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.
