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.
Stratospheric aerosol geoengineering (SAG) has been proposed to reduce some impacts of anthropogenic climate change. Previous studies examined annual mean climate responses to SAG. Here we use the Stratospheric Aerosol Geoengineering Large Ensemble simulations to explore the effects of SAG on the seasonal cycle of climate change. Simulations show that relative to the present-day climate, SAG diminishes the amplitude of the seasonal cycle of temperature at many high-latitude locations, with warmer winters and cooler summers. The seasonal temperature shift significantly influences the seasonal cycle of snow depth and sea ice, with Arctic sea ice recovery overcompensated in summer by 52% and undercompensated in winter by 8%. We identify that both the dynamic effects of aerosol-induced stratospheric heating and seasonal variations of sunlight contribute to the shifts in seasonal cycle. Shifts in the seasonal cycle have important ecological and environmental implications, which should be considered in geoengineering impact analysis.Plain Language Summary Stratospheric aerosol geoengineering, by releasing sulfate aerosol particles or their precursors (SO 2 ) into the stratosphere to scatter more sunlight back to space, is a potential climate intervention option to counteract anthropogenic global warming. Previous studies focused on the effect of aerosol injection on annual mean climate change. Here we assess seasonal climate shifts in response to aerosol injection using a large ensemble of sophisticated climate model simulations. Relative to the high-CO 2 scenario, stratospheric aerosol injection can stabilize many aspects of climate change on the annual mean basis including global mean temperature, interhemispheric temperature gradient, and equator-to-pole temperature gradient. However, we find that injection of SO 2 into the stratosphere would substantially alter the high-latitude seasonal cycle. Relative to the present-day climate, in a high-CO 2 world with additional aerosols in the stratosphere, many high-latitude locations are warmer in winter and cooler in summer. Meanwhile, stratospheric aerosol geoengineering overcompensated Arctic sea ice extent recovery in summer and undercompensated it in winter. These seasonal climate shifts have important ecological, economic, and aesthetic implications for a full assessment of benefits and risks of stratospheric aerosol geoengineering. Key Points: • We use a large ensemble of simulations to explore high-latitude climate seasonal shifts under stratospheric aerosol geoengineering • Stratospheric aerosol geoengineering would alter seasonal cycle of temperature, snow depth, and sea ice at high latitudes • Stratospheric heating and seasonal sunlight variations are two likely mechanisms that cause shifts in high-latitude seasonal cycle Supporting Information: • Supporting Information S1
Stratospheric sulfate aerosol geoengineering has been proposed as a potential strategy to reduce the impacts of climate change. Here we investigate the impact of stratospheric aerosol geoengineering on the terrestrial hydrological cycle. We use the Geoengineering Large Ensemble, which involves a 20‐member ensemble of simulations using the Community Earth System Model with the Whole Atmosphere Community Climate Model, in which sulfur dioxide (SO2) was injected into the stratosphere at four different locations, to maintain global mean surface temperature, and also the interhemispheric and equator‐to‐pole temperature gradients at values representative of 2020 (“baseline”) under the Representative Concentration Pathway 8.5. In our simulations, annual mean land precipitation and evapotranspiration (ET) increase by 12% each under Representative Concentration Pathway 8.5. Under the Geoengineering Large Ensemble, the hydrological cycle is suppressed compared to the baseline, with end‐of‐century decreases of 1.4% (12 ± 5 mm/year) and 3.3% (18 ± 2 mm/year) in global mean, annual mean precipitation, and ET over land, respectively. Geoengineering effectively maintains global mean soil moisture under a high CO2 scenario, although there is significant regional variability. Summertime soil moisture is reduced by 42 ± 11 kg/m2 (3.5%) and 27 ± 16 kg/m2 (2.1%) in India and the Amazon, respectively, which is dominated by the decrease in precipitation. We also compare these regional changes in soil moisture under the Geoengineering Large Ensemble with an equatorial‐only SO2 injection case and find a similar sign in residual changes, although the magnitude of the changes is larger in the equatorial run.
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