One of Earth’s most vital ocean currents is rapidly weakening in response to global warming. Its collapse could have devastating repercussions on the planet’s climate system.
The potential consequences of stopping this flow, called the Atlantic Meridional Overturning Circulation, or AMOC, are vast. The AMOC moves warm surface waters from the tropics northward toward Europe, carrying an enormous amount of heat.
If this transport stops, scientists say, the world could see more extreme cold and heat in Europe, warming in the Southern Hemisphere, stronger storms in the North Atlantic, increasing drought in Africa’s Sahel and reduced summer precipitation in Europe with potentially devastating consequences for agriculture.
Researchers have been sounding the alarm about AMOC for decades; its hypothetical stop was even the disastrous premise of the 2004 film The day after tomorrow. But scientific predictions of his fate to have been all over the map – and only in the last two years have scientists been able to understand in which direction the current is actually moving.
The consensus is not good. AMOC is weakening.
The issues are alarming enough that some scientists are even proposing to restore the AMOC to health by geoengineering. One idea: build a series of dams approximately 80 kilometers long to close the Bering Strait between Russia and Alaska. Cut off water flowing into the AMOC from the strait could help stabilize the strength of the currentthe researchers report on April 24 in Scientific advances.
AMOC will become around 50 percent weaker by 2100.Predicting how the AMOC will change over time is “not as simple as predicting global temperature,” says Stefan Rahmstorf, a physical oceanographer at the Potsdam Institute for Climate Impact Research in Germany. “It’s notoriously difficult to succeed.”
Uncertainty over the fate of the AMOC turns out not to be strongly linked to future greenhouse gas emissions scenarios, says Valentin Portmann, a climate data specialist at the University of Bordeaux in France and co-author of the study. Rather, the problem lies in the differences between the climate models themselves.
The United Nations Intergovernmental Panel on Climate Change uses around 50 different climate simulations from different scientists around the world in its comprehensive climate assessments. Each climate model integrates current climate observations differently into its projections.
“The IPCC uses these data to estimate what the future climate will be,” explains Portmann. “Around that mean, you have a standard deviation.” When it comes to projections of Earth’s future heat, climate models show little disagreement. But for AMOC, “this spread is very high”.
The biggest problem in projecting the fate of the AMOC, scientists say, is properly simulating the future salinity of the Atlantic Ocean, which makes the water denser and helps it sink. “You have to get the salinity correct throughout the Atlantic Ocean,” says Rahmstorf. This means, in turn, correctly simulating changes in precipitation as the climate changes. And with precipitation, he says, “we’re talking about clouds, the biggest uncertainty in atmospheric models.”
Given these challenges, past computer simulations have varied widely regarding the extent to which the AMOC will weaken by 2100. The average reduction hovers around 32%, or plus or minus 37%.
Portmann and his colleagues solved the AMOC conundrum with classical statistical approaches. One approach, called ridge-regularized linear regression, is “not often used in climate science, but it’s very well known in statistical science,” says Portmann. This approach is specifically designed to simplify complex statistical problems with many variables, many of which may be correlated with each other.
This is also the approach that has proven to offer the best statistical fit to the problem. This approach has favored climate simulations with darker forecasts, suggesting that the AMOC will weaken by about 51 percent, plus or minus 8 percent, by the end of the century.
“What they showed very convincingly is that, unfortunately, the simulations with the biggest drop are the most realistic,” says Rahmstorf, who is not the author of this study.
The most important factor in this correction comes from how one factor was weighted: the future surface salinity of the South Atlantic Ocean. Researchers have long suggested that the higher the salinity of South Atlantic surface waters, the weaker the AMOC should become – and this analysis confirms this link. Meanwhile, observational data shows that these waters have become saltier, both from increased evaporation from the ocean as the atmosphere warms and the seepage of saltier water from the Indian Ocean as wind patterns change.
The ins and outs of AMOC are many to understand
In addition to the surface salinity of the South Atlantic, scientists have identified many factors that impact the AMOC. These include circulation in the Nordic Seas and the influx of fresh water via places like the Bering Strait.
To understand such problems, oceanographers sometimes start by simplifying them into box models, simple representations of ocean regions divided into hypothetical boxes with a given chemical composition. Other parcels of water entering the box mix and the water leaving the box has a changed chemistry. Although highly simplified, box models can reveal the fundamental forces at play. For example, as early as 1996, Rahmstorf published a study based on a box model that highlighted the probable importance of the salinity of the South Atlantic Ocean when it comes to the strength of AMOC.
The influence of the melting Greenland ice sheet is also quite simple: a large amount of fresh water entering the ocean will have a huge impact on its density, the driving force of movement.
There is some data on this: a 2025 Earth ArXiv study found that hundreds of thousands of years ago, meltwater from a decaying Greenland ice sheet triggered a weakening of the AMOC it lasted for about 1,000 years. And the island’s rapidly melting ice cap it is believed to contribute enormous quantities of fresh water to the North Atlantic today.
The other entries are more difficult to understand. There is a northern branch of AMOC known as Nordic Seas Overturning Circulation, or NOC. Although the AMOC has weakened over the past century, the NOC appears stable. Some computer models even predict that the CNO will strengthen slightly in the future.
That may sound like good news, says Sasha Roewer, a physical oceanographer now at the Max Planck Institute for Meteorology in Hamburg, Germany. “It makes sense to say that if some of the current strengthens, maybe the system is stable and we don’t really have anything to worry about.”
However, strengthening the NOC could actually be another wake-up call, she believes.
Roewer, Rahmstorf and other researchers simulated what would happen as the North Atlantic Ocean became cooler. April 20 at Ocean Sciencesthey reported that this would – for a time – result in strengthen the NOC currentbecause it also changes the density difference between the North Atlantic Ocean and the waters further south.
But “eventually it reaches a tipping point where the convection in the Nordic Seas collapses. And that’s where both currents collapse,” Roewer explains.
Scientists mull controversial approach
The idea of building a dam across the Bering Strait “was really a very spontaneous idea; it just started with a thought experiment,” explains Soons. This was triggered by a study carried out in 2025 Geophysical Research Letters which analyzed what factors contributed to a strong AMOC during the Mid-Pliocene epoch, approximately 3 million years ago.
The Bering Strait is narrow and shallow, measuring only 82 kilometers wide and having an average depth of between 30 and 50 meters. Changes in sea level alter the amount of water that can flow into the strait. During ice ages, when sea levels fall, the Bering Strait becomes a land bridge, providing a route for human or animal migration between continents. About three million years ago, reduced freshwater flow in the Bering Strait helped maintain the stability of the AMOC, according to the study that inspired them.
“It got me thinking: Could we close the Bering Strait again? » said Soons.
Simulations by Soons and colleague Henk Dijkstra at Utrecht University suggest that building dams on the Bering Strait is a matter of time. If the shutdown is applied while the AMOC is already severely weakened, this stabilizing effect could become counterproductive, they found.
Soons acknowledges that reactions to the proposal have been mixed. “There are debates about whether we should even be researching geoengineering because it would distract from the real problem: giving people a way out” by focusing on reducing emissions.
He says he and Dijkstra also didn’t consider ecological impacts or other factors in their proposal. “It’s just another direction to explore instead of a finished solution.”
Even if AMOC rushes towards its baseit is not at all clear that a dam is the solution, other researchers say.
“I’m not a big fan [of the dam proposal]that’s an understatement,” says Rahmstorf. “The most important the thing we can do is stick to the Paris Agreement” to reduce emissions and prevent further warming, he says – although in 2026 the planet’s average temperature will likely exceed the 1.5 degree Celsius warming target set by that agreement.
The world – probably – hasn’t yet passed the AMOC tipping point, and it’s difficult to know when exactly that will happen, says Rahmstorf. But according to the most recent studies, this moment could arrive as early as the 2040s. “That means we have no time to lose.”
