Projected global sulfur deposition with climate intervention

Even with immediate implementation of global policies to mitigate carbon dioxide emissions, the impacts of climate change will continue to worsen over the next decades. One potential response is stratospheric aerosol injection (SAI), where sulfur dioxide is released into the stratosphere to block incoming solar radiation. SAI does not reduce the level of carbon dioxide in the atmosphere, but it can slow warming and act as a stopgap measure to give the world more time to pursue effective carbon reduction strategies. While SAI is controversial, it remains a technically feasible proposition. It ought to be thoroughly modeled both to characterize global risks better and to further the scientific community's understanding of stratospheric aerosol dynamics. SAI relies on sulfate aerosols which have a lifetime of several years in the stratosphere but will eventually be deposited back onto Earth's surface. While sulfate is an important nutrient for many ecosystems, high concentrations can cause acidification, eutrophication, and biodiversity loss. We use model outputs from the Geoengineering Model Intercomparison Project (GeoMIP) to track the impacts of sulfur deposition from SAI to various ecoregions through comparison with historical climate and future Shared Socioeconomic Pathway (SSP) scenarios. Our results demonstrate that dry sulfur deposition will continue to decline worldwide, regardless of scenario, from a high of 41 Tg S/yr in 1981 to under 20 Tg S/yr by 2100. Wet sulfur deposition, however, is much more uncertain and further work needs to be done in this area to harmonize model estimates. Under SAI, many ecoregions will experience notably different sulfur deposition regimes by the end of the century compared to historical trends. In some places, this will not be substantially different than the impacts of climate change under SSP2–4.5 or SSP5–8.5. However, in some ecoregions the model projections disagree dramatically on the magnitude of future trends in both emissions and deposition, with, for example, UKESM1–0-LL projecting that SO42- deposition in deciduous needleleaf forests under G6 Sulfur will reach 394 % of SSP2–4.5 deposition by the 2080 s while CESM2-WACCM projects that SO42- deposition will remain at 170 % of SSP2–4.5 deposition during that same time period. Our work emphasizes the lack of agreement between models and the importance of improving our understanding of SAI impacts for future climate decision-making.