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A modelling study published in Nature suggests that Atlantic Meridional Overturning Circulation (AMOC) may withstand climate extremes.
Dr Alessandro Silvano, Oceanographer, University of Southampton said:
“AMOC will control extreme weather events, sea level rise and temperature over many areas, including Europe, and communities will need to adapt to changes, especially in case of collapse. This new study shows that what will happen is still not completely clear and a more “global approach” is needed, an approach that looks at the ocean as one large scale system where changes on one side of the planet can control what happens on the other side.
“Whether an AMOC collapse could occur is one of the most pressing questions for the scientific community. Especially if this can happen over the next century. Some studies suggest the AMOC might be approaching a tipping point, others instead suggest AMOC to be more resilient to change in CO2 concentrations, melting of the Greenland Ice Sheet and changes in the precipitation. Therefore, at present, there is a debate about a potential collapse, while an AMOC weakening seems likely.”
Dr René van Westen, Postdoctoral Researcher, Royal Netherlands Meteorological Institute, (KNMI), said:
“The press release for this paper slightly oversells the point that the AMOC is ‘able to withstand future global warming’. In fact, the study still supports the conclusion that the AMOC is expected to severely weaken under extreme climate change, which is in line with the results from the latest IPCC report.
“The study’s results should certainly not be interpreted as showing that AMOC is a resilient system, given it finds that the AMOC still reduces to (very) weak strengths under human-caused global warming.
“In principle it is possible that all the AMOCs reached their collapsed state by the end of the 150-year long simulation. This can only be tested by continuing the simulation much longer to reach an equilibrium state, the simulations are too short to verify this. Nevertheless, the authors clearly demonstrate that the AMOC does not fully collapse (i.e. to 0 Sv strength) under 4xCO2 and show a prominent role for the Southern Ocean and Indo-Pacific Ocean.
“The study is still an exciting contribution to the literature. One of its key strengths is the inter-model comparison analysis under both 4xCO2 and hosing set-up. The authors show a clear relation in 34 different CMIP6 and demonstrate why the AMOC remains in a (very) weak state.
“It also demonstrates an important role for Southern Ocean dynamics, also suggested by previous research. However, Southern Ocean dynamics can only be adequately captured with high-resolution climate models in which large swirls (i.e., ocean eddies) are resolved. None of the 34 climate models used in this study have such a high resolution. It would be very interesting to see whether the proposed mechanism remains robust when resolving these swirls.
“The key message of this paper is that the AMOC may be partly stabilised by ‘remote’ (i.e. outside the Atlantic Ocean) feedback processes. It is therefore good to consider these remote feedback processes when analysing the AMOC in future work. This will help to understand the future AMOC trajectory under climate change.”
Prof Stefan Rahmstorf, Head of Research Department, Potsdam Institute for Climate Impact Research, said:
“This new paper does not (and does not claim to) contradict other modeling studies about future AMOC changes and their climatic impact.
It has been well-established since the 1990s that the AMOC has a smaller, shallower part which is driven by the winds, meaning that a part remains once the density-driven (thermohaline) overturning has stopped. However, that wind-driven part is not nearly as important for climate as the part driven by differences in sea-water density. It is the latter which has a tipping point.
In previous studies about the risk of future AMOC collapse, the wind-driven part also persists since the winds won’t stop blowing, so this is not new information. The new study investigates the remaining wind-driven overturning in more detail, which is a valuable contribution to the scientific literature. It does not, however, change the assessment of the risk and impact of future AMOC changes in response to human-caused global warming.
A false impression of contradicting our and other results may however easily arise from their different usage of the word ‘AMOC collapse’. To the new paper, this word implies zero or negative overturning in the North Atlantic north of the equator below 500 m, while in previous studies this term has been used for states with greatly weakened AMOC. The new study has used the same models as previous studies and its findings change nothing about the climate risk of a major AMOC weakening, which remains significant and would have global ramifications.”
Dr Joel Hirschi, Associate Head of Marine Systems Modelling, UK’s National Oceanography Centre (NOC), said:
Does the press release accurately reflect the science?
“Yes, it does. As it stands, the only bit that could be confusing is the statement saying that “…AMOC can only collapse if a Pacific meridional overturning circulation (PMOC) develops”.
“It would be clearer to say that for the AMOC to stop, the Southern Ocean upwelling must be entirely compensated in the Pacific Ocean.
“The authors clarify this later in the press release but it would be better to say this upfront.
Is this good quality research? Are the conclusions backed up by solid data?
“I enjoyed reading this article and I find the research to be of excellent quality. The work and methodology are closely related to an earlier study by the same authors in Geophysical Research Letters but the key message about AMOC stability is new.
“The authors used a large number of numerical models and the key results are robust across a range of model solutions. This enhances my confidence that the key findings of the study are robust.
How does this work fit with the existing evidence?
“This latest work fits nicely in the ongoing debate as to whether the AMOC is likely to shut down or not as climate warms. During the last two years, several studies have re-ignited the debate about whether the AMOC is likely to shut down, suggesting that the AMOC is more likely to shut down than we previously expected. This study provides a counterbalance and provides evidence for stabilising AMOC mechanisms linked to winds in the Southern Ocean.
“Direct observations of the AMOC do not suggest that the AMOC is shutting down and the results from this study are consistent with a view that the AMOC is not in immediate danger of shutting down.
Have the authors accounted for confounders? Are there important limitations to be aware of?
“The numerical models used in this study test the impact of a very strong greenhouse gas forcing (4xCO2) or a freshwater hosing north of 50N in the Atlantic. Neither the CO2 forcing nor the hosing on their own can cause the AMOC to shut down.
“In our warming world, both global CO2 concentrations and freshwater discharge into the North Atlantic, are increasing in parallel. It is not obvious how both effects put together would combine. The possibility of non-linear, amplifying AMOC interactions possible. To test that would require a new set of numerical experiments where CO2 and freshwater forcing are applied at the same time.
“The models used in the study typically have a low spatial resolution (in the order of 100km). Important features, such as ocean mesoscale eddies are missing and sharp temperature and salinity fronts are not realistically simulated. How strongly this affects the findings reported in this study, we do not yet know.
What are the implications in the real world? Is there any overspeculation?
“The study highlights the importance of the wind-driven Southern Ocean upwelling to understand the AMOC and its stability. Observations in the North and South Atlantic, where the AMOC is currently being observed may not be enough to decide where the AMOC is heading and knowing the amplitude and variability of the wind-driven Southern Ocean upwelling could be key.
“The authors are careful and their results should be considered when discussing the probability of a future AMOC shut down. The applied perturbations are large: 4xCO2 is higher an anomaly than what we will get – even in a pessimistic outlook. The freshwater discharge (0.3 Sv = 300000 m^3/s) applied during 100 years is roughly equivalent to melting about 1/3 of the Greenland ice sheet. Both perturbations are large compared with what we will likely experience in the real World.”
Sofia Palazzo Corner, PhD Researcher at the Centre for Environmental Policy, Imperial College London, said:
“This paper investigates the AMOC response to extreme climate change and finds that as waters continue to be pulled to the surface by wind in the Southern Ocean, so must waters sink elsewhere.
“This leads to two important results: an AMOC that weakens but doesn’t shut down completely, and the formation of a new overturning circulation in the Pacific: a PMOC.
“Though AMOC here shows resilience to complete collapse, ocean circulation definitely does not show a general resilience to climate change. Even a weakened AMOC will result in major impacts to global and regional climate, and the formation of a new overturning circulation in the Pacific is an extraordinary and dramatic change to global ocean dynamics.
“What’s unambiguous is that increasing carbon emissions are increasing the risk of major changes in global ocean circulation, including the AMOC. This study takes an extreme case to investigate the interactions between the Atlantic, the Southern Ocean and the Pacific, and finds that although the AMOC does not collapse completely, there is significant weakening, and a major transformation in the Pacific Ocean to accommodate the new balance between rising and sinking waters.
“These results are a signal to pay increased attention to other parts of the global ocean which may hold clues to the trajectory of AMOC in the 21st century.”
Prof Jonathan Bamber, Director of the Bristol Glaciology Centre, University of Bristol, said:
“This paper presents a careful and thorough analysis of how the AMOC responds to both extreme greenhouse gas and freshwater forcing that could result from accelerated fossil fuel consumption and increased melting of the Greenland Ice Sheet. Their analysis is based on examining 34 state of the art climate models and strongly suggests that the AMOC is not close to a tipping point for present-day and near-future climate. That is good news. While they find no evidence for a switch off or collapse of the AMOC they do find a weakening in all cases and this, alone, should be cause for concern. Because the AMOC is responsible for so much of the oceanic poleward heat transport, changes in its strength have a huge impact on the climate of northwest Europe and globally.
“A collapse of the AMOC would be devasting for civilisation so it is understandable that there has been a lot of focus on whether this might happen in the near future but a weakening of the AMOC should also be of concern. While it might not grab the headlines in the same way and its impact is a little more complicated to explain, it is still extremely important to model, understand, monitor and predict.”
Dr Lee de Mora, Marine Ecosystem Modeller, Plymouth Marine Laboratory, said:
“The Atlantic Meridional Overturning Circulation is hugely important to the global climate, influencing heat transport, carbon drawdown and deep water formation. Despite its importance, the future of the AMOC is not yet fully understood.”
“On one hand, the climate models from Coupled Model Intercomparison Project Phase 6 (CMIP6) universally projected a weakening in the AMOC as temperatures increase, but they did not project a full collapse to zero at any warming level. On the other hand, some experiments have suggested that the AMOC is too stable in those CMIP-style models, and the real AMOC may be more prone to collapse.”
“This paper from Baker et al. identifies AMOC-stabilizing mechanisms in the Southern Ocean and Pacific Ocean that may explain why the CMIP6 models have a stable AMOC”.
‘Continued Atlantic overturning circulation even under climate extremes’ by Baker et al. was published in Nature at 16:00 UK time on Wednesday 26 February.
DOI: 10.1038/s41586-024-08544-0
Declared interests
Dr Alessandro Silano “None”
Dr. René van Westen “None”
Prof Stefan Rahmstorf “None”
Dr Joel Hirischi “None”
Sofia Palazzo Corner “No interests to declare. I’m a PhD student funded by the Grantham Institute, and research assistant funded by ESM2025.”
Prof Johnathan Bamber “I am a member of the Advisory Committee for Earth Observation of the European Space Agency and a member of the European Space Science Committee, which receives funding from a number of national space agencies. I also receive funding from the European Commission.”
Dr Lee de Mora “LdM was supported by the UK Natural Environment Research Council through The UK Earth System Modelling Project (UKESM, grant no. NE/N017951/1) and by the UK Natural Environment Research Council through the TerraFIRMA: Future Impacts, Risks and Mitigation Actions in a changing Earth System project, Grant reference NE/W004895/1.”