Andrew Ackerman

There is considerable debate over the magnitude of the indirect radiative forcing of aerosols (through changes in clouds). A key unknown is how the total amount of cloud water changes with increasing levels of aerosol pollution. In situ observations show that there on average is no change in cloud water due to pollution in marine boundary layer clouds, but satellite observations show a surprisingly (and entirely unexpected) significant decrease in cloud water in polluted clouds. Using a detailed cloud model, we have resolved this apparent discrepancy by showing that the satellite data suffers from a bias. We also find that daytime cloud water changes little due to pollution in these clouds. This observational result (confirmed by detailed simulations) is in stark contrast to the results from general circulation models (GCMs) that are used to predict global climate change. The crude cloud parameterizations within the GCMs show a strong increase in cloud water due to pollution, and aerosols are thereby predicted to largely offset greenhouse warming. Our results do not confirm this, and instead imply that the indirect aerosol forcing is nowhere near as great as that in the GCM simulations. Extrapolated to a global scale, our analysis suggests that aerosols will not save us from greenhouse warming.

Collaborators: O.B. Toon, University of Colorado

                         D.E. Stevens, Lawrence Livermore National Laboratory

                          J.A. Coakley, Jr., Oregon State University

Point of Contact: Andrew Ackerman, (650) 604-3522, ack@sky.arc.nasa.gov