Quantifying sources of inter-model diversity in the cloud albedo effect

There is a large diversity in simulated aerosol forcing among models that participated in the fifth Coupled Model Intercomparison Project, particularly related to aerosol interactions with clouds. Here we use the reported model data and fitted aerosol-cloud relations to separate the main sources of inter-model diversity in the magnitude of the cloud albedo effect. There is a large diversity in the global load and spatial distribution of sulfate aerosol, as well as in global mean cloud top effective radius. The use of different parameterizations of aerosol-cloud interactions makes the largest contribution to diversity in modeled radiative forcing (-39%, +48% about the mean estimate). Uncertainty in preindustrial sulfate load also makes a substantial contribution (-15%, +61% about the mean estimate), with smaller contributions from inter-model differences in the historical change in sulfate load and in mean cloud fraction.