Articles | Volume 17, issue 11
Research article
14 Jun 2017
Research article |  | 14 Jun 2017

Radiative and climate effects of stratospheric sulfur geoengineering using seasonally varying injection areas

Anton Laakso, Hannele Korhonen, Sami Romakkaniemi, and Harri Kokkola

Abstract. Stratospheric sulfur injections have often been suggested as a cost-effective geoengineering method to prevent or slow down global warming. In geoengineering studies, these injections are commonly targeted to the Equator, where the yearly mean intensity of the solar radiation is the highest and from where the aerosols disperse globally due to the Brewer–Dobson Circulation. However, compensating for greenhouse gas-induced zonal warming by reducing solar radiation would require a relatively larger radiative forcing to the mid- and high latitudes and a lower forcing to the low latitudes than what is achieved by continuous equatorial injections. In this study we employ alternative aerosol injection scenarios to investigate if the resulting radiative forcing can be targeted to be zonally more uniform without decreasing the global the mean radiative forcing of stratospheric sulfur geoengineering. We used a global aerosol–climate model together with an Earth system model to study the radiative and climate effects of stratospheric sulfur injection scenarios with different injection areas. According to our simulations, varying the SO2 injection area seasonally would result in a similar global mean cooling effect as injecting SO2 to the Equator, but with a more uniform zonal distribution of shortwave radiative forcing. Compared to the case of equatorial injections, in the seasonally varying injection scenario where the maximum sulfur production from injected SO2 followed the maximum of solar radiation, the shortwave radiative forcing decreased by 27 % over the Equator (the latitudes between 20° N and 20° S) and increased by 15 % over higher latitudes. Compared to the continuous injections to the Equator, in summer months the radiative forcing was increased by 17 and 14 % and in winter months decreased by 14 and 16 % in Northern and Southern hemispheres, respectively. However, these forcings do not translate into as large changes in temperatures. The changes in forcing would only lead to 0.05 K warmer winters and 0.05 K cooler summers in the Northern Hemisphere, which is roughly 3 % of the cooling resulting from solar radiation management scenarios studied here.

Short summary
Based on simulations, equatorial stratospheric sulfur injections have shown to be an efficient strategy to counteract ongoing global warming. However, equatorial injections would result in relatively larger cooling in low latitudes than in high latitudes. This together with greenhouse-gas-induced warming would lead to cooling in the Equator and warming in the high latitudes. Results of this study show that a more optimal cooling effect is achieved by varying the injection area seasonally.
Final-revised paper