How Soon Might the Atlantic Ocean Break? Two Sibling Scientists Found an Answer—and Shook the World
The 2004 movie The Day After Tomorrow may have taken some serious artistic license with climate science, but it got one thing right. Climate change may indeed cause a collapse in global ocean currents, with devastating consequences. When the Hollywood blockbuster was made, researchers knew relatively little about the state of the Atlantic meridional overturning circulation (AMOC), an ocean system that transports warm water and nutrients around the Atlantic Ocean.
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.
The big takeaway is a Europe that’s cozier than geography says it should be. That warm gift—the one where the AMOC dumps much of its heat near Iceland—helps, for example, the Norwegian city of Tromsø to enjoy temperatures as warm as –1 degree Celsius in late January, while, at the same latitude in Canada, Cambridge Bay often gets down to –34 degrees Celsius (or 30 degrees Fahrenheit and –30 degrees Fahrenheit, respectively). The heat delivery is also why the northern hemisphere is a few degrees warmer than the southern hemisphere and why Earth’s warmest latitude is (on average) not the point closest to the sun—the equator—but 5 degrees north of it.
But, that warming hole. This spot isn’t feeling the full kapow of rising global temperatures because, in recent years, less heat has been arriving from the tropics. Which means the currents must be slowing. By some calculations, the AMOC’s flow has weakened by 15 percent since the middle of the 20th century. Looking back further, it is the weakest it has been in a millennium.
Which is alarming. To be sure, the worry is not that the AMOC is on the verge of a complete stop. The fear is that it will cross a pivotal threshold, and then begin a decline that is unstoppable.
At that point, it would take many decades for the currents to grind to a halt. Even so, a shutdown would trigger, as one paper put it, “a profound global-scale reorganization” in Earth’s climate systems. The effects would be devastating—plunging northern Europe into a deep cold spell, crushing food systems, condemning big regions to drought. It’s so, so bad.
It follows, then, that you’d wonder how close we humans are to that threshold. Perhaps you’d heard about the AMOC’s frailty; the shutdown threat; maybe even the decades of fighting among scientists as they try to fathom this gigantic, interconnected, barely understood current. But it was only rather recently that someone dared to go right to the core and ask: How much time do we have left before the AMOC breaks?