According to The Guardian, scientists have called the new discovery "very alarming," as a collapse would have catastrophic consequences for Europe, Africa, and America.
The Atlantic Meridional Overturning Circulation (AMOC) is a major part of the global climate system and was already known to be at its weakest level in 1,600 years due to the climate crisis. Scientists observed warning signs of a tipping point in 2021 and know that AMOC has collapsed in Earth's past.
Climate scientists use dozens of different computer models to estimate future climate. However, for the complex AMOC system, these produce widely varying results, ranging from some showing no further slowdown by 2100 to those suggesting a major slowdown of about 65%, even when carbon emissions from burning fossil fuels gradually decline to zero.
The research combined real-world ocean observations with models to determine the most reliable ones, significantly reducing the spread of uncertainty. They found an estimated slowdown of 42% to 58% by 2100, a level almost certain to end in collapse.
AMOC is a major part of the global climate system, bringing 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 50-100 cm to already rising sea levels around the Atlantic.
Dr. Valentin Portmann, at the Inria research center Bordeaux Sud-Ouest in France and who led the new research, said: "We found that AMOC will 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."
Professor Stefan Rahmstorf, at the Potsdam Institute for Climate Impact Research in Germany, said: "This is an important and very alarming result. It shows that the 'pessimistic' models, which indicate a strong weakening of AMOC by 2100, are, unfortunately, the realistic ones, in the sense that they best match the observational data."
"I am now increasingly concerned that we might cross that AMOC shutdown tipping point, where it becomes inevitable, by the middle of this century, which is quite close," he added.
Rahmstorf, who has studied AMOC for 35 years, has said that 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 said the risk is too high, given the massive impacts. Now it seems to be more than 50%. The most dramatic and drastic climate changes we see in the last 100,000 years of Earth's history have been when AMOC switched to another state," says Rahmstorf.
AMOC is slowing because air temperatures are rising rapidly in the Arctic due to global warming. This means the ocean cools more slowly there. Warmer water is less dense and therefore sinks more slowly. This slowdown allows more rainfall to accumulate in the salty surface waters, making it also less dense, and further slowing the sinking, forming a feedback loop in AMOC.
The AMOC system is very complex and subject to random natural variations, making accurate predictions impossible. However, a major weakening is now expected by scientists and could have serious impacts in the coming decades.
The new research, published in the journal Science Advances, explored four different ways of using real-world observations to evaluate models. They found that a method called ridge regression, which had been little used in climate science before, gave the best results.
"AMOC is difficult to model because it is driven by delicate changes in water density caused by salinity changes across the Atlantic. The reduction in uncertainty in the new analysis comes from identifying models that better reflect surface salinity in the South Atlantic, which scientists already knew was important. This makes the work 'very reliable,'" said Rahmstorf.
Rahmstorf said the slowdown of AMOC by 2100 could be even greater than in the new 'pessimistic' estimate. This is because computer models do not include meltwater from the Greenland ice cap, which is also cooling ocean waters: "This is an additional factor that means reality is probably even worse."