The Atlantic Meridional Overturning Circulation (AMOC) plays a crucial role in the global climate system. Various studies report both ongoing and projected reductions in AMOC strength, with important implications for climate and society. While Stratospheric Aerosol Injection (SAI) has been proposed to mitigate some impacts of a warming climate, model simulations disagree whether it could also be successful in ameliorating the projected AMOC decline. Using SAI sensitivity simulations with the Community Earth System Model, we demonstrate that whether SAI could restore AMOC depends on the details of SAI realization, particularly its latitude(s). Specifically, Northern-hemispheric SAI initially impacts upper-ocean densities in the North Atlantic through changes in surface heat flux and temperature, ultimately preventing AMOC decline. On the other hand, Southern-hemispheric SAI does not substantially impact AMOC strength even though global mean cooling is achieved. We show that different processes play different roles in determining the AMOC response between the initial (~10-15 years) and longer timescales, with the former dominated by the direct SAI effect and the latter influenced by feedbacks from AMOC adjustments. These processes may also offset each other, leading to a relatively stable evolution of AMOC under each SAI realization and a small, yet substantially different, subset of potential AMOC responses. Overall, our results demonstrate the potential for SAI to help avoid climatic tipping points, but also highlight the need to understand the dependence of the outcomes on the specific SAI realization as well as for a better process-based understanding of the many factors influencing such outcomes.

Rising global temperatures pose significant risks to marine ecosystems, biodiversity and fisheries. Recent comprehensive assessments suggest that large-scale mitigation efforts to limit warming below crucial thresholds are falling short, and all feasible future climate projections, including those that represent ideal emissions reductions, exceed the Paris Agreement’s aspirational <1.5{degree sign}C warming target, at least temporarily. As such, there are a number of proposed climate interventions that aim to deliberately manipulate the environment at large scales to counteract anthropogenic global warming. Yet, there is a high level of uncertainty in how marine ecosystems will respond to these interventions directly as well as how these interventions may impact marine ecosystems’ responses to climate change. Due to the key role the ocean plays in regulating Earth’s climate and ensuring global food security, understanding the effects that these interventions may have on marine ecosystems is crucial. This review provides an overview of proposed intervention methodologies for solar radiation modification and marine carbon dioxide removal and outlines the potential trade-offs and knowledge gaps associated with their impacts on marine ecosystems. Climate interventions have the potential to reduce warming-driven impacts, but could also substantially alter marine food systems, biodiversity and ecosystem function. Impact assessments are thus crucial to quantify trade-offs between plausible intervention scenarios and to identify and discontinue scaling efforts or commercial implementation for those with unacceptable risks.

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, walkerl@ucar.eduSubmitted 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.

This study examines West Africa’s climate vulnerability under stratospheric aerosol injection (SAI) using two climate models: UKESM1 and CESM2. We analyzed temperature and precipitation patterns between 2050-2069 compared to 2015-2034 under two scenarios: SSP2-4.5 and ARISE-SAI-1.5. Our methodology involved evaluating temperature and precipitation anomalies, using Signal-to-Noise Ratio (SNR) analysis to assess climate signal reliability, and analyzing precipitation extremes through Cumulative Distribution Function (CDF) and Probability Density Function (PDF). Under SSP2-4.5, UKESM1 showed temperature increases of 1.8{degree sign}C while CESM2 showed increases of 1.0-1.2{degree sign}C. However, under ARISE-SAI-1.5, UKESM1 showed cooling (-0.3{degree sign}C below reference period) while CESM2 maintained slight warming (0-0.3{degree sign}C above reference). SNR analysis revealed that SAI significantly dampened the warming signal in UKESM1, making precipitation and temperature trends less detectable. CDF and PDF analyses showed that while SAI may reduce warming, it increases precipitation variability and uncertainty. These results emphasize the importance of multi-model comparisons when assessing geoengineering impacts, as models can produce varying results based on their sensitivity to radiative forcing and other factors.

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

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.

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 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.

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 four latitudes (30°, 15° N/S), it might be possible not only to reduce global mean surface temperature but also to minimize changes in the equator-to-pole and inter-hemispheric gradients of temperature, further reducing some of the impacts arising from climate change relative to equatorial injection. This can happen only if the 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. However, the stratospheric heating produced by these aerosols modifies the circulation and strengthens the stratospheric polar vortex which acts as a barrier to the transport of air toward the poles. We show how the heating results in a feedback where increasing injection rates lead to stronger high-latitudinal transport barriers. This implies a potential limitation in the high-latitude aerosol burden and subsequent 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.