Satellite-based estimates of the aerosol indirect effect (AIE) are consistently smaller than the estimates from global aerosol models, and, partly as a result of these differences, the assessment of this climate forcing includes large uncertainties. Satellite estimates typically use the present-day (PD) relationship between observed cloud drop number concentrations (Nc) and aerosol optical depths (AODs) to determine the preindustrial (PI) values of Nc. These values are then used to determine the PD and PI cloud albedos and, thus, the effect of anthropogenic aerosols on top of the atmo- sphere radiative fluxes. Here, we use a model with realistic aerosol and cloud processes to show that empirical relationships for lnðNc Þ versus lnðAODÞ derived from PD results do not represent the atmo- spheric perturbation caused by the addition of anthropogenic aerosols to the preindustrial atmosphere. As a result, the model estimates based on satellite methods of the AIE are between a factor of 3 to more than a factor of 6 smaller than model estimates based on actual PD and PI values for Nc. Using lnðNcÞ versus lnðAIÞ (Aerosol Index, or the optical depth times angstrom exponent) to estimate preindustrial values for Nc provides estimates for Nc and forcing that are closer to the values predicted by the model. Never- theless, the AIE using lnðNcÞ versus lnðAIÞ may be substantially incorrect on a regional basis and may underestimate or overesti- mate the global average forcing by 25 to 35%.
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