Infrared active gases in the atmosphere exert a warming influence on climate by reducing the intensity of infrared radiation emitted from the earth-atmosphere system. The possible influence of increased concentrations of infrared absorbing gases such as carbon dioxide and methane on the radiative balance of the atmosphere, leading to a warming of global climate, has been appreciated for some time and is the subject of much active research. Aerosol particles exert a radiative influence on climate by scattering and absorbing shortwave radiation and, through their action as cloud condensation nuclei, by modifying cloud radiative properties. Climate forcing by anthropogenic aerosols is estimated to be comparable on global and annual average to that of anthropogenic greenhouse gases but of opposite sign, and is therefore likely to be offsetting a considerable fraction of the warming influence of anthropogenic greenhouse gases. However the spatial and temporal distribution of the aerosol forcing is quite different from that of the greenhouse gases, and therefore this forcing cannot a priori be treated as a scalable anti-greenhouse forcing. Assessment of the climate influence of the aerosol forcing requires studies with global climate models, but the accuracy of these analyses will be limited by present uncertainties in key parameters necessary to describe this forcing.
Return to Stephen E. Schwartz Publications Page