Our glossary explains words, terms and acronyms used frequently across the website. Use the A–Z listing or your browser page search to find your required term (usually CTRL+F, or Command+F on a Mac).




Adjustment in natural or human systems in response to actual or expected climatic stimuli or their effects, which moderates harm or exploits beneficial opportunities. Various types of adaptation can be distinguished.

  • Planned adaptation is the result of a deliberate policy decision, based on an awareness that conditions have changed or are about to change, and that action is required to maintain, or achieve, a desired state.
  • Reactive adaptation is adaptation that takes place in response to the consequences of a particular event.
  • Anticipatory adaptation is that which takes place before impacts of climate change are observed
  • Spontaneous (or autonomous) adaptation does not constitute a conscious response to climatic stimuli, but is triggered by ecological changes in natural systems, and by market or welfare changes in human systems.

Adaptation assessment

The practice of identifying options to adapt to climate change and evaluating them in terms of criteria such as availability, benefits, costs, effectiveness, efficiency and feasibility.

Adaptation to climate change

Actions to reduce the vulnerability of a system to the negative impacts of anticipated human-induced climate change.

Adaptation to climate variability

This involves taking action to reduce vulnerability to short-term climate shocks. Often, adaptation to climate variability will also result in adaptation to climate change. The objective of adaptation is to reduce vulnerability to climate change and variability, and enhance the capability to capture any benefits of climate change.

Adaptation measures

This refer to actual adjustments, or changes in decision environments, which might enhance resilience or reduce vulnerability to observed or expected changes in climate.

Adaptation Sub-Committee

The Adaptation Sub-Committee (ASC) is a sub-committee of the Committee on Climate Change (CCC), which was established under the Climate Change Act 2008 to provide advice and scrutiny on the Government’s programme for adaptation.

Adaptive capacity

Inherent capacity of a system or population to adjust to climate climate impacts or climate change, to moderate potential damages, exploit opportunities, and cope with the consequences.


Caused by human activity. Anthropogenic climate change refers to the changes caused by increased greenhouse gas emissions.

Assessment endpoints

These are chosen to help establish the acceptability of the risk posed to an exposure unit by future circumstances and decisions. They are often referred to as thresholds in the climate impacts assessment literature. Thresholds are determined by past records or experience of previous events that defined the edge of the coping range and the limit of a tolerable climate. Thresholds, or assessment endpoints, may be a fundamental property of a system (i.e. a biophysical discontinuity in space and time, such as the water level at which a river bursts its banks, or the wind speed that leads to the felling of large areas of forest), or behavioural (i.e. a point at which individuals, or society at large, responds to an issue by a change in behaviour that has an economic or social outcome). The concept of assessment endpoints is helpful in determining coping ranges for systems that are less readily quantifiable, such as biodiversity.

Atlantic Ocean Circulation (Gulf Stream)

Large-scale circulation in the ocean that transforms low-density upper ocean waters to higher-density intermediate and deep waters, and returns those waters back to the upper ocean. The Atlantic Ocean Circulation is driven by high surface water densities and mechanical forces such as wind or tides. It is sometimes less precisely referred to as the Gulf Stream.

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The baseline (or reference) period defines the climatology against which future changes are projected. For details of the baseline used for UKCP09, see the UKCP09 Index.

Building adaptive capacity (BAC)

BAC involves developing the institutional capacity to respond effectively to climate change. This means compiling the requisite information and creating the regulatory, institutional and managerial conditions that are needed for adaptation actions to be undertaken.

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Change factor

A set of change values calculated for all Weather Generator variables for a specified future time period. The change factors are obtained by calculating the difference between the modelled baseline climate and the climate given in the UKCP09 probabilistic projections, for each 25 km grid square.


Climate is typically defined as the average weather (or more rigorously a statistical description of the average in terms of the mean and variability) over a period of time, usually 30 years. These quantities are most often surface variables such as temperature, precipitation, and wind. Climate in a wider sense is the state, including a statistical description, of the climate system.

Climate Change Act 2008

The Climate Change Act 2008 is a legally binding long-term framework to cut carbon emissions, along with a framework for building the UK’s ability to adapt to climate change.

Climate change risk

Additional risk to investments (such as buildings and infrastructure) and actions from potential climate change impacts.

Climate Change Risk Assessment

The Climate Change Act commits the UK Government to carry out an assessment of the risks to the UK of climate change every five years (Climate Change Risk Assessment or CCRA). The first cycle reported to Parliament in January 2012.

Climate change impact

A specific change in a system caused by its exposure to climate change. Impacts may be harmful (threat) or beneficial (opportunity).

Climate model

A simplified mathematical representation of the climate system based on the physical, chemical, and biological properties of its components, their interactions and feedbacks between them. See the UKCP09 Index for further details.

Committee on Climate Change

The Committee on Climate Change is an independent body established under the Climate Change Act (2008) that advises the UK Government on setting and meeting carbon budgets and on preparing for the impacts of climate change.

Contingency planning

A plan devised for a specific situation when things could go wrong. Contingency plans are often devised by those who want to be prepared for any eventuality.

Cooling degree days

An annual measure of the extent to which temperatures suggest that buildings may require some form of cooling (e.g. air conditioning), based on the daily temperature being above a specified threshold (22ºC). See the UKCP09 Index for further details.

Cost-benefit analysis (CBA)

A term used to describe the appraisal of the merits associated with each option by quantifying in monetary terms as many costs and benefits as possible, including items for which the market does not provide a satisfactory measure of value. CBA is designed to help one select the option which offers the greatest excess of benefits over costs.

Critical threshold

The point in a system at which sudden or rapid change occurs.

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Delivering adaptation actions (DAA)

DAA involves taking practical actions to either reduce vulnerability to climate risks, or to exploit positive opportunities.

Department for Energy and Climate Change (DECC)

DECC is responsible for all aspects of UK energy policy, and for tackling global climate change on behalf of the UK.

Department for Environment, Food and Rural Affairs (Defra)

Defra is the UK government department responsible for policy and regulations on the environment, food and farming, the countryside and pets and wildlife. Defra is also responsible for domestic adaptation to climate change.


The observed range of a climatic variable in a single calendar day or 24 hour period.


The process of reducing coarse spatial scale model output to a finer scale. See the UKCP09 Index for further details.

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El Niño

El Niño is a large scale ocean-atmospheric phenomenon characterised by unusually warm ocean temperatures in the Equatorial Pacific, as opposed to La Niña, which characterised by unusually cold ocean temperatures in the Equatorial Pacific. After the seasons, El Niño and La Niña are the single largest cause of year-to-year natural climate variability on Earth.


A set of simulations (each one an ensemble member) made by either adjusting parameters within plausible limits in the model, or starting the model from different initial conditions. While many parameters are constrained by observations, some are subject to considerable uncertainty. The best way to investigate this uncertainty is to run an ensemble experiment in which each relevant parameter combination is investigated. This is known as a perturbed physics ensemble. See the UKCP09 Index for further details.

Exposure unit

Represents the system considered to be at risk, and may be defined in terms of geographical extent, location and distribution of a variety population of receptors at risk.

Extreme weather events

Extreme weather describes weather phenomena that are at the extremes of the historical distribution, especially severe or unseasonal weather. See the UKCP09 Index for further information.

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Glacial isostatic adjustment (GIA)

The vertical land movement associated with the melting of ice sheets.

Global climate model (GCM)

A General Circulation Model (GCM), more commonly called a global climate model, is a mathematical model of the general circulation of the planet’s atmosphere or oceans based on mathematic equations that represent physical processes. These equations are the basis for complex computer programs commonly used for simulating the atmosphere or oceans of the Earth. GCMs are widely applied for weather forecasting, understanding the climate, and projecting climate change. HadCM3 is the GCM used for the UKCP09 scenarios. See the UKCP09 Index for further information

Greenhouse gas

A gas within the atmosphere which absorbs and emits energy radiated by the Earth. Carbon dioxide (CO2) is the most important greenhouse gas being emitted by humans. The greenhouse effect is natural and without it the Earth would be considerably colder. The primary greenhouse gases are: Water vapour (H2O), carbon dioxide (CO2), nitrous oxide (N2O), methane (Ch2) and ozone (O3). Further information can be found in the UKCP09 Index.

Gulf Stream

See Atlantic Ocean Circulation.

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Heating degree days

An annual measure of the extent to which daily temperatures suggest that buildings may require some form of space heating, based on the daily temperature being below a certain threshold (15.5ºC). See the UKCP09 Index for further details.

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Inter-annual variability (IAV)

Climatic variations with periods of longer than 1 year (and normally less than 10 years).

Intergovernmental Panel on Climate Change

The Intergovernmental Panel on Climate Change (IPCC) is an international forum of experts brought together by the United Nations to undertake periodical assessments addressing how climate will change, what its impacts may be and how we can respond. It was originally formed in 1988 and published its Fourth Assessment Report (AR4) in 2007.

Internal climate variability

Variations in climate due to natural internal processes within the climate system. The Earth’s climate is not static, but varies on time scales of decades to millennia in response to interactions between the ocean and the atmosphere, changes in the Earth’s orbit, fluctuations in energy received from the sun and volcanic eruptions.

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Kyoto protocol

International legally-binding agreement adopted at Kyoto in 1997 under the UN Framework Convention on Climate Change to reduce the emissions of greenhouse gases. The Protocol was ratified in 2005.

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La Niña

El Niño is a large scale ocean-atmospheric phenomenon characterised by unusually warm ocean temperatures in the Equatorial Pacific, as opposed to La Niña, which characterised by unusually cold ocean temperatures in the Equatorial Pacific. After the seasons, El Niño and La Niña are the single largest cause of year-to-year natural climate variability on Earth.

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Action or investment that enhances vulnerability to climate change impacts rather than reducing them.

Marine Climate Change Impacts Partnership (MCCIP)

The Marine Climate Change Impacts Partnership (MCCIP) provides a co-ordinating framework within the UK for the transfer of high-quality marine climate change impacts evidence and advice to policy advisors and decision-makers.


Reducing greenhouse gas emissions in order to slow or stop global climate change. For further information, see DECC Mitigation page.

Multi-criteria analysis (MCA)

Any structured approach used to determine overall preference amongst alternative options, where the options accomplish multiple objectives.

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Natural climate variability

Variations in climate due to either natural processes either internal to the climate system or due to external factors.

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Receptors represent important aspects of the exposure unit. In some cases, the exposure unit and receptor may be synonymous.


The term used to refer to the administrative regions of England and the devolved administrations of Northern Ireland, Scotland and Wales.

Regional Climate Model (RCM)

A regional climate model (RCM) is a climate model of higher resolution than a global climate model (GCM). It can be nested within a global model to provide more detailed simulations for a particular location. HadRM3 is the RCM used for the UKCP09 scenarios. For further information, go to the UKCP09 Index.


The ability of a social or natural system to absorb disturbances while retaining the same basic structure and ways of functioning, the capacity of self-organisation and the capacity to adapt to stress and change.

Return period

The average time between events of a given magnitude. A 100-year return period is the equivalent of the event that has a 1 per cent probability of occurring in any given year. For further information, see the UKCP09 Index.

Risk assessment

The structured analysis of hazards and impacts to provide information for decision making. Risk assessment usually relates to a particular system which may be individual, population, infrastructure, building or environmental asset etc. The process usually involves identifying hazards that could have an impact, assessing the likelihoods and severities of impacts, and assessing the significance of the risk. This is usually related to the probability multiplied by the severity of the impact.


The term used to refer to the ability to cope with or recover from a range of inputs and situations in a given environment.

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A description of a plausible future state which is not associated with an ascribed likelihood. UKCP09 uses emissions scenarios to underpin probabilistic climate projections. For further information see the UKCP09 Index.


One of the averaging periods for which climate changes projections are provided in UKCP09, based on projected changes to seasonal average conditions. Winter is the average for the three months December, January and February; Spring is the average for March, April and May; Summer is the average for June, July and August; and Autumn is the average for September, October and November.

Special Report on Emissions Scenarios (SRES)

A report published by the IPCC which defined different future emissions pathways based on potential storylines for the future, including population, energy use etc. For more information, see the UKCP09 Index.

Storm surge

The temporary increase, at a particular place, in the height of the sea due to extreme meteorological conditions (low atmospheric pressure and/or strong winds). The storm surge is defined as being the excess above the level expected from the tidal variation alone at that time and place. See the UKCP09 Index for further details.

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Thermal growing season

The growing season is the period of time each year during which plants can grow. The length of the thermal growing season is defined as the longest period within a year that satisfies the two requirements of:

  • beginning at the start of a period when daily-average temperature is greater than 5.5°C for five consecutive days, and
  • ending on the day prior to the first subsequent period when daily-average temperature is less than 5.5°C for five consecutive days

The thermal definition of growing season length does not consider the impact of other climatic variables (e.g. wind, precipitation).

Time period

A period of 30 years over which climate averages are calculated. For details of the time periods used in UKCP09, see the UKCP09 Index.

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Uncertainty refers to a state of having limited knowledge. Uncertainty can result from lack of information or from disagreement over what is known or even knowable. Uncertainty may arise from many sources, such as quantifiable errors in data, or uncertain projections of human behaviour. Uncertainty can be represented by quantitative measures or by qualitative statements. See the UKCP09 Index for further details.

United Nations Framework Convention on Climate Change (UNFCCC)

The United Nations Framework Convention on Climate Change (UNFCCC) sets an overall framework for intergovernmental efforts to tackle the challenge posed by climate change. It recognises that the climate system is a shared resource whose stability can be affected by industrial and other emissions of carbon dioxide and other greenhouse gases.

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Vulnerability is the degree to which a system is susceptible to, and unable to cope with, adverse effects of climate change, including climate 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.

Vulnerability assessment

A vulnerability assessment is the process of identifying, quantifying, and prioritising (or ranking) the vulnerabilities in a system.

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Weather refers to the state of the atmosphere with regard to temperature, cloudiness, rainfall, wind, and other meteorological conditions.

Weather Generator

A Weather Generator is a statistical method of creating projections of future daily (or sub-daily) climate that are consistent with climate change projections for longer temporal averaging periods (e.g. monthly or seasonal). It DOES NOT generate weather forecasts for specific future dates! For further information, see the UKCP09 Index.