When will a vital system of currents in the Atlantic Ocean collapse? Depends on whom you ask. todayheadline

Just below Greenland is a menacing stretch of water known as the Cold Blob. As the planet heats up, the Cold Blob remains a spooky outlier — positioned right above the area where the Atlantic Ocean’s so-called conveyor belt is supposed to switch back and head south. 

The Atlantic Meridional Overturning Current, or AMOC for short, comprises an enormous system of currents that carries water and nutrients across the world and plays a large part in stabilizing the global climate. For years, scientists have warned that the AMOC was slowing down, possibly nearing collapse. The Cold Blob is the most immediately visible proof of its decline, likely a result of Greenland’s melting glaciers, but research on the deep water current’s strength over recent years has varied wildly. 

New studies published in Nature Geoscience last week and in Nature earlier this year offer some slightly encouraging news. AMOC’s decline could be “much more of a slow, gradual change, rather than an immediate change,” said David Bonan, a postdoctoral fellow at the University of Washington who served as the lead author on the Nature Geoscience study. The full collapse of the current might not be reached this century, rather than around the midcentury mark as other research has predicted. 

The stakes could hardly be higher. Should the current break down, the most frightening predictions describe a world thrown into chaos: Drought could destroy India, South America, and Africa; the Eastern Seaboard of the United States would see dramatic sea level rise; and an arctic chill would spread across Europe. 

“You cannot adapt to this,” said Peter Ditlevsen, the coauthor of a 2023 study and an ice and climate researcher at the University of Copenhagen, in an interview with Inside Climate News. “There’s some studies of what happens to agriculture in Great Britain, and it becomes like trying to grow potatoes in Northern Norway.” 

A more gradual change would still cause enormous upheaval, but would give the world more time to put mitigation efforts in place. 

Part of what makes AMOC’s behavior so hard to forecast is that consistent monitoring of the current didn’t begin until 2004, so the historical data is limited. When researchers run models to examine AMOC’s behavior in the past, they sometimes get baffling results. “The new models aren’t working for AMOC,” said David Thornalley, a paleoclimatologist at University College London, who wasn’t involved with the latest research. “Some people would say we don’t 100 percent know what AMOC did through the 20th century.” 

The February study in Nature found that the current is more stable than expected. Winds in the Southern Ocean churn the incoming Arctic water up to the surface and send it northward again. The research showed AMOC slowing between 20 to 80 percent by 2100, but not collapsing entirely. This is obviously a pretty big range, and “even a moderate weakening could affect rainfall patterns, sea level rise, and the ocean’s ability to take up carbon,” said Jonathan Baker, lead author of the study and a senior scientist in the Ocean, Cryosphere, and Climate group at the Met Office, the weather service for the U.K. Between 2009 and 2010, AMOC wobbled — slowing about 30 percent — and sea levels rose 5 inches between New York City and Newfoundland within a year. 

The most recent study in Nature Geosciences narrowed that range to a weakening of 18 to 43 percent by 2100 after investigating how previous models were making their calculations. Models that predict an imminent collapse tend to assume AMOC is fairly strong at the moment, extending to great depths within the ocean and forcing warm surface water deep into the sea. Models that presented the current as weaker, with a shallower reach into the deep ocean, were less affected by warming surface waters. 

Bonan and his team found that the North Atlantic is “a little bit more aligned with these weaker models,” he said. “If you warmed up the surface [water], or if you have increased sea ice melt or Greenland Ice Sheet melt, a lot of those surface properties are probably just going to stay at the upper ocean, rather than going deeper into the ocean.”

Still, Bonan highlighted the need for more advanced models that may be able to better forecast  interactions between ice sheets and the ocean. Thornalley underlined those concerns — without sophisticated modeling of meltwater coming off Greenland, he said, these studies might be painting an overly rosy picture. “If you look at what all the models do after 2100, a lot of them go on to collapse,” he said. 

One problem with estimating a drop-dead date for AMOC is that researchers still don’t understand when the current might reach a tipping point, a threshold that, when crossed, will have a cascading effect from which there is no return. Whether the current dies a slower or faster death won’t matter in the long run if the world breaches that threshold. “It’s a good study,” Thornalley said. “Does it make me calmer about the future? No.”