Note: This Glossary defines some specific terms as the Lead Authors intend them to be interpreted in the context of this Special Report. Blue italicisations indicate terms defined in this Glossary. Note that subterms are in bold italics beneath main terms.
Ablation (of glaciers, ice sheets, or snow cover)
All processes that reduce the mass of a glacier, ice sheet or snow cover. The main processes are melting, and for glaciers also calving (or, when the glacier nourishes an ice shelf, discharge of ice across the grounding line), but other processes such as sublimation and loss of wind-blown snow can also contribute to ablation. Ablation also refers to the mass lost by any of these processes. See also Mass balance/budget (of glaciers or ice sheets).
Abrupt climate change
Abrupt change refers to a change that is substantially faster than the rate of change in the recent history of the affected components of a system. Abrupt climate change refers to a large-scale 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 disruptions in human and natural systems. See also Climate change, Human system, Natural systems and Tipping point.
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). See also Avalanche and Mass balance/ budget (of glaciers or ice sheets).
Layer of ground above permafrost subject to annual thawing and freezing.
See Adaptive capacity.
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.
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).
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; and adaptation responses that will be required in the face of a global failure to mitigate the causes of anthropogenic climate change and are characterised by system-wide change or changes across more than one system, by a focus on the future and long-term change, or by a direct questioning of the effectiveness of existing systems, social injustices and power imbalances.
Adaptation limits The point at which an actor’s objectives (or system needs) cannot be secured from intolerable risks through adaptive actions.
Hard adaptation limit – No adaptive actions are possible to avoid intolerable risks.
Soft adaptation limit – Options may exist but are currently not available to avoid intolerable risks through adaptive action.
See also Adaptation options, Adaptive capacity, Justice, Maladaptive actions (Maladaptation) and Mitigation (of climate change).
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 behavioural. See also Adaptive capacity and Maladaptive actions (Maladaptation).
The ability of systems, institutions, humans and other organisms to adjust to potential damage, to take advantage of opportunities or to respond to consequences (IPCC, 2014; MA, 2005). See also Adaptation.
A suspension of airborne solid or liquid particles, with a typical size between a few nanometres and 10 μm, that reside in the atmosphere for at least several hours. The term aerosol, which includes both the particles and the suspending gas, is often used in this Special Report in its plural form to mean ‘aerosol particles’. Aerosols may be of either natural or anthropogenic origin. Aerosols can influence climate in several ways: directly through scattering and absorbing radiation, and indirectly by acting as cloud condensation nuclei or ice nuclei, modifying the optical properties and lifetime of clouds or upon deposition on snow or icecovered surfaces thereby altering their albedo and contributing to climate feedback. Atmospheric aerosols, whether natural or anthropogenic, originate from two different pathways: emissions of primary particulate matter (PM), and formation of secondary PM from gaseous precursors. The bulk of aerosols are of natural origin. Some scientists use group labels that refer to the chemical composition, namely: sea salt, organic carbon, black carbon (BC), mineral species (mainly desert dust), sulphate, nitrate and ammonium. These labels are, however, imperfect as aerosols combine particles to create complex mixtures. See also Short-lived climate forcers (SLCFs).
In this Special 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, Likelihood and Uncertainty.
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 varying cloudiness, snow, ice, leaf area and land cover changes.
Alien (non-native) species
An introduced species (alien species, exotic species, non-indigenous species, or non-native species) living outside its native distributional range, but which has arrived there by human activity, either deliberate or accidental. Non-native species can have various effects on and adversely affect the local ecosystem. See also Endemic species and Invasive species.
The deviation of a variable from its value averaged over a reference period.
Resulting from or produced by human activities. See also 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 processes. See also Anthropogenic.
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. See also Anthropogenic and Sea level change (sea level rise/sea level fall).
Atlantic Meridional Overturning Circulation (AMOC)
See Meridional Overturning Circulation (MOC).
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 (N2, 78.1% volume mixing ratio) and oxygen (O2, 20.9% volume mixing ratio), together with a number of trace gases, such as argon (Ar, 0.93% volume mixing ratio), helium (He) and radiatively active greenhouse gases (GHG) 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 Climate system, Hydrological cycle, Methane (CH4) and Radiative forcing.
Atmosphere-ocean general circulation model (AOGCM)
See Climate model.
See Detection and attribution.
A mass of snow, ice, earth or rocks, or a mixture of these, falling down a mountainside.
The community of organisms living on the bottom or in sediments of a body of water (such as an ocean, river or 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’.
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 service and Functional diversity.
Biological (carbon) pump
A series of ocean processes through which inorganic carbon (as carbon dioxide, CO2) is fixed as organic matter by photosynthesis in sunlit surface water and then transported to the ocean interior, and possibly the sediment, resulting in the storage of carbon. See also Carbonate pump, Dissolved organic carbon (DOC) and particulate organic carbon (POC), Microbial carbon pump and Solubility pump.
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).
Black carbon (BC)
A relatively pure form of carbon, also known as soot, arising from the incomplete combustion of fossil fuels, biofuel and biomass. It only stays in the atmosphere for days or weeks. BC is a climate forcing agent with strong warming effect, both in the atmosphere and when deposited on snow or ice. See also Aerosol, Albedo, Forcing and Short-lived climate forcers (SLCF).
All biologically-driven carbon fluxes and storage in marine systems that are amenable to management can be considered as blue carbon. 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 Carbon cycle, Coast, Ecosystem service and Sequestration.
Calving (of glaciers or ice sheets)
The process of mechanical destruction of a mass of ice usually typical of marine-terminating glaciers; in the latter case, the ice calving (or breaking away) from the glacier edge can lead to the formation of icebergs. See also Ice sheet and Marine ice cliff instability (MICI).
Ocean carbon fixation through the biological formation of carbonates, primarily by plankton that generate bio-mineral particles that sink to the ocean interior, and possibly the sediment. It is also called carbonate counter-pump, since the formation of calcium carbonate (CaCO3) is accompanied by the release of carbon dioxide (CO2) to surrounding water and subsequently to the atmosphere. See also Biological (carbon) pump, Blue carbon, Dissolved organic carbon (DOC) and particulate organic carbon (POC), Microbial carbon pump and Solubility pump.
refers to three concepts in the literature: (1) an assessment of carbon cycle sources and sinks on a global level, through the synthesis of evidence for fossil-fuel and cement emissions, land use change emissions, ocean and land carbon dioxide (CO2) sinks, and the resulting atmospheric CO2 growth rate. This is referred to as the global carbon budget; (2) the estimated cumulative amount of global CO2 emissions that is estimated to limit global surface temperature to a given level above a reference period, taking into account global surface temperature contributions of other GHGs and climate forcers; (3) the distribution of the carbon budget defined under (2) to the regional, national, or sub-national level based on considerations of equity, costs or efficiency. See also Atmosphere, Forcing and Land.
The flow of carbon (in various forms, e.g., as carbon dioxide (CO2), carbon in biomass, and carbon dissolved in the ocean as carbonate and bicarbonate) through the atmosphere, hydrosphere, ocean, terrestrial and marine biosphere and lithosphere. In this Special Report, the reference unit for the global carbon cycle is GtCO2 or GtC (one Gigatonne = 1 Gt = 1015 grams; 1 GtC corresponds to 3.667 GtCO2). See also Atmosphere, Blue carbon and Ocean acidification (OA).
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. See also Global warming, Greenhouse gas (GHG), Land and Ocean acidification (OA).
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 capture and storage but excludes natural CO2 uptake not directly caused by human activities. See also Greenhouse gas removal (GGR), Mitigation (of climate change) and Negative emissions.
The price for avoided or released carbon dioxide (CO2) or CO2-equivalent emissions. This may refer to the rate of a carbon tax, or the price of emission permits. In many models that are used to assess the economic costs of mitigation, carbon prices are used as a proxy to represent the level of effort in mitigation policies.
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 multidimensional, and are associated more with the magnitude of vulnerability than with that of the hazard (modified from Pescaroli & Alexander, 2015). See also Impacts (consequences, outcomes), Natural systems and Risk.
in a narrow sense 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.
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 (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 Global warming, Ocean acidification (OA) and Detection and attribution.
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. For simplicity, both extreme weather events and extreme climate events are referred to collectively as ‘climate extremes’. See also Extreme weather/ climate event.
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 variability. See also Climate variability and Forcing.
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 scenarios. 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 Climate sensitivity and Earth system model (ESM).
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. See also (Model) Ensemble, Projection and Radiative forcing.
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 2oC) and achieve desirable and liveable futures and well-being for all. See also Equality.
The change in the annual global mean surface temperature (GMST) in response to a change in the atmospheric carbon dioxide (CO2) concentration or other radiative forcing.
Equilibrium climate sensitivity The equilibrium (steady state) change in the globally-averaged near-surface temperature following a doubling of the atmospheric CO2 concentration from preindustrial conditions. Often estimated through experiments in atmosphere-ocean general circulation models (AOGCMs) where CO2 levels are either quadrupled or doubled from pre-industrial levels and which are integrated for 100-200 years. A related quantity, the climate feedback parameter (unit: W m–2 oC-1) refers to the top of atmosphere budget change per degree of globally-averaged near-surface temperature change. See also Climate model and Global mean surface temperature (GMST).
Global system consisting of five major components: the atmosphere, hydrosphere, cryosphere, lithosphere and 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.
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).
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. See also Ocean, Ocean deoxygenation and Sea level change (sea level rise/sea level fall).
The positive effects that a policy or measure aimed at one objective might have on other objectives, thereby increasing the total benefits for society or the environment. Co-benefits are often subject to uncertainty and depend on local circumstances and implementation practices, among other factors. Co-benefits are also referred to as ancillary benefits. See also Risk.
See Compound weather/climate events.
Compound weather/climate events
The combination of multiple drivers and/or hazards that contributes to societal and environmental risk (Zscheischler et al., 2018).
arise from the interaction of hazards, which may be characterised by single extreme events or multiple coincident or sequential events that interact with exposed systems or sectors. See also Extreme weather/ climate event and Risk.
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 Special Report, confidence is expressed qualitatively (Mastrandrea et al., 2010). See Section 1.8.3 for the list of confidence levels used. See also Likelihood and Uncertainty.
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. See also Ocean acidification (OA).
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 Discounting.
A measure of the cost at which a policy goal or outcome is achieved. The lower the cost, the greater the cost-effectiveness. See also Private costs.
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). See also Climate projection.
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. See also Climate system.
See Ecosystem services.
The total amount of emissions released over a specified period of time. See also Carbon budget.