There are many terms and special definitions related to climate and climate change. New and existing professionals working in the realm of climate education, research, and outreach need to be clear in their terminology and usage. This glossary compiles the most commonly used terms and definitions for academics, researchers, and educators to communicate effectively in this emerging arena. To enhance understanding, key terms include a separate interpretative explanation of the concept “Why this matters.”

Adaptation: Initiatives and measures to reduce the vulnerability of natural and human systems from existing or expected climate change effects. Various types of adaptation exist, e.g. anticipatory and reactive, private and public, and autonomous and planned. Examples are raising river or coastal dikes, the substitution of more temperature-shock resistant plants for sensitive ones, etc.

Autonomous adaptation: Actions taken voluntarily by decision makers (such as farmers or city leaders) whose risk management is motivated by information, market signals, co-benefits, and other factors.

Planned adaptation: Interventions by governments to address needs judged unlikely to be met by autonomous actions—often adaptations larger in scale and/or resource requirements.

Afforestation: Planting of new forests on lands that historically have not contained forests. See Reforestation & Land-use change

Anomaly: Positive or negative value indicating amount of departure from an average value, for example, the 2011 global average temperature anomaly was 2.61ºC (1.45ºF) relative to the 1901-2000 global average temperature, in other words, the global average temperature in 2011 was 2.61ºC (1.45ºF) warmer than the global temperature average from 1901-2000.

Anthropogenic emissions: Emissions of greenhouse gases, greenhouse gas precursors, and aerosols released by human activities, including the burning of fossil fuels, land-use changes, livestock, and fertilization.

Atmosphere: Gaseous envelope surrounding the Earth. The dry atmosphere consists almost entirely of nitrogen (78.1%) and oxygen (20.9%), together with a number of trace gases, such as argon (0.93%), helium, plus radiatively active greenhouse gases, such as carbon dioxide (0.04%), ozone, and water vapor, whose amounts are highly variable but typically around 1%. The atmosphere also contains clouds and aerosols.

Attribution: The process of establishing the most likely causes for the detected change with some defined level of confidence. See Detection

Baseline: Reference from which an alternative outcome can be measured. For example, a future scenario assuming current emission rates used as a baseline in the analysis of the impact of decreased or increased future emission rates; similar to a control run in a field experiment is used to measure the impact of applying different fertilizer amounts to a field study.

Carbon Flux: Transfer of carbon from one carbon pool to another in units of measurement of mass per unit area and time.

Carbon sequestration: Process that removes carbon dioxide from the atmosphere—trees, grasses, and other plants uptake carbon dioxide during photosynthesis and stores it in biomass (trunks, branches, foliage, roots) and soil (the process of increasing the carbon pools other than the atmosphere). See Uptake & Heterotrophic respiration

Climate: Usually defined as the average weather, or more rigorously, as the statistical description in terms of the mean and variability of interest over a period of time The World Meteorological Organization defines climate as a 30-year average. Climate in a wider sense is the state, including a statistical description, of the climate system.

Climate change: Statistically detectable changes in the mean and/or the variability of climate variables of interest that persists for an extended period, typically decades or longer. Climate changes can be attributable to natural internal processes, to natural external forcings, or to persistent anthropogenic changes in atmospheric composition or land use.

Climate feedback: An interaction between processes in the climate system when the result of an initial process triggers changes in a second process that in turn influences the initial one. A positive feedback intensifies the original process, and a negative feedback dampens it.

Climate model: A mathematical representation of the climate system based on the physical, chemical and biological properties of its components, their interactions and feedback processes, which accounts for all or some of its known behavior. The climate system can be represented by a hierarchy of models of different complexity which can differ in the number of spatial dimensions, the extent to which physical, chemical or biological processes are explicitly represented, or the level at which empirical parameterizations are involved.

• Why this matters Coupled Atmosphere-Ocean General Circulation Models (AOGCMs) provide a representation of the climate system - perhaps the most comprehensive model type currently in use. More complex models have evolved to include interactive chemistry and biology. Climate models can be used as a research tool to study and simulate the climate or operationally to make monthly, seasonal, or annual climate projections.

Climate projection: A projection of the response of the climate system to emission or concentration scenarios of greenhouse gases and aerosols, or radiative forcing scenarios, often based upon simulations by climate models. Climate projections differ from climate predictions by the inclusion of emission/concentration/radiative forcing scenario used, which are based on assumptions about future socio-economic and technological developments and thus may be subject to substantial uncertainty. See Radiative forcing

Climate scenario: A plausible representation of future climate state, based on an internally consistent set of relationships explicitly constructed for investigating the potential consequences of anthropogenic climate change, often serving as input to impact models. Climate projections often serve as the basis for constructing climate scenarios, but climate scenarios usually require additional information like current climate, assumptions about economics, politics, and land use change. A climate change scenario is the difference between a climate scenario and the current climate. See Representative Concentration Pathways (RCPs)

Climate sensitivity: In IPCC reports, equilibrium climate sensitivity refers to the equilibrium change in the annual mean global surface temperature following a doubling of the atmospheric equivalent carbon dioxide concentration. Due to computational constraints, the equilibrium climate sensitivity in a climate model is usually estimated by running an atmospheric general circulation model coupled to a mixed-layer ocean model, because equilibrium climate sensitivity is largely determined by atmospheric processes.

Climate shift: An abrupt shift or jump in mean values signaling a change in climate regime. Most widely used in conjunction with the 1976 - 1977 climate shift that seems to correspond to a change in El Niño-Southern Oscillation behavior. See Patterns of climate variability & El Niño-Southern Oscillation

Climate system: The climate system consisting of five major components: the atmosphere, the hydrosphere, the cryosphere, the land surface and the biosphere, and the interactions between them. The climate system evolves in time under the influence of its own internal dynamics and because of external forcings such as volcanic eruptions, solar variations and anthropogenic emissions.

Climate variability: Refers to variations in the mean state and other statistics (such as standard deviations, the occurrence of extremes, etc.) of the climate on all scales beyond that of individual weather events. Variability may be due to natural internal processes within the climate system (internal variability), or to variations in natural or anthropogenic external forcing (external variability). See Patterns of climate variability

Cloud feedback: A climate feedback involving changes in any of the properties of clouds as a response to other atmospheric changes.

• Why this matters: Understanding cloud feedbacks and determining their magnitude and sign require an understanding of how a change in climate may affect the spectrum of cloud types, the cloud fraction and height, and the radiative properties of clouds, and an estimate of the impact of these changes on the Earth’s radiation budget. At present, cloud feedbacks remain the largest source of uncertainty in climate sensitivity estimates.

Deforestation: Conversion of forest to non-forest land, such as cropland. See Afforestation

Detection: The process of demonstrating that climate has changed in some defined statistical sense (over a fixed time frame) without providing a reason for that change. See Attribution

Downscaling climate models: Process that attempts to refine global climate projections to regional and local scales. This can be done by layering local-level data over larger-scale climate models or statistically manipulating global model projections to local conditions, temperature and precipitation history, topography, etc.

• Why this matters: Downscaled modeling examines relatively small areas in detail—in some cases down to 25 square kilometers, a far higher resolution than that offered by global climate model simulations. The goal is to generate more locally relevant projections of long-term weather patterns for regions, states, and cities.

Drought: A prolonged absence or marked deficiency of precipitation, a deficiency that results in water shortage for some activity or for some group, or a period of abnormally dry weather sufficiently prolonged for the lack of precipitation to cause a serious hydrological imbalance⁸.

• Why this matters: Drought has been defined in a number of ways, but often involves an economic impact to a particular sector. Agricultural drought relates to moisture deficits in the topmost 1m or so of soil (the root zone) that affect crops. Meteorological drought is mainly a prolonged deficit of precipitation. Hydrologic drought is related to below-normal streamflow, lake and groundwater levels.

El Niño-Southern Oscillation (ENSO): Basin-wide warming of the tropical Pacific 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. When applied to time scales of two to seven years these events are collectively known as El Niño-Southern Oscillation, or ENSO. It is often measured by the surface pressure anomaly difference between Darwin and Tahiti and the sea surface temperatures in the central and eastern equatorial Pacific. See Anomaly

• Why this matters: During an El Niño event, the prevailing trade winds weaken, reducing upwelling and altering ocean currents such that the sea surface temperatures warm, further weakening the trade winds. This event 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, including the Southeast US. The opposite, cold phase of ENSO is called La Niña.

Emission scenario: A representation of the future emissions of potentially radiatively active substances such as greenhouse gases and aerosols based on an internally consistent set of assumptions about driving forces (such as demographic and socioeconomic development, technological change) and their key relationships.

• Why this matters: Concentration scenarios, derived from emission scenarios, are used as input to a climate model to compute climate projections. See SRES scenarios & Representative Concentration Pathways (RCPs)

Energy balance: The difference between the total incoming and total outgoing energy in the climate system. If this balance is positive, warming occurs; if it is negative, cooling occurs. Averaged over the globe and over long time periods, this balance must be zero to maintain climate homeostasis.

• Why this matters: Because the climate system derives virtually all its energy from the Sun, zero balance implies that, globally, the amount of incoming solar radiation on average must be equal to the sum of the outgoing reflected solar radiation and the outgoing thermal infrared radiation emitted by the climate system. A perturbation of this global radiation balance, be it anthropogenic or natural, is called radiative forcing.

External forcing: An agent outside the climate system causing a change in the climate system. For example, volcanic eruptions, solar variations, black carbon, and anthropogenic changes are external forcings.

Extreme weather event: An event that is rare at a particular place and time of year. An extreme weather event would normally be as rare as or rarer than the 10th or 90th percentile of the observed probability density function.

• Why this matters: Single extreme events cannot be simply and directly attributed to anthropogenic climate change, as there is always a finite chance the event in question might have occurred naturally. When a pattern of extreme weather persists for some time, such as a season, it may be classed as an extreme climate event, especially if it yields an average or total that is itself extreme (e.g., drought or heavy rainfall over a season).

Global Warming Potential (GWP): An index based upon radiative properties of greenhouse gases, measuring the radiative forcing of a given greenhouse gas in today’s atmosphere over a time period relative to that of carbon dioxide. The GWP represents the combined effect of the differing times these gases remain in the atmosphere and their relative effectiveness in absorbing outgoing thermal infrared radiation. See Greenhouse Gas (GHG)

• Why this matters: If nitrous oxide has a global warming potential of 289, it means that 1 kg of nitrous oxide has the same impact on climate change as 289 kg of carbon dioxide and thus 1 kg of nitrous oxide would count as 289 kg of carbon dioxide equivalent. The Kyoto Protocol is based on GWPs from pulse emissions over a 100-year time frame.

Greenhouse effect: Greenhouse gases effectively absorb thermal infrared radiation emitted by the Earth’s surface, by the atmosphere itself, and by clouds. Atmospheric radiation is emitted to all sides, including downward to the Earth’s surface. Thus, greenhouse gases trap heat within the surface-troposphere system.

• Why this matters: Thermal infrared radiation in the troposphere is strongly coupled to the temperature of the atmosphere at the altitude at which it is emitted. In the troposphere, the temperature generally decreases with height. Effectively, infrared radiation emitted to space originates from an altitude with a temperature of, on average, –19°C, in balance with the net incoming solar radiation, whereas the Earth’s surface is kept at a much higher temperature of, on average, +14°C. An increase in the concentration of greenhouse gases leads to an increased infrared opacity of the atmosphere, and therefore to an effective radiation into space from a higher altitude at a lower temperature. This causes a radiative forcing that leads to an enhancement of the greenhouse effect.

Greenhouse Gas (GHG): Gaseous constituents of the atmosphere, both natural and anthropogenic, that absorb and emit radiation within the spectrum of thermal infrared radiation emitted by the Earth’s surface, the atmosphere itself, and by clouds. See Greenhouse effect & Global Warming Potential

Heterotrophic respiration: The release of carbon dioxide from decomposition of organic matter. See Carbon sequestration

Impacts from climate change: The effects of climate change on natural and human systems. Depending on the consideration of adaptation, one can distinguish between potential impacts and residual impacts:

• Potential impacts: All impacts that may occur given a projected change in climate, without considering adaptation.
• Residual impacts: The impacts of climate change that would occur after adaptation.

Land-use change: A change in the use or management of land by humans, which may lead to a change in land cover.

• Why this matters: Land cover and land use change may have an impact on the surface albedo, evapotranspiration, sources and sinks of greenhouse gases, or other properties of the climate system and may thus have a radiative forcing and/or other impacts on climate, locally or globally.

Level of Scientific Understanding (LOSU): An index on a 5-step scale (high, medium, medium-low, low and very low) designed to characterize the degree of scientific understanding of the radiative forcing agents that affect climate change. For each agent, the index represents a subjective judgment about the evidence for the physical/chemical mechanisms determining the forcing and the consensus surrounding the quantitative estimate and its uncertainty.

Likelihood: The likelihood of an occurrence, an outcome, or a result, estimated probabilistically and expressed in IPCC reports using a standard terminology defined as follows:

Meridional Overturning Circulation (MOC): A zonally averaged, large-scale meridional (north-south) overturning circulation in the oceans. In the Atlantic, such a circulation transports relatively warm upper-ocean waters northward, and relatively cold deep waters southward. The Gulf Stream forms part of this Atlantic circulation.

Mitigation: Technological change and substitution that reduce resource inputs and emissions per unit of output.

• Why this matters: With respect to climate change, mitigation means implementing policies to reduce greenhouse gas emissions and enhance sinks.

Ocean acidification: A decrease in the pH of seawater due to the uptake of anthropogenic carbon dioxide. See Uptake

Paleoclimate: Climate during periods prior to the development of measuring instruments, including historic and geologic time, for which only proxy climate records are available.

Patterns of climate variability: Natural variability of the climate system fostered by dynamic atmospheric circulation and its interaction with the land and ocean surfaces. Such patterns are often called regimes, modes or teleconnections. Examples are the North Atlantic Oscillation (NAO), the Pacific-North American pattern (PNA), and the El Niño-Southern Oscillation (ENSO). See El Niño-Southern Oscillation

Radiative forcing: The change in the net irradiance at the tropopause due to a change in an external driver of climate change, such as a change in the concentration of carbon dioxide or the output of the Sun.

Reforestation: Planting of forests on lands that have previously contained forests but that have been converted to some other use. See Land-use change

Representative Concentration Pathways (RCPs): Scenarios that represent time series of emissions and concentrations of all of greenhouse gases (GHGs) and aerosols and chemically active gases, as well as land use/land cover⁵.

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 emphasizes that not only the long-term concentration levels are of interest, but also the trajectory taken over time to reach that outcome⁶. See Greenhouse Gas (GHG) & Radiative forcing

Resilience: The ability of a social or ecological system to absorb disturbances while retaining the same basic structure and ways of functioning, the capacity for self-organization, and the capacity to adapt to stress and change.

Risk: An assessment of the magnitude of the potential consequence(s) of climate change impact(s) and likelihood of occurrence. See Likelihood

Risk management: The process of planning, prioritizing and selecting approaches to climate change adaptation. See Adaptation

Saltwater intrusion: Displacement of fresh surface water or groundwater by the advance of saltwater due to its greater density. This usually occurs in coastal and estuarine areas and can result from: 1) reduced runoff and associated groundwater recharge, 2) excessive water withdrawals from aquifers 3) increasing marine influence, or all factors in combination.

Scenario: A plausible and often simplified description of how the future may develop based on an internally consistent set of assumptions about driving forces and key relationships. Scenarios may be derived from projections, but are often based on additional information from other sources. See SRES scenarios, & Representative Concentration Pathways (RCPs)

Sea level change/sea level rise: Global and local sea level can be altered due to: (i) changes in the shape of the ocean basins, (ii) changes in the total mass of water, and (iii) changes in water density. Global sea level rise results from increases in the total mass of water from the melting of land-based snow and ice, and changes in water density from an increase in ocean water temperatures and salinity changes.

Sensitivity: The degree to which a system is affected, either adversely or beneficially, by climate variability or climate change.

• Why this matters: The effect may be direct, for example, a change in crop yield in response to a change in the mean, range, or variability of temperature or indirect, such as damages caused by an increase in the frequency of coastal flooding due to sea level rise. See Climate sensitivity

Sink: Any process, activity or mechanism, which removes a greenhouse gas, an aerosol, or a precursor of a greenhouse gas or aerosol from the atmosphere, such as carbon sequestration in forests wood products. See Heterotrophic respiration & Uptake

Source: Any process, activity or mechanism that releases a greenhouse gas, an aerosol, or a precursor of a greenhouse gas or aerosol into the atmosphere, such as deforestation and forest fires. See Greenhouse Gas (GHG)

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).

SRES scenarios: Scenarios constructed to explore future developments in the global environment with special reference to the production of greenhouse gases and aerosol precursor emissions. They use the following terminology:

• Storyline: a narrative description of a scenario (or a family of scenarios), highlighting the main scenario characteristics and dynamics, and the relationships between key driving forces.
• Scenario: projections of a potential future, based on a clear logic and a quantified storyline.
• Scenario family: one or more scenarios that have the same demographic, politico-societal, economic and technological storyline. See Representative Concentration Pathways (RCPs)

Thermal expansion: In connection with sea-level rise, this refers to the increase in volume and decrease in density that result from warming water. A warming of the ocean leads to an expansion of the ocean volume and hence an increase in sea level. See Sea level change/Sea level rise.

Uncertainty: An expression of the degree to which a value, for example, the future state of the climate system, is unknown.

• Why this matters: Uncertainty may arise from quantifiable errors in the data to ambiguously defined concepts or terminology, or uncertain projections of human behavior. Uncertainty can therefore be represented by quantitative measures, for example, a range of values calculated by various models, or by qualitative statements, for example, reflecting the judgment of a team of experts⁹. See Likelihood

Uptake: The addition of a substance of concern to a reservoir. The long-term uptake of carbon containing substances, in particular carbon dioxide, is often called carbon sequestration. See Carbon sequestration

Vulnerability: The degree to which a system is susceptible to, or unable to cope with, adverse effects of climate changes, including variability and extremes. Vulnerability is a function of the character, magnitude, and rate of climate change and variation to which a system is exposed, its sensitivity, and its adaptive capacity.

References and resources for more information

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4. Ipcc. “Land Use, Land-Use Change, and Forestry.” Forestry (2000): 1–9. Web.
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6. Moss, Richard H et al. “The next Generation of Scenarios for Climate Change Research and Assessment.” Nature 463.7282 (2010): 747–756. Web.
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