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.”

Collapse of critical Atlantic current is no longer low-likelihood, study finds. Scientists say ‘shocking’ discovery shows rapid cuts in carbon emissions are needed to avoid catastrophic fallout

New modelling suggests the tipping point that makes an Amoc shutdown inevitable is likely to be passed within a few decades. Photograph: Henrik Egede-Lassen/Zoomedia/PA

 
by Damian Carrington Environment editor

 

The collapse of a critical Atlantic current can no longer be considered a low-likelihood event, a study has concluded, making deep cuts to fossil fuel emissions even more urgent to avoid the catastrophic impact.

The Atlantic meridional overturning circulation (Amoc) is a major part of the global climate system. It brings sun-warmed tropical water to Europe and the Arctic, where it cools and sinks to form a deep return current. The Amoc was already known to be at its weakest in 1,600 years as a result of the climate crisis.

Climate models recently indicated that a collapse before 2100 was unlikely but the new analysis examined models that were run for longer, to 2300 and 2500. These show the tipping point that makes an Amoc shutdown inevitable is likely to be passed within a few decades, but that the collapse itself may not happen until 50 to 100 years later.

 

The research found that if carbon emissions continued to rise, 70% of the model runs led to collapse, while an intermediate level of emissions resulted in collapse in 37% of the models. Even in the case of low future emissions, an Amoc shutdown happened in 25% of the models.

Scientists have warned previously that Amoc collapse must be avoided “at all costs”. It 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 50cm to already rising sea levels.

 

The new results are “quite shocking, because I used to say that the chance of Amoc collapsing as a result of global warming was less than 10%”, said Prof Stefan Rahmstorf, at the Potsdam Institute for Climate Impact Research in Germany, who was part of the study team. “Now even in a low-emission scenario, sticking to the Paris agreement, it looks like it may be more like 25%.

“These numbers are not very certain, but we are talking about a matter of risk assessment where even a 10% chance of an Amoc collapse would be far too high. We found that the tipping point where the shutdown becomes inevitable is probably in the next 10 to 20 years or so. That is quite a shocking finding as well and why we have to act really fast in cutting down emissions.”

Scientists spotted warning signs of a tipping point in 2021 and know that the Amoc has collapsed in the Earth’s past. “Observations in the deep [far North Atlantic] already show a downward trend over the past five to 10 years, consistent with the models’ projections,” said Prof Sybren Drijfhout, at the Royal Netherlands Meteorological Institute, who was also part of the team.

“Even in some intermediate and low-emission scenarios, the Amoc slows drastically by 2100 and completely shuts off thereafter. That shows the shutdown risk is more serious than many people realise.”

The study, published in the journal Environmental Research Letters, analysed the standard models used by the Intergovernmental Panel on Climate Change (IPCC). The scientists were particularly concerned to find that in many models the tipping point is reached in the next decade or two, after which the shutdown of the Amoc becomes inevitable owing to a self-amplifying feedback.

Air temperatures are rising rapidly in the Arctic because of the climate crisis, meaning 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, forming the feedback loop. Another new study, using a different approach, also found the tipping point is probably going to be reached around the middle of this century.

 

Only some of the IPCC models have been run beyond 2100, so the researchers also looked to see which of those running to the end of this century showed Amoc was already in terminal decline. This produced the 70%, 37% and 25% figures. The scientists concluded: “Such numbers no longer comply with the low-likelihood-high-impact event that is used to discuss an abrupt Amoc collapse in [the IPCC’s last report].”

Rahmstorf said the true figures could be even worse, because the models did not include the torrent of meltwater from the Greenland ice cap that is also freshening the ocean waters.

Dr Aixue Hu at the Global Climate Dynamics Laboratory in Colorado, US, who was not part of the study team, said the results were important. “But it is still very uncertain when Amoc collapse will happen or when the Amoc tipping point is going to crossed because of the lack of direct observations [of the ocean] and the varying results from the models.”

The study that found that a total collapse of the Amoc was unlikely this century was led by Dr Jonathan Baker at the Met Office Hadley Centre in the UK. “This new study highlights that the risk rises after 2100,” he said. “[But] these percentages should be treated with caution – the sample size is small, so more simulations [beyond 2100] are needed to better quantify the risk.”

Nonetheless, Baker said, “the ocean is already changing, and projected shifts in North Atlantic convection are a real concern. Even if a collapse is unlikely, a major weakening is expected, and that alone could have serious impacts on Europe’s climate in the decades to come. But the future of the Atlantic circulation is still in our hands.”

Total collapse of vital Atlantic currents unlikely this century, study finds. Climate scientists caution, however, that even weakened currents would cause profound harm to humanity

Global heating is weakening the Atlantic meridional overturning circulation (Amoc), which plays a crucial role in global weather conditions. Photograph: Henrik Egede-Lassen/Zoomedia/PA
 

Damian Carrington Environment editor

 

 

Vital Atlantic Ocean currents are unlikely to completely collapse this century, according to a study, but scientists say a severe weakening remains probable and would still have disastrous impacts on billions of people.

The Atlantic meridional overturning circulation (Amoc) is a system of currents that plays a crucial role in the global climate. The climate crisis is weakening the complex system, but determining if and when it will collapse is difficult.

Studies based on ocean measurements indicate that the Amoc is becoming unstable and approaching a tipping point, beyond which a collapse will be unstoppable. They have suggested this would happen this century, but there are only 20 years of direct measurements and data inferred from earlier times bring large uncertainties.

 

Climate models have indicated that a collapse is not likely before 2100, but they might have been unrealistically stable compared with the actual ocean system.

The latest study is important because it uses climate models to reveal the reason that the Amoc is more stable: winds in the Southern Ocean continuing to draw water up to the surface and drive the whole system. The study does not rule out an Amoc collapse after 2100, and other modelling research suggests collapses will occur after that time.

“We found that the Amoc is very likely to weaken under global warming, but it’s unlikely to collapse this century,” said Dr Jonathan Baker at the UK’s Met Office, who led the latest study. He said it was reassuring that an abrupt Amoc crash was improbable, and that the knowledge could help governments plan better for future climate impacts. Amoc weakening would still bring major climate challenges across the globe however, with more floods and droughts and faster sea level rise, he added.

“Of course, unlikely doesn’t mean impossible,” he said. “There’s still a chance that Amoc could collapse [this century], so we still need to cut greenhouse gas emissions urgently. And even a collapse in the next century would cause devastating impacts for climate and society.”

Prof Niklas Boers at the Potsdam Institute for Climate Impact Research (PIK) in Germany said the study delivered a substantial improvement in the understanding of Amoc. “But even a weakening that is not due to a tipping point could have similarly severe impacts on, for example, tropical rains,” he said. “One could even go as far as saying that, in the short term, it doesn’t really matter if we have a strong weakening, say 80%, or a collapse.”

The Amoc system brings warm, salty water northwards towards the Arctic where it cools, sinks, and flows back southwards. Global heating, however, is pushing water temperatures up and increasing the melting of the huge Greenland ice cap, which is flooding the area with fresh water. Both factors mean the water is less dense, reducing sinkage and slowing the currents.

The Amoc was already known to be at its weakest in 1,600 years as a result of global heating, and researchers spotted warning signs of a tipping point in 2021. The Amoc has collapsed in the Earth’s past, Baker said. “So it’s a real risk.”

A collapse of Amoc would have disastrous consequences around the world, severely disrupting the rains that billions of people depend on for food in India, South America and West Africa. It would increase the ferocity of storms and send temperatures plunging in Europe, while pushing up sea levels on the eastern coast of North America and further endangering the Amazon rainforest and Antarctic ice sheets. Scientists have previously said a collapse must be avoided at all costs.

The latest study, published in the journal Nature, used 34 state-of-the-art climate models to assess the Amoc. The researchers used extreme conditions – a quadrupling of carbon dioxide levels or a huge influx of meltwater into the North Atlantic – so that the changes in the modelled ocean currents were clear.

They found that while the Amoc slowed by between 20% and 80% this century, it did not collapse completely in any of the models. This was because winds in the Southern Ocean continued to draw water up to the surface. Balancing this, to the scientists’ surprise, were new downwelling areas in the Pacific and Indian oceans, but they were not strong enough to wholly compensate for the slowing of the Amoc, leaving it significantly weakened.

“Even just a 50% reduction in strength would result in a large drop in heat transport that would alter regional and global climates,” said Dr Aixue Hu at the Global Climate Dynamics Laboratory in Colorado, US. “There is therefore no reason to be complacent about Amoc weakening, and every effort must still be made to combat the global warming that drives it.”

Prof Stefan Rahmstorf, an Amoc expert at PIK, said the latest study considered a collapse to be the total cessation of the currents in the North Atlantic, while previous studies have termed a greatly weakened Amoc a collapse.

Amoc is partly driven by the sinking of dense water and partly by winds, and the latest study provides particular insights on the latter. “It does not, however, change the assessment of the risk and impact of future Amoc changes in response to human-caused global warming, as that is linked to the [density-driven] part of Amoc,” Rahmstorf said. His own research on post-2100 Amoc collapse, currently under review, concludes “a collapse cannot be considered a low-probability event any more”.

Despite the revelations in the latest study, the extent of future Amoc weakening and the timing of any collapse remain uncertain. “There’s a huge amount of work left to do, because there’s still a huge range across models in how much Amoc will weaken,” Baker said, with increasing the resolution of models one important requirement.

“We also show that the Southern Ocean and the Pacific Ocean are more important than we thought for Amoc, so we need better observations and modelling in those regions. That’s crucial to improving the projections so we can better inform policymakers,” he said.