The last time there was a significant slowdown in the massive network of climate-shaping ocean currents around the North Atlantic, they seemed to plunge Europe into a deep cold for more than a millennium.
That was nearly 12,800 years ago, when not many people had experienced it. But in recent decades, human-driven warming can slow the currents again, and scientists are working to determine if and when they will experience another major weakening, ripple effects on weather patterns across a swath of the globe.
This week a pair of researchers in Denmark put forth a bold answer: A severe weakening of the currents, or even a shutdown, could be upon us by the end of the century.
It came as a surprise even to the researchers that their analysis showed a possible collapse soon, one of them, Susanne Detlefsen, a professor of statistics at the University of Copenhagen, said in an interview. Climate scientists generally agree that Atlantic circulation will decrease this century, but there is no consensus on whether it will stop before 2100.
That’s why it was also a surprise, Dr. Detelefsen said, that she and her co-author were able to pinpoint the timing of the collapse at all. Scientists are bound to continue studying and debating this issue, but Dr Detlevsen said the new findings were reason enough not to view lockdown as an abstract, far-fetched concern. She said, “It is now.”
The new research, published Tuesday in the journal Nature Communications, adds to a growing body of scientific work describing how humanity’s continued emissions of greenhouse gases may lead to “tipping points” in climate, or rapid and hard-to-reversible changes in the environment.
Sudden melting of permafrost in the Arctic. Loss of the Amazon rainforest. Ice sheet collapse in Greenland and West Antarctica. Once the world warms past a certain point, scientists warn, these and other events could be set in rapid motion, although the exact thresholds at which that could occur are still highly uncertain.
In the Atlantic, researchers have been looking for signs of a tipping-point-like change in a tangle of ocean currents that goes by an unpopular name: the Atlantic overturning circulation, or AMOC (pronounced “EY-mock”).
These currents carry warm waters from the tropics across the Gulf Stream, past the southeastern United States, before curving toward northern Europe. As this water releases its heat into the air farther north, it becomes cooler and denser, causing it to sink into the ocean depths and back toward the equator. The sinking effect, or “inversion,” allows currents to transport huge amounts of heat around the planet, making them a major influence on the climate around the Atlantic Ocean and beyond.
As the human atmosphere warms, the melting of the Greenland ice sheet adds large amounts of fresh water to the North Atlantic, potentially upsetting the balance of heat and salinity that keeps the upturn moving. A patch of the Atlantic Ocean south of Greenland has visibly cooled in recent years, creating a “cold bubble” that some scientists see as a sign of a slowing down of the system.
If the circulation is in a much weaker state, the effects on climate would be far-reaching, though scientists are still working out its potential magnitude. Much of the northern hemisphere could cool down. The coasts of North America and Europe could see faster sea level rise. Northern Europe could experience windier winters, while Africa’s Sahel and monsoon regions of Asia are likely to experience less rain.
Evidence from ice and sediment samples indicates that the circulation of the Atlantic Ocean experienced abrupt stops and starts in the deep past. But scientists’ most advanced computer models of global climate have produced a wide range of predictions about how the currents will behave in the coming decades, in part because the mix of factors that shape them is so complex.
Dr. Detlefsen’s new analysis focused on a simple scale, based on sea surface temperatures, similar to those other scientists have used as proxies for the strength of the Atlantic Ocean’s circulation. She conducted the analysis with Peter Detlefsen, her brother, who is a climate scientist at the University of Copenhagen’s Niels Bohr Institute. They used data on their proxy scale from 1870 to 2020 to calculate statistical indicators that predict changes in inversion.
“Not only are we seeing an increase in these indicators, but we are seeing an increase in line with approaching this tipping point,” said Peter Detlefsen.
They then used the mathematical properties of a turning point-like system to extrapolate from these trends. This led them to predict that the Atlantic circulation could collapse around the middle of the century, although it could happen as early as 2025 and as late as 2095.
Their analysis did not include any specific assumptions about how much greenhouse gas emissions will rise in this century. He just assumed that the forces driving the collapse of the AMOC would continue at a steady pace—essentially, that atmospheric carbon dioxide concentrations would continue to rise as they have been since the Industrial Revolution.
In interviews, several researchers who study the inversion have praised 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 with computer models of global climate. But they have expressed reservations about some of its methods, and said more work is still needed to determine the timing with more certainty.
Sea surface temperatures in the North Atlantic near Greenland weren’t necessarily affected by changes in the overturning alone, said Susan Lozier, a physical oceanographer at Georgia Tech, making her a questionable proxy for inferring these changes. She points to a study published last year that showed much of the evolution of the cold bubble can be explained by changes in wind and atmospheric patterns.
Scientists are now using sensors dangling across the Atlantic Ocean to measure the inversion directly. Dr. Lozier is involved in one of these benchmarking efforts. The goal is to better understand what is driving the changes beneath the waves, and to improve predictions of future changes.
But the projects started collecting data in 2004 at the earliest, which isn’t enough time to draw firm, long-term conclusions. “It’s very difficult to look at a short record of ocean upending and talk about what it will do over 30, 40 or 50 years,” said Dr. Lozier.
Levke Kaiser, a postdoctoral researcher who studies inversion at the University of Bremen in Germany, has expressed concerns about the old temperature records used by Dr. She said these records, which date back to the late 19th and early 20th centuries, may not be reliable enough to be used in careful statistical analysis without careful adjustments.
However, the new study sent an urgent message about the need to continue collecting data on changing ocean currents, said Dr. Kaiser. “Something is going on,” she said, “and it’s probably out of the ordinary.” “Something that would not have happened without humans.”
Scientists’ uncertainty about the timing of the AMOC’s collapse should not be taken as an excuse for not reducing greenhouse gas emissions to try to avoid them, said Haley Kilburn, a research associate professor at the University of Maryland’s Center for Environmental Sciences.
“It’s very plausible that we actually fell off a cliff and just don’t know it,” said Dr. Kilburn. “I fear, frankly, that by the time any of this becomes settled science, it will be too late to act.”