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.

Walker Raymond Leea, Michael Diamondb, Pete Irvinec, Jesse Reynoldsd, Daniele VisionieaClimate and Global Dynamics Division, National Center for Atmospheric Research, Boulder, CO, USAbDepartment of Earth, Ocean, and Atmospheric Science, Florida State University, Tallahassee, FL, USAcEarth Sciences, University College London, London, UKdThe DEGREES Initiative, London, UKeDepartment of Earth and Atmospheric Sciences, Cornell University, Ithaca, NY, USACorr. author: Walker Raymond Lee, [email protected] 5/31/24 to Nature Communications Earth & Environment as Matters Arising regarding the study "Radiative forcing geoengineering causes higher risk of wildfires and permafrost thawing over the Arctic regions" (R. C. Müller, et al, 2024, https://doi.org/10.1038/s43247-024-01329-3)The study “Radiative forcing geoengineering causes higher risk of wildfires and permafrost thawing over the Arctic regions”1 (henceforth “Müller, et al.”) examines three scenarios of radiative forcing geoengineering (RFG) - stratospheric aerosol injection (SAI), marine cloud brightening (MCB), and cirrus cloud thinning (CCT) - as simulated by the Norwegian Earth System Model (NorESM2), comparing high-latitude (>50°N) boreal summer maximum temperatures (TXx) and winter minimum temperatures (TNn) for the RFG scenarios to the high-warming RCP8.5 and moderate-warming RCP4.5 scenarios. They conclude that all three RFG interventions worsen the risk of wildfire and permafrost thaw relative to RCP4.5. We have significant concerns about how this paper’s results and conclusions are framed. First and foremost, the title of the study claims that RFG increases risk of wildfires and permafrost thaw; instead, what the authors show is that RFG reduces these risks, but not as much as an equivalent scenario of emissions cuts. Secondly, the authors overgeneralize from a limited set of simulations even though it is now well known that regional impacts are highly dependent on the specific RFG strategy employed2.Our first concern relates to how Müller, et al. characterize “risk”. All three RFG interventions were simulated in a context of RCP8.5 emissions and designed to achieve the same global radiative balance as RCP4.5. It is clear from Figure 1 of Müller, et al. that the interventions substantially reduce global and Arctic mean temperatures relative to RCP8.5 by 2100. While it may be the case that, relative to RCP8.5, the greenhouse gas mitigation represented by RCP4.5 more efficiently reduces risk than any of the RFG interventions (assuming they were used as a substitute for that mitigation), the title misattributes the impacts of increased GHGs plus RFG to RFG alone; their Figures 2-6 present results with respect to RCP4.5, which is not, on its own, a suitable frame of reference to determine the impacts of RFG. International assessments of RFG underscore that such methods should not be considered as a substitute to emissions reduction3, not least because the environmental consequences of GHGs and RFG can be very different4. Thus, to have a clear and accurate sense of their potential consequences, an assessment of RFG’s potential climatic risks must consider them in relation to, not isolated from, the counterfactual risks of a world where warming is unabated by RFG. In Figure 1, we plot July maximum (TXx) and January minimum (TNn) temperature differences for each RFG realization to both RCP8.5 and RCP4.5 using the authors’ data. The authors’ data show a reduction of risk of wildfires and permafrost thaw in the RFG intervention scenarios compared to a world with the same CO2 concentrations but without RFG (in line with other studies5,6. Müller, et al. mischaracterize the response to RFG as an increase in these risks because they compare against the wrong baseline, ignoring the appropriate counterfactual.

A document by Daniele Visioni. Click on the document to view its contents.

The specifics of the simulated injection choices in the case of Stratospheric Aerosol Injections (SAI) are part of the crucial context necessary for meaningfully discussing the impacts that a deployment of SAI would have on the planet. One of the main choices is the desired amount of cooling that the injections are aiming to achieve. Previous SAI simulations have usually either simulated a fixed amount of injection, resulting in a fixed amount of warming being offset, or have specified one target temperature, so that the amount of cooling is only dependent on the underlying trajectory of greenhouse gases. Here, we use three sets of SAI simulations achieving different amounts of global mean surface cooling while following a middle-of-the-road greenhouse gas emission trajectory: one SAI scenario maintains temperatures at 1.5ºC above preindustrial levels (PI), and two other scenarios which achieve additional cooling to 1.0ºC and 0.5ºC above PI. We demonstrate that various surface impacts scale proportionally with respect to the amount of cooling, such as global mean precipitation changes, changes to the Atlantic Meridional Overturning Circulation (AMOC) and to the Walker Cell. We also highlight the importance of the choice of the baseline period when comparing the SAI responses to one another and to the greenhouse gas emission pathway. This analysis leads to policy-relevant discussions around the concept of a reference period altogether, and to what constitutes a relevant, or significant, change produced by SAI.

Owing to increasing greenhouse gas emissions, the West Antarctic Ice Sheet as well as a few subglacial basins in East Antarctica are vulnerable to rapid ice loss in the upcoming decades and centuries, respectively. This study examines the effectiveness of using Stratospheric Aerosol Injection (SAI) that minimizes global mean temperature (GMT) change to slow projected 21st century Antarctic ice loss. We use eleven different SAI cases which vary by the latitudinal location(s) and the amount(s) of the injection(s) to examine the climatic response near Antarctica in each case as compared to the reference climate at the turn of the last century. We demonstrate that injecting at a single latitude in the northern hemisphere or at the Equator increases Antarctic shelf ocean temperatures pertinent to ice shelf basal melt, while injecting only in the southern hemisphere minimizes this temperature change. We use these results to analyze the results of more complex multi-latitude injection strategies that maintain GMT at or below 1.5°C above the pre-industrial. All these cases will slow Antarctic ice loss relative to the mid-to-late 21st century SSP2-4.5 emissions pathway. Yet, to avoid a GMT threshold estimated by previous studies pertaining to rapid West Antarctic ice loss (~1.5°C above the pre-industrial), our study suggests SAI would need to cool below this threshold and predominately inject at low southern hemisphere latitudes. These results highlight the complexity of factors impacting the Antarctic response to SAI and the critical role of the injection strategy in preventing future ice loss.

Simulating whole atmosphere dynamics, chemistry, and physics is computationally expensive. It can require high vertical resolution throughout the middle and upper atmosphere, as well as a comprehensive chemistry and aerosol scheme coupled to radiation physics. An unintentional outcome of the development of one of the most sophisticated and hence computationally expensive model configurations is that it often excludes a broad community of users with limited computational resources. Here, we analyze two configurations of the Community Earth System Model Version 2, Whole Atmosphere Community Climate Model Version 6 (CESM2(WACCM6)) with simplified “middle atmosphere” chemistry at nominal 1 and 2 degree horizontal resolutions. Using observations, a reanalysis, and direct model comparisons, we find that these configurations generally reproduce the climate, variability, and climate sensitivity of the 1 degree nominal horizontal resolution configuration with comprehensive chemistry. While the background stratospheric aerosol optical depth is elevated in the middle atmosphere configurations as compared to the comprehensive chemistry configuration, it is comparable between all configurations during volcanic eruptions. For any purposes other than those needing an accurate representation of tropospheric organic chemistry and secondary organic aerosols, these simplified chemistry configurations deliver reliable simulations of the whole atmosphere that require 35% to 86% fewer computational resources at nominal 1 and 2 degree horizontal resolution, respectively.

By injecting SO2 into the stratosphere at different latitudes, it might be possible to reduce global mean surface temperature and to minimize changes in the equator-to-pole and inter-hemispheric gradients, reducing some of the impacts arising from climate change relative to equatorial SO2 injection. This can happen only if the resulting aerosols are transported to higher latitudes by the stratospheric circulation, ensuring that a greater part of the solar radiation is reflected back to space at higher latitudes, compensating for the reduced sunlight there. However, the stratospheric heating produced by these aerosols modifies the global circulation and strengthens the stratospheric polar vortex that acts as a barrier to the transport of air towards the poles. We show how this heating results in a nonlinear feedback where increasing injection rates lead to a stronger high latitudinal transport barrier. This implies a potential limitation in the high-latitude aerosol burden and subsequent high-latitude cooling.

Stratospheric aerosol injection (SAI) can provide global cooling by adding aerosols to the lower stratosphere, and thus is considered as a possible supplement to emission reduction. Previous studies have shown that injecting aerosols at different latitude(s) and season(s) can lead to differences in regional surface climate, and there are at least three independent degrees of freedom (DOF) that can be used to simultaneously manage three different climate goals. To understand the fundamental limits of how well SAI might compensate for anthropogenic climate change, we need to know the possible surface climate responses to SAI by evaluating the SAI design space. This research work quantifies the number of meaningfully-independent DOFs of the SAI design space. This number of meaningfully-independent DOF depends on both the climate metrics that we care about and the amount of cooling. From the available simulation data of different SAI strategies, we observe that between surface air temperature and precipitation, surface air temperature dominates the change of surface climate. The number of injection choices that produce detectably different surface temperature is more than the number of injection choices that produce detectably different precipitation. At low levels of cooling, only a small set of injection choices yield detectably different surface climate responses. As more cooling is needed, more injection choices produce detectably different surface climate. For a cooling level of 1-2C, we find that there are likely between 6 and 12 DOFs. This reveals new opportunities for exploring alternate SAI designs with different distributions of climate impacts and evaluating the underlying trade-offs between different climate goals.

Deliberately blocking out a small portion of the incoming solar radiation would cool the climate. One such approach would be injecting SO$_2$ into the stratosphere, which would produce sulfate aerosols that would remain in the atmosphere for 1–3 years, reflecting part of the incoming shortwave radiation. This would not affect the climate the same way as increased greenhouse gas (GHG) concentrations, leading to residual differences when a GHG increase is offset by stratospheric sulfate geoengineering. Many climate model simulations of geoengineering have used a uniform reduction of the incoming solar radiation as a proxy for stratospheric aerosols, both because many models are not designed to adequately capture relevant stratospheric aerosol processes, and because a solar reduction has often been assumed to capture the most important differences between how stratospheric aerosols and GHG would affect the climate. Here we show that dimming the sun does not produce the same surface climate effects as simulating aerosols in the stratosphere. By more closely matching the spatial pattern of solar reduction to that of the aerosols, some improvements in this idealized representation are possible, with further improvements if the stratospheric heating produced by the aerosols is included. This is relevant both for our understanding of the physical mechanisms driving the changes observed in stratospheric sulfate geoengineering simulations, and in terms of the relevance of impact assessments that use a uniform solar dimming.

The impacts of Stratospheric Aerosol Injection strategies on the Southern Annular Mode (SAM) are analysed with the Community Earth System Model (CESM). Using a set of simulations with fixed single-point SO2 injections 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 utilising more complex injection strategies - 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.

Stratospheric aerosol geoengineering focused on the Arctic could substantially reduce local and worldwide impacts of anthropogenic global warming. Because the Arctic receives little sunlight during the winter, stratospheric aerosols present in the winter at high latitudes have little impact on the climate, whereas stratospheric aerosols present during the summer achieve larger changes in radiative forcing. Injecting SO2 in the spring leads to peak aerosol optical depth (AOD) in the summer. We demonstrate that spring injection produces approximately twice as much summer AOD as year-round injection and restores approximately twice as much September sea ice, resulting in less increase in stratospheric sulfur burden, stratospheric heating, and stratospheric ozone depletion per unit of sea ice restored. We also find that differences in AOD between different seasonal injection strategies are small compared to the difference between annual and spring injection.

Stratospheric aerosol injection (SAI) has been shown in climate models to reduce some impacts of global warming in the Arctic, including the loss of sea ice, permafrost thaw, and reduction of Greenland Ice Sheet (GrIS) mass; SAI at high latitudes could preferentially target these impacts. In this study, we use the Community Earth System Model to simulate two Arctic-focused SAI strategies, which inject at 60°N latitude each spring with injection rates adjusted to either maintain September Arctic sea ice at 2030 levels (“Arctic Low”) or restore it to 2010 levels (“Arctic High”). Both simulations maintain or restore September Arctic sea ice to within 10% of their respective targets, reduce permafrost thaw, and increase GrIS surface mass balance by reducing runoff. Arctic High reduces these impacts more effectively than a globally-focused SAI strategy that injects similar quantities of SO2 at lower latitudes. However, Arctic-focused SAI is not merely a “reset button” for the Arctic climate, but brings about a novel climate state, including changes to the seasonal cycles of Northern Hemisphere temperature and sea ice and less high-latitude carbon uptake relative to SSP2-4.5. Additionally, while Arctic-focused SAI predominantly cools the Arctic, its effects are not confined to the Arctic, including detectable cooling throughout most of the northern hemisphere for both simulations, increased mid-latitude sulfur deposition, and a southward shift of the location of the Intertropical Convergence Zone (ITCZ).