In a recent paper, Han et al. analyzed satellite data radiances to retrieve cloud droplet effective radii and reported significant interhemispheric differences for both maritime and continental clouds. The mean cloud droplet radius in the Northern Hemisphere is smaller than in the Southern Hemisphere by about 0.7 μm. This hemispheric contrast suggests the presence of an aerosol effect on cloud droplet size and is consistent with higher cloud condensation nuclei number concentration in the Northern Hemisphere due to anthropogenic production of aerosol precursors. In the present study, we constrain a climate model with the satellite retrievals of Han et al. and discuss the climate forcing that can be inferred from the observed distribution of cloud droplet radius. Based on two sets of experiments, this sensitivity study suggests that the indirect radiative forcing by anthropogenic aerosols could he about −0.6 or −1 W m−2 averaged in the 0°–50°N latitude band. The uncertainty of these estimates is difficult to a assess but is at least 50%.

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