How Much Will the World Warm ?

A key challenge for climate science is predicting how the global-mean temperature will respond to anticipated changes in ra-diative forcing produced by human activities, such as emissions of long-lived greenhouse gases and other pollutants. While the practical effects of climate change will be felt locally and involve other variables besides surface temperature, the overall strength and significance of climate change will be determined by the magnitude of the global-mean warming. Estimating this surface warming in response to projected changes in atmospheric CO 2 concentration was the goal of the early classic studies of climate change – notably the first radiative-convective-equilibrium model studies in the mid-1960s by Suki Manabe, Fritz Möller and Richard Wetherald, and the first climate change studies with comprehensive general circulation models in the mid-1970s by Manabe and Wether-ald. One of the first formal assessments of the problem was by the 1979 US National Academy of Sciences (NAS) panel chaired by Jule Charney. The panel concluded that a doubling of atmospheric CO 2 concentration would lead to a global warming of the surface of between 1.5°C and 4.5°C. and 2007 have all included a review of predictions of the equilibrium warming to a doubling of atmospheric CO 2. The 1990 assessment included results from 22 atmospheric general circulation models (GCMs) that were mainly coupled with simplified representations of the ocean heat storage and transport. These various models predicted a warming of between 1.9°C and 4.8°C. By the Fourth IPCC report in 2007 (AR4) results from 19 fully coupled comprehensive ocean– atmosphere GCMs were available, and they predict equilibrium warming for double CO 2 conditions between 2.1°C and 4.4°C, a range not much narrower than that determined in the NAS study almost 3 decades earlier! Climate sensitivity is a measure of the strength of the connection between a perturbation to Earth's radiation balance due to greenhouse gases, aerosols, or other natural or anthropogenic forc-ings, and the resulting change in global mean surface temperature. High climate sensitivity means the climate system responds strongly to a radiative perturbation and produces a comparatively large temperature change (and correspondingly large changes in other climate variables). As noted above, the present array of state-of-the-art GCMs displays a disconcertingly wide range of sensitivity to the same radiative perturbation. Here I will discuss only what is sometimes called the " Charney sensitivity, " which considers just rapid feedbacks such as those from water vapor , clouds, …