Scientists have found that the Atlantic Meridional Overturning Circulation (Amoc) is significantly more likely to collapse than previously estimated. New research published in the journal Science Advances indicates that climate models predicting the biggest slowdown are the most realistic, leading to an estimated slowdown of 42% to 58% by 2100, a level almost certain to trigger a collapse. Dr Valentin Portmann, who led the research at the Inria Centre de Recherche Bordeaux Sud-Ouest in France, 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.”
The Amoc is a crucial part of the global climate system, carrying warm tropical water to Europe and the Arctic. A collapse would have catastrophic consequences, including shifting the tropical rainfall belt on which millions rely for food, plunging western Europe into extreme cold winters and summer droughts, and adding 50-100cm to rising sea levels around the Atlantic. The system is already at its weakest in 1,600 years due to climate change, and warning signs of a tipping point were identified in 2021.
Previous climate models produced widely varying results for the Amoc, ranging from no further slowdown by 2100 to a 65% deceleration, even with carbon emissions gradually cut to net zero. The new study combined real-world ocean observations with models to determine the most reliable, drastically reducing uncertainty. Prof Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research called the findings “very concerning” and noted that the pessimistic models are now seen as realistic. 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.”
The Amoc is slowing due to rapid Arctic warming, which reduces ocean cooling and causes saltier, denser water to sink more slowly. This creates a feedback loop further weakening the system. While exact predictions are impossible due to natural variations, scientists warn that a major weakening alone would bring serious impacts in coming decades. Rahmstorf stressed that a collapse must be avoided “at all costs”, stating: “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 research used a method called ridge regression, little used in climate science, to better reflect surface salinity in the south Atlantic, making the work “very credible” according to Rahmstorf. He also cautioned that the slowdown in 2100 could be even greater than the pessimistic assessment suggests, as computer models may still underestimate the feedbacks involved.
