Satellite methods underestimate indirect climate forcing by aerosols, PNAS, Joyce E. Penner, Li Xu & Minghuai Wang

Proceedings of the National Academy of Sciences, published online before print August 1, 2011; doi: 10.1073/pnas.1018526108

Satellite methods underestimate indirect climate forcing by aerosols

1. Joyce E. Penner^a,¹, 
2. Li Xu^a, and 
3. Minghuai Wang^b

Abstract

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 ln(N_c) versus ln(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 ln(N_c) versus ln(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 ln(N_c) versus ln(AI) may be substantially incorrect on a regional basis and may underestimate or overestimate the global average forcing by 25 to 35%.

http://www.pnas.org/content/early/2011/07/25/1018526108.abstract