The annual and global mean radiative forcing for the troposphere-surface system has previously been used to rank the warming influences of CH4, N2O, O3 and chlorofluorocarbons versus CO2. The approach was also used recently to compare the cooling influence of tropospheric sulfate aerosols with the global warming influence of greenhouse gases. However, differences among the greenhouse gases in their vertical radiative forcing distributions lead to different climate responses, and by extension, similar differences (other than just in sign) may be expected between climate responses to forcings by sulfate aerosol and by greenhouse gases. Here we use a general circulation model to examine the adequacy of global mean radiative forcing as a predictor of global and regional climate responses. The calculated responses indicate that differences in vertical partitioning between the trace gas longwave forcing and a sulfate shortwave forcing chosen so that the global annual net forcing at the tropopause was zero, induce global climate responses markedly cooler from those that would result from a simple cancellation of greenhouse gas forcing. The cooling is in part due to the fact that the sulfate aerosol cooling is nearly independent of surface temperature, whereas the greenhouse forcing is reduced in a cooler climate. Consequently, the cooling influence of sulfate aerosols may be underestimated if global mean tropopause forcing is used as a predictor. It is concluded that the concept of radiative forcing at the tropopause is not adequate to rank the relative importance of atmospheric constituents.
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