IPCC Fourth Assessment Report: Climate Change 2007

Glossary A-D

Acclimatisation The physiological adaptation to climatic variations.

Active layer The top layer of soil or rock in permafrost that is subjected to seasonal freezing and thawing.

Adaptability See adaptive capacity.

Autonomous adaptation – Adaptation that does not constitute a conscious response to climatic stimuli but is triggered by ecological changes in natural systems and by market orwelfare changes in human systems. Also referred to as spontaneous adaptation.

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

Adaptation benefits The avoided damage costs or the accrued benefits following the adoption and implementation of adaptation measures.

Adaptation costs Costs of planning, preparing for, facilitating, and implementingadaptation measures, including transition costs.

Adaptive capacity (in relation to climate change impacts) The ability of a system to adjust to climate change (including climate variability and extremes) to moderate potential damages, to take advantage of opportunities, or to cope with the consequences.

Aerosols A collection of air-borne solid or liquid particles, with a typical size between 0.01 and 10 µm, that reside in the atmosphere for at least several hours. Aerosols may be of either natural or anthropogenic origin. Aerosols may influence climate in two ways: directly through scattering and absorbing radiation, and indirectly through acting as condensation nuclei for cloud formation or modifying the optical properties and lifetime of clouds.

Afforestation Direct human-induced conversion of land that has not been forested for a period of at least 50 years to forested land through planting, seeding and/or the human-induced promotion of natural seed sources. See also reforestation and deforestation. For a discussion of the term forest and related terms such as afforestation, reforestation and deforestation, see the IPCC Special Report on Land Use, Land-Use Change, and Forestry (IPCC,2000).


Albedo The fraction of solar radiation reflected by a surface or object, often expressed as a percentage. Snow-covered surfaces have a high albedo; the albedo of soils ranges from high to low; vegetation-covered surfaces and oceans have a low albedo. The Earth’s albedo varies mainly through varying cloudiness, snow ,ice, leaf area, and land-cover changes.

Algae Photosynthetic, often microscopic and planktonic, organisms occurring in marine and freshwater ecosystems.

Algal bloom A reproductive explosion of algae in a lake, river or ocean.

Alpine The biogeographic zone made up of slopes above the tree line characterised by the presence of rosette-forming herbaceous plants and low, shrubby, slow-growing woody plants.

Anthropogenic Resulting from or produced by human beings.

AOGCM See climate model.





Arid region A land region of low rainfall, where ‘low’ is widely accepted to be <250 mm precipitation per year.


Attribution See Detection and attribution.

Baseline/reference The baseline (or reference) is the state against which change is measured. It might be a ‘current baseline’, in which case it represents observable, present-day conditions. It might also be a ‘future baseline’, which is a projected future set of conditions excluding the driving factor of interest. Alternative interpretations of the reference conditions can give rise to multiple baselines.

Basin The drainage area of a stream, river or lake.







Biota All living organisms of an area; the flora and fauna considered as a unit.

Bog Peat-accumulating acidic wetland.






Calcite A calcium carbonate (limestone) mineral, used by shell- or skeleton-forming, calcifying organisms such as foraminifera,some macroalgae, lobsters, crabs, sea urchins and starfish. Calcite is less sensitive to ocean acidification thanaragonite, also used by many marine organisms. See also aragonite and ocean acidification.




Carbon dioxide fertilisation The stimulation of plant photosynthesis due to elevated CO2concentrations, leading to either enhanced productivity and/or efficiency of primary production. In general, C3 plants show a larger response to elevated CO2 than C4 plants.

Carbon sequestration The process of increasing the carbon content of a reservoir/pool other than the atmosphere.

Catchment An area that collects and drains rainwater.




Climate Climate in a narrow sense is usually defined as the ‘average weather’, or more rigorously, as the statistical description interms of the mean and variability of relevant quantities over a period of time ranging from months to thousands or millions of 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. The classical period of time is 30 years, as defined by the World Meteorological Organization (WMO).

Climate change Climate change refers to any change in climate over time, whether due to natural variability or as a result of human activity. This usage differs from that in the United Nations Framework Convention on Climate Change (UNFCCC), which defines ‘climate change’ as: ‘a change of climate which is attributed directly or indirectly to human activity that alters the composition of the global atmosphere and which is in addition to natural climate variability observed over comparable time periods’. See also climate variability.

Climate change commitment Due to the thermal inertia of the ocean and slow processes in the biosphere, the cryosphere and land surfaces, the climate would continue to change even if the atmospheric composition was held fixed at today’s values. Past change in atmospheric composition leads to a ‘committed’ climate change which continues for as long as a radiative imbalance persists and until all components of the climate system have adjusted to a new state. The further change in temperature after the composition of the atmosphere is held constant is referred to as the committed warming or warming commitment. Climate change commitment includes other future changes, for example in the hydrological cycle, in extreme weather events, and in sea-level rise.


Climate prediction A climate prediction or climate forecast is the result of an attempt to produce an estimate of the actual evolution of the climate in the future, e.g., at seasonal, interannual or long-term time scales. See also climate projection and climate (change) scenario.

Climate projection The calculated response of the climate system to emissions or concentration scenarios of greenhouse gases and aerosols, or radiative forcing scenarios, often based on simulations by climate models. Climate projections are distinguished from climate predictions, in that the former critically depend on the emissions/concentration/radiative forcing scenario used, and therefore on highly uncertain assumptions of future socio-economic and technological development.

Climate (change) scenario A plausible and often simplified representation of the future climate, based on an internally consistent set of climatological relationships and assumptions of radiative forcing, typically constructed for explicit use as input to climate change impact models. A ‘climate change scenario’ is the difference between a climate scenario and the current climate.

Climate sensitivity The equilibrium temperature rise that would occur for a doubling of CO2 concentration above pre-industrial levels.

Climate system The climate system is defined by the dynamics and interactions of five major components: atmosphere, hydrosphere, cryosphere, land surface, and biosphere. Climate system dynamics are driven by both internal and external forcing, such as volcanic eruptions, solar variations, or human-induced modifications to the planetary radiative balance, for instance via anthropogenic emissions of greenhouse gases and/or land-use changes.

Climate threshold The point at which external forcing of the climate system, such as the increasing atmospheric concentration of greenhouse gases, triggers a significant climatic or environmental event which is considered unalterable, or recoverable only on very long times-cales, such as widespread bleaching of corals or a collapse of oceanic circulation systems.


CO2 fertilisation See carbon dioxide fertilisation.

Coastal squeeze The squeeze of coastal ecosystems (e.g., salt marshes, mangroves and mud and sand flats) between rising sea levels and naturally or artificially fixed shorelines, including hard engineering defences (see Chapter 6).

Coccolithophores Single-celled microscopic phytoplankton algae which construct shell-like structures from calcite (a form of calcium carbonate). See also calcite and ocean acidification.

Committed to extinction This term describes a species with dwindling population that is in the process of inescapably becoming extinct in the absence of human intervention. See also extinction.

Communicable disease An infectious disease caused by transmission of an infective biological agent (virus, bacterium, protozoan, or multicellular macroparasite).

Confidence In this Report, the level of confidence in a statement is expressed using a standard terminology defined in the Introduction. See also uncertainty.

Control run A model run carried out to provide a ‘baseline’ for comparison with climate-change experiments. The control run uses constant values for the radiative forcing due to greenhouse gases and anthropogenic aerosols appropriate to pre-industrial conditions.

Coral The term ‘coral’ has several meanings, but is usually the common name for the Order Scleractinia, all members of which have hard limestone skeletons, and which are divided into reef-building and non-reef-building, or cold- and warm-water corals.

Coral bleaching The paling in colour which results if a coral loses its symbiotic, energy-providing, organisms.

Coral reefs Rock-like limestone (calcium carbonate) structures built by corals along ocean coasts (fringing reefs) or on top of shallow, submerged banks or shelves (barrier reefs, atolls), most conspicuous in tropical and sub-tropical oceans.

Cryosphere The component of the climate system consisting of all snow and ice (including permafrost) on and beneath the surface of the Earth and ocean.

Cryptogams An outdated but still-used term, denoting a group of diverse and taxonomically unrelated organisms, including fungi and lower plants such as algae, lichens, hornworts, liverworts, mosses and ferns.

Deforestation Natural or anthropogenic process that converts forest land to non-forest. See afforestation and reforestation.

Dengue fever An infectious viral disease spread by mosquitoes, often called breakbone fever because it is characterised by severe pain in the joints and back. Subsequent infections of the virus may lead to dengue haemorrhagic fever (DHF) and dengue shock syndrome (DSS), which may be fatal.

Desert A region of very low rainfall, where ‘very low’ is widely accepted to be <100 mm per year.

Desertification Land degradation in arid, semi-arid, and dry sub-humid areas resulting from various factors, including climatic variations and human activities. Further, the United Nations Convention to Combat Desertification (UNCCD) defines land degradation as a reduction or loss in arid, semi-arid, and dry sub-humid areas of the biological or economic productivity and complexity of rainfed cropland, irrigated cropland, or range, pasture, forest and woodlands resulting from land uses or from a process or combination of processes, including those arising from human activities and habitation patterns, such as: (i) soil erosion caused by wind and/or water; (ii) deterioration of the physical, chemical, and biological or economic properties of soil; and (iii) long-term loss of natural vegetation.

Detection and attribution Detection of change in a system (natural or human) is the process of demonstrating that the system has changed in some defined statistical sense, without providing a reason for that change.
Attribution of such an observed change in a system to anthropogenic climate change is usually a two-stage process. First, the observed change in the system must be demonstrated to be associated with an observed regional climate change with a specified degree of confidence. Second, a measurable portion of the observed regional climate change, or the associated observed change in the system, must be attributed to anthropogenic climate forcing with a similar degree of confidence. Confidence in such joint attribution statements must be lower than the confidence in either of the individual attribution steps alone due to the combination of two separate statistical assessments.

Diadromous Fish that travel between salt water and freshwater.

Discount rate The degree to which consumption now is preferred to consumption one year hence, with prices held constant, but average incomes rising in line with GDP per capita.

Disturbance regime Frequency, intensity, and types of disturbances, such as fires, insect or pest outbreaks, floods and droughts.

Downscaling A method that derives local- to regional-scale (10 to 100 km ) information from larger-scale models or data analyses.

Drought The phenomenon that exists when precipitation is significantly below normal recorded levels, causing serious hydrological imbalances that often adversely affect land resources and production systems.

Dyke A human-made wall or embankment along a shore to prevent flooding of low-lying land.

Dynamic global vegetation model (DGVM) Models that simulate vegetation development and dynamics through space and time, as driven by climate and other environmental changes.