Critical Atlantic current significantly more likely to collapse than thought. Scientists say finding is ‘very concerning’ as collapse would be catastrophic for Europe, Africa and the Americas

The Atlantic meridional overturning circulation is a major part of global climate system and is known to be at its weakest for 1,600 years as a result of climate crisis. Photograph: Henrik Egede-Lassen/Zoomedia/PA

by  Damian Carrington Environment editor

 

The critical Atlantic current system appears significantly more likely to collapse than previously thought after new research found that climate models predicting the biggest slowdown are the most realistic. Scientists called the new finding “very concerning” as a collapse would have catastrophic consequences for Europe, Africa and the Americas.

The Atlantic meridional overturning circulation (Amoc) is a major part of the global climate system and was already known to be at its weakest for 1,600 years as a result of the climate crisis. Scientists spotted warning signs of a tipping point in 2021 and know that the Amoc has collapsed in the Earth’s past.

Climate scientists use dozens of different computer models to assess the future climate. However, for the complex Amoc system, these produce widely varying results, ranging from some that indicate no further slowdown by 2100 to those suggesting a huge deceleration of about 65%, even when carbon emissions from fossil fuel burning are gradually cut to net zero.

 

The research combined real-world ocean observations with the models to determine the most reliable, and this hugely reduced the spread of uncertainty. They found an estimated slowdown of 42% to 58% in 2100, a level almost certain to end in collapse.

The Amoc is a major part of the global climate system and brings sun-warmed tropical water to Europe and the Arctic, where it cools and sinks to form a deep return current. A collapse would shift the tropical rainfall belt on which many millions of people rely to grow their food, plunge western Europe into extreme cold winters and summer droughts, and add 50-100cm to already rising sea levels around the Atlantic.

Dr Valentin Portmann, at the Inria Centre de recherche Bordeaux Sud-Ouest in France and who led the new research, said: “We found that the Amoc is going to decline more than expected compared to the average of all climate models. This means we have an Amoc that is closer to a tipping point.”

Prof Stefan Rahmstorf, at the Potsdam Institute for Climate Impact Research in Germany, said: “This is an important and very concerning result. It shows that the ‘pessimistic’ models, which show a strong weakening of the Amoc by 2100, are, unfortunately, the realistic ones, in that they agree better with observational data.”

He added: “I now am increasingly worried that we may well pass that Amoc shutdown tipping point, where it becomes inevitable, in the middle of this century, which is quite close.”

Rahmstorf, who has studied the Amoc for 35 years, has said a collapse must be avoided “at all costs”. “I argued this when we thought the chance of an Amoc shutdown was maybe 5%, and even then we were saying that risk is too high, given the massive impacts. Now it looks like it’s more than 50%. The most dramatic and drastic climate changes we see in the last 100,000 years of Earth history have been when the Amoc switched to a different state.”

 

The Amoc is slowing because air temperatures are rising rapidly in the Arctic because of global heating. That means the ocean cools more slowly there. Warmer water is less dense and therefore sinks into the depths more slowly. This slowing allows more rainfall to accumulate in the salty surface waters, also making it less dense, and further slowing the sinking and forming an Amoc feedback loop.

The Amoc system is highly complex and subject to random natural variations, making precise predictions impossible. However, a major weakening is now expected by scientists and that alone could have serious impacts in the decades to come.

The new research, published in the journal Science Advances, explored four different ways of using real-world observations to assess the models. They found a method called ridge regression, which had been little used in climate science before now, provided the best results.

The Amoc is difficult to model because it is governed by subtle differences in water density caused by salinity changes over the entire Atlantic. The reduction in uncertainty in the new analysis results from identifying the models that better reflect surface salinity in the south Atlantic, which scientists already knew was important. This makes the work “very credible”, said Rahmstorf.

Rahmstorf said Amoc slowdown in 2100 may be even greater than in the new, pessimistic assessment. This is because the computer models do not include the meltwater from the Greenland ice cap that is also freshening the ocean waters: “That is one additional factor that means the reality is probably still worse.”

Slowing ocean current caused by melting Antarctic ice could have drastic climate impact, study says

 The Southern Ocean overturning circulation has ebbed 30% since the 90s, CSIRO scientist claims, leading to higher sea levels and changing weather

 

 

Melting ice in Antarctica has affected a key global ocean current, research suggests. Photograph: Anadolu Agency/Getty Images

A major global deep ocean current has slowed down by approximately 30% since the 1990s as a result of melting Antarctic ice, which could have critical consequences for Earth’s climate patterns and sea levels, new research suggests.

Known as the Southern Ocean overturning circulation, the global circulation system plays a key role in influencing the Earth’s climate, including rainfall and warming patterns. It also determines how much heat and carbon dioxide the oceans store.

 Donna Lu  @donnadlu Theguardian.com

Scientists warn that its slowdown could have drastic impacts, including increasing sea levels, altering weather patterns and depriving marine ecosystems of vital nutrients.

“Changes in the overturning circulation are a big deal,” said the study’s co-author, Dr Steve Rintoul, an oceanographer and expert on the Southern Ocean at the Australian government’s Commonwealth Scientific and Industrial Research Organisation (CSIRO).

“It’s something that is a concern because it touches on so many aspects of the Earth, including climate, sea level, and marine life.”

The finding comes months after modelling, which Rintoul was involved in, that predicted a 40% slowdown in the circulation by 2050.

“The model projections of rapid change in the deep ocean circulation in response to melting of Antarctic ice might, if anything, have been conservative,” Rintoul said. “We’re seeing changes have already happened in the ocean that were not projected to happen until a few decades from now.”

The overturning circulation originates in the cold and dense waters that plunge down deep off Antarctica’s continental shelf and spread to ocean basins globally. It brings oxygen to the deep ocean and returns nutrients to the surface ocean.

 

“What’s driven the slowing is the fact that that dense shelf water is not as dense as it used to be because it’s not as salty as it used to be,” Rintoul said.

The melting of Antarctic glacial ice, the researchers found, has resulted in additional freshwater, increasing buoyancy.

The study looked specifically at changes in overturning circulation in the Australian Antarctic basin, but the researchers believe a “circumpolar slowdown” is occurring.

“The Australian Antarctic basin is the best ventilated of all the deep basins in the sense that it gets more … oxygen-rich water getting to the the bottom,” Rintoul said. “The signal in that basin might provide a kind of early warning of changes that might happen around Antarctica.”

Dr Ariaan Purich of Monash University, who was not involved in the research, said the Australian Antarctic basin was downstream of the region in Antarctica experiencing the greatest melt of ice shelves and loss of land ice. “In that sense, it makes it an important region to study to see these meltwater impacts on the ocean circulation.”

Purich described the paper as significant for providing observational evidence for the slowing of large-scale ocean overturning as a result of the melting Antarctic ice sheet.“We’re now seeing lots of lines of evidence that this melt water isn’t only increasing sea levels – it’s affecting the climate system in lots of different ways,” Purich said.,

“This is pretty confronting. These are big changes that are happening in Antarctica that can affect our global climate.”

 

Between 1994 and 2017, there was a net slowdown in the circulation by 0.8 sverdrups per decade, the study found. One sverdrup is a flow rate equivalent to 1 million cubic metres per second.

The researchers found a temporary increase in the overturning circulation between 2009 and 2017, as a result of increased sea ice formation. “That was enough to compensate for the melt from glacial melt for a few years,” Rintoul said.

“We expect in the longer term that while there will be ups and downs related to sea ice formation, the overall trend is that Antarctica is losing more ice, is melting more, and that will gradually slow down this overturning circulation.

“Unless we act soon we will commit ourselves to changes that we’d really rather avoid,” he said. “We need to act to reduce emissions and we need to do everything we can as fast as we can.”

The study, whose first author is Kathryn Gunn of the CSIRO and the University of Southampton, was published in the journal Nature Climate Change.