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(Internal) Displacement (of humans)
The involuntary movement, individually or collectively, of persons from their country or community,
notably for reasons of armed conflict, civil unrest, or natural or man-made disasters (adapted from IOM,

2030 Agenda for Sustainable Development
A UN resolution in September 2015 adopting a plan of action for people, planet and prosperity in a new
global development framework anchored in 17 Sustainable Development Goals (UN, 2015).

Abrupt change
A change in the system that is substantially faster than the typical rate of the changes in its history. See also
Abrupt climate change, Tipping point

Abrupt climate change
A large-scale abrupt change in the climate system that takes place over a few decades or less, persists (or is
anticipated to persist) for at least a few decades and causes substantial impacts in human and/or natural
systems. See also Tipping point and Abrupt change.

Access to food
See Access under Food Security

A change in functional or morphological traits occurring once or repeatedly (e.g., seasonally) during the
lifetime of an individual organism in its natural environment. Through acclimatization the individual
maintains performance across a range of environmental conditions. For a clear differentiation between
findings in laboratory and field studies, the term acclimation is used in ecophysiology for the respective
phenomena when observed in well-defined experimental settings. The term (adaptive) plasticity
characterises the generally limited scope of changes in phenotype that an individual can reach through the
process of acclimatization.

Accumulation (of glaciers, ice sheets, or snow cover)
All processes that add to the mass of a glacier, an ice sheet, or snow cover. The main process of
accumulation is snowfall. Accumulation also includes deposition of hoar, freezing rain, other types of solid
precipitation, gain of wind-blown snow, avalanching, and basal accumulation (often beneath floating ice).

In human systems, the process of adjustment to actual or expected climate and its effects, in order to
moderate harm or exploit beneficial opportunities. In natural systems, the process of adjustment to actual
climate and its effects; human intervention may facilitate adjustment to expected climate and its effects.
See also Adaptation options, Adaptive capacity, Maladaptive actions (Maladaptation)

Adaptation deficit
The gap between the current state of a system and a state that minimizes adverse impacts from
existing climate conditions and variability.

Adaptation gap
The difference between actually implemented adaptation and a societally set goal, determined largely
by preferences related to tolerated climate change impacts and reflecting resource limitations and
competing priorities" (UNEP, 2014; UNEP, 2018).

Adaptation limits
The change in climate where adaptation is unable to prevent damaging impacts and further risk.

    · Soft limits occur when additional adaptation may be possible if constraints are able to be

    · Hard limits occur when no additional adaptation is possible.

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Adaptation needs
The circumstances requiring action to ensure safety of populations and security of assets in response
to climate impacts.

Adaptation options
The array of strategies and measures that are available and appropriate for addressing adaptation.
They include a wide range of actions that can be categorised as structural, institutional, ecological or

Autonomous adaptation
Adaptation in response to experienced climate and its effects, without planning explicitly or
consciously focused on addressing climate change. Also referred to as spontaneous adaptation.

Community-based adaptation
Local, community-driven adaptation. Community-based adaptation focuses attention on empowering
and promoting the adaptive capacity of communities. It is an approach that takes context, culture,
knowledge, agency, and preferences of communities as strengths.

Ecosystem-based adaptation (EBA)
The use of ecosystem management activities to increase the resilience and reduce the vulnerability of
people and ecosystems to climate change (Campbell et al., 2009).
See also Nature-based solutions

Evolutionary adaptation
The process whereby a species or population becomes better able to live in a changing environment,
through the selection of heritable traits. Biologists usually distinguish evolutionary adaptation from
acclimatisation, with the latter occurring within an organism's lifetime.

Incremental adaptation
Adaptation that maintains the essence and integrity of a system or process at a given scale (Park et al.,
2012). In some cases, incremental adaptation can accrue to result in transformational adaptation
(Tàbara et al., 2018; Termeer et al., 2017). Incremental adaptations to change in climate are
understood as extensions of actions and behaviours that already reduce the losses or enhance the
benefits of natural variations in extreme weather / climate events.

Transformational adaptation
Adaptation that changes the fundamental attributes of a social-ecological system in anticipation of
climate change and its impacts.

Adaptation Fund
A Fund established under the Kyoto Protocol in 2001 and officially launched in 2007. The Fund finances
adaptation projects and programmes in developing countries that are Parties to the Kyoto Protocol. Financing
comes mainly from sales of Certified Emissions Reductions (CERs) and a share of proceeds amounting to 2
% of the value of CERs issued each year for Clean Development Mechanism (CDM) projects. The
Adaptation Fund can also receive funds from government, private sector, and individuals.

Adaptation limits
See Adaptation

Adaptation needs
See Adaptation

Adaptation options
See Adaptation

Adaptation pathways
See Pathways

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Adaptive capacity
The ability of systems, institutions, humans and other organisms to adjust to potential damage, to take
advantage of opportunities, or to respond to consequences (MA, 2005).

Adaptive governance
See Governance

Adaptive management
A process of iteratively planning, implementing, and modifying strategies for managing resources in the face
of uncertainty and change. Adaptive management involves adjusting approaches in response to observations
of their effect and changes in the system brought on by resulting feedback effects and other variables.

Adverse side-effect
A negative effect that a policy or measure aimed at one objective has on another objective, thereby
potentially reducing the net benefit to society or the environment. See also Trade-off, Co-benefit

A suspension of airborne solid or liquid particles, with typical diameters between a few nanometres and a
few micrometres and atmospheric lifetimes of up to several days in the troposphere and up to years in the
stratosphere. The term aerosol, which includes both the particles and the suspending gas, is often used in this
report in its plural form to mean `aerosol particles'. Aerosols may be of either natural or anthropogenic
origin in the troposphere; stratospheric aerosol mostly stems from volcanic eruptions. Aerosols can cause an
effective radiative forcing directly through scattering and absorbing radiation (aerosol­radiation interaction),
and indirectly by acting as cloud condensation nuclei or ice nucleating particles that affect the properties of
clouds (aerosol­cloud interaction), and upon deposition on snow- or ice-covered surfaces. Atmospheric
aerosols may be emitted as primary particulate matter, and form within the atmosphere from gaseous
precursors (secondary production). The main classes of aerosol chemical composition are sea salt, organic
carbon, black carbon (BC), mineral species (mainly desert dust), sulphate, nitrate and ammonium. See also
Short-lived climate forcers (SLCF).

Conversion to forest of land that historically has not contained forests. See also Deforestation

[Note: For a discussion of the term forest and related terms such as afforestation, reforestation and
deforestation, see the 2006 IPCC Guidelines for National Greenhouse Gas Inventories and their 2019
Refinement, and information provided by the United Nations Framework Convention on Climate Change
(IPCC 2006, 2019; UNFCCC 2021a, 2021b)]

In this report, the degree of agreement within the scientific body of knowledge on a particular finding is
assessed based on multiple lines of evidence (e.g., mechanistic understanding, theory, data, models, expert
judgement) and expressed qualitatively (Mastrandrea et al., 2010). See also Confidence, Evidence,
Likelihood and Uncertainty.

Agricultural and ecological drought
See drought

The science and practice of applying ecological concepts, principles and knowledge (i.e., the interactions of,
and explanations for, the diversity, abundance and activities of organisms) to the study, design and
management of sustainable agroecosystems. It includes the roles of human beings as a central organism in
agroecology by way of social and economic processes in farming systems. Agroecology examines the roles
and interactions among all relevant biophysical, technical and socioeconomic components of farming
systems and their surrounding landscapes (IPBES, 2019).


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Collective name for land-use systems and technologies where woody perennials (trees, shrubs, palms,
bamboos, etc.) are deliberately used on the same land-management units as agricultural crops and/or
animals, in some form of spatial arrangement or temporal sequence. In agroforestry systems there are both
ecological and economical interactions between the different components. Agroforestry can also be defined
as a dynamic, ecologically based, natural resource management system that, through the integration of trees
on farms and in the agricultural landscape, diversifies and sustains production for increased social, economic
and environmental benefits for land users at all levels (FAO, 2015a).

Air pollution
Degradation of air quality with negative effects on human health or the natural or built environment due to
the introduction, by natural processes or human activity, into the atmosphere of substances (gases, aerosols)
which have a direct (primary pollutants) or indirect (secondary pollutants) harmful effect. See also Short-
lived climate forcers (SLCF).

The proportion of sunlight (solar radiation) reflected by a surface or object, often expressed as a percentage.
Clouds, snow and ice usually have high albedo; soil surfaces cover the albedo range from high to low;
vegetation in the dry season and/or in arid zones can have high albedo, whereas photosynthetically active
vegetation and the ocean have low albedo. The Earth's planetary albedo changes mainly through changes in
cloudiness, snow, ice, leaf area and land cover.

The deviation of a variable from its value averaged over a reference period.

A proposed new geological epoch resulting from significant human-driven changes to the structure and
functioning of the Earth System, including the climate system. Originally proposed in the Earth System
science community in 2000, the proposed new epoch is undergoing a formalization process within the
geological community based on the stratigraphic evidence that human activities have changed the Earth
System to the extent of forming geological deposits with a signature that is distinct from those of the
Holocene, and which will remain in the geological record. Both the stratigraphic and Earth System
approaches to defining the Anthropocene consider the mid-20th Century to be the most appropriate starting
date (Steffen et al., 2016), although others have been proposed and continue to be discussed. The
Anthropocene concept has already been informally adopted by diverse disciplines and the public to denote
the substantive influence of humans on the Earth System.

Resulting from or produced by human activities.

Anthropogenic emissions
See Emissions

Anthropogenic subsidence
Downward motion of the land surface induced by anthropogenic drivers (e.g., loading, extraction of
hydrocarbons and/or groundwater, drainage, mining activities) causing sediment compaction or
subsidence/deformation of the sedimentary sequence, or oxidation of organic material, thereby leading to
relative sea level rise.

Arid zone
Areas where vegetation growth is severely constrained due to limited water availability. For the most part,
the native vegetation of arid zones is sparse. There is high rainfall variability, with annual averages below
300 mm. Crop farming in arid zones requires irrigation.

The state of a long-term climatic feature characterised by low average precipitation or available water in a
region. Aridity generally arises from widespread persistent atmospheric subsidence or anticyclonic

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conditions, and from more localised subsidence in the lee side of mountains (adapted from Ogallo and
Gbeckor-Kove, 1989). See also Drought

The gaseous envelope surrounding the earth, divided into five layers ­ the troposphere which contains half
of the Earth's atmosphere, the stratosphere, the mesosphere, the thermosphere, and the exosphere, which is
the outer limit of the atmosphere. The dry atmosphere consists almost entirely of nitrogen (78.1% volume
mixing ratio) and oxygen (20.9% volume mixing ratio), together with a number of trace gases, such as argon
(0.93 % volume mixing ratio), helium and radiatively active greenhouse gases (GHGs) such as carbon
dioxide (CO2) (0.04% volume mixing ratio) and ozone (O3). In addition, the atmosphere contains the GHG
water vapour (H2O), whose amounts are highly variable but typically around 1% volume mixing ratio. The
atmosphere also contains clouds and aerosols. See also Hydrological cycle

Attribution is defined as the process of evaluating the relative contributions of multiple causal factors to a
change or event with an assessment of confidence.

Autonomous adaptation
See Adaptation

A mass of snow, ice, earth or rocks, or a mixture of these, falling down a mountainside.

Baseline scenario
see Reference scenario

Behavioural change
In this report, behavioural change refers to alteration of human decisions and actions in ways that mitigate
climate change and/or reduce negative consequences of climate change impacts.

Occurring at the bottom of a body of water; related to benthos (NOAA, 2018). See also Benthos

The community of organisms living on the bottom or in sediments of a body of water (such as an ocean, a
river or a lake). The ecological zone at the bottom of a body of water, including the sediment surface and
some sub-surface layers, is known as the `benthic zone'.

Beta Diversity
The change in species composition between different areas (spatial turnover) or times (temporal turnover)
due to habitat and environmental heterogeneity

Biodiversity or biological diversity means the variability among living organisms from all sources including,
among other things, terrestrial, marine and other aquatic ecosystems, and the ecological complexes of which
they are part; this includes diversity within species, between species, and of ecosystems (UN, 1992). See also
Ecosystem, Ecosystem services

Biodiversity Hotspots
Biodiversity hotspots are geographic areas exceptionally rich in species, ecologically distinct, and often
contain geographically rare endemic species. They are thus priorities for nature conservation action.

Energy derived from any form of biomass or its metabolic by-products. See also Biofuel, Biomass


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A fuel, generally in liquid form, produced from biomass. Biofuels include bioethanol from sugarcane, sugar
beet or maize, and biodiesel from canola or soybeans. See also Bioenergy

Organic material excluding the material that is fossilised or embedded in geological formations. Biomass
may refer to the mass of organic matter in a specific area (ISO, 2014). See also Bioenergy, Biofuel

Global-scale zones, generally defined by the type of plant life that they support in response to average
rainfall and temperature patterns. For example, tundra, coral reefs or savannas (IPBES, 2019).

Biosphere (terrestrial and marine)
The part of the earth system comprising all ecosystems and living organisms, in the atmosphere, on land
(terrestrial biosphere) or in the oceans (marine biosphere), including derived dead organic matter, such as
litter, soil organic matter and oceanic detritus.

Blue carbon
Biologically-driven carbon fluxes and storage in marine systems that are amenable to management. Coastal
blue carbon focuses on rooted vegetation in the coastal zone, such as tidal marshes, mangroves and
seagrasses. These ecosystems have high carbon burial rates on a per unit area basis and accumulate carbon in
their soils and sediments. They provide many non-climatic benefits and can contribute to ecosystem-based
adaptation. If degraded or lost, coastal blue carbon ecosystems are likely to release most of their carbon back
to the atmosphere. There is current debate regarding the application of the blue carbon concept to other
coastal and non-coastal processes and ecosystems, including the open ocean. See also Ecosystem services
and Sequestration

Blue Infrastructure
See Infrastructure

The total mass of a gaseous substance of concern in the atmosphere.

Business as usual (BAU)
The term business as usual scenario has been used to describe a scenario that assumes no additional policies
beyond those currently in place and that patterns of socio-economic development are consistent with recent
trends. The term is now used less frequently than in the past. See also Reference scenario

The process of biologically precipitating calcium carbonate minerals to create organism shells, skeletons,
otoliths, or other body structures. The chemical equation describing calcification is Ca2+(aq) + 2HCO3-(aq)
 CaCO3(s) + CO2 + H2O. Aragonite and calcite are two common crystalline forms of biologically
precipitated calcium carbonate minerals that have different solubilities.

Capacity building
The practice of enhancing the strengths and attributes of, and resources available to, an individual,
community, society, or organization to respond to change.

Carbon dioxide (CO2)
A naturally occurring gas, CO2 is also a by-product of burning fossil fuels (such as oil, gas and coal), of
burning biomass, of land use changes (LUC) and of industrial processes (e.g., cement production). It is the
principal anthropogenic greenhouse gas (GHG) that affects the Earth's radiative balance. It is the reference
gas against which other GHGs are measured and therefore has a Global Warming Potential (GWP) of 1.

Carbon dioxide (CO2) fertilisation
The increase of plant photosynthesis and water-use efficiency in response to increased atmospheric carbon
dioxide (CO2) concentration. Whether this increased photosynthesis translates into increased plant growth

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and carbon storage on land depends on the interacting effects of temperature, moisture and nutrient

Carbon dioxide removal (CDR)
Anthropogenic activities removing carbon dioxide (CO2) from the atmosphere and durably storing it in
geological, terrestrial, or ocean reservoirs, or in products. It includes existing and potential anthropogenic
enhancement of biological or geochemical CO2 sinks and direct air carbon dioxide capture and storage
(DACCS), but excludes natural CO2 uptake not directly caused by human activities. See also Afforestation

Carbon footprint
Measure of the exclusive total amount of emissions of carbon dioxide (CO2) that is directly and indirectly
caused by an activity or is accumulated over the life stages of a product (Wiedmann and Minx, 2008).

Carbon stock
The quantity of carbon in a carbon pool.

Cascading impacts
Cascading impacts from extreme weather/climate events occur when an extreme hazard generates a
sequence of secondary events in natural and human systems that result in physical, natural, social or
economic disruption, whereby the resulting impact is significantly larger than the initial impact. Cascading
impacts are complex and multi-dimensional, and are associated more with the magnitude of vulnerability
than with that of the hazard (modified from Pescaroli & Alexander, 2015).

An area that collects and drains precipitation.

Cities are open systems, continually exchanging resources, products and services, waste, people, ideas, and
finances with the hinterlands and broader world. Cities are complex, self-organizing, adaptive, and
constantly evolving. Cities also encompass multiple actors with varying responsibilities, capabilities and
priorities, as well as processes that transcend the institutional sector-based approach to city administration.
Cities are embedded in broader ecological, economic, technical, institutional, legal, and governance
structures that enable or constrain their systemic function, which cannot be separated from wider power
relations. Urban processes of physical, social, and economic nature are causally interlinked, with interactions
and feedbacks that result in both intended and unintended impacts on emissions. See also City region; Peri-
urban areas; Urban.

City region
The areal extent of an individual city's material associations and economic or political influence. The city
region concept accepts that rural livelihoods and land uses can be incorporated within the functional
activities of a city. This will include dormitory settlements, sources for critical inputs of water, some food,
and waste disposal. See also region, cities, urban, urban systems

In a narrow sense climate is usually defined as the average weather -or more rigorously, as the statistical
description in terms of the mean and variability of relevant quantities- over a period of time ranging from
months to thousands or millions of years. The classical period for averaging these variables is 30 years, as
defined by the World Meteorological Organization (WMO). The relevant quantities are most often surface
variables such as temperature, precipitation and wind. Climate in a wider sense is the state, including a
statistical description, of the climate system.

Climate change
A change in the state of the climate that can be identified (e.g., by using statistical tests) by changes in the
mean and/or the variability of its properties and that persists for an extended period, typically decades or
longer. Climate change may be due to natural internal processes or external forcings such as modulations of
the solar cycles, volcanic eruptions and persistent anthropogenic changes in the composition of the
atmosphere or in land use. Note that the United Nations Framework Convention on Climate Change

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(UNFCCC), in its Article 1, defines climate change as: 'a change of climate which is attributed directly or
indirectly to human activity that alters the composition of the global atmosphere and which is in addition to
natural climate variability observed over comparable time periods'. The UNFCCC thus makes a distinction
between climate change attributable to human activities altering the atmospheric composition and climate
variability attributable to natural causes. See also Climate variability, Detection, Attribution, and Ocean

Climate extreme (extreme weather or climate event)
The occurrence of a value of a weather or climate variable above (or below) a threshold value near the upper
(or lower) ends of the range of observed values of the variable.

By definition, the characteristics of what is called extreme weather may vary from place to place in an
absolute sense. When a pattern of extreme weather persists for some time, such as a season, it may be
classified as an extreme climate event, especially if it yields an average or total that is itself extreme (e.g.,
high temperature, drought, or heavy rainfall over a season). For simplicity, both extreme weather events and
extreme climate events are referred to collectively as 'climate extremes'.

Climate feedback
An interaction in which a perturbation in one climate quantity causes a change in a second and the change in
the second quantity ultimately leads to an additional change in the first. A negative feedback is one in which
the initial perturbation is weakened by the changes it causes; a positive feedback is one in which the initial
perturbation is enhanced. The initial perturbation can either be externally forced or arise as part of internal

Climate finance
There is no agreed definition of climate finance. The term 'climate finance' is applied to the financial
resources devoted to addressing climate change by all public and private actors from global to local scales,
including international financial flows to developing countries to assist them in addressing climate change.
Climate finance aims to reduce net greenhouse gas emissions and/or to enhance adaptation and increase
resilience to the impacts of current and projected climate change. Finance can come from private and public
sources, channelled by various intermediaries, and is delivered by a range of instruments, including grants,
concessional and non-concessional debt, and internal budget reallocations.

Climate governance
See Governance

Climate information
Information about the past, current state, or future of the climate system that is relevant for mitigation,
adaptation and risk management. It may be tailored or "co-produced" for specific contexts, taking into
account users' needs and values.

Climate justice
See Justice

Climate model
A qualitative or quantitative representation of the climate system based on the physical, chemical and
biological properties of its components, their interactions and feedback processes and accounting for some of
its known properties. The climate system can be represented by models of varying complexity; that is, for
any one component or combination of components a spectrum or hierarchy of models can be identified,
differing in such aspects as the number of spatial dimensions, the extent to which physical, chemical or
biological processes are explicitly represented, or the level at which empirical parametrisations are involved.
There is an evolution towards more complex models with interactive chemistry and biology. Climate models
are applied as a research tool to study and simulate the climate and for operational purposes, including
monthly, seasonal and interannual climate predictions. See also Earth System Model (ESM)

Climate prediction

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A climate prediction or climate forecast is the result of an attempt to produce (starting from a particular state
of the climate system) an estimate of the actual evolution of the climate in the future, for example, at
seasonal, interannual, or decadal time scales. Because the future evolution of the climate system may be
highly sensitive to initial conditions, such predictions are usually probabilistic in nature.

Climate projection
Simulated response of the climate system to a scenario of future emissions or concentrations of greenhouse
gases (GHGs) and aerosols and changes in land use, generally derived using climate models. Climate
projections depend on an emission / concentration / radiative forcing scenario, which is in turn based on
assumptions concerning, for example, future socioeconomic and technological developments that may or
may not be realised.

Climate refugium
A climate refugium is a geographic area that has had a stable climate on evolutionary timescales, or that is
projected to have a stable climate into the future. See also refugium

Climate scenario
See Scenarios

Climate services
Climate services involve the provision of climate information in such a way as to assist decision-making.
The service includes appropriate engagement from users and providers, is based on scientifically credible
information and expertise, has an effective access mechanism, and responds to user needs (Hewitt et al.

Climate simulation ensemble
A group of parallel model simulations characterising historical climate conditions, climate predictions, or
climate projections. Variation of the results across the ensemble members may give an estimate of
modelling-based uncertainty. Ensembles made with the same model but different initial conditions
characterise the uncertainty associated with internal climate variability, whereas multi-model ensembles
including simulations by several models also include the effect of model differences. Perturbed parameter
ensembles, in which model parameters are varied in a systematic manner, aim to assess the uncertainty
resulting from internal model specifications within a single model. Remaining sources of uncertainty
unaddressed with model ensembles are related to systematic model errors or biases, which may be assessed
from systematic comparisons of model simulations with observations wherever available.

Climate system
The global system consisting of five major components: the atmosphere, the hydrosphere, the cryosphere,
the lithosphere and the biosphere and the interactions between them. The climate system changes in time
under the influence of its own internal dynamics and because of external forcings such as volcanic eruptions,
solar variations, orbital forcing, and anthropogenic forcings such as the changing composition of the
atmosphere and land-use change.

Climate variability
Deviations of some climate variables from a given mean state (including the occurrence of extremes, etc.) at
all spatial and temporal scales beyond that of individual weather events. Variability may be intrinsic, due to
fluctuations of processes internal to the climate system (internal variability), or extrinsic, due to variations in
natural or anthropogenic external forcing (forced variability).

Climate velocity
The speed at which isolines of a specified climate variable travel across landscapes or seascapes due to
changing climate. For example, climate velocity for temperature is the speed at which isotherms move due to
changing climate (km yr-1) and is calculated as the temporal change in temperature (°C yr-1) divided by the
current spatial gradient in temperature (°C km-1). It can be calculated using additional climate variables such
as precipitation or can be based on the climatic niche of organisms.

Climate-resilient development

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in the WGII report climate-resilient development refers to the process of implementing greenhouse gas
mitigation and adaptation measures to support sustainable development for all.

Climate-resilient development pathways (CRDPs)
See Pathways

Climate-resilient pathways
See Pathways

Climate-smart agriculture (CSA)
An approach to agriculture that aims to transform and reorient agricultural systems to effectively support
development and ensure food security in a changing climate by: sustainably increasing agricultural
productivity and incomes; adapting and building resilience to climate change; and reducing and/or removing
greenhouse gas emissions, where possible (FAO, 2018).

Climatic driver (Climate driver)
A changing aspect of the climate system that influences a component of a human or natural system.

See Coupled Model Intercomparison Project (CMIP)

A positive effect that a policy or measure aimed at one objective has on another objective, thereby increasing
the total benefit to society or the environment. Co-benefits are also referred to as ancillary benefits. See also
Trade-off, Adverse side-effect

Coast The land near to the sea. The term `coastal' can refer to that land (e.g., as in `coastal communities'), or
to that part of the marine environment that is strongly influenced by land-based processes. Thus, coastal seas
are generally shallow and near-shore. The landward and seaward limits of the coastal zone are not
consistently defined, neither scientifically nor legally. Thus, coastal waters can either be considered as
equivalent to territorial waters (extending 12 nautical miles / 22.2 km from mean low water), or to the full
Exclusive Economic Zone, or to shelf seas, with less than 200 m water depth.

Coastal Erosion
Coastal erosion, sometimes referred to as shoreline retreat, occurs when a net loss of sediment or bedrock
from the shoreline results in landward movement of the high-tide mark.

Communicable diseases
Illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or
its products from an infected person, animal, or reservoir to a susceptible host, either directly or indirectly
through an intermediate plant or animal host, vector, or the inanimate environment." Communicable disease
pathogens include bacteria, viruses, fungi, parasites and prions.

Community-based adaptation
See Adaptation

Compound risks
See Risk

Compound weather/climate events
The terms `compound events', `compound extremes' and `compound extreme events' are used
interchangeably in the literature and this report and refer to the combination of multiple drivers and/or
hazards that contributes to societal and/or environmental risk (Zscheischler et al., 2018).

Concentrations scenario
See scenarios

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The robustness of a finding based on the type, amount, quality and consistency of evidence (e.g., mechanistic
understanding, theory, data, models, expert judgment) and on the degree of agreement across multiple lines
of evidence. In this Report, confidence is expressed qualitatively (Mastrandrea et al., 2010.

Conservation agriculture
A farming system that promotes minimum soil disturbance (e.g., by using no till practices), maintenance of a
permanent soil cover, and diversification of plant species. It aims to prevent land degradation and regenerate
degraded lands by enhancing biodiversity and natural biological processes above and below the ground
surface, that contribute to increased water and nutrient use efficiency and improved and sustained crop
production. (FAO, 2016)

The use of available skills, resources, and opportunities to address, manage, and overcome adverse
conditions, with the aim of achieving basic functioning of people, institutions, organizations, and systems in
the short to medium term. FTN: This glossary entry builds from the definition used in UNISDR (2009) and
IPCC (2012a).

Coping capacity
The ability of people, institutions, organizations, and systems, using available skills, values, beliefs,
resources, and opportunities, to address, manage, and overcome adverse conditions in the short to medium
term. (UNISDR, 2009; IPCC, 2012a) See also Resilience

Coral bleaching
Loss of coral pigmentation through the loss of intracellular symbiotic algae (known as zooxanthellae) and/or
loss of their pigments.

Coral reef
An underwater ecosystem characterised by structure-building stony corals. Warm-water coral reefs occur in
shallow seas, mostly in the tropics, with the corals (animals) containing algae (plants) that depend on light
and relatively stable temperature conditions. Cold-water coral reefs occur throughout the world, mostly at
water depths of 50-500 m. In both kinds of reef, living corals frequently grow on older, dead material,
predominantly made of calcium carbonate (CaCO3). Both warm and cold-water coral reefs support high
biodiversity of fish and other groups, and are considered to be especially vulnerable to climate change.

Cost-benefit analysis
Monetary assessment of all negative and positive impacts associated with a given action. Cost-benefit
analysis enables comparison of different interventions, investments or strategies and reveal how a given
investment or policy effort pays off for a particular person, company or country. Cost-benefit analyses
representing society's point of view are important for climate change decision-making, but there are
difficulties in aggregating costs and benefits across different actors and across timescales. See also

Coupled Model Intercomparison Project (CMIP)
A climate modelling activity from the World Climate Research Programme (WCRP) which coordinates and
archives climate model simulations based on shared model inputs by modelling groups from around the
world. The CMIP3 multi-model data set includes projections using Special Report on Emissions Scenarios
(SRES) scenarios. The CMIP5 data set includes projections using the Representative Concentration
Pathways (RCP). The CMIP6 phase involves a suite of common model experiments as well as an ensemble
of CMIP-endorsed Model Intercomparison Projects (MIPs).

The components of the Earth System at and below the land and ocean surface that are frozen, including
snow cover, glaciers, ice sheets, ice shelves, icebergs, sea ice, lake ice, river ice, permafrost and seasonally
frozen ground.

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Cultural Impacts
Impacts on material and ecological aspects of culture and the lived experience of culture, including
dimensions such as identity, community cohesion and belonging, sense of place, worldview, values,
perceptions, and tradition. Cultural impacts are closely related to ecological impacts, especially for iconic
and representational dimensions of species and landscapes. Culture and cultural practices frame the
importance and value of the impacts of change, shape the feasibility and acceptability of adaptation options,
and provide the skills and practices that enable adaptation.

Human actions to reduce carbon dioxide emissions from human activities.

Deep uncertainty
See uncertainty

Conversion of forest to non-forest. See also Afforestation and Reforestation

[Note: For a discussion of the term forest and related terms such as afforestation, reforestation and
deforestation, see the 2006 IPCC Guidelines for National Greenhouse Gas Inventories and their 2019
Refinement, and information provided by the United Nations Framework Convention on Climate Change
(IPCC 2006, 2019; UNFCCC 2021a, 2021b)]

Land degradation in arid, semi-arid, and dry sub-humid areas resulting from many factors, including climatic
variations and human activities (UNCCD, 1994).

Detection of change is defined as the process of demonstrating that climate or a system affected by climate
has changed in some defined statistical sense, without providing a reason for that change. An identified
change is detected in observations if its likelihood of occurrence by chance due to internal variability alone is
determined to be small, for example, <10%.

Detection and attribution
See attribution and detection

Developed / developing countries (Industrialised / developed / developing countries)
There is a diversity of approaches for categorizing countries on the basis of their level of development, and
for defining terms such as industrialised, developed, or developing. Several categorisations are used in this
Special Report. (1) In the United Nations (UN) system, there is no established convention for the designation
of developed and developing countries or areas. (2) The UN Statistics Division specifies developed and
developing regions based on common practice. In addition, specific countries are designated as least
developed countries, landlocked developing countries, small island developing states (SIDS), and transition
economies. Many countries appear in more than one of these categories. (3) The World Bank uses income as
the main criterion for classifying countries as low, lower middle, upper middle, and high income. (4) The
UN Development Programme (UNDP) aggregates indicators for life expectancy, educational attainment, and
income into a single composite Human Development Index (HDI) to classify countries as low, medium,
high, or very high human development.

Development pathways
See Pathways

Microscopic (2-200µm) unicellular photosynthetic algae that live in surface waters of lakes, rivers and
oceans and form shells of opal. In the global ocean, marine diatom species distribution is primarily driven by
nutrient availability. On regional scales, their species distribution in ocean sediment cores can be related to
past sea surface temperatures.

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The kinds of food that follow a particular pattern that a person or community eats (FAO, 2014).

A `serious disruption of the functioning of a community or a society at any scale due to hazardous events
interacting with conditions of exposure, vulnerability and capacity, leading to one or more of the following:
human, material, economic and environmental losses and impacts' (UNGA, 2016) See also Exposure,
Hazard, Risk and Vulnerability

Disaster management
Social processes for designing, implementing, and evaluating strategies, policies, and measures that promote
and improve disaster preparedness, response, and recovery practices at different organizational and societal

Disaster risk
The likelihood over a specified time period of severe alterations in the normal functioning of a community or
a society due to hazardous physical events interacting with vulnerable social conditions, leading to
widespread adverse human, material, economic, or environmental effects that require immediate emergency
response to satisfy critical human needs and that may require external support for recovery.

Disaster risk management (DRM)
Processes for designing, implementing, and evaluating strategies, policies, and measures to improve the
understanding of current and future disaster risk, foster disaster risk reduction and transfer, and promote
continuous improvement in disaster preparedness, prevention and protection, response, and recovery
practices, with the explicit purpose of increasing human security, wellbeing, quality of life, and sustainable
development (SD).

Disaster Risk Reduction (DRR)
Denotes both a policy goal or objective, and the strategic and instrumental measures employed for
anticipating future disaster risk; reducing existing exposure, hazard, or vulnerability; and improving

Discount rate
See Discounting

A mathematical operation that aims to make monetary (or other) amounts received or expended at different
times (years) comparable across time. If the discount rate is positive, future values are given less weight than
those today. The choice of discount rate(s) is debated as it is a judgement based on hidden and/or explicit

A method that derives local- to regional-scale (up to 100 km) information from larger-scale models or data
analyses. Two main methods exist: dynamical downscaling and empirical/statistical downscaling. The
dynamical method uses the output of regional climate models, global models with variable spatial resolution,
or high-resolution global models. The empirical/statistical methods are based on observations and develop
statistical relationships that link the large-scale atmospheric variables with local/regional climate variables.
In all cases, the quality of the driving model remains an important limitation on quality of the downscaled
information. The two methods can be combined, e.g., applying empirical/statistical downscaling to the
output of a regional climate model, consisting of a dynamical downscaling of a global climate model.

Artificial lowering of the soil water table (IPCC, 2013).

Any natural or human-induced factor that directly or indirectly causes a change in a system (adapted from
MA, 2005). See also Climatic driver

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An exceptional period of water shortage for existing ecosystems and the human population (due to low
rainfall, high temperature, and/or wind).

       A very lengthy and pervasive drought, lasting much longer than normal, usually a decade or more.

       Hydrological drought
       A period with large runoff and water deficits in rivers, lakes and reservoirs.

       Agricultural and ecological drought
       Agricultural and ecological drought (depending on the affected biome): a period with abnormal soil
       moisture deficit, which results from combined shortage of precipitation and excess evapotranspiration,
       and during the growing season impinges on crop production or ecosystem function in general.

       Meteorological drought
       A period with an abnormal precipitation deficit.

Early warning systems (EWS)
The set of technical and institutional capacities to forecast, predict, and communicate timely and meaningful
warning information to enable individuals, communities, managed ecosystems, and organisations threatened
by a hazard to prepare to act promptly and appropriately to reduce the possibility of harm or loss. Dependent
upon context, EWS may draw upon scientific and/or indigenous knowledge, and other knowledge types.
EWS are also considered for ecological applications, e.g., conservation, where the organisation itself is not
threatened by hazard but the ecosystem under conservation is (e.g., coral bleaching alerts), in agriculture
(e.g., warnings of heavy rainfall, drought, ground frost, and hailstorms) and in fisheries (e.g., warnings of
storm, storm surge, and tsunamis) (UNISDR 2009; IPCC, 2012a).

Earth system model (ESM)
A coupled atmosphere­ocean general circulation model (AOGCM) in which a representation of the carbon
cycle is included, allowing for interactive calculation of atmospheric carbon dioxide (CO2) or compatible
emissions. Additional components (e.g., atmospheric chemistry, ice sheets, dynamic vegetation, nitrogen
cycle, but also urban or crop models) may be included.

Eastern boundary upwelling system (EBUS)
Eastern boundary upwelling system (EBUS) are located at the eastern (landward) edges of major ocean
basins in both hemispheres, where equatorward winds drive upwelling currents that bring cool, nutrient-rich
(and often oxygen-poor) waters from the deep ocean to the surface near the coast.

A functional unit consisting of living organisms, their non-living environment and the interactions within
and between them. The components included in a given ecosystem and its spatial boundaries depend on the
purpose for which the ecosystem is defined: in some cases, they are relatively sharp, while in others they are
diffuse. Ecosystem boundaries can change over time. Ecosystems are nested within other ecosystems and
their scale can range from very small to the entire biosphere. In the current era, most ecosystems either
contain people as key organisms, or are influenced by the effects of human activities in their environment.
See also Ecosystem services and Ecosystem Health

Ecosystem Health
Ecosystem health is a metaphor used to describe the condition of an ecosystem, by analogy with human
health. Note that there is no universally accepted benchmark for a healthy ecosystem. Rather, the apparent
health status of an ecosystem is judged on the ecosystems resilience to change, with details depending upon
which metrics are employed in judging it, and which societal aspirations are driving the assessment.
(following IPBES 2019)

Ecosystem services

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Ecological processes or functions having monetary or non-monetary value to individuals or society at large.
These are frequently classified as (1) supporting services such as productivity or biodiversity maintenance,
(2) provisioning services such as food or fibre, (3) regulating services such as climate regulation or carbon
sequestration, and (4) cultural services such as tourism or spiritual and aesthetic appreciation. See also
Ecosystem and Ecosystem Health

Ecosystem-based adaptation (EBA)
See Adaptation

El Niño-Southern Oscillation (ENSO)
The term El Niño was initially used to describe a warm-water current that periodically flows along the coast
of Ecuador and Peru, disrupting the local fishery. It has since become identified with warming of the tropical
Pacific Ocean east of the dateline. This oceanic event is associated with a fluctuation of a global-scale
tropical and subtropical surface pressure pattern called the Southern Oscillation. This coupled atmosphere­
ocean phenomenon, with preferred time scales of two to about seven years, is known as the El Niño-
Southern Oscillation (ENSO). The warm and cold phases of ENSO are called El Niño and La Niña,
respectively. ENSO is often measured by the surface pressure anomaly difference between Tahiti and
Darwin and/or the sea surface temperatures in the central and eastern equatorial Pacific. This phenomenon
has a great impact on the wind, sea surface temperature and precipitation patterns in the tropical Pacific. It
has climatic effects throughout the Pacific region and in many other parts of the world through global
teleconnections. See Section AIV.2.3 in Annex IV of the AR6 WGI report.

Emergence (of the Climate Signal)
Emergence of a climate change signal or trend refers to when a change in climate (the `signal') becomes
larger than the amplitude of natural or internal variations (defining the `noise'), This concept is often
expressed as a `signal-to-noise' ratio and emergence occurs at a defined threshold of this ratio (e.g., S/N > 1
or 2). Emergence can refer to changes relative to a historical or modern baseline (usually at least 20 years
long) and can also be expressed in terms of time (time of emergence) or in terms of a global warming level.
Emergence is also used to refer to a time when we can expect to see a response of reducing greenhouse gas
(GHG) emissions (emergence with respect to mitigation). Emergence can be estimated using observations
and/or model simulations.

Emission pathways
See Pathways

Emission scenario
See Scenario


       Anthropogenic emissions: Emissions of greenhouse gases (GHGs), precursors of GHGs and aerosols
       caused by human activities. These activities include the burning of fossil fuels, deforestation, land use
       and land use changes (LULUC), livestock production, fertilisation, waste management, and industrial

       Fossil fuel emissions: Emissions of greenhouse gases (in particular carbon dioxide), other trace gases
       and aerosols resulting from the combustion of fuels from fossil carbon deposits such as oil, gas and

       Non-CO2 emissions and radiative forcing: Non-CO2 emissions included in this report are all
       anthropogenic emissions other than CO2 that result in radiative forcing. These include short-lived
       climate forcers, such as methane (CH4), some fluorinated gases, ozone (O3) precursors, aerosols or
       aerosol precursors, such as black carbon and sulphur dioxide, respectively, as well as long-lived
       greenhouse gases, such as nitrous oxide (N2O) or other fluorinated gases. The radiative forcing
       associated with non-CO2 emissions and changes in surface albedo is referred to as non-CO2 radiative

Enabling conditions (for adaptation and mitigation options)

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Conditions that enhance the feasibility of adaptation and mitigation options. Enabling conditions include
finance, technological innovation, strengthening policy instruments, institutional capacity, multi-level
governance, and changes in human behaviour and lifestyles.

Endemic species
Plants and animals that are only found in one geographic region.

Energy access
Access to clean, reliable and affordable energy services for cooking and heating, lighting, communications,
and productive uses (with special reference to Sustainable Development Goal 7) (AGECC, 2010).

Energy efficiency
The ratio of output or useful energy or energy services or other useful physical outputs obtained from a
system, conversion process, transmission or storage activity to the input of energy (measured as kWh kWh-
1, tonnes kWh-1 or any other physical measure of useful output like tonne-km transported). Energy
efficiency is often described by energy intensity.

Energy security
The goal of a given country, or the global community as a whole, to maintain an adequate, stable and
predictable energy supply. Measures encompass safeguarding the sufficiency of energy resources to meet
national energy demand at competitive and stable prices and the resilience of the energy supply; enabling
development and deployment of technologies; building sufficient infrastructure to generate, store and
transmit energy supplies and ensuring enforceable contracts of delivery.

Energy system
The energy system comprises all components related to the production, conversion, delivery, and use of

A principle that ascribes equal worth to all human beings, including equal opportunities, rights, and
obligations, irrespective of origins. See also Equity and Fairness

Uneven opportunities and social positions, and processes of discrimination within a group or society,
based on gender, class, ethnicity, age, and (dis)ability, often produced by uneven development.
Income inequality refers to gaps between highest and lowest income earners within a country and
between countries.

The principle of being fair and impartial, and a basis for understanding how the impacts and responses to
climate change, including costs and benefits, are distributed in and by society in more or less equal ways.
Often aligned with ideas of equality, fairness and justice and applied with respect to equity in the
responsibility for, and distribution of, climate impacts and policies across society, generations, and gender,
and in the sense of who participates and controls the processes of decision-making.

Ethics involves questions of justice and value. Justice is concerned with right and wrong, equity and fairness,
and, in general, with the rights to which people and living beings are entitled. Value is a matter of worth,
benefit, or good.

Over-enrichment of water by nutrients such as nitrogen and phosphorus. It is one of the leading causes of
water quality impairment. The two most acute symptoms of eutrophication are hypoxia (or oxygen
depletion) and harmful algal blooms.

The physical process by which a liquid (e.g., water) becomes a gas (e.g., water vapour).

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The combined processes through which water is transferred to the atmosphere from open water and ice
surfaces, bare soil, and vegetation that make up the Earth's surface.

Data and information used in the scientific process to establish findings. In this report, the degree of
evidence reflects the amount, quality and consistency of scientific/technical information on which the Lead
Authors are basing their findings. See also Agreement, Confidence, Likelihood and Uncertainty

Evolutionary adaptation
See Adaptation

The presence of people; livelihoods; species or ecosystems; environmental functions, services, and resources;
infrastructure; or economic, social, or cultural assets in places and settings that could be adversely affected.

Externality/external cost/external benefit
Externalities arise from a human activity, when agents responsible for the activity do not take full account of
the activity's impact on others' production and consumption possibilities, and no compensation exists for
such impacts. When the impact is negative, they are external costs. When positive they are referred to as
external benefits. See also co-benefits

A population, species or more inclusive taxonomic group has gone extinct when all its individuals have died.
A species may go extinct locally (population extinction), regionally (e.g., extinction of all populations in a
country, continent or ocean) or globally (IPBES, 2019). See also extirpation

The disappearance of a species from an area, sometimes also referred to as local extinction. Its use implies
that the species still occurs elsewhere. See also extinction

Extreme sea level (ESL)
The occurrence of an exceptionally low or high local sea-surface height, arising from (a combination of)
short term phenomena (e.g. storm surges, tides and waves). Relative sea-level changes affect extreme sea
levels directly by shifting the mean water levels and indirectly by modulating the propagation of tides, waves
and/or surges due to increased water depth. In addition, extreme sea levels can be influenced by changes in
the frequency, tracks, or strength of weather systems and storms, or due to anthropogenically induced
changes such as the modification of coastlines or dredging. In turn, changes in any or all of the contributions
to extreme sea levels may lead to long term relative sea-level changes. Alternate expressions for ESL may be
used depending on the processes resolved.

Extreme Still Water Level (ESWL) refers to the combined contribution of relative sea-level change, tides
and storm-surges. Wind-waves also contribute to coastal sea level via three processes: infragravity waves
(lower frequency gravity waves generated by the wind waves); wave setup (time-mean sea-level elevation
due to wave energy dissipation); and swash (vertical displacement up the shore-face induced by individual
waves). Extreme Total Water Level (ETWL) is the ESWL plus wave setup. When considering coastal
impacts, swash is also important, and Extreme Coastal Water Level (ECWL) is used. See also Sea level
change (sea level rise/ sea level fall)

Extreme weather event
An event that is rare at a particular place and time of year. Definitions of `rare' vary, but an extreme weather
event would normally be as rare as or rarer than the 10th or 90th percentile of a probability density function
estimated from observations. By definition, the characteristics of what is called extreme weather may vary
from place to place in an absolute sense. See also Heatwave and Climate extreme

Extreme/ heavy precipitation event

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An extreme/heavy precipitation event is an event that is of very high magnitude with a very rare occurrence
at a particular place. Types of extreme precipitation may vary depending on its duration, hourly, daily or
multi-days (e.g., 5 days), though all of them qualitatively represent high magnitude. The intensity of such
events may be defined with block maxima approach such as annual maxima or with peak over threshold
approach, such as rainfall above 95th or 99th percentile at a particular space.

Impartial and just treatment without favouritism or discrimination in which each person is considered of
equal worth with equal opportunity. See also Equality and Equity

In this report, feasibility refers to the potential for a mitigation or adaptation option to be implemented.
Factors influencing feasibility are context-dependent, temporally dynamic, and may vary between different
groups and actors. Feasibility depends on geophysical, environmental-ecological, technological, economic,
socio-cultural and institutional factors that enable or constrain the implementation of an option. The
feasibility of options may change when different options are combined, and increase when enabling
conditions are strengthened. See also Enabling conditions (for adaptation and mitigation options)

Fire weather
Weather conditions conducive to triggering and sustaining wildfires, usually based on a set of indicators and
combinations of indicators including temperature, soil moisture, humidity, and wind. Fire weather does not
include the presence or absence of fuel load.

The overflowing of the normal confines of a stream or other water body, or the accumulation of water over
areas that are not normally submerged. Floods can be caused by unusually heavy rain, for example during
storms and cyclones. Floods include river (fluvial) floods, flash floods, urban floods, rain (pluvial) floods,
sewer floods, coastal floods, and glacial lake outburst floods (GLOFs).

A movement (a flow) of matter (e.g., water vapor, particles), heat or energy from one place to another, or
from one medium (e.g., land surface) to another (e.g., atmosphere).

Food security
   A situation that exists when all people, at all times, have physical, social and economic access to
   sufficient, safe and nutritious food that meets their dietary needs and food preferences for an active and
   healthy life. The four pillars of food security are availability, access, utilization and stability. The
   nutritional dimension is integral to the concept of food security {FAO, 2018/ 2009 #6619}.

   Physical availability of food. Food availability addresses the supply side of food security and is
   determined by the levels of food production, stocks and net trade.

   Economic and/ or physical access to food. Economic access is determined by disposable income, food
   prices and the provision of and access to social support. Physical access is determined by the availability
   and quality of land and other infrastructure, property rights or the functioning of markets.

   The way in which the body uses the various nutrients in food. Individuals achieve sufficient energy and
   nutrient intake through good care and feeding practices, food preparation, diet diversity and
   intrahousehold distribution of food. Combined with biological utilization of the food consumed, energy
   and nutrient intake determine the nutrition status of individuals.

   The stability of the other three dimensions over time. Even if individuals' food intake is adequate today,
   they are still considered food-insecure if periodically they have inadequate access to food, risking
   deterioration of their nutrition status. Adverse weather conditions, political instability or economic factors
   (unemployment, rising food prices) may have an impact on individuals' food security status.

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Food system
All the elements (environment, people, inputs, processes, infrastructures, institutions, etc.) and activities that
relate to the production, processing, distribution, preparation and consumption of food, and the output of
these activities, including socio-economic and environmental outcomes (HLPE, 2017). [Note: Whilst there is
a global food system (encompassing the totality of global production and consumption), each location's food
system is unique, being defined by that place's mix of food produced locally, nationally, regionally or

Food-borne disease
Illnesses transmitted through consumption of unsafe or contaminated food, that contamination can come
from a variety of sources, including contaminated water. (adapted from UNEP, 2018)

A vegetation type dominated by trees. Many definitions of the term forest are in use throughout the world,
reflecting wide differences in biogeophysical conditions, social structure and economics. See also
Afforestation, Deforestation and Reforestation

[Note: For a discussion of the term forest in the context of National GHG inventories, see the 2006 IPCC
Guidelines for National GHG Inventories and their 2019 Refinement, and information provided by the
United Nations Framework Convention on Climate Change (IPCC 2006, 2019; UNFCCC, 2021a, 2021b).]

Forest degradation
A reduction in the capacity of a forest to produce ecosystem services such as carbon storage and wood
products as a result of anthropogenic and environmental changes.

Forest Dieback
See Forest degradation

Forest line
The upper limit of the closed upper montane forest or forest at high latitudes. It is less elevated or less
poleward than the tree line

Forest management
See Sustainable Forest Management

Fossil fuels
Carbon-based fuels from fossil hydrocarbon deposits, including coal, oil, and natural gas.

Glacial lake outburst flood (GLOF) / Glacier lake outburst
A sudden release of water from a glacier lake, including any of the following types ­ a glacier-dammed lake,
a pro-glacial moraine-dammed lake or water that was stored within, under or on the glacier.

A perennial mass of ice, and possibly firn and snow, originating on the land surface by accumulation and
compaction of snow and showing evidence of past or present flow. A glacier typically gains mass by
accumulation of snow and loses mass by ablation. Land ice masses of continental size (>50,000 km2) are
referred to as ice sheets (Cogley et al., 2011).

Global change
A generic term to describe global scale changes in systems, including the climate system, ecosystems, and
social-ecological systems.

Global mean sea level change
Global mean sea level (GMSL) change is the increase or decrease in the volume of the ocean divided by the
ocean surface area. It is driven by changes in ocean density through temperature changes (global mean

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thermosteric sea level change) and changes in the ocean mass as a result of changes in the cryosphere or
terrestrial water storage (barystatic sea level change).

Global mean surface air temperature (GSAT)
Global average of near-surface air temperatures over land, oceans and sea ice. Changes in GSAT are often
used as a measure of global temperature change in climate models. See also Global mean surface
temperature (GMST).

Global mean surface temperature (GMST)
Estimated global average of near-surface air temperatures over land and sea ice, and sea surface temperature
(SST) over ice-free ocean regions, with changes normally expressed as departures from a value over a
specified reference period. See also Global mean surface air temperature (GSAT).

Global monsoon
The global monsoon (GM) is a global-scale solstitial mode that dominates the annual variation of tropical
and sub-tropical precipitation and circulation. The GM domain is defined as the area where the annual range
of precipitation (local summer minus winter mean precipitation rate) is greater than 2.5 mm day-1, following
on from the definition as in Kitoh et al. (2013). Further details on how the GM is defined, used and related to
regional monsoons throughout the Report are provided by AR6 WGI Annex V.

Global warming
Global warming refers to the increase in global surface temperature relative to a baseline reference period,
averaging over a period sufficient to remove interannual variations (e.g., 20 or 30 years). A common choice
for the baseline is 1850­1900 (the earliest period of reliable observations with sufficient geographic
coverage), with more modern baselines used depending upon the application. See also Climate Change and
Climate variability

The structures, processes, and actions through which private and public actors interact to address
societal goals. This includes formal and informal institutions and the associated norms, rules, laws and
procedures for deciding, managing, implementing and monitoring policies and measures at any
geographic or political scale, from global to local.

Adaptive governance
Adjusting to changing conditions, such as climate change, through governance interactions that
seek to maintain a desired state in a social-ecological system.

Climate governance
The structures, processes, and actions through which private and public actors seek to mitigate
and adapt to climate change.

Multilevel governance
The dispersion of governance across multiple levels of jurisdiction and decision-making,
including, global, regional, national and local, as well as trans-regional and trans-national levels.

Polycentric governance
Polycentric governance involves multiple centres of decision-making with overlapping jurisdictions.
While the centres have some degree of autonomy, they also take each other into account, coordinating
their actions and seeking to resolve conflicts (Carlisle and Gruby, 2017; Jordan et al., 2018; McGinnis
and Ostrom, 2012).

Governance capacity
The ability of governance institutions, leaders, and non-state and civil society to plan, co-ordinate, fund,
implement, evaluate and adjust policies and measures over the short, medium and long term, adjusting for
uncertainty, rapid change and wide-ranging impacts and multiple actors and demands.

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Green Climate Fund (GCF)
The Green Climate Fund was established by the 16th Session of the Conference of the Parties (COP) in 2010
as an operating entity of the financial mechanism of the United Nations Framework Convention on Climate
Change (UNFCCC), in accordance with Article 11 of the Convention, to support projects, programmes and
policies and other activities in developing country Parties. The Fund is governed by a board and will receive
guidance of the COP.

Green infrastructure
See infrastructure

Greenhouse gases (GHG)
Gaseous constituents of the atmosphere, both natural and anthropogenic, that absorb and emit radiation at
specific wavelengths within the spectrum of radiation emitted by the Earth's ocean and land surface, by the
atmosphere itself, and by clouds. This property causes the greenhouse effect. Water vapour (H2O), carbon
dioxide (CO2), nitrous oxide (N2O), methane (CH4) and ozone (O3) are the primary GHGs in the Earth's
atmosphere. Human-made GHGs include sulphur hexafluoride (SF6), hydrofluorocarbons (HFCs),
chlorofluorocarbons (CFCs) and perfluorocarbons (PFCs); several of these are also O3-depleting (and are
regulated under the Montreal Protocol).

Grey infrastructure
See infrastructure

Gross domestic product (GDP)
The sum of gross value added, at purchasers' prices, by all resident and non-resident producers in the
economy, plus any taxes and minus any subsidies not included in the value of the products in a country or a
geographic region for a given period, normally one year. GDP is calculated without deducting for
depreciation of fabricated assets or depletion and degradation of natural resources.

Groundwater recharge
The process by which external water is added to the zone of saturation of an aquifer, either directly into a
geologic formation that traps the water or indirectly by way of another formation.

Habitability (human)
The ability of a place to support human life by providing protection from hazards which challenge human
survival, and by assuring adequate space, food and freshwater.

The potential occurrence of a natural or human-induced physical event or trend that may cause loss of life,
injury, or other health impacts, as well as damage and loss to property, infrastructure, livelihoods, service
provision, ecosystems and environmental resources. See also Impacts and Risk

Health is a state of complete physical, mental and social well-being and not merely the absence of disease or
infirmity (WHO).

Heat index
A measure of how hot the air feels to the human body. The index is mainly based on surface air temperature
and relative humidity; thus, it reflects the combined effect of high temperature and humidity on human
physiology and provides a relative indication of potential health risks. See also Heatwave

Heat stress
A range of conditions in, for example, terrestrial or aquatic organisms when the body absorbs excess heat
during overexposure to high air or water temperatures or thermal radiation. In aquatic water-breathing
animals, hypoxia and acidification can exacerbate vulnerability to heat. Heat stress in mammals (including
humans) and birds, both in air, is exacerbated by a detrimental combination of ambient heat, high humidity
and low wind speeds, causing regulation of body temperature to fail.

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A period of abnormally hot weather often defined with reference to a relative temperature threshold, lasting
from two days to months. Heatwaves and warm spells have various and, in some cases, overlapping
definitions. See also Heat index, Heat stress and Marine Heatwave

Heavy precipitation event
See Extreme/heavy precipitation event.

Human mobility
The permanent or semi-permanent move by a person for at least one year and involving crossing an
administrative, but not necessarily a national, border.

Human rights
Rights that are inherent to all human beings, universal, inalienable, and indivisible, typically expressed and
guaranteed by law. They include the right to life, economic, social, and cultural rights, and the right to
development and self-determination (UNOHCHR, 2018).

Human security
A condition that is met when the vital core of human lives is protected, and when people have the freedom
and capacity to live with dignity. In the context of climate change, the vital core of human lives includes the
universal and culturally specific, material and non-material elements necessary for people to act on behalf of
their interests and to live with dignity.

Human system
Any system in which human organisations and institutions play a major role. Often, but not always, the term
is synonymous with society or social system. Systems such as agricultural systems, urban systems, political
systems, technological systems, and economic systems are all human systems in the sense applied in this

Hydrological cycle
The cycle in which water evaporates from the ocean and the land surface, is carried over the Earth in
atmospheric circulation as water vapour, condenses to form clouds, precipitates over the ocean and land as
rain or snow, which on land can be intercepted by trees and vegetation, potentially accumulating as snow or
ice, provides runoff on the land surface, infiltrates into soils, recharges groundwater, discharges into streams,
and ultimately, flows into the oceans as rivers, polar glaciers and ice sheets, from which it will eventually
evaporate again. The various systems involved in the hydrological cycle are usually referred to as
hydrological systems.

Hydrological drought
See drought

Power harnessed from the flow of water.

Hyperthermal Event
Geologically abrupt global warming events of the past associated with disturbances of the carbon cycle and
impacts on the biosphere.

Conditions of low dissolved oxygen in shallow water ocean and freshwater environments. There is no
universal threshold for hypoxia. A value around 60 mol kg-1 has commonly been used for some estuarine
systems, although this does not necessarily directly translate into biological impacts. Anoxic conditions
occur where there is no oxygen present at all. See also Eutrophication.

Hypoxic events
Events that lead to deficiencies of oxygen in water bodies.

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Ice sheet
An ice body originating on land that covers an area of continental size, generally defined as covering
>50,000 km2, and that has formed over thousands of years through accumulation and compaction of snow.
An ice sheet flows outward from a high central ice plateau with a small average surface slope. The margins
usually slope more steeply, and most ice is discharged through fast-flowing ice streams or outlet glaciers,
often into the sea or into ice shelves floating on the sea. There are only two ice sheets in the modern world,
one on Greenland and one on Antarctica. The latter is divided into the East Antarctic Ice Sheet (EAIS), the
West Antarctic Ice Sheet (WAIS) and the Antarctic Peninsula ice sheet. During glacial periods, there were
other ice sheets.

The consequences of realised risks on natural and human systems, where risks result from the interactions of
climate-related hazards (including extreme weather / climate events), exposure, and vulnerability. Impacts
generally refer to effects on lives, livelihoods, health and wellbeing, ecosystems and species, economic,
social and cultural assets, services (including ecosystem services), and infrastructure. Impacts may be
referred to as consequences or outcomes, and can be adverse or beneficial. See also Adaptation, Exposure,
Loss and Damage and losses and damages, Vulnerability and Risk

The maximum amount that a household, or other unit, can consume without reducing its real net worth. Total
income is the broadest measure of income and refers to regular receipts such as wages and salaries, income
from self-employment, interest and dividends from invested funds, pensions or other benefits from social
insurance, and other current transfers receivable. FTN: This glossary entry builds from the definition used in
OECD (2003).

Incremental adaptation
See Adaptation

Indigenous knowledge
The understandings, skills and philosophies developed by societies with long histories of interaction with
their natural surroundings. For many indigenous peoples, IK informs decision-making about fundamental
aspects of life, from day-to-day activities to longer term actions. This knowledge is integral to cultural
complexes, which also encompass language, systems of classification, resource use practices, social
interactions, values, ritual and spirituality. These distinctive ways of knowing are important facets of the
world's cultural diversity (UNESCO, 2018). See also Local knowledge

Indigenous Peoples
Indigenous Peoples and Nations are those that, having a historical continuity with pre-invasion and pre-
colonial societies that developed on their territories, consider themselves distinct from other sectors of the
societies now prevailing on those territories, or parts of them. They form at present principally non-dominant
sectors of society and are often determined to preserve, develop, and transmit to future generations their
ancestral territories, and their ethnic identity, as the basis of their continued existence as peoples, in
accordance with their own cultural patterns, social institutions, and common law system. FTN: This glossary
entry builds from the definitions used in Cobo (1987) and previous IPCC reports.

Indirect land-use change (iLUC)
See land-use-change

See Equality

Informal settlement
A term given to settlements or residential areas that by at least one criterion fall outside official rules and
regulations. Most informal settlements have poor housing (with widespread use of temporary materials) and
are developed on land that is occupied illegally with high levels of overcrowding. In most such settlements,
provision for safe water, sanitation, drainage, paved roads, and basic services is inadequate or lacking. The

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term slum is often used for informal settlements, although it is misleading as many informal settlements
develop into good quality residential areas, especially where governments support such development.

The designed and built set of physical systems and corresponding institutional arrangements that mediate
between people, their communities, and the broader environment to provide services that support economic
growth, health, quality of life, and safety (Chester, 2019; Dawson et al., 2018) There are four categories of

Blue infrastructure
Blue infrastructure includes bodies of water, watercourses, ponds, lakes and storm drainage, that
provide ecological and hydrological functions including evaporation, transpiration, drainage,
infiltration, and temporarily storage of runoff and discharge.

Green infrastructure
The strategically planned interconnected set of natural and constructed ecological systems, green
spaces and other landscape features that can provide functions and services including air and water
purification, temperature management, floodwater management and coastal defence often with co-
benefits for human and ecological well-being. Green infrastructure includes planted and remnant
native vegetation, soils, wetlands, parks and green open spaces, as well as building and street level
design interventions that incorporate vegetation. (after Culwick and Bobbins, 2016)

Grey infrastructure
Engineered physical components and networks of pipes, wires, roads, tracks that underpin energy,
transport, communications (including digital), built form, water and sanitation and solid waste
management systems.

Social infrastructure
The social, cultural, and financial activities and institutions as well as associated property, buildings
and artefacts and policy domains such as social protection, health and education that support
wellbeing and public life. (Frolova et al., 2016; Latham and Layton, 2019).

Institutional capacity
Building and strengthening individual organisations and providing technical and management training to
support integrated planning and decision-making processes between organisations and people, as well as
empowerment, social capital, and an enabling environment, including the culture, values and power relations
(Willems and Baumert, 2003).

Rules, norms and conventions that guide, constrain or enable human behaviours and practices.
Institutions can be formally established, for instance through laws and regulations, or informally
established, for instance by traditions or customs. Institutions may spur, hinder, strengthen, weaken or
distort the emergence, adoption and implementation of climate action and climate governance.

[Note: Institutions can also refer to a large organisation]

A family of financial instruments for sharing and transferring risk among a pool of at-risk households,
businesses, and/or governments.

Integrated assessment
A method of analysis that combines results and models from the physical, biological, economic and social
sciences and the interactions among these components in a consistent framework to evaluate the status and
the consequences of environmental change and the policy responses to it.

Integrated assessment model (IAM)

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Models that integrate knowledge from two or more domains into a single framework. They are one of the
main tools for undertaking integrated assessments. One class of IAM used in respect of climate change
mitigation may include representations of: multiple sectors of the economy, such as energy, land use and
land use change; interactions between sectors; the economy as a whole; associated greenhouse gas (GHG)
emissions and sinks; and reduced representations of the climate system. This class of model is used to assess
linkages between economic, social and technological development and the evolution of the climate system.
Another class of IAM additionally includes representations of the costs associated with climate change
impacts, but includes less detailed representations of economic systems. These can be used to assess impacts
and mitigation in a cost-benefit framework and have been used to estimate the social cost of carbon.

Invasive species
A species that is not native to a specific location or nearby, lacking natural controls, and has a tendency to
rapidly increase in abundance, displacing native species. Invasive species may also damage the human
economy or human health.

Justice is concerned with ensuring that people get what is due to them, setting out the moral or legal
principles of fairness and equity in the way people are treated, often based on the ethics and values of

Climate justice
Justice that links development and human rights to achieve a human-centred approach to addressing
climate change, safeguarding the rights of the most vulnerable people and sharing the burdens and
benefits of climate change and its impacts equitably and fairly (MRFJC, 2018).

Procedural justice
Justice in the way outcomes are brought about including who participates and is heard in the processes
of decision-making.

Social justice
Just or fair relations within society that seek to address the distribution of wealth, access to resources,
opportunity, and support according to principles of justice and fairness.

Key risk
Those risks that are especially relevant to the interpretation of `dangerous anthropogenic interference with
the climate system' (DAI) in the terminology of UNFCCC, Article 2, meriting particular attention by policy
makers in that context. Key risks are potentially severe adverse consequences for humans and social-
ecological systems resulting from the interaction of climate related hazards with vulnerabilities of societies
and systems exposed. Risks are considered "key" due to high hazard or high vulnerability of societies and
systems exposed, or both.

The terrestrial portion of the biosphere that comprises the natural resources (soil, near-surface air, vegetation
and other biota, and water), the ecological processes, topography, and human settlements and infrastructure
that operate within that system (FAO, 2007; UNCCD, 1994).

Land cover
The biophysical coverage of land (e.g., bare soil, rocks, forests, buildings and roads or lakes). Land cover is
often categorised in broad land-cover classes (e.g., deciduous forest, coniferous forest, mixed forest,
grassland, bare ground). Note: In some literature assessed in this report, land cover and land use are used
interchangeably, but the two represent distinct classification systems. For example, the land cover class
woodland can be under various land uses such as livestock grazing, recreation, conservation, or wood

Land cover change
Change from one land cover class to another, due to change in land use or change in natural conditions
(Pongratz et al., 2018). See also Land cover and Land-use change

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Land degradation
A negative trend in land condition, caused by direct or indirect human-induced processes including
anthropogenic climate change, expressed as long-term reduction or loss of at least one of the following:
biological productivity, ecological integrity or value to humans. [Note: This definition applies to forest and
non-forest land. Changes in land condition resulting solely from natural processes (such as volcanic
eruptions) are not considered to be land degradation. Reduction of biological productivity or ecological
integrity or value to humans can constitute degradation, but any one of these changes need not necessarily be
considered degradation.]

Land management
Sum of land-use practices (e.g., sowing, fertilizing, weeding, harvesting, thinning, clear-cutting) that take
place within broader land-use categories. (Pongratz et al., 2018)

Land use
The total of arrangements, activities and inputs applied to a parcel of land. The term land use is also used in
the sense of the social and economic purposes for which land is managed (e.g., grazing, timber extraction,
conservation and city dwelling). In national greenhouse gas (GHG) inventories, land use is classified
according to the IPCC land-use categories of forest land, cropland, grassland, wetlands, settlements, other
lands (see the 2006 IPCC Guidelines for National GHG Inventories and their 2019 Refinement for details
(IPCC, 2006, 2019)).

Land-use change
The change from one land use category to another. Note that in some scientific literature, land-use change
encompasses changes in land-use categories as well as changes in land management. See also Afforestation,
Agriculture, Forestry and Other Land Use (AFOLU), Deforestation, Land use, land-use change and forestry
(LULUCF) and Reforestation.

       Indirect land-use change (iLUC)
       Land use change outside the area of focus, that occurs as a consequence of change in use or
       management of land within the area of focus, such as through market or policy drivers. For example, if
       agricultural land is diverted to biofuel production, forest clearance may occur elsewhere to replace the
       former agricultural production. See Land-use change (LUC).

Least Developed Countries (LDCs)
A list of countries designated by the Economic and Social Council of the United Nations (ECOSOC) as
meeting three criteria: (1) a low income criterion below a certain threshold of gross national income per
capita of between USD 750 and USD 900, (2) a human resource weakness based on indicators of health,
education, adult literacy, and (3) an economic vulnerability weakness based on indicators on instability of
agricultural production, instability of export of goods and services, economic importance of non-traditional
activities, merchandise export concentration, and the handicap of economic smallness. Countries in this
category are eligible for a number of programmes focused on assisting countries most in need. These
privileges include certain benefits under the articles of the United Nations Framework Convention on
Climate Change (UNFCCC).

The chance of a specific outcome occurring, where this might be estimated probabilistically. Likelihood is
expressed in this Special Report using a standard terminology (Mastrandrea et al., 2010). See also
Agreement, Confidence, Evidence and Uncertainty

The resources used and the activities undertaken in order for people to live. Livelihoods are usually
determined by the entitlements and assets to which people have access. Such assets can be categorised as
human, social, natural, physical, or financial.

Local extinction
See extirpation

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Local knowledge (LK)
The understandings and skills developed by individuals and populations, specific to the places where they
live. Local knowledge informs decision-making about fundamental aspects of life, from day-to-day activities
to longer term actions. This knowledge is a key element of the social and cultural systems which influence
observations of and responses to climate change; it also informs governance decisions (UNESCO, 2018).
See also Indigenous knowledge

A situation in which the future development of a system, including infrastructure, technologies, investments,
institutions, and behavioural norms, is determined or constrained (`locked in') by historic developments. See
also Path dependence

Loss and Damage, and losses and damages
Research has taken Loss and Damage (capitalised letters) to refer to political debate under the United
Nations Framework Convention on Climate Change (UNFCCC) following the establishment of the Warsaw
Mechanism on Loss and Damage in 2013, which is to `address loss and damage associated with impacts of
climate change, including extreme events and slow onset events, in developing countries that are particularly
vulnerable to the adverse effects of climate change.' Lowercase letters (losses and damages) have been taken
to refer broadly to harm from (observed) impacts and (projected) risks and can be economic or non-
economic. (Mechler et al., 2018).

Low-likelihood, high impact outcomes
Outcomes/events whose probability of occurrence is low or not well known (as in the context of deep
uncertainty) but whose potential impacts on society and ecosystems could be high. To better inform risk
assessment and decision-making, such low-likelihood outcomes are considered if they are associated with
very large consequences and may therefore constitute material risks, even though those consequences do not
necessarily represent the most likely outcome. See also impacts

Maladaptive actions (Maladaptation)
Actions that may lead to increased risk of adverse climate-related outcomes, including via increased
greenhouse gas (GHG) emissions, increased or shifted vulnerability to climate change, more inequitable
outcomes, or diminished welfare, now or in the future. Most often, maladaptation is an unintended

Deficiencies, excesses, or imbalances in a person's intake of energy and/or nutrients. The term malnutrition
addresses three broad groups of conditions: undernutrition, which includes wasting (low weight-for-height),
stunting (low height-for-age) and underweight (low weight-for-age); micronutrient-related malnutrition,
which includes micronutrient deficiencies (a lack of important vitamins and minerals) or micronutrient
excess; and overweight, obesity and diet-related noncommunicable diseases (such as heart disease, stroke,
diabetes and some cancers) (WHO, 2018). Micronutrient deficiencies are sometimes termed `hidden hunger'
to emphasise that people can be malnourished in the sense of deficient without being deficient in calories.
Hidden hunger can apply even where people are obese.

Marine heatwave
A period during which water temperature is abnormally warm for the time of the year relative to historical
temperatures, with that extreme warmth persisting for days to months. The phenomenon can manifest in any
place in the ocean and at scales of up to thousands of kilometres. See also Heatwave

Mean sea level
The surface level of the ocean at a particular point averaged over an extended period of time such as a month
or year. Mean sea level is often used as a national datum to which heights on land are referred.


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Processes of data collection over time, providing basic datasets, including associated accuracy and precision,
for the range of relevant variables. Possible data sources are field measurements, field observations,
detection through remote sensing and interviews (UN-REDD, 2009).

Urban agglomerations with 10 million inhabitants or more. (United Nations, Department of Economic and
Social Affairs, Population Division (2019).

See Drought

Meteorological drought
See Drought

Mental health
The state of well-being in which an individual realizes his or her own abilities, can cope with the normal
stresses of life, can work productively and is able to contribute to his or her community.

Methane (CH4)
One of the six greenhouse gases (GHGs) to be mitigated under the Kyoto Protocol. Methane is the major
component of natural gas and associated with all hydrocarbon fuels. Significant anthropogenic emissions
also occur as a result of animal husbandry and paddy rice production. Methane is also produced naturally
where organic matter decays under anaerobic conditions, such as in wetlands. Under future global warming,
there is risk of increased methane emissions from thawing permafrost, coastal wetlands and sub-sea gas
hydrates. See also Short-lived climate forcers (SLCFs)

A consistent measurement of a characteristic of an object or activity that is otherwise difficult to quantify.
Within the context of the evaluation of climate models, this is a quantitative measure of agreement between a
simulated and observed quantity which can be used to assess the performance of individual models.

Metropolitan Region
See city region

Local climate at or near the Earth's surface.

Any person who is moving or has moved across an international border or within a State away from his/her
habitual place of residence, regardless of (1) the person's legal status; (2) whether the movement is voluntary
or involuntary; (3) what the causes for the movement are; or (4) what the length of the stay is. (IOM, 2018).

Migration (of humans)
Movement of a person or a group of persons, either across an international border, or within a State. It is a
population movement, encompassing any kind of movement of people, whatever its length, composition and
causes; it includes migration of refugees, displaced persons, economic migrants, and persons moving for
other purposes, including family reunification (IOM, 2018).

Mitigation (of climate change)
A human intervention to reduce emissions or enhance the sinks of greenhouse gases.

Mitigation measures
In climate policy, mitigation measures are technologies, processes or practices that contribute to mitigation,
for example renewable energy technologies, waste minimization processes, and public transport commuting

Mitigation option

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A technology or practice that reduces greenhouse gas (GHG) emissions or enhances sinks.

Mitigation scenario
A plausible description of the future that describes how the (studied) system responds to the implementation
of mitigation policies and measures.

Model Ensemble
See Climate simulation ensemble

Structured imitations of a system's attributes and mechanisms to mimic appearance or functioning of
systems, for example, the climate, the economy of a country, or a crop. Mathematical models assemble
(many) variables and relations (often in a computer code) to simulate system functioning and performance
for variations in parameters and inputs.

Monitoring and evaluation (M&E)
Mechanisms put in place to respectively monitor and evaluate efforts to reduce greenhouse gas emissions
and/or adapt to the impacts of climate change with the aim of systematically identifying, characterizing and
assessing progress over time.

See Global monsoon

A mountain is a landform formed through plate tectonics that rises above its surrounding area, characterized
by verticality and ruggedness such as gentle or steep sloping sides, sharp or rounded ridges, and a high point
called a peak or a summit. Mountain regions consist of mountains and mountain ranges as defined by
ruggedness, intermontane valleys, plateaus and tablelands, and hills and hilly forelands, together forming a
complex terrain.

To delineate mountain regions a combination of terrain characteristics is used, such as elevation above sea
level, steepness of slope and relative relief or local elevational range.

Three mountain characterizations using different combinations of the above criteria applied to digital
elevation models have been developed to arrive at mountain area statistics, described and analysed in detail
in Sayre et al, (2018), namely K1 (Kapos et al., 2000), K2 (Körner et al., 2011) and K3 (Karagulle et al.,

Multilevel governance
See Governance

See Storyline. See also Pathways.

Native species
Indigenous species of animals or plants that naturally occur in a given region or ecosystem. Under climate
change many species colonize new areas where they may become native over time (following IPBES 2019).
See also Invasive species

Natural systems
The dynamic physical, physicochemical and biological components of the Earth system that would operate
independently of human activities.

Nature-based solution (NBS)
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) See also Biodiversity and Ecosystem

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Net Primary Production (NPP)
See Primary Production

Net zero CO2 emissions
Condition in which anthropogenic carbon dioxide (CO2) emissions are balanced by anthropogenic CO2
removals over a specified period. See also Land use

[Note: Carbon neutrality and net zero CO2 emissions are overlapping concepts. The concepts can be applied
at global or sub-global scales (e.g., regional, national and sub-national). At a global scale, the terms carbon
neutrality and net zero CO2 emissions are equivalent. At sub-global scales, net zero CO2 emissions is
generally applied to emissions and removals under direct control or territorial responsibility of the reporting
entity, while carbon neutrality generally includes emissions and removals within and beyond the direct
control or territorial responsibility of the reporting entity. Accounting rules specified by GHG programmes
or schemes can have a significant influence on the quantification of relevant CO2 emissions and removals.]

New Urban Agenda
The New Urban Agenda was adopted at the United Nations Conference on Housing and Sustainable Urban
Development (Habitat III) in Quito, Ecuador, on 20 October 2016. It was endorsed by the United Nations
General Assembly at its sixty-eighth plenary meeting of the seventy-first session on 23 December 2016.

Non-climatic driver (Non-climate driver)
An agent or process outside the climate system that influences a human or natural system.

Non-communicable diseases
Non-communicable diseases (NCDs), also known as chronic diseases, tend to be of long duration and are the
result of a combination of genetic, physiological, environmental and behaviours factors. The main types of
NCDs are cardiovascular diseases (like heart attacks and stroke), cancers, chronic respiratory diseases (such
as chronic obstructive pulmonary disease and asthma) and diabetes (WHO)

The interconnected body of saline water that covers 71% of the Earth's surface, contains 97% of the Earth's
water and provides 99% of the Earth's biologically-habitable space. It includes the Arctic, Atlantic, Indian,
Pacific and Southern Oceans, as well as their marginal seas and coastal waters.

Ocean acidification (OA)
A reduction in the pH of the ocean, accompanied by other chemical changes (primarily in the levels of
carbonate and bicarbonate ions), over an extended period, typically decades or longer, which is caused
primarily by uptake of carbon dioxide (CO2) from the atmosphere, but can also be caused by other chemical
additions or subtractions from the ocean. Anthropogenic OA refers to the component of pH reduction that is
caused by human activity (IPCC, 2011, p. 37).

Ocean deoxygenation
The loss of oxygen in the ocean. It results from ocean warming, which reduces oxygen solubility and
increases oxygen consumption and stratification, thereby reducing the mixing of oxygen into the ocean
interior. Deoxygenation can also be exacerbated by the addition of excess nutrients in the coastal zone.

Ocean stratification
See stratification

Often used synonymously with "epidemic", usually to indicate localised as opposed to generalised epidemics
(WHO, 2020)

Overshoot pathways
See pathways

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Oxygen Minimum Zone (OMZ)
The midwater layer (200­1000 m) in the open ocean in which oxygen saturation is the lowest in the ocean.
The degree of oxygen depletion depends on the largely bacterial consumption of organic matter and the
distribution of the OMZs is influenced by large-scale ocean circulation. In coastal oceans, OMZs extend to
the shelves and may also affect benthic ecosystems.

Ozone (O3)
The triatomic form of oxygen, and a gaseous atmospheric constituent. In the troposphere, O3 is created both
naturally and by photochemical reactions involving gases resulting from human activities (e.g., smog).
Tropospheric O3 acts as a greenhouse gas (GHG). In the stratosphere, O3 is created by the interaction
between solar ultraviolet radiation and molecular oxygen (O2). Stratospheric O3 plays a dominant role in the
stratospheric radiative balance. Its concentration is highest in the ozone layer. See also Short-lived climate
forcers (SLCFs)

A worldwide outbreak of a disease in humans in numbers clearly in excess of normal (WHO, 2020)

Particulate matter (PM)
Very small solid particles emitted during the combustion of biomass and fossil fuels. PM may consist of a
wide variety of substances. Of greatest concern for health are particulates of diameter less than or equal to 10
nanometers, usually designated as PM10.

Area covered with grass or other plants used or suitable for grazing of livestock; grassland.

Path dependence
The generic situation where decisions, events, or outcomes at one point in time constrain adaptation,
mitigation, or other actions or options at a later point in time. See also Lock in

The temporal evolution of natural and/or human systems towards a future state. Pathway concepts range
from sets of quantitative and qualitative scenarios or narratives of potential futures to solution-oriented
decision-making processes to achieve desirable societal goals. Pathway approaches typically focus on
biophysical, techno-economic, and/or socio-behavioural trajectories and involve various dynamics, goals,
and actors across different scales. See also Scenario

Adaptation pathways
A series of adaptation choices involving trade-offs between short-term and long-term goals and
values. These are processes of deliberation to identify solutions that are meaningful to people in the
context of their daily lives and to avoid potential maladaptation.

Climate-resilient development pathways (CRDPs)
Trajectories that strengthen sustainable development and efforts to eradicate poverty and reduce
inequalities while promoting fair and cross-scalar adaptation to and resilience in a changing climate.
They raise the ethics, equity, and feasibility aspects of the deep societal transformation needed to
drastically reduce emissions to limit global warming (e.g., to well below 2°C) and achieve desirable
and liveable futures and wellbeing for all.

Climate-resilient pathways
Iterative processes for managing change within complex systems in order to reduce disruptions and
enhance opportunities associated with climate change. See also Development pathways and Pathways

Development pathways
Development pathways evolve as the result of the countless decisions being made and actions being
taken at all levels of societal structure, as well due to the emergent dynamics within and between
institutions, cultural norms, technological systems and other drivers of behavioural change.

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Emission pathways
Modelled trajectories of global anthropogenic emissions over the 21st century are termed emission

Overshoot pathways
Pathways that first exceed a specified concentration, forcing, or global warming level, and then return
to or below that level again before the end of a specified period of time (e.g., before 2100). Sometimes
the magnitude and likelihood of the overshoot is also characterized. The overshoot duration can vary
from one pathway to the next, but in most overshoot pathways in the literature and referred to as
overshoot pathways in the AR6, the overshoot occurs over a period of at least one decade and up to
several decades.

Representative Concentration Pathways (RCPs)
Scenarios that include time series of emissions and concentrations of the full suite of greenhouse
gases (GHGs) and aerosols and chemically active gases, as well as land use/land cover (Moss et al.,
2008; van Vuuren et al., 2011),The word representative signifies that each RCP provides only one of
many possible scenarios that would lead to the specific radiative forcing characteristics. The term
pathway emphasises the fact that not only the long-term concentration levels, but also the trajectory
taken over time to reach that outcome are of interest (Moss et al., 2010; van Vuuren et al., 2011).

RCPs usually refer to the portion of the concentration pathway extending up to 2100, for which
Integrated assessment models produced corresponding emission scenarios. Extended concentration
pathways describe extensions of the RCPs from 2100 to 2300 that were calculated using simple rules
generated by stakeholder consultations, and do not represent fully consistent scenarios. Four RCPs
produced from Integrated assessment models were selected from the published literature and are used
in the Fifth IPCC Assessment and also used in this Assessment for comparison, spanning the range
from approximately below 2°C warming to high (>4°C) warming best-estimates by the end of the 21st
century: RCP2.6, RCP4.5 and RCP6.0 and RCP8.5.

    · RCP2.6: One pathway where radiative forcing peaks at approximately 3 W m-2 and then
         declines to be limited at 2.6 W m-2 in 2100 (the corresponding Extended Concentration
         Pathway, or ECP, has constant emissions after 2100).

    · RCP4.5 and RCP6.0: Two intermediate stabilisation pathways in which radiative forcing is
         limited at approximately 4.5 W m-2 and 6.0 W m-2 in 2100 (the corresponding ECPs have
         constant concentrations after 2150).

    · RCP8.5: One high pathway which leads to >8.5 W m-2 in 2100 (the corresponding ECP has
         constant emissions after 2100 until 2150 and constant concentrations after 2250).

Shared socio-economic pathways (SSPs)
Shared socio-economic pathways (SSPs) have been developed to complement the Representative
concentration pathways (RCPs). By design, the RCP emission and concentration pathways were
stripped of their association with a certain socio-economic development. Different levels of emissions
and climate change along the dimension of the RCPs can hence be explored against the backdrop if
different socio-economic development pathways (SSPs) on the other dimension in a matrix. This
integrative SSP-RCP framework is now widely used in the climate impact and policy analysis
literature (see e.g., where climate projections obtained under the RCP scenarios
are analysed against the backdrop of various SSPs.
As several emission updates were due, a new set of emission scenarios was developed in conjunction
with the SSPs. Hence, the abbreviation SSP is now used for two things: On the one hand SSP1, SSP2,
..., SSP5 is used to denote the five socio-economic scenario families. On the other hand, the
abbreviations SSP1-1.9, SSP1-2.6, ..., SSP5-8.5 are used to denote the newly developed emission
scenarios that are the result of an SSP implementation within an integrated assessment model. Those
SSP scenarios are bare of climate policy assumption, but in combination with so-called shared policy
assumptions (SPAs), various approximate radiative forcing levels of 1.9, 2.6, ..., or 8.5 W m-2 are
reached by the end of the century, respectively.

Sustainable development pathways (SDPs)

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Trajectories aimed at attaining the Sustainable Development Goals (SDGs) in the short term and the
goals of sustainable development in the long term. In the context of climate change, such pathways
denote trajectories that address social, environmental, and economic dimensions of sustainable
development, adaptation and mitigation, and transformation, in a generic sense or from a particular
methodological perspective such as integrated assessment models and scenario simulations.

Soft, porous or compressed, sedentary deposit of which a substantial portion is partly decomposed plant
material with high water content in the natural state (up to about 90 percent) (IPCC, 2013).

Peatlands are wetland ecosystems where soils are dominated by peat. In peatlands net primary production
exceeds organic matter decomposition as a result of waterlogged conditions, which leads to the accumulation
of peat.

The pelagic zone consists of the entire water column of the open ocean. It is subdivided into the 'epipelagic
zone' (<200 m, the uppermost part of the ocean that receives enough sunlight to allow photosynthesis), the
'mesopelagic zone' (200­1000 m depth) and the 'bathypelagic zone' (>1000 m depth). The term `pelagic' can
also refer to organisms that live in the pelagic zone.

Organisms large and small living in the pelagic zones. Includes plankton (small) and nekton (free swimming,
large). See Benthos.

A partition value in a population distribution that a given percentage of the data values are below or equal to.
The 50th percentile corresponds to the median of the population. Percentiles are often used to estimate the
extremes of a distribution. For example, the 90th (10th) percentile may be used to refer to the threshold for
the upper (lower) extremes.

Peri-urban areas
Dynamic transition zones that have intense interaction between rural and urban economies, activities,
households, and lifestyles. Neither fully rural or urban. (following Seto, K.C., Sánchez-Rodríguez, R.,
Fragkias, M. 2010.)

Ground (soil or rock, and included ice and organic material) that remains at or below 0°C for at least two
consecutive years (Harris et al., 1988). Note that permafrost is defined via temperature rather than ice
content and, in some instances, may be ice-free.

Permafrost degradation
Decrease in the thickness and/or areal extent of permafrost.

Permafrost thaw
Progressive loss of ground ice in permafrost, usually due to input of heat. Thaw can occur over decades to
centuries over the entire depth of permafrost ground, with impacts occurring while thaw progresses. During
thaw, temperature fluctuations are subdued because energy is transferred by phase change between ice and
water. After the transition from permafrost to non-permafrost, ground can be described as thawed.

A dimensionless measure of the acidity of a solution given by its concentration of hydrogen ions (H+). pH is
measured on a logarithmic scale where pH = -log10(H+). Thus, a pH decrease of 1 unit corresponds to a 10-
fold increase in the concentration of H+, or acidity.


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The relationship between biological phenomena that recur periodically (e.g., development stages, migration)
especially related to climate and seasonal changes.

The production of carbohydrates in plants, algae and some bacteria using the energy of light. Carbon dioxide
(CO2) is used as the carbon source.

Free-floating organisms living in the upper layers of aquatic systems. Their distribution and migration are
primarily determined by water currents. A distinction is made between phytoplankton, which depend on
photosynthesis for their energy supply, and zooplankton, which feed on phytoplankton, other zooplankton,
and bacterioplankton.

Planned relocation (of humans)
A form of human mobility response in the face of sea level rise and related impacts. Planned relocation is
typically initiated, supervised and implemented from national to local level and involves small communities
and individual assets but may also involve large populations. Also termed resettlement, managed retreat, or
managed realignment.

Plasticity (biology)
Change in organismal trait values in response to an environmental cue, and which does not require change in
underlying DNA sequence.

Policies (for climate change mitigation and adaptation)
Strategies that enable actions to be undertaken to accelerate adaptation and mitigation. Policies include those
developed by national and subnational public agencies, and with the private sector. Policies for adaptation
and mitigation often take the form of economic incentives, regulatory instruments, and decision-making and
engagement processes.

Political economy
The set of interlinked relationships between people, the state, society and markets as defined by law, politics,
economics, customs and power that determine the outcome of trade and transactions and the distribution of
wealth in a country or economy.

Polycentric governance
See Governance

Potential Evapotranspiration
The potential rate of water loss without any limits imposed by the water supply.

A complex concept with several definitions stemming from different schools of thought. It can refer to
material circumstances (such as need, pattern of deprivation or limited resources), economic conditions (such
as standard of living, inequality or economic position) and/or social relationships (such as social class,
dependency, exclusion, lack of basic security or lack of entitlement). See also Poverty trap

Poverty trap
Poverty trap is understood differently across disciplines. In the social sciences, the concept, primarily
employed at the individual, household, or community level, describes a situation in which escaping poverty
becomes impossible due to unproductive or inflexible resources. A poverty trap can also be seen as a critical
minimum asset threshold, below which families are unable to successfully educate their children, build up
their productive assets, and get out of poverty. Extreme poverty is itself a poverty trap, since poor persons
lack the means to participate meaningfully in society. In economics, the term poverty trap is often used at
national scales, referring to a self-perpetuating condition where an economy, caught in a vicious cycle,
suffers from persistent underdevelopment (Matsuyama, 2008). Many proposed models of poverty traps are
found in the literature.

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Pre-industrial (period)
The multi-century period prior to the onset of large-scale industrial activity around 1750. The reference
period 1850­1900 is used to approximate pre-industrial global mean surface temperature (GMST).

Atmospheric compounds that are not greenhouse gases (GHGs) or aerosols, but that have an effect on GHG
or aerosol concentrations by taking part in physical or chemical processes regulating their production or
destruction rates.

The extent to which future states of a system may be predicted based on knowledge of current and past states
of the system. Because knowledge of the climate system's past and current states is generally imperfect, as
are the models that utilize this knowledge to produce a climate prediction, and because the climate system is
inherently nonlinear and chaotic, predictability of the climate system is inherently limited. Even with
arbitrarily accurate models and observations, there may still be limits to the predictability of such a nonlinear
system (AMS, 2000).

Primary production
The synthesis of organic compounds by plants and microbes, on land or in the ocean, primarily by
photosynthesis using light and carbon dioxide (CO2) as sources of energy and carbon respectively. It can
also occur through chemosynthesis, using chemical energy, e.g., in deep sea vents.

Net Primary production (NPP)
The difference between how much CO2 vegetation takes in during photosynthesis (gross primary
production) minus how much CO2 the plants release during respiration (IPBES, 2019. Global

Procedural justice
See Justice

A potential future evolution of a quantity or set of quantities, often computed with the aid of a model. Unlike
predictions, projections are conditional on assumptions concerning, for example, future socio-economic and
technological developments that may or may not be realised. See also Pathways and Scenario

A proxy climate indicator is a record that is interpreted, using physical and biophysical principles, to
represent some combination of climate-related variations back in time. Climate-related data derived in this
way are referred to as proxy data. Examples of proxies include pollen analysis, tree ring records,
speleothems, characteristics of corals, and various data derived from marine sediments and ice cores. Proxy
data can be calibrated to provide quantitative climate information

Radiative forcing
The change in the net, downward minus upward, radiative flux (expressed in W m-2) at the tropopause or
top of atmosphere due to a change in an [external] driver of climate change, such as a change in the
concentration of carbon dioxide (CO2), the concentration of volcanic aerosols or in the output of the Sun.
The traditional radiative forcing is computed with all tropospheric properties held fixed at their unperturbed
values, and after allowing for stratospheric temperatures, if perturbed, to readjust to radiative-dynamical
equilibrium. Radiative forcing is called instantaneous if no change in stratospheric temperature is accounted
for. The radiative forcing once rapid adjustments are accounted for is termed the effective radiative forcing.
Radiative forcing is not to be confused with cloud radiative forcing, which describes an unrelated measure of
the impact of clouds on the radiative flux at the top of the atmosphere.

Reasons for Concern (RFCs)
Elements of a classification framework, first developed in the IPCC Third Assessment Report, which aims to
facilitate judgments about what level of climate change may be dangerous (in the language of Article 2 of

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the UNFCCC) by aggregating risks from various sectors, considering hazards, exposures, vulnerabilities,
capacities to adapt, and the resulting impacts.

Reference period
A time period of interest, or a period over which some relevant statistics are calculated. A reference period
can be used as a baseline period or as a comparison to a baseline period.

Conversion to forest of land that has previously contained forests but that has been converted to some other
use. See also Afforestation and Forest

[Note: For a discussion of the term forest and related terms such as afforestation, reforestation and
deforestation, see the 2006 IPCC Guidelines for National Greenhouse Gas Inventories and their 2019
Refinement, and information provided by the United Nations Framework Convention on Climate Change
(IPCC 2006, 2019; UNFCCC 2021a, 2021b)]

A refugium is a geographic area where a population found safety from some threat to its existence. E.g.,
climate refugia, glacial refugia (refuge from glaciations). See also Climate refugium

Land and/or ocean area characterized by specific geographical and/or climatological features. The climate of
a region emerges from a multi-scale combination of its own features, remote influences from other regions,
and global climate conditions.

A rule or order issued by governmental executive authorities or regulatory agencies and having the force of
law. Regulations implement policies and are mostly specific for particular groups of people, legal entities or
targeted activities. Regulation is also the act of designing and imposing rules or orders. Informational,
transactional, administrative and political constraints in practice limit the regulator's capability for
implementing preferred policies.

Relative humidity
The relative humidity specifies the ratio of actual water vapour pressure to that at saturation with respect to
liquid water or ice at the same temperature.

The process of formal reporting of assessment results to the UNFCCC, according to predetermined formats
and according to established standards, especially the Intergovernmental Panel on Climate Change (IPCC)
Guidelines and GPG (Good Practice Guidance) (UN REDD, 2009).

Representative Concentration Pathways (RCPs)
See Pathways

A component or components of the climate system where a greenhouse gas (GHG) or a precursor of a
greenhouse gas is stored (UNFCCC Article 1.7).

Residual risk
The risk related to climate change impacts that remains following adaptation and mitigation efforts.
Adaptation actions can redistribute risk and impacts, with increased risk and impacts in some areas or
populations, and decreased risk and impacts in others. See also Loss and Damage

The capacity of interconnected social, economic and ecological systems to cope with a hazardous event,
trend or disturbance, responding or reorganising in ways that maintain their essential function, identity and

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structure. Resilience is a positive attribute when it maintains capacity for adaptation, learning and/or
transformation (Arctic Council, 2016). See also Hazard, Risk, and Vulnerability.

In climate models, this term refers to the physical distance (metres or degrees) between each point on the
grid used to compute the equations. Temporal resolution refers to the time step or time elapsed between each
model computation of the equations.

The process whereby living organisms convert organic matter to carbon dioxide (CO2), releasing energy and
consuming molecular oxygen.

In environmental context, restoration involves human interventions to assist the recovery of an ecosystem
that has been previously degraded, damaged or destroyed.

Return period
An estimate of the average time interval between occurrences of an event (e.g., flood or extreme rainfall) of
(or below/above) a defined size or intensity.

The potential for adverse consequences for human or ecological systems, recognising the diversity of values
and objectives associated with such systems. In the context of climate change, risks can arise from potential
impacts of climate change as well as human responses to climate change. Relevant adverse consequences
include those on lives, livelihoods, health and wellbeing, economic, social and cultural assets and
investments, infrastructure, services (including ecosystem services), ecosystems and species.
In the context of climate change impacts, risks result from dynamic interactions between climate-related
hazards with the exposure and vulnerability of the affected human or ecological system to the hazards.
Hazards, exposure and vulnerability may each be subject to uncertainty in terms of magnitude and likelihood
of occurrence, and each may change over time and space due to socio-economic changes and human

In the context of climate change responses, risks result from the potential for such responses not achieving
the intended objective(s), or from potential trade-offs with, or negative side-effects on, other societal
objectives, such as the Sustainable Development Goals (SDGs). Risks can arise for example from
uncertainty in implementation, effectiveness or outcomes of climate policy, climate-related investments,
technology development or adoption, and system transitions. See also Hazard and Impacts

Risk assessment
The qualitative and/or quantitative scientific estimation of risks. See also Risk management and Risk

Risk management
Plans, actions, strategies or policies to reduce the likelihood and/or magnitude of adverse potential
consequences, based on assessed or perceived risks.

Risk perception
The subjective judgment that people make about the characteristics and severity of a risk. See also Risk
assessment and Risk management

Risk transfer
The process of formally or informally shifting the financial consequences of particular risks from one party
to another whereby a household, community, enterprise, or state authority will obtain resources from the
other party after a disaster occurs, in exchange for ongoing or compensatory social or financial benefits
provided to that other party.


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The flow of water over the surface or through the subsurface, which typically originates from the part of
liquid precipitation and/or snow/ice melt that does not evaporate, transpire or refreeze, and returns to water

Salt-water intrusion/encroachment
Displacement of fresh surface water or groundwater by the advance of salt water due to its greater density.
This usually occurs in coastal and estuarine areas due to decreasing land-based influence (e.g., from reduced
runoff or groundwater recharge, or from excessive water withdrawals from aquifers) or increasing marine
influence (e.g., relative sea level rise).

Sand and dust storms

A plausible description of how the future may develop based on a coherent and internally consistent set of
assumptions about key driving forces (e.g., rate of technological change (TC), prices) and relationships. Note
that scenarios are neither predictions nor forecasts, but are used to provide a view of the implications of
developments and actions. See also Pathways

Baseline scenario
See Reference Scenario

Concentration scenario
A plausible representation of the future development of atmospheric concentrations of substances that
are radiatively active (e.g., greenhouse gases (GHGs), aerosols, tropospheric ozone), plus human-
induced land cover changes that can be radiatively active via albedo changes, and often used as input
to a climate model to compute climate projections.

Emission scenario
A plausible representation of the future development of emissions of substances that are radiatively
active (e.g., greenhouse gases (GHGs), or aerosols) based on a coherent and internally consistent set
of assumptions about driving forces (such as demographic and socio-economic development,
technological change, energy and land use) and their key relationships. Concentration scenarios,
derived from emission scenarios, are often used as input to a climate model to compute climate

Reference scenario
The scenario used as starting or reference point for a comparison between two or more scenarios.

[Note 1: In many types of climate change research, reference scenarios reflect specific assumptions
about patterns of socio-economic development and may represent futures that assume no climate
policies or specified climate policies, for example those in place or planned at the time a study is
carried out. Reference scenarios may also represent futures with limited or no climate impacts or
adaptation, to serve as a point of comparison for futures with impacts and adaptation. These are also
referred to as baseline scenarios in the literature.

Note 2: Reference scenarios can also be climate policy or impact scenarios, which in that case are
taken as a point of comparison to explore the implications of other features, e.g., of delay,
technological options, policy design and strategy or to explore the effects of additional impacts and
adaptation beyond those represented in the reference scenario.

Note 3: The term business as usual scenario has been used to describe a scenario that assumes no
additional policies beyond those currently in place and that patterns of socio-economic development
are consistent with recent trends. The term is now used less frequently than in the past.

Note 4: In climate change attribution or impact attribution research reference scenarios may refer to
counterfactual historical scenarios assuming no anthropogenic greenhouse gas emissions (climate
change attribution) or no climate change (impact attribution)]

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       Socio-economic scenario
       A scenario that describes a possible future in terms of population, gross domestic product (GDP), and
       other socio-economic factors relevant to understanding the implications of climate change.

Sea ice
Ice found at the sea surface that has originated from the freezing of seawater. Sea ice may be discontinuous
pieces (ice floes) moved on the ocean surface by wind and currents (pack ice), or a motionless sheet attached
to the coast (land-fast ice). Sea ice concentration is the fraction of the ocean covered by ice. Sea ice less than
one year old is called first-year ice. Perennial ice is sea ice that survives at least one summer. It may be
subdivided into second-year ice and multi-year ice, where multiyear ice has survived at least two summers.

Sea level change (sea level rise/sea level fall)
Change to the height of sea level, both globally and locally (relative sea level change) [at seasonal, annual,
or longer time scales] due to (1) a change in ocean volume as a result of a change in the mass of water in the
ocean [(e.g., due to melt of glaciers and ice sheets)], (2) changes in ocean volume as a result of changes in
ocean water density [(e.g., expansion under warmer conditions)], (3) changes in the shape of the ocean
basins and changes in the Earth's gravitational and rotational fields, and (4) local subsidence or uplift of the
land. Global mean sea level change resulting from change in the mass of the ocean is called barystatic. The
amount of barystatic sea level change due to the addition or removal of a mass of water is called its sea level
equivalent (SLE). Sea level changes, both globally and locally, resulting from changes in water density are
called steric. Density changes induced by temperature changes only are called thermosteric, while density
changes induced by salinity changes are called halosteric. Barystatic and steric sea level changes do not
include the effect of changes in the shape of ocean basins induced by the change in the ocean mass and its
distribution. See also Extreme sea level (ESL)

Sea surface temperature (SST)
The subsurface bulk temperature in the top few metres of the ocean, measured by ships, buoys, and drifters.
From ships, measurements of water samples in buckets were mostly switched in the 1940s to samples from
engine intake water. Satellite measurements of skin temperature (uppermost layer; a fraction of a millimetre
thick) in the infrared or the top centimetre or so in the microwave are also used, but must be adjusted to be
compatible with the bulk temperature.

Semi-arid zone
Areas where vegetation growth is constrained by limited water availability, often with short growing seasons
and high interannual variation in primary production. Annual precipitation ranges from 300 to 800 mm,
depending on the occurrence of summer and winter rains.

Sendai Framework for Disaster Risk Reduction
The Sendai Framework for Disaster Risk Reduction 2015­2030 outlines seven clear targets and four
priorities for action to prevent new, and to reduce existing, disaster risks. The voluntary, non-binding
agreement recognises that the State has the primary role to reduce disaster risk but that responsibility should
be shared with other stakeholders, including local government and the private sector. Its aim is to achieve
`substantial reduction of disaster risk and losses in lives, livelihoods and health and in the economic,
physical, social, cultural and environmental assets of persons, businesses, communities and countries.'

The degree to which a system or species is affected, either adversely or beneficially, by climate variability or
change. The effect may be direct (e.g., a change in crop yield in response to a change in the mean, range, or
variability of temperature) or indirect (e.g., damages caused by an increase in the frequency of coastal
flooding due to sea level rise).

The process of storing carbon in a carbon pool. See also Sink


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Places of concentrated human habitation. Settlements can range from isolated rural villages to urban regions
with significant global influence. They can include formally planned and informal or illegal habitation and
related infrastructure. See also Cities, Urban and Urbanisation

Shared Socio-economic Pathways (SSPs)
See Pathways

Shelf seas
Relatively shallow water covering the shelf of continents or around islands. The limit of shelf seas is
conventionally considered as 200 m water depth at the continental shelf edge, where there is usually a steep
slope to the deep ocean floor. During glacial periods, most shelf seas are lost since they become land as the
build-up of ice sheets caused a decrease of global sea level.

Any process, activity or mechanism which removes a greenhouse gas, an aerosol or a precursor of a
greenhouse gas from the atmosphere (UNFCCC Article 1.8 (UNFCCC, 1992)). See also Sequestration

Small island developing states (SIDS)
Small island developing states (SIDS), as recognised by the United Nations OHRLLS (Office of the High
Representative for the Least Developed Countries, Landlocked Developing Countries and Small Island
Developing States), are a distinct group of developing countries facing specific social, economic and
environmental vulnerabilities (UN-OHRLLS, 2011). They were recognized as a special case both for their
environment and development at the Rio Earth Summit in Brazil in 1992. Fifty-eight countries and territories
are presently classified as SIDS by the UN OHRLLS, with 38 being UN member states and 20 being Non-
UN Members or Associate Members of the Regional Commissions (UN-OHRLLS, 2018).

Snow cover extent
The areal extent of snow covered ground.

Snow water equivalent (SWE)
The depth of liquid water that would result if a mass of snow melted completely.

Social inclusion
A process of improving the terms of participation in society, particularly for people who are disadvantaged,
through enhancing opportunities, access to resources, and respect for rights (UN, DESA 2016).

Social infrastructure
See infrastructure

Social justice
See Justice

Social learning
A process of social interaction through which people learn new behaviours, capacities, values, and attitudes.

Social protection
In the context of development aid and climate policy, social protection usually describes public and private
initiatives that provide income or consumption transfers to the poor, protect the vulnerable against livelihood
risks, and enhance the social status and rights of the marginalized, with the overall objective of reducing the
economic and social vulnerability of poor, vulnerable, and marginalized groups (Devereux and Sabates-
Wheeler, 2004). In other contexts, social protection may be used synonymously with social policy and can
be described as all public and private initiatives that provide access to services, such as health, education, or
housing, or income and consumption transfers to people. Social protection policies protect the poor and
vulnerable against livelihood risks and enhance the social status and rights of the marginalized, as well as
prevent vulnerable people from falling into poverty.

Social-ecological system

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An integrated system that includes human societies and ecosystems, in which humans are part of nature. The
functions of such a system arise from the interactions and interdependence of the social and ecological
subsystems. The system's structure is characterised by reciprocal feedbacks, emphasising that humans must
be seen as a part of, not apart from, nature (Arctic Council, 2016; Berkes and Folke, 1998).

Societal (social) transformation
See Transformation

Socio-economic scenario
See scenarios

Socio-technical transitions
Where technological change is associated with social systems and the two are inextricably linked.

Soil erosion
The displacement of the soil by the action of water or wind. Soil erosion is a major process of land

Soil moisture
Water stored in the soil in liquid or frozen form. Root-zone soil moisture is of most relevance for plant

Soil organic carbon
Carbon contained in soil organic matter.

Soil organic matter
The organic component of soil, comprising plant and animal residue at various stages of decomposition, and
soil organisms.

Solar Radiation Modification (SRM)
Refers to a range of radiation modification measures not related to greenhouse gas (GHG) mitigation that
seek to limit global warming. Most methods involve reducing the amount of incoming solar radiation
reaching the surface, but others also act on the longwave radiation budget by reducing optical thickness and
cloud lifetime.

Solution space
The set of biophysical, cultural, socio-economic, and political-institutional dimensions within which
opportunities and constraints determine why, how, when, and who acts to reduce climate risks. Within these
dimensions, there are `hard' (unsurpassable) limits and `soft' (surpassable) limits. The boundaries of the
solution space are path dependent, contested, and in constant flux (Haasnoot et. al. 2020).

Any process or activity which releases a greenhouse gas, an aerosol or a precursor of a greenhouse gas into
the atmosphere (UNFCCC Article 1.9). See also Sink and Sequestration

Southern Ocean
The ocean region encircling Antarctica that connects the Atlantic, Indian and Pacific Oceans together,
allowing inter-ocean exchange. This region is the main source of much of the deep water of the world's
ocean and also provides the primary return pathway for this deep water to the surface (Marshall and Speer,
2012; Toggweiler and Samuels, 1995). The drawing up of deep waters and the subsequent transport into the
ocean interior has major consequences for the global heat, nutrient, and carbon balances, as well as the
Antarctic cryosphere and marine ecosystems.

Spatial and temporal scales

Climate may vary on a large range of spatial and temporal scales. Spatial scales may range from local (less
than 100 000 km2), through regional (100 000 to 10 million km2) to continental (10 to 100 million km2).

Temporal scales may range from seasonal to geological (up to hundreds of millions of years).

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Set of rules or codes mandating or defining product performance (e.g., grades, dimensions, characteristics,
test methods, and rules for use). Product, technology or performance standards establish minimum
requirements for affected products or technologies. Standards impose reductions in greenhouse gas (GHG)
emissions associated with the manufacture or use of the products and/or application of the technology.

Storm surge
The temporary increase, at a particular locality, in the height of the sea due to extreme meteorological
conditions (low atmospheric pressure and/or strong winds). The storm surge is defined as being the excess
above the level expected from the tidal variation alone at that time and place. See also Extreme sea level and
Seal level change (sea level rise/ sea level fall)

A way of making sense of a situation or a series of events through the construction of a set of explanatory
elements. Usually it is built on logical or causal reasoning. In climate research, the term storyline is used
both in connection to scenarios as related to a future trajectory of the climate and human systems or to a
weather or climate event. In this context, storylines can be used to describe plural, conditional possible
futures or explanations of a current situation, in contrast to single, definitive futures or explanations.

Stranded assets
Assets exposed to devaluations or conversion to `liabilities' because of unanticipated changes in their
initially expected revenues due to innovations and/or evolutions of the business context, including changes in
public regulations at the domestic and international levels.

Process of forming of layers of (ocean) water with different properties such as salinity, density and
temperature that act as barrier for water mixing. The strengthening of near-surface stratification generally
results in warmer surface waters, decreased oxygen levels in deeper water, and intensification of ocean
acidification (OA) in the upper ocean.

Water flow within a river channel, for example, expressed in m3 s-1. A synonym for river discharge.

Events and trends, often not climate-related, that have an important effect on the system exposed and can
increase vulnerability to climate-related risk.

Involves ensuring the persistence of natural and human systems, implying the continuous functioning of
ecosystems, the conservation of high biodiversity, the recycling of natural resources and, in the human
sector, successful application of justice and equity.

Sustainable development (SD)
Development that meets the needs of the present without compromising the ability of future generations to
meet their own needs (WCED, 1987) and balances social, economic and environmental concerns. See also
Development pathways and Sustainable Development Goals (SDGs)

Sustainable Development Goals (SDGs)
The 17 global goals for development for all countries established by the United Nations through a
participatory process and elaborated in the 2030 Agenda for Sustainable Development, including ending
poverty and hunger; ensuring health and wellbeing, education, gender equality, clean water and energy, and
decent work; building and ensuring resilient and sustainable infrastructure, cities and consumption; reducing
inequalities; protecting land and water ecosystems; promoting peace, justice and partnerships; and taking
urgent action on climate change. See also Development pathways and Sustainable Development

Sustainable development pathways (SDPs)

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See Pathways

Sustainable forest management
The stewardship and use of forests and forest lands in a way, and at a rate, that maintains their biodiversity,
productivity, regeneration capacity, vitality and their potential to fulfil, now and in the future, relevant
ecological, economic and social functions, at local, national, and global levels, and that does not cause
damage to other ecosystems (Forest Europe, 1993).

Sustainable land management
The stewardship and use of land resources, including soils, water, animals and plants, to meet changing
human needs, while simultaneously ensuring the long-term productive potential of these resources and the
maintenance of their environmental functions (Adapted from WOCAT, undated).

Organisms and habitats related to the sea ice, analogous to `pelagic' (> water-column') or `benthic' (> `sea-

Association between climate variables at widely separated, geographically fixed locations related to each
other through physical processes and oceanic and/or atmospheric dynamical pathways. Teleconnections can
be caused by several climate phenomena, such as Rossby wave-trains, mid-latitude jet and storm track
displacements, fluctuations of the Atlantic Meridional Overturning Circulation, fluctuations of the Walker
circulation, etc. They can be initiated by modes of climate variability thus providing the development of
remote climate anomalies at various temporal lags.

Temperature overshoot
Exceedance of a specified global warming level, followed by a decline to or below that level during a
specified period of time (e.g., before 2100). Sometimes the magnitude and likelihood of the overshoot is also
characterized. The overshoot duration can vary from one pathway to the next but in most overshoot
pathways in the literature and referred to as overshoot pathways in the AR6, the overshoot occurs over a
period of at least one and up to several decades. See also Pathways

In the context of the IPCC Guidelines for National Greenhouse Gas Inventories, a tier represents a level of
methodological complexity. Usually three tiers are provided. Tier 1 is the basic method, Tier 2 intermediate
and Tier 3 most demanding in terms of complexity and data requirements. Tiers 2 and 3 are sometimes
referred to as higher tier methods and are generally considered to be more accurate (IPCC, 2019).

Tipping element
A component of the Earth system that is susceptible to a tipping point. See also Abrupt climate change and
Tipping point

Tipping point
A critical threshold beyond which a system reorganizes, often abruptly and/or irreversibly. See also abrupt
climate change

A competition between different objectives within a decision situation, where pursuing one objective will
diminish achievement of other objective(s). A trade-off exists when a policy or measure aimed at one
objective (e.g., reducing GHG emissions) reduces outcomes for other objective(s) (e.g., biodiversity
conservation, energy security) due to adverse side effects, thereby potentially reducing the net benefit to
society or the environment. See also Adverse side-effect, Co-benefit

A change in the fundamental attributes of natural and human systems.

Deliberate Transformations

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A profound shift towards sustainability, envisioned and intended by at least some societal actors,
facilitated by changes in individual and collective values and behaviours, and a fairer balance of
political, cultural, and institutional power in society.

Societal (social) Transformations
A change in the fundamental attributes of human systems advanced by societal actors

Transformational adaptation
See Adaptation

Transformative change
A system-wide change that requires more than technological change through consideration of social and
economic factors that, with technology, can bring about rapid change at scale.

The process of changing from one state or condition to another in a given period of time. Transition can
occur in individuals, firms, cities, regions and nations, and can be based on incremental or transformative

Just transitions
A set of principles, processes and practices that aim to ensure that no people, workers, places, sectors,
countries or regions are left behind in the transition from a high-carbon to a low-carbon economy. It stresses
the need for targeted and proactive measures from governments, agencies, and authorities to ensure that any
negative social, environmental or economic impacts of economy-wide transitions are minimized, whilst
benefits are maximized for those disproportionally affected. Key principles of just transitions include:
respect and dignity for vulnerable groups; fairness in energy access and use, social dialogue and democratic
consultation with relevant stakeholders; the creation of decent jobs; social protection; and rights at work. Just
transitions could include fairness in energy, land use and climate planning and decision-making processes;
economic diversification based on low-carbon investments; realistic training/retraining programs that lead to
decent work; gender specific policies that promote equitable outcomes; the fostering of international
cooperation and coordinated multilateral actions; and the eradication of poverty. Lastly, just transitions may
embody the redressing of past harms and perceived injustices. (ILO 2015; UNFCCC 2016)

Tree line
The upper limit of tree growth in mountains or at high latitudes. It is more elevated or more poleward than
the forest line.

Tropical cyclone
The general term for a strong, cyclonic-scale disturbance that originates over tropical oceans. Distinguished
from weaker systems (often named tropical disturbances or depressions) by exceeding a threshold wind
speed. A tropical storm is a tropical cyclone with one-minute average surface winds between 18 and 32 m s-
1. Beyond 32 m s-1, a tropical cyclone is called a hurricane, typhoon, or cyclone, depending on geographic

A wave, or train of waves, produced by a disturbance such as a submarine earthquake displacing the sea
floor, a landslide, a volcanic eruption, or an asteroid impact.

A treeless biome characteristic of polar and alpine regions.

A state of incomplete knowledge that can result from a lack of information or from disagreement about what
is known or even knowable. It may have many types of sources, from imprecision in the data to ambiguously
defined concepts or terminology, incomplete understanding of critical processes, or uncertain projections of
human behaviour. Uncertainty can therefore be represented by quantitative measures (e.g., a probability
density function) or by qualitative statements (e.g., reflecting the judgment of a team of experts) (see Moss

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and Schneider, 2000; IPCC, 2004; Mastrandrea et al., 2010). See also Agreement, Confidence, and

Deep uncertainty
A situation of deep uncertainty exists when experts or stakeholders do not know or cannot agree on:
(1) appropriate conceptual models that describe relationships among key driving forces in a system;
(2) the probability distributions used to represent uncertainty about key variables and parameters;
and/or (3) how to weigh and value desirable alternative outcomes (Lempert et al., 2003).

United Nations Framework Convention on Climate Change (UNFCCC)
The UNFCCC was adopted in May 1992 and opened for signature at the 1992 Earth Summit in Rio de
Janeiro. It entered into force in March 1994 and as of May 2018 had 197 Parties (196 States and the
European Union). The Convention's ultimate objective is the `stabilisation of greenhouse gas concentrations
in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate
system'. The provisions of the Convention are pursued and implemented by two treaties: the Kyoto Protocol
and the Paris Agreement.

The transfer of substances (such as carbon) or energy (e.g., heat) from one compartment of a system to
another; for example, in the Earth system from the atmosphere to the ocean or to the land. See also
Sequestration, Sink and Source

Upwelling region
A region of an ocean where cold, typically nutrient-rich waters well up from the deep ocean.

The categorisation of areas as "urban" by government statistical departments is generally based either on
population size, population density, economic base, provision of services, or some combination of the above.
Urban systems are networks and nodes of intensive interaction and exchange including capital, culture, and
material objects. Urban areas exist on a continuum with rural areas and tend to exhibit higher levels of
complexity, higher populations and population density, intensity of capital investment, and a preponderance
of secondary (processing) and tertiary (service) sector industries. The extent and intensity of these features
varies significantly within and between urban areas. Urban places and systems are open with much
movement and exchange between more rural areas as well as other urban regions. Urban areas can be
globally interconnected facilitating rapid flows between them ­ of capital investment, of ideas and culture,
human migration, and disease. See also city, city region urbanisation, urban systems,

Urban and Peri-urban agriculture
The cultivation of crops and rearing of animals for food and other uses within and surrounding the
boundaries of cities, including fisheries and forestry (EPRS, 2014).

Urban heat island (UHI)
The relative warmth of a city compared with surrounding rural areas, associated with heat trapping due to
land use, the configuration and design of the built environment, including street layout and building size, the
heat-absorbing properties of urban building materials, reduced ventilation, reduced greenery and water
features, and domestic and industrial heat emissions generated directly from human activities.

Urbanisation is a multi-dimensional process that involves at least three simultaneous changes: 1) land use
change: transformation of formerly rural settlements or natural land into urban settlements; 2) demographic
change: a shift in the spatial distribution of a population from rural to urban areas; and 3) infrastructure
change: an increase in provision of infrastructure services including electricity, sanitation, etc. Urbanisation
often includes changes in lifestyle, culture, and behaviour, and thus alters the demographic, economic, and
social structure of both urban and rural areas. (based on World Urbanization Prospects 2018; IPCC 2014;
Stokes and Seto, 2019) See also Settlements, Urban, Urban areas and Urban Systems

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Urban Systems
Urban systems refer to two interconnected systems-- first, the comprehensive collections of city elements
with multiple dimensions and characteristics: a) encompass physical, built, socio-economic-technical,
political, and ecological subsystems; b) integrate social agent/constituency/processes with physical structure
and processes; and c) exist within broader spatial and temporal scales and governance and institutional
contexts; and second, the global system of cities and towns. See also City region, Urban and Urban areas

Values and Beliefs
Fundamental attitudes about what is important, good, and right; strongly held principles or qualities
intrinsically valuable or desirable, often enshrined in laws, traditions, and religions. Examples include human
rights, subsistence, and equitable distribution of costs and benefits of climate policies (Hulme, 2009, 2018;
Nakashima et al., 2012; UNFCCC, 1992; UN Universal Declaration of Human Rights, 1948).

Vector-borne disease
Illnesses caused by parasites, viruses and bacteria that are transmitted by various vectors (e.g. mosquitoes,
sandflies, triatomine bugs, blackflies, ticks, tsetse flies, mites, snails and lice) (UNEP 2018).

Ventilation (ocean)
The exchange of ocean properties with the atmospheric surface layer such that property concentrations are
brought closer to equilibrium values with the atmosphere (AMS, 2000), and the processes that propagate
these properties into the ocean interior.

The propensity or predisposition to be adversely affected. Vulnerability encompasses a variety of concepts
and elements including sensitivity or susceptibility to harm and lack of capacity to cope and adapt. See also
Exposure, Hazard and Risk

Vulnerability index
A metric characterizing the vulnerability of a system. A climate vulnerability index is typically derived by
combining, with or without weighting, several indicators assumed to represent vulnerability.

Water-borne disease
Illnesses that transmitted through contact with or consumption of unsafe or contaminated water (UNEP

Water security
The capacity of a population to safeguard sustainable access to adequate quantities of acceptable quality
water for sustaining livelihoods, human well-being, and socio-economic development, for ensuring
protection against water-borne pollution and water-related disasters, and for preserving ecosystems in a
climate of peace and political stability (UN-Water, 2013).

Water-use efficiency
Carbon gain by photosynthesis per unit of water lost by evapotranspiration. It can be expressed on a short-
term basis as the ratio of photosynthetic carbon gain per unit transpirational water loss, or on a seasonal basis
as the ratio of net primary production or agricultural yield to the amount of water used.

The gradual removal of atmospheric CO2 through dissolution of silicate and carbonate rocks. Weathering
may involve physical processes (mechanical weathering) or chemical activity (chemical weathering).

A state of existence that fulfils various human needs, including material living conditions and quality of life,
as well as the ability to pursue one's goals, to thrive, and feel satisfied with one's life. Ecosystem well-being
refers to the ability of ecosystems to maintain their diversity and quality.

Land that is covered or saturated by water for all or part of the year (e.g., peatland).

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