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Abstract. Despite offsetting global mean surface temperature, various studies demonstrated that stratospheric aerosol injection (SAI) could influence the recovery of stratospheric ozone and have important impacts on stratospheric and tropospheric circulation, thereby potentially playing an important role in modulating regional and seasonal climate variability. However, so far, most of the assessments of such an approach have come from climate model simulations in which SO2 is injected only in a single location or a set of locations. Here we use CESM2-WACCM6 SAI simulations under a comprehensive set of SAI strategies achieving the same global mean surface temperature with different locations and/or timing of injections, namely an equatorial injection, an annual injection of equal amounts of SO2 at 15∘ N and 15∘ S, an annual injection of equal amounts of SO2 at 30∘ N and 30∘ S, and a polar strategy injecting SO2 at 60∘ N and 60∘ S only in spring in each hemisphere. We demonstrate that despite achieving the same global mean surface temperature, the different strategies result in contrastingly different magnitudes of the aerosol-induced lower stratospheric warming, stratospheric moistening, strengthening of stratospheric polar jets in both hemispheres, and changes in the speed of the residual circulation. These impacts tend to maximise under the equatorial injection strategy and become smaller as the aerosols are injected away from the Equator into the subtropics and higher latitudes. In conjunction with the differences in direct radiative impacts at the surface, these different stratospheric changes drive different impacts on the extratropical modes of variability (Northern and Southern Annular modes), including important consequences on the northern winter surface climate, and on the intensity of tropical tropospheric Walker and Hadley circulations, which drive tropical precipitation patterns. Finally, we demonstrate that the choice of injection strategy also plays a first-order role in the future evolution of stratospheric ozone under SAI throughout the globe. Overall, our results contribute to an increased understanding of the fine interplay of various radiative, dynamical, and chemical processes driving the atmospheric circulation and ozone response to SAI and lay the foundation for designing an optimal SAI strategy that could form a basis of future multi-model intercomparisons.

 

Abstract. Solar climate intervention using stratospheric aerosol injection (SAI) has been proposed as a method which could offset some of the adverse effects of global warming. The Assessing Responses and Impacts of Solar climate intervention on the Earth system with Stratospheric Aerosol Injection (ARISE-SAI) set of simulations is based on a moderate-greenhouse-gas-emission scenario and employs injection of sulfur dioxide at four off-equatorial locations using a control algorithm which maintains the global-mean surface temperature at 1.5 K above pre-industrial conditions (ARISE-SAI-1.5), as well as the latitudinal gradient and inter-hemispheric difference in surface temperature. This is the first comparison between two models (CESM2 and UKESM1) applying the same multi-target SAI strategy. CESM2 is successful in reaching its temperature targets, but UKESM1 has considerable residual Arctic warming. This occurs because the pattern of temperature change in a climate with SAI is determined by both the structure of the climate forcing (mainly greenhouse gases and stratospheric aerosols) and the climate models' feedbacks, the latter of which favour a strong Arctic amplification of warming in UKESM1. Therefore, research constraining the level of future Arctic warming would also inform any hypothetical SAI deployment strategy which aims to maintain the inter-hemispheric and Equator-to-pole near-surface temperature differences. Furthermore, despite broad agreement in the precipitation response in the extratropics, precipitation changes over tropical land show important inter-model differences, even under greenhouse gas forcing only. In general, this ensemble comparison is the first step in comparing policy-relevant scenarios of SAI and will help in the design of an experimental protocol which both reduces some known negative side effects of SAI and is simple enough to encourage more climate models to participate.

 

Abstract Stratospheric aerosol geoengineering has been proposed as a potential solution to reduce climate change and its impacts. Here, we explore the responses of the Hadley circulation (HC) intensity and the intertropical convergence zone (ITCZ) using the strategic stratospheric aerosol geoengineering, in which sulfur dioxide was injected into the stratosphere at four different locations to maintain the global-mean surface temperature and the interhemispheric and equator-to-pole temperature gradients at present-day values (baseline). Simulations show that, relative to the baseline, strategic stratospheric aerosol geoengineering generally maintains northern winter December–January–February (DJF) HC intensity under RCP8.5, while it overcompensates for the greenhouse gas (GHG)-forced southern winter June–July–August (JJA) HC intensity increase, producing a 3.5 ± 0.4% weakening. The residual change of southern HC intensity in JJA is mainly associated with stratospheric heating and tropospheric temperature response due to enhanced stratospheric aerosol concentrations. Geoengineering overcompensates for the GHG-driven northward ITCZ shifts, producing 0.7° ± 0.1° and 0.2° ± 0.1° latitude southward migrations in JJA and DJF, respectively relative to the baseline. These migrations are affected by tropical interhemispheric temperature differences both at the surface and in the free troposphere. Further strategies for reducing the residual change of HC intensity and ITCZ shifts under stratospheric aerosol geoengineering could involve minimizing stratospheric heating and restoring and preserving the present-day tropical tropospheric interhemispheric temperature differences. 

The impacts of Stratospheric Aerosol Injection (SAI) on the atmosphere and surface climate depend on when and where the sulfate aerosol precursors are injected, as well as on how much surface cooling is to be achieved. We use a set of CESM2(WACCM6) SAI simulations achieving three different levels of global mean surface cooling and demonstrate that unlike some direct surface climate impacts driven by the reflection of solar radiation by sulfate aerosols, the SAI‐induced changes in the high latitude circulation and ozone are more complex and could be non‐linear. This manifests in our simulations by disproportionally larger Antarctic springtime ozone loss, significantly larger intra‐ensemble spread of the Arctic stratospheric jet and ozone responses, and non‐linear impacts on the extratropical modes of surface climate variability under the strongest‐cooling SAI scenario compared to the weakest one. These potential non‐linearities may add to uncertainties in projections of regional surface impacts under SAI.

 

The impacts of Stratospheric Aerosol Injection (SAI) strategies on the Southern Annular Mode (SAM) are analyzed with the Community Earth System Model. Using a set of simulations with fixed single‐point SO₂injections we demonstrate the first‐order dependence of the SAM response on the latitude of injection, with the northern hemispheric and equatorial injections driving a response corresponding to a positive phase of SAM and the southern hemispheric injections driving a negative phase of SAM. We further demonstrate that the results can to first order explain the differences in the SAM responses diagnosed from the two recent large ensembles of geoengineering simulations utilizing more complex injection strategies – Geoengineering Large Ensemble and Assessing Responses and Impacts of Solar climate intervention on the Earth system with Stratospheric Aerosol Injection (GLENS and ARISE‐SAI) – as driven by the differences in the simulated sulfate aerosol distributions. Our results point to the meridional extent of aerosol‐induced lower stratospheric heating as an important driver of the sensitivity of the SAM response to the injection location.

 

Dichloromethane (CH₂Cl₂) and perchloroethylene (C₂Cl₄) are chlorinated very short lived substances (Cl‐VSLS) with anthropogenic sources. Recent studies highlight the increasing influence of such compounds, particularly CH₂Cl₂, on the stratospheric chlorine budget and therefore on ozone depletion. Here, a multiyear global‐scale synthesis inversion was performed to optimize CH₂Cl₂(2006–2017) and C₂Cl₄(2007–2017) emissions. The approach combines long‐term surface observations from global monitoring networks, output from a three‐dimensional chemical transport model (TOMCAT), and novel bottom‐up information on prior industry emissions. Our posterior results show an increase in global CH₂Cl₂emissions from 637 ± 36 Gg yr⁻¹in 2006 to 1,171 ± 45 Gg yr⁻¹in 2017, with Asian emissions accounting for 68% and 89% of these totals, respectively. In absolute terms, Asian CH₂Cl₂emissions increased annually by 51 Gg yr⁻¹over the study period, while European and North American emissions declined, indicating a continental‐scale shift in emission distribution since the mid‐2000s. For C₂Cl₄, we estimate a decrease in global emissions from 141 ± 14 Gg yr⁻¹in 2007 to 106 ± 12 Gg yr⁻¹in 2017. The time‐varying posterior emissions offer significant improvements over the prior. Utilizing the posterior emissions leads to modeled tropospheric CH₂Cl₂and C₂Cl₄abundances and trends in good agreement to those observed (including independent observations to the inversion). A shorter C₂Cl₄lifetime, from including an uncertain Cl sink, leads to larger global C₂Cl₄emissions by a factor of ~1.5, which in some places improves model‐measurement agreement. The sensitivity of our findings to assumptions in the inversion procedure, including CH₂Cl₂oceanic emissions, is discussed.