A Powerful Current Controls Our Weather Patterns—and It's Dangerously Slowing Down

• The Atlantic meridional overturning circulation (AMOC) is an ocean current that delivers warm water from the tropics all the way to the Arctic circle.
• Scientists have warned of AMOC’s collapse for years, but a new study suggests that as freshwater is added to the ocean via melting glaciers, the current could face a swift collapse and all but disappear within a century.
• This would have disastrous affects around the world, but especially in northern Europe, which AMOC helps keep reasonably mild compared to similar latitudes around the world.

When compared to cities in North America, London is shockingly far north—in fact, it’s farther north than any U.S. city in the Lower 48. However, visit both London and northern Maine in the very depths of winter, and you’ll be shocked how different the experiences are. This incredible contrast in temperature is thanks to a major ocean current known as the Atlantic meridional overturning circulation, or AMOC.

This current brings heat, carbon, and nutrients from the tropics all the way up to the arctic circle, hugging along the west coast of the British Isles in the process. This current makes many northern cities in Europe much more livable, which is why AMOC is one of the most important components of the world’s climate regulation system. But for the first time in 10,000 years, that system is under threat.

The disastrous effects if the AMOC were to collapse are well-known—so well known that its disappearance serves as one of the catastrophic plot points of the 2004 film The Day After Tomorrow. Although the impact of AMOC’s collapse gets a bit of the Hollywood treatment, streamlining its life-threatening effects into a comfortable 90-minute runtime, the real life consequences of AMOC’s demise are no less dire.

Scientists have been tracking the impact of climate change on AMOC for decades, and previous studies have looked at past climatic events and discerned a tipping point that would likely cause AMOC’s end. Now, a new study published earlier this month in the journal Science Advances discovers that things are even more grim than previously believed.

“In a new study using the latest generation of Earth’s climate models, we simulated the flow of fresh water until the ocean circulation reached that tipping point,” the authors write for The Conversation. “The results showed that the circulation could fully shut down within a century of hitting the tipping point, and that it’s headed in that direction.”

The main problem is that melting glaciers are injecting too much fresh water into this oceanic conveyor belt. This dilutes the current’s saltiness, which then inhibits sinking and thus weakens the current overall. Once too much freshwater enters the system (a.k.a. the tipping point), then the AMOC’s collapse happens quickly.

So, what does the AMOC’s demise mean for the world at large? Well, nothing good. While things won’t go from bad to worse quite as rapidly as in The Day After Tomorrow, temperatures could drop 5 to 10 degrees Celsius in Western Europe over the course of a century. While that may sound nice in an era of rising temperatures worldwide, such a climatic event would have devastating impacts on agriculture. As one climate scientist told Inside Climate News, it’d be like “trying to grow potatoes in Northern Norway.” Other places (i.e. Norway) would simply become too cold for human habitation, according to Slate.

Although some studies have suggested that such a collapse could happen as early as next year, this new study calls that approximation into question. While they don’t know for sure when it could happen, the team has developed a kind of early warning system to give the world a head’s up when AMOC is heading toward the point of no return.

“We were able to develop a physics-based and observable early warning signal involving the salinity transport at the southern boundary of the Atlantic Ocean,” the authors wrote. “Once a threshold is reached, the tipping point is likely to follow in one to four decades.”

While this certainly isn’t good news, it’s still not too late to stave off the worst effects of climate change by lowering emissions, transitioning to renewable energy, and doing our part to keep the Earth habitable for future generations.

Darren lives in Portland, has a cat, and writes/edits about sci-fi and how our world works. You can find his previous stuff at Gizmodo and Paste if you look hard enough.