Ocean currents in the Atlantic could slow by the end of the century, research suggests

The last time there was a major slowdown in the powerful network of ocean currents that shape the climate around the North Atlantic, it appears to have plunged Europe into deep cold for more than a millennium.

This was around 12,800 years ago, when few people were around to experience it. But in recent decades, human-induced warming could slow currents againand scientists worked to determine if and when they might experience another big weakening, which would have ripple effects on weather patterns across part of the globe.

A pair of researchers in Denmark presented this week a bold answer: A sharp weakening of currents, or even a standstill, could be upon us by the end of the century.

It was a surprise even to the researchers that their analysis showed a potential collapse to come so soon, one of them, Susanne Ditlevsen, professor of statistics at the University of Copenhagen, said in an interview. Climatologists generally agree that The Atlantic circulation will decrease this centurybut there is no consensus on whether it will stop before 2100.

That’s why it was also a surprise, Dr. Ditlevsen said, that she and her co-author could pinpoint the timing of a collapse. Scientists are bound to continue to study and debate the issue, but Dr Ditlevsen said the new findings are reason enough not to consider a shutdown an abstract and distant concern. “It’s now,” she said.

The new research published 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 trigger climate.”tipping pointsor rapid and difficult-to-reverse changes in the environment.

Sudden thaw of arctic permafrost. Loss of the Amazon rainforest. Collapse of the Greenland and West Antarctic ice sheets. Once the world warms beyond a certain point, these and other events could be triggered quickly, scientists warn, although the exact thresholds at which this would occur are still highly uncertain.

In the Atlantic, researchers have been looking for warning signs of a tipping point-like shift in a tangle of ocean currents that goes by an unkindly name: the meridional overturning circulation of the Atlanticor AMOC (pronounced “EY-mock”).

These currents carry warm waters from the tropics through the Gulf Stream past the southeastern United States before heading into northern Europe. As this water releases its heat into the air further north, it becomes colder and denser, causing it to sink to the depths of the ocean and back toward the equator. This sinking, or “overturning” effect allows the currents to transfer huge amounts of heat around the planet, making them hugely influential on the climate around the Atlantic and beyond.

However, as humans warm the atmosphere, the melting Greenland Ice Sheet is adding large amounts of fresh water to the North Atlantic, which could upset the heat-salinity balance that keeps the overturning in motion. Part of the Atlantic south of Greenland has cooled considerably in recent years, creating a “cold drop” that some scientists believe is a sign that the system is slowing down.

If the circulation were to switch to a much weaker state, the effects on climate would be considerable, although scientists are still examining their potential magnitude. Much of the northern hemisphere could cool down. The coasts of North America and Europe could experience faster sea level rise. Northern Europe could experience stormier winters, while the Sahel in Africa and monsoon regions in Asia would most likely receive less rain.

Evidence from ice and sediment cores indicates that the Atlantic Circulation experienced abrupt stops and restarts in the distant past. But scientists’ most advanced computer models of global climate have produced a wide range of predictions of how the currents might behave in coming decades, in part because the mix of factors that shape them is so complex.

Dr Ditlevsen’s new analysis 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 climatologist at the Niels Bohr Institute at the University of Copenhagen. They used data on their indirect measurement from 1870 to 2020 to calculate statistical indicators that predict changes in the reversal.

“Not only are we seeing an increase in these indicators,” said Peter Ditlevsen, “but we are seeing an increase that is consistent with approaching a tipping point.”

They then used the mathematical properties of a tipping point-like system to extrapolate from these trends. This led them to predict that the Atlantic circulation could collapse around mid-century, although this could potentially occur as early as 2025 and into 2095.

Their analysis did not include any specific assumptions about the increase in greenhouse gas emissions during this century. He only assumed that the forces causing the AMOC to collapse would continue at an unchanging rate – essentially, that atmospheric carbon dioxide concentrations would continue to rise as they have since the Industrial Revolution.

In interviews, several researchers who study the reversal applauded the new analysis for using a novel approach to predicting when we might cross a tipping point, especially given how difficult it is to do so using computer models of the global climate. But they expressed reservations about some of his methods and said more work was still needed to set the timeline with more certainty.

Susan Lozier, a physical oceanographer at Georgia Tech, said sea surface temperatures in the North Atlantic near Greenland were not necessarily influenced by changes in overturning alone, making it a questionable indicator for inferring such changes. She pointed a study published last year showing that much of the development of the cold drop could be explained by changes in wind and atmospheric patterns.

Scientists are now using sensors suspended across the Atlantic to directly measure the overthrow. Dr. Lozier is involved in one of those measurement efforts. The goal is to better understand what drives changes under the waves and improve projections of future changes.

But the projects started collecting data in 2004 at the earliest, which is not enough to draw definitive long-term conclusions. “It’s extremely difficult to look at a brief history of ocean overturning and say what it’s going to do over 30, 40, or 50 years,” Dr Lozier said.

Levke Caesar, a postdoctoral researcher studying reversal at the University of Bremen in Germany, expressed concern about the older temperature records that Dr. Ditlevsen and Dr. Ditlevsen used to calculate their proxy. These records, dating from the late 19th and early 20th centuries, might not be reliable enough to be used for fine 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, Dr Caesar said. “There’s something going on, and it’s probably out of the ordinary,” she said. “Something that wouldn’t have happened if it wasn’t for us humans.”

Scientists’ uncertainty about the timing of the AMOC collapse should not be taken as an excuse for not reducing greenhouse gas emissions to try to avoid it, said Hali Kilbourne, associate research professor at the University of Maryland Environmental Science Center.

“It’s very plausible that we’ve already fallen off a cliff and don’t know it,” Dr Kilbourne said. “Honestly, I’m afraid that by the time this all becomes settled science, it will be far too late to act. »

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