A study published Monday concluded that melting ice in Greenland caused by climate change could cause the Atlantic Meridional Overturning Circulation (AMOC) to collapse as soon as 2025, ushering in dramatic consequences for the planet.

The AMOC brings warm water north and east from the Caribbean, while delivering colder Arctic water south. If it were to suddenly shut down, scientists believe North America would experience weather changes such as more severe hurricanes and northern Europe would get a lot colder.

But you’re not likely to see igloos in London very soon. While climatologists say the collapse of the AMOC is a real threat, and that the new study raises a legitimate alarm that we may pass a key climate change tipping point sooner than previously thought, there’s a lot we still don’t know about exactly when it might happen.

“I think the authors in this case are on to something real,” Michael Mann, a University of Pennsylvania climate scientist, told Axios. “We could be talking decades rather than a century.”

Why the AMOC is so important

The AMOC is part of what the National Oceanic and Atmospheric Administration calls “the global ocean conveyor belt.” That keeps northern Europe several degrees warmer than it would otherwise be at that latitude — Paris is farther north than notoriously frigid-in-winter Montreal, for example.

Why it’s at risk

In recent years, studies have shown that the current is at its weakest in 1,000 years. Although scientists are not certain why, several studies have attributed that weakening to an influx of fresh water from the melting of Arctic sea ice, including the Greenland ice sheet, and increasing precipitation — both of which are results of global warming. The AMOC is driven by heavier cold water sinking, which raises warm water to the surface, but since fresh water is lighter than salt water, it has reduced the tendency of colder water near the surface to sink.

What the new study found

Researchers used sea surface temperature data dating back to 1870 to estimate AMOC current strength over time. Projecting forward from the gradual weakening of the system, they estimated it will collapse between 2025 and 2095, with the most likely date being around midcentury.

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What a collapse would mean

In addition to colder weather at northern latitudes, an AMOC collapse would have wide-ranging effects including increased sea level rise in the Atlantic, a drop in precipitation over Europe and North America, and shifts in monsoon patterns in South America and Africa, Reuters reported in 2021, citing a British government agency.

Such events are not unprecedented. The Washington Post looked back at the end of the last ice age, when studies suggest a “flood of freshwater spilled into the Atlantic, halting the AMOC and plunging much of the Northern Hemisphere — especially Europe — into deep cold” that lasted 1,000 years.

Why some scientists are pushing back

Previous estimates about when a possible collapse of the current might occur have been much less dire. In 2019, the Intergovernmental Panel on Climate Change projected that the AMOC would weaken in this century, but that total collapse within the next 300 years was likely only under the worst-case scenarios.

Some scientists say it is premature to revise that assessment. They argue that there are too many different factors affecting the current to project exactly if and when it will collapse.

“We know that there is a possibility that AMOC could stop what it’s doing now at some point, but it’s really hard to have certainty about that,” Penny Holliday, head of the Marine Physics and Ocean Climate group at the National Oceanography Centre, a British research institute, told the BBC.

The fact that the study used past ocean surface temperatures as a proxy for AMOC strength — since actual data on the AMOC itself only goes back to 2004 — is a limitation, according to some.

The points of consensus among researchers are that imminent AMOC collapse is a very disturbing possibility and further study is needed.

“There is still large uncertainty where the AMOC tipping point is, but the new study adds to the evidence that it is much closer than we thought,” Stefan Rahmstorf, a professor of physics of the oceans at the University of Potsdam in Germany, told the Guardian.

“Although the study probably overstates odds this century, uncertainty is high & the potential consequences of an AMOC disruption are extremely high,” tweeted Daniel Swain, a climate scientist at UCLA.

However, Peter Ditlevsen, a professor of climate physics at the University of Copenhagen and one of the authors of the report, told CNN that he and his co-authors are “very confident that this is a robust result.”

“It’s really scary,” he said. “This is not something you would lightly put into papers.”