The effect of smoke particles on clouds and climate forcing

Smoke particles from biomass burning can generate forcing of climate by modifying cloud microphysics and reflectance of sunlight. Cloud modification, critical to an un-derstanding of climate change, is uncertain and variable. Satellite data over the Amazon Basin and Cerrado were analyzed for cloud reflectance and droplet size and for smoke concentration. Smoke increased cloud reflectance from 0.35 to 0.45, while reducing droplet size from 14 to 9 micrometers. The regional variability of the smoke effect was correlated to the availability of water vapor. During the 3 months of biomass burning in the dry season, the smoke-cloud forcing of climate was only 22 watts per square meter in this region, much smaller than what can be inferred from model predictions. Cloud modification by aerosol particles (1, 2) may be the missing process in reconciling climate models with observations (3). Yet the chemical and physical processes in-volved may be too complex for parameter-ization in climate models (4), and som