The vital Atlantic current system appears much more likely to collapse than previously thought after a new study found that climate models predicting the greatest slowdown are the most realistic. Scientists said the new findings were “very concerning” as a collapse could have devastating consequences for Europe, Africa and the Americas.
The Atlantic Meridional Overturning Circulation (Amoc) is a major part of Earth’s climate system and was already known to be at its weakest in 1,600 years as a result of the climate crisis. Scientists have discovered warning signs of a tipping point in 2021 and know that Amok has collapsed in Earth’s past.
Climate scientists use dozens of different computer models to assess future climate. However, for complex Amoc systems, these results range from indicating no further slowdown by 2100 to suggesting a significant slowdown of about 65%, even if carbon emissions from fossil fuel combustion are gradually reduced to net zero.
The study combined real-world ocean observations and models to determine the most reliable, significantly reducing the spread of uncertainty. They found an estimated 42% to 58% slowdown in 2100, a level that would almost certainly end in collapse.
The Amok River is a major part of Earth’s climate system, bringing solar-warmed tropical water to Europe and the Arctic, where it cools and sinks, forming deep return streams. A collapse would shift the tropical rain belt that millions of people rely on to grow food, plunge Western Europe into frigid winters and summer droughts, and raise already rising sea levels around the Atlantic Ocean by 50 to 100 centimeters.
“We found that the Amoc is decreasing more than expected compared to the average of all climate models, which means that the Amoc is approaching a tipping point,” said Dr. Valentin Portman of the Inria Center in southwestern Bordeaux, France, who led the new study.
“This is an important and very worrying result,” said Professor Stefan Rahmstorff of Germany’s Potsdam Institute for Climate Impact Research. “This result shows that ‘pessimistic’ models showing a significant weakening of Amok by 2100 are unfortunately realistic in that they better match observational data.”
He added: “I’m increasingly concerned that mid-century we’ll cross the tipping point where Amoc closures are inevitable, and we’re pretty close.”
Rahmstorff, who has studied Amok for 35 years, said collapse must be avoided “at all costs.” “I argued this when we thought there was maybe a 5% chance of Amoc closure, but even then I said the risk was too high given the large-scale impact. Now it looks like it’s over 50%. The most drastic and drastic climate change we’ve seen in the last 100,000 years of Earth’s history is when Amoc switched to another nation.”
Amok is slowing down because temperatures in the Arctic are rising rapidly due to global warming. That means the ocean cools more slowly there. Warmer water is less dense, so it sinks to deeper depths more slowly. This reduction in velocity causes more rainfall to accumulate in the saline surface water, reducing its density and further slowing subsidence, forming an Amoc feedback loop.
Amoc systems are highly complex and subject to random natural fluctuations, making accurate predictions impossible. But now scientists are predicting a significant weakening, which alone could have serious implications for decades to come.
The new study, published in the journal Science Advances, looked at four different ways to evaluate models using real-world observations. They found that a method called ridge regression, which until now has been little used in climate science, produced the best results.
The Amoc is difficult to model because it is dominated by subtle differences in water density caused by changes in salinity across the Atlantic Ocean. The reduced uncertainty in the new analysis is a result of identifying a model that better reflects the surface salinity of the South Atlantic, something scientists already knew was important. This makes the study “very reliable,” Rahmstorff said.
Rahmstorff said Amok’s slowdown in 2100 could be even greater than the new pessimistic assessment. That’s because the computer model doesn’t include meltwater from the Greenland ice sheet that freshens the seawater. “This is one additional factor that means the reality is probably even worse.”
