A smaller global estimate of the second indirect aerosol effect

Abstract

Global estimates of the indirect aerosol effect much larger than 1 W m-2 in magnitude are difficult to reconcile with observations, yet climate models give estimates between -1 and -4.4 W m-2. We use a climate model with a new treatment of autoconversion to reevaluate the second indirect aerosol effect. We obtain a global-mean value of -0.28 W m-2, compared to -0.71 W m-2 with the autoconversion treatment most often used in climate models. The difference is due to (1) the new scheme's smaller autoconversion rate, and (2) an autoconversion threshold that increases more slowly with cloud droplet concentration. The impact of the smaller autoconversion rate shows the importance of accurately modeling this process. Our estimate of the total indirect aerosol effect on liquid-water clouds changes from -1.63 to -1.09 W m-2. Copyright 2005 by the American Geophysical Union.