A long-term global daily data set of satellite-derived radiances has been applied over oceans to investigate the effect of aerosol particles on marine stratus cloud physical parameters and cloud shortwave radiative forcing. Tropospheric aerosol can indirectly influence cloud reflectance and radiative forcing by acting as cloud condensation nuclei, thus modifying the droplet size distribution and cloud optical properties. The NOAA polar-orbiting satellite advanced very high resolution radiometer Pathfinder Atmosphere (PATMOS) data provide estimates of aerosol optical thickness (AOT) which were utilized to represent the amount of aerosol available to modify marine stratus. Periods of stratospheric aerosol loading were avoided for this analysis. PATMOS multichannel radiances for cloudy pixels were combined with radiative transfer modeling to estimate cloud droplet effective radius and cloud optical thickness for several months and years in (110 km)2 grid cells between 50 ¿S and 50 ¿N. The results indicate an inverse relationship between aerosol burden and cloud droplet size, including seasonal and regional variability that compares well with known aerosol sources. Analyses suggest an AOT value above which the stratus properties do not change; that is, the indirect effect reaches a microphysical saturation limit. Radiative flux modeling was used to evaluate the impact of observed cloud characteristics on shortwave forcing. Decreases in droplet size and increases in cloud albedo associated with enhanced AOT produce significantly increased shortwave radiative forcing estimates as compared with clean background cloud and aerosol conditions. These results provide observational evidence for the global influence of aerosol on the shortwave radiative forcing by stratus. ¿ 1999 American Geophysical Union