Chilling news: a mysterious patch of cold water at the bottom of the north Atlantic is caused by the weakening of a vital ocean current that, if it collapses, would trigger devastating climate shifts across the globe.

At least, that’s the conclusion of a new study published in the journal Geophysical Research Letters, which seeks to settle the debate on the ocean’s so-called “cold blob” that has mystified scientists for years. 

While the rest of the world heats up, this region to the southeast of Greenland keeps getting colder. Some theories propose that atmospheric phenomena like shifting wind patterns are responsible, causing the water to lose heat through its surface. 

But a growing body of evidence has explored the possibility that the cold blob is a symptom of a much more troubling malady: the impending collapse of a key artery of the global ocean called the Atlantic Meridional Overturning Circulation, or AMOC.

The AMOC cycles warm water northwards and dumps colder water southwards, influencing weather systems along the way. If it collapses, scientists predict that Europe would be plunged into even colder temperatures, rainfall patterns across the tropics would be thrown into chaos and imperil harvests, and sea levels near the US east coast would rise.

Whether the AMOC is weakening or near the verge of collapse is still hotly debated by scientists. This latest work enters the fray by painting a long-term picture of the region’s climate using direct observations such as temperature readings taken between 1995 and 2024, and then feeding them into a climate model, in a process known as climate reanalysis.

The reanalysis found that the heat loss from the ocean surface has actually decreased since 1955, cutting against the theory that the cold blob formed from atmospheric phenomena. But the smoking gun was that the temperatures were also plunging deep underwater, at up to 1,000 meters below the surface. For this to be the case, the AMOC must be bringing less warm water into this region — a sign that the current is in decline.

Surface heat loss caused by winds and clouds “only explain a modest fraction of the warming hole,” study lead author Stefan Rahmstorf, an oceanologist at the Potsdam Institute for Climate Impact Research in Germany, told New Scientist.

“Even if, in some modelling approaches, it seems possible that the cold blob is caused by the atmosphere, in fact, the data show it is caused by the ocean,” he added.

It’s compelling work. It’s too soon, however, to say if the debate on the cold blob is settled. 

“It won’t be the final word,” David Thornalley at University College London, told New Scientist.

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