A new study suggests that the Gulf Stream system could collapse as soon as 2025.
The shutting down of the vital ocean currents, called the Atlantic Meridional Overturning Circulation (Amoc) by scientists, could bring catastrophic climate impacts.
In 2021, Amoc was already known to be at its weakest in 1,600 years and researchers spotted warning signs of a tipping point, due to global heating.
The new analysis estimates a timescale for the collapse between 2025 and 2095, with a central estimate of 2050, if global carbon emissions are not reduced. Evidence from past collapses indicates changes of temperature of 10C in a few decades, although these occurred during ice ages.
Other scientists said the assumptions about how a tipping point would play out and uncertainties in the underlying data are too large for a reliable estimate of the timing of the tipping point. All said the prospect of an Amoc collapse was extremely concerning and should encourage rapid cuts in carbon emissions.
Amoc carries warm ocean water northwards towards the pole where it cools and sinks, driving the Atlantic's currents. But an influx of fresh water from the accelerating melting of Greenland's ice cap and other sources is increasingly smothering the currents.
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 storms and drop temperatures in Europe, and lead to a rising sea level on the eastern coast of North America. It would also further endanger the Amazon rainforest and Antarctic ice sheets.
Professor Peter Ditlevsen, who led the study at the University of Copenhagen, expressed concerns, saying: "This would be a very, very large change. The Amoc has not been shut off for 12,000 years".
The Amoc collapsed and restarted repeatedly in the cycle of ice ages that occurred from 115,000 to 12,000 years ago. It is one of the climate tipping points scientists are most concerned about as global temperatures continue to rise.
Research in 2022 showed that five dangerous tipping points may already have been passed due to the 1.1C of global heating to date, including the:
The new study, published in Nature Communications, uses sea surface temperate data stretching back to 1870 as a proxy for the change in the strength of Amoc currents over time.
The researchers then mapped this data onto the path seen in systems that are approaching a particular type of tipping point called a "saddle-node bifurcation". Ditlevsen said the data fitted "surprisingly well" and that the researchers were then able to extrapolate the data to estimate when the tipping point was likely to occur. Further statistical analysis provided a measure of the uncertainty in the estimate.
The analysis is based on greenhouse gas emissions rising as they have done to date. If emissions do start to fall, as intended by current climate policies, then the world would have more time to try to keep global temperature below the Amoc tipping point.
The most recent assessment by the Intergovernmental Panel on Climate Change concluded that Amoc would not collapse this century, but Ditlevsen said the models used have coarse resolution and are not adept at analysing the non-linear processes involved, which may make them overly conservative. The potential collapse of Amoc is intensely debated by scientists who have previously said it must be avoided at all costs.
Professor Niklas Boers, from the Potsdam Institute for Climate Impact Research, said: "The results of the new study sound alarming but if the uncertainties in the heavily oversimplified model [of the tipping point] and in the underlying [sea temperature] data are included, then it becomes clear that these uncertainties are too large to make any reliable estimate of the time of tipping".
Professor David Thornalley, at University College London, agreed the study had large caveats and unknowns, and said further research was essential, but if the statistics are robust and a relevant way to describe how the Amoc behaves, then this is "a very concerning result".
Dr Levke Caesar, at the University of Bremen, said using sea surface temperatures as proxy data for the strength of the Amoc currents was a key source of uncertainty, and that we only have direct observational data of the Amoc since 2004. However, the extrapolation in the new analysis was deemed reasonable according to Professor Tim Lenton at the University of Exeter. He said the tipping point could lead to a partial Amoc collapse, for example only in the Labrador Sea, but that this would still cause major impacts.
Professor Stefan Rahmstorf, at the University of Potsdam, said: "There is still large uncertainty where the Amoc tipping point is, but the new study adds to the evidence that it is much closer than we thought. A single study provides limited evidence, but when multiple approaches have led to similar conclusions this must be taken very seriously, especially when we’re talking about a risk that we really want to rule out with 99.9% certainty. Now we can’t even rule out crossing the tipping point in the next decade or two".
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