When Might the AMOC Collapse?

I've posted before about the possibility that the AMOC (Atlantic Meridional Overturning Circulation) might collapse. The AMOC is the Atlantic's circulatory system that brings warm water up from the equatorial region north and returns cold water south. Among other things, it's responsible for Europe's relatively mild climate. Were it to weaken or collapse completely, it would wreak havoc on global climate and the world's weather systems. 

Wired has a long and very readable article about research into the current and future state of the AMOC. It's one of the best articles I've read about it and well worth taking the time to read.

Off the southwest tip of Iceland, you’ll find what’s often called a “marginal” body of water. This part of the Atlantic, the Irminger Sea, is one of the stormiest places in the northern hemisphere. On Google Maps it gets three stars: “very windy,” says one review. It’s also where something rather strange is happening. As the rest of the planet has warmed since the 20th century—less in the tropics, more near the poles—temperatures in this patch of ocean have hardly budged. In some years they’ve even cooled. If you get a thrill from spooky maps, check out one that compares the average temperatures in the late 19th century with those of the 2010s. All of the planet is quilted in pink and red, the familiar colors of climate change. But in the North Atlantic, there’s one freak splotch of blue. If global warming were a blanket, the Irminger Sea and its neighboring waters are where the moths ate through. Scientists call it the warming hole.

The warming hole could be a very big problem. That’s because it’s a sign that something may be wrong with the Atlantic Meridional Overturning Circulation. The AMOC is the main current system that crisscrosses the ocean. It flows like a big river up, down, and across the two hemispheres. All that moving water performs an amazing service—it’s basically a supremely massive, 1-petawatt heat pump for the North Atlantic.

The mega-current hauls warm, salty surface water from the tropics near the Americas up to northern Europe. There the warm water meets cold air and evaporates. The atmosphere heats up. The water that’s left in the AMOC is now colder and saltier—which is to say, it’s much denser than the surrounding water. And if you’re a cod swimming west of Iceland, you’re in for an astonishing show. Here the heavy AMOC water doesn’t merely sink, it plummets nearly 3 kilometers down. (Two miles!) Some 3 million cubic meters of water fall per second, in what amounts to the world’s most record-smashing, invisible waterfall. This cold river joins up with other falling water—more underwater cataracts—and crawls through the depths of the ocean, following the topography of the seabed, all the way to Antarctica. The flow intersects other currents, things get messy, and eventually the current rises to the surface near South America and continues its loop.