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What would happen if the Atlantic Meridional Overturning Circulation (AMOC) collapses? How likely is it?

The idea that the AMOC is headed to collapse is very controversial, but it is clearly weakening. If the circulation did collapse, the consequences on both sides of the Atlantic Ocean would be immense—including large changes in temperature and a spike in weather-related disasters.

November 7, 2024

The Atlantic Meridional Overturning Circulation, or AMOC, is part of a “global conveyor belt” of ocean currents, exchanging cool and warm water in a path around the Atlantic Ocean.¹ The surface branch of the AMOC merges with the famous Gulf Stream, and carries warm water coming all the way from the Southern Ocean near Antarctica up to the Arctic Sea near the North Pole.

Along this journey, the waters become saltier, thanks to evaporation in the subtropics and an influx of salty waters from the Mediterranean Sea. Upon reaching the Arctic, they also get colder. Cold, salty waters are very dense and plunge to the bottom of the ocean, leaving the surface waters on the European side of the Atlantic warm. Eventually, the cold water winds its way back south to the Southern Ocean, where the westerly winds pull it back to the surface, and the process begins again.

The AMOC doesn’t just move water around; it plays a huge role in transporting heat to different parts of the world. This means any changes in its circulation also affect weather and climate conditions up and down the Atlantic.² And today, the AMOC is changing, thanks to warming oceans, increased rainfall, and melting sea ice.

The history of our planet reveals how dramatic these changes can be. “We know that the AMOC at times, during the ice ages, has collapsed. We know that the AMOC at other times was weaker,” says Raffaele Ferrari, a professor of oceanography at MIT.

So what is likely to happen today?

Data around whether the AMOC has already weakened in modern times is a bit uncertain, says Ferrari, because we only have direct measurements going back about a half century. But current information suggests the circulation may have slowed by as much as 15 percent since the 1950s.³

How much more it could slow—or even how accurate that 15% figure is—is still debated. But there is consensus that climate change is likely to further weaken the AMOC. Warming temperatures heat up the ocean, which makes water lighter, less dense, and less likely to sink. Meanwhile, increased rainfall and melting ice will add more freshwater to the ocean, reducing its salinity. “If there isn't enough increase in salinity and decrease in temperature, this water, instead of sinking down, will essentially veer to the right and come back at the surface,” says Ferrari. That would reduce the amount of heat released to the atmosphere—a crucial function of the AMOC, because it plays an important role in keeping Northern Europe’s climate mild, even at latitudes that are frigid in other parts of the world.

As a result, even a slight slowdown in the AMOC can cool Europe, change precipitation patterns in parts of Europe, South America, and Africa, affect the timing of the Indian monsoon, and lead the tropical rain belt to shift southward, resulting in droughts over the African Sahel.⁴

We also know that it’s possible for the AMOC to collapse altogether. Around the end of the last ice age, a mass melting of ice sheets inundated the Northern Atlantic with freshwater. That changed the density of the water in the northern latitudes, which made it more difficult for water to sink and cool down before heading south. The result was massive cooling in the Northern Hemisphere, and warming in the south, in a matter of just decades.⁵

Such a collapse would be what scientists call a “climate tipping point”—an event that would lead to sudden, wide-reaching impacts that are difficult if not impossible to reverse. Changing currents would cause sea levels to rise swiftly in areas like the U.S. East Coast, storms would grow more severe, the rainy and dry seasons in the Amazon may flip, and the ice age pattern of a cooling north and warming south would play out once again.⁶

Is that in store for us? A recent study, using a climate model in a novel way, projected that an AMOC collapse could come as early as mid century, assuming global warming continues at its current rate.⁶ But that finding is not widely accepted, says Ferrari. In his view, today’s pace of climate change doesn’t appear substantial enough to spur a near-term collapse of the AMOC. “The likelihood that the AMOC collapses, let's say, before the end of the century, according to numerical models and our understanding, is pretty small. Most likely the weakening will be modest,” he says.

“That said, a weakening of the circulation, and the impacts of a slowing AMOC, are worrying enough.” This system of currents is just one way that small pressures on our climate tend to feed on each other and produce ever more extreme changes. The sooner we shift the world economy away from climate-warming fossil fuels and halt further warming, the more secure we can be that the AMOC will continue providing the Atlantic with the steady, predictable climate of the past 10,000 years.

Thank you to Hugh Bakhurst from Toronto, Canada for the question.

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