The last time there was a major slowdown in the massive network of ocean currents that shape the climate around the North Atlantic, it appears to have caused a severe cold spell in Europe for more than a millennium.
That was about 12,800 years ago, when not many people experienced it. But in recent decades, human warming may have caused the tides to weaken again, and scientists are working to determine if and when they might undergo another severe weakening, which would have ripple effects for weather patterns in one part of the world.
A pair of researchers in Denmark this week released bold answer: A sharp weakening of currents, or even a shutdown, may be upon us by the end of the century.
It was a surprise even to the researchers that their analysis showed a potential collapse coming, 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’s also a surprise, said Dr. Ditlevsen, that he and his co-author were able to pin down the timing of a collapse. Scientists will certainly continue to study and debate the issue, but Dr. Ditlevsen said the new findings are reason enough not to treat a shutdown as an abstract, distant concern. “Now,” he said.
The new research, published on Tuesday in the journal Nature Communications, adds to a growing body of scientific work that describes how humanity’s continued emissions of heat-trapping gases could cause “tipping points,” or rapid and difficult-to-reverse changes in the environment.
Sudden melting of Arctic permafrost. Loss of the Amazon rain forest. Collapse of the Greenland and West Antarctic ice sheets. Once the world warms to a certain point, these and other events could be set into rapid motion, scientists warn, although the exact limits at which this will happen are still uncertain.
In the Atlantic, researchers look for harbingers of a tipping-point-like change in a tangle of ocean currents that goes by an ominous 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 bending toward northern Europe. When this water releases its heat into the air farther north, it becomes colder and denser, causing it to sink into the deep ocean and return to the Equator. This sinking effect, or “overturning,” allows the currents to transfer enormous amounts of heat across the planet, making it a major influence for the climate around the Atlantic and beyond.
As humans warm the atmosphere, however, the melting of Greenland’s ice sheet adds large amounts of fresh water to the North Atlantic, which can disrupt the balance of heat and salinity that keeps the fall moving. A patch of the Atlantic south of Greenland has cooled dramatically in recent years, creating a “cold blob” that some scientists see as a sign that the system has slowed.
If the circulation were to shift to a weaker state, the effects on the climate would be far-reaching, though scientists are still assessing their potential magnitude. Most of the Northern Hemisphere can get cold. The coasts of North America and Europe may see even faster sea level rise. Northern Europe may experience stormier winters, while the Sahel in Africa and the monsoon regions of Asia tend to see less rainfall.
Evidence from ice and sediment cores indicates that the Atlantic circulation underwent abrupt stops and starts in the deep past. But global climate scientists’ most advanced computer models make wide-ranging predictions for how the currents might behave in the coming decades, in part because the mix of factors shaping them is so complex.
The new analysis by Dr. Ditlevsen focused on a simple metric, based on sea surface temperatures, similar to those used by other scientists as proxies for the strength of the Atlantic circulation. He conducted the analysis with Peter Ditlevsen, his brother, who is a climate scientist at the Niels Bohr Institute of the University of Copenhagen. They used data in their proxy measure from 1870 to 2020 to calculate statistical indicators that indicate changes in the fall.
“We’re not only seeing an increase in these indicators,” says Peter Ditlevsen, “but we’re 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 by mid-century, though it could happen as soon as 2025 and as late as 2095.
Their analysis did not include specific assumptions about how much greenhouse-gas emissions will increase this century. This simply assumes that the forces causing the collapse of the AMOC will continue at an unchanging pace – essentially, atmospheric carbon dioxide concentrations will continue to rise as they have since the Industrial Revolution.
In interviews, some researchers who study overturning applauded the new analysis for using a novel approach to predict when we might cross a tipping point, especially given how difficult it is to do so using computer models of global climate. But they expressed reservations about some of its methods, and said more work is needed to determine the time with greater certainty.
Susan Lozier, a physical oceanographer at Georgia Tech, said that sea surface temperatures in the North Atlantic near Greenland are not necessarily influenced by inversion changes alone, making them a questionable proxy for inferring those changes. He taught a study published last year showing that much of the cold drop development can be explained by changes in wind and atmospheric patterns.
Scientists are now using sensors slung in the Atlantic to directly measure the fall. Dr. Lozier is involved in one of them effort measurement. The goal is to better understand what drives changes in the bottom of the waves, and to improve projections of future changes.
But the projects started collecting data in 2004 at the earliest, which is not enough time to draw firm long-term conclusions. “It’s very difficult to look at a short record for ocean subsidence and say what it’s going to do in 30, 40 or 50 years,” said Dr. Lozier.
Levke Caesar, a postdoctoral researcher studying overturning at the University of Bremen in Germany, expressed concerns about the older temperature records used by Dr. Ditlevsen and Dr. Ditlevsen to calculate their proxy. These records, from the late 19th and early 20th centuries, may not be reliable enough to be used for fine-toothed statistical analysis without careful editing, he said.
However, the new study sends an urgent message about the need to continue collecting data on changing ocean currents, said Dr. Caesar. “Something is going on, and it’s probably unusual,” he said. “Something that wouldn’t have happened if it wasn’t for our people.”
Scientists’ uncertainty about the timing of the AMOC collapse should not be used as a reason not to reduce greenhouse-gas emissions to try to prevent it, said Hali Kilbourne, an associate research professor at the University of Maryland Center for Environmental Science.
“It’s very true that we’ve fallen off the cliff and we don’t know it,” said Dr. Kilbourne. “I’m afraid, frankly, that by the time any of this is scientifically settled, it’s too late to act.”