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

Abstract

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 SO 2 injection area seasonally would result in a similar global mean cooling effect as injecting SO 2 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 SO 2 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.

Changing transport processes in the stratosphere by radiative heating of sulfate aerosols

“…The decreased forcing efficiency simulated in this study would increase the injected amount to 70 Tg(S) yr −1 to be injected for such a forcing. Adapting a strategy of Laakso et al (2017) with injections following the zenith of the sun or injecting at 15 • N and 15 • S, may slightly reduce the injection rate. However, the spread in the forcing simulated by different models is large (Niemeier and Tilmes, 2017), as is the amount of injected sulfur necessary to generate a certain forcing.…”

Sensitivity of Aerosol Distribution and Climate Response to Stratospheric SO2 Injection Locations

“…Improved adjustments of the aerosol mass distribution and AOD may be possible by varying the location and timing of injections aligned with the location of the divergence of the stream functions of the BDC and considerations of large-scale mixing and transport barriers. Since the TOA imbalance and temperature changes are not directly correlated to AOD changes, and therefore to shortwave radiative forcing, considering transport characteristics of the stratosphere for identifying injection regions may be more efficient than following the maximum intensity of solar radiation, as suggested by Laakso et al (2017).…”

The Climatic Effects of Hygroscopic Growth of Sulfate Aerosols in the Stratosphere

“…In this method, a part of solar radiation reaching the surface is back‐scattered by injecting aerosols or their gaseous precursors into the stratosphere, thus resulting in surface cooling. Climate modeling studies have shown that the efficiency (cooling per unit aerosol amount) related to aerosol injection can depend on multiple factors, including type of aerosols (Pope et al, ; Weisenstein et al, ), the aerosol size (Heckendorn et al, ; Rasch et al, ), the altitude, the latitude, and the timing of injection (Kravitz et al, ; Laakso et al, ; Tilmes et al, ).…”