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Discussion papers | Copyright
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 05 Feb 2018

Research article | 05 Feb 2018

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This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Upper tropospheric ice sensitivity to sulfate geoengineering

Daniele Visioni1,2, Giovanni Pitari1, and Glauco di Genova2 Daniele Visioni et al.
  • 1Department of Physical and Chemical Sciences, Universitá dell'Aquila, 67100 L'Aquila, Italy
  • 2CETEMPS, Universitá dell'Aquila, 67100 L'Aquila, Italy

Abstract. Aside from the direct surface cooling sulfate geoengineering (SG) would produce, the investigation on possible side-effects 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 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 influencing the amount of sulfur needed to counteract the positive RF due to greenhouse gases. In our study, given a 8Tg-SO2 equatorial injection in the lower stratosphere, an all-sky net tropopause RF of −2.13W/m2 is calculated, of which −0.96W/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 −1.45W/m2, of which −0.21W/m2 (14%) from cirrus thinning. Relatively to the clear-sky net tropopause RF due to SG aerosols (−2.06W/m2), the cumulative effect of background clouds and cirrus thinning accounts for −0.07W/m2, due to close compensation of large positive shortwave (+1.85W/m2) and negative longwave adjustments (−1.92W/m2). When the surface cooling is ignored, the net cloud adjustment becomes +0.71W/m2, with the shortwave contribution (+1.97W/m2) significantly larger in magnitude than the longwave one (−1.26W/m2). This highlights the importance of including all dynamical feedbacks of SG aerosols.

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Daniele Visioni et al.
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Daniele Visioni et al.
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Publications Copernicus
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Short summary
Many side effects of sulfate geoengineering have to be analyzed before the world can even consider deploying this method of solar radiation management. In particular, we show that ice clouds in the upper troposphere are modified by a sulfate injection, producing a change that, by allowing for more planetary radiation to escape to space, would produce a further cooling. This might be important when considering the necessary amount of sulfate that needs to be injected to achieve a certain target.
Many side effects of sulfate geoengineering have to be analyzed before the world can even...