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. Reduction of surface temperatures of the planet by injecting sulfate aerosolsin the stratosphere has been suggested as an option to reduce the amount ofhuman-induced climate warming. Several previous studies have shown that for aspecified amount of injection, aerosols injected at a higher altitude in thestratosphere would produce more cooling because aerosol sedimentation wouldtake longer. In this study, we isolate and assess the sensitivity ofstratospheric aerosol radiative forcing and the resulting climate change tothe altitude of the aerosol layer. We study this by prescribing a specifiedamount of sulfate aerosols, of a size typical of what is produced byvolcanoes, distributed uniformly at different levels in the stratosphere. Wefind that stratospheric sulfate aerosols are more effective in coolingclimate when they reside higher in the stratosphere. We explain thissensitivity in terms of effective radiative forcing: volcanic aerosols heatthe stratospheric layers where they reside, altering stratospheric watervapor content, tropospheric stability, and clouds, and consequently theeffective radiative forcing. We show that the magnitude of the effectiveradiative forcing is larger when aerosols are prescribed at higher altitudesand the differences in radiative forcing due to fast adjustment processes canaccount for a substantial part of the dependence of the amount of cooling onaerosol altitude. These altitude effects would be additional to dependenceson aerosol microphysics, transport, and sedimentation, which are outside thescope of this study. The cooling effectiveness of stratospheric sulfateaerosols likely increases with the altitude of the aerosol layer both becauseaerosols higher in the stratosphere have larger effective radiative forcingand because they have higher stratospheric residence time; these two effectsare likely to be of comparable importance.

Climate system response to stratospheric sulfate aerosols: sensitivity to altitude of aerosol layer