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A vast system of Atlantic currents appears to be weakening, and scientists say its collapse could lead to dramatic cooling in northern Europe. A new study suggests an additional risk: a surge in warming across the rest of the globe.

The Atlantic Meridional Overturning Circulation, or AMOC, carries warm water from the tropics toward Europe. As the water flows north, it cools, becoming more dense, and descends to the depths, where it flows back south. But as oceans warm, those waters are not so cool, nor so dense, as they once were, and measurements suggest the system of circulation is slowing down.

Complicating matters further, the melting Greenland ice sheet is unleashing into the North Atlantic large amounts of fresh water, which is less dense than salty water, further slowing circulation. If the AMOC were to collapse, temperatures in northern Europe would drop by 9 to 27 degrees F (5 to 15 degrees C), according to one estimate .

The weakening is not a new phenomenon.…