Comment on this storyComment
The Atlantic Ocean’s delicate 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 in the next few decades.
Scientists have long regarded the Atlantic Meridional Overturning Circulation (AMOC) as one of the planet’s most vulnerable “tipping elements” – This means that the system could change abruptly and irreversibly, with dramatic consequences for the rest of the world. Under Earth’s current climate, this water conveyor belt transports warm, salty water from the tropics to the North Atlantic and then sends colder water back south along the sea floor. But as rising global temperatures melt Arctic ice, the resulting influx of cold freshwater has taken its toll on the system — and could cripple it entirely.
The study, published Tuesday in the journal Nature Communications, suggests that continued warming will push the AMOC past its “tipping point” around mid-century. The shift would be as abrupt and irreversible as turning off a light switch and could result in dramatic weather changes on both sides 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. “It really shows that we need to put a big brake on greenhouse gas emissions.”
Ditlevsen’s analysis contradicts the recent report by the United Nations Intergovernmental Panel on Climate Change, which relied on multiple climate models and concluded with “moderate certainty” that the AMOC will not collapse completely this century.
Other AMOC experts also warned that the new study’s conclusions should be taken with caution, as they do not represent new observations of the entire ocean system, but rather extrapolate into the future based on previous data from a limited region of the Atlantic.
“The qualitative statement that AMOC has lost stability over the last century remains true even when all uncertainties are taken into account,” said Niklas Boers, a scientist at the Potsdam Institute for Climate Impact Research in Germany. “But the uncertainties are too great for a reliable estimate of the timing of the AMOC tipping.”
The new study adds to the growing evidence that this important marine system is under threat. Since 2004, observations of a network of sea buoys show that the AMOC is weakening – although the limited time frame of this dataset makes it difficult to identify a trend. Scientists have also analyzed several “proxy” indicators of the strength of the current, including microscopic organisms and tiny sediments from the seafloor, to show the system is at its weakest state in more than 1,000 years.
For their analysis, Peter Ditlevsen and his colleague Susanne Ditlevsen (Peter’s sister) examined records of sea surface temperatures dating back to 1870. They found that temperatures in the northernmost waters of the Atlantic had fluctuated more widely in recent years and were taking longer to return to normal. These are “early warning signs” that the AMOC is becoming critically unstable, the scientists said — like the increasingly wild swings before a tower of Jenga blocks begins to collapse.
Susanne Ditlevsen, a statistician at the University of Copenhagen, then developed an advanced mathematical model to predict how much more wobble the AMOC system could handle. The results suggest the AMOC could collapse anytime between 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 flow leads to a sudden freeze in the northern hemisphere. But it could lead to a temperature drop in northern Europe and more warming in the tropics, Peter Ditlevsen said, as well as stronger storms on North America’s east coast.
Marilena Oltmanns, an oceanographer at the National Oceanography Center in the UK, noted in a statement that North Atlantic temperatures are “only part of a highly complex, dynamic system”. Although her own research into marine physics supports the Ditlevsens’ conclusion that this particular region may reach a turning point this century, she is wary of associating that transition with a major change in the Atlantic circulation.
But the dangers of even a partial AMOC shutdown mean it’s worth investigating any signs of instability, said Stefan Rahmstorf, another oceanographer at the Potsdam Institute who wasn’t involved in the new study.
“As always in science, a single study provides limited evidence, but when multiple approaches lead to similar conclusions, it needs to be taken very seriously,” he said. “The scientific findings now show that we cannot even rule out that a turning point will be reached in the next one to two decades.”
Chris Mooney contributed to this report.
Give this item as a gift