Climate engineering to mitigate the projected 21st-century terrestrial drying of the Americas: a direct comparison of carbon capture and sulfur injection
Authors: Xu Yangyang , Lin Lei , Tilmes Simone , Dagon Katherine , Xia Lili , Diao Chenrui , Cheng Wei , Wang Zhili , Simpson Isla , Burnell Lorna 
Published: July 2020

Abstract. To mitigate the projected global warming in the 21st century, it is well-recognized that society needs to cut CO2 emissions and other short-lived
warming agents aggressively. However, to stabilize the climate at a warming
level closer to the present day, such as the
“well below 2 ∘C” aspiration in the Paris Agreement, a net-zero carbon emission by 2050 is
still insufficient. The recent IPCC special report calls for a massive
scheme to extract CO2 directly from the atmosphere, in addition to
decarbonization, to reach negative net emissions at the mid-century mark.
Another ambitious proposal is solar-radiation-based geoengineering
schemes, including injecting sulfur gas into the stratosphere. Despite being
in public debate for years, these two leading geoengineering schemes
have not been directly compared under a consistent analytical framework
using global climate models. Here we present the first explicit analysis of the hydroclimate impacts of these
two geoengineering approaches using two recently available large-ensemble
(>10 members) model experiments conducted by a family of
state-of-the-art Earth system models. The CO2-based mitigation simulation is
designed to include both emission cuts and carbon capture. The solar-radiation-based mitigation simulation is designed to inject sulfur gas
strategically at specified altitudes and latitudes and run a feedback
control algorithm to avoid common problems previously identified such as
the overcooling of the tropics and large-scale precipitation shifts. Our analysis focuses on the projected aridity conditions over the Americas
in the 21st century in detailed terms of the potential mitigation
benefits, the temporal evolution, the spatial distribution (within North and
South America), the relative efficiency, and the physical mechanisms. We
show that sulfur injection, in contrast to previous notions of leading to
excessive terrestrial drying (in terms of precipitation reduction) while
offsetting the global mean greenhouse gas (GHG) warming, will instead
mitigate the projected drying tendency under RCP8.5. The surface energy
balance change induced by sulfur injection, in addition to the well-known
response in temperature and precipitation, plays a crucial role in
determining the overall terrestrial hydroclimate response. However, when
normalized by the same amount of avoided global warming in these
simulations, sulfur injection is less effective in curbing the worsening trend
of regional land aridity in the Americas under RCP8.5 when compared with
carbon capture. Temporally, the climate benefit of sulfur injection will
emerge more quickly, even when both schemes are hypothetically started in
the same year of 2020. Spatially, both schemes are effective in curbing the
drying trend over North America. However, for South America, the sulfur
injection scheme is particularly more effective for the sub-Amazon region
(southern Brazil), while the carbon capture scheme is more effective for the
Amazon region. We conclude that despite the apparent limitations (such as
an inability to address ocean acidification) and potential side effects (such
as changes to the ozone layer), innovative means of sulfur injection should
continue to be explored as a potential low-cost option in the climate
solution toolbox, complementing other mitigation approaches such as
emission cuts and carbon capture (Cao et al., 2017). Our results demonstrate
the urgent need for multi-model comparison studies and detailed regional
assessments in other parts of the world.