Stratospheric Ozone Response to Sulfate Aerosol and Solar Dimming Climate Interventions based on the G6 Geoengineering Model Intercomparison Project (GeoMIP) Simulations

Abstract

Abstract. This study assesses the impacts of sulfate aerosol intervention (SAI) and solar dimming on stratospheric ozone based on the G6 Geoengineering Model Intercomparison Project (GeoMIP) experiments, called G6sulfur and G6solar. For G6sulfur the stratospheric sulfate aerosol burden is increased to reflect some of the incoming solar radiation back into space in order to cool the surface climate, while for G6solar the global solar constant is reduced to achieve the same goal. The high emissions scenario SSP5-8.5 is used as the baseline experiment and surface temperature from the medium emission scenario SSP2-4.5 is the target. Based on three out of six Earth System Models (ESMs) that include interactive stratospheric chemistry, we find significant differences in the ozone distribution between G6solar and G6sulfur experiments compared to SSP5-8.5 and SSP2-4.5, which differ by both region and season. Both SAI and solar dimming methods reduce incoming solar insolation and result in tropospheric temperatures comparable to SSP2-4.5 conditions. G6sulfur increases the concentration of absorbing sulfate aerosols in the stratosphere, which increases lower tropical stratospheric temperatures by between 5 to 13 K for six different ESMs, leading to changes in stratospheric transport. The increase of the aerosol burden also increases aerosol surface area density, which is important for heterogeneous chemical reactions. The resulting changes in ozone include a significant reduction of total column ozone (TCO) in the Southern Hemisphere polar region in October of 10 DU at the onset and up to 20 DU by the end of the century. The relatively small reduction in TCO for the multi-model mean in the first two decades results from variations in the required sulfur injections in the models and differences in the complexity of the chemistry schemes, with no significant ozone loss for 2 out of 3 models. The decrease in the second half of the 21st century counters increasing TCO between SSP2-4.5 and SSP5-8.5 due to the super-recovery resulting from increasing greenhouse gases. In contrast, in the Northern Hemisphere (NH) high latitudes, only a small initial decline in TCO is simulated, with little change in TCO by the end of the century compared to SSP5-8.5. All models consistently simulate an increase in TCO in the NH mid-latitudes up to 20 DU compared to SSP5-8.5, in addition to 20 DU increase resulting from increasing greenhouse gases between SSP2-4.5 and SSP5-8.5. G6solar counters zonal wind and tropical upwelling changes between SSP2-4.5 and SSP5-8.5 but does not change stratospheric temperatures. Solar dimming results in little change in TCO compared to SSP5-8.5 and does not counter the effects of the ozone super-recovery. Only in the tropics, G6solar results in an increase of TCO of up to 8 DU compared to SSP2-4.5, which may counter the projected reduction due to climate change in the high forcing future scenario. This work identifies differences in the response of SAI and solar dimming on ozone, which are at least partly due to differences and shortcomings in the complexity of aerosol microphysics, chemistry, and the description of ozone photolysis in the models. It also identifies that solar dimming, if viewed as an analog to SAI using a predominantly scattering aerosol, would, for the most part, not counter the potential harmful increase in TCO beyond historical values induced by increasing greenhouse gases.

Potential Non‐Linearities in the High Latitude Circulation and Ozone Response to Stratospheric Aerosol Injection

“…Unlike in the SH, the NH ozone columns largely increase under SAI during boreal winter and spring (Figures 4b and 4c) due to the SAI-induced changes in the BDC and the resulting ozone transport (Section 3; see also Tilmes et al, 2022;Bednarz et al, 2023b). Owing to the Arctic vortex being climatologically weaker and more variable than its SH counterpart, the chemical impacts from the SAI-induced enhancement of the heterogenous halogen processing on the elevated SAD are generally smaller.…”

Identifying Climate Impacts From Different Stratospheric Aerosol Injection Strategies in UKESM1

“…Changes to stratospheric temperatures as a result of SAI can drive changes in stratospheric ozone, due to changes in both stratospheric dynamics and chemistry. Studies have shown that enhancements of the stratospheric sulfate aerosol layer from SAI would increase the aerosol surface area density, influencing halogen activation in the lower stratosphere and the removal of active nitrogen species in the middle stratosphere, thereby modulating chemical ozone loss (Bednarz et al, 2023a(Bednarz et al, , 2023bTilmes et al, 2018aTilmes et al, , 2022. In addition, the SAI-induced lower stratospheric heating will also influence ozone via changes in the large scale transport as well as through increased stratospheric water vapor levels and thus chemical ozone loss.…”

A subpolar-focused stratospheric aerosol injection deployment scenario

“…Similar to the results from global or tropical injections, subpolar SAI will also result in heating of the lower stratosphere (Niemeier et al 2013, Ferraro et al 2015 , although because the aerosols would be focused at higher latitudes, there would be less heating per unit injection. In addition, the introduction of aerosols into the stratosphere enhances the aerosol-induced surface area density which leads to an increase in heterogeneous reactions required for halogen activations (Solomon 1999, Tilmes et al 2021, though as the aerosols would primarily be present during the summer this may be less of an effect than for global SAI. Consequently, subpolar SAI may impact stratospheric ozone concentrations through a combination of dynamical and chemical effects, In addition to the direct climatic impacts resulting from the interaction of injected aerosols with the stratosphere, the carbon footprint associated with the deployment program also carries environmental risks.…”