POTENTIAL EFFECTS OF CLOUD OPTICAL-THICKNESS ON CLIMATE WARMING

CLIMATE warming can cause changes in the optical properties of low clouds, which may in turn amplify or diminish the warming1,2. But both the sign and magnitude of such feedbacks have been uncertain, largely because the observational evidence for variations in the large-scale optical properties of clouds has been very limited. Recently, analysis of data from the International Satellite Cloud Climatology Project yielded a relationship between low-cloud optical thickness and cloud temperature that implies a positive feed-back between clouds and climate3. Here we use a two-dimensional radiative-convective model to assess the effect of such a feedback on the climate change associated with a doubling of the atmospheric carbon dioxide concentration. We find that, zonally averaged, the feedback is positive in the Northern Hemisphere and is stronger at lower than at higher latitudes. The positive feedback amplifies the overall global climate sensitivity, and the latitudinal gradient in the strength of the feedback acts to eliminate the high-latitude amplification of the greenhouse warming predicted by most climate models.