Climate sensitivity

In IPCC reports, equilibrium climate sensitivity refers to the equilibrium change in global mean surface temperature following a doubling of the atmospheric (equivalent) CO2 concentration. This value is estimated, by the IPCC Third Assessment Report primarily on the basis of climate models, to be "likely to be in the range of 1.5 to 4.5°C" [1]. More generally, equilibrium climate sensitivity refers to the equilibrium change in surface air temperature following a unit change in radiative forcing (°C/Wm-2). In practice, the evaluation of the equilibrium climate sensitivity from models requires very long simulations with coupled global climate models, or it may be deduced from observations.

Constraints are available from observations, but also dependent on estimates of pre-existing average heat flux into the oceans based on models: Gregory et al. (2002) estimate a lower bound of 1.6°C by estimating the change in Earth's radiation budget and comparing it to the global warming observed over the 20th century. Recent work by Annan and Hargreaves [2] combines independent observational and model based estimates to produce a mean of about 3°C, and only a 5% chance of exceeding 4.5°C. A general discussion of some recent work is given here. Shaviv (2005) carried out a similar analysis for 6 different time scales, ranging from the 11-yr solar cycle to the climate variations over geological time scales. He found a typical sensitivity of 2.0°C (ranging between 0.9°C and 2.9°C at 99% confidence) if there is no cosmic-ray climate connection, or a typical sensitivity of 1.3°C (between 0.9°C and 2.5°C at 99% confidence), if the cosmic-ray climate link is real.

Schlesinger and Andronova (using simple climate models) found that it could lie between 1 and 10 °C, with a 54 percent likelihood that the climate sensitivity lies outside the IPCC range [3]. The exact range depends on which factors are most important during the instrumental period: "At present, the most likely scenario is one that includes anthropogenic sulfate aerosol forcing but not solar variation. Although the value of the climate sensitivity in that case is most uncertain, there is a 70 percent chance that it exceeds the maximum IPCC value. This is not good news." said Schlesinger.

Forest et al. (2002; [4]) using patterns of change and the MIT EMIC estimated a 95% confidence interval of 1.4–7.7°C for the climate sensitivity, and a 30% probability that sensitivity was outside the 1.5 to 4.5°C range.

Frame et al. (2005) and Allen et al. [5] note that the size of the confidence limits are dependent on the nature of the prior assumptions made.

Climate sensitivity is not the same as the expected climate change at, say 2100: the TAR reports this to be an increase of 1.4 to 5.8 °C over 1990.

The Transient climate response (TCR) - a term first used in the TAR - is the temperature change at the time of CO2 doubling in a run with CO2 increasing at 1%/year.

The effective climate sensitivity is a related measure that circumvents this requirement. It is evaluated from model output for evolving non-equilibrium conditions. It is a measure of the strengths of the feedbacks at a particular time and may vary with forcing history and climate state. Details are discussed in Section 9.2.1 of Chapter 9 in the TAR [6].