Climate sensitivity

Climate sensitivity is a measure of how responsive the temperature of the climate system is to a change in the radiative forcing. It is usually expressed as the temperature change associated with a doubling of the concentration of carbon dioxide in Earth's atmosphere.

The equilibrium climate sensitivity refers to the equilibrium change in global mean near-surface air temperature that would result from a sustained doubling of the atmospheric (equivalent) CO2 concentration (ΔTx2). This value is estimated, by the IPCC Fourth Assessment Report (AR4) as 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.[1] This is a slight change from the IPCC Third Assessment Report (TAR), which said it was "likely to be in the range of 1.5 to 4.5°C".[2] More recent work continues to support a best-guess value around 3°C.[3]

A model estimate of equilibrium sensitivity thus requires a very long model integration. A measure requiring shorter integrations is the transient climate sensitivity which is defined as the average temperature response over a twenty year period centered at CO2 doubling in a transient simulation with CO2 increasing at 1% per year.[4] The transient sensitivity is lower than the equilibrium sensitivity, due to the "inertia" of ocean heat uptake. Fully equilibrating ocean temperatures would require integrations of thousands of model years.

An estimate of the equilibrium climate sensitivity may be made from combining the effective 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".[5]

Although climate sensitivity is usually used in the context of radiative forcing by 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 expressed in units of °C/(W/m2). For this to be so, 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.

For a coupled atmosphere-ocean global climate model 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 paramter.

\Delta T_s = \lambda \cdot RF

The terms represented in the equation relate radiative forcing of any cause to linear changes in global surface temperature change.

It is also possible to estimate climate sensitivity from observations; however, this is difficult due to uncertainties in the forcing and temperature histories.

Climate sensitivity can be a useful summary of the sensitivity of the real climate, or of a given model climate. But it is not the same as the expected climate change at, say 2100: the TAR forecasts this to be an increase of 1.4 to 5.8°C over 1990.

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[edit] SRES scenarios and climate change initiatives

While some scenarios assume a more environmentally friendly world than others, none include any climate-specific initiatives, such as the Kyoto Protocol.[7]
[edit] Criticism

The SRES scenarios were criticised by Ian Castles, and David Henderson.[8][9][10] The core of their critique was the use of market exchange rates (MER) for international comparison, in lieu of the theoretically favoured PPP exchange rate which corrects for differences in purchasing power.[11] The IPCC rebutted this criticism[12][13][14]

The positions in the debate can be summarised as follows. Using MER, the SRES scenarios overstate income differences in past and present, and overestimate future economic growth in developing countries. This, Castles and Henderson argue, leads to an overestimate of future greenhouse gas emissions. The IPCC would have made climate change more dramatic than it is.


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The equilibrium climate sensitivity refers to the equilibrium change in global mean near-surface air temperature that would result from a sustained doubling of the atmospheric (equivalent) CO2 concentration (ΔTx2). This value is estimated, by the IPCC Fourth Assessment Report (AR4) as 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.[1] This is a slight change from the IPCC Third Assessment Report (TAR), which said it was "likely to be in the range of 1.5 to 4.5°C".[2] More recent work continues to support a best-guess value around 3°C.[3]










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