Sulfate Geoengineering Impact on Methane Transport and Lifetime: Results from the Geoengineering Model Intercomparison Project (GeoMIP)

Sulfate geoengineering (SG), made by sustained injection of SO 2 in the tropical lower stratosphere, may impact the CH 4 abundance through several photochemical mechanisms affecting tropospheric OH and hence the methane lifetime. (a) The reflection of incoming solar radiation increases the planetary albedo and cools the surface, with a tropospheric H 2 O decrease. (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( 1 D). (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-to-upper troposphere, thus reducing the amount of NO x and O 3 production. (d) The tropical lower stratosphere is warmed by solar and planetary radiation absorption by the aerosols. The heating rate perturbation is highly latitude dependent, producing a stronger meridional component of the Brewer–Dobson circulation. The net effect on tropospheric OH due to the enhanced stratosphere–troposphere exchange may be positive or negative depending on the net result of different superimposed species perturbations (CH 4 , NO y , O 3 , SO 4 ) in the extratropical upper troposphere and lower stratosphere (UTLS). In addition, the atmospheric stabilization resulting from the tropospheric cooling and lower stratospheric warming favors an additional decrease of the UTLS extratropical CH 4 by lowering the horizontal eddy mixing. Two climate–chemistry coupled models are used to explore the above radiative, chemical and dynamical mechanisms affecting CH 4 transport and lifetime (ULAQ-CCM and GEOSCCM). The CH 4 lifetime may become significantly longer (by approximately 16 %) with a sustained injection of 8 Tg-SO 2  yr −1 starting in the year 2020, which implies an increase of tropospheric CH 4 (200 ppbv) and a positive indirect radiative forcing of sulfate geoengineering due to CH 4 changes (+0.10 W m −2 in the 2040–2049 decade and +0.15 W m −2 in the 2060–2069 decade).