The last time there was a major slowdown in the mighty network of ocean currents that shape the climate around the North Atlantic, Europe seems to have been plunged into a deep cold for more than a millennium.
That was about 12,800 years ago when there weren’t many people around to experience it. But in recent decades, human-induced warming could cause the currents to slow again, and scientists have been working to determine if and when they could undergo another major weakening, which would have ripple effects on weather patterns across part of the world.
A pair of researchers in Denmark came up with a bold answer this week: By the end of the century, a sharp weakening of the current or even a closure could be upon us.
It came as a surprise even to the researchers that their analysis showed that a possible collapse was coming so soon, one of them, Susanne Ditlevsen, a professor of statistics at the University of Copenhagen, said in an interview. Climate scientists generally agree that the Atlantic circulation will decrease this century, but there is no consensus on whether it will stop before 2100.
That’s why it came as a surprise, said Dr. Ditlevsen, that she and her co-author could even pinpoint the timing of a collapse. Scientists will no doubt continue to study and debate the issue, but Dr. Ditlevsen said the new findings were reason enough not to view a closure as an abstract, distant concern. “It is now,” she said.
The new research, published Tuesday in the journal Nature Communications, adds to a growing body of scientific work describing how humanity’s continued release of heat-trapping gases could create climate “tipping points,” or rapid and hard-to-reverse changes in the environment.
Abrupt thaw of the Arctic permafrost. Loss of the Amazon rainforest. Collapse of the Greenland and West Antarctic ice sheets. Once the world warms to a certain point, this and other events could be set in motion quickly, scientists warn, though the exact thresholds at which this would happen are still highly uncertain.
In the Atlantic Ocean, researchers have been looking for harbingers of tipping point-like changes in a tangle of ocean currents that has an unappealing name: the Atlantic Meridional Overturning Circulation, or AMOC (pronounced “EY-mock”).
These currents carry warm water from the tropics through the Gulf Stream, past the southeastern United States, before veering into northern Europe. When this water gives off its heat to the air farther north, it gets colder and denser, causing it to sink to the deep ocean and back toward the equator. This sinking effect, or “tilting,” allows the currents to carry massive amounts of heat around the planet, having a huge impact on the climate around the Atlantic Ocean and beyond.
However, as humans warm the atmosphere, the melting of the Greenland ice sheet is adding large amounts of fresh water to the North Atlantic, which could upset the balance of heat and salinity that keeps the tilt moving. A stretch of the Atlantic Ocean south of Greenland has cooled markedly in recent years, creating a “cold blob” that some scientists see as a sign that the system is slowing down.
If the circulation moved into a much weaker state, the effects on climate would be far-reaching, though scientists are still exploring their potential magnitude. Much of the Northern Hemisphere could cool down. The coastlines of North America and Europe could see faster sea level rise. Northern Europe could experience stormier winters, while the Sahel in Africa and the monsoon regions of Asia are likely to receive less rain.
Evidence from ice and sediment cores indicates that Atlantic circulation stopped and started abruptly in the deep past. But scientists’ most sophisticated computer models of the global climate have yielded a wide range of predictions about how the currents might behave over the coming decades, in part because the mix of factors that shape them is so complex.
The new analysis from Dr. Ditlevsen focused on a simple metric, based on sea surface temperatures, similar to those other scientists have used as proxies for the strength of the Atlantic circulation. She conducted the analysis with Peter Ditlevsen, her brother, who is a climate scientist at the University of Copenhagen’s Niels Bohr Institute. They used data on their proxy measure from 1870 to 2020 to calculate statistical indicators that predict changes in the tilt.
“Not only are we seeing an increase in these indicators,” said Peter Ditlevsen, “but we are seeing an increase consistent with approaching a tipping point.”
They then used the mathematical properties of a tipping point-like system to extrapolate from these trends. That led them to predict that the Atlantic circulation could collapse around mid-century, though it could possibly happen as early as 2025 and as late as 2095.
Their analysis did not include specific assumptions about how much greenhouse gas emissions will rise in this century. It only assumed that the forces that would trigger an AMOC collapse would continue at an unchanged rate — essentially that atmospheric carbon dioxide concentrations would continue to rise, as they have since the industrial revolution.
In interviews, several researchers studying the tilt applauded the new analysis for using a new approach to predict when we might cross a tipping point, especially given how difficult it was to do so using computer models of global climate. But they expressed reservations about some of his methods, saying more work was needed to establish the timing with more certainty.
Susan Lozier, a physical oceanographer at Georgia Tech, said sea surface temperatures in the North Atlantic near Greenland weren’t necessarily affected by tilt changes alone, making them a questionable proxy for inferring those changes. She pointed to a study published last year that found that much of the cold blob’s development could be explained by shifts in wind and atmospheric patterns.
Scientists are now using sensors hurled across the Atlantic Ocean to directly measure the tilt. Dr. Lozier is involved in one of these measurement efforts. The aim is to better understand what causes the changes beneath the waves, and to improve projections of future changes.
But the projects didn’t start collecting data until 2004 at the earliest, which isn’t enough time to draw definitive long-term conclusions. “It’s extremely difficult to look at a short record of the ocean flipping and say what it’s going to be like 30, 40 or 50 years from now,” said Dr. Lozier.
Levke Caesar, a postdoctoral researcher who studied the tilt at the University of Bremen in Germany, expressed concern about the older temperature records Dr. Ditlevsen and Dr. Ditlevsen used to calculate their proxy. This data, from the late 19th and early 20th centuries, may not be reliable enough to be used for fine-toothed statistical analysis without careful adjustments, she said.
Still, the new study sent an urgent message about the need to continue collecting data on changing ocean currents, said Dr. Caesar. “Something is happening, and it’s probably unusual,” she said. “Something that wouldn’t have happened if it weren’t for us humans.”
Scientists’ uncertainty about the timing of an AMOC collapse should not be taken as an excuse not to reduce greenhouse gas emissions to try to avoid it, said Hali Kilbourne, an associate professor at the University of Maryland Center for Environmental Science.
“It’s very likely that we’ve already fallen off a cliff and don’t know it,” said Dr. Kilbourne. “I fear, frankly, that by the time this is all solid science, it will be far too late to act.”
