Geo-engineering research: Stratospheric aerosol injection and AMOC

By Rob | September 1, 2025

Ewa M. Bednarz, Paul Brent Goddard, Douglas G MacMartin, et al. Stratospheric Aerosol Injection could prevent future Atlantic Meridional Overturning Circulation decline, but injection location is key. ESS Open Archive . December 21, 2024.
DOI: 10.22541/essoar.173482135.52320055/v1

link to scientific article in American Geophysical Union

Stratospheric Aerosol Injection (SAI) can prevent the decline of the Atlantic Meridional Overturning Circulation (AMOC), but only if the aerosols are injected in the Northern Hemisphere. Northern-hemispheric injections influence surface heat flux and ocean densities in the North Atlantic, which sustains AMOC strength. In contrast, Southern-hemispheric SAI does not significantly impact the AMOC, even though it still achieves global cooling.

Why Northern Hemisphere Injection Matters

Direct Impact on North Atlantic: SAI in the Northern Hemisphere directly affects upper-ocean densities in the North Atlantic.
Heat Flux and Temperature: This injection strategy alters surface heat flux and temperature, which is key to preventing the weakening of the AMOC.
AMOC Stability: By maintaining these conditions, Northern Hemisphere SAI helps to prevent the decline of the AMOC, a crucial component of the global climate system.

Why Southern Hemisphere Injection is Ineffective for AMOC

Minimal Impact: Injecting aerosols in the Southern Hemisphere, while producing global cooling, does not have a substantial effect on the strength of the AMOC.
Different Processes: This is because the processes that influence the AMOC are located primarily in the North Atlantic, and Southern-hemispheric injections do not alter these processes sufficiently.

Implications for Climate Intervention

Targeted Application: The findings highlight that the effectiveness of SAI for managing specific climatic risks, like AMOC decline, depends critically on the location of aerosol injection.
Need for Process-Based Understanding: More research is needed to understand the precise processes and feedbacks that determine the outcomes of different SAI strategies.
Potential for Climate Intervention: SAI has the potential to help avoid critical climate tipping points, but the details of its implementation, including the injection location, are crucial for achieving desired climate outcomes.

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 idealized SAI sensitivity simulations with the Community Earth System Model, we demonstrate that whether SAI could restore AMOC depends on the details of SAI implementation, 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. Our results demonstrate the potential for SAI to help avoid some climatic tipping points, but also highlight the need to understand the dependence of the outcomes on the specifics of SAI as well as for a better process‐based understanding of the many factors influencing such outcomes.

Plain Language Summary

The Atlantic Meridional Overturning Circulation (AMOC) consists of warmer tropical surface Atlantic waters moving northward, becoming colder and denser, and sinking in subpolar Atlantic to the lower ocean where they move southward again. AMOC plays a crucial role in global climate system, redistributing heat, carbon, and nutrients. Various studies report both ongoing and projected reductions in the AMOC strength under global warming, with important implications for climate and society. Stratospheric Aerosol Injection (SAI) is a proposed form of climate intervention based on injection of sulfate aerosols into the lower stratosphere. While some studies demonstrated that SAI could effectively mitigate certain risks of anthropogenic global warming, climate model simulations disagree whether this climate intervention could also be successful in ameliorating the projected AMOC decline. Here, we demonstrate that whether SAI could prevent the decline of AMOC depends on the details of SAI implementation, particularly its latitude(s), and not solely on model-dependent representation of climate processes. Our results demonstrate the potential for SAI to help avoid some climatic tipping points, but also highlight the need to understand the dependence of outcomes on the specifics of SAI, as well as for a better process-based understanding of the many factors influencing such outcomes.

Key Points

Whether Stratospheric Aerosol Injection (SAI) could restore Atlantic Meridional Overturning Circulation (AMOC) depends heavily on details of SAI implementation, particularly its latitude(s)
SAI in the Northern Hemisphere leads to AMOC recovery predominantly by altering surface heat fluxes
Initial (∼10–15 years) responses are driven directly by SAI-related forcing, while longer-term responses involve AMOC feedbacks

1 Introduction

The Atlantic Meridional Overturning Circulation (AMOC) plays a pivotal role in the Earth’s thermohaline circulation, arguably representing the largest component of the global ocean circulation system. It transports warm, saline waters from the Southern Hemisphere and the tropics toward the Northern Hemisphere. This process involves the movement of warmer surface waters along the Gulf Stream and the North Atlantic Current toward the coast of Europe, where the waters become denser via loss of heat and moisture to the atmosphere. In the subpolar North Atlantic, significantly cooled and salt-enriched waters descend to deep and abyssal ocean. These denser waters then move southwards at depth, eventually resurfacing in the South Atlantic, thus maintaining a vital cycle that affects global climate by redistributing heat, freshwater, and tracers around the globe (Bower et al., 2019; Buckley & Marshall, 2016).

There are indications that AMOC had remained relatively stable for the last 8,000 years (Fox-Kemper, 2021). Nevertheless, it has long been suggested that AMOC may present more than one stable regime (Stommel, 1961), something confirmed by paleoclimate records that indicate large, sudden weakenings in the overturning circulation during previous abrupt periods of climate change (Lynch-Stieglitz, 2017); a similar weakening under current climate change can thus not be excluded. Owing to the substantial warming of the Arctic region, and the resulting reduced downwelling of cold and salty water in the subpolar North Atlantic, recent research indicates that AMOC could be at its weakest in over a millennium (Caesar et al., 2021) and may be approaching a critical tipping point (e.g., van Westen et al., 2024), possibly by as early as around the mid 21st century (Ditlevsen & Ditlevsen, 2023). A substantial slowdown or complete halt of this circulation on decadal timescales would drastically affect global temperature and precipitation patterns (Barker & Knorr, 2016; Rahmstorf, 2002), sea-ice coverage (Delworth et al., 2016; Liu et al., 2020), sea level (particularly along the North American coast; Goddard et al., 2015; Little et al., 2019), agriculture (Benton, 2020; Jägermeyr et al., 2021), and both marine and terrestrial ecosystems (Bozbiyik et al., 2011; Schmittner, 2005). Such climatic consequences of a potential substantial AMOC weakening and its interactions with other climate system tipping points are synthesized in, for example, Fox-Kemper, 2021; Wunderling et al., 2021; R. Zhang et al., 2019.

Stratospheric Aerosol Injection (SAI) is a proposed form of climate intervention that aims to temporarily alter the global radiative balance, thereby reducing some of the negative impacts of rising greenhouse gas (GHG) levels (e.g., Crutzen, 2006). SAI is based on the injection of aerosols—typically sulfate – or their precursors into the lower stratosphere to reflect a small portion of the incoming solar radiation and, thus, reduce surface temperatures. While some studies demonstrated that SAI could effectively mitigate certain risks of anthropogenic global warming (e.g., Irvine et al., 2019; Kravitz et al., 2019; Lee et al., 2023; Moore et al., 2024), its impacts on AMOC are not well understood. In particular, climate model simulations show disagreement regarding the effectiveness of SAI, and other climate intervention methods, in mitigating the projected AMOC slowdown under rising GHG levels (Xie et al., 2022). The issue is complicated further by uncertainties related to long timescales of ocean processes, and how effective SAI would be if applied after considerable global warming and the resulting AMOC slowdown has already occurred (Pfluger et al., 2024). The matter is also exacerbated by the substantial inter-model uncertainties in the projected AMOC decline under GHG forcing alone (i.e., no-SAI; Weijer et al., 2020; Xie et al., 2022). Similarly, while modeling studies have also linked changes in anthropogenic tropospheric aerosols to impacts on AMOC, the drivers of those changes, as well as their inter-model differences, remain yet to be addressed (e.g., Menary et al., 2020; Robson et al., 2022).

Li et al. (2023) and Fasullo and Richter (2023) analyzed the behavior of AMOC in two versions of the Community Earth System Model (CESM) with the Whole Atmosphere Chemistry Climate Model (WACCM)—CESM1(WACCM5) and CESM2(WACCM6)—under both climate change only scenarios (RCP8.5 in CESM1; and SSP2-4.5 in CESM2) and under SAI scenarios (Geoengineering Lange Ensemble—GLENS, Tilmes et al., 2018—in CESM1; and Assessing Responses and Impacts of Solar climate intervention on the Earth system with SAI—ARISE-SAI1.5, Richter et al., 2022—in CESM2). Those SAI scenarios utilized the same injection strategy which injects SO₂ at four tropical/sub-tropical locations (30°S, 15°S, 15°N, and 30°N; in each Case 180°E) and the injection magnitude is adjusted interactively at the beginning of every year to offset background GHG forcing and maintain the global-mean near-surface temperatures and their large-scale horizontal gradients at predetermined quasi-present day levels. Yet, despite this similar injection strategy, the two SAI simulations showed contrastingly different AMOC responses. CESM1 showed substantial AMOC strengthening under SAI in GLENS compared to both the corresponding no-SAI scenario as well as its target quasi-present day levels; in contrast, CESM2 showed only a slight reduction in the rate of AMOC decline under SAI in ARISE-SAI1.5 compared to the no-SAI case (Figure 1 here). Li et al. (2023) concluded that differences in the representation of physical processes and ocean climate states within the models are the primary drivers of the contrasting AMOC responses to SAI between CESM1 and CESM2. Fasullo and Richter (2023) further argued that these different AMOC states and responses to both GHGs and SAI contributed to significant differences in the partitioning of aerosol injections across the four latitudes.

Here, we utilize idealized SAI sensitivity simulations with the CESM2(WACCM6) model and demonstrate that while inter-model differences in model physics are relevant, the details of the SAI realization alone—in particular aerosol injection location and hence the resulting latitudinal aerosol distribution—can be of primary importance in determining the AMOC response to SAI. The remainder of the paper is structured as follows: Section 2 describes the model and experiments performed. Section 3 discusses the simulated evolution of AMOC in the different SAI experiments. Section 4 analyzes whether the SAI influence on AMOC is through changes in the North Atlantic surface heat flux and/or salinity, and Section 5 further breaks down the simulated changes in surface heat flux into individual components. Finally, Section 6 summarizes and discusses the main results.

Simulated evolution of the annual mean Atlantic Meridional Overturning Circulation (AMOC) strength [Sv], defined as the maximum annual‐mean AMOC transport in the NH computed in depth space, in the CESM1(WACCM5) RCP8.5 and Stratospheric Aerosol Injection (SAI) (GLENS) simulations (Tilmes et al., 2018), the CESM2(WACCM6) SSP2‐4.5 and SAI (ARISE‐SAI1.5) simulations (Richter et al., 2022), and the CESM2(WACCM6) SSP5‐8.5 and SAI (GEO SSP5‐8.5 1.5) simulations (Tilmes et al., 2020). Individual lines denote individual ensemble members and thick lines denote the corresponding ensemble means. After Li et al. (2023).

Left panels: Simulated evolution of the annual mean North Atlantic (a) surface heat flux [W/m²] (with negative values representing heat flux from the ocean to the atmosphere), (d) surface salinity [g/kg], and (g) surface temperature [°C]. The North Atlantic region defined as in Figure 3, also indicated as a black box in panel (b). Middle and right panels show the corresponding differences between the SAI‐45N and SSP2‐4.5 simulations averaged over 2040–2049 (middle) and 2060–2069 (right) periods. Stippling denotes regions where the difference is not statistically significant at the 95% level.

References

Barker, S., & Knorr, G. (2016). A paleo-perspective on the AMOC as a tipping element. Past Global Change Magazine, 24/1(1), 14–15. https://doi.org/10.22498/pages.24.1.14