Vital Ocean Current May Be Approaching Ruinous Tipping Point - Newsweek

Vital Ocean Current May Be Approaching Ruinous Tipping Point

A major current that runs the length of the Atlantic Ocean and plays an important role in the planet's climate may be approaching a tipping point that would lead to its collapse, researchers have suggested.

The Atlantic Meridional Overturning Circulation (AMOC) is a current that carries warm water north and cool water south, and is part of a network of currents across the world's oceans that influences global weather patterns and temperatures.

But due to increases in freshwater entering the Atlantic and warming temperatures, the volume of water the AMOC transports is dropping and is "currently in its weakest state in over a millennium," according to a paper published in Science Advances at the start of February.

Researchers at Utrecht University's Institute for Marine and Atmospheric Research in the Netherlands modeled this drop in volume from pre-industrial levels and found that the introduction of freshwater over time would lead to a slow decrease before it reached a tipping point that would lead to a sudden collapse of the current's flow.

René Van Westen, the paper's lead author, told Newsweek that a subsequent study had found that it would be "very hard to recover the AMOC back to its present-day state" if it collapsed, and would be "irreversible on human time scales."

The Big Freeze

If it were to stop, the effects on the climate would be sudden and profound. David Thornalley, a professor of ocean and climate science at University College London, told Newsweek that there would be "cooling of several degrees across large parts of the northern hemisphere" while tropical rainfall belts would shift and rainfall patterns in general would change.

Modelling suggests that temperatures for major European cities would not reach above freezing year-round, plunging the continent effectively into a localized Ice Age. Meanwhile, rainfall patterns in the Amazon would shift, with little precipitation in the first three months of the year.

North America would see slightly lower temperatures, with marginally more precipitation in winter months and less in the summer. Coastal regions of the Atlantic Ocean would see up to 70 centimeters (27.6 inches) of sea-level rise.

"A collapse in AMOC would cause a southward shift in atmospheric circulation belts and a strengthening of some features," Thornalley said. "For example, the westerly winds coming off the Atlantic to the U.K. would be much stronger and so the U.K. would get more winter storms.

"The inter-tropical convergence zone (tropical rainfall belt) would shift south and so southern subtropical areas that are desert would now get tropical rainfall, and parts of the tropics would dry out. This would have really bad effects on agriculture and water supplies."

While these sorts of shifts have occurred in the past and the natural ecosystem has adapted, he added, "the problem is for humans, with our fixed political boundaries and cities fixed in place."

The researchers in the Netherlands did not mimic the real-world flow of the Atlantic current, but rather modelled how the AMOC would behave when freshwater was progressively added to the system over several centuries from pre-industrial levels to demonstrate if its flow would reach a tipping point or continue on a gradual decline.

Thornalley described it as "a very idealized experiment" that "showed that a relatively complex climate model did show tipping point behaviour," and provided a measure of how to predict if the AMOC was reaching that tipping point.

The modelling demonstrates that the more freshwater that is added to the ocean, the more its power to transport saline water decreases. When the AMOC's strength reaches 10 Sverdrups—a measure of volumetric flow, where one is equivalent to a million cubic meters a second—within a hundred-year period it is expected to drop rapidly to 2 Sverdrups.

The researchers also found that the salinity transport across a certain latitude in the Atlantic was a strong indicator of the strength of the AMOC, reaching a minimum 25 years before the tipping point. "When we then look at real-world observations, this measure...would indicate that we might be approaching a tipping point," Thornalley said.

While a study produced last summer suggested the tipping point could occur this century, Van Westen said: "We cannot say anything about the likelihood of AMOC tipping during this century, we can only say that we are moving towards the tipping point," and that how soon it might occur was "still an ongoing discussion" among scientists.

Tipping Point

Raffaele Ferrari, a professor of oceanography at the Massachusetts Institute of Technology, who specializes in ocean circulation, expressed uncertainty about the imminence of such a tipping point.

He told Newsweek that he felt "the freshwater forcing used in the model appears quite large" and was "not applied in the latitude band where one may expect ice melt from Greenland and increased precipitation due to warming."

He cited a recent report by the Intergovernmental Panel on Climate Change which judged with medium confidence that a collapse before 2100 was "will not occur."

Thornalley said, though, that IPCC models "often struggle to correctly simulate key ocean processes, and they may be biased to be too stable" but that there was also "evidence from the past that the AMOC may be more resilient to changes such as meltwater input to the ocean.

"The science is far from certain, but it would be a very dramatic change in climate if it did collapse, and there are valid reasons for us to be concerned about it."

All three experts and the IPCC say that a significant weakening of the AMOC is more likely due to climate change. As average global temperatures warm, more sea ice melts while increased precipitation near the Arctic puts more freshwater into the ocean, preventing water from sinking to form the colder, deep flow that transports water south.

Even though a warmer atmosphere is expected to see more evaporation from the ocean, this will be outweighed by freshwater entering the oceans, while ocean warming in places like the Arctic would prevent water from sinking.

The scientists all agreed that curbing greenhouse gas emissions would avoid further destabilization of the major current. Van Westen said her team were currently conducting experiments in which carbon dioxide was added to the atmosphere to simulate an AMOC collapse under climate change conditions.

Researchers believe the AMOC may have collapsed previously in the Earth's history, but this "was during the last Ice Age when the whole climate system was very different," Thornalley said. At the time, the global average temperature was 5-8 degrees Celsius lower than now "and there were large ice sheets on the North American and European continent," Van Westen said.

"Once the AMOC transitions to a collapsed state, it is not sufficient to return to the pre-collapse freshwater budget," Ferrari said. "The AMOC will not restart until the freshwater forcing is reduced substantially below its pre-collapsed state."

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About the writer

Aleks Phillips is a Newsweek U.S. News Reporter based in London. His focus is on U.S. politics and the environment. He has covered climate change extensively, as well as healthcare and crime. Aleks joined Newsweek in 2023 from the Daily Express and previously worked for Chemist and Druggist and the Jewish Chronicle. He is a graduate of Cambridge University. Languages: English.

You can get in touch with Aleks by emailing aleks.phillips@newsweek.com.
Aleks Phillips is a Newsweek U.S. News Reporter based in London. His focus is on U.S. politics and the environment. ... Read more

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