Comparing Surface and Stratospheric Impacts of Geoengineering With Different SO₂ Injection Strategies

Abstract

Geoengineering with stratospheric sulfate aerosols can, to some extent, be designed to achieve different climate objectives. Here we use the state‐of‐the‐art Community Earth System Model, version 1, with the Whole Atmosphere Community Climate Model as its atmospheric component (CESM1(WACCM)), to compare surface climate and stratospheric effects of two geoengineering strategies. In one, SO2 is injected into the tropical lower stratosphere at the equator to keep global mean temperature nearly constant under an RCP8.5 scenario, as has been commonly simulated in previous studies. In another, the Geoengineering Large Ensemble (GLENS), SO2 is injected into the lower stratosphere at four different locations (30°N/S and 15°N/S) to keep global mean temperature, the interhemispheric temperature gradient, and the equator‐to‐pole temperature gradient nearly unchanged. Both simulations are effective at offsetting changes in global mean temperature and the interhemispheric temperature gradient that result from increased greenhouse gases, but only GLENS fully offsets changes in the equator‐to‐pole temperature gradient. GLENS results in a more even aerosol distribution, whereas equatorial injection tends to result in an aerosol peak in the tropics. Moreover, GLENS requires less total injection than in the equatorial case due to different spatial distributions of the aerosols. Many other aspects of surface climate changes, including precipitation and sea ice coverage, also show reduced changes in GLENS as compared to equatorial injection. Stratospheric changes, including heating, circulation, and effects on the quasi‐biennial oscillation are greatly reduced in GLENS as compared to equatorial injection.

The Regional Hydroclimate Response to Stratospheric Sulfate Geoengineering and the Role of Stratospheric Heating

“…Details of the regional hydroclimate response will also depend on the geoengineering strategy employed. For example, experiments with CESM1(WACCM) in which only equatorial injection was used and interhemispheric temperature gradients were not maintained, actually exhibit an increase in precipitation over India during JJA Kravitz et al ( their Fig. 9b).…”

Stratospheric Sulfate Aerosol Geoengineering Could Alter the High‐Latitude Seasonal Cycle

“…Under RCP8.5, sea ice extents in high latitudes experience rapid decrease during this century, while GLENS largely maintains the high‐latitude sea ice in both hemispheres (Figure S4, also see Kravitz et al, ). However, the seasonal temperature shift at high latitudes under GLENS has a significant effect on the seasonal cycle of sea ice change.…”

“…The stratospheric heating could have significant dynamic effects on many aspects of the climate system (Ferraro et al, ; Ferraro et al, ; Richter et al, ; Visioni et al, ), including Northern Hemisphere (NH) winter warming that has been observed after large volcanic eruptions (Driscoll et al, ; Robock & Mao, ; Shindell et al, ; Wunderlich & Mitchell, ). Although volcanic eruptions are an imperfect analog of SAG, SAG‐induced stratospheric dynamic change could also lead to the NH winter warming (Rasch et al, ; Kravitz et al, ).…”

Uncertainty and the basis for confidence in solar geoengineering research

“…However, for off-equatorial injection that still results in similar levels of global mean cooling, the phase and magnitude of the QBO remain relatively unchanged ( fig. 2), and some of the side effects experienced under equatorial injection do not materialize 58 .…”

“…By modifying several of these degrees of freedom -that is, factors related to the deployment strategy -it may be possible to design SAG to achieve climate outcomes beyond solely reducing global mean temperature 19,38,49,59 . For example, if SAG was to be deployed only in the equatorial regions, this would prevent the rise in global mean temperature 58 but have the side effect of residual polar warming 8 ( fig. 3).…”