State dependence of stratospheric aerosol chemistry-climate impacts in GFDL-ESM4.1

Projecting the chemistry-climate effects of stratospheric aerosols within general circulation models (GCMs) requires simulating multiple coupled processes, which are subject to large uncertainties. Here, we utilize an updated version of the GFDL Earth System Model (GFDL-ESM4.1) with an interactive representation of the stratospheric sulfur cycle to explore the state dependence of stratospheric aerosol chemistry-climate impacts in GFDL-ESM4.1. Understanding this state dependence is crucial for assessing the volcanic chemistry-climate impacts under global warming and is beneficial for evaluating the effectiveness of stratospheric aerosol injection as a geoengineering approach.

We first conduct a baseline simulation from 1989 to 2014, driven by observed sea-surface temperature and sea ice, and including volcanic emissions of sulfur into the stratosphere. Then, we perform sensitivity simulations with sea surface temperature uniformly increased or decreased by 4K to examine the chemistry-climate impacts of stratospheric aerosols under warmer and cooler climate conditions. Our results show that stratospheric aerosol optical depth (SAOD) and burden are sensitive to surface temperature, in our simulations with prescribed volcanic injection heights. In a warmer climate, the accelerated Brewer-Dobson Circulation causes a rapid decay of stratospheric sulfate lifetime and lower SAOD in the 3 years following a the Mt. Pinatubo eruption. The warmer climate also produces a continuously lower SAOD during periods without major eruptions. Changes in SAOD from major eruptions are more sensitive to warming (approximately -11%/K) than to cooling (approximately -5%/K) from the baseline climate. Moreover, the lower SAOD from major eruptions is compensated by higher natural aerosol emissions under a warmer climate, buffering the changes in total AOD. Combined changes—decreased SAOD, albedo feedback, and increased natural aerosols emissions—result in an increase of clear-sky shortwave radiative effect by up to ~2.8 W/m2 in a warmer climate compared to the baseline. We will also explore the follow-on effects on ozone chemistry from the sensitivity of stratospheric aerosols to surface temperature in GFDL-ESM4.1. These results highlights the importance of the interactive sulfur cycle approach in GCMs.