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. To mitigate the projected global warming in the 21st century, it is well-recognized that society needs to cut CO2 emissions and other short-livedwarming agents aggressively. However, to stabilize the climate at a warminglevel closer to the present day, such as the“well below 2  ∘ C” aspiration in the Paris Agreement, a net-zero carbon emission by 2050 isstill insufficient. The recent IPCC special report calls for a massivescheme to extract CO2 directly from the atmosphere, in addition todecarbonization, to reach negative net emissions at the mid-century mark.Another ambitious proposal is solar-radiation-based geoengineeringschemes, including injecting sulfur gas into the stratosphere. Despite beingin public debate for years, these two leading geoengineering schemeshave not been directly compared under a consistent analytical frameworkusing global climate models. Here we present the first explicit analysis of the hydroclimate impacts of thesetwo geoengineering approaches using two recently available large-ensemble( >10  members) model experiments conducted by a family ofstate-of-the-art Earth system models. The CO2 -based mitigation simulation isdesigned to include both emission cuts and carbon capture. The solar-radiation-based mitigation simulation is designed to inject sulfur gasstrategically at specified altitudes and latitudes and run a feedbackcontrol algorithm to avoid common problems previously identified such asthe overcooling of the tropics and large-scale precipitation shifts. Our analysis focuses on the projected aridity conditions over the Americasin the 21st century in detailed terms of the potential mitigationbenefits, the temporal evolution, the spatial distribution (within North andSouth America), the relative efficiency, and the physical mechanisms. Weshow that sulfur injection, in contrast to previous notions of leading toexcessive terrestrial drying (in terms of precipitation reduction) whileoffsetting the global mean greenhouse gas (GHG) warming, will insteadmitigate the projected drying tendency under RCP8.5. The surface energybalance change induced by sulfur injection, in addition to the well-knownresponse in temperature and precipitation, plays a crucial role indetermining the overall terrestrial hydroclimate response. However, whennormalized by the same amount of avoided global warming in thesesimulations, sulfur injection is less effective in curbing the worsening trendof regional land aridity in the Americas under RCP8.5 when compared withcarbon capture. Temporally, the climate benefit of sulfur injection willemerge more quickly, even when both schemes are hypothetically started inthe same year of 2020. Spatially, both schemes are effective in curbing thedrying trend over North America. However, for South America, the sulfurinjection scheme is particularly more effective for the sub-Amazon region(southern Brazil), while the carbon capture scheme is more effective for theAmazon region. We conclude that despite the apparent limitations (such asan inability to address ocean acidification) and potential side effects (suchas changes to the ozone layer), innovative means of sulfur injection shouldcontinue to be explored as a potential low-cost option in the climatesolution toolbox, complementing other mitigation approaches such asemission cuts and carbon capture (Cao et al., 2017). Our results demonstratethe urgent need for multi-model comparison studies and detailed regionalassessments in other parts of the world.

earth system dynamics

Climate engineering to mitigate the projected 21st-century terrestrial drying of the Americas: a direct comparison of carbon capture and sulfur injection