'Serious risk' of vital ocean current collapse by 2100, warn scientists

A group of 44 climate scientists from 15 different countries warn there is a "serious risk" that soaring global temperatures will trigger the "catastrophic" collapse of a crucial system of ocean currents—and possibly sooner than established estimates considered likely.

The Atlantic Meridional Overturning Circulation, or AMOC, moves warm water up from the tropics to the North Atlantic, where it sinks and cools before returning south. It is, as letter signatory and oceanographer Stefan Rahmstorf toldThe Guardian, "one of our planet's largest heat transport systems." If it collapsed, it could lower temperatures in some parts of Europe by up to 30°C.

That's why the scientists sent a letter to the Council of Nordic Ministers over the weekend urging them to take action to understand and prevent a potential collapse.

"A string of scientific studies in the past few years suggests that this risk has so far been greatly underestimated," the scientists wrote. "Such an ocean circulation change would have devastating and irreversible impacts especially for Nordic countries, but also for other parts of the world."

In the letter, the scientists detailed some of the potential "catastrophic" impacts of such a collapse, including "major cooling" in northern Europe, extreme weather, and changes that would "potentially threaten the viability of agriculture in northwestern Europe."

One study cited in the letter shows that London could cool by 10°C and Bergen, Norway by 15°C.

"If Britain and Ireland become like northern Norway, (that) has tremendous consequences. Our finding is that this is not a low probability," Peter Ditlevsen, a University of Copenhagen professor who signed the letter, toldReuters. "This is not something you easily adapt to."

Globally, the scientists said, the end of AMOC could cause the ocean to absorb less carbon dioxide, thereby increasing its presence in the atmosphere. It could also further augment sea-level rise along the U.S. Atlantic coast and alter tropical rainfall patterns.

The most recent synthesis report from the Intergovernmental Panel on Climate Change (IPCC) expressed "medium confidence" that the current would not cease functioning before 2100. Since its publication in March 2023, however, a rash of studies have come out upping the risk.

"Given that the outcome would be catastrophic and impacting the entire world for centuries to come, we believe more needs to be done to minimize this risk."

A Nature Communications study, also published last year, looked at 150 years of temperature data and determined with 95% confidence that AMOC would collapse between 2025 and 2095 if greenhouse gas emissions continue to rise as currently predicted.

Another, published in Science Advances in February, concluded that AMOC was currently "on route to tipping."

There are already signs that AMOC has begun to stall over the last six to seven decades, Rahmstorf told The Guardian, such as the cold blob in the North Atlantic that is defying global warming trends. The water in North Atlantic is also becoming less salty due to meltwater from the Greenland ice sheets and increased precipitation due to climate change. Less salty water is lighter and does not sink, interrupting the process that makes AMOC flow.

"It is an amplifying feedback: As AMOC gets weaker, the subpolar oceans gets less salty, and as the oceans gets less salty then AMOC gets weaker," Rahmstorf explained. "At a certain point this becomes a vicious circle which continues by itself until AMOC has died, even if we stop pushing the system with further emissions."

"The big unknown here—the billion-dollar question—is how far away this tipping point is," Rahmstorf said.

The scientists acknowledged that the chance of the AMOC tipping "remains highly uncertain."

They continued:

The purpose of this letter is to draw attention to the fact that only 'medium confidence' in the AMOC not collapsing is not reassuring, and clearly leaves open the possibility of an AMOC collapse during this century. And there is even greater likelihood that a collapse is triggered this century but only fully plays out in the next.

Given the increasing evidence for a higher risk of an AMOC collapse, we believe it is of critical importance that Arctic tipping point risks, in particular the AMOC risk, are taken seriously in governance and policy. Even with a medium likelihood of occurrence, given that the outcome would be catastrophic and impacting the entire world for centuries to come, we believe more needs to be done to minimize this risk.

To respond to this threat, the scientists urged the council—a group that includes Denmark, Finland, Iceland, Norway, Sweden, Greenland, the Faroe Islands, and Åland—to launch a study of the risk posed to these countries by an AMOC collapse and to take measures to counter that risk.

"This could involve leveraging the strong international standing of the Nordic countries to increase pressure for greater urgency and priority in the global effort to reduce emissions as quickly as possible, in order to stay close to the 1.5°C target set by the Paris agreement," they wrote.

Johan Rockström, a letter signatory who leads the Potsdam Institute for Climate Impact Research, wrote on social media that global politics, "particularly in [the] Nordic region, can no longer exclude [the] risk of AMOC collapse."

And there is one way that political leaders can stave off such a collapse, as well as other climate tipping points, according to Rahmstorf.

"This is all driven mainly by fossil fuel emissions and also deforestation, so both must be stopped," he told The Guardian. "We must stick to the Paris agreement and limit global heating as close to 1.5°C as possible."