AMOC tipping response

Collapse or abrupt weakening of the Atlantic Meridional Overturning Circulation (AMOC) could cause severe, hemispheric-scale impacts including cooling of Northern Europe and disruption to tropical rainfall. Modeling suggests that Stratospheric Aerosol Injection (SAI) would likely reduce AMOC decline (except under Southern Hemisphere-only injection) by counteracting the key drivers of weakening in the North Atlantic Ocean—surface warming and freshening (reduced saltiness)—that control how warm water transported north by AMOC sinks before circulating back south, a process known as deep convection. However, the modeled effectiveness of SAI at preventing AMOC slowdown or eventual collapse remains highly uncertain and depends critically on deployment strategy, timing, and magnitude (Bednarz et al., 2025; Futerman et al., 2025; Pflüger et al., 2024; Xie et al., 2022; Zhao et al., 2025). In summary, there is confidence that SAI would reduce or prevent AMOC slowdown, but the extent to which it would be able to do this, and how this would depend on injection strategy, warming scenario, and timing of SAI onset, is not well constrained.

A second, related tipping risk applies to deep-water formation in the North Atlantic subpolar gyre (SPG), which may be subject to a tipping point after which convection collapses (Loriani et al., 2025). SPG tipping could occur on time-scales much faster than full AMOC collapse, and would both contribute to AMOC decline and cause North Atlantic cooling and associated climate impacts (Sgubin et al., 2017). At present, there is very little direct modeling evidence for SAI's impact on SPG tipping, but, as with AMOC as a whole, it is likely that SAI would reduce the risk of collapse (Futerman et al., 2025; Pflüger et al., 2024).

This research activity will fund research over multiple years to develop mechanistic understanding of how SAI affects AMOC and North Atlantic subpolar gyre stability, and to identify how this sensitivity varies across different injection strategies, magnitudes, and deployment timelines.

This work could involve a variety of approaches, but will most likely entail some combination of the following:

1. Earth system model analysis of AMOC and SPG responses — assessing drivers of SAI-induced changes in recent GeoMIP simulations, including how different SAI injection locations and magnitudes affect surface temperature and the freshening drivers of deep-water formation (ice sheet melt, precipitation minus evaporation, and others), and identifying which factors dominate circulation changes under different scenarios. This analysis could also compare AMOC responses when SAI deployment occurs before versus after tipping thresholds are crossed.
2. Idealized modeling and theoretical work — developing understanding of how SAI could prevent or reverse ocean circulation collapse, including whether AMOC becomes permanently weakened once it starts to fail ("hysteresis," where a system does not return to its original state even after the initial cause is removed), how long SAI would need to be sustained to prevent or reverse circulation breakdown, and what deployment strategies and timelines would be necessary under different warming scenarios.
3. Subpolar gyre responses to SAI — an area that currently lacks model analysis. This work will determine whether the SPG responds similarly to AMOC or exhibits distinct sensitivities to different injection strategies.

This research will represent a meaningful advancement in the mechanistic understanding of ocean circulation responses to SAI across different deployment strategies and timelines, and enable future assessments of whether and under what conditions SAI could reliably mitigate ocean circulation tipping risk. An open-source workflow for assessing AMOC and SPG responses is part of this research activity to support rapid evaluation of circulation impacts as new SAI simulations become available.

This area of study will be ongoing with multiple impact studies over time. This research activity is focused on the initial scope of work needed to meaningfully advance scientific understanding of these processes and make future research faster and more robust. Reflective has funded the following projects in this area of study:

• Radiative Controls on Polar Tipping and Extremes in GeoMIP simulations. Dr. Louise Sime, British Antarctic Survey
• The Effect of Stratospheric Aerosol Injection on North Atlantic Subpolar Gyre Tipping. Dr. Matthew Henry, University of Exeter and Prof. Claudia E. Wieners & Dr. Swinda K.J. Falkena, Utrecht University