Anthropogenic SO(2) emissions may exert a significant cooling effect on climate in the Northern Hemisphere through backscattering of solar radiation by sulfate particles. Earlier estimates of the sulfate climate forcing were based on a limited number of sulfate-scattering correlation measurements from which a high sulfate-scattering efficiency was derived. Model results suggest that cloud processing of air is the underlying mechanism. Aqueous phase oxidation of SO(2) into sulfate and the subsequent release of the dry aerosol by cloud evaporation render sulfate a much more efficient scatterer than through gas-phase SO(2) oxidation.