Critical ocean circulation won't entirely collapse because of climate crisis - but it's still in trouble

Independent

26-02-2025

One of the world's most important ocean currents is dangerously weakening but new research suggests it is unlikely to stop entirely in the near future.
The Atlantic Meridional Overturning Circulation, or AMOC, a massive system of ocean currents, helps regulate global weather patterns by transporting warm water north and cold water south.
Concerns about its stability have grown, with recent studies warning the system might be approaching a tipping point that could prompt catastrophic climate shifts.
However, a new study published in Nature on Wednesday suggests that while the AMOC is still in trouble, a key mechanism, wind-driven upwelling in the Southern Ocean, can prevent its total collapse this century.
The study, led by researchers from the UK's Met Office and the University of Exeter, analysed 34 climate models under two extreme scenarios: a quadrupling of atmospheric Carbon dioxide and a major freshwater influx into the North Atlantic.
While all models showed a weakening of the AMOC by 20-81 per cent over 90 years, none predicted a complete collapse.
The main reason for this resilience is persistent upwelling in the Southern Ocean, driven by strong westerly winds, which compensates for AMOC weakening, the study says.
A Pacific Meridional Overturning Circulation does develop in most models as the AMOC weakens, but it is too weak to completely replace the AMOC's role.
Scientists say the findings do not imply that the AMOC is safe from dangerous weakening.
'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,' Dr René van Westen, postdoctoral researcher at Royal Netherlands Meteorological Institute, says.
The 'key message of this paper', he says, is that the AMOC may be partly stabilised by 'remote' feedback processes, such as those outside the Atlantic Ocean.
While the AMOC does not shut down in the study's 150-year simulations, Dr van Westen says, this does not rule out a full collapse if simulations are extended further.
He points out a key limitation of the study: '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.'
Several studies in recent years have found that AOMC is likely to shut down completely, and earlier than we previously expected. Dr Joel Hirischi, associate head of Marine Systems Modelling at UK's National Oceanography Centre says this new study provides 'counterbalance' to the debate.
However, even without a complete collapse, a weaker AMOC could have serious economic and environmental consequences, experts warn.
'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,' Dr Jonathan Bamber, director of the Bristol Glaciology Centre, University of Bristol, says about the study.
A recent study by the University of Hamburg's climate research group found that an AMOC slowdown could accelerate global heating by reducing CO2 absorption in the ocean.
This would, in turn, intensify extreme weather events such as heatwaves, floods, and hurricanes, potentially costing the global economy trillions of euros by 2100.
'The formation of a new overturning circulation in the Pacific, as seen in this study, signals a major transformation in global ocean dynamics,' says Dr Sofia Palazzo Corner, a researcher at the Centre for Environmental Policy at Imperial College London.
Observations from the National Oceanography Centre indicate that the AMOC has already slowed by about 15 per cent since the mid-20th century, and its continued weakening could disrupt weather patterns, sea level rise, and ocean ecosystems.
Scientists say the AMOC's long-term stability depends on future emissions and warming trajectories. 'This is not a reason to relax - on the contrary, it's a warning that the AMOC is already in decline, and we need to act decisively to prevent worse outcomes,' Dr Rahmstorf says.