The indirect shortwave effect of anthropogenic sulfate aerosol augmentation in marine stratocumulus clouds has been evaluated using global cloud climatology, sulfate aerosol data from a chemical model, and cloud albedo augmentation obtained from a large-eddy simulation (LES) cloud model with explicit microphysics. It is found that the annually and globally (over the oceans) averaged indirect shortwave forcing is -1.1 Wm-2, with a hemispheric difference of 0.4 Wm-2. Given the many uncertainties in the aerosol-CCN-cloud microstructure links, this estimate, most likely, should be considered as an upper limit of the aerosol indirect forcing. The hemispheric forcing has a strong seasonal cycle with the Northern Hemisphere (NH) forcing exceeding the Southern Hemisphere (SH) forcing during the NH spring and summer and the SH forcing prevailing during the SH spring and summer. We also estimate that the single layer St/Sc contributes most to the aerosol indirect forcing (60%) with an additional 25% from the St/Sc overlapped with Ci/Cs/Cc, and 15% from St/Sc overlapped with As/Ac. ¿ American Geophysical Union 1996