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. The AMOC has undergone hundreds of weak periods over the last 2.7 million years as the climate shifted. And scientists have long assumed that each time the North Atlantic cooled, southern latitudes warmed by an equal amount. The total amount of heat remained roughly constant, they thought. It merely shifted from place to place.
But the new modeling study reveals a more dramatic change, showing that when AMOC previously weakened, the Earth grew hotter overall.
"The AMOC works like a heat valve that controls the energy budget of the planet," said lead author Christo Buizert, of Oregon State University. With a strong AMOC, warm water flows to the North Atlantic, where the heat is lost to the atmosphere. But with a weak AMOC, the study found, heat instead builds up in the ocean.
Buizert said that past weak periods produced the same amount of warming as 10 years of human emissions would today. The research was published in Nature Geoscience.
Buizert noted that studies of past shifts in the AMOC are not a perfect comparison for changes underway today, but they can help scientists make sense of what may be in store as temperatures rise.
Why Fears Are Growing Over the Fate of a Key Atlantic Current