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

Abstract

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.