Satellite-based estimate of the direct and indirect aerosol climate forcing

The main uncertainty in anthropogenic forcing of the Earth's climate stems from pollution aerosols, particularly their "indirect effect'' whereby aerosols modify cloud properties. We develop a new methodology to derive a measurement-based estimate using almost exclusively information from an Earth radiation budget instrument (CERES) and a radiometer ( MODIS). We derive a statistical relationship between planetary albedo and cloud properties, and, further, between the cloud properties and column aerosol concentration. Combining these relationships with a data set of satellite-derived anthropogenic aerosol fraction, we estimate an anthropogenic radiative forcing of -0.9 +/- 0.4 Wm(-2) for the aerosol direct effect and of -0.2 +/- 0.1 Wm(-2) for the cloud albedo effect. Because of uncertainties in both satellite data and the method, the uncertainty of this result is likely larger than the values given here which correspond only to the quantifiable error estimates. The results nevertheless indicate that current global climate models may overestimate the cloud albedo effect.