Injection strategy – a driver of atmospheric circulation and ozone response to stratospheric aerosol geoengineering

Abstract

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: 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. In conjunction with the differences in direct radiative impacts at the surface, these drive different impacts on the extratropical modes of variability (Northern and Southern Annular Mode), including important consequences on the northern winter surface climate, as well as 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 response to SAI, as well as lay the ground for designing an optimal SAI strategy that could form a basis of future multi-model intercomparisons.

Quantifying the Efficiency of Stratospheric Aerosol Geoengineering at Different Altitudes

“…In 2021, the National Academies recommended that a research agenda be established to investigate the efficacy, feasibility, and risks of SAI and other methods of solar geoengineering, with the ultimate purpose of informing future decision making (National Academies of Sciences, Engineering, and Medicine, 2021). The effects of SAI on the surface climate and atmospheric circulation would depend on the quantity, latitude, seasonality, and altitude of SO 2 injection (Bednarz, Butler, et al., 2023; Bednarz, Visioni, et al., 2023; Visioni et al., 2023; Zhang et al., 2023). The choice of injection altitude can affect the impacts of SAI through various mechanisms (Tilmes, Richter, Mills, et al., 2018), which we divide into two broad categories: Lifetime and size effects, which affect aerosol optical depth (AOD) produced per unit SO 2 injection: a higher injection altitude places aerosols deeper into the upper branch of the Brewer‐Dobson circulation (BDC), resulting in longer aerosol lifetime against sedimentation (Niemeier et al., 2011) and thus more forcing per unit injection.…”

“…First, a sulfate aerosol layer closer to the tropopause heats the tropical tropopause layer, allowing more water vapor to transport into the stratosphere. The increase in lower stratospheric water vapor results in a net increase of trapped terrestrial infrared radiation, requiring additional SO 2 injection to compensate and decreasing the efficacy of SAI as a whole (Bednarz, Butler, et al., 2023; Bednarz, Visioni, et al., 2023; Heckendorn et al., 2009; K.‐P. S.‐P.…”

“…First, a sulfate aerosol layer closer to the tropopause heats the tropical tropopause layer, allowing more water vapor to transport into the stratosphere. The increase in lower stratospheric water vapor results in a net increase of trapped terrestrial infrared radiation, requiring additional SO 2 injection to compensate and decreasing the efficacy of SAI as a whole (Bednarz, Butler, et al, 2023;Heckendorn et al, 2009; K.-P. S.-P. Krishnamohan et al, 2019;Visioni et al, 2017;Quaglia et al, 2022). Second, a decrease in the vertical temperature gradient in the upper troposphere can result in a thinning of high cirrus clouds which trap outgoing terrestrial radiation, increasing the overall cooling effect of SAI (Visioni et al, 2018).…”

The Choice of Baseline Period Influences the Assessments of the Outcomes of Stratospheric Aerosol Injection

“…Here we have focused on the amount of cooling that SO 2 is chosen to produce, a method that is similar to the scenario exploration under different GHG concentrations in the IPCC scenarios (Meinshausen et al., 2020). In other works, the way in which some impacts are driven by different SO 2 injection locations (the injection strategy) has been explored (Bednarz et al., 2023; Kravitz et al., 2019; Visioni, MacMartin, Kravitz, Richter, et al., 2020; Zhang et al., 2023). Together these studies provide an overview of the possible design space of SAI that form a foundation for future SAI explorations in a multi‐model framework.…”

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

“…The effectiveness of SAI in reducing surface temperatures and mitigating regional climate change will depend on where and when the aerosol precursors are injected (Bednarz et al., 2023a, 2023b; Visioni et al., 2023a; Zhang et al., 2023). In addition, the effectiveness of parallel GHG emission reductions will determine the overall magnitude of SAI needed to maintain or cool the temperatures to a desired level, and the resulting SAI impacts will thus also depend on this desired temperature target (MacMartin et al., 2022; Visioni et al., 2023b).…”