Indirect sulphate aerosol forcing in a climate model with an interactive sulphur cycle

The effects of anthropogenic sulphate aerosol on cloud albedo and on precipitation efficiency (the first and second indirect effects, respectively) are investigated using a new version of the Hadley Centre climate model. This version includes a new cloud microphysics scheme, an interactive sulphur cycle, and a parameterization of the effects of sea salt aerosol. The combined global mean radiative impact from both indirect effects is estimated to be approximately -1.9 W m(-2) in terms of the change in net cloud forcing, with the albedo effect dominating: we obtain values of -1.3 and -0.5 W m(-2) for the first and second effects, respectively, when calculated separately. The estimate for the combined effect has at least a factor of 2 uncertainty associated with it: for example, alternative assumptions which affect the concentration of natural background sulphate aerosol reduce the forcing by over 25%, and different parameterizations of the autoconversion of cloud droplets to rainwater can double the forcing.