100-year flood

A flood or storm that has a 1% probability of occurring in any given year. The 100-year flood zone is the extent of the area of a flood that has a 1% chance of occurring or being exceeded in a given year.

Related terms: 100-year storm
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A1B emissions scenario

A medium emissions scenario in which greenhouse gas emissions increase, with reductions in the rate of increase in emissions after 2070.

A2 emissions scenario

A high emissions scenario assuming continued increases in greenhouse gas emissions.


Adjustment in natural or human systems to a new or changing environment that exploits beneficial opportunities or moderates negative effects.

Related terms: adapt
Adaptation science

Integrated scientific research that directly contributes to enabling adjustments in natural or human systems to a new or changing environment in a way that exploits beneficial opportunities or moderates negative effects.

Adaptive capacity

The potential of a system to adjust to climate change (including climate variability and extremes) to moderate potential damages, take advantage of opportunities, and cope with the consequences.

Adaptive management

A structured process of flexible decision-making that incorporates learning from outcomes and new scientific information. The process facilitates decision-making by resource managers to manage and respond to climate change impacts.

Aerosol (Atmospheric)

Aerosols are fine solid or liquid particles, caused by people or occurring naturally, that are suspended in the atmosphere. Aerosols can cause cooling by scattering incoming radiation or by affecting cloud cover. Aerosols can also cause warming by absorbing radiation.

Related terms: Aerosol Effect, aerosols
Algal bloom

A sudden, rapid growth of algae in lakes and coastal oceans caused by a variety of factors including, for example, warmer surface waters or increased nutrient levels. Some algal blooms may be toxic or harmful to humans and ecosystems.

Assisted migration

The intentional movement of individuals into areas assumed to be their future habitats.

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B1 emissions scenario

A lower emissions scenario in which emissions are reduced rapidly and substantially.

B2 emissions scenario

A low emissions scenario in which emissions are reduced substantially, but not as rapidly as B1.


The diversity of life on earth, from genes to species to ecosystems.


Energy produced using plant or animal matter such as corn or manure.


Fuel produced from plant or animal matter such as corn or manure.

Biogeochemical cycles

Fluxes, or flows, of chemical elements among different parts of the Earth: from living to non-living, from atmosphere to land to sea, from soils to plants.

Related terms: biogeochemical cycle

The mass of living organisms in a given area, or material derived from organisms.

Black carbon

Soot produced from coal burning, diesel engines, cooking fires, wildfires, and other combustion sources. These particles absorb solar energy and have a warming influence on the climate.

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Carbon capture and storage

The process of capturing carbon dioxide and injecting it into geologic formations underground for long-term storage.

Carbon cycle

Circulation of carbon atoms through the Earth systems as a result of photosynthetic conversion of carbon dioxide into complex organic compounds by plants, which are consumed by other organisms, and return of the carbon to the atmosphere as carbon dioxide as a result of respiration, decay of organisms, and combustion of fossil fuels.

Carbon sequestration

Storage of carbon through natural or technological processes in biomass or in deep geological formations.

Climate change

Changes in average weather conditions that persist over multiple decades or longer. Climate change encompasses both increases and decreases in temperature, as well as shifts in precipitation, changing risk of certain types of severe weather events, and changes to other features of the climate system. [See also global change]

Climate change refugia

Areas relatively buffered from contemporary climate change that are likely to increase species or ecosystem persistence.

Climate variability

Natural changes in climate that fall within the observed range of extremes for a particular region, as measured by temperature, precipitation, and frequency of events. Drivers of climate variability include the El Niño Southern Oscillation and other phenomena.

Related terms: natural variability
Cold wave

A period of abnormally cold weather lasting days to weeks.

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The movement of individual organisms away from their birth site or breeding site to another location.

Related terms: Disperse, dispersal ability

A period of abnormally dry weather marked by little or no rain that lasts long enough to cause water shortage for people and natural systems.

Dry spell

A period with little or no rain. Whether a dry spell becomes a drought depends on how long it lasts, expectations based on historical data and perceptions, and the water needs of people and natural systems.

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Ecological corridor

An area of habitat connecting wildlife populations separated by human activities or structures.


All the living things in a particular area as well as components of the physical environment with which they interact, such as air, soil, water, and sunlight.

Ecosystem services

The benefits produced by ecosystems on which people depend, including, for example, fisheries, drinking water, fertile soils for growing crops, climate regulation, and aesthetic and cultural value.

Related terms: ecosystem service
El Niño-Southern Oscillation

A natural variability in ocean water surface pressure that causes periodic changes in ocean surface temperatures in the tropical Pacific ocean. El Niño Southern Oscillation (ENSO) has two phases: the warm oceanic phase, El Niño, accompanies high air surface pressure in the western Pacific, while the cold phase, La Niña, accompanies low air surface pressure in the western Pacific. Each phase generally lasts for 6 to 18 months. ENSO events occur irregularly, roughly every 3 to 7 years. The extremes of this climate pattern's oscillations cause extreme weather (such as floods and droughts) in many regions of the world.

Related terms: La Niña, El Niño
Emergent properties

Properties of a complex system (or group) that arise from the interactions among the individual parts or changing membership of a larger system that are not possible when individual parts of the system act alone.

Emissions scenarios

Quantitative illustrations of how the release of different amounts of climate altering gases and particles into the atmosphere from human and natural sources will produce different future climate conditions. Scenarios are developed using a wide range of assumptions about population growth, economic and technological development, and other factors.

Related terms: emissions scenario, emission scenario
Energy systems

The infrastructure and systems of electricity production, transport, storage and consumption.

Environmental justice

The fair treatment and meaningful involvement of all people regardless of race, color, national origin, or income with respect to the development, implementation, and enforcement of environmental laws, regulations, and policies.


Evaporation of water from soil and plant leaves.


Change in heritable characteristics over successive generations.

Extreme events

A weather event that is rare at a particular place and time of year, including, for example, heat waves, cold waves, heavy rains, periods of drought and flooding, and severe storms.

Related terms: Extreme weather
Extreme precipitation (events)

An episode of abnormally high rain or snow. The definition of "extreme" is a statistical concept that varies depending on location, season, and length of the historical record.

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The process through which a system is controlled, changed, or modulated in response to its own output. Positive feedback results in amplification of the system output; negative feedback reduces the output of a system.


An individual organism’s reproductive success.

Food security

When all people at all times have both physical and economic access to sufficient food to meet their dietary needs for a productive and healthy life.


Factors that affect the Earth's climate. For example, natural factors such as volcanoes and human factors such as the emission of heat-trapping gases and particles through fossil fuel combustion.

Frontline communities

Those communities that experience climate change first and often feel the worst effects. They are communities that have higher exposures, are more sensitive, and are less able to adapt to climate change impacts for a variety of reasons.

Frost-free season

The time period between the last occurrence of an air temperature of 32°F in spring and the first occurrence of 32°F in the subsequent fall.

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Genetic rescue

A conservation tool used to increase the fitness of a small, imperiled population by adding genetic variation through a small number of immigrants.


Intentional modifications of the Earth system, usually technological, as a means to reduce future climate change.

Global change

Changes in the global environment that may alter the capacity of the Earth to sustain life. Global change encompasses climate change, but it also includes other critical drivers of environmental change that may interact with climate change, such as land use change, the alteration of the water cycle, changes in biogeochemical cycles, and biodiversity loss. [See also climate change]

Global Climate Models (GCM)

Mathematical models that simulate the physics, chemistry, and biology that influence the climate system.

Global warming

The observed increase in average temperature near the Earth’s surface and in the lowest layer of the atmosphere. In common usage, “global warming” often refers to the warming that has occurred as a result of increased emissions of greenhouse gases from human activities. Global warming is a type of climate change; it can also lead to other changes in climate conditions, such as changes in precipitation patterns.

Greenhouse gases

Gases that absorb heat in the atmosphere near the Earth's surface, preventing it from escaping into space. If the atmospheric concentrations of these gases rise, the average temperature of the lower atmosphere will gradually increase, a phenomenon known as the greenhouse effect. Greenhouse gases include, for example, carbon dioxide, water vapor, and methane.

Related terms: Heat trapping gases, Greenhouse Effect
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Heat stress

The negative health impacts, such as heat stroke or heat exhaustion, caused by exposure to extreme heat or long periods in hot environments.

Heat wave

A period of abnormally hot weather lasting days to weeks.

Heavy precipitation events

An episode of abnormally high rain or snow. The definition of "extreme" is a statistical concept that varies depending on location, season, and length of the historical record.

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An observation or calculation that allows scientists, analysts, decision makers, and others to track environmental trends, understand key factors that influence the environment, and identify effects on ecosystems and society.

Indigenous knowledges

In brief, refers to Indigenous peoples' systems of observing, monitoring, researching, recording, communicating, and learning that are required, as for any group, to support survival and flourishing in an ecosystem and the social adaptive capacity to adjust to or prepare for changes.

Invasive species

A non-native organism whose introduction within a particular ecosystem causes or is likely to cause economic or environmental harm, or harm to human, animal, or plant health.

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Land cover

The physical characteristics of the land surface, such as crops, trees, or concrete.

Land use

Activities taking place on land, such as growing food, cutting trees, or building cities.

Life history

The sequence of events experienced by an organism, from birth to reproduction to death.

Related terms: Life histories
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Measures to reduce the amount and speed of future climate change by reducing emissions of heat-trapping gases or removing carbon dioxide from the atmosphere.

Related terms: mitigate
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Chemicals (such as nitrogen and phosphorus) that plants and animals need to live and grow. At high concentrations, particularly in water, nutrients can become pollutants.

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Ocean acidification

The process by which ocean waters have become more acidic due to the absorption of human-produced carbon dioxide, which interacts with ocean water to form carbonic acid and lower the ocean’s pH. Acidity reduces the capacity of key plankton species and shelled animals to form and maintain shells.


A colorless gas consisting of three atoms of oxygen, readily reacting with many other substances. Ozone in the upper atmosphere protects the Earth from harmful levels of ultraviolet radiation from the Sun. In the lower atmosphere ozone is an air pollutant with harmful effects on human health.

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The climate that existed during the period before modern record-keeping. Paleoclimate can be measured with "natural thermometers" such as ice cores or tree rings.


Microorganisms (such as a bacteria or viruses) that causes disease.


Ground that remains at or below freezing for at least two consecutive years.


The pattern of seasonal life cycle events in plants and animals, such as timing of blooming, hibernation, and migration.

Phenotypic plasticity

The ability of an organism to change its behavior, physiology, or physical characteristics in response to its environment. This change occurs within an organism’s lifetime and therefore does not require genetic change.


Microscopic organisms (single-celled plants, bacteria, and protists) capable of photosynthesis. Phytoplankton are found in oceans, seas, and freshwater, and are an essential component of aquatic ecosystems.


Actions taken to build, apply, and sustain the capabilities necessary to prevent, protect against, and ameliorate negative effects.

Primary productivity

Energy captured from the atmosphere through photosynthesis.

Related terms: Primary producers

A proxy is a way to indirectly measure aspects of climate. Biological or physical records from ice cores, tree rings, and soil boreholes are good examples of proxy data.

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Radiative forcing

The change in energy flux (expressed in Watts/square meter) at the tropopause or top of the atmosphere due to a change in a climate driver (such as changes in atmospheric carbon dioxide concentrations).

Range shift

Change in the total areal extent of a species or the geographic limits within which a species can be found.

Related terms: Range shifts
Representative Concentration Pathways

Scenarios that include time series of emissions and concentrations of the full suite of greenhouse gases, and aerosols and other chemically active gases, as well as land use/land cover. 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 outcome. 

Related terms: RCP

A capability to anticipate, prepare for, respond to, and recover from significant multi-hazard threats with minimum damage to social well-being, the economy, and the environment.

Resource mismatch

The decoupling of a previously synchronized ecological relationship, such as changes in timing within a trophic (food-web) relationship.


Risks are threats to life, health and safety, the environment, economic well-being, and other things of value. Risks are often evaluated in terms of how likely they are to occur (probability) and the damages that would result if they did happen (consequences).

Risk assessment

Studies that estimate the likelihood of specific sets of events occurring and their potential positive or negative consequences.

Risk management

Planning to manage the effects of climate change to increase positive impacts and decrease negative impacts.

Risk perception

The psychological and emotional factors that affect people’s behavior and beliefs about potential negative hazards or consequences.

Risk-based framing

Planning based on the pros and cons of a given set of possibilities; includes assessment of a risk in terms of the likelihood of its occurrence and the magnitude of the impact associated with the risk.

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Sets of assumptions used to help understand potential future conditions such as population growth, land use, and sea level rise. Scenarios are neither predictions nor forecasts. Scenarios are commonly used for planning purposes.

Seed sourcing

To assist with climate adaptation, seed sources are taken from areas where the climate is similar to the predicted future climate in the planting location.


A natural or technological process that removes carbon from the atmosphere and stores it.

Related terms: Carbon sink
Snow water equivalent (SWE)

The amount of water held in a volume of snow, which depends on the density of the snow and other factors.


Snow that accumulates over the winter, and slowly melts to release water in spring and summer.

Special Report on Emissions Scenarios (SRES)

A set of emission scenarios from the IPCC Special Report on Emission Scenarios released in 2000 that describe a wide range of potential future socioeconomic conditions and resulting emissions.


An individual or group that is directly or indirectly affected by or interested in the outcomes of decisions.

Storm surge

The sea height during storms such as hurricanes that is above the normal level expected at that time and place based on the tides alone.


The layering of water by temperature and density that can occur in lakes or other bodes of water, often seasonally.


Something that has an effect on people and on natural, managed, and socioeconomic systems. Multiple stressors can have compounded effects, such as when economic or market stress combines with drought to negatively impact farmers.


Taken broadly, refers to any human system that seeks to secure survival and flourishing within particular ecosystems. In some legal and policy contexts, the term may be used more narrowly to refer to the provision of food that is a necessary part of a household's or a community's regular diet or legal entitlements to harvesting rights in particular situations. In some contexts subsistence means the harvest or use of naturally-produced renewable resources for direct personal or family consumption as food, shelter, fuel, clothing, tools, transportation, or production of handicrafts for customary and traditional trade, barter or sharing.

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Tipping point

The point at which a change in the climate triggers a significant environmental event, which may be permanent, such as widespread bleaching of corals or the melting of very large ice sheets.

Related terms: Threshold
Traditional knowledge

Knowledge, practices and beliefs that have been handed down through generations.


The evaporation of water through plant leaves.

Trophic level

An organism’s hierarchical level on the food chain, determined by the organisms it eats and what organisms eat it.

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An expression of the degree to which future climate is unknown. Uncertainty about the future climate arises from the complexity of the climate system and the ability of models to represent it, as well as the inability to predict the decisions that society will make. There is also uncertainty about how climate change, in combination with other stressors, will affect people and natural systems.

Urban heat island effect

The tendency for higher air temperatures to persist in urban areas as a result of heat absorbed and emitted by buildings and asphalt, tending to make cities warmer than the surrounding countryside.

Related terms: Heat island
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To establish or verify accuracy. For example, using measurements of temperature or precipitation to determine the accuracy of climate model results.


Belief or ideal held by individuals or society about what is important or desirable.

Value (economic)

The benefit, usually expressed in monetary terms, gained from use or enjoyment from a good or service.

Vector (disease)

An organism, such as an insect, that transmits disease-causing microorganisms such as viruses or bacteria. Vector-borne diseases include, for example, malaria, dengue fever and lyme disease.

Related terms: Vector-borne disease

The degree to which physical, biological, and socio-economic systems are susceptible to and unable to cope with adverse impacts of climate change.

Vulnerability assessment

An analysis of the degree to which a system is susceptible to or unable to cope with the adverse effects of climate change.

Related terms: Vulnerability analysis
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Water security

Reliable availability of water in sufficient quantity and quality to sustain human health, livelihoods, and the environment.

Water stress

Water stress occurs when demand for water by people and ecosystems exceeds available supply.

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