Looking out into the Atlantic Ocean from the jetty at Camp Ellis in Saco on Tuesday. Shawn Patrick Ouellette/Staff Photographer
The Atlantic Ocean’s sensitive circulation system has become slower and less resilient, according to a new analysis of 150 years of temperature data – raising the possibility that this crucial element of the climate system could collapse within the next few decades.
Scientists have long seen the Atlantic Meridional Overturning Circulation, or AMOC, as one of the planet’s most vulnerable “tipping elements” – meaning the system could undergo an abrupt and irreversible change, with dramatic consequences for the rest of the globe. Under Earth’s current climate, this aquatic conveyor belt transports warm, salty water from the tropics to the North Atlantic, and then sends colder water back south along the ocean floor. But as rising global temperatures melt Arctic ice, the resulting influx of cold freshwater has thrown a wrench in the system – and could shut it down entirely.
The study published Tuesday in the journal Nature Communications suggests that continued warming will push the AMOC over its “tipping point” around the middle of this century. The shift would be as abrupt and irreversible as turning off a light switch, and it could lead to dramatic changes in weather on either side of the Atlantic.
“This is a really worrying result,” said Peter Ditlevsen, a climate physicist at the University of Copenhagen and lead author of the new study. “This is really showing we need a hard foot on the brake” of greenhouse gas emissions.
Ditlevsen’s analysis is at odds with the most recent report from the United Nations Intergovernmental Panel on Climate Change, which drew on multiple climate models and concluded with “medium confidence” that the AMOC will not fully collapse this century.
Other experts on the AMOC also cautioned that because the new study doesn’t present new observations of the entire ocean system – instead, it is extrapolating about the future based on past data from a limited region of the Atlantic – its conclusions should be taken with a grain of salt.
“The qualitative statement that AMOC has been losing stability in the last century remains true even taking all uncertainties into account,” said Niklas Boers, a scientist at the Potsdam Institute for Climate Impact Research in Germany. “But the uncertainties are too high for a reliable estimate of the time of AMOC tipping.”
The new study adds to a growing body of evidence that this crucial ocean system is in peril. Since 2004, observations from a network of ocean buoys has showed the AMOC getting weaker – though the limited time frame of that data set makes it hard to establish a trend. Scientists have also analyzed multiple “proxy” indicators of the current’s strength, including microscopic organisms and tiny sediments from the seafloor, to show the system is in its weakest state in more than 1,000 years.
For their analysis, Peter Ditlevsen and his colleague Susanne Ditlevsen (who is Peter’s sister) examined records of sea surface temperatures going back to 1870. In recent years, they found, temperatures in the northernmost waters of the Atlantic have undergone bigger fluctuations and taken longer to return to normal. These are “early warning signals” that the AMOC is becoming critically unstable, the scientists said – like the increasingly wild wobbles before a tower of Jenga blocks starts to fall.
Susanne Ditlevsen, a statistician at the University of Copenhagen, then developed an advanced mathematical model to predict how much more wobbling the AMOC system can handle. The results suggest that the AMOC could collapse any time between now and 2095, and as early as 2025, the authors said.
The consequences would not be nearly as dire as they appear in the 2004 sci-fi film “The Day After Tomorrow,” in which a sudden shutdown of the current causes a flash freeze across the northern hemisphere. But it could lead to a drop in temperatures in northern Europe and elevated warming in the tropics, Peter Ditlevsen said, as well as stronger storms on the East Coast of North America.
Marilena Oltmanns, an oceanographer at the National Oceanography Center in Britain, noted in a statement that the temperatures in the north Atlantic are “only one part of a highly complex, dynamical system.” Though her own research on marine physics supports the Ditlevsens’ conclusion that this particular region could reach a tipping point this century, she is wary of linking that transition to a full-scale change in Atlantic Ocean circulation.
Yet the dangers of even a partial AMOC shutdown mean any indicators of instability are worth investigating, said Stefan Rahmstorf, another oceanographer at the Potsdam Institute who was not involved in the new study.
“As always in science, a single study provides limited evidence, but when multiple approaches lead to similar conclusions this must be taken very seriously,” he said. “The scientific evidence now is that we can’t even rule out crossing a tipping point already in the next decade or two.”
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