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Climate sensitivity is the equilibrium temperature change in response to changes of the radiative forcing. Therefore climate sensitivity depends on the initial climate state, but potentially can be accurately inferred from precise palaeoclimate data. Slow climate feedbacks, especially changes of ice sheet size and atmospheric CO2, amplify the total Earth system sensitivity by an amount that depends on the time scale considered. Although climate sensitivity is usually used in the context of radiative forcing by carbon dioxide (CO2), it is thought of as a general property of the climate system: the change in surface air temperature (ΔTs) following a unit change in radiative forcing (RF), and thus is expressed in units of °C/(W/m2). For this to be useful, the measure must be independent of the nature of the forcing (e.g. from greenhouse gases or solar variation); to first order this is indeed found to be so. The climate sensitivity specifically due to is often expressed as the temperature change in °C associated with a doubling of the concentration of carbon dioxide in Earth's atmosphere. For coupled atmosphere-ocean global climate models (e.g. CMIP5) the climate sensitivity is an emergent property: it is not a model parameter, but rather a result of a combination of model physics and parameters. By contrast, simpler energy-balance models may have climate sensitivity as an explicit parameter. The terms represented in the equation relate radiative forcing (RF) to linear changes in global surface temperature change (ΔTs) via the climate sensitivity λ. It is also possible to estimate climate sensitivity from observations; however, this is difficult due to uncertainties in the forcing and temperature histories. ==Equilibrium and transient climate sensitivity== The equilibrium climate sensitivity (ECS) refers to the equilibrium change in global mean near-surface air temperature that would result from a sustained doubling of the atmospheric (equivalent) carbon dioxide concentration (ΔTx2). As estimated by the IPCC Fifth Assessment Report (''AR5'') "there is ''high confidence'' that ECS is ''extremely unlikely'' less than 1°C and ''medium confidence'' that the ECS is ''likely'' between 1.5°C and 4.5°C and ''very unlikely'' greater than 6°C." This is a change from the IPCC Fourth Assessment Report (''AR4''), which said it was ''likely to be in the range 2 to 4.5 °C with a best estimate of about 3 °C, and is very unlikely to be less than 1.5 °C. Values substantially higher than 4.5 °C cannot be excluded, but agreement of models with observations is not as good for those values''. The IPCC Third Assessment Report (''TAR'') said it was "likely to be in the range of 1.5 to 4.5 °C". Other estimates of climate sensitivity are discussed later on. A model estimate of equilibrium sensitivity thus requires a very long model integration; fully equilibrating ocean temperatures requires integrations of thousands of model years. A measure requiring shorter integrations is the transient climate response (TCR) which is defined as the average temperature response over a twenty-year period centered at doubling in a transient simulation with increasing at 1% per year. The transient response is lower than the equilibrium sensitivity, due to the "inertia" of ocean heat uptake. Over the 50–100 year timescale, the climate response to forcing is likely to follow the TCR; for considerations of climate stabilization, the ECS is more useful. An estimate of the equilibrium climate sensitivity may be made from combining the transient climate sensitivity with the known properties of the ocean reservoirs and the surface heat fluxes; this is the effective climate sensitivity. This "may vary with forcing history and climate state". A less commonly used concept, the Earth system sensitivity (ESS), can be defined which includes the effects of slower feedbacks, such as the albedo change from melting the large ice sheets that covered much of the northern hemisphere during the last glacial maximum. These extra feedbacks make the ESS larger than the ECS — possibly twice as large — but also mean that it may well not apply to current conditions. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Climate sensitivity」の詳細全文を読む スポンサード リンク
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