Glossary E-O
Ecosystem A system of living organisms interacting with each other and their physical environment. The boundaries of what could be called an ecosystem are somewhat arbitrary, depending on the focus of interest or study. Thus, the extent of an ecosystem may range from very small spatial scales to, ultimately, the entire Earth.
Efficacy A measure of how effective a radiative forcing from a given anthropogenic or natural mechanism is at changing the equilibrium global surface temperature compared to an equivalent radiative forcing from carbon dioxide. A carbon dioxide increase by definition has an efficacy of 1.0.
Ekman pumping Frictional stress at the surface between two fluids (atmosphere and ocean) or between a fluid and the adjacent solid surface (Earth’s surface) forces a circulation. When the resulting mass transport is converging, mass conservation requires a vertical flow away from the surface. This is called Ekman pumping. The opposite effect, in case of divergence, is called Ekman suction. The effect is important in both the atmosphere and the ocean.
Ekman transport The total transport resulting from a balance between the Coriolis force and the frictional stress due to the action of the wind on the ocean surface. See also Ekman pumping.
El Niño-Southern Oscillation (ENSO) The term El Niño was initially used to describe a warm-water current that periodically flows along the coast of Ecuador and Perú, disrupting the local fishery. It has since become identified with a basin-wide warming of the tropical Pacific Ocean east of the dateline. This oceanic event is associated with a fluctuation of a global-scale tropical and subtropical surface pressure pattern called the Southern Oscillation. This coupled atmosphere-ocean phenomenon, with preferred time scales of two to about seven years, is collectively known as the El Niño-Southern Oscillation (ENSO). It is often measured by the surface pressure anomaly difference between Darwin and Tahiti and the sea surface temperatures in the central and eastern equatorial Pacific. During an ENSO event, the prevailing trade winds weaken, reducing upwelling and altering ocean currents such that the sea surface temperatures warm, further weakening the trade winds. This event has a great impact on the wind, sea surface temperature and precipitation patterns in the tropical Pacific. It has climatic effects throughout the Pacific region and in many other parts of the world, through globalteleconnections. The cold phase of ENSO is called La Niña.
Emission scenario A plausible representation of the future development of emissions of substances that are potentially radiatively active (e.g., greenhouse gases, aerosols), based on a coherent and internally consistent set of assumptions about driving forces (such as demographic and socioeconomic development, technological change) and their key relationships. Concentration scenarios, derived from emission scenarios, are used as input to a climate model to compute climate projections. In IPCC (1992) a set of emission scenarios was presented which were used as a basis for the climate projections in IPCC (1996). These emission scenarios are referred to as the IS92 scenarios. In the IPCC Special Report on Emission Scenarios (Nakićenović and Swart, 2000) new emission scenarios, the so-called SRES scenarios, were published, some of which were used, among others, as a basis for the climate projections presented in Chapters 9 to 11 of IPCC (2001) and Chapters 10 and 11 of this report. For the meaning of some terms related to these scenarios, seeSRES scenarios.
Energy balance The difference between the total incoming and total outgoing energy. If this balance is positive, warming occurs; if it is negative, cooling occurs. Averaged over the globe and over long time periods, this balance must be zero. Because the climate systemderives virtually all its energy from the Sun, zero balance implies that, globally, the amount of incoming solar radiation on average must be equal to the sum of the outgoing reflected solar radiation and the outgoing thermal infrared radiation emitted by the climate system. A perturbation of this global radiation balance, be it anthropogenic or natural, is called radiative forcing.
Ensemble A group of parallel model simulations used for climate projections. Variation of the results across the ensemble members gives an estimate of uncertainty. Ensembles made with the same model but different initial conditions only characterise the uncertainty associated with internal climate variability, whereas multi-model ensembles including simulations by several models also include the impact of model differences. Perturbed-parameter ensembles, in which model parameters are varied in a systematic manner, aim to produce a more objective estimate of modelling uncertainty than is possible with traditional multi-model ensembles.
Equilibrium and transient climate experiment An equilibrium climate experiment is an experiment in which a climate modelis allowed to fully adjust to a change in radiative forcing. Such experiments provide information on the difference between the initial and final states of the model, but not on the time-dependent response. If the forcing is allowed to evolve gradually according to a prescribedemission scenario, the time-dependent response of a climate model may be analysed. Such an experiment is called a transient climate experiment. See Climate projection.
Equilibrium line The boundary between the region on a glacier where there is a net annual loss of ice mass (ablation area) and that where there is a net annual gain (accumulation area). The altitude of this boundary is referred to as equilibrium line altitude.
Equivalent carbon dioxide (CO2) concentration
The concentration of carbon dioxide that would cause the same amount of radiative forcing as a given mixture of carbon dioxide and other greenhouse gases.
Equivalent carbon dioxide (CO2) emission The amount of carbon dioxide emission that would cause the same integratedradiative forcing, over a given time horizon, as an emitted amount of a well mixed greenhouse gas or a mixture of well mixed greenhouse gases. The equivalent carbon dioxide emission is obtained by multiplying the emission of a well mixed greenhouse gas by itsGlobal Warming Potential for the given time horizon. For a mix of greenhouse gases it is obtained by summing the equivalent carbon dioxide emissions of each gas. Equivalent carbon dioxide emission is a standard and useful metric for comparing emissions of different greenhouse gases but does not imply exact equivalence of the corresponding climate change responses (see Section 2.10).
Evapotranspiration The combined process of evaporation from the Earth’s surface and transpiration from vegetation.
External forcing External forcing refers to a forcing agent outside the climate system causing a change in the climate system. Volcanic eruptions, solar variations and anthropogenic changes in the composition of the atmosphere and land use change are external forcings.
Extreme weather event An extreme weather event is an event that is rare at a particular place and time of year. Definitions of rare vary, but an extreme weather event would normally be as rare as or rarer than the 10th or 90th percentile of the observed probability density function. By definition, the characteristics of what is called extreme weather may vary from place to place in an absolute sense. Single extreme events cannot be simply and directly attributed to anthropogenic climate change, as there is always a finite chance the event in question might have occurred naturally. When a pattern of extreme weather persists for some time, such as a season, it may be classed as an extreme climate event, especially if it yields an average or total that is itself extreme (e.g., drought or heavy rainfall over a season).
Faculae Bright patches on the Sun. The area covered by faculae is greater during periods of high solar activity.
Feedback See Climate feedback.
Fingerprint The climate response pattern in space and/or time to a specific forcing is commonly referred to as a fingerprint. Fingerprints are used to detect the presence of this response in observations and are typically estimated using forced climate modelsimulations.
Flux adjustment To avoid the problem of coupled Atmosphere-Ocean General Circulation Models (AOGCMs) drifting into some unrealistic climate state, adjustment terms can be applied to the atmosphere-ocean fluxes of heat and moisture (and sometimes the surface stresses resulting from the effect of the wind on the ocean surface) before these fluxes are imposed on the model ocean and atmosphere. Because these adjustments are pre-computed and therefore independent of the coupled model integration, they are uncorrelated with the anomalies that develop during the integration. Chapter 8 of this report concludes that most models used in this report (Fourth Assessment Report AOGCMs) do not use flux adjustments, and that in general, fewer models use them.
Forest A vegetation type dominated by trees. Many definitions of the term forest are in use throughout the world, reflecting wide differences in biogeophysical conditions, social structure and economics. For a discussion of the term forest and related terms such asafforestation, reforestation and deforestation see the IPCC Report on Land Use, Land-Use Change and Forestry (IPCC, 2000). See also the Report on Definitions and Methodological Options to Inventory Emissions from Direct Human-induced Degradation of Forests and Devegetation of Other Vegetation Types (IPCC, 2003).
Fossil fuel emissions Emissions of greenhouse gases (in particular carbon dioxide) resulting from the combustion of fuels from fossil carbon deposits such as oil, gas and coal.
Framework Convention on Climate Change See United Nations Framework Convention on Climate Change(UNFCCC).
Free atmosphere
The atmospheric layer that is negligibly affected by friction against the Earth’s surface, and which is above the atmospheric boundary layer.
Frozen ground Soil or rock in which part or all of the pore water is frozen (Van Everdingen, 1998). Frozen ground includespermafrost. Ground that freezes and thaws annually is called seasonally frozen ground.
General circulation The large-scale motions of the atmosphere and the ocean as a consequence of differential heating on a rotating Earth, which tend to restore the energy balance of the system through transport of heat and momentum.
General Circulation Model (GCM) See Climate model.
Geoid The equipotential surface (i.e., having the same gravity potential at each point) that best fits the mean sea level (see relative sea level) in the absence of astronomical tides; ocean circulations; hydrological, cryospheric and atmospheric effects; Earth rotation variations and polar motion; nutation and precession; tectonics and other effects such as post-glacial rebound. The geoid is global and extends over continents, oceans and ice sheets, and at present includes the effect of the permanent tides (zero-frequency gravitational effect from the Sun and the Moon). It is the surface of reference for astronomical observations, geodetic levelling, and for ocean, hydrological, glaciological and climate modelling. In practice, there exist various operational definitions of the geoid, depending on the way the time-variable effects mentioned above are modelled.
Geostrophic winds or currents A wind or current that is in balance with the horizontal pressure gradient and the Coriolis force, and thus is outside of the influence of friction. Thus, the wind or current is directly parallel to isobars and its speed is inversely proportional to the spacing of the isobaric contours.
Glacial isostatic adjustment See Post-glacial rebound.
Glacier A mass of land ice that flows downhill under gravity (through internal deformation and/or sliding at the base) and is constrained by internal stress and friction at the base and sides. A glacier is maintained by accumulation of snow at high altitudes, balanced by melting at low altitudes or discharge into the sea. See Equilibrium line; Mass balance.
Global dimming Global dimming refers to perceived widespread reduction of solar radiation received at the surface of the Earth from about the year 1961 to around 1990.
Global surface temperature The global surface temperature is an estimate of the global mean surface air temperature. However, for changes over time, only anomalies, as departures from a climatology, are used, most commonly based on the area-weighted global average of the sea surface temperature anomaly and land surface air temperature anomaly.
Global Warming Potential (GWP) An index, based upon radiative properties of well-mixed greenhouse gases, measuring theradiative forcing of a unit mass of a given well-mixed greenhouse gas in the present-day atmosphere integrated over a chosen time horizon, relative to that of carbon dioxide. The GWP represents the combined effect of the differing times these gases remain in the atmosphere and their relative effectiveness in absorbing outgoing thermal infrared radiation. The Kyoto Protocol is based on GWPs from pulse emissions over a 100-year time frame.
Greenhouse effect Greenhouse gases effectively absorb thermal infrared radiation, emitted by the Earth’s surface, by theatmosphere itself due to the same gases, and by clouds. Atmospheric radiation is emitted to all sides, including downward to the Earth’s surface. Thus, greenhouse gases trap heat within the surface-troposphere system. This is called the greenhouse effect. Thermal infrared radiation in the troposphere is strongly coupled to the temperature of the atmosphere at the altitude at which it is emitted. In the troposphere, the temperature generally decreases with height. Effectively, infrared radiation emitted to space originates from an altitude with a temperature of, on average, –19°C, in balance with the net incoming solar radiation, whereas the Earth’s surface is kept at a much higher temperature of, on average, +14°C. An increase in the concentration of greenhouse gases leads to an increased infrared opacity of the atmosphere, and therefore to an effective radiation into space from a higher altitude at a lower temperature. This causes aradiative forcing that leads to an enhancement of the greenhouse effect, the so-called enhanced greenhouse effect.
Greenhouse gas (GHG) Greenhouse gases are those gaseous constituents of the atmosphere, both natural andanthropogenic, that absorb and emit radiation at specific wavelengths within the spectrum of thermal infrared radiation emitted by the Earth’s surface, the atmosphere itself, and by clouds. This property causes the greenhouse effect. Water vapour (H2O), carbon dioxide (CO2), nitrous oxide (N2O), methane (CH4) and ozone (O3) are the primary greenhouse gases in the Earth’s atmosphere. Moreover, there are a number of entirely human-made greenhouse gases in the atmosphere, such as the halocarbons and other chlorine- and bromine-containing substances, dealt with under the Montreal Protocol. Beside CO2, N2O and CH4, the Kyoto Protocol deals with the greenhouse gases sulphur hexafluoride (SF6), hydrofluorocarbons (HFCs) and perfluorocarbons (PFCs).
Gross Primary Production (GPP) The amount of energy fixed from the atmosphere through photosynthesis.
Ground ice A general term referring to all types of ice contained in freezing and seasonally frozen ground and permafrost (Van Everdingen, 1998).
Ground temperature The temperature of the ground near the surface (often within the first 10 cm). It is often called soil temperature.
Grounding line/zone The junction between a glacier or ice sheet and ice shelf; the place where ice starts to float.
Gyre Basin-scale ocean horizontal circulation pattern with slow flow circulating around the ocean basin, closed by a strong and narrow (100–200 km wide) boundary current on the western side. The subtropical gyres in each ocean are associated with high pressure in the centre of the gyres; the subpolar gyres are associated with low pressure.
Hadley Circulation A direct, thermally driven overturning cell in the atmosphere consisting of poleward flow in the uppertroposphere, subsiding air into the subtropical anticyclones, return flow as part of the trade winds near the surface, and with rising air near the equator in the so-called Inter-Tropical Convergence Zone.
Halocarbons A collective term for the group of partially halogenated organic species, including the chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs), halons, methyl chloride, methyl bromide, etc. Many of the halocarbons have large Global Warming Potentials. The chlorine- and bromine-containing halocarbons are also involved in the depletion of theozone layer.
Halosteric See Sea level change.
HCFC See Halocarbons.
HFC See Halocarbons.
Heterotrophic respiration The conversion of organic matter to carbon dioxide by organisms other than plants.
Holocene The Holocene geological epoch is the latter of two Quaternary epochs, extending from about 11.6 ka to and including the present.
Hydrosphere The component of the climate system comprising liquid surface and subterranean water, such as oceans, seas, rivers, fresh water lakes, underground water, etc.
Ice age An ice age or glacial period is characterised by a long-term reduction in the temperature of the Earth’s climate, resulting in growth of continental ice sheets and mountain glaciers (glaciation).
Ice cap A dome shaped ice mass, usually covering a highland area, which is considerably smaller in extent than an ice sheet.
Ice core A cylinder of ice drilled out of a glacier or ice sheet.
Ice sheet A mass of land ice that is sufficiently deep to cover most of the underlying bedrock topography, so that its shape is mainly determined by its dynamics (the flow of the ice as it deforms internally and/or slides at its base). An ice sheet flows outward from a high central ice plateau with a small average surface slope. The margins usually slope more steeply, and most ice is discharged through fast-flowing ice streams or outlet glaciers, in some cases into the sea or into ice shelves floating on the sea. There are only three large ice sheets in the modern world, one on Greenland and two on Antarctica, the East and West Antarctic Ice Sheets, divided by the Transantarctic Mountains. During glacial periods there were others.
Ice shelf A floating slab of ice of considerable thickness extending from the coast (usually of great horizontal extent with a level or gently sloping surface), often filling embayments in the coastline of the ice sheets. Nearly all ice shelves are in Antarctica, where most of the ice discharged seaward flows into ice shelves.
Ice stream A stream of ice flowing faster than the surrounding ice sheet. It can be thought of as a glacier flowing between walls of slower-moving ice instead of rock.
Indirect aerosol effect Aerosols may lead to an indirect radiative forcing of the climate system through acting as cloud condensation nuclei or modifying the optical properties and lifetime of clouds. Two indirect effects are distinguished:
Cloud albedo effect A radiative forcing induced by an increase in anthropogenic aerosols that cause an initial increase in droplet concentration and a decrease in droplet size for fixed liquid water content, leading to an increase in cloud albedo. This effect is also known as the first indirect effect or Twomey effect.
Cloud lifetime effect A forcing induced by an increase in anthropogenic aerosols that cause a decrease in droplet size, reducing the precipitation efficiency, thereby modifying the liquid water content, cloud thickness and cloud life time. This effect is also known as the second indirect effect or Albrecht effect.
Apart from these indirect effects, aerosols may have a semi-direct effect. This refers to the absorption of solar radiation by absorbing aerosol, which heats the air and tends to increase the static stability relative to the surface. It may also cause evaporation of cloud droplets.
Industrial revolution A period of rapid industrial growth with far-reaching social and economic consequences, beginning in Britain during the second half of the eighteenth century and spreading to Europe and later to other countries including the United States. The invention of the steam engine was an important trigger of this development. The industrial revolution marks the beginning of a strong increase in the use of fossil fuels and emission of, in particular, fossil carbon dioxide. In this report the terms pre-industrial and industrial refer, somewhat arbitrarily, to the periods before and after 1750, respectively.
Infrared radiation See Thermal infrared radiation.
Insolation The amount of solar radiation reaching the Earth by latitude and by season. Usually insolation refers to the radiation arriving at the top of the atmosphere. Sometimes it is specified as referring to the radiation arriving at the Earth’s surface. See also:Total Solar Irradiance.
Interglacials The warm periods between ice age glaciations. The previous interglacial, dated approximately from 129 to 116 ka, is referred to as the Last Interglacial (AMS, 2000)
Internal variability See Climate variability.
