Cool, but Different: Climate Response to Solar Geo-Engineering Mediated by the AMOC

Abstract

Stratospheric Aerosol Injection (SAI) is a contentious geo-engineering proposal to lessen the impacts of global heating via an artificial shade of aerosols. While model results suggest that Earth’s global mean surface temperature (GMST) can indeed be stabilised via SAI, the same does not apply to other parts of the climate system. Especially the fate of a weakening Atlantic Meridional Overturning Circulation (AMOC) under SAI remains unclear.We simulate two SAI deployment scenarios in the Community Earth System Model 2 (CESM2) to study whether a weakened AMOC can be stabilised or recovered. To obtain a strong AMOC response, both scenarios follow a high GHG emission pathway. At the same time, proportionally chosen aerosol injections stabilise the GMST at 1.5K above pre-industrial conditions. The scenarios only differ in their deployment times: aerosol injections start either early (SAI 2020) or late century (SAI 2080).Both SAI scenarios reach the target GMST. However, we find that SAI only mitigates rather than decisively reverses AMOC decline. This relatively mild oceanic response stands in contrast to efficient surface cooling. As a result, both deployment scenarios lead to drastically different climate states. In particular, late-century deployment (SAI 2080) creates a striking temperature gradient from a cold northern to a warm southern hemisphere. This phenomenon potentially stems from impaired meridional heat transport of a much weaker AMOC in SAI 2080 compared to SAI 2020.Our findings mirror recent results on fast negative emission scenarios displaying a similar inter-hemispheric gradient likely connected to slow AMOC recovery. This affirms the need for carefully tailoring SAI deployment strategies should the technology ever be considered as part of the climate action portfolio.