Representing Aerosols in Global Models: from Micrometers to Megameters

Anthropogenically induced climate change is of great current interest because of increases in atmospheric loading of infrared active (greenhouse) gases over the past 150 years and the inferred resultant increase in the flux of infrared radiation in the troposphere. In parallel with the increase in greenhouse gas loadings there has been an increase in loadings of tropospheric aerosols derived from industrial activities. These aerosols scatter shortwave (solar) radiation leading to a decrease in the shortwave radiation absorbed by the earth-atmosphere system, thereby exerting a cooling influence on climate, the direct aerosol effect. These aerosols are thought also to modify the microphysical and radiative properties of clouds, enhancing their albedo and perhaps suppressing precipitation, thereby enhancing cloud lifetimes, both of which effects would also exert a cooling influence, the indirect aerosol effect.