Evaluation of Climate Sensitivity Related to Different Cloud Feedbacks in Global Climate Models

For the current generation of climate models, climate response varies from model to model even when the radiative forcing used to drive the models is similar. This difference in the climate models ’ response is mainly the result of differing climate sensitivities. According to the Intergovernmental Panel on Climate Change (IPCC) Third Assessment Report, the climate sensitivity is likely to be in the range of 1.5 to 4.5°C. This estimate is unchanged from the first IPCC Assessment Report in 1990 and the Second Assessment Report (IPCC, 2001). The range in estimated climate sensitivity of 1.5 to 4.5°C for a CO2 doubling arises mainly from uncertainties in the climate models and their internal feedbacks, particularly those related to clouds and related processes. Clouds can both reflect solar radiation (thereby cooling the surface) and absorb and emit long-wave radiation (thereby warming the surface). The potential complexity of the response of clouds to climate change makes clouds a dominant source of uncertainty. Although there have been a number of improvements in the simulation of both the cloud distribution and in the radiative properties of clouds, the uncertaint