A new study affirms that a critical system of Atlantic Ocean currents that shunt warm and cold water between the poles is “on course” to a tipping point. If the Atlantic Meridional Overturning Circulation fails because of increasing freshwater inflows from melting ice sheets and rivers swelled by global warming, the authors said it would disrupt the climate globally, shifting Asian monsoon rainfall patterns and even reversing the rainy and dry seasons in the Amazon.
“It’s a global shift,” said Utrecht University climate and physics researcher René van Westen, co-author of the research published today in Science Advances. Along with changes in rain distribution, an AMOC collapse could also make some other related ocean currents in the Atlantic, like the Gulf Stream, “partly vanish,” he said.
“This leads to a lot of dynamic sea level rise, up to a meter in the North Atlantic under an AMOC collapse,” he said. “And you need to add that on top of the sea level rise already caused by global warming. So the problems are really severe.”
The East Coast of the United States would be one of the regions most affected by rising sea levels if the AMOC shuts down, he explained, because warming waters, which expand and increase sea level, would pile up there instead of flowing northward. Warming coastal oceans can also contribute to extreme heat waves over land and fuel more intense storms and rainfall.
Without warm water flowing toward the Arctic, he added, winter sea ice could expand as far south as England, and some regions of Europe would quickly dry out and cool by as much as 1.5 degrees Celsius per decade.
Some of the projected impacts would be nearly impossible to adapt to, said Peter Ditlevsen, an ice and climate researcher with the University of Copenhagen Niels Bohr Institute and the author of a 2023 paper in Nature Communications that warned of a mid-century AMOC tipping point.
“A lot of discussion is, how should agriculture prepare for this,” he said. But a collapse of the heat-transporting circulation is a going-out-of-business scenario for European agriculture, he added. “You cannot adapt to this. There’s some studies of what happens to agriculture in Great Britain, and it becomes like trying to grow potatoes in Northern Norway.”
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Under the current global warming trend, “It will be about 1 to 2 degrees Celsius warmer by 2050, and then maybe the AMOC tips and results in a slight cooling,” he said. The impact on the average global temperature wouldn’t be extreme, but Western Europe could cool to pre-industrial levels, and would get substantially less precipitation, he added.
Other parts of the planet will warm faster, especially the southern hemisphere and tropics, since the heat transport system won’t be able to convey the increasing ocean warmth northward, he added.
“It’s not science fiction,” van Westen said. Alarmist or not, “We need to show this is not only a Hollywood blockbuster, ‘The Day After Tomorrow.’ This is real, this can happen. And I think it’s important and urgent to keep saying to people, okay, we need to really tackle our emissions.”
The AMOC distributes both warmer and colder water between both poles via a network of deep and near-surface ocean currents. The twin engines for the network are at high latitudes, where dense, cold and salty water sinks deep and pushes water horizontally across the seafloor. Those dynamics maintain the Circulation’s strength and the relative warmth of the Northern Hemisphere.
The new study takes a detailed look at what happens when the balance is disrupted by greater quantities of freshwater flowing into the ocean, and the findings are a “major advance in AMOC stability science,” said Stefan Rahmstorf, head of earth system analysis with the Potsdam Institute for Climate Impact Research, and a professor of physics of the oceans at Potsdam University.
The findings confirm that the addition of freshwater from increasing rainfall, river runoff and melting ice can push the AMOC past its tipping point, which has been suggested by basic climate models since the early 1960s, he said.
The new research “crushes the hope that some feedback might prevent an AMOC collapse,” he said. The hope that more refined models would identify something that could prevent the disruptions to the system of currents was not convincing to start with, he said, because paleoclimate records clearly show “abrupt AMOC shifts, including full AMOC breakdowns triggered by meltwater input.”
The last AMOC breakdown occurred about 12,000 years ago and most climate scientists think it triggered the Younger Dryas cold event around the northern Atlantic, during which temperatures over Greenland dropped by 4 to 10 degrees Celsius in a matter of decades and glaciers temporarily advanced, while drier conditions spread across parts of the Northern Hemisphere.
As of 2021, the melting Greenland Ice Sheet was adding about 400 billion tons of water to the North Atlantic each year. Rivers flowing to the Arctic are discharging increasing amounts of fresh water, according to a 2021 study.
Sharp reductions of atmosphere-cooling industrial aerosols, both over the Atlantic, as well from sources in Asia and Europe, may have also slowed the AMOC toward a shutdown in recent decades, potentially even affecting the stability of Antarctic Ice.
In a 2023 paper on the recent acceleration of global warming, climate scientists James Hansen and co-authors wrote that an “AMOC shutdown is not unusual and occurred in the Eemian (when global temperature was similar to today), and also that sea level in the Eemian rose a few meters within a century with the likely source being collapse of the West Antarctic ice sheet.”
Another 2023 study by led by Australian oceanographer Matt England showed the Southern Hemisphere AMOC engine may also be failing, for the same reason as in the Arctic—an increase of Antarctic meltwater that disrupts the vertical part of the circulation by hindering the formation of heavier “abyssal” water that then drives currents horizontally across the seafloor.
The Risk of Collapse
“The billion-dollar question is, how far away is this tipping point?” said Rahmstorf. “Three recent studies, using different data and methods, have argued that we are approaching the tipping point and that it might be rather close, even posing a risk of crossing it in the next decades.” The reliability of the methods, however, have been questioned, he added.
The new research used freshwater transport by the AMOC at the entrance of the South Atlantic, across the latitude of the southern tip of Africa, as a “physics-based and observable type of early warning signal,” but it does not try to pin down the timing of a shutdown. Rahmstorf said he explored a similar approach in a 1996 study, but determining the tipping point requires more observations of the ocean circulation at this latitude.
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In the study, van Westen’s team modeled a theoretical span of about 2,200 years, starting with pre-industrial climate conditions and simulating a gradual increase in surface freshwater input to the North Atlantic, to trigger an abrupt AMOC tipping event at model year 1,758.
Even though his own paper doesn’t identify a real-life timeline for an AMOC breakdown, he said the 2050 tipping point identified in last year’s paper by Ditlevsen could, “to some degree, be an accurate projection.”
The new study also confirms past concerns that climate models systematically overestimate the stability of the AMOC because they don’t accurately account for freshwater input, said Rahmstorf.
He said such model weaknesses are “Why, in my view, the IPCC has so far underestimated the risk of an AMOC collapse. The most recent climate assessment from the Intergovernmental Panel on Climate Change, estimates the chances of an AMOC breakdown this century at less than 10 percent.
“The new study adds significantly to the rising concern about an AMOC collapse in the not too distant future,” Rahmstorf said. “We will ignore this risk at our peril.”