Are major ocean currents moving to turn Earth into a snowball?

One of Earth’s most vital ocean currents is rapidly weakening. The cause: global warming. A collapse of this current could send devastating ripples through the planet’s climate.

The current is known as the Atlantic Meridional Overturning Circulation. Most scientists just call it AMOC. If it were to shut down, the consequences could be catastrophic and far-ranging. In the worst case: Earth’s warming climate could flip to a deep-freeze. Scientists refer to this scenario as a “snowball Earth.”

Why might that happen?

AMOC moves warm surface waters in the Atlantic Ocean from the tropics northward toward Europe. This water ferries with it a massive amount of heat. If that transport were to shut down, scientists say, the world might see a host of dramatic weather. Europe could experience more temperature extremes, of both chilling and heating. The Southern Hemisphere would warm even more than it is now. The North Atlantic would trigger stronger storms. The drought in Africa’s Sahel would worsen. And summer rains in Europe could sharply decrease, with devastating outcomes for farmers.

Researchers have rung warning bells about the AMOC for decades. Its possible shutdown even was the theme of The Day After Tomorrow, a 2004 snowball-Earth disaster movie. 

But predictions of how long AMOC will hold out have been all over the map. And it’s only in the last couple of years that scientists have been able to figure out which way the current is really trending.

Their consensus: It’s not good — AMOC is weakening.

The stakes are so alarming that some scientists have proposed engineering changes to AMOC. One is building a series of dams to seal off the Bering Strait between Russia and Alaska. By cutting off the northward flow of water from the Strait into the AMOC, the current’s strength might stabilize. At least that’s what researchers claim in a report April 24 in Science Advances.

Such proposed changes are controversial. No one can be sure what unknown impacts they might trigger. And the Science Advances paper “is very much a conceptual study,” says Jelle Soons of Utrecht University in the Netherlands. By that he means it’s a clever idea — but no one yet knows how it might be achieved. And Soons should know. He’s not only a physical oceanographer but also a coauthor of that new report.

What such suggestions can do is focus attention on how serious a threat an AMOC shutdown is — and how little time remains to address it.

The role of sea salt

A system of currents — including the AMOC — moves ocean waters around the planet. The water’s density is what drives them.

Saltier masses of water sink below less-salty ones. Colder waters also sink relative to warmer ones. The constant shifting between these masses keeps those currents on the move, almost as if they were riding a conveyor belt.

As the AMOC releases heat to the atmosphere at its North Atlantic destination, the water chills and sinks. That colder, denser water then heads back southward along the seafloor. To replace it, warmer water gets pulled northward along the surface.

For decades, researchers have worried that Earth’s changing climate could weaken AMOC’s strength.

Computer models have suggested the AMOC might weaken as the planet warms. But those models found it hard to know when and by how much — at least without data measured from the current. And such data had been lacking.

But scientists now have about two decades of direct observations to work with. These seem enough to start identifying a trend.

An April 15 paper in Science Advances that reviewed those data now issues a bleak conclusion: AMOC will become about 50 percent weaker by 2100.

Turning to the models

Predicting how the AMOC will change over time is “not as easy as predicting global temperature,” says Stefan Rahmstorf. He’s a physical oceanographer at the Potsdam Institute for Climate Impact Research in Germany. “It’s notoriously difficult to get [AMOC predictions] right,” he says.

The uncertainty is not strongly linked to future predictions of greenhouse-gas releases, says Valentin Portmann. He’s a climate-data scientist at the University of Bordeaux in France. He’s also lead author of the April 15 paper. Instead, he says, the issue comes down to differences between the many climate models.

The U.N. Intergovernmental Panel on Climate Change (IPCC) regularly updates its prediction of how Earth’s climate may change over the next few decades. To do this, it uses about 50 different computer models of climate. They’ve been developed by different scientists across the globe. Each model uses data on the current climate differently as it seeks to predict the future.

“The IPCC takes the mean of these to estimate what will be the future climate,” Portmann says. But the estimates can vary a bit above and below the mean, or average. This is known as standard deviation. For projections of Earth’s future heat, climate models show little disagreement. But for AMOC, the “spread is very high.”

The biggest problem, scientists say, is correctly projecting how salty different parts of the Atlantic Ocean’s future water will be. Where saltier and less-salt parts will be also matters, since saltier waters are more likely to sink, helping to drive currents.

To make an accurate prediction, Rahmstorf says, “you have to get the salinity all over the Atlantic Ocean right.”

To do that, scientists must also predict how precipitation will vary as the climate changes. Why? Rains can dilute the ocean’s saltiness. To predict rains, he says, “you’re talking about clouds — the biggest uncertainty in [climate] models.”

That’s one reason past models have varied widely when predicting how much AMOC will weaken over the next 75 years. The average drop hovers around 32 percent — give or take another 37 percent!

Portmann’s team used a branch of math known as statistics to get a less fuzzy estimate. One approach helped simplify complex problems with a lot of variables, like this one. This approach is “not often used in climate science,” Portmann notes, “but is very well known in statistical science.” Called ridge-regularized linear regression, it offered the best match to the problem.

It showed the likelier outcomes were those seen in models whose predictions were more dire. They now suggest AMOC will weaken by about 51 percent (plus or minus 8 percent) by the end of the century.

“What they have shown very convincingly is that, unfortunately, the simulations with the strongest decline are the most realistic ones,” says Rahmstorf. He did not take part in that analysis.

The biggest factor in this correction came from how one factor was weighted: how salty the surface of the future South Atlantic Ocean will be. Researchers had long suspected that the saltier those surface waters would be, the weaker the AMOC would become.

The new analysis now supports that link.

And measurements of these waters show they have been getting saltier. One reason is increased evaporation as the oceans have been warming. Another reason: More salty water has been leaking into the South Atlantic from the Indian Ocean as wind patterns shift.

The AMOC’s ins and outs are a lot to grasp

Scientists have identified many things that impact the AMOC besides the salinity of the South Atlantic. Those include circulation in the Nordic Seas and the release of freshwater into the oceans. This is especially likely in places such as the Arctic’s Bering Strait or where meltwater from Greenland’s ice sheet is flowing into the sea.

In fact, any big glut of freshwater entering the ocean is going to have a huge impact on its density. That in turn will drive changes in ocean currents.

Consider one 2025 study in the Earth ArXiv. Hundreds of thousands of years ago, it showed, meltwater from a decaying Greenland ice sheet triggered a weakening of the AMOC. That weakening lasted about 1,000 years!

Rapid melting of that ice sheet today again appears to be flooding huge amounts of freshwater into the North Atlantic.

Still, AMOC is one complex beast. It has a northern branch known as the Nordic Seas Overturning Circulation, or the NOC. Even as AMOC has been weakening, NOC appears to be stable. Some computer models even project it could strengthen a bit in coming years.

That might seem like good news, says Sasha Roewer. She’s a physical oceanographer at the Max Planck Institute for Meteorology in Hamburg, Germany. However, she warns, NOC’s strengthening might actually be another warning sign.

She, Rahmstorf and other researchers modeled what would happen as the North Atlantic Ocean became less salty. On April 20 in Ocean Science, they reported this would — for a time — strengthen the NOC current.

But “eventually it reaches a tipping point where convection in the Nordic Seas breaks down,” Roewer says. And then, she says, “both currents collapse.”

A damming proposal

The idea of damming the Bering Strait “really was a very spontaneous one,” says Soons at Utrecht. “It just started with a thought experiment.”

A study last year in Geophysical Research Letters analyzed what had played a role in a strong AMOC some 3 million years ago. And the Arctic’s Bering Strait was one factor.

It is narrow, just 82 kilometers (51 miles) wide. It’s also shallow, on average just 30 to 50 meters (100 to 160 feet) deep. So changing sea levels will alter how much water flows through this strait.

During ice ages, sea levels drop. That allows the Bering Strait to become a land bridge  between Asia and North America. Three million or so years ago, reduced freshwater flow through the Strait helped keep AMOC stable.

And that finding was like a light bulb going on, says Soons. “It made me think: Could we close the Bering Strait again?”

His simulations with Utrecht colleague Henk Dijkstra now suggest that damming the Bering Strait might work. But it’s also time sensitive. If the closure occurs during a very weak AMOC, they found, this stabilizing effect could backfire.

The damming idea has triggered mixed opinions. Notes Soons, “There are debates of whether we should even [study] geoengineering.” Why? “It would distract from the real problem,” says Soons — “give people a way out” of focusing on reducing the pollution behind climate change. (Soons admits that he and Dijkstra didn’t consider ecological and other impacts of damming the Strait.)

Even if the AMOC is speeding toward its tipping point, it’s in no way clear that a dam is the solution, other researchers say.

“I’m not a great fan [of the dam proposal], to put it mildly,” says Rahmstorf. “The most important thing we can do is stick to the Paris Agreement,” he says. This United Nations treaty aims to reduce emissions of greenhouse gases and slow global warming.

In fact, average temps this year are likely to blow through that treaty’s target: a warming of 1.5 degrees Celsius (2.7 degrees Fahrenheit) above what was typical in the 1700s. (That’s when heavy use of fossil fuels began.)

The world has likely not passed the point of triggering AMOC to stall, Rahmstorf says. When exactly that will be, he adds, is hard to know. But based on the most recent studies, that time might come as soon as the 2040s. That suggests, he says, “We have no time to lose.”

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