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The critical Atlantic current system appears significantly more likely to collapse than previously thought after new research found that climate models predicting the biggest slowdown are the most realistic. Scientists called the new finding “very concerning” as a collapse would have catastrophic consequences for Europe, Africa and the Americas.
The Atlantic meridional overturning circulation (Amoc) is a major part of the global climate system and was already known to be at its weakest for 1,600 years as a result of the climate crisis. Scientists spotted warning signs of a tipping point in 2021 and know that the Amoc has collapsed in the Earth’s past.
Climate scientists use dozens of different computer models to assess the future climate. However, for the complex Amoc system, these produce widely varying results, ranging from some that indicate no further slowdown by 2100 to those suggesting a huge deceleration of about 65%, even when carbon emissions from fossil fuel burning are gradually cut to net zero.
The research combined real-world ocean observations with the models to determine the most reliable, and this hugely reduced the spread of uncertainty. They found an estimated slowdown of 42% to 58% in 2100, a level almost certain to end in collapse.
The Amoc is a major part of the global climate system and brings sun-warmed tropical water to Europe and the Arctic, where it cools and sinks to form a deep return current. A collapse would shift the tropical rainfall belt on which many millions of people rely to grow their food, plunge western Europe into extreme cold winters and summer droughts, and add 50-100cm to already rising sea levels around the Atlantic.
Dr Valentin Portmann, at the Inria Centre de recherche Bordeaux Sud-Ouest in France and who led the new research, said: “We found that the Amoc is going to decline more than expected compared to the average of all climate models. This means we have an Amoc that is closer to a tipping point.”
Prof Stefan Rahmstorf, at the Potsdam Institute for Climate Impact Research in Germany, said: “This is an important and very concerning result. 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.”
He added: “I now am 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, who has studied the Amoc for 35 years, has said a collapse must be avoided “at all costs”. “I argued this when we thought the chance of an Amoc shutdown was maybe 5%, and even then we were saying that risk is too high, given the massive impacts. Now it looks like it’s more than 50%. The most dramatic and drastic climate changes we see in the last 100,000 years of Earth history have been when the Amoc switched to a different state.”
The Amoc is slowing because air temperatures are rising rapidly in the Arctic because of global heating. That means the ocean cools more slowly there. Warmer water is less dense and therefore sinks into the depths more slowly. This slowing allows more rainfall to accumulate in the salty surface waters, also making it less dense, and further slowing the sinking and forming an Amoc feedback loop.
The Amoc system is highly complex and subject to random natural variations, making precise predictions impossible. However, a major weakening is now expected by scientists and that alone could have serious impacts in the decades to come.
The new research, published in the journal Science Advances, explored four different ways of using real-world observations to assess the models. They found a method called ridge regression, which had been little used in climate science before now, provided the best results.
The Amoc is difficult to model because it is governed by subtle differences in water density caused by salinity changes over the entire Atlantic. The reduction in uncertainty in the new analysis results from identifying the models that better reflect surface salinity in the south Atlantic, which scientists already knew was important. This makes the work “very credible”, said Rahmstorf.
Rahmstorf said Amoc slowdown in 2100 may be even greater than in the new, pessimistic assessment. This is because the computer models do not include the meltwater from the Greenland ice cap that is also freshening the ocean waters: “That is one additional factor that means the reality is probably still worse.”
