Photo by Dimitry B
Two new studies paint an alarming picture of the Atlantic meridional overturning circulation (AMOC) – the vast ocean current system that regulates climate across Europe, Africa and the Americas. Scientists say the AMOC is both already weakening and significantly more likely to collapse than previously thought.
The future: models were too optimistic
The first study, published in the journal Science Advances, combined climate models with real-world ocean temperature and salinity data to assess which models best reflect reality. It found that most models underestimate the AMOC’s decline. The AMOC is on course to slow by more than 50% by the end of the century — a weakening 60% stronger than the average of all climate models had suggested.
Dr Valentin Portmann, at the Inria Centre de recherche Bordeaux Sud-Ouest and lead author, 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, said the study showed 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 warned: “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 also cautioned that even this revised assessment may understate the risk, as the models do not account for meltwater from the Greenland ice cap further freshening Atlantic surface waters: “That is one additional factor that means the reality is probably still worse.”
The present: weakening already observed
A second study, published the previous week by scientists from the University of Miami, examined what is happening to the AMOC right now. Researchers analysed data from four moorings along the western boundary of the North Atlantic, measuring water temperature, salinity and current velocity since 2004, and found the AMOC has been weakening at all four latitudes over the past two decades.
Shane Elipot, a physical oceanographer at the University of Miami and report author, said the western boundary of the Atlantic is “the canary in a coal mine” for what is happening to the AMOC more broadly. “The worrying part is that the same models are predicting that the AMOC is likely going towards a tipping point where it eventually shuts down,” he said.
René van Westen, a marine and atmospheric researcher at Utrecht University who was not involved in either study, said the results of both were “very worrying.” He added: “This also means that the risk of AMOC tipping is getting more substantial, as every additional AMOC weakening pushes the system towards the tipping point.”
What collapse would mean
A full AMOC collapse would shift the tropical rainfall belt on which millions depend for food, drive extreme cold winters and summer droughts across western Europe, and add 50 to 100cm to already rising sea levels around the Atlantic. The last collapse occurred roughly 12,000 years ago. The AMOC is already known to be at its weakest for 1,600 years.