Satellite methods underestimate indirect climate forcing by aerosols

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 (N-c) and aerosol optical depths (AODs) to determine the preindustrial (PI) values of N-c. These values are then used to determine the PD and PI cloud albedos and, thus, the effect of anthropogenic aerosols on top of the atmosphere radiative fluxes. Here, we use a model with realistic aerosol and cloud processes to show that empirical relationships for In (N-c) versus In (AOD) derived from PD results do not represent the atmospheric 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 N-c. Using In(N-c) versus In(AI) (Aerosol Index, or the optical depth times angstrom exponent) to estimate preindustrial values for N-c provides estimates for N-c and forcing that are closer to the values predicted by the model. Nevertheless, the AIE using In(N-c) versus In(AI) may be substantially incorrect on a regional basis and may underestimate or overestimate the global average forcing by 25 to 35%.