of chemical and biological
                                  decontamination of soils and          aquatic processes that capture
                                  sediments                             pollutants and particles (e.g.,
                                                                        adsorption by minerals, microbial
                                                                        breakdown of pollutants).

                                  Regulation of hazards and extreme     Ability of coastal environments to
                                  events                                serve as wave energy dissipators,
                                                                        barriers, and wave breaks.

                                  Regulation of key elements            Status of aquatic processes that
                                                                        maintain and balance stocks of carbon,
                                                                        nitrogen, phosphorus, and other
                                                                        elements critical for life.

                                  Physical and psychological            Existence of and access to recreational
                                  experiences                           opportunities including visiting
                                                                        beaches and coastal environments; and
                                                                        aquatic activities such as fishing,
                                                                        boating, swimming, and diving.

                                  Supporting identities                 Existence of and access to cultural,
                                                                        heritage, and religious activities, and
                                                                        opportunities for intergenerational
                                                                        knowledge transfer. Sense of place.

                                  Learning and inspiration              Existence of educational opportunities
                                                                        and characteristics to be emulated, as
                                                                        in biomimicry.

                                  Maintenance of options                Existence of opportunities to develop
                                                                        new medicines, materials, foods, and
                                                                        resources, or to adapt to a warmer
                                                                        climate and emergent diseases.

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1 Table 3.26: Conclusions from previous IPCC assessments about observed and projected climate impacts to ocean and
2 coastal biodiversity and ecosystem services.

Ecosystem service and chapter     Observed Impacts                       Projected Impacts

All (Section 3.5)                 Climate change has affected marine "Long-term loss and degradation of

                                  "ecosystem services with regionally marine ecosystems compromises the

                                  diverse outcomes, challenging their ocean's role in cultural, recreational,

                                  governance (high confidence). Both and intrinsic values important for

                                  positive and negative impacts result for human identity and well-being

                                  food security through fisheries        (medium confidence) (3.2.4, 3.4.3,

                                  (medium confidence), local cultures 5.4.1, 5.4.2, 6.4)" (SROCC SPM B.8,

                                  and livelihoods (medium confidence), IPCC, 2019c).

                                  and tourism and recreation (medium

                                  confidence). The impacts on ecosystem

                                  services have negative consequences

                                  for health and well-being (medium

                                  confidence), and for Indigenous

                                  Peoples and local communities

                                  dependent on fisheries (high

                                  confidence) (1.1, 1.5, 3.2.1, 5.4.1,

                                  5.4.2, Figure SPM.2)" (SROCC SPM

                                  A.8, IPCC, 2019c).

Biodiversity (Section 3.5.2)      "[Climate] Impacts are already         "Risks of severe impacts on

                                  observed on [coastal ecosystem]        biodiversity, structure and function of

                                  habitat area and biodiversity, as well as coastal ecosystems are projected to be

                                  ecosystem functioning and services higher for elevated temperatures under

                                  (high confidence) (4.3.2, 4.3.3, 5.3, high compared to low emissions

                                  5.4.1, 6.4.2, Figure SPM.2)" (SROCC scenarios in the 21st century and

                                  SPM A.6, IPCC, 2019c).                 beyond." (SROCC SPM B.6, IPCC,


Food provision (Section 3.5.3)    "Warming-induced changes in the "Future shifts in fish distribution and

                                  spatial distribution and abundance of decreases in their abundance and

                                  some fish and shellfish stocks have fisheries catch potential due to climate

                                  had positive and negative impacts on change are projected to affect income,

                                  catches, economic benefits,            livelihoods, and food security of

                                  livelihoods, and local culture (high marine resource-dependent

                                  confidence). There are negative        communities (medium confidence).

                                  consequences for Indigenous Peoples Long-term loss and degradation of

                                  and local communities that are         marine ecosystems compromises the

                                  dependent on fisheries (high           ocean's role in cultural, recreational,

                                  confidence). Shifts in species         and intrinsic values important for

                                  distributions and abundance has        human identity and well-being

                                  challenged international and national (medium confidence) (3.2.4, 3.4.3,

                                  ocean and fisheries governance,        5.4.1, 5.4.2, 6.4)" (SROCC SPM

                                  including in the Arctic, North Atlantic B.8,IPCC, 2019c).

                                  and Pacific, in terms of regulating

                                  fishing to secure ecosystem integrity

                                  and sharing of resources between

                                  fishing entities (high confidence)

                                  (3.2.4, 3.5.3, 5.4.2, 5.5.2, Figure

                                  SPM.2)". (SROCC SPM A.8.1 IPCC,


Non-food consumable provisioning  Observed impacts on non-food           "Reductions in marine biodiversity due
services (Section        provisioning services not previously   to climate change and other
                                  assessed.                              anthropogenic stressors (Tittensor et
                                                                         al., 2010), such as ocean acidification
                                                                         (CBD, 2009) and pollution, might
                                                                         reduce the discovery of genetic
                                                                         resources from marine species useful
                                                                         in pharmaceutical, aquaculture,
                                                                         agriculture, and other industries
                                                                         (Arrieta et al., 2010), leading to a loss

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Renewable energy (Section
                                                                            of option value from marine
Habitat creation and maintenance                                            ecosystems." (WGII AR5 Section
(Section                                                 , Pörtner et al., 2014)

Climate regulation and air quality  Observed impacts on ocean renewable "Ocean renewable energy can support
                                    energy not previously assessed.         climate change mitigation, and can

                                                                            comprise energy extraction from

                                                                            offshore winds, tides, waves, thermal

                                                                            and salinity gradient and algal

                                                                            biofuels. The emerging demand for

                                                                            alternative energy sources is expected

                                                                            to generate economic opportunities for

                                                                            the ocean renewable energy sector

                                                                            (high confidence), although their

                                                                            potential may also be affected by

                                                                            climate change (low confidence)

                                                                            (5.4.2, 5.5.1, Figure 5.23)". (SROCC

                                                                            SPM C.2.5, IPCC, 2019c).

                                    "[Climate] Impacts are already          "In the Southern Ocean, the habitat of

                                    observed on [coastal ecosystem]         Antarctic krill, a key prey species for

                                    habitat area and biodiversity, as well as penguins, seals and whales, is

                                    ecosystem functioning and services projected to contract southwards under

                                    (high confidence) (4.3.2, 4.3.3, 5.3, both RCP2.6 and RCP8.5 (medium

                                    5.4.1, 6.4.2, Figure SPM.2)" (SROCC confidence) (3.2.2, 3.2.3, 5.2.3)"

                                    SPM A.6, IPCC, 2019c).                  (SROCC SPM B5.3, IPCC, 2019c).

                                                                            "Ocean warming, oxygen loss,

                                    "In polar regions, ice associated       acidification and a decrease in flux of

                                    marine mammals and seabirds have organic carbon from the surface to the

                                    experienced habitat contraction linked deep ocean are projected to harm

                                    to sea ice changes (high confidence)." habitat-forming cold-water corals,

                                    (SROCC SPM A.5.2, IPCC, 2019c). which support high biodiversity, partly

                                                                            through decreased calcification,

                                                                            increased dissolution of skeletons, and

                                                                            bioerosion (medium confidence)."

                                                                            (SROCC SPM B5.4, IPCC, 2019c).

                                    "Global ocean heat content continued    "The increase in global ocean heat
                                    to increase throughout [the 1951-       content (TS2.4) will likely continue
                                    present] period, indicating continuous  until at least 2300 even for low-
                                    warming of the entire climate system    emission scenarios." (WGI AR6 Box
                                    (very high confidence)" (WGI AR6        TS.9, Arias et al., 2021).
                                    TS1.2.3, Arias et al., 2021).

                                    "Land and ocean have taken up a near- "While natural land and ocean carbon

                                    constant proportion (globally about sinks are projected to take up, in

                                    56% year­1) of CO2 emissions from absolute terms, a progressively larger

                                    human activities over the past six      amount of CO2 under higher compared

                                    decades, with regional differences to lower CO2 emissions scenarios, they

                                    (high confidence)" (WGI AR6 SPM become less effective, that is, the

                                    A1.1, IPCC, 2021b).                     proportion of emissions taken up by

                                                                            land and ocean decrease with

                                                                            increasing cumulative CO2 emissions.

                                                                            This is projected to result in a higher

                                                                            proportion of emitted CO2 remaining

                                                                            in the atmosphere (high confidence)"

                                                                            (WGI AR6 SPM B4.1, IPCC, 2021b).

                                    Observed impacts on marine              "The effect of climate change on
                                    organisms' contribution to climate      marine biota will alter their
                                    regulation not previously assessed.     contribution to climate regulation, that
                                                                            is, the maintenance of the chemical
                                                                            composition and physical processes in
                                                                            the atmosphere and oceans (high
                                                                            confidence) (Beaumont et al., 2007)"

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                                                                              (WGII AR5 Section, Pörtner et
                                                                              al., 2014).

Provision of fresh water, maintenance  Observed climate impacts on            "In the absence of more ambitious
of water quality, regulation of        salinisation of coastal soil and       adaptation efforts compared to today,
pathogens (Section            groundwater not previously assessed.   and under current trends of increasing
                                                                              exposure and vulnerability of coastal
                                                                              communities, risks, such as erosion
                                                                              and land loss, flooding, salinisation,
                                                                              and cascading impacts due to mean sea
                                                                              level rise and extreme events are
                                                                              projected to significantly increase
                                                                              throughout this century under all
                                                                              greenhouse gas emissions scenarios
                                                                              (very high confidence)." (SROCC
                                                                              SPM B9.1, IPCC, 2019c)

                                       "Global warming compromises            "[Risks from marine-borne pollutants

                                       seafood safety (medium confidence) and pathogens] are projected to be

                                       through human exposure to elevated particularly large for human

                                       bioaccumulation of persistent organic communities with high consumption of

                                       pollutants and mercury in marine       seafood, including coastal Indigenous

                                       plants and animals (medium             communities (medium confidence),

                                       confidence), increasing prevalence of and for economic sectors such as

                                       waterborne Vibrio sp. pathogens        fisheries, aquaculture, and tourism

                                       (medium confidence), and heightened (high confidence) (3.4.3, 5.4.2, Box

                                       likelihood of harmful algal blooms 5.3)" (SROCC SPM B.8.3, IPCC,

                                       (medium confidence)." (SROCC SPM 2019c).

                                       B.8.3, IPCC, 2019c).

                                       "Since the early 1980s, the occurrence "Overall, the occurrence of HABs,

                                       of harmful algal blooms (HABs) and their toxicity and risk on natural and

                                       pathogenic organisms (e.g., Vibrio) has human systems are projected to

                                       increased in coastal areas in response continue to increase with warming and

                                       to warming, deoxygenation and          rising CO2 in the 21st century (Glibert

                                       eutrophication, with negative impacts et al., 2014; Martín-García et al., 2014;

                                       on food provisioning, tourism, the McCabe et al., 2016; Paerl et al., 2016;

                                       economy and human health (high         Gobler et al., 2017; McKibben et al.,

                                       confidence)." (SROCC Chapter 5 2017; Rodríguez et al., 2017; Paerl et

                                       Executive Summary, Bindoff et al., al., 2018; Riebesell et al., 2018) (high

                                       2019).                                 confidence)." (SROCC Box 5.4,

                                                                              Bindoff et al., 2019).

Regulation of physical hazards         "Coastal ecosystems are already        "The decline in warm water coral reefs
                                       impacted by the combination of sea is projected to greatly compromise the

                                       level rise, other climate-related ocean services they provide to society, such

                                       changes, and adverse effects from as...coastal protection (high

                                       human activities on ocean and land confidence)..." (SROCC SPM B.8.2,

                                       (high confidence)... Coastal and near- IPCC, 2019c).

                                       shore ecosystems including

                                       saltmarshes, mangroves, and vegetated

                                       dunes in sandy beaches, ... provide

                                       important services including coastal

                                       protection... (high confidence)"

                                       (SROCC Chapter 4 Executive

                                       Summary, Oppenheimer et al., 2019).

Ocean and coastal carbon storage       "Recent observations show that ocean "Emission scenarios SSP4-6.0 and
                                       carbon processes are starting to change SSP5-8.5 lead to warming of the

                                       in response to the growing ocean sink, surface ocean and large reductions of

                                       and these changes are expected to the buffering capacity, which will slow

                                       contribute significantly to future     the growth of the ocean sink after

                                       weakening of the ocean sink under 2050. Scenario SSP1-2.6 limits further

                                       medium- to high-emission scenarios. reductions in buffering capacity and

                                       However, the effects of these changes warming, and the ocean sink weakens

                                       is not yet reflected in a weakening in response to the declining rate of

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                                      trend of the contemporary (1960­     increasing atmospheric CO2. There is
                                      2019) ocean sink (high confidence)"  low confidence in how changes in the
                                      (WGI AR6 Chapter 5 Executive         biological pump will influence the
                                      Summary, Canadell et al., 2021).     magnitude and direction of the ocean
                                                                           carbon feedback" (WGI AR6 Chapter
                                                                           5 Executive Summary, Canadell et al.,

                                      "Mangrove, seagrass, and salt marsh "...under high emission scenarios, sea

                                      ecosystems offer important carbon level rise and warming are expected to

                                      storage and sequestration opportunities reduce carbon sequestration by

                                      (limited evidence, medium agreement), vegetated coastal ecosystems (medium

                                      in addition to ecosystem goods and confidence); however, under

                                      services such as protection against conditions of slow sea level rise, there

                                      coastal erosion and storm damage and may be net increase in carbon uptake

                                      maintenance of habitats for fisheries by some coastal wetlands (medium

                                      species." (WGII AR5 Technical        confidence)" (SROCC Chapter 5,

                                      Summary).                            Bindoff et al., 2019).

   Cultural Services (Section 3.5.6)  "Climate change impacts on marine "Future shifts in fish distribution and

                                      ecosystems and their services put key decreases in their abundance and

                                      cultural dimensions of lives and     fisheries catch potential due to climate

                                      livelihoods at risk (medium          change are projected to affect income,

                                      confidence), including through shifts in livelihoods, and food security of

                                      the distribution or abundance of     marine resource-dependent

                                      harvested species and diminished     communities (medium confidence).

                                      access to fishing or hunting areas. This Long-term loss and degradation of

                                      includes potentially rapid and       marine ecosystems compromises the

                                      irreversible loss of culture and local ocean's role in cultural, recreational,

                                      knowledge and Indigenous knowledge, and intrinsic values important for

                                      and negative impacts on traditional human identity and well-being

                                      diets and food security, aesthetic   (medium confidence)" (SROCC SPM

                                      aspects, and marine recreational     B.8, IPCC, 2019c).

                                      activities (medium confidence)"

                                      (SROCC SPM B.8.4, IPCC, 2019c).



3 3.5.2 Biodiversity


 5 Climate change is a key agent of biodiversity change in numerous ocean and coastal ecosystems (very high
 6 confidence) (Table 3.26, Worm and Lotze, 2021), and climate change and biodiversity loss reinforce each
 7 other (Pörtner et al., 2021b). Biodiversity has changed in association with ocean warming and loss of sea ice
 8 (Sections,; Section CCP6 2.4.2), SLR (Section 3.4.2; Cross-Chapter Box SLR in Chapter
 9 3), coral bleaching (Section, MHWs (Sections, and upwelling changes (high
10 confidence) (Section Overlapping non-climate drivers (Section 3.1) also decrease ocean and coastal
11 ecosystem biodiversity (very high confidence) (O'Hara et al., 2021; Pörtner et al., 2021b). There is medium
12 confidence that local and regional marine biodiversity losses from climate disrupt ecosystem services
13 provided by specific ocean and coastal species or places (Sections 3.5.3­3.5.6, Figure 3.23, Table 3.26, Box
14 3.3, Dee et al., 2019a; Hossain, 2019; Smale et al., 2019; Teixeira et al., 2019; Martin et al., 2020; Pathak,
15 2020; Weiskopf et al., 2020; Zunino et al., 2020; Archer et al., 2021). However, adaptive capacity varies
16 greatly among ecosystems, and ecological functions sometimes remain, despite changes in species
17 assemblages, as in certain coral reef communities (Richardson et al., 2020). Projected changes in biodiversity
18 due to climate change (Section are expected to alter the flow and array of ocean and coastal
19 ecosystem services (high confidence) (Smale et al., 2019; Cavanagh et al., 2021; Ruthrof et al., 2021; Worm
20 and Lotze, 2021), but data gaps hinder developing projections of ecosystem service changes detailed enough
21 to support decision making (Rosa et al., 2020).


23 Non-indigenous marine species are major agents of ocean and coastal biodiversity change, and climate and
24 non-climate drivers interact to support their movement and success (high confidence) (Iacarella et al., 2020).
25 At times, non-indigenous species act invasively and outcompete indigenous species, causing regional
26 biodiversity shifts and altering ecosystem function, as seen in the Mediterranean region (high confidence)

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 1 (e.g., Mannino et al., 2017; Bianchi et al., 2019; Hall-Spencer and Harvey, 2019; Verdura et al., 2019;
 2 García-Gómez et al., 2020; Dimitriadis et al., 2021). Warming-related range expansions of non-indigenous
 3 species have directly or indirectly decreased commercially important fishery species and nursery habitat
 4 (Booth et al., 2018). Non-indigenous species outperform indigenous species in coastal zones experiencing
 5 warming and freshening (McKnight et al., 2021). Non-climate drivers, especially marine shipping in newly
 6 ice-free locations (Chan et al., 2019), fishing pressure (Last et al., 2011), aquaculture of non-indigenous
 7 species (Mach et al., 2017; Ruby and Ahilan, 2018), and marine pollution and debris (Gall and Thompson,
 8 2015; Carlton et al., 2018; Carlton and Fowler, 2018; Lasut et al., 2018; Miralles et al., 2018; Rech et al.,
 9 2018; Therriault et al., 2018), promote range shifts and movement of non-indigenous species (high
10 confidence). Non-climate drivers can also intensify the ecological effects of non-indigenous species (Geraldi
11 et al., 2020). Invasive marine species can alter species behaviour, reduce indigenous species abundance,
12 reduce water clarity, bioaccumulate more heavy metals than indigenous species, and inhibit ecosystem
13 resilience in the face of extreme events (medium confidence) (McDowell et al., 2017; Geburzi and
14 McCarthy, 2018; Anton et al., 2019; Ruthrof et al., 2021). Risks from invasive species to the sources of other
15 ecosystem services or aquatic goods, including valuable materials, mining activities, shipping, or ocean
16 energy installations, have not been evaluated.


18 Reducing risk to ecosystem functions and services that depend on biodiversity requires an integrated
19 approach that acknowledges the close linkages between the climate and biodiversity crises and common
20 governance challenges (Pörtner et al., 2021b). Climate-focused solutions that employ nature-based solutions
21 (NbS), technological interventions, and socio-institutional interventions (Section 3.6.2) can also safeguard
22 biodiversity (Pörtner et al., 2021b), which in turn will help ocean and coastal ecosystems adapt to climate
23 impacts as well as help sustain the services they provide to people (Sections 3.5.3­3.5.6).


25 3.5.3 Food Provision


27 Globally, about 17% of humans' average per capita intake of animal protein in 2017 came from marine and
28 freshwater wild-caught and aquacultured aquatic animals (Costello et al., 2020; FAO, 2020a). Per capita
29 intake of seafood is 50% or more in some Small Island Developing States (SIDS) (Vannuccini et al., 2018),
30 and consumption per capita is 15 times higher in Indigenous Peoples than non-Indigenous Peoples (Cisneros-
31 Montemayor et al., 2016). Fishery products also supply critical dietary micronutrients worldwide (Section
32, Hicks et al., 2019; Vianna et al., 2020). Marine and freshwater fisheries and aquaculture provide
33 livelihoods for an estimated 10­12% of the world's population (Barange et al., 2018). Fishing and
34 aquaculture provide women and their families with substantial amounts of food and income (Harper et al.,
35 2020b), because at least 11% of small-scale fishers (Harper et al., 2020b) and up to half of all fishery and
36 aquaculture workers (FAO, 2018) are women. This section assesses how climate-driven alterations of the
37 abundance or nutritional quality of food from the sea could affect humans. Aquaculture, catch potential
38 changes, and human adaptations to changes in wild and cultured harvests, are assessed in Section 5.9.


40 Ocean and coastal fauna are moving towards higher latitudes globally due to warming (high confidence)
41 (Section, Table 3.26), challenging fishers and fisheries management (high confidence) as fishers also
42 move poleward and diversify harvests (medium evidence, high agreement) (Table 3.26, Sections,
43 5.8.4, Leitão et al., 2018; Liang et al., 2018; Ottosen et al., 2018; Peck and Pinnegar, 2018; Pinsky et al.,
44 2018; Erauskin-Extramiana et al., 2019; Free et al., 2019; Gianelli et al., 2019; Scott et al., 2019; Smith et
45 al., 2019; Gervais et al., 2021). Model hindcasts have identified temperature-associated fisheries reductions
46 worldwide (Free et al., 2019), and they have implicated overfishing as the primary non-climate driver
47 increasing fishery vulnerability (Section 5.8.4, Peck and Pinnegar, 2018; Das et al., 2020). Catch
48 composition is changing in many locations fished by smaller-scale, less mobile commercial, artisanal, and
49 recreational fisheries (high confidence) (Booth et al., 2018; Townhill et al., 2019; Young et al., 2019b;
50 Robinson et al., 2020; Champion et al., 2021). Limited exceptions have been noted, with wild harvests in
51 some places remaining stable or increasing (e.g., Arreguín-Sánchez, 2019; Robinson et al., 2019b; Kainge et
52 al., 2020). Where possible, fishers are maintaining harvests by broadening catch diversity, traveling
53 poleward, and changing gear and strategies (high confidence) (Section, Barange et al., 2018; Dubik
54 et al., 2019; Townhill et al., 2019). Fisheries and aquaculture adaptations, including management, are
55 comprehensively assessed in Sections, 5.8.4, and 5.9.4.


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 1 Ocean acidification and deoxygenation caused by climate change are thought to influence fishing and
 2 aquaculture harvests, but limited evidence prevents assessing their present global impact on harvests.
 3 Substantial economic losses in the North American Pacific Coast shellfish aquaculture industry in the 2000s
 4 assessed in SROCC (Bindoff et al., 2019) and WGII AR5 (Pörtner et al., 2014) remain the clearest example
 5 of human harm from ocean acidification. Technology-based adaptations (Section 3.6.3) have minimised
 6 aquaculture losses from ocean acidification, including early warning systems to guide hatchery operations
 7 and culturing resilient shellfish strains (Section 5.9.4, Barton et al., 2015a). Laboratory studies show that
 8 ocean acidification decreases the fitness, growth, or survival of many economically and culturally important
 9 larval or juvenile shelled mollusks (high confidence) (Cao et al., 2018; Onitsuka et al., 2018; Stevens and
10 Gobler, 2018; Griffith et al., 2019a; Mellado et al., 2019), and of several valuable wild-harvest crab species
11 (Barton et al., 2015a; Punt et al., 2015; Miller et al., 2016; Swiney et al., 2017; Gravinese et al., 2018;
12 Tomasetti et al., 2018; Long et al., 2019; Trigg et al., 2019). Ocean acidification alters larval settlement and
13 metamorphosis of fish in laboratory studies (high confidence) (Cattano et al., 2018; Espinel-Velasco et al.,
14 2018), suggesting possible changes in fish survival and thus fishery characteristics. Deoxygenation can
15 decrease size and abundance of marine species and suppress trophic interactions (Levin, 2003), decrease the
16 diversity within marine ecosystems (Sperling et al., 2016) while temporarily increasing catchability and
17 increasing the risk of overfishing (Breitburg et al., 2018), and decrease the ecosystem services provided by
18 specific fisheries (Orio et al., 2021). The chronic effects of deoxygenation on wild fisheries are complex and
19 highly interactive with co-occurring drivers and overall ecosystem responses (medium evidence, high
20 agreement) (Townhill et al., 2017; Rose et al., 2019). Detecting and attributing marine ecosystem responses
21 to ocean acidification and deoxygenation outside of laboratory studies remains challenging because of the
22 strong influence of co-occurring environmental changes on natural systems (Section 3.3.5, Rose et al., 2019;
23 Doo et al., 2020).


25 Ocean and coastal organisms will continue moving poleward under RCP8.5 (high confidence) (Section
26, Figure 3.18), and this is expected to decrease fisheries harvests in low latitudes and alter species
27 composition and abundance in higher latitudes (high confidence) (Table 3.26, Figure 3.23, Asch et al., 2018;
28 Morley et al., 2018; Tai et al., 2019; Erauskin-Extramiana et al., 2020; Shelton et al., 2021). Species that
29 succeed in new ranges or conditions may offer opportunities to diversify regional fisheries or aquaculture
30 (Sections, 5.8.4, 5.9.4, Bindoff et al., 2019), or they may outcompete indigenous species and act as
31 invasive species (Sections, 3.5.2).


33 Temperature will continue to be a major driver of fisheries changes globally, but other non-climate factors
34 like organism physiology and ecosystem response (Section 3.3) and fishing pressure (Chapter 5), as well as
35 other climate-impact drivers like acidification, deoxygenation, and sea-ice loss (Section 3.2), will play
36 critical roles in future global and local fisheries changes (high confidence). Warming, acidification, and
37 business-as-usual fishing policy under RCP8.5 are projected to place around 60% of global fisheries at very
38 high risk (medium confidence) (Cheung et al., 2018). Model intercomparison showed that ocean acidification
39 and protection affect ecosystems more than fishing pressure, and ecological adaptation greatly determines
40 impacts on fishery biomass, catch and value until approximately 2050 (medium confidence) (Olsen et al.,
41 2018). Ecosystem responses to warming water, fishing pressure, food-web changes, MHWs, and sea ice
42 algal populations have been responsible for highly variable or collapsing populations of Northern
43 Hemisphere high-latitude forage fish species including sand lances (Ammodytes spp.), Arctic cod
44 (Boreogadus saida), capelin (Mallotus catervarius), and herring (Clupea spp.) (Lindegren et al., 2018;
45 Steiner et al., 2019; Arimitsu et al., 2021; Suca et al., 2021). Declining stocks of forage fish are expected to
46 have detrimental effects on seabirds, pelagic fish, and marine mammals (medium confidence) (Lindegren et
47 al., 2018; Steiner et al., 2019), which may harm dependent human communities, including Arctic Indigenous
48 Peoples (low confidence) (Arctic Monitoring and Assessment Programme, 2018; Steiner et al., 2019).
49 Modelled fishery futures and revenue depend on environmental scenario, fishing fleet composition and
50 management, and ocean acidification and temperature responses of harvested species (high confidence) (Punt
51 et al., 2014; Punt et al., 2015; Seung et al., 2015; Fernandes et al., 2017; Rheuban et al., 2018; Tai et al.,
52 2019; Punt et al., 2020). Detrimental effects of ocean acidification are projected to begin emerging in
53 specific fisheries by 2030 (limited evidence, high agreement) [(southern Tanner crab (Chionoecetes bairdi)
54 (Punt et al., 2015); sea scallop (Placopecten magellanicus) (Rheuban et al., 2018); Northeast Arctic cod
55 (Gadus morhua) (Hänsel et al., 2020); Arctic fisheries (Lam et al., 2016)]. At the same time, projected
56 hypoxic conditions of ~2 mg l­1 of oxygen will be consistently detrimental across taxonomic groups,
57 developmental stages, and climate regions (high confidence) (Sampaio et al., 2021). Ecosystem-based

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 1 management (Section shows promise for decreasing risk from interacting climate and non-climate
 2 drivers to forage species and fished species.


 4 3.5.4 Other Provisioning Services


 6 Non-Food Consumable Products


 8 The interaction of climate and non-climate drivers endangers the supply of non-food consumable products
 9 developed from marine organisms (limited evidence, high agreement). This broad class includes
10 nutraceuticals (derived from fish, krill, shellfish, seaweeds, microbes), food preservatives or additives
11 (derived from crustaceans, fish, microalgae and seaweeds, cyanobacteria), pharmaceuticals (derived from
12 fish, shellfish, microbes, cyanobacteria, corals, sponges), or cosmetic products (derived from sponges,
13 phytoplankton and seaweeds, fish, etc.) (Freitas et al., 2012; Dewapriya and Kim, 2014; Leal and Calado,
14 2015; Stengel and Connan, 2015; Greene et al., 2016; Ciavatta et al., 2017; Gutiérrez-Rodríguez et al.,
15 2018). But biodiversity changes, warming, acidification, and non-climate drivers (especially fishing
16 pressure) may decrease the availability of these organisms or the potency of the compounds they produce
17 (Section, Table 3.26, Figure 3.23, Webster and Taylor, 2012; Mehbub et al., 2014; Kotta et al., 2018;
18 Martins et al., 2018; Conrad et al., 2021). Observed and projected declines and movement of fish stocks due
19 to fishing pressure and climate change impacts (IPCC, 2019b) have generated concerns that the supply and
20 safety of fish and krill oil for human dietary supplements may decline (Section, Gribble et al., 2016;
21 Lloret et al., 2016). This risk can be lowered by technological adaptations (Section such as
22 increasing the use of alternative sources, like marine phytoplankton, macroalgae, marine microbes
23 (Dewapriya and Kim, 2014; Greene et al., 2016; Dave and Routray, 2018; Nguyen et al., 2020) and
24 underutilised resources such as fish, seal, crab and shrimp byproducts (Dave and Routray, 2018), and by
25 improving extraction and processing efficiency(Cashion et al., 2017). Climate effects on non-food
26 consumable products could be widespread yet poorly detected, complicating assessment of impacts, risks,
27 and vulnerability reduction.


29 There is insufficient evidence to develop global projections of future climate impacts on humans through
30 changes in non-food consumable marine products, but specific local examples have been investigated, such
31 as the Arctic ooligan (eulachon) (Thaleichthys pacificus), a small smelt fish. Ooligan grease has been used
32 by Indigenous Peoples of the North Pacific coast (Phinney et al., 2009) for at least 5000 years to treat
33 stomach aches, colds and skin conditions and as a traditional food source high in omega-3 fatty acids (Byram
34 and Lewis, 2001; Cranmer, 2016; Patton et al., 2019). Analysis of remains have shown that ooligan could
35 comprise up to 67% of traditional historical fisheries catches (Patton et al., 2019). Because ooligan spawning
36 relies on the timing of the spring freshet, and because the species has declined in the last 25 years due to
37 fishing pressure and predation, the species may be at risk from combined climate and non-climate drivers
38 (medium confidence) (Talloni-Álvarez et al., 2019). Projections under RCP2.6 or RCP8.5 estimate
39 reductions by 21% or 31% by 2050 in essential nutrients from traditional seafood for Indigenous Peoples in
40 Canada, relative to 2000, with a modelled nutritional deficit that includes non-traditional dietary
41 substitutions (Marushka et al., 2019).


43 Non-Consumable Goods


45 Limited evidence about climate impacts exists for valuable non-food aquatic materials. Ocean warming and
46 acidification harm red coral (Corallium rubrum) (Bramanti et al., 2013) and communities hosting black coral
47 (Antipatharian spp.), both used for jewellery (Ross et al., 2020). While no-take MPAs (Section
48 enhance red coral structural complexity, they only weakly compensate for warming effects (Cerrano et al.,
49 2013; Montero-Serra et al., 2019). Antipatharian spp. are not well studied or monitored (Gress and Andradi-
50 Brown, 2018). Acidification and warming negatively impact pearl oysters (Welladsen et al., 2010; Liu and
51 He, 2012; Liu et al., 2012; Hoegh-Guldberg et al., 2014; Zhang et al., 2019b). Projected climate impacts for
52 2035 would decrease the average net present value of French Polynesia's pearl aquaculture industry by
53 29.1% compared to the present (Hilsenroth et al., 2021). Climate impacts on ornamental species sought by
54 aquarists have not been well studied (Dee et al., 2019b).


56 Decreasing the vulnerability of renewable energy installations, particularly wind turbines, to climate risks
57 (Table 3.26, Bindoff et al., 2019) could include technological adaptations (Section such as storm

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 1 "survival mode" settings (Penalba et al., 2018); preparation for hazards such as icing, SLR, drifting sea ice,
 2 and wave activity (Neill et al., 2018; Goodale and Milman, 2019; Solaun and Cerdá, 2019), and biofouling
 3 (medium confidence) (Want and Porter, 2018; Joyce et al., 2019; Vinagre et al., 2020), which is expected to
 4 increase in response to warming and acidification (medium confidence) (Dobretsov et al., 2019; Khosravi et
 5 al., 2019; Liu et al., 2020d; Lamim and Procópio, 2021). Macroalgae and fish processing byproducts are
 6 being tested for biofuel use (Greene et al., 2016; Alamsjah et al., 2017; Saifuddin and Boyce, 2017;
 7 Sakthivel et al., 2018; Sudhakar et al., 2019; Nguyen et al., 2020; Ramachandra and Hebbale, 2020; Tan et
 8 al., 2020), but weather variability could pose financial risk to this sector (Kleiman et al., 2021).


10 3.5.5 Supporting and Regulating Services


12 Ocean and coastal regulating services are detailed in Table 3.25. The economic value of all regulating
13 ecosystem services in 2015 was estimated at 29.1 trillion USD, with water- and climate-regulating services
14 contributing the most (Balasubramanian, 2019).


16 Habitat Creation and Maintenance and Larval Dispersal


18 Climate impacts have already altered ocean and coastal habitats (Section 3.4.2, Table 3.26, Gissi et al., 2021)
19 in ways that have led to species range shifts, biodiversity changes, phenology changes, and regime shifts
20 (Section 3.4.3) from the surface ocean to the seafloor (very high confidence) (Box 3.3, Figure 3.22).
21 Continued ocean and coastal habitat impacts are projected, and their severities will depend on emissions
22 scenario and co-occurring drivers (Section 3.4.3, Qiu et al., 2019) or extremes (e.g., Babcock et al., 2019).
23 Warming and physical circulation are projected to change larval dispersal, a habitat-related service
24 (Bashevkin et al., 2020), but identifying probable outcomes remains challenging owing to the high
25 variability among species, locations, and recruitment (Schilling et al., 2020; King et al., 2021; Le Corre et
26 al., 2021; Raventos et al., 2021). Climate risks to habitat can be decreased by reducing non-climate drivers,
27 preserving ecosystems, or restoring habitat (Sections 3.6.2, Risk to larval dispersal cannot be
28 meaningfully addressed at scale by human-implemented adaptations; instead, declines in this service will
29 pressure natural systems to adapt via physiological plasticity or evolution (Section 3.3, Bashevkin et al.,
30 2020).


32 Climate Regulation and Air Quality


34 Climate regulation by the ocean depends on physical and biogeochemical processes (Sections 3.2­3.4) that
35 create, move, and store heat, water vapor and other climate-active compounds including CO2, methane, and
36 dimethyl sulfide and methane (WGI AR6 Chapter 6, Naik et al., 2021). Over the 21st century, ocean heat
37 and CO2 uptake will continue (WGI AR6 SPMB4.1, B5.1, IPCC, 2021b) and sea ice loss from warming will
38 allow some additional CO2 uptake (Armstrong et al., 2019), but the ocean will take up a smaller fraction of
39 CO2 emissions as atmospheric CO2 concentrations rise (high confidence) (Table 3.26, WGI AR6 SPM B4.1,
40 IPCC, 2021b).


42 There is very limited evidence on climate-driven air-quality changes in the coastal zone. Increased humidity
43 decreases the lifetime of ozone and increases particulate matter and indoor mold levels (USGCRP, 2016),
44 potentially affecting near-shore air quality. However, coastal zone air pollution can enhance coastal climate
45 impacts by increasing risk of acid rain, which worsens ocean acidification (nitrogen oxides, sulphur oxides,
46 and mercury, Doney, 2010; Northcott et al., 2019).


48 Provision of Fresh Water, Maintenance of Water Quality, and Regulation of Pathogens


50 The salinities of many estuaries, deltas, coastal fresh water aquifers, and soils around the world are
51 increasing, and this decrease in water quality is endangering human health and agricultural yields (very high
52 confidence) (Table 3.26, Section, Bindoff et al., 2019; Bouderbala, 2019; Rahman et al., 2019; Naser
53 et al., 2020; Rakib et al., 2020; Mastrocicco and Colombani, 2021). Coastal salinisation is attributed to
54 regionally varying combinations of climate-impact drivers, like SLR and storm-related flooding by seawater,
55 and non-climate drivers, like water withdrawal and land use changes (very high confidence) (Islam et al.,
56 2019; Rahman et al., 2019; Paldor and Michael, 2021). Monitoring-related adaptations (Section
57 including advances in modeling and monitoring are providing decision-relevant, regional-scale information

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 1 (Colombani et al., 2016; Mukhopadhyay et al., 2019; Slama et al., 2020; Corwin, 2021). For example, new
 2 projections indicate which drinking water intake stations on China's Pearl River Estuary will be unable to
 3 meet demands by 2100 due to SLR and drought (Wang and Hong, 2021), while others show that SLR effects
 4 on seawater intrusion into the coastal aquifer in Kerala, India under both RCP4.5 and RCP8.5 scenarios are
 5 negligible (Sithara et al., 2020). Salinisation-associated changes may disproportionately burden women
 6 responsible for securing drinking water and fuel, such as in the Indian Sundarbans (Mukhopadhyay et al.,
 7 2019). Salinisation will continue to endanger coastal water and soil quality in the future (high confidence)
 8 (Islam et al., 2019; Paldor and Michael, 2021), but the evidence assessed above shows that subsequent
 9 impacts to human health and agriculture will depend heavily on regional variations in environment and
10 human behaviour (medium confidence).


12 Together, climate and non-climate drivers can mobilise toxins and contaminants in ways that harm human
13 and marine species health (very high confidence) (Box 3.2), and climate change is altering these relationships
14 (high confidence) (Table 3.26, Bindoff et al., 2019). Under warming or ocean acidification, marine molluscs
15 exposed to pharmaceuticals via wastewater experience more detrimental biological consequences or greater
16 bioaccumulation (limited evidence, high agreement) (Costa et al., 2020a; Costa et al., 2020b; Dionísio et al.,
17 2020; Freitas et al., 2020; Kibria et al., 2021). Physical circulation, temperature, and biogeochemical
18 characteristics (Bowman et al., 2020; Liu et al., 2020a; Liu et al., 2020b; Sun et al., 2020; Zhang et al.,
19 2020b) control the ubiquitous oceanic distribution of methylmercury, and ocean acidification- and warming-
20 driven changes in planktonic speciation and interactions can promote additional food-web bioaccumulation
21 of methylmercury (Tada and Marumoto, 2020; Wu et al., 2020b; Zhang et al., 2020b; Zhang et al., 2021a).
22 Interactions among drivers also matter: temperature plus overfishing increased tissue methylmercury
23 concentrations in Atlantic bluefin tuna from the 1970s to the 2000s more than the decreases in the late 1990s
24 and 2000s from lower environmental mercury levels (Schartup et al., 2019). This appears true for persistent
25 organic pollutants as well, but their bioaccumulation is related more to temperature effects on animal
26 behaviour than on pollutant dynamics (Houde et al., 2019; Wagner et al., 2019; Kalia et al., 2021). By 2100
27 under RCP8.5, productivity changes and community structure shifts are expected to increase methylmercury
28 concentrations in polar oceans and high-latitude phytoplankton and decrease it in low latitudes (Zhang et al.,
29 2021a). The estimated average global cost of mercury-related health effects by 2050, mainly from seafood
30 consumption during 2010­2050, will be 19 trillion USD (2020), assuming a 3% discount rate, if
31 methylmercury emissions are not reduced (Zhang et al., 2021b).


33 Since previous assessments, evidence has increased that climate impacts such as warming, extreme weather,
34 and SLR are increasing the geographic spread and risk of marine-borne human pathogen outbreaks,
35 including Vibrio spp. (very high confidence) (Table 3.26, Bindoff et al., 2019; Logar-Henderson et al., 2019;
36 Froelich and Daines, 2020; Montánchez and Kaberdin, 2020; Semenza, 2020; Ferchichi et al., 2021).
37 Climate change affects at least 30 human pathogens with aquatic-system infection routes (e.g., ingestion of
38 contaminated water or seafood, or contact with wounds, see Table SM3.2, Cross-Chapter Box ILLNESS in
39 Chapter 2, Nichols et al., 2018). Conditions favourable for Vibrio cholerae are increasing globally, which
40 raises risk to humans (Cross-Chapter Box ILLNESS in Chapter 2). Increased storm-related flooding and
41 SLR further increase human encounters with Vibrio spp. (Froelich and Daines, 2020). Aquatic diseases,
42 particularly Vibrio spp., have caused large economic losses in aquaculture by decreasing the quality or
43 survival of cultured species (Lafferty et al., 2015; Novriadi, 2016). Temperature-based model projections
44 show that all Canadian shellfish beds will experience conditions that promote high risk of Vibrio spp. growth
45 by 2100 for both RCP4.5 and RCP8.5 scenarios (Ferchichi et al., 2021). Climate-impact drivers may increase
46 Vibrio spp. loads in seafood species: laboratory-simulated heatwaves increase Vibrio spp. abundance in
47 Pacific oyster (Crassostrea gigas) (Green et al., 2019) and simulated ocean acidification increases hard clam
48 (Mercenaria mercenaria) susceptibility to Vibrio spp. infection (Schwaner et al., 2020). Projected increases
49 in temperature, extreme and variable rainfall conditions, coastal flooding, and SLR (Section 3.2, Cross-
50 Chapter Box SLR in Chapter 3) strongly increase the risk of frequent and severe aquatic human pathogen
51 outbreaks in ocean and coastal areas that will continue to harm human health and cause economic losses
52 (high confidence) (Cross-Chapter Box ILLNESS in Chapter 2, Froelich and Daines, 2020; Semenza, 2020;
53 Ferchichi et al., 2021). Section assesses human adaptations to increasing risk of marine-borne
54 pathogens.


56 Climate-driven changes in temperature, salinity (from ice melt and precipitation changes), deoxygenation,
57 and ocean acidification can alter dynamics of infectious diseases that target ocean and coastal species by

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 1 increasing hosts' susceptibility or pathogens' abundance or virulence (high confidence) (Burge and
 2 Hershberger, 2020; Byers, 2021). Coral and urchin diseases have increased over time driven by warming-
 3 related declines in organism recovery and survival or immunity (medium confidence) (Cohen et al., 2018;
 4 Tracy et al., 2019). Seagrass and sea star wasting disease outbreaks have occurred under combinations of
 5 ocean warming or MHWs and non-climate drivers (e.g., eutrophication, bottom trawling), but attribution of
 6 these outbreaks to specific drivers is still not resolved (Harvell et al., 2019; Jakobsson-Thor et al., 2020;
 7 Krause-Jensen et al., 2021). Disease outbreaks threaten marine biodiversity, species that create habitat or
 8 dampen wave action, and keystone species (Harvell and Lamb, 2020). Attributing observed changes in
 9 marine disease patterns to climate remains extremely difficult owing to interacting climate and non-climate
10 drivers (Burge and Hershberger, 2020) and lack of baseline data (Tracy et al., 2019). Projected increases in
11 the frequency, duration, and intensity of warming events would reduce survival and recovery of some
12 species from hot events, reduce immunity of other species to pathogens, extend poleward ranges of some
13 pathogens, and increase infection risk when host species congregate in scarce habitat (Cohen et al., 2018).
14 Pathogens that target ocean and coastal organisms may themselves be sensitive to future climate conditions
15 or subsequent ecosystem changes, which challenges development of projections (Cohen et al., 2018; Burge
16 and Hershberger, 2020).


18 Following the high confidence assessment of SROCC Table 3.26, Bindoff et al. (2019) that risks associated
19 with HABs will continue to increase with warming and rising CO2 in the 21st century, new examples have
20 illustrated how toxic HABs interfere with regulating, provisioning (Section 3.5.3), and cultural ecosystem
21 services (Section 3.5.6) in interconnected ways (limited evidence, high agreement). A massive toxic Pseudo-
22 nitzschia spp. bloom in 2013­2016 along the United States (US) West Coast triggered Dungeness crab, rock
23 crab, and razor clam fishery closures to protect human consumers (Sections 3.6.2,, McCabe et al.,
24 2016), and this disproportionately harmed fishers, especially small-vessel owners, and fishing-support
25 service industries, primarily through lost revenue (Ritzman et al., 2018; Moore et al., 2019; Trainer et al.,
26 2019; Jardine et al., 2020; Moore et al., 2020a). Toxic Alexandrium spp. blooms promoted by climate-driven
27 coastal extremes (e.g., MHWs, stratification, runoff) in Tasmania, Australia, in 2012 and Chile in 2016
28 caused fish kills, shellfish product recalls, substantial economic losses, and human sickness and death
29 (Trainer et al., 2019). The Chile event caused an estimated loss of 800 million USD in the farmed salmon
30 industry (Díaz et al., 2019) and resulted in a series of large, long-lasting regional protests calling for national
31 aid (Delgado et al., 2019). New evidence, however, suggests that the perceived global increase in harmful
32 algal blooms results from better monitoring and more detrimental bloom impacts rather than a climate-linked
33 mechanism (Hallegraeff et al., 2021).


35 Natural and engineered systems have long been used effectively to manage precipitation and wastewater
36 safely (Chapter 4, Box 4.5), and maintaining and enhancing them is a key nature-based adaptation strategy
37 for coastal communities (Section, Cross-Chapter Paper 2). Estimated values of water purification and
38 stormwater management provided by coastal ecosystems are in the hundreds to thousands of USD per
39 hectare [e.g., 272 Euro per 0.01 km2 yr­1 from the Mediterranean's sandy coastline (Hérivaux et al., 2018);
40 1100­2800 USD per 0.01 km2 yr­1 from the state of Maryland, USA (Campbell et al., 2020b); 600 USD per
41 0.01 km2 yr­1 in Zhuzhou City, China (Zhan et al., 2020)]. Both wild and cultured organisms also provide
42 filtration services. Seagrasses' ability to purify water is well recognised by coastal residents and ocean
43 resource users in tropical and temperate locations (Ambo-Rappe et al., 2019; Quevedo et al., 2020; Heckwolf
44 et al., 2021; McKenzie et al., 2021a). Globally, aquacultured shellfish remove an estimated 49,000 tonnes of
45 nitrogen and 6000 tonnes of phosphorus from coastal waters, worth a potential 1.20 billion USD, and they
46 may help improve existing engineered wastewater treatment systems (van der Schatte Olivier et al., 2020).
47 Climate change, especially episodic extreme rains and RSLR (Romero-Lankao et al., 2014), is challenging
48 management and design of wastewater and stormwater systems (high confidence) (Flood and Cahoon, 2011;
49 Trtanj et al., 2016; Hummel et al., 2018; Kirshen et al., 2018; Nazarnia et al., 2020; Reznik et al., 2020;
50 McKenzie et al., 2021b) and integrity of coastal landfills (Beaven et al., 2020). Without substantial
51 adaptation that addresses projected wastewater management challenges and community needs (Section
52, Kirshen et al., 2018; Kirchhoff and Watson, 2019; Kool et al., 2020; Nazarnia et al., 2020; Hughes et
53 al., 2021), coastal water quality in many areas will decrease because of more frequent or severe releases of
54 untreated wastes (high confidence) (Flood and Cahoon, 2011; Hummel et al., 2018; Hughes et al., 2021;
55 McKenzie et al., 2021b), and this will have harmful consequences for human and coastal ecosystem health
56 (high confidence) (Cross-Chapter Box ILLNESS in Chapter 2, Section, Bindoff et al., 2019).


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 1 Regulation of Physical Hazards


 3 Coastal ecosystems physically protect people and property from storms and flooding, and climate change
 4 threatens this protection function (Table 3.26, Figure 3.22). Increasingly detailed models show how warm-
 5 water coral reefs (Reguero et al., 2019; Storlazzi et al., 2019; Reguero et al., 2021) mangroves (Blankespoor
 6 et al., 2017; Menéndez et al., 2020; Trégarot et al., 2021) and wetlands (Sun and Carson, 2020) prevent
 7 billions of USD of direct and indirect damage to private and public property and shield millions of people
 8 from flooding each year. Protection by mangroves provides more economic benefits in higher-income
 9 nations and shields more people in lower-income nations (Menéndez et al., 2020). Seagrasses (James et al.,
10 2020; James et al., 2021), kelp (Morris et al., 2020b; Zhu, 2020), suspended shellfish aquaculture (Gentry et
11 al., 2020; Zhu et al., 2020a), oyster reefs (Chowdhury et al., 2019) coastal wetlands (Möller, 2019; Keimer et
12 al., 2021), and sandy coastlines (Section, Hérivaux et al., 2018) also measurably decrease wave
13 energy. Non-climate drivers (e.g., invasive species (James et al., 2020), sediment supply changes (Ganju,
14 2019; Ladd et al., 2019; Ilia, 2020), erosion and storm damage (Mehvar et al., 2019; Bacopoulos and Clark,
15 2021)], acting together with climate-impact drivers and impacts (e.g., sea level rise (Cross-Chapter Box SLR
16 in Chapter 3), changes in plant biodiversity (Section 3.5.2, Lee Smee, 2019; Silliman et al., 2019; Schoutens
17 et al., 2020), MHWs (Section, and acidification (Section compromise physical protection
18 by coastal ecosystems (very high confidence). See Cross-Chapter Box SLR in Chapter 3 and Sections
19 and for assessment of adaptations that address this ecosystem service.


21 Regulation of Carbon Cycling in Ocean and Coastal Ecosystems


23 Current and future total carbon storage and cycling in the ocean are governed by past and future CO2
24 emissions trajectories (Table 3.26), but regional ocean and coastal carbon stocks and cycling vary over time
25 and space due to processes being altered by climate, including ocean circulation, sea-ice cover, coastal
26 upwelling, and thermal stratification (Section; ocean primary production and export (Sections 3.2.3,
27 3.4.4); and marine ecosystem biodiversity (high confidence) (Figure 3.22, Section 3.5.2). Quantifying
28 regional carbon fluxes and stocks is still challenging and relies on indirect measures (e.g., Fennel et al.,
29 2019; Clay et al., 2020), especially in coastal ecosystems where drivers interact. Carbon cycling and storage
30 co-occurs with other regulating services such as habitat provision, water quality maintenance, and coastal
31 protection (Ouyang et al., 2018), particularly in vegetated coastal ecosystems (Box 3.4). Adaptations to
32 support regional carbon cycling and storage generally focus on area-based management and conservation
33 (Section, but interventions to enhance ocean carbon storage are being explored for mitigation
34 (WGIII AR6 Chapter 7).





39 Box 3.4: Blue Carbon Ecosystems


41 Climate change and other anthropogenic drivers, including eutrophication, land-use changes, and
42 overexploitation, directly and indirectly threaten blue carbon ecosystems (Annex II: Glossary). Commonly
43 considered blue carbon ecosystems include vegetated coastal ecosystems (Sections­, whose
44 mangroves, saltmarshes and seagrass beds host rooted, vascular plants known to store large amounts of
45 carbon for long periods and to be amenable to management (Lovelock and Duarte, 2019). Other ocean and
46 coastal taxa including rooted or floating macroalgae (e.g., non-vascular multicellular kelp or seaweed genera
47 such as Macrocystis spp., Sargassum spp., or Laminaria spp., Filbee-Dexter and Wernberg, 2020),
48 phytoplankton, and even pelagic fauna (e.g., finfish or whales, Chami et al., 2019) have also been proposed
49 as blue carbon ecosystems. Terrestrial vascular-plant-derived material can also carry and store significant
50 amounts of carbon in marine environments (Cragg et al., 2020). There is increasing evidence about the
51 coverage and carbon content of macroalgal, planktonic, and faunal taxa, but low agreement about their long-
52 term carbon storage potential and manageability (Alongi, 2018b; Wernberg and Filbee-Dexter, 2018;
53 Lovelock and Duarte, 2019; Ortega et al., 2019; Pfister et al., 2019; Queirós et al., 2019; Filbee-Dexter et al.,
54 2020a; Gallagher, 2020; Mariani et al., 2020; Thorhaug et al., 2020; van Son et al., 2020; Bach et al., 2021;
55 Bayley et al., 2021; Cavanagh et al., 2021; Frontier et al., 2021; Martin et al., 2021; Pedersen et al., 2021;
56 Weigel and Pfister, 2021). This section focuses on the array of ecosystem services and adaptation
57 opportunities provided by vegetated coastal blue carbon ecosystems, where consensus and evidence are most

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1 abundant. Mitigation potential of blue carbon ecosystems is assessed with land-based mitigation options in
2 WGIII AR6 Section 7.4.


 4 Carbon storage and burial in mangroves, saltmarshes and seagrass meadows (Table Box3.4.1) help regulate
 5 ocean and coastal carbon cycling and may contribute to nature-based mitigation, although regional estimates
 6 vary widely based on climatic and edaphic conditions (WGIII AR6 Section 7.4). In addition, coastal
 7 vegetated ecosystems provide substantial and interdependent regulating, provisioning and cultural ecosystem
 8 services. These include disproportionately high biodiversity per unit area (Pörtner et al., 2021a); abundant
 9 habitat (Section and nurseries for aquatic, terrestrial, aerial, and microbial species; natural filtration
10 of waste and stormwater runoff into the coastal ocean (Sections, 4.2.7, Cross-Chapter Box ILLNESS
11 in Chapter 2); coastal protection (Section, Ouyang et al., 2018; Quevedo et al., 2020); food and
12 natural materials (Sections 3.5.3, 3.5.4); and support for tourism, livelihoods, and cultural activities (Section
13 3.5.6). Global estimates of services provided by coastal blue carbon ecosystems depend on the quality of
14 available mapping, which is currently best developed for mangroves (Macreadie et al., 2019), and improving
15 for saltmarshes and seagrasses (McOwen et al., 2017; McKenzie et al., 2020; Young et al., 2021).



18 Table Box3.4.1: Estimates of organic carbon storage and burial rates in mangroves, saltmarshes, and seagrass
19 meadows. Estimates are the mean ± 95% confidence interval, where available (indicating the extremely likely range)
20 and range. Carbon stocks for mangroves include above- and below-ground storage up to 3 m depth (sampling period
21 2007­2017). The estimates for saltmarsh and seagrass stocks are soil stocks up to 1 m depth (observations spanning
22 1983­2016 for saltmarshes and until 2016 for seagrass meadows). Date ranges for the burial rates are: 1989­2020,
23 1975­2020 and 1956­2016 for mangroves, saltmarshes and seagrass meadows, respectively.

                 Mangroves                                       Saltmarshes             Seagrass meadows

    Carbon stocks (MgC ha­1) 856 ± 64.2 (79­2208)                317.2 ± 38.2 (27­1900)  139.7 (9.1­628) (Fourqurean
                                        (Kauffman et al., 2020)  (Alongi, 2018c)         et al., 2012; Alongi, 2018d)

    Carbon burial rate (g C m­2 194 ± 30 (6.2­1722) (Wang 168 ± 14 (1.2­1167.5)          220.7 ± 40.2 (-2094­2124)
                                                                                         (Alongi, 2018d)
    yr­1)        et al., 2020)                                   (Wang et al., 2020)

    Global Carbon burial rate 41 (Wang et al., 2020)             12.63 (Wang et al., 2020) 35.31 (Alongi, 2020)
    (TgC yr­1)

    Global areal coverage (Mha) 13.7 (Richards et al., 2020) 5.5 (McOwen et al., 2017) 16 (McKenzie et al., 2020)



26 Coastal vegetated ecosystems are vulnerable to harm from multiple climate and non-climate drivers, and
27 together these have reduced wetland area globally (high confidence) (Section and endangered the
28 services provided by these ecosystems (high confidence). Loss of coastal vegetated ecosystems changes
29 biodiversity (Sections 3.5.2,­ (Numbere, 2019; Parreira et al., 2021), increases risk of damage
30 and erosion from SLR and storms (Sections­, Cross-Chapter Box SLR in Chapter 3, Galeano
31 et al., 2017), and impacts provisioning (Sections 3.5.3­3.5.4, Li et al., 2018b; Maina et al., 2021). These
32 changes also strongly determine the quantity and longevity of blue carbon storage (high confidence)
33 (Macreadie et al., 2019; Lovelock and Reef, 2020). Specific site characteristics and ecosystem responses to
34 climate change will determine future local blue carbon storage or loss (high confidence) (Table Box3.4.2).
35 For instance, poleward migration of mangroves to areas dominated by salt marshes is expected to increase
36 carbon storage (Kelleway et al., 2016); however, this change in the dominant vegetation and associated
37 faunal changes can modify carbon stocks and sequestration, as well as other ecosystem services (Martinetto
38 et al., 2016; Kelleway et al., 2017; Smee et al., 2017; Macreadie et al., 2019; Macy et al., 2019). Landward
39 range expansion of mangroves, marshes, and seagrass in response to gradual RSLR can enhance carbon
40 sequestration (Cross-Chapter Box SLR in Chapter 3, Section, Macreadie et al., 2019), but coastal
41 squeeze can limit this (Phan et al., 2015; Schuerch et al., 2018) and RSLR can either submerge and bury or
42 erode and release stored blue carbon (Section, Macreadie et al., 2019; Lovelock and Reef, 2020).
43 Gains and losses of mangrove habitat area (and therefore carbon storage) projected for nations under RCP4.5
44 and RCP8.5 depend primarily on the combination of SLR rate, adaptation scenario (including coastal
45 development), and island or continental status (Lovelock and Reef, 2020). The influence of warming,
46 MHWs, and acidification on seagrass meadows (Kendrick et al., 2019; Strydom et al., 2020) and associated
47 coralligenous reefs (Zunino et al., 2019) suggests that future warming and especially MHWs will cause more
48 widespread loss of services from these ecosystems (Section Loss of blue carbon ecosystems will not
49 only halt carbon storage, but also release stored carbon: emissions after 2000 due to global mangrove

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1 deforestation have been estimated at 23.5­38.7 Tg Cyr­1 (Ouyang and Lee, 2020). Mitigation estimates for
2 avoided conversion and restoration of coastal wetlands and the implications of the impacts of climate change
3 are assessed in WGIII AR6 Section 7.4.


 5 To date, initiatives aiming to restore coastal wetland ecosystems primarily address ecosystem characteristics
 6 other than carbon storage (Herr et al., 2017; de los Santos et al., 2019; Lovelock and Duarte, 2019; Friess et
 7 al., 2020a). But recovery of coastal vegetated ecosystems is expected to bring back the full suite of
 8 ecosystem services they provide, not just carbon storage (medium confidence) (Marbà et al., 2015a; Burden
 9 et al., 2019; Friess et al., 2020a), making coastal restoration a low-risk action that offers both adaptation and
10 mitigation benefits (Steven et al., 2020; Gattuso et al., 2021). Successful restoration requires using
11 appropriate plant species in suitable environmental settings (Wodehouse and Rayment, 2019; Friess et al.,
12 2020a) with favourable geomorphology and biophysical conditions (Cameron et al., 2019; Ochoa-Gómez et
13 al., 2019) and considering social, economic, policy, and operational constraints (Section, Cross-
14 Chapter Box NATURAL in Chapter 2), now and in the future (high confidence) (Duarte et al., 2020;
15 Lovelock and Reef, 2020). Nevertheless, restored spaces may not store carbon at rates equal to those of
16 undisturbed spaces (Yang et al., 2020), and it may take decades to determine or achieve carbon storage
17 outcomes of restoration (Sasmito et al., 2019; Duarte et al., 2020; Oreska et al., 2020). Integration improves
18 efforts to restore or conserve coastal wetland ecosystems to accomplish both adaptation and mitigation
19 outcomes (Steven et al., 2020). Government-led conservation of blue carbon ecosystems as part of national
20 and subnational climate strategies (e.g., Friess et al., 2020a; Kelleway et al., 2020; Wedding et al., 2021)
21 benefits from coordination with private activities, such as incentivising conservation with payments for
22 ecosystem services (Muenzel and Martino, 2018; Friess et al., 2020a). Moreover, successful area-based
23 protection measures consider both environmental and social issues (Section Continued integration
24 and alignment of policies at international to local levels (Section 3.6.5) will also support achieving the
25 adaptation and mitigation benefit of blue carbon spaces (Friess et al., 2020a; Steven et al., 2020; Wu et al.,
26 2020a).



29 Table Box 3.4.2: Examples of vegetated blue carbon ecosystem carbon storage gains and losses in response to climate-
30 impact drivers, and key actions contributing to maintained or and increased carbon storage. "+C" indicates potential
31 positive effects on blue carbon stocks, "­C" indicates negative effects, "0" indicates no effects, and "C" indicates
32 positive or negative effects. Effects on carbon stocks from, Macreadie et al. (2019); Lovelock and Reef (2020);
33 Wang et al. (2020). Key actions to sustain blue carbon storage from Duarte et al. (2020); Wedding et al. (2021).

                                      Mangroves             Saltmarshes  Seagrasses

    Sea-level rise

    Landward expansion by             +C                    +C           +C


    Coastal squeeze                   ­C                    ­C           ­C

    Loss of low-lying or              ­C                    ­C           ­C

    submerged land or


    Human adaptation to               +C                    +C

    increase accommodation


    Extreme storms

    Erosion/ loss of area/            ­C                    ­C           0 to ­C


    Enhanced sedimentation            +C                    +C           +C

    Vegetation damage and             ­C to +C                           ­C



    Increased productivity            +C                                 +C

    Vegetation mortality                                                 ­C

    Increased decomposition of        ­C                    ­C to +C


    Poleward expansion of             +C                    ­C


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Poleward expansion of                                +C


Poleward expansion of             C


Change in dominant species        C

Rising concentrations of
atmospheric CO2

Increased productivity of         C              C   +C

some species

Biodiversity loss                                    ­C

Altered precipitation

Vegetation mortality              ­C

Reduced productivity              ­C             ­C

Increased productivity            +C                 +C

Increased remineralisation        ­C             ­C

Low salinity events                                  0 to ­C

Key actions to sustain blue
carbon storage

Protect ecosystems                X              X   X

Develop alternative               X


Provide space for landward        X              X


Restore hydrological              X              X


Maintain/restore sediment         X              X


Restore ecosystems                X                  X

Plant indigenous species                         X

Reduce nutrient inputs                               X


3 [END BOX 3.4 HERE]


6 3.5.6 Cultural Services


 8 Cultural services provided by ocean and coastal ecosystems help maintain psychological well-being, cultural
 9 development, human identities, educational opportunities, and reserves that could support development of
10 future goods or activities (Table 3.25). Most recent studies of ocean and coastal cultural services simply
11 detail local benefits using replicable methods (e.g., Drakou et al., 2018; Folkersen, 2018; Förster et al., 2019;
12 Lau et al., 2019; Pouso et al., 2019; Weitzman, 2019; Yang et al., 2019), focusing on diverse ocean and
13 coastal environments and ecosystems (Jobstvogt et al., 2014; Balzan et al., 2018; Drakou et al., 2018; Ingram
14 et al., 2018; Pouso et al., 2018; Zapata et al., 2018; Ghermandi et al., 2019; Pouso et al., 2019; Tanner et al.,
15 2019; Turner et al., 2019; Ortíz Liñán and Vázquez Solís, 2021). Cultural ecosystem services may directly
16 benefit from marine development activities, such as marine aquaculture (e.g., Alleway et al., 2018), and
17 indirectly benefit from marine activities that increase biodiversity (e.g., Causon and Gill, 2018). Cultural
18 services are generally quantified using interviews and revealed-preference or stated-preference valuation
19 (National Research Council, 2005; Sangha et al., 2019), but people often are especially reluctant to evaluate
20 cultural ecosystem services in monetary terms, given the spiritual and community linkages to these services
21 (Oleson et al., 2018).


23 Additional evidence since previous assessments (Table 3.26) confirms that climate-change impacts on ocean
24 and coastal cultural ecosystem services have already disrupted people's place-based emotional attachments
25 and cultural activities (limited evidence, high agreement) (Figure 3.22). Bleaching and mortality of corals in

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 1 the Great Barrier Reef have induced measurable "reef grief," a type of solastalgia, among reef visitors and
 2 researchers (Conroy, 2019; Curnock et al., 2019; Marshall et al., 2019). The mental health of people in
 3 Tuvalu (Gibson et al., 2020), Alaska (Allen, 2020), and Honduras (Kent and Brondo, 2020) have suffered
 4 from both the experience of climate impacts on ocean and coastal ecosystems (e.g., SLR and changes in
 5 fisheries and wildlife), and the anticipation of more in the future. The climate-associated MHWs and harmful
 6 algal bloom events in 2014­2016 in the US Pacific Northwest (Moore et al., 2019) prevented seasonal razor
 7 clam harvests culturally important to Indigenous Peoples and the local community (Section, Crosman
 8 et al., 2019). SLR and storm-driven coastal erosion endanger coastal archaeological and heritage sites around
 9 the world (very high confidence) (Hoque and Hoque, 2008; Carmichael et al., 2018; Reimann et al., 2018;
10 Elliott and Williams, 2019; Ravanelli et al., 2019; Anzidei et al., 2020; Chemeli et al., 2020; García Sánchez
11 et al., 2020; Harkin et al., 2020; Hil, 2020; Rivera-Collazo, 2020).


13 Disruptions in ocean and coastal ecosystem services partly attributable to climate change have also caused
14 economic losses (limited evidence, high agreement). Water quality deterioration over 24 years in a US
15 temperate bay due to nutrient enrichment and warming caused 0.08­0.67 million USD per decade in lost
16 recreational shellfish revenues (Luk et al., 2019). In southwestern Florida, USA, where nutrient enrichment,
17 lake hydrology, and rainfall conditions control cyanobacterial HAB formation (Havens et al., 2019), toxic
18 HAB events deterred visitors and recreation, leading to lodging and restaurant revenue losses (Bechard,
19 2020), decreased domestic and international arrivals and overall visitor spending (a 99 million USD loss
20 from August to October 2018, Scanlon, 2019), and lost recreational spending from loss of boat-ramp access
21 (a 3 million USD economic loss from June to September 2018, Alvarez et al., 2019). In Cornwall, England,
22 HABs from 2009­2016 disrupted residents' sense of place, identity, and well-being by interrupting
23 recreational and economic activities, and by creating feelings of uncertainty and unease around the safety or
24 dependability of future ocean-related activities (Willis et al., 2018). Increasingly abundant Sargassum spp.
25 floating macroalgae from the central Atlantic Ocean and Caribbean Sea, whose proliferation has been
26 attributed to high sea surface temperatures and nutrient enrichment (Wang et al., 2019a), has substantially
27 disrupted beach tourism in the Caribbean and Mexico and imposes millions of dollars of clean-up costs
28 annually on affected beaches (Milledge and Harvey, 2016).


30 Observed disruption of ocean and coastal cultural services by climate impacts, plus increasingly severe and
31 widespread projected climate change impacts on ocean and coastal ecosystems, imply that risk to cultural
32 ecosystem services will remain constant or grow (medium confidence) (Figure 3.22, Table 3.26). Recent
33 studies assert that cultural ecosystem services are at risk from climate change (high confidence) (Singh et al.,
34 2019a; Koenigstein, 2020). However, limited evidence and complex social-ecological interactions (e.g.,
35 Ingram et al., 2018) challenge development of specific projections. For instance, the little auk (Alle alle) in
36 the North Water Polynya is traditionally harvested by Indigenous Inughuit for food and community-wide
37 celebrations and seasonal activities, but harvests are threatened to an undetermined degree as the seabird
38 competes for food with recovering bowhead whale (Balaena mysticetus) populations and northward range
39 shifts of capelin (Mallotus villosus) due to warming (Mosbech et al., 2018). Section 3.6 assesses the cultural
40 implications of implemented human adaptations.



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 2 Figure 3.22: Observed global influence of climate-impact drivers on ecosystem services. (Coloured cells) The
 3 "observed impact" indicates the total effect of all climate-impact drivers on a specific ecosystem service, using expert
 4 judgement based on summary statements throughout Section 3.5. (Grey cells) Co-occurring non-climate drivers that
 5 affect the service. Cell colour shows whether the observed impact of the climate-impact driver on a group of ecosystem
 6 services is positive (beneficial), negative (detrimental) or mixed (usually resulting from location, the presence of
 7 interacting drivers, or changing effects over time). No assessment indicates that not enough evidence is available to
 8 assess the direction of impact.



11 3.6 Planned Adaptation and Governance to Achieve the Sustainable Development Goals (SDGs)


13 3.6.1 Point of Departure


15 Human adaptation comprises an array of measures (adaptation options, IPCC, 2014a) that modulate harm or
16 exploit opportunities from climate change (Section Adaptation options that respond to key ocean
17 and coastal risks (Section 3.4) focus on individuals, livelihoods, and economic sectors that benefit from
18 ocean and coastal ecosystem services (Section 3.5). AR5 concluded that local adaptation measures would not
19 alone be enough to offset global effects of increased climate change on marine and coastal ecosystems, and
20 that mitigation of emissions would also be necessary (high confidence) (Pörtner et al., 2014; Oppenheimer et
21 al., 2019, Table 3.27). SROCC assessed that ecosystem-based adaptation, including MPAs (high confidence)
22 (Bindoff et al., 2019) and adaptive management are effective to reduce climate change impacts (IPCC, 2018;
23 IPCC, 2019b), but that existing marine governance is insufficient to provide an effective adaptation response
24 in the marine ecosystem (high confidence) (IPCC, 2019c).

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3 Table 3.27: Conclusions from previous IPCC assessments about implemented adaptation, enablers and limits and

4 contribution to SDGs.

                               AR5                             SR15                       SROCC

   Degree of Implementation    The analysis and                Adaptation (to SLR) is     A diversity of adaptation
                               implementation of coastal already happening (high responses to coastal impacts

                               adaptation toward climate- confidence) and will remain and risks have been

                               resilient and sustainable important over                   implemented around the

                               coasts has progressed more multi-centennial time scales world, but mostly as a

                               significantly in developed (Hoegh-Guldberg et al., reaction to current coastal

                               countries than in developing 2018a).                       risk or experienced disasters

                               countries (high confidence)                                (high confidence)

                               (Wong et al., 2014).                                       (Oppenheimer et al., 2019).

   Conservation and            With continuing climate Existing and restored natural Ecosystem restoration may
   (Section           change, local adaptation coastal ecosystems may be be able to locally reduce

                               measures (such as               effective in reducing the climate risks (medium

                               conservation) or a reduction adverse impacts of rising sea confidence) but at relatively

                               in human activities (such as levels and intensifying       high cost and effectiveness

                               fishing) may not sufficiently storms by protecting coastal limited to low emissions

                               offset global-scale effects on and deltaic regions (medium scenarios and to less

                               marine ecosystems (high confidence)                        sensitive ecosystems (high

                               confidence)                     (Hoegh-Guldberg et al., confidence)

                               (Pörtner et al., 2014).         2018a).                    (Bindoff et al., 2019).

   Enablers, Barriers and      Adaptation strategies for       Lower rates of change      There are a broad range of
   Limits of Adaptation        ocean regions beyond
   (Section           coastal waters are generally    [associated with a 1.5ºC identified barriers and limits
                               poorly developed but will
                               benefit from international      temperature increase]      for adaptation to climate
                               legislation and expert
                               networks, as well as marine     enhance the ability of natural change in ecosystems and
                               spatial planning (high
                               agreement)                      and human systems to adapt, human systems (high
                               (Hoegh-Guldberg et al.,
                               2014).                          with substantial benefits for confidence). Limitations

                                                               a wide range of terrestrial, include [...] availability of

                                                               freshwater, wetland, coastal technology, knowledge and

                                                               and ocean ecosystems       financial support and

                                                               (including coral reefs) (high existing governance

                                                               confidence)                structures (medium

                                                               (Hoegh-Guldberg et al., confidence).

                                                               2018a).                    (Bindoff et al., 2019).

                                                                                          Existing ocean governance

                                                                                          structures are already facing

                                                                                          multi-dimensional, scale-

                                                                                          related challenges because of

                                                                                          climate change [...] (high


                                                                                          (Bindoff et al., 2019).

   SDGs and Other Policy       Overall, there is a strong Adaptation strategies can       Achieving [the SDGs] and
   Frameworks (Section 3.6.4)                                                             charting Climate Resilient
                               need to develop ecosystem- result in trade-offs with and   Development Pathways
                                                                                          depends in part on ambitious
                               based monitoring and            among the SDGs (medium     and sustained mitigation
                                                                                          efforts to contain SLR
                               adaptation strategies to        evidence, high agreement)  coupled with effective
                                                                                          adaptation actions to reduce
                               mitigate rapidly growing (Roy et al., 2018).               SLR impacts and risk
                                                                                          (medium evidence, high
                               risks and uncertainties to the                             agreement)
                                                                                          (Oppenheimer et al., 2019).
                               coastal and oceanic

                               industries, communities, and

                               nations (high agreement)

                               (Hoegh-Guldberg et al.,




7 This section builds on the SROCC assessment of the portfolio of available solutions, their applicability, and
8 their effectiveness in reducing climate change-induced risks to ocean and coastal ecosystems. Section 3.6.2
9 assesses the set of planned adaptation measures. Section 3.6.3 assesses implementation of adaptation

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 1 solutions and the enablers, barriers, and limitations that affect their feasibility. Section 3.6.4 evaluates the
 2 contribution of planned adaptation to the Sustainable Development Goals (SDGs) and other policy-relevant
 3 frameworks, and Section 3.6.5 synthesises emerging evidence about best practices.


 5 3.6.2 Adaptation Solutions


 7 Adaptation in ocean and coastal ecosystems continues to be informed primarily by theory, as there is still
 8 limited evidence about implemented solutions (high agreement) (Seddon et al., 2020) and their success
 9 across regions, especially in low-income nations (Chausson et al., 2020). Adapting to climate change
10 depends on society's ability and willingness to anticipate the change, recognise its effects, plan to
11 accommodate its consequences (Ling and Hobday, 2019; Wilson et al., 2020b), and implement a coordinated
12 portfolio of informed solutions. Here, the complete portfolio of adaptation solutions is assessed using the
13 taxonomy of Abram et al. (2019): (1) socio-institutional adaptation, (2) built infrastructure and technology,
14 and (3) marine and coastal nature-based solutions (NbS) (Figure 3.23).




18 Figure 3.23: Adaptation solutions for ocean and coastal ecosystems that address climate-change risk in different ocean
19 ecosystems, communities and economic sectors. Box-outline weights indicate confidence in the solution's potential to
20 reduce mid-term risks (based on the amount of evidence and agreement supporting the solutions, see SM3.5.1 for full
21 assessment). The feasibility and effectiveness of each solution (low, medium or high) indicates its ability to support
22 ecosystems and societies as they adapt to climate change impacts, based on Table SM3.3.

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3 Socio-Institutional Adaptation


 5 Increasing evidence shows that an effective solution portfolio includes social and institutional adaptation
 6 (Figure 3.23, top; Table 3.28). Social adaptation to climate change is already occurring, as people use
 7 strategies ranging from accommodating change, to coping, adapting and transforming their livelihoods (Béné
 8 and Doyen, 2018; Fedele et al., 2019; Galappaththi et al., 2019; Barnes et al., 2020; Ojea et al., 2020; Green
 9 et al., 2021c). Although management and institutions have major roles in adaptation (Gaines et al., 2018;
10 Barange, 2019), marine governance is impeded by increasing numbers of often-competing users and uses
11 (Boyes and Elliott, 2014); sector-led, fragmented, efforts (Nunan et al., 2020); and a legal framework less
12 clear than those on land (Crespo et al., 2019; Guggisberg, 2019). Future social responses depend on warming
13 levels and on the institutional, socio-economic and cultural constructs that allow or limit livelihood changes
14 (medium confidence) (Chapter 18, Galappaththi et al., 2019; Ford et al., 2020; Green et al., 2021c). Both
15 social and institutional transformations are needed to change the structures of power, culture, politics and/or
16 identity associated with marine ecosystems (Section 1.5.2, Wilson et al., 2020b). Ideally, institutional and
17 social adaptation will work together to sustain knowledge systems and education, enhance participation and
18 social inclusion, facilitate livelihood support and transformational change of dependent coastal communities,
19 provide economic and financial instruments, and include polycentric and multi-level governance of
20 transboundary management (Fedele et al., 2019; Fulton et al., 2019).



23 Table 3.28: Assessment of socio-institutional adaptation solutions to reduce mid-term climate impacts in oceans and
24 coastal ecosystems. Confidence is assessed in SM3.5.1. Feasibility and effectiveness are assessed in Figure 3.24.

    Solution            Confidence in solution Contribution to       Selected references Examples of
                        (mid-term potential) adaptation                                          implementation

    Knowledge diversity High confidence    Consideration of IK (Norström et al., 2020; Ecotourism (Section

                                           and LK systems is Petzold et al., 2020;,

                                           beneficial to             Gianelli et al., 2021; conservation (Section

                                           communities, increases Schlingmann et al.,

                                           their resilience, and is 2021)

                                           relevant and

                                           transferable beyond

                                           the local scale.

    Socially inclusive  High confidence    Policies that promote (Brodie Rudolph et al., Finance (Section
                                           participation of a        2020; Ford et al.,

                                           diversity of groups are 2020; Friedman et al.,

                                           able to address           2020)

                                           existing vulnerabilities

                                           in coastal

                                           communities, and

                                           promote adaptation

                                           and transformational


    Participation       Medium confidence  Participation in          (Brodie Rudolph et al., Fisheries and

                                           decision making and 2020; Claudet et al., mariculture (Section

                                           adaptation processes is 2020a; Hügel and, Indigenous

                                           recommended across a Davies, 2020; Sumaila Peoples (Section

                                           range of different        et al., 2021)

                                           hazards and contexts

                                           and has the potential to

                                           improve adaptation


    Livelihood          Medium confidence  Livelihood                (Blanchard et al.,    Fisheries and
                                           diversification in        2017; Cinner and      mariculture (Section

                                           communities               Barnes, 2019;, coastal

                                           dependent on marine Mohamed Shaffril et communities (Cross-

                                           and coastal ecosystems al., 2020; Owen, 2020; Chapter Box SLR in

                                           reduces climate risks Pinsky, 2021; Taylor Chapter 3), tourism

                                           and confers flexibility et al., 2021)           (Section

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                                       to individuals, which is
Migration                              key to adaptive
Finance and market
mechanisms          Medium confidence  When individuals are (Barnett and              Fisheries and
                    Low confidence     given the choice about McMichael, 2018)        mariculture (Section
Disaster response                      mobility, they may                   
programs            High confidence    elect to use this
                    High confidence    response to minimise
Multi-level ocean   High confidence    climate risks and
governance          Medium confidence  benefit their
transboundary                          Migration often           (Maharjan et al., 2020; Coastal livelihoods
                                       involves different Biswas and Mallick, (Section

                                       spatio-temporal scales 2021; Zickgraf, 2021)

                                       than mobility.

                                       Migration could be

                                       considered an

                                       adaptation solution for

                                       some coastal and

                                       island populations in

                                       the cases of extreme

                                       events, but also as a

                                       response to more

                                       gradual changes (e.g.,

                                       coastal erosion from

                                       sea-level rise (SLR)).

                                       Financial mechanisms (Shaffril et al., 2017; Economic dimensions

                                       and credit provision Dunstan et al., 2018; (Section

                                       for marine-dependent Hinkel et al., 2018;

                                       livelihoods are           Moser et al., 2019;

                                       effective for             Sainz et al., 2019;

                                       overcoming impacts Woodruff et al., 2020)

                                       from SLR, extreme

                                       events and other

                                       climate-impact drivers.

                                       Disaster response (Nurhidayah and              Climate services
                                       programs confer           McIlgorm, 2019)      tourism cruise ship
                                                                                      sector (Section
                                       resilience to                        

                                       communities and

                                       contribute to

                                       adaptation, when

                                       designed to be

                                       inclusive, participatory

                                       and adaptive.

                                       The multi-scale nature (Miller et al., 2018;   Policy frameworks
                                       of ocean and coastal Gilfillan, 2019;          (Section
                                       climate-change risk Holsman et al., 2019;
                                       demands adaptation Obura et al., 2021).
                                       solutions at multiple
                                       levels of governance
                                       that consider the
                                       objectives and
                                       perceptions of all
                                       stakeholders to support
                                       local implementation
                                       of broad strategies.

                                       Institutional             (Engler, 2020; Mason Fisheries (Section

                                       agreements for the et al., 2020; Oremus et, Cross-

                                       management of             al., 2020; Melbourne- Chapter Box

                                       transboundary marine Thomas et al., 2021) MOVING SPECIES in

                                       resources are key for a                        Chapter 5)

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                                            sustainable future
                                            given current impacts
                                            on marine species
                                            distribution due to
                                            climate change



3 Built Infrastructure and Technology


 5 Engineering and technology support marine and coastal adaptation (Table 3.29). Built infrastructure includes
 6 engineered solutions that protect, accommodate or relocate coastal assets using hard engineering, like
 7 seawalls, and soft engineering, such as beach and shore nourishment (Cross-Chapter Box SLR in Chapter 3).
 8 Technological tools include early-warning systems for extreme events (Bindoff et al., 2019; Collins et al.,
 9 2019a), improved forecast and hindcast models (Winter et al., 2020; Davidson et al., 2021; Spillman and
10 Smith, 2021) and environmental monitoring (Claudet et al., 2020a; Wilson et al., 2020a; Melbourne-Thomas
11 et al., 2021) that support informed decision making (Tommasi et al., 2017; Rilov et al., 2020; A. Maureaud
12 et al., 2021). Emerging adaptation technologies, such as habitat development, active restoration and assisted
13 evolution (Boström-Einarsson et al., 2020; Kleypas et al., 2021), intend to accelerate recovery of damaged
14 ecosystems and promote ecological adaptation to climate change (Jones et al., 2018a; Boström-Einarsson et
15 al., 2020; Kleypas et al., 2021).



18 Table 3.29: Assessment of built infrastructure and technology solutions to reduce mid-term climate impacts in oceans
19 and coastal ecosystems. Confidence is assessed in SM3.5.1. Feasibility and effectiveness are assessed in Figure 3.24.

    Solution     Confidence in solution Contribution to            Selected references Examples of
                 (mid-term potential) adaptation                                               implementation

    Accommodation and High confidence       Asset modification and (Hanson and Nicholls, Cross-Chapter Box
                                            relocation of          2020; Monios and SLR in Chapter 3,

                                            livelihoods to adapt to Wilmsmeier, 2020; coastal development

                                            sea-level rise (SLR), Zickgraf, 2021)           (Section

                                            extreme events and

                                            coastal erosion.

    Protection and beach Medium confidence  Protection of coastal  (Pinto et al., 2020; de  Cross-Chapter Box
    and shore nourishment                   ecosystems with        Schipper et al., 2021;   SLR in Chapter 3,
                                            interventions such as  Elko et al., 2021)       coastal development
                                            beach and shore                                 (Section
                                            nourishment is a
                                            common response to
                                            beach erosion around
                                            the world, and an
                                            alternative to hard
                                            protection structures
                                            such as seawalls.

    Early-warning systems High confidence   Early-warning systems (Bindoff et al., 2019;    Coastal development
                                            can support decision- Collins et al., 2019a;    human health (Section
                                            making, limit          Winter et al., 2020;

                                            economic losses from Neußner, 2021)

                                            extreme events, and

                                            aid in the enterprise

                                            and development of

                                            adaptive management


    Seasonal and dynamic High confidence    The proliferation of (Payne et al., 2017; Fisheries and
                                            real-time and seasonal Hazen et al., 2018; mariculture (Section

                                            forecasts of           Fernández-Montblanc, MPAs

                                            temperature extremes, et al., 2019; Holbrook (Section,

                                            marine heatwaves et al., 2020; Winter et climate services

                                            (MHWs), storm          al., 2020; Bever et al., (Section

                                            surges, harmful algal 2021; Davidson et al.,

                                            blooms (HABs), and

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                                          the distribution of        2021; Spillman and
                                          living marine              Smith, 2021)
                                          resources greatly
                                          contribute to
                                          adaptation through
                                          monitoring, early-
                                          warning systems,
                                          adaptive management
                                          and ecosystem-based

    Monitoring systems Medium confidence  Monitoring systems (Nichols et al., 2019;         MPAs (Section
                                                                                  , climate
                                          that address climate- Claudet et al., 2020a;      services (Section
                                                                                  , fisheries
                                          impact drivers,            Wilson et al., 2020a)  (Section

                                          ecosystem impacts and

                                          social vulnerabilities in

                                          marine social-

                                          ecological systems are

                                          key for adaptation.

    Habitat development Low confidence    Accelerates the            (Jones et al., 2018a;  Restoration (Section
                                          recovery of damaged Boström-Einarsson et

                                          ecosystems and             al., 2020; Kleypas et

                                          promotes ecological or al., 2021)

                                          biological adaptation

                                          to future climate


    Active restoration High confidence    Reintroduces species (Boström-Einarsson et Restoration (

                                          or augments existing al., 2020; Rinkevich,

                                          populations. For           2021)

                                          example, propagating

                                          and transplanting heat-

                                          tolerant coral species.

    Assisted evolution High confidence    Manipulates species'       (Bulleri et al., 2018; Restoration (Section
                                          genes to accelerate        National Academies of
                                          natural selection.         Sciences, 2019; Morris
                                                                     et al., 2020c)



3 Marine and Coastal Nature-Based Solutions (NbS)


 5 The ocean and coastal adaptation portfolio (Figure 3.23) also includes marine and coastal NbS (Table 3.30).
 6 NbS that contribute to climate adaptation, also known as ecosystem-based adaptations (EBA), are cross-
 7 cutting actions that harness ecosystem functions to restore, protect, and sustainably manage marine
 8 ecosystems facing climate change impacts, while also benefiting social systems and human security
 9 (Abelson et al., 2015; Barkdull and Harris, 2019) and supporting biodiversity (high confidence) (Annex II:
10 Glossary, Cross-Chapter Box NATURAL in Chapter 2, Seddon et al., 2021). NbS are expected to contribute
11 to global adaptation and mitigation goals (high confidence) (Beck et al., 2018; Cooley et al., 2019; Hoegh-
12 Guldberg et al., 2019b; Menéndez et al., 2020; Morris et al., 2020a) by protecting coastal environments from
13 SLR and storms (Cross-Chapter Box SLR, Reguero et al., 2018), and by storing substantial quantities of
14 carbon (Sections,, WGIII AR6 Chapter 7, Howard et al., 2017; Chow, 2018; Smale et al.,
15 2018; Singh et al., 2019b; Soper et al., 2019). Marine NbS are cost-effective, can generate social, economic
16 and cultural co-benefits and can contribute to the conservation of biodiversity in the near- to mid-term (high
17 confidence) (Secretariat of the Convention on Biological Diversity, 2009; Gattuso et al., 2018; Barkdull and
18 Harris, 2019; McLeod et al., 2019).



21 Table 3.30: Assessment of marine and coastal nature-based solutions to reduce mid-term climate impacts in oceans and

22 coastal ecosystems. Confidence is assessed in SM3.5.1. Feasibility and effectiveness are assessed in Figure 3.24.

    Solution     Confidence in solution Contribution to              Selected references Examples of

                 (mid-term potential) adaptation                                            implementation

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Habitat restoration
                     High confidence    Marine habitat           (Colls et al., 2009;  Restoration (Section
                                        restoration increases Arkema et al., 2017;

                                        biodiversity and         Espeland and

                                        protects shorelines and Kettenring, 2018;

                                        coastal livelihoods McLeod et al., 2019)

                                        from climate oceanic


Marine protected areas High confidence  MPAs and MPA             (Section, Conservation (Section
(MPAs) and other                        networks that are        Queirós et al., 2016;
effective area-based                    carefully designed to    Roberts et al., 2017;
conservation measures                   address climate          Maxwell et al., 2020a;
(OECMs)                                 change, strategically    Arafeh-Dalmau et al.,
                                        placed, and well         2021; Gurney et al.,
                                        enforced, hold great     2021; Sala et al., 2021)
                                        potential to deliver
                                        adaptation outcomes.
                                        OECMs can confer
                                        climate resilience to
                                        communities outside
                                        of MPAs.

Conservation of      Medium confidence  Protecting areas that (Section, Conservation (Section
climate refugia
                                        retain climate and Cross-Chapter Box

                                        biodiversity conditions MOVING SPECIES in

                                        for longer durations Chapter 5, Rilov et al.,

                                        under climate-change 2020; Wilson et al.,

                                        can increase the         2020a; Arafeh-Dalmau

                                        resilience of marine et al., 2021)

                                        ecosystems to

                                        warming and marine

                                        heatwaves (MHWs)

                                        and facilitate marine

                                        species range shifts.

Transboundary marine Low confidence     Transboundary MSP (Rosendo et al., 2018; Tourism (Section
spatial planning (MSP)
and integrated coastal                  and ICZM that            Tittensor et al., 2019;,
zone management
(ICZM)                                  incorporate climate- Frazão Santos et al., conservation, (Section

Sustainable harvesting High confidence  change impacts and 2020; Rilov et al.,

                                        adaptation in their 2020; Pinsky et al.,

                                        design can support 2021)

                                        climate adaptation and

                                        to foster international

                                        ocean cooperation.

                                        Sustainable harvesting (Gattuso et al., 2018;  Fisheries and
                                                                                       mariculture (Section
                                        is a nature-based        Burden and Fujita,

                                        solution (NbS) that 2019; Duarte et al.,

                                        contributes to           2020)

                                        adaptation by

                                        safeguarding the

                                        provision of marine

                                        food and cultural

                                        services, while

                                        reducing the ecological

                                        vulnerability of marine


Climate-adaptive     High confidence    Incorporating climate- (Cross-Chapter Box Fisheries and
                                        adaptive management MOVING SPECIES in mariculture (Section

                                        allows climate           Chapter 5, Rilov et al.,,

                                        knowledge and            2019; Free et al., 2020; conservation, (Section

                                        information available Wilson et al., 2020a;, restoration

                                        for the system to be Melbourne-Thomas et (Section

                                        iteratively updated in al., 2021)

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                                       the management plan.
                                       It also facilitates
                                       consideration of
                                       species distribution
                                       shifts and other

    Ecosystem-based   High confidence  EbM focuses on          (Fernandino et al.,   Fisheries and
                                                                                     mariculture (Section
    management (EbM)                   ecosystems. By          2018; Lowerre-

                                       incorporating many of Barbieri et al., 2019)

                                       the above tools,


                                       adaptation (EbA)

                                       benefits adaptation of

                                       marine ecosystems and

                                       supports provision of

                                       ecosystem services to




3 3.6.3 Implementation and Effectiveness of Adaptation and Mitigation Measures


5 This section assesses implemented adaptations introduced in Section 3.6.2 for selected marine sectors
6 (Section and ecosystems (Section, using case studies to emphasise characteristics that enable
7 or inhibit adaptation (Section The feasibility and effectiveness of these adaptations is assessed in
8 Figure 3.24.


10 Degree of Implementation and Evidence of Effectiveness Across Sectors


12 Coastal community development and settlement


14 Coastal adaptation often addresses the risk of flooding and erosion from SLR, changes in storm activity and
15 degradation of coastal ecosystems and their services (high confidence) (Sections 3.4.2, 3.5, Oppenheimer et
16 al., 2019). Without coastal protection, people and property will be increasingly exposed to coastal flooding
17 after 2050, especially under RCP8.5 (Cross-Chapter Box SLR in Chapter 3, Bevacqua et al., 2020; Kirezci et
18 al., 2020). This section assesses adaptation responses for coastal ecosystems, addressing loss of natural
19 coastal protection (Sections,­, and the need for relocation (Section
20 Adaptation responses specific to SLR are assessed in detail in Cross-Chapter Box SLR in Chapter 3, while
21 adaptation in coastal cities and settlements is assessed in Chapter 6.


23 Coastal conservation tends to involve cost-effective, low-impact actions that aim to support both adaptation
24 and mitigation by conserving a wide array of ecosystem functions and services (Gattuso et al., 2018; Gattuso
25 et al., 2021), and that are achievable by nations with extensive coastlines or low-income status (Herr et al.,
26 2017; Taillardat et al., 2018). Where coastlines are undeveloped, the lowest-risk option is to avoid new
27 development, but elsewhere, coastal conservation includes protection of key assets, accommodation of SLR,
28 advancing defences seawards or upwards, or planned retreat from the coast (Cross-Chapter Box SLR in
29 Chapter 3).


31 Hard engineered structures like seawalls are generally more costly than nature-based adaptations (high
32 confidence) (Hérivaux et al., 2018; Haasnoot et al., 2019; Nicholls et al., 2019; Oppenheimer et al., 2019)
33 and can lock communities into engineered responses in the future (Cross-Chapter Box SLR in Chapter 3),
34 creating trade-offs with mitigation goals, which constitutes maladaptation (Nunn et al., 2021) that carries
35 ecological and cultural costs (Sections,, 3.5.6). As a result there is high agreement on the
36 importance of shifting from hard infrastructure to soft infrastructure for coastal defence (Toimil et al., 2020;
37 Nunn et al., 2021). The common remedy for beach erosion is beach nourishment (Oppenheimer et al., 2019;
38 Pinto et al., 2020; Elko et al., 2021), which provides rapid results but poorly quantified trade-offs between
39 efficacy, long-term cost, utility to beach users and ecological damage (de Schipper et al., 2021).


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 1 Since SROCC, coastal adaptation using NbS, like restoration of coastal vegetation, has advanced
 2 substantially (Cohen-Shacham et al., 2019; Kuhl et al., 2020; Kumar et al., 2020; Morris et al., 2020a). Field
 3 and modelling studies confirm that wetland restoration and preservation are key actions to restore coastal
 4 protection and reduce community vulnerability to flooding (very high confidence) (see also Section 3.6,
 5 Chapter 15, Cross-Chapter Box SLR in Chapter 3, Jones et al., 2020; Menéndez et al., 2020; Van Coppenolle
 6 and Temmerman, 2020), while maintaining coastal ecosystem services (Section 3.5). Restoring coral reefs,
 7 oyster reefs and mangroves (Section, and protecting macrophyte meadows, dissipate wave energy
 8 (Section, Yates et al., 2017; Beck et al., 2018; Wiberg et al., 2019; Menéndez et al., 2020), accrete
 9 sediment, and elevate shorelines, which reduces exposure to waves and storm surges, and offsets erosional
10 losses (medium confidence) (Kench and Mann, 2017; Pomeroy et al., 2018; Dasgupta et al., 2019; James et
11 al., 2019; Morris et al., 2019; David and Schlurmann, 2020; Masselink et al., 2020). However, irreversible
12 regime shifts in ocean ecosystems due to SLR and extreme events such as MHW can limit or compromise
13 restoration in the long term (high confidence) (Section, Cross-Chapter Box SLR in Chapter 3,
14 Marzloff et al., 2016; Johnson et al., 2017a). Under all warming scenarios, coastal wetlands will be impacted
15 by warming and MHWs (Sections,, Cross-Chapter Box 9.1 in WGI Chapter 9, Fox-Kemper et
16 al., 2021), while also being pressed inland by RSLR (Section, Cross-Chapter Box SLR in Chapter 3).
17 Therefore, restoration and conservation are more successful when non-climate drivers are also minimised
18 (high confidence) (Brodie et al., 2020; Duarte et al., 2020; Liu et al., 2021).


20 For highly exposed human settlements, migration is an adaptation option (e.g., for some island populations
21 under extreme circumstances), but there are important uncertainties (Section, as international
22 regimes develop around human rights, migration (Scobie, 2019a), displacement (George Puthucherril, 2012)
23 and the implications for national sovereignty (Yamamoto and Esteban, 2014) of disappearing land spaces
24 caused by climate change. Colonial power dynamics can influence climate change responses (Chapter 18),
25 for example when external funders favour migration over local desires to adapt in place to preserve national
26 identity and sovereignty (Bordner et al., 2020). Examples of relocation within livelihoods' customary land
27 show some successes (Section


29 Evidence since SROCC (Section,, Bindoff et al., 2019) continues to show that built infrastructure
30 cannot address all of the adaptation challenges that coastal communities face. Coastal squeeze creates
31 tensions between coastal development, armouring, and habitat management (Sections­
32 Managed realignment is the best option to reduce risks from SLR (high confidence) (Cross-Chapter Box
33 SLR in Chapter 3) but requires transformative changes in coastal development and settlement (Felipe Pérez
34 and Tomaselli, 2021; Fitton et al., 2021; Mach and Siders, 2021; Siders et al., 2021). Implementation of
35 protective measures varies among nations and lack of financial resources limits the options available (very
36 high confidence) (Cross-Chapter Box SLR in Chapter 3, Hinkel et al., 2018; Klöck and Nunn, 2019).





41 Cross-Chapter Box SLR: Sea Level Rise


43 Authors: Gonéri Le Cozannet (France, Chapter 13, CCP4), Judy Lawrence (New Zealand, Chapter 11),
44 David Schoeman (Australia, Chapter 3), Ibidun Adelekan (Nigeria, Chapter 9), Sarah Cooley (USA, Chapter
45 3), Bruce Glavovic (New Zealand/South Africa, Chapter 18, CCP2), Marjolijn Haasnoot (The Netherlands,
46 Chapter 13, CCP2), Rebecca Harris (Australia, Chapter 2), Wolfgang Kiessling (Germany, Chapter 3),
47 Robert E. Kopp (USA, WGI), Aditi Mukherji (Nepal, Chapter 4), Patrick Nunn (Australia, Chapter 15),
48 Dieter Piepenburg (Germany, Chapter 13), Daniela Schmidt (UK/Germany, Chapter 13), Craig T. Simmons
49 (Australia), Chandni Singh (India, Chapter 10, CCP2), Aimée Slangen (The Netherlands, WGI), Seree
50 Supratid (Thailand, Chapter 4).



53 Sea-level rise (SLR) is already impacting ecosystems, human livelihoods, infrastructure, food security and
54 climate mitigation at the coast and beyond. Ultimately, it threatens the existence of cities and settlements in
55 low lying areas, and some island nations and their cultural heritage (Chapters 9­ 15, Cross-Chapter Paper 2
56 and 4 Oppenheimer et al., 2019). The challenge can be addressed by mitigation of climate change and coastal
57 adaptation.

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 2 Current impacts of sea level rise
 3 The rate of global mean SLR was 1.35 mm yr­1 (0.78­1.92 mm yr­1, very likely range) during 1901­1990,
 4 faster than during any century in at least 3000 years (high confidence) (WGI AR6 Chapter 9, Stanley and
 5 Warne, 1994; Woodroffe et al., 2016; Fox-Kemper et al., 2021). Global mean SLR has accelerated to 3.25
 6 mm yr­1 (2.88­3.61 mm yr­1, very likely range) during 1993­2018 (high confidence). Extreme sea levels
 7 have increased consistently across most regions (WGI AR6 Chapter 9, Fox-Kemper et al., 2021). The largest
 8 observed changes in coastal ecosystems are being caused by the concurrence of human activities, waves,
 9 current-induced sediment transport, and extreme storm events (medium confidence) (Chapters 3, 15 and 16,
10 Takayabu et al., 2015; Mentaschi et al., 2018; Duvat, 2019; Murray et al., 2019; Oppenheimer et al., 2019).
11 Early impacts of accelerating SLR detected at sheltered or subsiding coasts include chronic flooding at high
12 tides, wetland salinisation and ecosystem transitions, increased erosion and coastal flood damage (Chapters
13 3, 11 and 13­16, WGI AR6 Chapter 9, Sweet and Park, 2014; Moftakhari et al., 2015; Nunn et al., 2017;
14 Oppenheimer et al., 2019; Sharples et al., 2020; Fox-Kemper et al., 2021; Strauss et al., 2021). The exposure
15 of many coastal populations and ecosystems to SLR is high: economic development is disproportionately
16 concentrated in and around coastal cities and settlements (virtually certain) (Chapters 3 and 9­15, Cross-
17 Chapter Papers 2 and 4).


19 Projected risks to coastal communities, infrastructure and ecosystems
20 Risks from SLR are very likely to increase by one order of magnitude well before 2100 without adaptation
21 and mitigation action as agreed by parties to the Paris Agreement (very high confidence). Global mean SLR
22 is likely to continue accelerating under SSP1-2.6 and more strongly forced scenarios (Figure SLR1, WGI
23 AR6 Chapter 9, Oppenheimer et al., 2019; Fox-Kemper et al., 2021), increasing the risk of chronic coastal
24 flooding at high-tide, extreme flooding during extreme events such as swells, storms and hurricanes, and
25 erosion, and coastal ecosystem losses across many low-lying and erodible coasts (very high confidence)
26 (Chapters 3 and 9­15, Cross-Chapter Paper 2, Hinkel et al., 2014; McLachlan and Defeo, 2018; Kulp and
27 Strauss, 2019; Vousdoukas et al., 2020b). The compounding of rainfall, river flooding, rising water tables,
28 coastal surges, and waves are projected to exacerbate SLR impacts on low-lying areas and rivers further
29 inland (Chapters 4 and 11­15) (Bevacqua et al., 2020).


31 There is high confidence that coastal risks will increase by at least one order of magnitude over the 21st
32 century due to committed SLR (Hinkel et al., 2013; Hinkel et al., 2014; IPCC, 2019b). Exposure of
33 population and economic assets to coastal hazards is projected to increase over the next decades, particularly
34 in coastal regions with fast-growing populations in Africa, Southeast Asia, and Small Islands (medium
35 evidence) (Chapters 9­15, Cross-Chapter Papers 2 and 4, Neumann et al., 2015; Jones and O'Neill, 2016;
36 Merkens et al., 2016; Merkens et al., 2018; Rasmussen et al., 2020). For RCP8.5, 2.5­9% of the global
37 population and 12­20% of the global gross domestic product (GDP) is projected to be exposed to coastal
38 flooding by 2100 (Kulp and Strauss, 2019; Kirezci et al., 2020; Rohmer et al., 2021). Above 3°C global
39 warming levels (GWL) and with low adaptation, SLR may cause disruptions to ports and coastal
40 infrastructure (Camus et al., 2019; Christodoulou et al., 2019; Verschuur et al., 2020; Yesudian and Dawson,
41 2021), which in turn may cascade and amplify across sectors and regions, generating impacts to financial
42 systems (Chapters 11, 13, Mandel et al., 2021). Depending on the hydrogeological context, SLR causes
43 salinisation of groundwater, estuaries, wetlands and soils, adding constraints to water management and
44 livelihoods in agriculture sectors, for example, in deltas (Chapters 9, 15, Cross-Chapter Paper 4,
45 Oppenheimer et al., 2019; Nicholls et al., 2020).


47 Coastal ecosystems can migrate landward or grow vertically in response to SLR, but their resilience and
48 capacity to keep up with SLR will be compromised by ocean warming and other drivers, depending on
49 regions and species, e.g., above 1.5°C for coral reefs (high confidence) (Chapter 3, 16, IPCC, 2018;
50 Melbourne et al., 2018; Perry et al., 2018; IPCC, 2019b; Cornwall et al., 2021). Sediments and space for
51 landward retreat are crucial for mangroves, saltmarshes and beach ecosystems (high confidence) (Chapter 3,
52 Peteet et al., 2018; Schuerch et al., 2018; FitzGerald and Hughes, 2019; Friess et al., 2019; Leo et al., 2019;
53 Schuerch et al., 2019). Loss of habitat is accompanied by loss of associated ecosystem services, including
54 wave-energy attenuation, habitat provision for biodiversity, food production and carbon storage (Chapter 3,
55 Cross-Chapter Box NATURAL in Chapter 2).


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 1 Under a high-emissions, low-likelihood/high-impact scenario, where low confidence ice-sheet mass loss
 2 occurs, global mean SLR could exceed the likely range by more than one additional metre in 2100 (Figure
 3 SLR1b, Cross-Chapter Box DEEP in Chapter 17, WGI AR6 Technical Summary and Chapter 9, Arias et al.,
 4 2021; Fox-Kemper et al., 2021). This is a reason for concern given that rapid SLR after the last glacial-
 5 interglacial transition caused a drowning of coral reefs (high confidence) (Camoin and Webster, 2015;
 6 Sanborn et al., 2017; Webster et al., 2018), extensive loss of coastal land and islands, habitats and associated
 7 biodiversity (high confidence) (AR6 WGI Chapter 9, Fruergaard et al., 2015; Fernández-Palacios et al.,
 8 2016; Hamilton et al., 2019; Helfensdorfer et al., 2019; Kane and Fletcher, 2020; Fox-Kemper et al., 2021)
 9 and triggered Neolithic migrations in Europe and Australia (medium confidence) (Cross-Chapter Box
10 PALEO in Chapter 1, Turney and Brown, 2007; Brisset et al., 2018; Williams et al., 2018).


12 At centennial timescales, projected SLR represents an existential threat for island nations, low-lying coastal
13 zones, and the communities, infrastructure, and cultural heritage therein (Chapters 9­15, Cross-Chapter
14 Paper 4). Even if climate warming is stabilised at 2°C to 2.5°C GWL, coastlines will continue to reshape
15 over millennia, affecting at least 25 megacities and drowning low-lying areas where 0.6­1.3 billion people
16 lived in 2010 (medium confidence) (WGI AR6 Chapter 9 Marzeion and Levermann, 2014; Clark et al., 2016;
17 Kulp and Strauss, 2019; Fox-Kemper et al., 2021; Strauss et al., 2021).


19 Solutions, opportunities and limits to adaptation
20 The ability to adapt to current coastal impacts, to cope with future coastal risks, and to prevent further
21 acceleration of SLR beyond 2050 depends on immediate mitigation and adaptation actions (very high
22 confidence). The most urgent adaptation challenge is chronic flooding at high tide (Chapters 10, 11, and 13­
23 15). Reducing the acceleration of SLR beyond 2050 will only be achieved with fast and profound mitigation
24 of climate change (Nicholls et al., 2018; Oppenheimer et al., 2019). Until 2050, adaptation planning and
25 implementation needs are projected to increase significantly in most inhabited coastal regions (Figure SLR1,
26 WGI AR6 Chapter 9, IPCC, 2019b; Fox-Kemper et al., 2021). For SSP1-2.6 and more strongly forced
27 scenarios, SLR rates continue to increase (WGI AR6 Chapter 9, Fox-Kemper et al., 2021), and so do the
28 scale and the frequency of adaptation interventions needed in coastal zones (Figure SLR1, Haasnoot et al.,
29 2020).


31 Risks can be anticipated, planned, and decided upon, and adaptation interventions can be implemented over
32 the coming decades, considering their often long lead- and life-times, irrespective of the large uncertainty
33 about SLR beyond 2050 (high confidence) (Figure SLR1, Cross-Chapter Box DEEP in Chapter 17, Cross-
34 Chapter Paper 2, Chapters 11, 13, Haasnoot et al., 2018; Stephens et al., 2018; Stammer et al., 2019).
35 Adaptation capacity and governance to manage risks from projected SLR typically require decades to
36 implement and institutionalise (high confidence) (Chapters 11,13, Haasnoot et al., 2021). Without
37 considering both short- and long-term adaptation needs, including beyond 2100, communities are
38 increasingly confronted with a shrinking solution space, and adverse consequences are disproportionately
39 borne by exposed and socially vulnerable people (Chapters 1, 8). SLR is likely to compound social conflict
40 in some settings (high confidence) (Oppenheimer et al., 2019).


42 Coastal impacts of SLR can be avoided by preventing new development in exposed coastal locations
43 (Chapters 3, 9­15, Cross-Chapter Paper 2, Doberstein et al., 2019; Oppenheimer et al., 2019). For existing
44 developments, a range of near-term adaptation options exists, including (1) engineered, sediment or
45 ecosystem-based protection; (2) accommodation and land use planning, to reduce the vulnerability of people
46 and infrastructure; (3) advance through, e.g., land reclamation; and (4) retreat through planned relocation or
47 displacements and migrations due to SLR (Chapters 9­15, Cross-Chapter Paper 2, Oppenheimer et al.,
48 2019). Only avoidance and relocation can remove coastal risks for the coming decades, while other measures
49 only delay impacts for a time, have increasing residual risk or perpetuate risk and create ongoing legacy
50 effects and virtually certain property and ecosystem losses (high confidence) (Cross-Chapter Paper 2, Siders
51 et al., 2019). Large-scale relocation has immense cultural, political, social and economic costs, and equity
52 implications, which can be reduced by fast implementation of climate mitigation and adaptation policies
53 (Chapter 8, Cross-Chapter Paper 2, Gibbs, 2015; Haasnoot et al., 2021). While relocation may currently
54 appear socially unacceptable, economically inefficient, or technically infeasible today (Lincke and Hinkel,
55 2021), it becomes the only feasible option as protection costs become unaffordable and the limits to
56 accommodation become obvious (Chapters 11, 13, 15, Hino et al., 2017; Siders et al., 2019; Strauss et al.,
57 2021). Effective responses to rising sea level involve locally applicable combinations of decision analysis,

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 1 land use planning, public participation and conflict resolution approaches; together these can anticipate
 2 change and help to chart adaptation pathways, over time addressing the governance challenges due to rising
 3 sea level (high confidence) (Oppenheimer et al., 2019).


 5 Ecosystem-based adaptation can reduce impacts on human settlements and bring substantial co-benefits such
 6 as ecosystem services restoration and carbon storage, but they require space for sediment and ecosystems
 7 and have site-specific physical limits, at least above 1.5°C GWL (high confidence) (Cross-Chapter Box
 8 NATURAL in Chapter 2, Chapters 3, 9, 11, 15, Herbert et al., 2015; Brown et al., 2019; Van Coppenolle and
 9 Temmerman, 2019; Watanabe et al., 2019; Neijnens et al., 2021). For example, planting and conserving
10 vegetation helps sediment accumulation by dissipating wave energy and reducing impacts of storms, at least
11 at present-day sea levels (high confidence) (Temmerman et al., 2013; Narayan et al., 2016; Romañach et al.,
12 2018; Laengner et al., 2019; Leo et al., 2019). Coastal wetlands and ecosystems can be preserved by
13 landward migration (Schuerch et al., 2018; Schuerch et al., 2019) or sediment supply (VanZomeren et al.,
14 2018), but they can be seriously damaged by coastal defences designed to protect infrastructure (Chapters 3,
15 13, Cooper et al., 2020b). Sediment nourishment can prevent erosion, but it can also negatively impact beach
16 amenities and ecosystems through ongoing dredging, pumping and deposition of sand and silts (VanZomeren
17 et al., 2018; de Schipper et al., 2021; Harris et al., 2021).


19 There is increasing evidence that current governance and institutional arrangements are unable to address the
20 escalating risks in low-lying coastal areas worldwide (high confidence). Barriers to adaptation such as
21 decision-making driven by short-term thinking or vested interests, funding limitations, and inadequate
22 financial policies and insurance can be addressed equitably and sustainably through implementation of suites
23 of adaptation options and pathways, (Chapters 11, 13, 17­18, Cross-Chapter Paper 2). Improved coastal
24 adaptation governance can be supported by approaches that consider changing risks over time, such as
25 "dynamic adaptation pathways" planning (Chapters 11, 13, 18, Cross-Chapter Box DEEP in Chapter 17).
26 Integrated Coastal Zone Management and land-use and infrastructure planning are starting to consider SLR
27 by, for example, monitoring early signals (Haasnoot et al., 2018; Stephens et al., 2018; Kool et al., 2020),
28 updating sea-level projections (Stephens et al., 2017; Hinkel et al., 2019; Kopp et al., 2019; Stammer et al.,
29 2019), considering uncertainties of sea-level projections and coastal impacts (e.g., Stephens et al., 2017;
30 Jevrejeva et al., 2019; Rohmer et al., 2019), as well as engaging with communities, practitioners and
31 scientists, recognising the values of current and future generations (e.g., Nicholls et al., 2014; Buchanan et
32 al., 2016b). While there is high agreement that the majority of adaptation needs are not met yet, there is
33 robust evidence of sea level rise increasingly being considered in coastal adaptation decision making and
34 being embedded in national and local guidance and regulations (Nicholls et al., 2014; Le Cozannet et al.,
35 2017; Lawrence et al., 2018; Kopp et al., 2019; McEvoy et al., 2021).


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 2 Figure Cross-Chapter Box SLR.1: The challenge of coastal adaptation in the era of sea-level rise (SLR): (a): typical
 3 timescales for the planning, implementation (grey triangles) and operational lifetime of current coastal risk-management
 4 measures (blue bars); (b): global sea-level projections, which are representative of relative SLR projected for 60 to 70%
 5 of global shorelines, within ±20% errors (WGI AR6 Chapter 9, Fox-Kemper et al., 2021); (c): Frequency of illustrative
 6 adaptation decisions to +0.5 m of SLR under different SSP-RCP scenarios. In response to accelerated SLR, adaptation
 7 either occurs earlier and faster, or accounts for higher amounts of SLR (e.g., to +1 m instead of to +0.5 m). Adaptation
 8 to +0.5 m from today's sea-levels have a lifetime of 90 years for SSP1-2.6, but lifetime is reduced to 60 years for SSP5-
 9 8.5 and 30 years for a high-end scenario involving low confidence processes. Adaptations to +0.5 m are comparable to
10 e.g., the Thames Barrier in the United Kingdom, or the Delta Works in the Netherlands, which primarily had an
11 intended lifetime of 100­200 years. Adaptation measures to +0.2 m may include nourishment or wetland or setback
12 zones.






18 Fisheries and mariculture
19 SROCC (Bindoff et al., 2019) assessed adaptation in fisheries and mariculture (marine aquaculture), and
20 socioeconomically focused updates are provided in Section 5.8.4 and Cross-Chapter Box MOVING
21 SPECIES in Chapter 5. Here, we present a brief synthesis of how fisheries and mariculture adaptations
22 interact with the natural environment, with further detail and supporting material in SM3.5.2.

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 2 Mobility allows fishing fleets and fishers to adapt to shifts in marine species distributions (high agreement)
 3 (Sections, 3.5.3, Peck and Pinnegar, 2018; Pinsky et al., 2018; Frazão Santos et al., 2020), but with
 4 limits and unintended consequences (Pinsky and Fogarty, 2012; Bell et al., 2021). Diversification of target
 5 species, harvest tactics and employment sectors, including transitions from fisheries to mariculture and
 6 ecotourism, allows some fishers to accommodate some impacts on their livelihoods (Miller et al., 2018;
 7 Robinson et al., 2020; Gonzalez-Mon et al., 2021). Technology and infrastructure adaptations can improve
 8 marine harvest efficiency, reduce risk, and support resource management goals (Friedman et al., 2020; Bell
 9 et al., 2021; Melbourne-Thomas et al., 2021), but their ability to overcome climate-change impacts remains
10 uncertain (Bell et al., 2020). Improving capacity to predict anomalous conditions in coastal and marine
11 ecosystems (Jacox et al., 2019; Holbrook et al., 2020; Jacox et al., 2020), storm-driven flooding in reef-lined
12 coasts (Scott et al., 2020; Winter et al., 2020) and fisheries stocks (Payne et al., 2017; Tommasi et al., 2017;
13 Muhling et al., 2018) can improve forecasts of coastal and marine resources. These can enhance
14 sustainability of wild-capture fisheries under climate change (high confidence) (Blanchard et al., 2017;
15 Tommasi et al., 2017; Pinsky et al., 2020a; Bell et al., 2021). Limiting overexploitation is the central goal of
16 fishery management, and it very likely benefits fisheries adaptation to climate change (Burden and Fujita,
17 2019; Free et al., 2019; Sumaila and Tai, 2020). Conventional tools include catch and size limits, spatial
18 management and adaptive management. Ecosystem-based fisheries management outperforms single-species
19 management (Fulton et al., 2019), is widely legislated (Bryndum-Buchholz et al., 2021), and can reduce
20 climate impacts in fisheries in the near-term, especially under low-emission scenarios (Karp et al., 2019;
21 Holsman et al., 2020). Transboundary agreements on shifting fisheries will reduce the risk of overharvesting
22 (medium confidence) (Gaines et al., 2018). Permits tradable across political boundaries could also address
23 this challenge, but limited evidence is available regarding their efficacy (Cross-Chapter Box MOVING
24 SPECIES in Chapter 5, Pinsky et al., 2018). Climate-smart conservation (Section under the
25 negotiations on areas beyond national jurisdiction (ABNJ) (Pinsky et al., 2018; Tittensor et al., 2019; Frazão
26 Santos et al., 2020), and in the Convention on Biological Diversity (CBD) areas designed as other effective
27 area-based conservation measures (OECMs, Tittensor et al., 2019) provide further benefits. Despite the
28 potential for of adaptive management to achieve sustainable fisheries, outcomes will very likely be
29 inequitable (Gaines et al., 2018; Lam et al., 2020) with lower-income countries suffering the greater biomass
30 and economic losses, increasing inequalities especially under higher emission scenarios (high confidence)
31 (Boyce et al., 2020). Flexible and polycentric governance approaches have facilitated some short-term
32 successes in achieving equitable, sustainable fisheries practices, but these may be challenging to implement
33 where other governance systems, especially hierarchical systems, are well-established (Cvitanovic et al.,
34 2018; Bell et al., 2020).


36 Tourism
37 Coastal areas, coastal infrastructure and beaches, sustaining tourism that contributes significantly to local
38 economies (James et al., 2019; Ruiz-Ramírez et al., 2019), are under threat from development, SLR and
39 increased wave energy during storms and (high confidence) (Sections, 3.4.4­3.4.6, 3.5.6, Lithgow et
40 al., 2019; Ruiz-Ramírez et al., 2019). Engineered solutions like seawalls and revetments have traditionally
41 been used to address coastal erosion (Section, but soft infrastructure approaches, including beach
42 nourishment, submerged breakwaters and groins, and NbS (Section, are becoming more common,
43 partly due to demand from the tourism industry (medium confidence) (Pranzini, 2018; Pranzini et al., 2018).


45 Elsewhere, interactions between tourism and climate impacts worsen outcomes for coastal and ocean
46 environments (Section Climate change is opening up new cruise-ship routes in the Arctic (Sun et
47 al., 2018), increasing number of visitors and associated stressors, such as litter, to previously undisturbed
48 areas (Anfuso et al., 2020; Hovelsrud et al., 2020; Suaria et al., 2020). Risk reduction for cruise-ship tourism
49 includes disaster response management, improved mapping, and passenger codes of conduct ensuring social,
50 cultural and ecological sustainability (Stewart et al., 2015; Dawson et al., 2016).


52 Marine ecotourism, integrating conservation, education and provision of benefits to local communities
53 (Donohoe and Needham, 2006), can provide significant economic benefits (Wabnitz, 2019), and is among
54 the most common livelihood alternatives to support both marine conservation and climate change adaptation
55 (Kutzner, 2019; Pham, 2020; Prasetyo et al., 2020). Ecotourism can enhance social and political will for
56 marine conservation (Cisneros-Montemayor and Sumaila, 2014), and facilitates integration of local and
57 Indigenous Peoples in employment, ownership, and industry governance. The community of Cabo Pulmo,

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 1 Mexico, self-imposed an MPA and replaced fishing with ecotourism, which now generates millions of USD
 2 yr­1, sustains locally owned and operated tour companies, and has increased some fish populations ten-fold
 3 (Knowlton, 2020). In Misool, Indonesia, local ecotourism incorporates IK by including local communities'
 4 preferences and sustainable resource use (Prasetyo et al., 2020).


 6 Unintended consequences of ecotourism, such as detrimental ecological impacts on reefs (Giglio et al.,
 7 2020), sharks, marine birds (Monti et al., 2018), and whales (Higham et al., 2016; Barra et al., 2020; Hoarau
 8 et al., 2020), can be minimised by relying on evidence-based management of associated activities (Blumstein
 9 et al., 2017). Public perception of climate change connections to tourism can create obstacles (Meynecke et
10 al., 2017; Atzori et al., 2018) such as deterring long-term investment in SIDS tourism initiatives (Santos-
11 Lacueva et al., 2017), or benefits like inclining tourists to participate in conservation projects (Curnock et al.,
12 2019; Miller et al., 2020b; Ziegler et al., 2021). Social and cultural networks may decrease climate
13 vulnerability, as with Indigenous tourism operators in SIDS (Parsons et al., 2018). Tourism-based adaptation
14 can also be improved by equitable access to resources, and recognition and inclusion of all stakeholders
15 during policy planning and implementation. The principles of marine spatial planning (Papageorgiou, 2016)
16 provide for effectively incorporating stakeholders and could inform development of activities to assess
17 climate-associated risks (e.g., Tzoraki et al., 2018; Loehr, 2020). The recent decrease in global tourism due
18 to the COVID-19 pandemic may offer opportunities to transform existing practices to more sustainable
19 approaches (Cross-Chapter Box COVID in Chapter 7, Gössling et al., 2021).


21 Maritime transport
22 Increased maritime transport and cruise-ship tourism in the Arctic are already impacting local and
23 Indigenous Peoples, revealing conflicts over the uses of the ocean and the governance needed to support
24 local people and a sustainable blue economy (high confidence) (Debortoli et al., 2019; Palma et al., 2019;
25 Berman et al., 2020; Dundas et al., 2020). While shipping and its associated environmental impacts are
26 projected to grow (Palma et al., 2019; Dawson et al., 2020), adaptation efforts are only at the planning stage
27 (Debortoli et al., 2019). Increased Arctic traffic due to ice loss can benefit trade, transportation and tourism
28 (medium confidence), but will also affect Arctic marine ecosystems and livelihoods (high confidence) (Palma
29 et al., 2019; Dawson et al., 2020). Increasing search-and-rescue activities (Ford and Clark, 2019) reveal
30 capacity gaps to support future demands (Ford and Clark, 2019; Palma et al., 2019). The Low-Impact
31 Shipping Corridors initiative has been developed as an adaptation strategy in the Arctic, although with
32 limited inclusion of IK and LK (Dawson et al., 2020).


34 RSLR and the increased frequency and severity of storms are already affecting port activity, infrastructure,
35 and supply chains, sometimes disrupting trade and transport (Monios and Wilmsmeier, 2020), but these
36 hazards are not systematically incorporated into adaptation planning (medium evidence) (Monios and
37 Wilmsmeier, 2020; O'Keeffe et al., 2020). Climate-change impacts that increase food insecurity, income
38 loss, and poverty can exacerbate maritime criminal activity including illegal fishing, drug trafficking or
39 piracy (medium evidence) (Germond and Mazaris, 2019). A transformational adaptation approach to address
40 climate impacts on maritime activities and increase security (Germond and Mazaris, 2019) would relocate
41 ports, change centers of demand, reduce shipping distances, or shorten supply chains (medium agreement)
42 (Walsh et al., 2019; Monios and Wilmsmeier, 2020) as well as decrease marginalization of vulnerable
43 groups, develop polycentric governance systems and eliminate maladaptive environmental policies and
44 resource loss (Belhabib et al., 2020; O'Keeffe et al., 2020).


46 Human Health
47 Health-focused adaptations to climate-driven changes in ocean and coastal water quality (Section
48 mainly leverage technology and infrastructure (Section to improve water-quality monitoring and
49 forecasting to inform socio-institutional adaptation (Section and NbS (Section Seafood
50 quality and safety are decreasing due to climate-driven increases in marine-borne diseases (Cross-Chapter
51 Box ILLNESS in Chapter 2), toxic HABs, or toxin bioaccumulation (high agreement) (Karagas et al., 2012;
52 Krabbenhoft and Sunderland, 2013; Rafaj et al., 2013; Curtis et al., 2019; Schartup et al., 2019; Thackray
53 and Sunderland, 2019). Future exposure to seafood-borne contaminants also depends partly on consumers'
54 seafood preferences (Elsayed et al., 2020) and seafood supply (Sunderland et al., 2018). Reducing this risk
55 by decreasing seafood consumption increases risk of eating less nutritious foods, and loss of cultural
56 practices (Chapter 5, Cross-Chapter Box MOVING SPECIES in Chapter 5, Donatuto et al., 2011; Bindoff et
57 al., 2019). Models incorporating high-resolution satellite images, field survey data, meteorological

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 1 observations and historical records can provide early-warning forecasts of HABs or conditions that favour
 2 microbial pathogen outbreaks (Cross-Chapter Box ILLNESS in Chapter 2, Semenza et al., 2017; Franks,
 3 2018; Hattenrath-Lehmann et al., 2018; Borbor-Cordova et al., 2019; Davis et al., 2019; Campbell et al.,
 4 2020a; Davidson et al., 2021). Forecasts facilitate preventive public health measures (World Health
 5 Organisation and United Nations Children's Fund, 2017), or seafood harvest guidance (Maguire et al., 2016;
 6 Leadbetter et al., 2018; Anderson et al., 2019; Bolin et al., 2021), reducing risks of disease outbreaks, waste,
 7 and contaminated seafood entering the market (medium confidence) (Cross-Chapter Box ILLNESS in
 8 Chapter 2, Nichols et al., 2018). Monitoring of water quality and seafood safety (Cross-Chapter Box
 9 ILLNESS in Chapter 2), paired with effective public communication and education (Ekstrom et al., 2020)
10 inform individual and local adaptations, including use of personal protective equipment, seafood selection
11 and preparation (Elsayed et al., 2020; Froelich and Daines, 2020; Fielding et al., 2021), income
12 diversification (Section, Moore et al., 2020b), public education (Borbor-Cordova et al., 2019), or
13 community-level actions to decrease risk from coastal aquifer and soil salinisation (Slama et al., 2020;
14 Mastrocicco and Colombani, 2021), HAB toxins (Ekstrom et al., 2020) and other contaminants (e.g.,
15 methylmercury, metals, persistent organic pollutants) in seafood (Chan et al., 2021). A full assessment of
16 climate-change impacts on human health is found in Chapter 7 and Cross-Chapter Box ILLNESS in Chapter
17 2.




21 Figure 3.24: Assessment of feasibility and effectiveness of adaptation solutions for ocean and coastal ecosystems.

22 Feasibility dimensions assessed include: technical and economic capacity to deliver and implement the solution, the

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 1 institutional and geophysical capacity to implement a solution; and associated social and ecological implications that
 2 make a solution more feasible. The general feasibility level is obtained from assessment of the three dimensions
 3 together. Note that feasibility is assessed for marine and coastal ecosystems as a whole and not by ecosystem type or
 4 region. Feasibility dimensions and assessment are updated and adapted from IPCC (2018) and Singh et al. (2020).
 5 Effectiveness: ability of the adaptation solution to reduce climate change mid-term risks. Main solutions are assessed
 6 per sector. Underlying data are available in Table SM3.3.



 9 Cross-Cutting Solutions for Coastal and Ocean Ecosystems


11 SROCC concluded that protection, restoration and pollution reduction can support ocean and coastal
12 ecosystems (high confidence), and that EbA lowers climate risks locally and provides multiple societal
13 benefits (high confidence) (IPCC, 2019c). This section updates the assessment of the effectiveness of these
14 strategies for addressing climate impacts.


16 Area-based protection: MPAs for adapting to climate change
17 Marine protected areas (MPAs) are the most widely implemented area-based management approach (Section
18, commonly intended to conserve, preserve, or restore biodiversity and habitats, protect species, or
19 manage resources (especially fisheries) (National Research Council, 2001). By August 2021, 7.74% of the
20 ocean was protected (in both MPAs and other effective conservation measures, OECMs) (UNEP-WCMC
21 and IUCN, 2021), primarily within nations' Exclusive Economic Zones (EEZs). These MPAs support
22 adaptation by sustaining nearshore ecosystems that provide natural erosion barriers (Sections 3.4.2.­,
23 Cross-Chapter Box SLR in Chapter 3), ecosystem function (Cheng et al., 2019), habitat, natural filtration,
24 carbon storage, livelihoods, and cultural opportunities (Sections 3.5.5, 3.5.6, Erskine et al., 2021) and help
25 ecosystems and livelihoods recover after extreme events (Roberts et al., 2017; Aalto et al., 2019; Wilson et
26 al., 2020a). However, in 2021 only 2.7% of the ocean was in fully or highly protected MPAs (Marine
27 Conservation Institute, 2021), the hard-to-achieve states that most effectively rebuild biomass and fish
28 community structure (Sala and Giakoumi, 2017; Bergseth, 2018; Zupan et al., 2018; Ohayon et al., 2021).
29 Only 1.18% of ABNJ is protected (UNEP-WCMC and IUCN, 2021), mostly due to governance limitations
30 (O'Leary and Roberts, 2017; Vijayaraghavan, 2021), but calls to protect more ABNJ emphasise the need to
31 protect habitat of long-range pelagic fish and marine mammals, maintain the ocean's regulating functions,
32 and minimise impacts from uses such as maritime shipping or deep-sea mining (Table 3.30).


34 MPAs are theorised to facilitate ecological climate adaptation and contribute to SDG14 ("Life below water",
35 Table 3.30, Figure 3.26, Bates et al., 2014; Lubchenco and Grorud-Colvert, 2015; Gattuso et al., 2018)
36 because they alleviate non-climate drivers and promote biodiversity (i.e., "managed resilience hypothesis",
37 Bruno et al., 2019; Maestro et al., 2019; Cinner et al., 2020). Current MPAs offer conservation benefits such
38 as increases in biomass and diversity of habitats, populations, and communities (high confidence) (Pendleton
39 et al., 2018; Bates et al., 2019; Stevenson et al., 2020; Lenihan et al., 2021; Ohayon et al., 2021), and these
40 benefits may last after some (possibly climate-enhanced) disturbances (e.g., tropical cyclones, McClure et
41 al., 2020). But current MPAs do not provide resilience against observed warming and heatwaves in tropical
42 to temperate ecosystems (medium confidence) (Bates et al., 2019; Bruno et al., 2019; Freedman et al., 2020;
43 Graham et al., 2020; Rilov et al., 2020). There is robust evidence that processes around MPA design and
44 implementation strongly influence whether outcomes are beneficial or harmful for adjacent human
45 communities (McNeill et al., 2018; Zupan et al., 2018; Ban et al., 2019).


47 Current placement and extent of MPAs will not provide substantial protections against projected climate
48 change past 2050 (high confidence), as the placement of MPAs has been driven more often by political
49 expediency (e.g., Leenhardt et al., 2013) than by managing key drivers of biodiversity loss (Cockerell et al.,
50 2020; Stevenson et al., 2020) or climate-impact drivers (Bruno et al., 2018). Only 3.5% of the area currently
51 protected will provide refuges from both SST and deoxygenation by 2050 under both RCP4.5 and RCP8.5
52 (Bruno et al., 2018) and MPAs are more exposed to climate change under RCP8.5 than non-MPAs (Section
53, Figure 3.20d). Community thermal tolerances will be exceeded by 2050 in the tropics and by 2150
54 for many higher-latitude MPAs (Bruno et al., 2018). Most MPA design has focused on the surface ocean, but
55 MPAs are assumed to protect the entire water column and benthos. Climate-impact drivers (Section 3.2)
56 throughout the water column and rapidly accelerating climate velocities at depths below 200 m (Johnson et
57 al., 2018; Brito-Morales et al., 2020), are projected to affect virtually all North Atlantic deep-water and open
58 ocean area-based management zones in the next 20­50 years (Johnson et al., 2018) and the conservation

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 1 goals of benthic MPAs in the North Sea are not expected to be fulfilled (Weinert et al., 2021). Heightened
 2 risk of non-indigenous species immigration from vessel traffic plus climate change further endangers MPA
 3 success (Iacarella et al., 2020), a particular concern in the Mediterranean (D'Amen and Azzurro, 2020;
 4 Mannino and Balistreri, 2021), where the current MPA network is already highly vulnerable to climate
 5 change (Kyprioti et al., 2021). This new evidence supports SROCC's high confidence assessment that
 6 present governance arrangements including MPAs are too fragmented to provide integrated responses to the
 7 increasing and cascading risks from climate change in the ocean (SROCC SPMC1.2, IPCC, 2019c).


 9 Strategic conservation planning can yield future MPA networks substantially more ready for climate change
10 (e.g., Section, SROCC SPM C2.1, IPCC, 2019c; Frazão Santos et al., 2020; Rassweiler et al.,
11 2020). Global protection is increasing (Worm, 2017; Claudet et al., 2020b) as nations pursue international
12 targets (e.g., SDG14, "Life below water" aimed to conserve 10% of the ocean by 2020), and the UN CBD
13 proposes to protect 30% by 2030 (Section 3.6.4, SM3.5.3, CBD, 2020). A growing body of evidence
14 (Tittensor et al., 2019; Cabral et al., 2020; Zhao et al., 2020a; Pörtner et al., 2021b; Sala et al., 2021)
15 underscores the urgent need to pursue biodiversity, ecosystem-service provision, and climate-adaptation
16 goals simultaneously, while acknowledging inherent tradeoffs (Claudet et al., 2020a; Sala et al., 2021).
17 Frameworks to create "climate-smart" MPAs (Tittensor et al., 2019) generally include: defining conservation
18 goals that embrace resource vulnerabilities and co-occurring hazards; carefully selecting adaptation
19 strategies that include LK and IK while respecting Indigenous rights and accommodating human behaviour
20 (Kikiloi et al., 2017; Thomas, 2018; Yates et al., 2019; Failler et al., 2020; Wilson et al., 2020a; Croke, 2021;
21 Reimer et al., 2021; Vijayaraghavan, 2021); developing protection that is appropriate for all ocean depths
22 (Brito-Morales et al., 2018; Frazão Santos et al., 2020; Wilson et al., 2020a), especially considering climate
23 velocity (Arafeh-Dalmau et al., 2021); using dynamic national and international management tools to
24 accommodate extreme events or species distribution shifts (Gaines et al., 2018; Pinsky et al., 2018; Bindoff
25 et al., 2019; Scheffers and Pecl, 2019; Tittensor et al., 2019; Cashion et al., 2020; Crespo et al., 2020; Frazão
26 Santos et al., 2020; Maxwell et al., 2020b), which could build on dynamic regulations already in place for
27 fishing or ship strikes (Maxwell et al., 2020b); and seeking to increase connectivity (Wilson et al., 2020a),
28 using genomic or multispecies model insights (Xuereb et al., 2020; Friesen et al., 2021; Lima et al., 2021).


30 There is growing international support for a 30% conservation target for 2030 (Gurney et al., 2021), that will
31 need efforts beyond protected areas. For example, Other Effective area-based Conservation Measures
32 (OECMs) recognise management interventions that sustain biodiversity, irrespective of their main objective
33 (Maxwell et al., 2020b; Gurney et al., 2021). There is high agreement on the potential of OECMs to
34 contribute to conservation and equity, for example by recognising Indigenous territories as OECMs
35 (Maxwell et al., 2020b; Gurney et al., 2021). However, the capacity of these conservation tools to provide
36 adaptation outcomes remains unexplored.


38 In summary, MPAs and other marine spatial planning tools have great potential to address climate change
39 mitigation and adaptation in ocean and coastal ecosystems, if they are designed and implemented in a
40 coordinated way that takes into account ecosystem vulnerability and responses to projected climate
41 conditions, considers existing and future ecosystem uses and non-climate drivers, and supports effective
42 governance (high confidence).


44 Ecological restoration, interventions and their limitations
45 Restoration of degraded ecosystems is a common NbS increasingly deployed at local scales in response to
46 climate change (Cross-Chapter Box NATURAL in Chapter 2, Duarte et al., 2020; Bertolini and da Mosto,
47 2021; Braun de Torrez et al., 2021). Despite covering limited areas and having uncertain efficacy under
48 future climate change (Gordon et al., 2020), these actions have successfully restored marine populations and
49 ecosystems at regional to global scales (Duarte et al., 2020), and enhanced livelihoods and wellbeing of
50 coastal peoples as well as biodiversity and resilience of ecological communities (Silver et al., 2019; Gordon
51 et al., 2020; Braun de Torrez et al., 2021). Technology-based approaches like active restoration, assisted
52 evolution, and ecological forecasting can aid in moving beyond restoring ecosystems (Section
53 towards enhancing resilience, reviving biodiversity and guarding against loss of foundational, ornamental or
54 iconic species (Bulleri et al., 2018; Collins et al., 2019a; da Silva et al., 2019; National Academies of
55 Sciences, 2019; Boström-Einarsson et al., 2020; Fredriksen et al., 2020; Morris et al., 2020c; Kleypas et al.,
56 2021).


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 1 Local restoration projects often target vegetated ecosystems like mangroves, seagrasses and saltmarshes that
 2 are valued and used by coastal communities (Veettil et al., 2019; Duarte et al., 2020; Wu et al., 2020a;
 3 Bertolini and da Mosto, 2021) Detail on mangroves and corals as EbA and protection/restoration hotspots is
 4 provided in SuppMat 3.8. Common and effective actions (Sasmito et al., 2019; Duarte et al., 2020; Oreska et
 5 al., 2020) include securing accommodation space (Sections­, restoring hydrological (Kroeger
 6 et al., 2017; Al-Haj and Fulweiler, 2020) and sediment dynamics; managing harvesting (particularly in
 7 mangroves); reducing pollution (especially in seagrasses, de los Santos et al., 2019); and replanting
 8 appropriate species in suitable environmental settings (Wodehouse and Rayment, 2019; Friess et al., 2020a).
 9 Although efficacy is context dependent (Zeng et al., 2020; Krause-Jensen et al., 2021), and implementation
10 is most often local (Alongi, 2018a), such projects allow tangible community engagement in climate action.
11 Moreover, because these ecosystems sequester disproportionate amounts of carbon (blue carbon, Annex II:
12 Glossary, Box 3.4), restoration supports climate-change mitigation (Lovelock and Reef, 2020; Gattuso et al.,
13 2021). Yet, constraints remain. For instance, Southeast Asia has 1.21 million km2 of terrestrial, freshwater
14 and mangrove area biophysically suitable for reforestation, which could mitigate 3.43 ± 1.29 Pg CO2e yr-1
15 through 2030; however, reforestation is only feasible in a small fraction of this area (0.3­18%) given
16 financial, land-use and operational constraints (Zeng et al., 2020). Nevertheless, the multiple benefits offered
17 by ecosystem restoration will likely outweigh competing costs, and increase its relevance as part of
18 adaptation strategy portfolios (Silver et al., 2019; Wedding et al., 2021), national carbon-accounting systems,
19 and NDCs (Friess et al., 2020a; Wu et al., 2020a).


21 Restoration efficacy of coral reefs, kelp forests and other habitat-forming coastal ecosystems (Section
22­ are jeopardised by the near-term nature of climate-driven risks (McLeod et al., 2019;
23 National Academies of Sciences, 2019; Coleman et al., 2020b). Modelling studies indicate that available
24 practices will not prevent degradation of coral reefs from >1.5°C of global average surface warming (Figure
25 3.25, National Academies of Sciences Engineering and Medicine, 2019; Condie et al., 2021; Hafezi et al.,
26 2021). Proposed interventions, not yet implemented, include assisted migration (Boström-Einarsson et al.,
27 2020; Fredriksen et al., 2020; Morris et al., 2020c), assisted evolution (Bay et al., 2019; National Academies
28 of Sciences, 2019) and other engineering solutions like artificial shading and enhanced upwelling (Condie et
29 al., 2021; Kleypas et al., 2021).


31 Transplanting heat-tolerant coral colonies can increase reef resistance to bleaching (Morikawa and Palumbi,
32 2019; Howells et al., 2021), but potentially lowering species diversity and altering ecosystem function
33 (Section Genetic manipulation or assisted evolution that propagates genes from heat-tolerant
34 populations could enhance restoration of corals (Anthony et al., 2017; Epstein et al., 2019) and kelp (medium
35 agreement, limited evidence) (Coleman and Goold, 2019; Coleman et al., 2020b; Fredriksen et al., 2020;
36 Wade et al., 2020). Managed breeding of corals has also had limited success in the laboratory and at small
37 local scales (National Academies of Sciences, 2019). There is also limited evidence that physiological
38 interventions like algal-symbiont or microbiome manipulation could increase coral thermal tolerance in the
39 field (National Academies of Sciences, 2019). Employing the natural adaptive capacity of species or
40 individuals in active restoration for corals and kelps with current technology involves fewer risks than
41 assisted evolution or long-distance relocation (high confidence) (Filbee-Dexter and Smajdor, 2019; National
42 Academies of Sciences, 2019). More ambitious engineered interventions like reef shading remain theoretical
43 and not scalable to the reef level (Condie et al., 2021). Debate continues on how to apply planned adaptation
44 in cost-effective ways that will accomplish the intended goals (National Academies of Sciences, 2019;
45 Duarte et al., 2020; Kleypas et al., 2021).


47 Models show that a combination of available management approaches (restoration, reducing non-climate
48 drivers) and speculative interventions (enhanced corals, reef shading) can contribute to sustaining some coral
49 reefs beyond 1.5°C of global warming with declining effectiveness beyond 2°C of global warming (medium
50 confidence) (Figure 3.25, WGII Chapter 17). These proposed interventions are also currently theoretical and
51 impractical over large scales; for example, engineered solutions like reef shading are untested and not
52 scalable at the reef level (Condie et al., 2021). Existing projects suggest that restoration and ecological
53 interventions to habitat-forming ecosystems have additional benefits of raising local awareness, promoting
54 tourism, and creating jobs and economic benefits (Fadli et al., 2012; Boström-Einarsson et al., 2020; Hafezi
55 et al., 2021), provided communities are involved in planning, operation and monitoring (Boström-Einarsson
56 et al., 2020).


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 3 Figure 3.25: Implemented and potential future adaptations in ocean and coastal ecosystems. (a) Global implementation
 4 since 1970 of (top) cumulative habitat-restoration projects (Duarte et al., 2020), (middle) cumulative area-based
 5 conservation protected area (MPA total, Boonzaier and Pauly, 2016), no-take areas (UN Environment World
 6 Conservation Monitoring Centre et al., 2018; UNEP-WCMC, 2019), and (bottom) percentage of total fish stocks rebuilt
 7 (Kleisner et al., 2013). (b) Adaptation pathways for coral reefs to maintain healthy cover (line weight: solid lines, likely
 8 effectiveness; dashed lines, more likely than not to likely; dotted lines = unlikely to more likely than not), with
 9 confidence noted for each intervention (VH = Very High, H = High, M = Medium) (Section,, Anthony et
10 al., 2019; National Academies of Sciences Engineering and Medicine, 2019). Circles denote when other measures must
11 also be implemented. (c) As in (b), but for mangrove ecosystems. Underlying data are available in Tables SM3.4­3.6.



14 Enablers, Barriers and Limitations of Adaptation and Mitigation


16 Not only is mitigation necessary to support ocean and coastal adaptation (Pörtner et al., 2014; Oppenheimer
17 et al., 2019), but the global emission pathways also impose limits to ocean and coastal adaptation, with lower
18 warming levels enabling greater effectiveness of adaptations (high confidence) (Figure 3.25). Chapter 17
19 broadly assesses the limits to adaptation, while this section focuses on barriers and limits to adaptation
20 imposed by cultural (Section, economic (Section and governance (Section
21 dimensions (Hinkel et al., 2018). Globally, these factors more strongly influence ocean development than
22 does local natural resource availability (Cisneros-Montemayor et al., 2021), and are key to avoiding
23 maladaptation. This section also assesses enablers and limits to mitigation (Section


25 Sociocultural dimensions (culture, ethics, identity, behaviour)
26 Every coastal community values marine ecosystems for more than the material and intangible resources they
27 deliver, or the physical protection they offer (Díaz et al., 2018). Cultural services that provide identity,
28 spiritual and cultural continuity, religious meaning, or options for the future (e.g., genetic or mineral
29 resources, Bindoff et al., 2019), are not substitutable. Furthermore, interactions between climate impacts and
30 existing inequalities can threaten the human rights of already-marginalised peoples by disrupting livelihoods
31 and food security, which further erodes people's social, economic, and cultural rights (Finkbeiner et al.,

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 1 2018). For instance, European colonisation and ongoing development blocked the Cucapá Indigenous
 2 People's access and rights to resources in the Colorado River Delta, USA, over the 20th century. Recent
 3 reallocation of water rights and fishing access is allowing the Cucapá people to reconstruct their cultural
 4 identity (Sangha et al., 2019), but future climate change impacts could reverse the community's recovery of
 5 their cultural heritage. Adaptations that consider local needs may help sustain cultural services (Ortíz Liñán
 6 and Vázquez Solís, 2021).


 8 Interactions with oceans are fundamental to the identities of many coastal Indigenous Peoples (Norman,
 9 2017) and this influences Indigenous responses to climate hazards and adaptation. Around 30 million
10 Indigenous Peoples live along coasts (Cisneros-Montemayor et al., 2016). Seafood consumption among
11 Indigenous Peoples is much higher than for non-Indigenous populations, and marine species support many
12 cultural, medicinal and traditional activities contributing to public health (Section, Kenny et al.,
13 2018). Perpetuation of Indigenous cultures depends on protecting marine ecosystems and on adapting to
14 changes in self-led ways (see Section 3.5.6, Sangha et al., 2019) that promote self-determination (von der
15 Porten et al., 2019). Indigenous resurgence, or reinvigorating Indigenous ways of life and traditional
16 management, can include marine resource protection and ocean-sector development founded on culturally
17 appropriate strategies and partnerships, that are consistent with traditional norms and beneficial to local
18 communities (von der Porten et al., 2019). Successful adaptation would simultaneously improve ecosystem
19 health and address current and historical inequities (Bennett, 2018). Examples include practicing traditional
20 resource management, protecting traditional territories, engaging with monitoring, collaborations with non-
21 Indigenous partners, and reinvesting benefits into capacity-building within communities (von der Porten et
22 al., 2019; Equator Initiative, 2020). The legitimacy of different adaptation strategies depends on local and
23 Indigenous Peoples' acceptance, which is based on cultural values (Adger et al., 2017); financial gain cannot
24 compensate for loss of IK or LK (Wilson et al., 2020b). Palau's recent goal of shifting seafood consumption
25 away from reef fishes (Remengesau Jr., 2019) and limiting and closely monitoring the expansion of
26 ecotourism was prompted by the cultural importance of protecting these reefs and associated traditional
27 fisheries for local consumption, a recognition of the importance of tourism, and the hazard of climate change
28 (Wabnitz et al., 2018a).


30 Adaptations implemented at the local level that consider IK and LK systems are beneficial (high confidence)
31 (Nalau et al., 2018; Sultana et al., 2019). Studies in SIDS and the Arctic have shown how IK and LK
32 facilitate the success of EbA (Nalau et al., 2018; Peñaherrera-Palma et al., 2018; Raymond-Yakoubian and
33 Daniel, 2018), reinforce and improve institutional approaches and enhance the provision of ecosystem
34 services (Ross et al., 2019; Terra Stori et al., 2019). Perspectives on adaptation also vary among groups of
35 age, race, (dis)ability, class, caste, and gender (Wilson et al., 2020b), so engaging different groups results in
36 more robust and equitable adaptation to climate change (Cross-Chapter Box GENDER in Chapter 18,
37 McLeod et al., 2018). Some coastal communities have developed substantial social capital and dense local
38 networks based on trust and reciprocity (Petzold and Ratter, 2015), with individual and community
39 flexibility to learn, adapt, and organise themselves to help local adaptation governance (Silva et al., 2020).
40 Recent evidence suggests that policies supporting local institutions can improve adaptation outcomes
41 (medium confidence) (Berman et al., 2020). Coastal communities can be engaged using novel approaches to
42 co-generate adaptation solutions (van der Voorn et al., 2017; Flood et al., 2018) that benefit education
43 (Koenigstein et al., 2020) and engagement in adaptation processes (Rumore et al., 2016). Successful
44 adaptation implementation in line with climate-resilient development pathways (WGII Chapter 18) depends
45 on bottom-up, participatory and inclusive processes (Section that engage diverse stakeholders
46 (Basel et al., 2020; McNamara et al., 2020; Ogier et al., 2020; Williams et al., 2020), and that protect
47 Indigenous customary rights (Farbotko and McMichael, 2019; Ford et al., 2020), empower women, and give
48 rights to climate refugees (McLeod et al., 2018).


50 Economic dimensions (planning, finance, costs)
51 Finance is a key barrier globally for ocean health, governance and adaptation to climate change (high
52 agreement) (Annex II: Glossary, Cross-Chapter Box FINANCE in Chapter 17, Hinkel et al., 2018; Miller et
53 al., 2018; Wabnitz and Blasiak, 2019; Woodruff et al., 2020; Sumaila et al., 2021). Global adaptation finance
54 was estimated to total 30 billion USD yr­1 in 2017­2018, or 5% of all climate finance (CPI, 2019), with no
55 tracking specifically for coastal or marine adaptation in low- to middle-income countries. Marine-focused
56 adaptation finance is difficult to trace and label due to the cross-sectoral nature of many projects (Blasiak
57 and Wabnitz, 2018) and the lack of clear definitions about what qualifies as adaptation or as new and

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 1 additional finance (Donner et al., 2016; Weikmans and Roberts, 2019). Finance for marine conservation
 2 from Overseas Development Assistance doubled between 2003 and 2016, reaching 634 million USD in
 3 2016, similar to the level provided by philanthropic foundations (Berger et al., 2019). Yet coastal adaptation
 4 to SLR alone is projected to cost hundreds of billions of USD yr­1, depending on the model and emission
 5 scenario (e.g., Wong et al., 2014; Nicholls et al., 2019). Economic and financing barriers to marine
 6 adaptation are often higher in low- to middle-income countries, where resources influence governance and
 7 constrain options for implementation and maintenance (high confidence) (Hinkel et al., 2018; Klöck and
 8 Nunn, 2019; Tompkins et al., 2020) and impacts on their coastal and marine ecosystems could total several
 9 percentage points of their gross domestic product (Wong et al., 2014). Current financial flows are
10 insufficient to meet the costs of coastal and marine impacts of climate change (very high confidence) and
11 ocean-focused finance is unevenly distributed, with higher flows within and to developed countries (very
12 high confidence).


14 Development assistance can help resolve resource constraints, but additional governance and coordination
15 challenges can arise from short-term, project-based funding, shifting priorities of donor institutions, and
16 pressures placed on human resources in the receiving nation (Parsons and Nalau, 2019; Nunn et al., 2020).
17 Innovative policy instruments like concessional loans, tax-policy reforms, climate bonds and public-debt
18 forgiveness can supplement traditional financial instruments (Bisaro and Hinkel, 2018; McGowan et al.,
19 2020). Mechanisms for solving the persistent problem of securing upfront investments for coastal protection
20 and other adaptation measures (Bisaro and Hinkel, 2018; Moser et al., 2019; Kok et al., 2021) include
21 integrating adaptation investments into insurance schemes (Reguero et al., 2020) and using debt financing to
22 bridge the time until benefits are realised (Ware and Banhalmi-Zakar, 2020). Insurance mechanisms that link
23 payments to losses from a trigger event (e.g., MHW) can confer resilience to marine-dependent communities
24 (Sumaila et al., 2021). All innovative financial instruments are most effective when they are inclusive and
25 reach vulnerable groups and marginalised communities (low evidence, high agreement) (Claudet et al.,
26 2020a; Sumaila et al., 2021).


28 Countries with large ocean areas within their EEZs have opportunities to develop "blue-green economies" to
29 reduce emissions and finance adaptation pathways (Chen et al., 2018a; Lee et al., 2020). Shifting from grants
30 to results-based financing can help attract more private capital to ocean adaptation (Lubchenco et al., 2016;
31 Claudet et al., 2020a). Public-private partnerships can also increase ocean adaptation finance (Goldstein et
32 al., 2019; Sumaila et al., 2021). For example, the financial benefits that biodiversity conservation confers to
33 seafood harvest resilience could be used to leverage industry participation in adaptation and conservation
34 finance (Barbier et al., 2018). Connecting restoration of blue carbon ecosystems with offset markets (e.g.,
35 Vanderklift et al., 2019) shows potential, but uncertainties remain about the international emissions trading
36 under the UN Framework Convention on Climate Change and climate impacts on blue-carbon ecosystems
37 (Section, Lovelock et al., 2017a; Macreadie et al., 2019).


39 Transparency, coherence between different actors and initiatives, and project monitoring and evaluation
40 enhance success in adapting and achieving SDG14 (Life below water) (Blasiak et al., 2019). Maladaptation
41 (WGII Chapter 16, Magnan et al., 2016), is a common risk of current project-based funding due to the
42 pressure to produce concrete results (medium confidence) (Parsons and Nalau, 2019; Nunn et al., 2020; Nunn
43 et al., 2021). Maladaptation can be avoided through a focus on building adaptive capacity, community-based
44 management, drivers of vulnerability and site-specific measures (low confidence) (Magnan and Duvat, 2018;
45 Piggott-McKellar et al., 2020; Schipper, 2020). More research is needed to identify ways that governance
46 and financing agreements can help overcome financial barriers and socio-cultural constraints to avoid
47 maladaptation in coastal ecosystems (high confidence) (Hinkel et al., 2018; Miller et al., 2018; Piggott-
48 McKellar et al., 2020; Schipper, 2020).


50 Governance dimension (institutional settings, decision making)
51 Ocean governance has become increasingly complex as new initiatives, new international agreements,
52 institutions, and scientific evidence arise at global, national, and sub-national scales (high agreement)
53 (Bindoff et al., 2019; Scobie, 2019b), limiting the present effectiveness of adaptation (IPCC, 2019c). Marine
54 climate governance is within the normatively contested marine governance space (Frazão Santos et al.,
55 2020), which is influenced by geopolitics (Gray et al., 2020) and profit maximisation (Flannery et al., 2016;
56 Haas et al., 2021) in ways that can entrench exclusionary processes in decision making, science management
57 and funding (Levin et al., 2018). This limits just and inclusive ocean governance (Bennett, 2018),

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1 perpetuates historical and cultural extractive practices and climate inaction, and leaves little space for
2 Indigenous-led adaptation frameworks and approaches (Nursey-Bray et al., 2019). At the national level,
3 ocean governance for climate-change adaptation is often transversal, requiring consideration of biophysical

4 and environmental conditions (Furlan et al., 2020), while fitting into existing economic (Kim, 2020) and

5 political processes. Adaptation governance that couples existing top-down structures with decentralised and
6 participatory approaches generates shared goals and unlocks required resources and monitoring (Gupta et al.,
7 2016; Haas et al., 2021).


 9 Communities and governments at all levels increasingly use decision-making frameworks (e.g., structured
10 decision making) or decision-analysis tools to evaluate trade-offs between different responses, rather than
11 applying generic best practices to different physical, technical or cultural contexts (high confidence)
12 (Watkiss et al., 2015; Haasnoot et al., 2019; Palutikof et al., 2019). Increased effort has also been devoted to
13 developing climate services (actionable information and data products) that bridge the gap between climate
14 prediction and decision-making (Hewitt et al., 2020). Climate services have the potential to inform decision
15 making related to disaster-risk reduction, adaptation responses, marine environmental management (e.g.,
16 fisheries management and MPA management) and ocean-based climate mitigation (e.g., renewable energy
17 installations, Le Cozannet et al., 2017; Gattuso et al., 2019; Gattuso et al., 2021). Although improving

18 observational and modelling capacity is important to developing ocean-focused services, particularly in high-
19 risk regions like SIDS where regional climate projections are scarce (WGI AR6 Chapter 9, Morim et al.,
20 2019; Fox-Kemper et al., 2021), data is not the only limiting factor in decision-making (Weichselgartner and

21 Arheimer, 2019). Focusing on user engagement, relationship-building and the decision-making context

22 ensures that climate services are useful to and used by different stakeholders (high confidence) (Soares et al.,
23 2018; Mackenzie et al., 2019; Weichselgartner and Arheimer, 2019; Findlater et al., 2021; West et al., 2021).


25 Mitigation

26 Ocean and coastal NbS can contribute to global mitigation efforts, especially with ocean renewable energy
27 and restoration and preservation of carbon ecosystems (Box 3.4, Section Technological, economic
28 and financing barriers presently hamper development of renewable ocean energy (AR6 WGIII Chapter 6).
29 Such development could help small nations reliant on imported fuel meet their climate-mitigation goals and
30 decrease risk from global fuel supply dynamics (Millar et al., 2017; Chen et al., 2018a), but progress is
31 limited by lack of investment (Millar et al., 2017; Lee et al., 2020) or equipment (Aderinto and Li, 2018;
32 Rusu and Onea, 2018). Wave-energy installations, possibly co-located with wind turbines(Perez-Collazo et
33 al., 2018), are promising for both low- to middle-income nations and areas with significant island or remote

34 coastal geographies (Lavidas and Venugopal, 2016; Bergillos et al., 2018; Jakimavicius et al., 2018; Kompor
35 et al., 2018; Penalba et al., 2018; Saprykina and Kuznetsov, 2018; Lavidas, 2019). Wave-energy capture may
36 also diminish storm-induced coastal erosion (Abanades et al., 2018; Bergillos et al., 2018). Tidal energy is a
37 relatively new technology (Haslett et al., 2018; Liu et al., 2018; Neill et al., 2018) with limiting siting

38 requirements (Mofor et al., 2013). Ocean renewable energy expansion faces other technological obstacles

39 including lack of implementable or scalable energy-capture devices, access to offshore sites, competing
40 coastal uses, potential environmental impacts, and lack of power-grid infrastructure at the coast (Aderinto

41 and Li, 2018; Neill et al., 2018).


43 3.6.4 Contribution to the Sustainable Development Goals and Other Relevant Policy Frameworks


45 The impacts of climate change on ocean and coastal ecosystems and their services threaten achievement of

46 the UN SDGs by 2030 (high confidence), particularly ocean targets (Table 3.31, Nilsson et al., 2016; Pecl et
47 al., 2017; IPCC, 2018; Singh et al., 2019a; Claudet et al., 2020a). Nevertheless, local to international
48 decision-making bodies have assigned the lowest priority to SDG14, Life Below Water (Nash et al., 2020).



51 Table 3.31: Sustainable Development Goals, grouped into broader categories as discussed in this section

52 (

    Category                           Goal

    Society                            SDG1: No Poverty
                                       SDG2: Zero Hunger
                                       SDG3: Good Health & Well-Being
                                       SDG4: Quality Education
                                       SDG5: Gender Equality

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   Economy                                           SDG6: Clean Water & Sanitation
                                                     SDG7: Affordable & Clean Energy
   Environment                                       SDG8: Decent Work & Economic Growth
   Governance                                        SDG9: Industry, Innovation & Infrastructure
                                                     SDG10: Reduced Inequality
                                                     SDG11: Sustainable Cities & Communities
                                                     SDG12: Responsible Consumption & Production
                                                     SDG13: Climate Action
                                                     SDG14: Life Below Water
                                                     SDG15: Life on Land
                                                     SDG16: Peace and Justice Strong Institutions
                                                     SDG17: Partnerships to achieve the Goals



3 Climate Mitigation Effects on Ocean-Related SDGs


 5 SROCC underscored the need for ambitious mitigation to control climate hazards in the ocean to achieve
 6 SDGs (medium evidence, high agreement) (Bindoff et al., 2019; Oppenheimer et al., 2019). Delays in
 7 achieving ocean-dependent SDGs observed in SROCC and SR15 can be addressed with ambitious planned
 8 adaptation and mitigation action (high agreement) (Hoegh-Guldberg et al., 2019b). Since the ocean can
 9 contribute substantially to the attainment of mitigation targets aiming to limit warming to 1.5ºC above pre-
10 industrial (Hoegh-Guldberg et al., 2019b), and to adaptation solutions facilitating attainment of social and
11 economic SDGs, climate policy is treating the ocean less as a victim of climate change and more as a central
12 participant in solving the global climate challenge (Cooley et al., 2019; Hoegh-Guldberg et al., 2019a;
13 Dundas et al., 2020).


15 Relationships between Climate Action (SDG13) targets and SDG14 targets are mostly synergistic (Figure
16 3.26, Fuso Nerini et al., 2019). Responding to climate-change impacts requires transformative governance
17 (high confidence) (Chapters 1 and 18, Collins et al., 2019a; Brodie Rudolph et al., 2020; Claudet et al.,
18 2020a), especially for extreme events and higher-impact scenarios (e.g., higher emissions) (Fedele et al.,
19 2019), and for achieving SDGs through one of the global ecosystems transitions (Chapter 18, Sachs et al.,
20 2019; Brodie Rudolph et al., 2020). Opportunities to transform ocean governance exist in developing new
21 international and local agreements, regulations and policies that reduce the risks of relocating ocean and
22 coastal activities (Section or in reinventing established practices (Section Policy
23 transformations improving ocean sustainability under SDG14 also help address SDG13 (Brodie Rudolph et
24 al., 2020; Dundas et al., 2020; Claudet, 2021; Sumaila et al., 2021). Emergent situations such as the COVID-
25 19 pandemic may provide opportunities to implement transformative "green recovery plans" that support
26 achievement of the SDGs and NDCs (Cross-Chapter Box COVID in Chapter 7).


28 Contribution of Ocean Adaptation to SDGs


30 Marine-focused adaptations show promise in helping achieve social SDGs, especially when they are
31 designed to achieve multiple benefits (medium confidence) (Figure 3.26, Ntona and Morgera, 2018; Claudet
32 et al., 2020a). Technology- and infrastructure-focused adaptations (Section can help relieve coastal
33 communities from risks associated with poverty (SDG1), hunger (SDG2), health and water sanitation (SDG3
34 and SDG6), and inequality (SDG10) by supporting aquaculture (Sections 3.5.3,, alerting the public
35 about poor water quality (Sections,, and empowering marginalised groups, such as women
36 and Indigenous Peoples, with decision-relevant information (medium evidence, high agreement) (Sections
37, Effectively implemented and managed marine NbS (Section contribute to
38 attainment of social SDGs by preserving biodiversity (Carlton and Fowler, 2018; Warner, 2018; Scheffers
39 and Pecl, 2019), which benefits most ocean and coastal ecosystem services (Section 3.5.3, Figure 3.22), by
40 increasing marine fishery and aquaculture sustainability (Section 3.6.3), by including vulnerable people and
41 communities in management (Section, by lowering risk of flooding from storms and SLR (Cross-
42 Chapter Box SLR in Chapter 3, Sections, and by implementing spatial management tools that
43 make room for new uses like renewable energy development (Section NbS can therefore help
44 support achievement of No Poverty (SDG1) (Ntona and Morgera, 2018), Zero Hunger (SDG2), Good Health
45 and Well-Being (SDG3) (Duarte et al., 2020), Affordable and Clean Energy (SDG7) (Fuso Nerini et al.,
46 2019; Levin et al., 2020), and Reduced Inequality (SDG10). Socio-institutional marine adaptations (Section

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 1 that support current livelihoods and help develop alternatives can contribute to attainment of social
 2 SDGs by enhancing social equity and supporting societal transformation (medium confidence) (Cisneros-
 3 Montemayor et al., 2019; Pelling and Garschagen, 2019; Nash et al., 2021). Even societal changes that are
 4 not directly marine-related can decrease human vulnerability to ocean and coastal climate risks by improving
 5 overall human adaptive capacity (Section 1.2).


 7 Marine adaptation also shows promise for helping support achievement of economic SDGs (medium
 8 confidence) (Figure 3.26). Marine NbS could help blue economy frameworks achieve Decent Work and
 9 Economic Growth (SDG8) (Lee et al., 2020), by sustainably and equitably incorporating ecosystem-based
10 fisheries management, restoration or conservation (Sections, and (Voyer et al.,
11 2018; Cisneros-Montemayor et al., 2019; Cohen et al., 2019; Okafor-Yarwood et al., 2020). NbS that involve
12 active restoration or accommodation can contribute to Sustainable Cities and Communities (SDG11) and
13 Infrastructure (SDG9) (Section Newly developed marine industries and livelihoods associated
14 with NbS might support attainment of Sustainable Communities (SDG11) (Cisneros-Montemayor et al.,
15 2019). Finance and market mechanisms to support disaster relief or ocean ecosystem services, such as blue
16 carbon or food provisioning, and innovations (SDG9) including new technologies like vessel-monitoring
17 systems (Kroodsma et al., 2018), can contribute to Responsible Consumption and Production (SDG12)
18 (Sumaila and Tai, 2020). Blue economy growth that includes sustainable shipping, tourism, renewable ocean
19 energy, and transboundary fisheries management (Pinsky et al., 2018) have the potential to contribute to
20 Economic Development (SDG8), affordable and clean energy (SDG7) (as well as global mitigation efforts,
21 SDG13, (Hoegh-Guldberg et al., 2019b; Duarte et al., 2020)). Participatory approaches and co-management
22 systems (Section in many maritime sectors can contribute to SDG11 and SDG12 while helping align
23 the blue economy and the SDGs (high agreement) (Lee et al., 2020; Okafor-Yarwood et al., 2020).


25 Developing marine adaptation pathways that offer multiple benefits requires transformational adaptation
26 (high confidence) (Claudet et al., 2020a; Friedman et al., 2020; Wilson et al., 2020b; Nash et al., 2021) that
27 avoids risky and maladaptive actions (Magnan and Duvat, 2018; Ojea et al., 2020). Ocean and coastal
28 extreme events and other hazards disproportionately harm the most vulnerable communities in SIDS, tropical
29 and Arctic regions, and Indigenous Peoples (Chapter Presently implemented adaptation activity, at
30 the aggregate level, adversely affects multiple gender targets under SDG5 (high confidence) (Cross-Chapter
31 Box GENDER in Chapter 18). Although women make up over half of the global seafood production
32 workforce (fishing and processing sectors), provide more than half the artisanal landings in Pacific region
33 (Harper et al., 2013), dominate some seafood sectors such as seaweed (Howard and Pecl, 2019) and shellfish
34 harvesting (Turner et al., 2020a), and account for 11% of global artisanal fisheries participants (Harper et al.,
35 2020b), they are often not specifically counted in datasets and excluded from decision-making and support
36 programs (Cross-Chapter Box GENDER in Chapter 18, Harper et al., 2020b; Michalena et al., 2020).
37 Targeted efforts to incorporate knowledge diversity, and include artisanal fishers, women and Indigenous
38 Peoples within international, regional, and local policy planning promote marine adaptation that supports
39 achievement of gender equality (SDG5) and reduces inequalities (SDG10) (limited evidence, high
40 agreement) (FAO, 2015). Integrated planning, financing, and implementation can help overcome these
41 limitations (Section, Cross-Chapter Box FINANCE in Chapter 17), ensuring that marine
42 adaptations do not compromise overall human equity or specific SDGs (Österblom et al., 2020; Nash et al.,
43 2021), but are in fact fully synergistic with these goals (Bennett et al., 2021).



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 2 Figure 3.26: Synergies and trade-offs between SDG13 Climate Action, SDG14 Life Below Water and social, economic
 3 and governance SDGs. Achieving SDG13 provides positive outcomes and supports the achievement of all SDG14
 4 targets. In turn, meeting SDG14 drives mostly positive interactions with social, economic and governance SDGs. The
 5 interaction types, `Indivisible' (inextricably linked to the achievement of another goal), `Reinforcing' (aids the
 6 achievement of another goal), `Enabling' (creates conditions that further another goal), `Consistent' (no significant
 7 positive or negative interactions), `Constraining' (limits options on another goal), follows Nilsson et al. (2016)'s scoring
 8 system based on authors' assessment, and agreement denotes consistency across author ratings. Full data available in
 9 Table SM3.7.



12 Relevant Policy Frameworks for Ocean Adaptation


14 The intricacy, scope, timescales, and uncertainties associated with climate change challenge ocean
15 governance, which already is extremely complex because it encompasses a variety of overlapping spatial
16 scales, concerns, and governance structures (Figure CB3.1 in SROCC Chapter 1, Prakash et al., 2019).
17 Assessment of how established global agreements and regional, sectoral, or scientific bodies address climate
18 adaptation and resilience, and how current practices can be improved, is found in SM3.5.3.


20 There is growing momentum to include the ocean in international climate policy (robust evidence), paving
21 the way for a more integrated approach to both mitigation and adaptation. Following adoption of the Paris
22 Agreement in 2015, the UN SDGs (Table 3.31) came into force in 2016, including SDG14 specifically
23 dedicated to life below water (Table 3.31). In 2017, the first UN Ocean Conference was held (United
24 Nations, 2017), the UNFCCC adopted the Ocean Pathway to increase ocean-targeted multilateral climate
25 action (COP23, 2017), and the UN Assembly declared 2021­2030 the Decade for Ocean Science for
26 Sustainable Development (Visbeck, 2018; Lee et al., 2020). Next, 14 world leaders formed the High-Level
27 Panel for a Sustainable Ocean Economy to produce the New Ocean Action Agenda, founded on 100%
28 sustainable management of national ocean spaces by 2025 (Ocean Panel, 2020). All of these initiatives
29 position oceans centrally within the climate-policy and biodiversity-conservation landscapes and seek to
30 develop a coherent effort and common frameworks to achieve marine sustainability (Visbeck, 2018; Lee et
31 al., 2020), new economic opportunities (Konar and Ding, 2020; Lee et al., 2020), more equitable outcomes
32 (Österblom et al., 2020), and decisive climate mitigation and adaptation (Hoegh-Guldberg et al., 2019a), to
33 achieve truly transformative change (Claudet et al., 2020a).


35 There is high confidence in the literature that multilateral environmental agreements need better alignment
36 and integration to support achievement of ambitious international development, climate mitigation, and
37 adaptation goals (Swilling et al., 202; Duarte et al., 2020; Friedman et al., 2020; Conservation International

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 1 and IUCN, 2021; Pörtner et al., 2021b; Sumaila et al., 2021). The ocean targets of the CBD (e.g., the Post-
 2 2020 Global Biodiversity Framework), the SDGs (Agenda 2030) and the Paris Agreement are already
 3 inclusive and synergistic (Duarte et al., 2020). However, specific policy instruments and sectors within them
 4 could be additionally integrated, especially to address such cross-cutting impacts as ocean acidification and
 5 deoxygenation (Gallo et al., 2017; Bindoff et al., 2019), increasing plastic pollution (Ostle et al., 2019;
 6 Duarte et al., 2020), high-seas governance (Johnson et al., 2019; Leary, 2019), or deep sea uses (Wright et
 7 al., 2019; Levin et al., 2020; Orejas et al., 2020). National adaptation plans present opportunities to
 8 synergistically build on mitigation to support equitable development (Morioka et al., 2020), economic
 9 planning (Dundas et al., 2020; Lee et al., 2020), and ocean stewardship (von Schuckmann et al., 2020).
10 Alignment of multilateral agreements is expected to increase mitigation impact as well as increase adaptation
11 options (Section 3.6.3, Figure 3.25, Roberts et al., 2020). Opportunities to improve multilateral
12 environmental agreements and policies beyond UNFCCC and CBD processes are discussed in SM3.5.3, and
13 an assessment of commercial species management initiatives and needs is in Chapter 5.


15 3.6.5 Emerging Best Practices for Ocean and Coastal Climate Adaptation


17 There is robust evidence that a combination of global and local solutions offers the greatest benefit in
18 reducing climate risk (Gattuso et al., 2018; Hoegh-Guldberg et al., 2019a; Hoegh-Guldberg et al., 2019b).
19 Ambitious and swift global mitigation offers more adaptation options and pathways to sustain ecosystems
20 and their services (Figure 3.25). Some solutions target both mitigation and adaptation (e.g., blue carbon
21 conservation, Cross-Chapter Box NATURAL in Chapter 2, Box 3.4), and cross-cutting solutions
22 simultaneously support several ocean-related sectors (e.g., area-based measures support fishing, tourism;
23 Section or ecosystem functions (e.g., NbS support coastal protection, biodiversity, habitat, etc.,
24 Section, Sala et al., 2021). Combined solutions also leverage a variety of existing policies and
25 governance systems (Section, Duarte et al., 2020) to advance climate mitigation and adaptation. Even
26 communities that face the limits of adaptation, like those who must relocate to cope with rising seas
27 (McMichael et al., 2019; Bronen et al., 2020), urgently require solutions that combine scientific projections,
28 IK and LK, cultural and community values, and ways to preserve cultural identity to support planning and
29 implementation of relocation (McMichael and Katonivualiku, 2020).


31 NbS are showing promising results in achieving adaptation and mitigation outcomes across marine and
32 coastal ecosystems (Sections­, but NbS have different degrees of readiness in marine
33 ecosystems (Duarte et al., 2020). Habitat restoration and recovery are highly effective in specific settings and
34 conditions (McLeod et al., 2019). Restoring and conserving vegetated coastal habitats (Sections­
35 represent robust NbS, especially in the tropics, and particularly when paired with restoration and
36 conservation of terrestrial ecosystems (robust evidence) (e.g., peatlands and forests, WGIII AR6 Chapter 7,
37 Hoegh-Guldberg et al., 2019b; Duarte et al., 2020; Griscom et al., 2020). Although most of the focus on NbS
38 efficacy has been on coastal and shelf ecosystems (Section, recent advances point to an emerging
39 role of NbS beyond coastal waters in the form of area-based management tools in marine areas beyond
40 national jurisdiction (Section, Gaines et al., 2018; Pinsky et al., 2018; Crespo et al., 2020; O'Leary et
41 al., 2020; Visalli et al., 2020; Wagner et al., 2020), because sustainable fisheries and aquaculture and
42 climate-responsive MPAs have high potential to adapt (Tittensor et al., 2019).


44 Adaptation efforts (Sections­ have three common characteristics that facilitate
45 implementation and success and contribute to climate-resilient development pathways (Chapter 18). First,
46 availability of multiple types of information (e.g., monitoring, models, climate services, Section
47 exposes the magnitude and nature of the adaptation challenge. Well-developed observation and modelling
48 capabilities (Reusch et al., 2018) offer insights on climate-associated risks at different timescales (Cvitanovic
49 et al., 2018; Hobday et al., 2018), and this facilitates adaptation within multiple areas (e.g., industries over
50 shorter timescales, societies over longer scales) (Hobday et al., 2018). Environmental data has supported
51 building societal and political (socio-institutional) will to adopt national and subnational adaptive
52 management principles (Hobday et al., 2016b; Champion et al., 2018; McDonald et al., 2019). However,
53 incorporating IK and LK at the same time provides more diverse social-environmental insight (Section
54, Goeldner-Gianella et al., 2019; Petzold and Magnan, 2019; Wilson et al., 2020b). This can help
55 align adaptation solutions with cultural values and increase their legitimacy with Indigenous and local
56 communities (Chapter, achieving climate resilient development pathways (Chapter 18, Adger et al.,
57 2017; Nalau et al., 2018; Peñaherrera-Palma et al., 2018; Raymond-Yakoubian and Daniel, 2018; Wamsler

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 1 and Brink, 2018). Second, implementation of multiple low-risk options (Hoegh-Guldberg et al., 2019a;
 2 Gattuso et al., 2021) such as economic diversification (Section can provide culturally acceptable
 3 livelihood alternatives and food supplies (e.g., fishing to ecotourism and mariculture, (Froehlich et al., 2019)
 4 while also providing environmental benefits (e.g., seaweed mariculture's potential carbon storage co-benefits
 5 (WGIII AR6 Chapter 7, Hoegh-Guldberg et al., 2019a; Gattuso et al., 2021). Third, inclusive governance
 6 that is well-aligned to the systems at risk from climate change is fundamental for effective adaptation
 7 (Barange et al., 2018). Solutions implemented within polycentric governance systems (Section 3.6.3,
 8 Bellanger et al., 2020) benefit from synergies between knowledge, action and socio-ecological contexts and
 9 stimulate governance responses at appropriate spatial and temporal scales (Cvitanovic and Hobday, 2018).
10 Governance aligned with Indigenous structures and local structures supports successful outcomes that
11 prioritise the concerns and rights of involved communities (Section 3.6.3, Mawyer and Jacka, 2018) and
12 better leverages existing social organisation (i.e., network structures), learning processes and power
13 dynamics (Barnes et al., 2020).


15 There is an opportunity to improve current practices when developing new ocean and coastal adaptation
16 efforts so that they routinely contain these successful characteristics and resolve technical, economic,
17 institutional, geophysical, ecological and social constraints (Figure 3.25, Section, IPCC, 2018; Singh
18 et al., 2020). Enhancements are needed in human, technical and financial resources; regulatory frameworks
19 (Ojwang et al., 2017); political support (Rosendo et al., 2018); institutional conditions and resources for fair
20 governance (Gupta et al., 2016; Scobie, 2018); political leadership; stakeholder engagement;
21 multidisciplinary data availability (Gopalakrishnan et al., 2018); funding and public support for adaptation
22 (Cross-Chapter Box FINANCE in Chapter 17, Ford and King, 2015); and incorporating IK and LK in
23 decision making (Nalau et al., 2018; Jabali et al., 2020; Petzold et al., 2020). As climate change continues to
24 challenge ocean and coastal regions, there is high confidence associated with the benefits of developing
25 robust, equitable adaptation strategies that incorporate scientific projections, employ portfolios of low-risk
26 options, internalise IK and LK, and address social aspects of governance from international to local scales
27 (Finkbeiner et al., 2018; Gattuso et al., 2018; Miller et al., 2018; Raymond-Yakoubian and Daniel, 2018;
28 Cheung et al., 2019; Gattuso et al., 2021).




32 FAQ3.2: Are we approaching so-called tipping points in the ocean and what can we do about it?


34 A tipping point is a threshold beyond which an abrupt or rapid change in a system occurs. Tipping points
35 that have already been reached in ocean systems include the melting of sea ice in the Arctic, thermal
36 bleaching of tropical coral reefs and the loss of kelp forests. Human-induced climate change will continue to
37 force ecosystems into abrupt and often irreversible change, absent strong mitigation and adaptation action.


39 A gradual change in water temperature or oxygen concentration can lead to a fundamental shift in the
40 structure and/or composition of an ecosystem when a tipping point is exceeded. For example, all species
41 have upper temperature limits below which they can thrive. In the tropics, prolonged warm temperatures can
42 cause fatal `bleaching' of tropical corals, leading reef ecosystems to degrade and become dominated by
43 algae. In temperate regions, marine heatwaves can kill or reduce the growth of kelp, threatening the other
44 species that depend on the tall canopy-forming marine plants for habitat. In the Arctic, rising temperatures
45 are melting sea ice, and reducing the available habitat for communities of ice-dependent species.


47 Once a tipping point is passed, the effects can be long-lasting and/or irreversible over timescales of decades
48 or longer. An ecosystem or a population can remain in the new state, even if the driver of the change returns
49 to previous levels. For example, once a coral reef has been affected by bleaching, it can take decades for
50 corals to grow back, even if temperatures remain below the bleaching threshold. Crossing a tipping point can
51 cause entire populations to collapse, causing local extinctions.


53 Tipping points are widespread across oceanic provinces and their ecosystems for climate variables like water
54 temperature, oxygen concentration and acidification. Evidence suggests that ocean tipping points are being
55 surpassed more frequently as the climate changes; scientists have estimated that abrupt shifts in communities
56 of marine species occurred over 14% of the ocean in 2015, up from 0.25% of the ocean in the 1980s. Other
57 human stresses to the ocean, including habitat destruction, overfishing, pollution and the spread of diseases,

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 1 combine with climate change to push marine systems beyond tipping points. As an example, nutrient
 2 pollution from land together with climate change can lead to low-oxygen coastal areas referred to as "dead
 3 zones".


 5 Human communities can also experience tipping points that alter people's relationships with marine
 6 ecosystem services. Indigenous Peoples and local communities may be forced to move from a particular
 7 location due to sea-level rise, erosion, or loss of marine resources. Current activities that help sustain
 8 Indigenous Peoples and their cultures may no longer be possible in the coming decades, and traditional diets
 9 or territories may have to be abandoned. These tipping points have implications for physical and mental
10 health of marine-dependent human communities.


12 Adaptation solutions to the effects of ecological tipping points are rarely able to reverse their environmental
13 impacts, and instead often require human communities to transform their livelihoods in different ways.
14 Examples include diversifying income by shifting from fishing to tourism and relocating communities
15 threatened by flooding to other areas to continue their livelihoods. Tipping points are being passed already in
16 coral reefs and polar systems, and more will probably be reached in the near future, given climate-change
17 projections. Nevertheless, the chances of moving beyond additional tipping points in the future will be
18 minimised if we reduce greenhouse gas emissions, and we also act to limit other human impacts on the
19 ocean, such as overfishing and nutrient pollution.




23 Figure FAQ3.2: Global map with examples of tipping points that have been passed in ocean systems around the world.
24 Tipping points in ecological systems are linked to increasing impacts and vulnerability of dependent human
25 communities. SES: semi-enclosed sea, EBUS: eastern boundary upwelling system, CBC; coastal boundary current.



28 [END FAQ3.2 HERE]




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 2 FAQ3.3: How are marine heatwaves affecting marine life and human communities?


 4 Heatwaves happen in the ocean as well as in the atmosphere. Marine heatwaves (MHWs) are extended
 5 periods of unusually warm ocean temperatures, relative to the typical temperatures for that location and
 6 time of year. Due to climate change, the number of days with MHWs have increased by 54% over the past
 7 century. MHWs cause mortalities in a wide variety of marine species, from corals to kelp to seagrasses to
 8 fish to seabirds, and have consequent effects on ecosystems and industries like aquaculture and fisheries.


10 Extreme events in the ocean can have damaging effects on marine ecosystems and the human communities
11 that depend on them. The most common form of ocean extremes are marine heatwaves (MHWs), extended
12 periods of unusually warm ocean temperatures, which are becoming more frequent and intense due to global
13 warming. Because seawater absorbs and releases heat more slowly than air, temperature extremes in the
14 ocean are not as pronounced as over land, but they can persist for much longer, often for weeks to months
15 over areas covering hundreds of thousands of square kilometres. These MHWs can be more detrimental for
16 marine species, in comparison to land species, because marine species are usually adapted to relatively stable
17 temperatures.


19 A commonly used definition of MHWs is a period of at least five days whose temperatures are warmer than
20 90% of the historical records for that location and time of year. MHWs are described by their abruptness,
21 magnitude, duration, intensity, and other metrics. In addition, targeted methods are used to characterize
22 MHWs that threaten particular ecosystems; for example, the accumulated heat stress above typical summer
23 temperatures, described by "degree heating weeks", is used to estimate the likelihood of coral bleaching.


25 Over the past century, MHWs have doubled in frequency, become more intense, lasted for longer and
26 extended over larger areas. MHWs have occurred in every ocean region over the past few decades, most
27 markedly in association with regional climate phenomena such as the El Niño/Southern Oscillation. During
28 the 2015­2016 El Niño event, 70% of the world's ocean surface encountered MHWs.


30 MHWs cause mortality of a wide variety of marine species, from corals to kelp to seagrasses to fish to
31 seabirds, and they have consequent effects on ecosystems and industries such as mariculture and fisheries.
32 Warm-water coral reefs, estuarine seagrass meadows and cold-temperate kelp forests are among the
33 ecosystems most threatened by MHWs since they are attached to the seafloor (FAQ 3.2). Unusually warm
34 temperatures cause bleaching and associated death of warm-water corals, which can lead to shifts to low-
35 diversity or algae-dominated reefs, changes in fish communities, and deterioration of the physical reef
36 structure, which causes habitat loss and increases the vulnerability of nearby shorelines to large-wave events
37 and sea-level rise. Since the early 1980s, the frequency and severity of mass coral bleaching events have
38 increased sharply worldwide. For example, from 2016 through 2020, the Great Barrier Reef experienced
39 mass coral bleaching three times in five years.


41 Mass loss of kelp from MHWs effects on the canopy-forming species has occurred across ocean basins,
42 including the coasts of Japan, Canada, Mexico, Australia and New Zealand. In southern Norway and the
43 northeast U.S., mortality from MHWs contributed to the decline of sugar kelp over the last two decades, and
44 to the spread of turf algal ecosystems that prevent recolonisation by the original canopy-forming species.


46 One of the largest and longest-duration MHWs, nicknamed the 'Blob,' occurred in the Northeast Pacific
47 Ocean, extending from California north towards the Bering Sea, from 2013 through 2015. Warming from the
48 MHW persisted into 2016 off the U.S. West Coast and into 2018 in the deeper waters of a Canadian fjord.
49 The consequent effects of this expansive MHW included widespread shifts in abundance, distribution and
50 nutritional value of invertebrates and fish, a bloom of toxic algae off the US west coast that impacted
51 fisheries, the decline of California kelp forests that contributed to the collapse of the abalone fishery, and
52 mass mortality of seabirds.


54 The projected increase in the frequency, severity, duration, and areal extent of MHWs threaten many marine
55 species and ecosystems. MHWs may exceed the thermal limits of species, and they may occur too frequently
56 for the species to acclimate or for populations to recover. The majority of the world's coral reefs are
57 projected to decline and begin eroding due to more frequent bleaching-level MHWs if the world warms by

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 1 more than 1.5°C. Recent research suggests possible shifts to more heat-tolerant coral communities, but at the
 2 expense of species and habitat diversity. Other systems, including kelp forests, are most threatened near the
 3 edges of their ranges, although more research is needed into the effect of re-occurring MHWs on kelp forests
 4 and other vulnerable systems.


 6 The projected ecological impacts of MHWs threaten local communities' and Indigenous Peoples' cultures,
 7 incomes, fisheries, tourism, and, in the case of coral reefs, shoreline protection from waves. High-resolution
 8 forecasts and early-warning systems, currently most advanced for coral reefs, can help people and industries
 9 prepare for MHWs and also collect data on their effects. Identifying and protecting locations and habitats
10 with reduced exposure to MHWs is a key scientific endeavour. For example, corals may be protected from
11 MHWs in tidally-stirred waters or in reefs where cooler water upwells from subsurface. Marine protected
12 areas and no-take zones, in addition to terrestrial protection surrounding vulnerable coastal ecosystems,
13 cannot prevent MHWs from occurring. But, depending on the location and adherence by people to
14 restrictions on certain activities, the cumulative effect of other stressors on vulnerable ecosystems can be
15 reduced, potentially helping to enhance the rate of recovery of marine life.




20 Figure FAQ 3.3: Impact pathway of a massive extreme marine heatwave, the NW Pacific "Blob," from causal
21 mechanisms, to initial effects, resulting non-linear effects, and the consequent impacts for humans. Lessons learnt from
22 the "Blob" include the need to advance seasonal forecasts, real-time predictions, monitoring responses, education,
23 possible fisheries impacts and adaptation.


26 [END FAQ3.3 HERE]





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1 FAQ3.4: Which industries and jobs are most vulnerable to the impacts of climate change in the

2   oceans?


4 The global ocean underpins human well-being through the provision of resources that directly and indirectly

5 feed and employ many millions of people. In many regions, climate change is degrading ocean health and
6 altering stocks of marine resources. Together with over-harvesting, climate change is threatening the future
7 of the sustenance provided to Indigenous Peoples, the livelihoods of artisanal fisheries, and marine-based

8 industries including tourism, shipping and transportation.


10 The ocean is the lifeblood of the planet. In addition to regulating planetary cycles of carbon, water and heat,
11 the ocean and its vast resources support human livelihoods, cultural practices, jobs and industries. The
12 impacts of climate change on the ocean can influence human activities and employment by altering resource
13 availability, spreading pathogens, flooding shorelines and degrading ocean ecosystems. Fishing and
14 mariculture are highly exposed to change. The global ocean and inland waters together provide more than
15 3.3 billion people at least 20% of the protein they eat and provide livelihoods for 60 million people. Changes
16 in the nutritional quality or abundance of food from the oceans could influence billions of people.


18 Substantial economic losses for fisheries resulting from recent climate-driven harmful algal blooms and
19 marine pathogen outbreaks have been recorded in Asia, North America and South America. A 2016 event in
20 Chile caused an estimated loss of 800 million USD in the farmed salmon industry and led to regional

21 government protests. The recent closure of the U.S. Dungeness crab and razor clam fishery due to a climate-

22 driven algal bloom harmed 84% of surveyed residents from 16 California coastal communities. Fishers and
23 service industries that support commercial and recreational fishing experienced the most substantial
24 economic losses, and fishers were the least able to recover their losses. This same event also disrupted

25 subsistence and recreational fishing for razor clams, important activities for Indigenous Peoples and local

26 communities in the Pacific Northwest of the U.S.A.


28 Other goods from the ocean, including non-food products like dietary supplements, food preservatives,
29 pharmaceuticals, biofuels, sponges and cosmetic products, as well as luxury products like jewellery coral,
30 cultured pearls, and aquarium species, will change in abundance or quality due to climate change. For
31 instance, ocean warming is endangering the "candlefish" ooligan (Thaleichthys pacificus), whose oil is a
32 traditional food source and medicine of Indigenous Peoples of the Pacific Northwest of North America.
33 Declines in tourism and real estate values have also been recorded in the United States, France, and England

34 associated with climate-driven harmful algal blooms.


36 Small-scale fisheries livelihoods and jobs are the most vulnerable to climate-driven changes in marine
37 resources and ecosystem services. The abundance and composition of their harvest depend on suitable

38 environmental conditions and on Indigenous knowledge and local knowledge developed over generations.

39 Large-scale fisheries, though still vulnerable, are more able to adapt to climate change due to greater
40 mobility and greater resources for changing technologies. These fisheries are already adapting by broadening

41 catch diversity, increasing their mobility to follow shifting species, and changing gear, technology and

42 strategies. Adaptation in large-scale fisheries, however, is at times constrained by regulations and

43 governance challenges.


45 Jobs, industries and livelihoods which depend on particular species or are tied to the coast can also be at risk

46 to climate change. Species-dependent livelihoods (e.g., a lobster fishery or oyster farm) are vulnerable due to
47 a lack of substitutes if the fished species are declining, biodiversity is reduced, or mariculture is threatened
48 by climate change or ocean acidification. Coastal activities and industries ranging from fishing (e.g.,
49 gleaning on a tidal flat) to tourism to shipping and transportation are also vulnerable to sea-level rise and
50 other climate-change impacts on the coastal environment. The ability of coastal systems to protect the

51 shoreline will decline due to sea-level rise and simultaneous degradation of nearshore systems including
52 coral reefs, kelp forests and coastal wetlands.


54 The vulnerability of communities to losses in marine ecosystem services varies within and among

55 communities. Tourists seeking to replace lost cultural services can adapt by engaging in the activity

56 elsewhere. But communities who depend on tourism for income or who have strong cultural identity linked
57 to the ocean have a more difficult time. Furthermore, climate-change impacts exacerbate existing inequalities

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 1 already experienced by some communities, including Indigenous Peoples, Pacific Island countries and
 2 territories, and marginalized peoples, like migrants and women in fisheries and mariculture. These inequities
 3 increase the risk to their fundamental human rights by disrupting livelihoods and food security, while leading
 4 to loss of social, economic, and cultural rights. These maladaptive outcomes can be avoided by securing
 5 tenure and access rights to resources and territories for all people depending on the ocean, and by supporting
 6 decision-making processes that are just, participatory and equitable.


 8 A key adaptation solution is improving access to credit and insurance in order to buffer against variability in
 9 resource access and abundance. Further actions that decrease social and institutional vulnerability are also
10 important, such as inclusive decision-making processes, access to resources and land to Indigenous Peoples,
11 and participatory approaches in management. For the fishing industry, international fisheries agreements and
12 investing in sustainable mariculture and fisheries reforms is often recommended. Immediate adaptations to
13 other challenges, such as harmful algal blooms, frequently include fishing-area closures. These can be
14 informed by early-warning forecasts, public communications, and education. These types of adaptations are
15 more effective when built on trusted relationships and effective coordination among involved parties, and are
16 inclusive of the diversity of actors in a coastal community.




20 Figure FAQ3.4: Illustration that identifies vulnerable groups and stresses the hazards or impacts over coastal and ocean
21 systems.


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8 FAQ3.5: How can nature-based solutions, including marine protected areas, help us to adapt to

9   climate driven changes in the oceans?


11 Coastal habitats like mangroves or vegetated dunes protect coastal communities from sea-level rise and
12 storm surges, while supporting fisheries, sequestering carbon and providing other ecosystem services as
13 well. Efforts to restore, conserve and/or recover these natural habitats help people confront the impacts of

14 climate change. These marine nature-based solutions like marine protected areas, habitat restoration and
15 sustainable fisheries are cost-effective and provide myriad benefits to society.


17 In the oceans, nature-based solutions comprise attempts to recover, restore or conserve coastal and marine
18 habitats to reduce the impacts of climate change on nature and society. Marine habitats such as seagrasses
19 and coral reefs provide services like food and flood regulation, in the same way as forests do so on land.
20 Coastal habitats like mangroves or vegetated dunes protect coastal communities from sea-level rise and
21 storm surges, while supporting fisheries, recreational and aesthetic services as well. Seagrasses, coral reefs
22 and kelp forests also provide important benefits that help humans adapt to climate change, including

23 sustainable fishing, recreation and shoreline protection services. By recognizing these services and benefits
24 of the ocean, nature-based solutions can improve the quality and integrity of the marine ecosystems.


26 Nature-based solutions offer a wide range of potential benefits, including protecting ecosystem services,

27 supporting biodiversity and mitigating climate change. Coastal and marine examples include marine

28 protected areas, habitat restoration, habitat development and maintaining sustainable fisheries. While local
29 communities with limited resources might find nature-based solutions challenging to implement, they are

30 generally "no-regret" options, which bring societal and ecological benefits regardless of the level of climate

31 change.


33 Carefully designed and placed marine protected areas, especially when they exclude fishing, can increase
34 resilience to climate change by removing additional stressors on ecosystems. While marine protected areas

35 do not prevent extreme events like marine heatwaves (FAQ3.3), they can provide marine plants and animals
36 with a better chance to adapt to a changing climate. Current marine protected areas, however, are often too
37 small, too poorly connected and too static to account for climate-induced shifts in the range of marine
38 species. Marine protected area networks that are large, are connected, have adaptable boundaries, and are
39 designed following systematic analysis of future climate projections can better support climate resilience.


41 Habitat restoration and development in coastal systems can support biodiversity, protect communities from
42 flooding and erosion, support the local economy, and enhance the livelihoods and wellbeing of coastal

43 peoples. Restoration of mangroves, saltmarshes and seagrass meadows provide effective ways to remove

44 carbon dioxide from the atmosphere and at the same time to protect coasts from the impacts of storms and

45 sea-level rise. Active restoration techniques that target heat-resistant individuals or species are increasingly
46 recommended for coral reefs and kelp forests, which are highly vulnerable to marine heatwaves and climate

47 change.


49 Sustainable fishing is also seen as a nature-based solution because managing marine commercial species
50 within sustainable limits maximizes the catch and food production, thus contributing to the UN's Sustainable
51 Development Goal 2 ­ Zero Hunger. Currently, the oceans provide 17% of the animal protein eaten by the
52 global population, but the contribution could be larger if fisheries were managed sustainably. Aquaculture,
53 such as oyster farming, can be efficient and sustainable means of food production and also provide additional
54 benefits like shoreline protection. Through nature-based solutions that conserve and restore marine habitats
55 and species, we can sustain marine biodiversity, respond to climate change, and provide benefits to society.

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FINAL DRAFT                       Chapter 3  IPCC WGII Sixth Assessment Report


 3 Figure FAQ3.5: Contributions of nature-based solutions in the oceans to the Sustainable Development Goals. The
 4 icons in the bottom show the Sustainable Development Goals to which nature-based solutions in the ocean possibly
 5 contribute. [Placeholder figure -- authoritative version on FMS]



 8 [END FAQ3.5 HERE]



11 Acknowledgements
12 We acknowledge the kind contributions of Rita Erven (GEOMAR Helmholtz Centre for Ocean Research
13 Kiel, Germany), Miriam Seifert (Alfred Wegener Institute for Polar and Marine Research, Germany),
14 Sebastian Rokitta (Alfred Wegener Institute for Polar and Marine Research, Germany), Amy Marie
15 Campbell (National Oceanography Centre, Southampton/Centre for Environment, Fisheries and Aquaculture
16 Science, United Kingdom), Mariana Castaneda-Guzman (Virginia Polytechnic Institute and State University,
17 USA), Stephen Goult (Plymouth Marine Laboratory/National Centre for Earth Observation, United
18 Kingdom), Josh Douglas (Plymouth Marine Laboratory, United Kingdom), Carl Reddin (Museum für
19 Naturkunde, Berlin, Germany) and the PML Communications and Graphics Team (Plymouth Marine
20 Laboratory, United Kingdom) who assisted in drafting figures and tables.




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