Sensitivity of the Methane Lifetime to Sulfate Geoengineering: Results from the Geoengineering Model Intercomparison Project (GeoMIP)

Sulfate geoengineering, made by sustained injection of SO2 in the tropical lower stratosphere, may impact the abundance of tropospheric methane through several photochemical mechanisms affecting the tropospheric OH abundance and hence the methane lifetime. Geoengineered sulfate aerosols in the stratosphere are responsible of important radiative and chemical effects: (a) solar radiation scattering increases the planetary albedo and cools the surface, with a tropospheric water vapor decrease as a response to this cooling:  less OH. (b) The tropospheric UV budget is upset by the additional aerosol scattering and stratospheric ozone changes: the net effect is meridionally not uniform, with a net decrease in the tropics, thus producing less tropospheric O(1D):  less OH. (c) The extratropical downwelling motion from the lower stratosphere tends to increase the sulfate aerosol surface area density available for heterogeneous chemical reactions in the mid-upper troposphere, thus reducing the amount of NOx and tropospheric O3 production:  less OH. (d) The tropical lower stratosphere is warmed by solar and planetary radiation absorption by the aerosols. The heating rates perturbation are strongly latitude dependent, producing a significant change of the pole-to-equator temperature gradient and mean zonal wind distribution, with a net increase of tropical upwelling. A stronger meridional component of the Brewer-Dobson circulation may affect the abundance of mid-upper tropospheric sulfate aerosols, NOy, and O3 (all possibly affecting the budget of tropospheric OH), as well as CH4 transport directly. Three climate-chemistry coupled models are used here to explore the above radiative, chemical and dynamical mechanisms affecting the methane lifetime (ULAQ-CCM, CCSM4, GEOSCCM). First results show that the CH4 lifetime may become significantly longer (Fig.1) with a sustained injection of 2.5 Tg-S/yr started in year 2020 (exp. G4), which implies an increase of tropospheric CH4 (Fig. 2) and a positive indirect RF of sulfate geoengineering due to CH4 changes, of the order of 10% the aerosols direct forcing, but with opposite sign.