Seasonal Injection Strategies for Stratospheric Aerosol Geoengineering

Simulations of stratospheric aerosol geoengineering have typically considered injections at a constant rate over the entire year. However, the seasonal variability of both sunlight and the stratospheric circulation suggests seasonally dependent injection strategies. We simulated single-point injections of the same amount of SO2 in each of the four seasons and at five different latitudes (30 degrees S, 15 degrees S, equator, 15 degrees N, and 30 degrees N), 5 km above the tropopause. Our findings suggest that injecting only during one season reduces the amount of SO2 needed to achieve a certain aerosol optical depth, thus potentially reducing some of the side effects of geoengineering. We find, in particular, that injections at 15 degrees N or 15 degrees S in spring of the corresponding hemisphere results in the largest reductions in incoming solar radiation. Compared to annual injections, by injecting in the different seasons we identify additional distinct spatiotemporal aerosol optical depth patterns, thanks to seasonal differences in the stratospheric circulation.