The collapse of a critical ocean current system—an event that would upend Earth’s climate and wreak havoc on the Americas, Europe, and Africa—has been the subject of heated scientific debate for more than a decade. Experts have differing opinions on how imminent this disaster really is, but a new study may have just confirmed their worst fears.
The findings, published Wednesday in the journal Science Advances, suggest that the Atlantic Meridional Overturning Circulation (AMOC) is on track for a 43% to 58% slowdown by 2100—a substantial weakening that is 60% stronger than than the average estimate of all climate models.
This is “seriously bad news,” Stefan Rahmstorf, an oceanographer at the Potsdam Institute for Climate Impact Research in Germany, wrote on social media. He was not involved in the research, but he has studied the AMOC system for more than 35 years.
“I am now increasingly worried that we may well pass that AMOC shutdown tipping point, where it becomes inevitable, in the middle of this century, which is quite close,” Rahmstorf told The Guardian.
A catastrophic collapse
The AMOC is a critical part of an even larger system of currents that transport water throughout the world’s oceans. Driven by changes in salinity and ocean temperature, it circulates water from north to south and back again within the Atlantic Ocean. This circulation redistributes heat from the tropics to the Arctic, and therefore plays an essential role in regulating Earth’s climate.
The AMOC’s collapse would trigger global climatic shifts with catastrophic regional impacts. Changing currents would cause sea levels to rise dramatically along the U.S. East Coast and other densely populated shorelines. Storms would become more extreme, and the tropical rain belt would shift, causing widespread drought in some areas and excessive rainfall and flooding in others.
Scientists know this because it’s happened before. Toward the end of the last ice age, mass melting of ice sheets released a deluge of freshwater into the Northern Atlantic. The resulting change in salinity forced the AMOC to slow down and ultimately collapse, leading to rapid cooling in the Northern Hemisphere and warming in the Southern Hemisphere.
In recent years, some studies have found evidence to suggest that the AMOC is weakening again, this time due to human-driven climate change. But because scientists use dozens of different computer models to assess the stability of this highly complex system, they often get wildly different results, which is why the future of the AMOC has been subject to debate. This new study sought to reduce that uncertainty.
Mounting evidence for instability
Researchers led by Valentin Portmann of the Inria Centre de recherche Bordeaux Sud-Ouest in France compared climate-model projections with real-world data to determine which projections best matched observed ocean conditions.
After testing four observational constraint methods, the best-performing approach estimated—with 90% probability—that the AMOC will weaken by roughly 50% by the end of the century.
“This is an important and very concerning result,” Rahmstorf told The Guardian. “It shows that the ‘pessimistic’ models, which show a strong weakening of the AMOC by 2100, are, unfortunately, the realistic ones, in that they agree better with observational data.”
The alarming finding follows research published last week that analyzed water temperature, salinity, and ocean current velocity data collected since 2004 by four moorings along the western boundary of the North Atlantic. That study concluded that the AMOC has been weakening at multiple latitudes over the past two decades.
As a growing body of evidence points to the AMOC’s instability, many scientists are sounding the alarm. While researchers are still working to understand its status and rate of weakening, it’s becoming increasingly clear that human-driven climate change could push this system to the brink of collapse.
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