Upper tropospheric ice sensitivity to sulfate geoengineering

Aside from the direct surface cooling sulfate geoengineering (SG) would produce, the investigation on possible sideeffects of this method is still ongoing, as for instance on upper tropospheric cirrus cloudiness. Goal of the present study is to better understand the SG thermo-dynamical effects on the homogeneous freezing ice formation process. This is done by comparing SG model simulations against a RCP4.5 reference case: in one case the aerosol-driven surface cooling is included and coupled to the 5 stratospheric warming resulting from aerosol absorption of longwave radiation. In a second SG perturbed case, surface temperatures are kept unchanged with respect to the reference RCP4.5 case. Surface cooling and lower stratospheric warming, together, tend to stabilize the atmosphere, thus decreasing turbulence and water vapor 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 would go in the same direction as the direct effect of solar radiation scattering by the aerosols, thus 10 influencing the amount of sulfur needed to counteract the positive RF due to greenhouse gases. In our study, given a 8 Tg-SO2 equatorial injection in the lower stratosphere, an all-sky net tropopause RF of -2.13 W/m2 is calculated, of which -0.96 W/m2 (45%) from the indirect effect on cirrus thinning (7.5% reduction in ice optical depth). When the surface cooling is ignored, the ice optical depth reduction is lowered to 5%, with an all-sky net tropopause RF of -1.45 W/m2, of which -0.21 W/m2 (14%) from cirrus thinning. Relatively to the clear-sky net tropopause RF due to SG aerosols (-2.06 W/m2), the cumulative effect of 15 background clouds and cirrus thinning accounts for -0.07 W/m2, due to close compensation of large positive shortwave (+1.85 W/m2) and negative longwave adjustments (-1.92 W/m2). When the surface cooling is ignored, the net cloud adjustment becomes +0.71 W/m2, with the shortwave contribution (+1.97 W/m2) significantly larger in magnitude than the longwave one (-1.26 W/m2). This highlights the importance of including all dynamical feedbacks of SG aerosols.