Glossary

Climate change is a complex topic that spans multiple scientific, economic, and social fields. Understanding its mechanisms and impacts requires acquiring a precise and rigorous vocabulary. This glossary aims to provide clear and accessible definitions of key climate-related terms, based on the latest scientific knowledge.

Intended for teachers, students, and curious citizens, this lexicon helps improve understanding of climate mechanisms, environmental challenges, and possible solutions for mitigating and adapting to the effects of climate change. The definitions are based on authoritative sources such as the reports of the Intergovernmental Panel on Climate Change (IPCC), as well as scientific and institutional publications.

 

 - A - B - C - D - E - F - G - H - I - J - K - L - M - N - O - P - Q - R - S - T - U - V - W - X - Y - Z

 

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• Adaptation

The process of adjusting to current or expected climate change impacts. In human systems, the aim of adaptation is to reduce risks, increase resilience or seize beneficial opportunities. In natural systems, human intervention may facilitate adjustments to expected climate change impacts.

• Afforestation

The establishment of a forest through tree planting or seeding on land that has lacked forest cover for a very long time or has never been forested.

• Agroecology

A type of sustainable farming that applies ecological concepts and principles in agriculture.

• Agroforestry

A method of using agricultural land combining trees and crops or animal husbandry.

• Albedo

Meaning “whiteness”, albedo is the reflective power of an object or surface. For instance, ice and fresh snow have a high albedo, ranging from 40-80%. This means that they reflect 40-80% of the incoming sunlight. The ocean is darker, so it has an albedo of less than 10%.

• Anthropogenic emissions

Greenhouse gases emitted by human activities.

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• Biodiversity

Biodiversity first refers to the variety of species (flora and fauna) that live on Earth or in a particular ecosystem. More precisely, we distinguish between three levels of biodiversity: intraspecific biodiversity (differences between members of the same species), interspecific biodiversity (differences between species) and ecosystem diversity (the environment and species that live in it).

• Biodiversity hotspot

Region of the world where the level of biodiversity is significantly highly threatened by human habitation. Biodiversity hotspots often contain high levels of local and specific (endemic) biodiversity. Biodiversity hotspots are commonly found in tropical regions. 

• Biomass

Organic matter used as a fuel, especially in a power station for the generation of electricity.

• Biome

The ensemble of flora and fauna in a specific region.

• Blue carbon

Carbon dioxide (CO₂) removed from the atmosphere by world’s coastal ecosystems (such as mangroves, salt marshes, seagrasses and algae). Aquatic plants grow, accumulate CO₂ that is then bury as organic matter (OM) in the soil. 

• Browning

Browning is a systematic decrease in vegetation growth or the death of vegetation that results in a loss of productivity over a period of time.

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• Calving

The breaking off or detachment of a glacier, ice sheet or an iceberg.

• Carbonic acid (H₂CO₃)

This acid is formed when carbon dioxide dissolves in water, causing an increase in the acidity of the water.

• Carbon cycle

Carbon is a chemical element which can be found in many molecules, in living beings as well as in non-living materials. Carbon is stored in huge amounts in what are called “reservoirs” on Earth–the most important being the ocean and the soil. But the carbon in these reservoirs does not stay there forever; it moves between reservoirs: these movements are called “flows”. The natural flows are perfectly balanced, leading to a carbon cycle.

• Carbon dioxide (CO₂)

A gas produced by the combustion of carbon (for example: fossil fuels). It is also produced by living organisms while breathing. CO₂ contributes to the greenhouse effect.

• Carbon fertilisation

Carbon fertilisation is also known as carbon dioxide fertilisation. It is the phenomenon by which the increase of carbon dioxide in the atmosphere increases the rate of photosynthesis in plants.

• Carbon footprint

A carbon footprint (in CO₂-eq) is defined as the total amount of greenhouse gases produced directly or indirectly by human activities. It can be calculated for an individual, a particular event or an organisa

• Carbon sequestration

Long term storage of carbon dioxide (CO₂) – or other forms of carbon – to mitigate climate change, by slowing down atmospheric and marine accumulation of greenhouse gases. 

• Carbon sink

This is a natural reservoir that stores carbon-containing chemical compounds accumulated over time. Carbon sinks help reduce the amount of atmospheric CO₂. Natural sinks are soil–the largest carbon store –and part of the biosphere via photosynthesis by terrestrial plants and marine phytoplankton and algae, a process that incorporates atmospheric CO₂ into sugars, using solar energy.

• Carbonic acid (H₂CO₃)

This acid is formed when carbon dioxide dissolves in water, causing an increase in the acidity of the water.

• Cascading effects

Inevitable and sometimes unforeseen chain of events due to an act affecting part of the climate system. These cascading effects are often negative.

• Citizen's convention 

A democratic practice that allows non-expert citizens to give their opinion and make proposals on reform projects, public policies, or controversial issues that require building consensus. It is a way to involve citizens in the development of public policies while considering the diversity of their opinions and expectations.

• Climate

Climate is defined as the average weather conditions over long timescales in a given region.

• Climate change

Generic term encompassing several global phenomena, including: changes in temperature, precipitation, melting ice (glaciers and pack ice), extreme events, sea level rise, and so on. The term is most used to describe the current human-induced climate change that started around 1850 due to an increase in the global average temperature.

• Climate justice

This term is used to acknowledge the social and political dimensions of the challenges associated with climate change, rather than considering only their environmental dimension. It relates the differences observed between those more responsible for climate change and those more affected by its consequences, to the notion of justice (in particular, social and environmental justice).

• Climate projection

Simulation of a future climate based on a scenario.

• Climate simulation

Experimentation with a digital model.

• Climate-resilient pathways

Climate-resilient pathways are trajectories of development that combine mitigation and adaptation to aim for sustainable human development and help avoiding dangerous interference with the climate system.

• Climate zones

Areas with distinct climates that can be classified using different parameters, such as temperature, precipitation, etc.

• CO₂ uptake

All the processes that contribute to the removal of CO₂ from the atmosphere. CO₂ can be removed by biological processes, such as ocean or land photosynthesis, or by more physical processes, such as carbon absorption in water.

• Coastal ecosystems

Coastal ecosystems exist where land and water meet. In these areas, the mixture of freshwater and seawater creates unique environments and ecosystems with distinct structures and diversity. They include saltmarshes, mangroves, coastal wetlands, estuaries and bays.

• Coastal erosion

Coastal erosion is a natural process that can be exacerbated by human action. It consists in the removal and transport of sediments or rocks along the coastal zone, through the action of waves, currents, winds and tides. A long-term loss of sediment from beaches results in beach erosion and consequent shoreline retreat. Climate change (in particular sea level rise and increased precipitation) can enhance coastal erosion.

• Complex system

A system (such as the climate system) regulated by many factors that interact with and influence each other: atmosphere, ocean, land and biosphere, for example.

• Compound event

An event that has more than one possible outcomes. 

• Confidence (level of confidence)

In the IPCC report, the level of confidence refers to the degree of certainty in the validity of a finding, based on the type, amount, quality, and consistency of evidence and the degree of agreement among experts. The level of confidence is expressed using five qualifiers: ‘very low’, ‘low’, ‘medium’, ‘high’ and ‘very high’. It summarises the judgements of the teams of authors on the validity of the results. Confidence should not be interpreted probabilistically, and is distinct from ‘statistical confidence’.

• Consensus

A scientific consensus is based on a high level of confidence (agreements between scientists, weight of evidence, etc). A scientific consensus on a given subject is not incompatible with all aspects of the subject or the notion of uncertainty.

• Continental ice

All the ice on land. Continental ice is formed by the accumulation and compaction of snow over a long period of time.

• Cooling solution

Nature-based, technical-based, or behavioural-based transformation of the city aiming at a diminution of the Urban Heat Island effect (UHI).

• Cryosphere

All the ice and snow on Earth, on land and in the water (glaciers, ice sheets, seasonal snow, sea ice, permafrost, and frozen lakes and rivers).

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• Deforestation

Destruction of a forest, often with the aim of turning it into agricultural land.

• Desertification

Land degradation in arid or semi-arid areas by human or climatic actions.

• Digital Model

Simplified representation of reality that uses mathematics and computer science.

• Displacement

Transfer of groups of people from one place of leaving to another. This movement can be temporary or permanent. People are moved away from a place that had became dangerous or that cannot sustain the concerned population due to climate change effects; to a safer place. 

• Drought

A drought is an event of prolonged shortages in the water supply. A drought can last for months or years, or may be declared after as few as fifteen days.

• Dryland

Drylands are ecosystems characterised by a lack of water. They include cultivated lands, scrublands, shrublands, grasslands, savannas, semi-deserts and true deserts.

• Dust storms

Masses of sand and dust raised by the wind in very dry areas such as deserts.

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• Eco-anxiety

Climate change can generate different feelings and emotions; some may leave us feeling helpless or hopeless. This is known as eco-anxiety.

• Ecosystem

The totality of living beings in a given environment, plus the environment itself. In an ecosystem, everything is interconnected and interdependent.

• Ecosystem services

Humans can directly or indirectly benefit from ecosystems, which provide them with services. They are grouped into four categories: provisioning, regulating, supporting and cultural. For example, ecosystems produce oxygen (through photosynthesis) and food, and they provide us with raw materials. Ecosystems also preserve soil fertility, fertilise plants and protect coasts.

• Emission pathways

Emission pathways refer to the modelled trajectories of global anthropogenic GHG emissions for the future following different scenarios possible.

• Equity

Justice, fairness: when the same opportunities are given to all – education, health, rights, etc.

• Evapotranspiration

The process by which water is transferred from the land to the atmosphere by evaporation, either from the soil and other surfaces, or by transpiration from plants.

• Exposure

How much a population is exposed to a certain climate hazard, due, for example, to its geographic location. Example: low-lying lands are more exposed to sea level rise than mountain regions.

• Extreme events

Rare meteorological events with a strong negative impact on human society and ecosystems (e.g. tornadoes, major fires, droughts, or heatwaves). Climate change is increasing the frequency and amplitude of some extreme events worldwide (extreme cold events are reduced however).

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• Feedback loop

Process by which the consequence can modify its own causes. Positive feedback will amplify the consequence. Negative climate feedback will limit it.

• Fermentation

Fermentation is a chemical reaction that happens naturally in some plants and animal substances. It needs the presence of very small living organisms, such as bacteria, fungi or yeast, to occur. This phenomenon is also used for human activities and can produce methane, lactic acid (in yogurts) and alcohol (in wine or beer).

• Food system

The food system includes every step that our food goes through, from farm to fork: food production, processing, storage, distribution, consumption and recycling.

• Food web

A food web is a representation of the “prey–predator” relationship in an ecosystem. It is a web rather than a chain, meaning that an organism may eat several species and a species may be eaten by multiple organisms.

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• Glacier

A large mass of ice on land that slowly moves downhill.

• Global warming

See Climate Change.

• Global warming potential (GWP)

The term Global Warming Potential is used to compare the “power” of a gas to warm the atmosphere and the duration of its effect. By definition, CO₂ has a global warming potential of 1 regardless of the time period used. CO₂ remains in the atmosphere for a century. CH4 is estimated to have a GWP of 28-36 over 100 years, meaning it absorbs much more energy than CO₂. Nevertheless, it “only” remains in the atmosphere about a decade on average. The net effect of the shorter lifetime and higher energy absorption is reflected in the GWP. N₂O has a GWP 265-298 times that of CO₂ for a 100-year timescale.

• Governance

Structures, processes, and actions through which societies are governed. This includes institutions with the rules, laws, and decision-making, management, implementation, and oversight procedures for policies and measures at all geographical or political scales, from local to global.

• Greenhouse effect

Solar radiation crosses the atmosphere, is absorbed by the Earth’s surface and warms it. The absorbed solar radiation is transformed into infrared radiation (heat). Some of this infrared radiation is ‘trapped’ on its escape towards space by greenhouse gases in the atmosphere and is sent back towards the Earth’s surface – heating it up even more. This is called the greenhouse effect.

• Greenhouse gas (GHG)

Greenhouse gases cause the greenhouse effect. Greenhouse gases are mainly water vapour, carbon dioxide, methane, nitrous oxide and ozone.

• Greening

Greening is an observed increase in vegetation productivity over a certain period of time. Trees, shrubs, herbaceous plants and ground cover vegetation are taken into account.

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• Heat capacity 

The thermal capacity of a material is the amount of heat required to raise its temperature by one degree Celsius.

• Heat island effect

An urban area has a higher average temperature than its rural surroundings owing to the greater absorption, retention and generation of heat by its buildings, pavements and human activities.

• Heat sink

In the context of climate change, a heat sink is a body – for example a forest or the ocean – that absorbs heat from a warmer body – such as the atmosphere. This results in cooling of the warmer body. The most important heat sink in the climate system is the ocean which, so far, has absorbed over 90% of the heat that has resulted from global warming.

• Heatwave

A period of abnormally hot weather with high daytime temperatures and no or little cooling down at night. A heatwave can last up to several weeks.

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• Ice sheet

A very large and thick layer of ice on a continent.

• Indigenous local knowledge

Indigenous communities often live a lifestyle based on a complex and important relationship with their direct environment, with low impact on it and on the climate. Their local knowledge of nature management and agriculture is important to adapt and mitigate climate change.

• Industrial Revolution

The historical period between 1760 and the 1840s. It has marked the transition from agricultural to industrial societies. The Industrial Revolution started in Europe and the United States and led to a rapid development of productivity, technologies and science, and therefore to population growth.

• Infrared radiation

Infrared radiation is the invisible part of light that we can feel as heat. It plays a key role in the greenhouse effect.

• Input on a model

Data entered in the model.

• Interactions 

The Earth’s envelopes (atmosphere, hydrosphere, etc.) continually exchange matter and energy. These fluxes help redistribute the uneven incoming solar energy. There are also interactions between different terrestrial environments (the melting of the Antarctic ice cap has repercussions on global ocean and on European coasts for example).

• Intergovernmental Panel on Climate Change (IPCC)

The Intergovernmental Panel on Climate Change (IPCC) was established in 1988 to provide comprehensive assessments of the state of scientific, technical, and socio-economic knowledge on climate change, its causes, potential impacts, and response strategies. Since then, the IPCC has produced six multi-volume assessment reports. It also publishes special reports on specific topics such as disasters and extreme events, the ocean and cryosphere, land use, and more. The IPCC is open to all United Nations member countries and includes 195 states. Its reports are written by hundreds of scientists from these member states. The IPCC's work also results in Summaries for Policymakers, which are reviewed line by line and formally approved unanimously by state delegates, with the consent of the scientific authors.

• Invasive species

Species that is not native to a specific location and has the tendency to spread to a degree that ca cause damage to the environment, human economy or human health. A lot of factors can help the spreading of invasive species such as the increase of the intensity of global commercial fluxes.

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• Land degradation

Temporary or permanent decline in quality of soil, vegetation, water resources or wildlife–or the deterioration of the economic productivity of the land, such as the ability to farm the land.

• Landslide

A mass of rock and earth moving suddenly and quickly down a steep slope. 

                           - M -

• Marine currents

A flow of water through the ocean. Warm and cold currents redistribute heat and nutrients around the globe.

• Marine heatwave

Period of time of minimum 5 days when seawater temperatures exceed a seasonally based maximum temperature (minimum, an exceeding average 2°C).

• Mass Mortality Event (MME) 

An incident (caused, for example, by poisoning, extreme heat, disease, pollution, etc.) that kills a large number of individuals of the same species within a short period of time. The event can endanger a species with extinction or disrupt an ecosystem's balance.

• Migration

See the definition of displacement. 

• Mitigation

Human intervention to reduce global warming by reducing GHG emissions or by enhancing GHG sinks.

• Model

Simplified representation of reality. Using models has many advantages, but it also has its limitations. It can not represent the full complexity of reality. Climate models are analogies for reality to make predictions/projections. Climate models are evolving thanks to the integration of new parameters and improvements in computers and technical power, leading to greater resolution and accuracy. There are different types of models: an 'analog model' is a simplified representation of reality using physical objects, and a 'digital model' is a simplified representation of reality using numeric data (the accuracy and resolution determine the quality of the model.

                           - N -

• Natural variability

Variations in the climate system that are not related to human activities (for example, alternation of glacial and interglacial eras).

• Negative emissions

A situation in which carbon sinks, whether natural (forests, oceans, etc.) or human-made (such as carbon capture and storage), remove more greenhouse gases (GHGs) than human activities emit, leading to a decrease in atmospheric GHG concentration.

• Net-zero emission

A situation in which anthropogenic greenhouse gas emissions into the atmosphere are balanced by anthropogenic removals over a given period. Net-zero CO₂ emissions refer exclusively to anthropogenic CO₂ emissions.

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• Ocean acidification

Increase in the acidity of seawater, caused by the dissolution of CO₂ from the atmosphere in the ocean’s surface water. When CO₂ reacts with water, the water becomes more acidic.

• Output of a model

Data produced by a numerical model when running a simulation.

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• Paleoclimate 

Description of the climate of past eras.

• Parameters

Measurable values used to characterise a system (e.g. for characterising the atmosphere system: level of CO2 in ppm). Climatologists must study a set of parameters simultaneously.

• Paris Agreement 

An international agreement on climate change, approved in 2015 during the COP21 in Paris and signed by 196 countries. The main goal is to limit the increase of global temperature to 1.5°C by the end of the century.

• Peatlands

Wetlands where the soil is highly organic because it is primarily composed of partially decomposed plants. This soil is called peat, and its presence is what defines peatlands.

• Permaculture

Development of an agricultural ecosystem intended to be sustainable and self-sufficient.

• Permafrost

Soil, rock or sediment that is permanently frozen (for at least two consecutive years).

• Perturbed climate 

Since the industrial revolution (1850-1900), the Earth's climate has been changing rapidly due to increased human activity.

• pH

pH is a measure of the acidity and alkalinity of a solution on a scale from 0 to 14, in which 7 represents neutrality. A lower value indicates a more acidic solution whereas a higher value means a more alkaline one.

• Photosynthesis

When they are exposed to light, plants are able to use this light, in addition to carbon dioxide, water and minerals, to produce their own organic matter and to grow. This process is called photosynthesis.

• Primary production

Primary production is the process by which a primary producer (a cell or an organism) produces its own organic matter using mineral materials. For example, photosynthetic living beings only use water, CO₂ and light to grow.

• Projection

The simulation of a given scenario. Climate projections show the potential future evolution of a quantity or set of quantities, calculated by a model. They are based on physics and are used to quantify global and regional climate change, and its impact on phenomena such as monsoons.

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                           - R -

• Reforestation

The planting of forests on lands that have previously contained forests but have since been converted for some other use.

• Reliability

Not all sources of information are trustworthy; they should always be checked. The Intergovernmental Panel on Climate Change (IPCC) assesses and compiles the latest scientific information on climate change and relays the consensus of the scientific community, making it one of the most reliable sources of information on climate change.

• Representative Concentration Pathway (RCP)

Trajectories for ‘Representative Concentration Pathways’ were used to evaluate different scenarios of GHG emissions. However, unlike SSPs scenarios, RCPs do not take into account socio-economic changes. Nevertheless, there are similarities between these two types of scenarios. RCPs were used in the previous IPCC report (e.g. AR5).

• Resilience 

The ability to cope with a disturbance by responding or reorganizing in a way that maintains its function, identity, and structure.

• Risks

The likelihood of a climate hazard (drought, cyclone, flood, etc.) occurring in a vulnerable area. The most significant risks cause potentially severe damage to humans and socio-ecological systems due to the interaction between climate hazards and the vulnerabilities of exposed societies and systems.

• Runoff

The draining away of water or snow from the surface of an area of land, a building or a structure. 

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• Sampling

Reduction of continuous information to a finite set of values.

• Scenario

Selection of inputs based on social/ethical reasons. A climate scenario is a plausible and often simplified representation of the future climate. IPCC scenarios are based on societal choice.

• Sea ice

Frozen seawater that floats on the ocean’s surface.

• Sea level rise

Because of climate change, the global mean sea level has increased by about 15cm from 1900 to 2018. The current rate of increase is between 3 and 4mm/yr. The sea level is projected to rise by a further 20cm to over one metre by the end of this century, depending on how much greenhouse gases we emit.

• Sea surface temperature

The average water temperature at a depth range from 1mm to 20 meters, according to the type of measurement.  

• Shared socio-economic pathways (SSPs)

Scenario of projected global socio-economic changes that depend on societal choices. These scenarios cover all kinds of projections, from very optimistic (SSP1-1.9: drastic reduction of global greenhouse gas emissions) to pessimistic (SSP5-8.5: further increase of greenhouse gas emissions). These scenarios are used as input data for climate models, which in turn calculate the corresponding change in climate. SSPs are used in the latest IPCC report (AR6).

• Simulation

One run of a digital model.

• Societal choices

Possibilities for the evolution of a society. This evolution is based, among other things, on demographic changes, future energy choices, the type of development, or the application of environmental policies.

• Soil degradation

Soil degradation means the loss of arable land, and can be a consequence of water erosion, coastal erosion, wind erosion, salinity, loss of organic matter, fertility decline, soil acidity, etc.

• Storm surge

Local rising of the sea as a result of wind and atmospheric pressure changes due to a storm. 

• Stratification

Layers of seawater masses with different properties: difference of salinity, oxygenation, density and/or temperature. Water of the ocean will then not mix leading to anoxia and lack of nutrients (the water is depleted in O2 and/or in nutrients, which has dramatic consequences on the marine wildlife).

• Sustainable development

Development that meets the needs of the present generations without compromising the ability of future generations to meet their own needs.

• Sustainable Development Goals (SDGs)

Sustainable development meets the needs of the present without compromising the ability of future generations to meet their own needs. The SDGs have different objectives across sectors to support sustainable development.

• Synergies

Implementing solutions (mitigation, adaptation) to face the UHI effect can also limit the temperature increase by the end of the century.

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• Temperature anomaly

Difference between an observed or simulated temperature and a specific reference.

• Temperature overshoot

The temporary overshoot of a certain level of global warming.

• Thermal expansion

The increase in the volume of a material as a result of rising temperature. Global warming leads to seawater thermal expansion: when the ocean gets warmer, it expands and occupies more space.

• Thermal inertia

Property of matter to approach the temperature of its surroundings. The slower its temperature changes, the higher the thermal inertia.

• Tipping point

The point at which a series of small changes or incidents becomes significant enough to cause a larger, more important change.  

• Transformation

An adaptation that changes the fundamental attributes of a system in response to climate and its effects. 

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• Uncertainty

Every scientific data is associated with uncertainty. Climate projections established by models are therefore always associated with uncertainties. There are two primary causes for uncertainty about the magnitude of climate change in the future: ~ climate feedbacks (involving clouds, carbon uptake, ocean heat uptake, water vapour, sea ice, etc.); ~ future human emissions (linked to climate policy, technology, politics, people, etc.).

• Unperturbed climate 

The unperturbed climate is considered here to be the climate prior to the industrial period. Climate has varied a great deal over long time scales, but has been relatively more stable over the last 10,000 years.

• Urban Heat Island (UHI) effect

An urban area has a higher average temperature than its rural surroundings owing to greater absorption, retention and generation of heat by buildings, pavement and human activity.

                           - V -

• Validation

Act of proving the accuracy of a model.

• Vulnerability

Sensitivity of a population when exposed to climate change hazards and its consequences. Example: a low-lying region with coastal protection infrastructures and resources is less vulnerable to sea level rise than a low-lying region with no coastal protection infrastructures and few economical resources.

                           - W -

• Weather

The state of the atmosphere at a particular time and place. To define it, many variables such as temperature, precipitation, cloudiness or wind are taken into account.

• Weather prediction

Forecasts for the weather on a given date. Climate projections, on the other hand, calculate the average weather conditions over much longer timescales (decades, centuries, millenia) in a given region. However, both forecasts and projections use models.

• Wildfire

A fire that is burning strongly and out of control on an area of grass in the countryside.

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