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Upper tropospheric ice sensitivity to sulfate geoengineering

Abstract. Aside from the direct surface cooling that sulfate geoengineering (SG) would
produce, investigations of the possible side effects of this method are still
ongoing, such as the exploration of the effect that SG may have on upper
tropospheric cirrus cloudiness. The goal of the present study is to better
understand the SG thermodynamical effects on the freezing mechanisms leading
to ice particle formation. This is undertaken by comparing SG model
simulations against a Representative Concentration Pathway 4.5 (RCP4.5)
reference case. In the first case, the aerosol-driven surface cooling is
included and coupled to the stratospheric warming resulting from the aerosol
absorption of terrestrial and solar near-infrared radiation. In a second SG
perturbed case, the surface temperatures are kept unchanged with respect to
the reference RCP4.5 case. When combined, surface cooling and lower
stratospheric warming tend to stabilize the atmosphere, which decreases the
turbulence and updraft velocities (−10 % in our modeling study). The
net effect is an induced cirrus thinning, which may then produce a
significant indirect negative radiative forcing (RF). This RF would go in the
same direction as the direct effect of solar radiation scattering by
aerosols, and would consequently influence the amount of sulfur needed to
counteract the positive RF due to greenhouse gases. In our study, given an
8 Tg-SO2 yr−1 equatorial injection into the lower
stratosphere, an all-sky net tropopause RF of −1.46 W m−2 is
calculated, of which −0.3 W m−2 (20 %) is from the indirect
effect on cirrus thinning (6 % reduction in ice optical depth). When
surface cooling is ignored, the ice optical depth reduction is lowered to
3 %, with an all-sky net tropopause RF of −1.4 W m−2, of which
−0.14 W m−2 (10 %) is from cirrus thinning. Relative to the
clear-sky net tropopause RF due to SG aerosols (−2.1 W m−2), the
cumulative effect of the background clouds and cirrus thinning accounts for
+0.6 W m−2, due to the partial compensation of large positive
shortwave (+1.6 W m−2) and negative longwave adjustments
(−1.0 W m−2). When surface cooling is ignored, the net cloud
adjustment becomes +0.8 W m−2, with the shortwave contribution
(+1.5 W m−2) almost twice as much as that of the longwave
(−0.7 W m−2). This highlights the importance of including all of the
dynamical feedbacks of SG aerosols.
