Radiative forcing and climate response

We examine the sensitivity of a climate model to a wide range of radiative forcings, including changes ofsolar irradiance, atmospheric CO2, 03, CFCs, clouds, aerosols, urface •bedo, and a "ghost" forcing introduced at arbitrary heights, latitudes, longitudes, easons, and times of day. We show that, in general, the climate response, specifically the global mean temperature change, issensitive tothe altitude, latitude, and nature of the forcing; that is, the response to a given forcing can vary by 50% or more depending upon characteristics of the forcing other than its magnitude measured in watts per square meter. The consistency of the response among different forcings is higher, within 20% or better, for most of the globally distributed forcings uspected of influencing global mean temperature in the past century, but exceptions occur for certain changes of ozone or absorbing aerosols, for which the climate response isless well behaved. In •1 cases the physic• basis for the variations of the response can be understood. The principal •nechanisms involve •terations of lapse rate and decrease (increase) oflarge-sc•e cloud cover in layers that are preferentially heated (cooled). Although the magnitude of these ffects must be model-dependent, the existence and sense of the mechanisms appear to be reasonable. Overall, we reaffirm the value of the radiative forcing concept for predicting climate response and for comparative studies of different forcings; indeed, the present results can help improve the accuracy of such an•yses and define error estimates. Our results also emphasize the need for measurexnents having the specificity and precision eeded to define poorly known forcings uch as absorbing aerosols and ozone change. Available data on aerosol single scatter •bedo ixnply that anthropogenic aerosols cause less cooling than has co•nmonly been asstuned. However, negative forcing due to the net ozone change since 1979 appears tohave counterbalanced 30-50% of the positive forcing due to the increase of well-xnixed greenhouse gases in the same period. As the net ozone change includes halogendriven ozone depletion with negative radiative forcing and a tropospheric ozone increase with positive radiative forcing, it is possible that the h•ogen-driven ozone depletion has counterbalanced more than h•f of the radiative forcing due to wellxnixed greenhouse gases since 1979.