IPCC Report.pdf - Adam Curry

Annex IIGlossary of Termswater vapor) to temperature in a system in which two phases of thesubstance (water) are in equilibrium.ClimateClimate in a narrow sense is usually defined as the average weather, ormore rigorously, as the statistical description in terms of the mean andvariability of relevant quantities over a period of time ranging from monthsto thousands or millions of years. The classical period for averagingthese variables is 30 years, as defined by the World MeteorologicalOrganization. The relevant quantities are most often surface variablessuch as temperature, precipitation, and wind. Climate in a wider senseis the state, including a statistical description, of the climate system. Invarious chapters in this report different averaging periods, such as aperiod of 20 years, are also used.Climate changeA change in the state of the climate that can be identified (e.g., byusing statistical tests) by changes in the mean and/or the variabilityof its properties and that persists for an extended period, typicallydecades or longer. Climate change may be due to natural internalprocesses or external forcings, or to persistent anthropogenic changesin the composition of the atmosphere or in land use. 1 See also Climatevariability and Detection and attribution.Climate extreme (extreme weather or climate event)The occurrence of a value of a weather or climate variable above (orbelow) a threshold value near the upper (or lower) ends of the range ofobserved values of the variable. For simplicity, both extreme weatherevents and extreme climate events are referred to collectively as ‘climateextremes.’ The full definition is provided in Section 3.1.2.Climate feedbackAn interaction mechanism between processes in the climate system iscalled a climate feedback when the result of an initial process triggerschanges in a second process that in turn influences the initial one. Apositive feedback intensifies the original process, and a negative feedbackreduces it.Climate modelA numerical representation of the climate system that is based on thephysical, chemical, and biological properties of its components, theirinteractions, and feedback processes, and that accounts for all or some ofits known properties. The climate system can be represented by modelsof varying complexity, that is, for any one component or combination of____________1 This definition differs from that in the United Nations Framework Convention onClimate Change (UNFCCC), where climate change is defined as: “a change of climatewhich is attributed directly or indirectly to human activity that alters the compositionof the global atmosphere and which is in addition to natural climate variabilityobserved over comparable time periods.” The UNFCCC thus makes a distinctionbetween climate change attributable to human activities altering the atmosphericcomposition, and climate variability attributable to natural causes.components a spectrum or hierarchy of models can be identified, differingin such aspects as the number of spatial dimensions, the extent to whichphysical, chemical, or biological processes are explicitly represented, orthe level at which empirical parameterizations are involved. CoupledAtmosphere-Ocean Global Climate Models (AOGCMs), also referred to asAtmosphere-Ocean General Circulation Models, provide a representationof the climate system that is near the most comprehensive end of thespectrum currently available. There is an evolution toward more complexmodels with interactive chemistry and biology. Climate models areapplied as a research tool to study and simulate the climate, and foroperational purposes, including monthly, seasonal, and interannual climatepredictions.Climate projectionA projection of the response of the climate system to emissions orconcentration scenarios of greenhouse gases and aerosols, or radiativeforcing scenarios, often based upon simulations by climate models.Climate projections are distinguished from climate predictions in orderto emphasize that climate projections depend upon the emission/concentration/radiative-forcing scenario used, which are based onassumptions concerning, e.g., future socioeconomic and technologicaldevelopments that may or may not be realized and are thereforesubject to substantial uncertainty.Climate scenarioA plausible and often simplified representation of the future climate,based on an internally consistent set of climatological relationships thathas been constructed for explicit use in investigating the potentialconsequences of anthropogenic climate change, often serving as inputto impact models. Climate projections often serve as the raw materialfor constructing climate scenarios, but climate scenarios usually requireadditional information such as about the observed current climate.Climate systemThe climate system is the highly complex system consisting of five majorcomponents: the atmosphere, the oceans, the cryosphere, the landsurface, the biosphere, and the interactions between them. The climatesystem evolves in time under the influence of its own internal dynamicsand because of external forcings such as volcanic eruptions, solarvariations, and anthropogenic forcings such as the changing compositionof the atmosphere and land use change.Climate thresholdA critical limit within the climate system that induces a non-linearresponse to a given forcing. See also Abrupt climate change.Climate variabilityClimate variability refers to variations in the mean state and otherstatistics (such as standard deviations, the occurrence of extremes, etc.) ofthe climate at all spatial and temporal scales beyond that of individualweather events. Variability may be due to natural internal processeswithin the climate system (internal variability), or to variations in natural557