Global Warming Glossary *

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Abrupt climate change - There are essentially two definitions of abrupt climate change: (1) In terms of physics, it is a transition of the climate system into a different mode on a time scale that is faster than the responsible forcing. (2) In terms of impacts, an abrupt change is one that takes place so rapidly and unexpectedly that human or natural systems have difficulty adapting to it.

Acclimatization - The physiological adaptation to climatic variations.

Active layer - The layer of ground that is subject to annual thawing and freezing in areas underlain by permafrost.

Adaptation - 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, including anticipatory, autonomous and planned adaptation.   ► Anticipatory adaptation - Adaptation that takes place before impacts of climate change are observed (also referred to as proactive adaptation).   ► 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 or welfare changes in human systems (also referred to as spontaneous adaptation).   ► Planned adaptation - Adaptation that 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 return to, maintain, or achieve a desired state.

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 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 implementing adaptation measures, including transition costs.

Adaptive capacity - 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.

Advection - Transport of water or air along with its properties (e.g., temperature, chemical tracers) by the motion of the fluid. Regarding the general distinction between advection and convection, the former describes the predominantly horizontal, large-scale motions of the atmosphere or ocean, while convection describes the predominantly vertical, locally induced motions.

Aerosols - A collection of airborne solid or liquid particles, with a typical size between 0.01 and 10 micrometer (~0.00000039 and ~0.00039 inch) that reside in the atmosphere for at least several hours. Aerosols may be of either natural or human origin. Aerosols may influence climate in several ways: directly through scattering and absorbing radiation, and indirectly by acting as cloud condensation nuclei or modifying the optical properties and lifetime of clouds (see Indirect aerosol effect).

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.

Aggregate impacts - Total impacts integrated across sectors and/or regions. The aggregation of impacts requires knowledge of (or assumptions about) the relative importance of impacts in different sectors and regions. Measures of aggregate impacts include, for example, the total number of people affected, or the total economic costs.

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 surface albedo of soils ranges from high to low, and vegetation-covered surfaces and oceans have a low albedo. The Earth's planetary 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.

Alliance of Small Island States (AOSIS) - Formed at the Second World Climate Conference (1990). AOSIS comprises small-island and low-lying coastal developing countries that are particularly vulnerable to the adverse consequences of climate change, such as sea-level rise, coral bleaching, and the increased frequency and intensity of tropical storms. With more than 35 states from the Atlantic, Caribbean, Indian Ocean, Mediterranean, and Pacific, AOSIS share common objectives on environmental and sustainable development matters in the UNFCCC process.

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

Ancillary benefits - Policies aimed at some target, e.g. climate change mitigation, may be paired with positive side effects, such as increased resource-use efficiency, reduced emissions of air pollutants associated with fossil fuel use, improved transportation, agriculture, land-use practices, employment, and fuel security. See also co-benefits.

Anthropogenic - Resulting from or produced by human beings.

Aquaculture - The managed cultivation of aquatic plants or animals such as salmon or shellfish held in captivity for the purpose of harvesting.

Aquifer - A stratum of permeable rock that bears water. An unconfined aquifer is recharged directly by local rainfall, rivers and lakes, and the rate of recharge will be influenced by the permeability of the overlying rocks and soils.

Aragonite - A calcium carbonate (limestone) mineral, used by shell- or skeleton-forming, calcifying organisms such as corals (warm- and coldwater corals), some macroalgae, pteropods (marine snails) and non-pteropod molluscs such as bivalves (e.g., clams, oysters), cephalopods (e.g., squids, octopuses). Aragonite is more sensitive to ocean acidification than calcite, also used by many marine organisms.

Arbovirus - Any of various viruses transmitted by blood-sucking arthropods (e.g., mosquitoes, ticks, etc.) and including the causative agents of dengue fever, yellow fever, and some types of encephalitis.

Arid region - A land region of low rainfall, where "low" is widely accepted to be <250 millimeters (~10 inches) precipitation per year.

Atmosphere - The gaseous envelope surrounding the Earth. The dry atmosphere consists almost entirely of nitrogen (78.1% volume mixing ratio) and oxygen (20.9% volume mixing ratio), together with a number of trace gases, such as argon (0.93% volume mixing ratio), helium and radiatively active greenhouse gases such as carbon dioxide (0.035% volume mixing ratio) and ozone. In addition, the atmosphere contains the greenhouse gas water vapor, whose amounts are highly variable but typically around 1% volume mixing ratio. The atmosphere also contains clouds and aerosols.

Attribution - The process of assigning causes to detected climate change, whether man-made or natural.

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