Dangers of Atlantic Ocean current collapse have been ‘greatly underestimated’, scientists warn

Scientists have warned that the dangers of the collapse of a key Atlantic Ocean current that helps regulate the Earth's climate have been "greatly underestimated".

In an open letter published earlier this week, 44 leading climate scientists from 15 countries said that the collapse of the Atlantic meridional overturning circulation (AMOC) would have devastating and irreversible impacts. They write that the risks require urgent action from policymakers.

The most recent Intergovernmental Panel on Climate Change report says there is "medium confidence" that the AMOC will not collapse abruptly by 2100. But the group of experts says this is an underestimate.

"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," they write in the open letter.

"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 minimise this risk."

The letter is addressed to the Nordic Council of Ministers, an intergovernmental forum which aims to promote cooperation among the Nordic countries. It urges policymakers to consider the risks posed by an AMOC collapse and put pressure on governments to stay within Paris Agreement targets.

What is the Atlantic Ocean Circulation?

The AMOC is an important system of ocean currents. It transports warm water, carbon and nutrients north via the Atlantic Ocean where the water cools and sinks into the deep.

This helps to distribute energy around the planet, moving heat through the ocean like a conveyor belt and regulating our climate.

Warm water - more salty due to evaporation - flows north on the surface of the ocean keeping Europe milder than it would otherwise be. When this water cools it sinks because its high salinity increases its density. It then flows back to the southern hemisphere along the bottom of the ocean.

But studies of past episodes of dramatic cooling in Europe over the last 100,000 years suggest melting ice sheets could weaken the AMOC due to changes in salinity and temperature.

Fresh water reduces the saltiness - and therefore the density of the water- on the surface of the ocean. This means less of the surface water sinks, potentially slowing the flow of the current.

Are we heading for a catastrophic tipping point?

Some research has suggested that climate change may be slowing the flow of the current. One study from 2023, based on sea surface temperatures, suggested that a complete collapse could happen between 2025 and 2095.

There is huge uncertainty about how, when or even if this ‘tipping point’ could actually happen, however, and modelling the scenario is tricky. Most previous computer simulations that showed a collapse involved adding huge, unrealistic quantities of fresh water all at once.

In February this year, scientists from Utrecht University in the Netherlands used a complex climate model to simulate the collapse of the AMOC and discovered that it could be closer than previously thought.

The Dutch team used a supercomputer to carry out the most sophisticated modelling so far to look for warning signs of this tipping point. They added water gradually, finding that a slow decline could eventually lead to a sudden collapse over less than 100 years.

Previously, the paper published in February said, an AMOC tipping point was only a “theoretical concept” and its authors found that the rate at which the tipping of this vital current occurred in their modelling was "surprising".

But researchers had to run the simulation for more than 2,000 years to get this result and still added significantly more water than is currently entering the ocean as Greenland’s ice sheet melts.

"The research makes a convincing case that the AMOC is approaching a tipping point based on a robust, physically-based early warning indicator," said University of Exeter climate scientist Tim Lenton, who wasn't involved in the research, at the time.

"What it cannot (and does not) say is how close the tipping point is because it shows that there is insufficient data to make a statistically reliable estimate of that."

Lead author of the study, René van Westen also added that there wasn’t enough data to say anything definitive about a potential future AMOC collapse. More research is needed to work out a timeframe - including models that incorporate increasing levels of carbon dioxide and global warming.

“We can only say that we’re heading towards the tipping point and that AMOC tipping is possible.”

Some of the changes seen in the model before the collapse do, however, correspond with changes we’ve seen in the Atlantic Ocean in recent decades.

“When the AMOC loses stability, as we know from the available reconstructions, it is more likely that abrupt transitions may develop in the future,” van Westen added.

Lenton said that we have to "hope for the best but prepare for the worst" by investing in more research to improve the estimate of how close a tipping point is, assess the potential impacts and work out how we can manage and adapt to those impacts.

What would a collapse of the ocean current mean for Europe?

If the AMOC collapses, previous research has shown the resulting climate impacts would be nearly irreversible in human timescales. It would mean severe global climate repercussions, with Europe bearing the brunt of the consequences.

Some parts of Europe could see temperatures plunge by up to 30C. On average, the model shows London cooling by 10C and Bergen by 15C.

The report’s authors say that “no realistic adaptation measures can deal with such rapid temperature changes”.

Temperatures in the southern hemisphere would rise with wet and dry seasons in the Amazon rainforest flipping.

Van Westen also explained earlier this year that it could mean less rainfall and a sea level rise of up to one metre in coastal areas of Europe.

"The overall picture that AMOC collapse would be catastrophic fits with my own group’s recent work showing that it would likely cause a widespread food and water security crisis," according to Lenton.