Climate system response to stratospheric sulfate aerosols: sensitivity to altitude of aerosol layer
Krishna-Pillai Sukumara-Pillai Krishnamohan Govindasamy Bala Long Cao Lei Duan Ken Caldeira
Abstract. Reduction of surface temperatures of the planet by injecting sulfate aerosols
in the stratosphere has been suggested as an option to reduce the amount of
human-induced climate warming. Several previous studies have shown that for a
specified amount of injection, aerosols injected at a higher altitude in the
stratosphere would produce more cooling because aerosol sedimentation would
take longer. In this study, we isolate and assess the sensitivity of
stratospheric aerosol radiative forcing and the resulting climate change to
the altitude of the aerosol layer. We study this by prescribing a specified
amount of sulfate aerosols, of a size typical of what is produced by
volcanoes, distributed uniformly at different levels in the stratosphere. We
find that stratospheric sulfate aerosols are more effective in cooling
climate when they reside higher in the stratosphere. We explain this
sensitivity in terms of effective radiative forcing: volcanic aerosols heat
the stratospheric layers where they reside, altering stratospheric water
vapor content, tropospheric stability, and clouds, and consequently the
effective radiative forcing. We show that the magnitude of the effective
radiative forcing is larger when aerosols are prescribed at higher altitudes
and the differences in radiative forcing due to fast adjustment processes can
account for a substantial part of the dependence of the amount of cooling on
aerosol altitude. These altitude effects would be additional to dependences
on aerosol microphysics, transport, and sedimentation, which are outside the
scope of this study. The cooling effectiveness of stratospheric sulfate
aerosols likely increases with the altitude of the aerosol layer both because
aerosols higher in the stratosphere have larger effective radiative forcing
and because they have higher stratospheric residence time; these two effects
are likely to be of comparable importance.