Model test of CCN‐cloud albedo climate forcing

Cloud condensation nuclei (CCN) influence cloud albedo through their effect on the cloud droplet size distribution. A number of studies have evaluated the climatic impact of the CCN‐cloud albedo feedback, but all have assumed that cloud distributions, cloud thicknesses, and cloud liquid water contents would remain constant as the climate adjusted. We have tested this assumption using the Livermore version of the National Center for Atmospheric Research (NCAR) Community Climate Model (CCM1). Our results indicate that there are no significant compensating changes in cloud properties that would counteract the 1.7% global albedo increase resulting from a fourfold increase in marine CCN concentration. Furthermore, when ocean surface temperatures are decreased 4° C in a manner broadly consistent with the enhanced cloud albedos, we predict an increase in cloud fraction of 3.5% and a reduction in cloud altitude, leading to a positive feedback from clouds that would imply a climate impact roughly double that calculated from cloud droplet size distribution change alone.