2023 News & Events
For Stratospheric Aerosol Injection, All Strategies are Not Created Equal
6 November 2023
A new study finds that injecting SO2 at higher latitudes, rather than in the tropics, could mitigate some undesirable side effects of SAI – but all options come with trade-offs.
As the world continues to witness the effects of climate change and substantial emissions reductions remain unrealized, discussions on climate mitigation have increasingly considered some form of intervention as a stop-gap measure to stave off the worst impacts of rising temperatures.
One such potential method of climate intervention, known as stratospheric aerosol injection (SAI), aims to mimic the planet cooling effects of volcanic eruptions by injecting sulfur dioxide (SO2) directly into the stratosphere where it forms sunlight-reflecting sulfate aerosols. Although the overall goal of SAI is straightforward (reflect more sunlight), when it comes to considering how such an intervention could or would be implemented, a complex patchwork of side effects and trade-offs emerges.
In a new study published in Atmospheric Chemistry and Physics, scientists from NOAA CSL and CIRES, in collaboration with Cornell and Indiana University, carefully examined a range of potential injection strategies by using a chemistry-climate model to simulate SAI while varying both the amount of SO2 injected into the stratosphere and the latitudes where it is injected.
What they found was a complicated picture: a diverse range of outcomes beyond just decreased surface temperatures, that included impacts on the stratospheric ozone layer, large-scale circulation patterns, and regional weather and precipitation, that vary both spatially and seasonally.
On top of this complexity, however, a general pattern also became apparent: many of the well established undesirable side effects of SAI associated with circulation and precipitation changes appear strongest when the injections are made at tropical latitudes around the equator, but are lessened if injections occur outside of the tropics.
"The major finding is that regional changes in weather patterns that may occur in response to SAI in the tropics, such as Eurasian winter warming and reduced tropical precipitation, are substantially reduced if instead the aerosols are injected at mid-latitudes or near the poles," explained lead author Ewa Bednarz, a CIRES research scientist at NOAA CSL.
The devil in the details
Although there are multiple different SAI strategies that can achieve a desired temperature decrease, the other unintended consequences of these strategies are not all equal.
In their study, Bednarz and coauthors considered a comprehensive set of SAI scenarios that all achieve the same global mean surface temperature (1°C above pre-industrial levels) 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.
The different strategies they examined resulted in contrastingly different magnitudes of aerosol-induced warming of the lower stratosphere and stratospheric moistening, which can thus partially offset the intended cooling effect since water vapor acts as a greenhouse gas. These different strategies also led to different magnitudes of strengthening of polar jets and weakening of the large-scale circulation flows (e.g., Hadley and Walker circulation), which are linked to regional temperature and precipitation patterns and will thus directly affect how people would experience SAI.
All of these effects are minimized in the 30° and polar strategies. However, additional impacts for these strategies also need to be considered, such as their impact on stratospheric ozone. In fact, all strategies considered resulted in a significant reduction of ozone over Antarctica, but via different mechanisms. The UV-shielding of stratospheric ozone is critically important to protect life on this planet; thus, any impacts to ozone must be examined carefully.
"There is a fine interplay of radiative, dynamical, and chemical processes that drive the circulation and ozone responses to SAI," added CSL scientist and coauthor Amy Butler. "All of the different regional and seasonal effects need to be considered, highlighting the complexity and trade-offs in evaluating which strategy is the most optimal."
Bednarz, E.M., A.H. Butler, D. Visioni, Y. Zhang, B. Kravitz, and D.G. MacMartin, Injection strategy - a driver of atmospheric circulation and ozone response to stratospheric aerosol geoengineering, Atmospheric Chemistry and Physics, doi:10.5194/acp-23-13665-2023, 2023.
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.