Assessing GFDL-ESM4.1 Climate Responses to a Stratospheric Aerosol Injection Strategy Intended to Avoid Overshoot 2.0°C Warming

In this work, we apply the GFDL Earth System Model (GFDL-ESM4.1) to explore the climate responses to a stratospheric aerosol injection (SAI) scenario that aims to restrict global warming to 2.0°C above pre-industrial levels (1850–1900) under the CMIP6 overshoot scenario (SSP5-34-OS). Simulations of this SAI scenario with the CESM Whole Atmosphere Community Climate Model (CESM2-WACCM6) showed nearly unchanged interhemispheric and pole-to-Equator surface temperature gradients relative to present-day conditions around 2020, and reduced global impacts, such as heatwaves, sea ice melting, and shifting precipitation patterns (Tilmes et al., 2020, https://doi.org/10.5194/esd-11-579-2020). However, model structural uncertainties can lead to varying climate projections under the same forcing. Implementing identical stratospheric aerosol radiative properties in GFDL-ESM4.1, which has a much lower Effective Climate Sensitivity compared to CESM2-WACCM6, resulted in a decrease in global-mean surface temperature by more than 1.5°C and a corresponding reduction in precipitation responses. Two major reasons contribute to the different temperature response between the two models: first, GFDL-ESM4.1 has less warming in the SSP534-OS scenario; second, GFDL-ESM4.1 has shown more pronounced cooling in response to the same stratospheric AOD perturbation. Notably, the Southern Hemisphere experiences substantial cooling compared to the Northern Hemisphere, accompanied by a northward shift of the Intertropical Convergence Zone (ITCZ). Furthermore, our analysis reveals that spatially heterogeneous forcing within the SAI scenario results in diverse climate feedback parameters in the GFDL-ESM4.1 model, through varying surface warming/cooling patterns. This research highlights the importance of considering model structural uncertainties and forcing spatial patterns for a comprehensive evaluation of future scenarios and geoengineering strategies.