The collapse of a critical Atlantic current can no longer be considered a low-likelihood event, a study has concluded, making deep cuts to fossil fuel emissions even more urgent to avoid the catastrophic impact.
The Atlantic meridional overturning circulation (Amoc) is a major part of the global climate system. It brings sun-warmed tropical water to Europe and the Arctic, where it cools and sinks to form a deep return current. The Amoc was already known to be at its weakest in 1,600 years as a result of the climate crisis.
Climate models recently indicated that a collapse before 2100 was unlikely but the new analysis examined models that were run for longer, to 2300 and 2500. These show the tipping point that makes an Amoc shutdown inevitable is likely to be passed within a few decades, but that the collapse itself may not happen until 50 to 100 years later.
The research found that if carbon emissions continued to rise, 70% of the model runs led to collapse, while an intermediate level of emissions resulted in collapse in 37% of the models. Even in the case of low future emissions, an Amoc shutdown happened in 25% of the models.
Scientists have warned previously that Amoc collapse must be avoided “at all costs”. It 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 50cm to already rising sea levels.
The new results are “quite shocking, because I used to say that the chance of Amoc collapsing as a result of global warming was less than 10%”, said Prof Stefan Rahmstorf, at the Potsdam Institute for Climate Impact Research in Germany, who was part of the study team. “Now even in a low-emission scenario, sticking to the Paris agreement, it looks like it may be more like 25%.
“These numbers are not very certain, but we are talking about a matter of risk assessment where even a 10% chance of an Amoc collapse would be far too high. We found that the tipping point where the shutdown becomes inevitable is probably in the next 10 to 20 years or so. That is quite a shocking finding as well and why we have to act really fast in cutting down emissions.”
Scientists spotted warning signs of a tipping point in 2021 and know that the Amoc has collapsed in the Earth’s past. “Observations in the deep [far North Atlantic] already show a downward trend over the past five to 10 years, consistent with the models’ projections,” said Prof Sybren Drijfhout, at the Royal Netherlands Meteorological Institute, who was also part of the team.
“Even in some intermediate and low-emission scenarios, the Amoc slows drastically by 2100 and completely shuts off thereafter. That shows the shutdown risk is more serious than many people realise.”
The study, published in the journal Environmental Research Letters, analysed the standard models used by the Intergovernmental Panel on Climate Change (IPCC). The scientists were particularly concerned to find that in many models the tipping point is reached in the next decade or two, after which the shutdown of the Amoc becomes inevitable owing to a self-amplifying feedback.
Air temperatures are rising rapidly in the Arctic becauseof the climate crisis, meaning 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, forming the feedback loop. Another new study, using a different approach, also found the tipping point is probably going to be reached around the middle of this century.
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Only some of the IPCC models have been run beyond 2100, so the researchers also looked to see which of those running to the end of this century showed Amoc was already in terminal decline. This produced the 70%, 37% and 25% figures. The scientists concluded: “Such numbers no longer comply with the low-likelihood-high-impact event that is used to discuss an abrupt Amoc collapse in [the IPCC’s last report].”
Rahmstorf said the true figures could be even worse, because the models did not include the torrent of meltwater from the Greenland ice cap that is also freshening the ocean waters.
Dr Aixue Hu at the Global Climate Dynamics Laboratory in Colorado, US, who was not part of the study team, said the results were important. “But it is still very uncertain when Amoc collapse will happen or when the Amoc tipping point is going to crossed because of the lack of direct observations [of the ocean] and the varying results from the models.”
The study that found that a total collapse of the Amoc was unlikely this century was led by Dr Jonathan Baker at the Met Office Hadley Centre in the UK. “This new study highlights that the risk rises after 2100,” he said. “[But] these percentages should be treated with caution – the sample size is small, so more simulations [beyond 2100] are needed to better quantify the risk.”
Nonetheless, Baker said, “the ocean is already changing, and projected shifts in North Atlantic convection are a real concern. Even if a collapse is unlikely, a major weakening is expected, and that alone could have serious impacts on Europe’s climate in the decades to come. But the future of the Atlantic circulation is still in our hands.”
