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1. High‐Latitude Stratospheric Aerosol Geoengineering Can Be More Effective if Injection Is Limited to Spring

   https://doi.org/10.1029/2021GL092696  

   Lee, Walker Raymond; MacMartin, Douglas G.; Visioni, Daniele; Kravitz, Ben (May 2021, Geophysical Research Letters)

   Abstract Stratospheric aerosol geoengineering focused on the Arctic could substantially reduce local and worldwide impacts of anthropogenic global warming. Because the Arctic receives little sunlight during the winter, stratospheric aerosols present in the winter at high latitudes have little impact on the climate, whereas stratospheric aerosols present during the summer achieve larger changes in radiative forcing. Injecting SO₂in the spring leads to peak aerosol optical depth (AOD) in the summer. We demonstrate that spring injection produces approximately twice as much summer AOD as year‐round injection and restores approximately twice as much September sea ice, resulting in less increase in stratospheric sulfur burden, stratospheric heating, and stratospheric ozone depletion per unit of sea ice restored. We also find that differences in AOD between different seasonal injection strategies are small compared to the difference between annual and spring injection. 

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2. Identifying the sources of uncertainty in climate model simulations of solar radiation modification with the G6sulfur and G6solar Geoengineering Model Intercomparison Project (GeoMIP) simulations

   https://doi.org/10.5194/acp-21-10039-2021  

   Visioni, Daniele; MacMartin, Douglas G.; Kravitz, Ben; Boucher, Olivier; Jones, Andy; Lurton, Thibaut; Martine, Michou; Mills, Michael J.; Nabat, Pierre; Niemeier, Ulrike; et al (January 2021, Atmospheric Chemistry and Physics)

   Abstract. We present here results from the Geoengineering Model Intercomparison Project (GeoMIP) simulations for the experiments G6sulfur and G6solar for six Earth system models participating in the Climate Model Intercomparison Project (CMIP) Phase 6. The aim of the experiments is to reduce the warming that results from a high-tier emission scenario (Shared Socioeconomic Pathways SSP5-8.5) to that resulting from a medium-tier emission scenario (SSP2-4.5). These simulations aim to analyze the response of climate models to a reduction in incoming surface radiation as a means to reduce global surface temperatures, and they do so either by simulating a stratospheric sulfate aerosol layer or, in a more idealized way, through a uniform reduction in the solar constant in the model. We find that over the final two decades of this century there are considerable inter-model spreads in the needed injection amounts of sulfate (29 ± 9 Tg-SO2/yr between 2081 and 2100), in the latitudinal distribution of the aerosol cloud and in the stratospheric temperature changes resulting from the added aerosol layer. Even in the simpler G6solar experiment, there is a spread in the needed solar dimming to achieve the same global temperature target (1.91 ± 0.44 %). The analyzed models already show significant differences in the response to the increasing CO2 concentrations for global mean temperatures and global mean precipitation (2.05 K ± 0.42 K and 2.28 ± 0.80 %, respectively, for SSP5-8.5 minus SSP2-4.5 averaged over 2081–2100). With aerosol injection, the differences in how the aerosols spread further change some of the underlying uncertainties, such as the global mean precipitation response (−3.79 ± 0.76 % for G6sulfur compared to −2.07 ± 0.40 % for G6solar against SSP2-4.5 between 2081 and 2100). These differences in the behavior of the aerosols also result in a larger uncertainty in the regional surface temperature response among models in the case of the G6sulfur simulations, suggesting the need to devise various, more specific experiments to single out and resolve particular sources of uncertainty. The spread in the modeled response suggests that a degree of caution is necessary when using these results for assessing specific impacts of geoengineering in various aspects of the Earth system. However, all models agree that compared to a scenario with unmitigated warming, stratospheric aerosol geoengineering has the potential to both globally and locally reduce the increase in surface temperatures. 

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3. Is Turning Down the Sun a Good Proxy for Stratospheric Sulfate Geoengineering?

   https://doi.org/10.1029/2020JD033952  

   Visioni, Daniele; MacMartin, Douglas G.; Kravitz, Ben (March 2021, Journal of Geophysical Research: Atmospheres)

   Abstract Deliberately blocking out a small portion of the incoming solar radiation would cool the climate. One such approach would be injecting SO₂into the stratosphere, which would produce sulfate aerosols that would remain in the atmosphere for 1–3 years, reflecting part of the incoming shortwave radiation. The cooling produced by the aerosols can offset the warming produced by increased greenhouse gas (GHG) concentrations, but it would also affect the climate differently, leading to residual differences compared to a climate not affected by either. Many climate model simulations of geoengineering have used a uniform reduction of the incoming solar radiation as a proxy for stratospheric aerosols, both because many models are not designed to adequately capture relevant stratospheric aerosol processes, and because a solar reduction has often been assumed to capture the most important differences between how stratospheric aerosols and GHG would affect the climate. Here we show that dimming the sun does not produce the same surface climate effects as simulating aerosols in the stratosphere. By more closely matching the spatial pattern of solar reduction to that of the aerosols, some improvements in this idealized representation are possible, with further improvements if the stratospheric heating produced by the aerosols is included. This is relevant both for our understanding of the physical mechanisms driving the changes observed in stratospheric‐sulfate geoengineering simulations, and in terms of the relevance of impact assessments that use a uniform solar dimming. 

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4. Injection strategy – a driver of atmospheric circulation and ozone response to stratospheric aerosol geoengineering

   https://doi.org/10.5194/acp-23-13665-2023  

   Bednarz, Ewa M.; Butler, Amy H.; Visioni, Daniele; Zhang, Yan; Kravitz, Ben; MacMartin, Douglas G. (January 2023, Atmospheric Chemistry and Physics)

   Abstract. Despite offsetting global mean surface temperature, various studies demonstrated that stratospheric aerosol injection (SAI) could influence the recovery of stratospheric ozone and have important impacts on stratospheric and tropospheric circulation, thereby potentially playing an important role in modulating regional and seasonal climate variability. However, so far, most of the assessments of such an approach have come from climate model simulations in which SO2 is injected only in a single location or a set of locations. Here we use CESM2-WACCM6 SAI simulations under a comprehensive set of SAI strategies achieving the same global mean surface temperature with different locations and/or timing of injections, namely an equatorial injection, an annual injection of equal amounts of SO2 at 15∘ N and 15∘ S, an annual injection of equal amounts of SO2 at 30∘ N and 30∘ S, and a polar strategy injecting SO2 at 60∘ N and 60∘ S only in spring in each hemisphere. We demonstrate that despite achieving the same global mean surface temperature, the different strategies result in contrastingly different magnitudes of the aerosol-induced lower stratospheric warming, stratospheric moistening, strengthening of stratospheric polar jets in both hemispheres, and changes in the speed of the residual circulation. These impacts tend to maximise under the equatorial injection strategy and become smaller as the aerosols are injected away from the Equator into the subtropics and higher latitudes. In conjunction with the differences in direct radiative impacts at the surface, these different stratospheric changes drive different impacts on the extratropical modes of variability (Northern and Southern Annular modes), including important consequences on the northern winter surface climate, and on the intensity of tropical tropospheric Walker and Hadley circulations, which drive tropical precipitation patterns. Finally, we demonstrate that the choice of injection strategy also plays a first-order role in the future evolution of stratospheric ozone under SAI throughout the globe. Overall, our results contribute to an increased understanding of the fine interplay of various radiative, dynamical, and chemical processes driving the atmospheric circulation and ozone response to SAI and lay the foundation for designing an optimal SAI strategy that could form a basis of future multi-model intercomparisons. 

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5. Seasonal Injection Strategies for Stratospheric Aerosol Geoengineering

   https://doi.org/10.1029/2019GL083680  

   Visioni, Daniele; MacMartin, Douglas G.; Kravitz, Ben; Tilmes, Simone; Mills, Michael J.; Richter, Jadwiga H.; Boudreau, Matthew P. (July 2019, Geophysical Research Letters)

   Abstract 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 SO₂in each of the four seasons and at five different latitudes (30°S, 15°S, equator, 15°N, and 30°N), 5 km above the tropopause. Our findings suggest that injecting only during one season reduces the amount of SO₂needed 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°N or 15°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. 

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