A Model Intercomparison of Stratospheric Solar Geoengineering by Accumulation-Mode Sulfate Aerosols

Abstract. Analyses of stratospheric solar geoengineering have focused on sulfate aerosol, and almost all climate model experiments on sulfate aerosol have assumed injection of SO2. Yet continuous injection of SO2 may produce overly large aerosols. Injection of SO3 or H2SO4 from an aircraft in stratospheric flight is expected to produce new accumulation-mode particles (AM-H2SO4), and such injection may allow the sulfate aerosol size distribution to be nudged towards higher radiative efficacy. We report the first multi-model intercomparison of AM-H2SO4 injection. We compare three models: CESM2(WACCM), MAECHAM5-HAM, and SOCOL-AER coordinated as a testbed experiment within the Geoengineering Model Intercomparison Project (GeoMIP). The intercomparison explores how the injection of new accumulation-mode particles changes the large-scale particle size distribution and thus the overall radiative and dynamical response to sulfate aerosol injection. Each model used the same injection scenarios testing AM-H2SO4 and SO2 injections at 5 and 25 Tg(S) yr−1 to test linearity and climate response sensitivity. All three models find that AM-H2SO4 injection increases the radiative efficacy, defined as the radiative forcing per unit of sulfur injection, relative to SO2 injection. Increased radiative efficacy means that when compared to the use of SO2 to produce the same radiative forcing, AM-H2SO4 emissions could reduce some side-effects of sulfate aerosol geoengineering such as stratospheric heating. We explore the sensitivity to injection pattern by comparing injection at two points at 30° N and 30° S to injection in a belt along the equator between 30° S and 30° N, and find opposite impacts on radiative efficacy for AM-H2SO4 and SO2, suggesting that prior model results for concentrated injection of SO2 may be strongly dependent on model resolution. Model differences arise from differences in aerosol formulation and differences in model transport and resolution, factors whose interplay cannot be easily untangled by this intercomparison.