Estimating the cloud feedback between 2000-2008

Much of the warming we expect over the next century comes from feedbacks rather than the direct warming from greenhouse gases. Recently, our knowledge of feedbacks has improved markedly, and we now have high confidence that the sum of the water vapor, lapse rate, and ice-albedo feedbacks is strongly positive. The cloud feedback, however, remains a substantial uncertainty. Despite its importance, there are few attempts to measure its magnitude. Here I show an estimate of the magnitude of the cloud feedback based on an analysis of variations in the top-of-atmosphere radiation budget over the period March 2000 to December 2008. Over this period, we see a strongly positive cloud feedback. Comparisons to control runs of climate models reveals a similar feedback in them. While the long-term cloud feedback may be different from this short-term cloud feedback, the agreement between the measurements and the models provides some confidence that their simulation of the long-term cloud feedback is reasonable.