CLIMATE RESPONSE TO RADIATIVE FORCINGS BY SULFATE AEROSOLS AND GREENHOUSE GASES

The annual, global mean radiative forcing for the troposphere-surface system has been used to rank the global warming influences of atmospheric trace gases. The approach was also used recently to compare the cooling influence of tropospheric sulfate aerosols with the warming influence of greenhouse gases. However, the spatial inhomogeneity of sulfate aerosols (concentrated mainly in the continental Northern Hemisphere) may induce climate responses which differ other than just in sign from those induced by increased concentrations of the more homogeneously distributed greenhouse gases. Here we use a general circulation model to further examine the suitability of global mean radiative forcing as a predictor of differences in global, hemispheric, and regional climate responses to differing spatial and temporal forcing patterns. The calculated responses indicate that changes of the global and annual mean surface air temperature depend only on global average net forcing and are not highly sensitive either to the details of the spatial and seasonal patterns in forcing or to the nature of the forcing (shortwave vs. longwave). Thus in global and annual mean the negative aerosol forcing may be viewed as a scaleable anti-greenhouse forcing. However substantial responses to nonuniformly distributed aerosol forcing were observed at hemispheric and regional scales. Further, the patterns of response differ from the patterns of forcing, leading to the conclusion that the spatial distribution of all significant forcings must be accurately represented when studying regional climate changes.