Why This is One of the Planetary Shifts Scientists are Most Worried about

By Sarah Kaplan / The Washington Post

16:52 JST, February 10, 2024

René van Westen watched in alarm as freshwater from a melting Arctic poured into the North Atlantic, diluting the normally dense and salty sea. The influx gradually slowed the aquatic conveyor belt that circulated water throughout the ocean basin, until, in the blink of an eye, the entire system shut down. Temperatures in North America and Europe dropped several degrees in a matter of decades. Global weather patterns shifted, depriving Africa and Asia of vital monsoon rains.

This cataclysm was happening only in a computer simulation – the product of a complex model that scientists use to understand the effects of climate change. But it was the first time anyone had used this particular model to show how the Atlantic Ocean’s sensitive circulatory system can collapse, said Van Westen, a climate scientist at Utrecht University in the Netherlands. And it hinted that the ocean might be on course for a “tipping point” ending in abrupt and irreversible change.

Van Westen’s study, published Friday in the journal Science Advances, is the latest attempt to understand what scientists call the Atlantic Meridional Overturning Circulation, or AMOC. Evidence from Earth’s past shows that this crucial and complex ocean system has shut down before, and modeling studies such as Van Westen’s suggest that it could happen again as human greenhouse gas emissions cause the planet to warm.

But the likelihood of a collapse – and the timeline on which it might occur – are still open questions. Here’s what you need to know.

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The AMOC is a complex system

Scientists often compare the AMOC to a conveyor belt, driven by differences in water density, which transports water, heat and nutrients throughout the Atlantic Ocean.

It starts near the equator, where the surface of the ocean is warmed by the tropical sun. As that water moves northward, some of it evaporates, which increases the salt concentration – and the density – of the water that is left behind. By the time the water nears Greenland, it has also cooled down, which makes it even more dense.

This cold, salty water sinks to the seafloor, pushing the water that was already down there out of its path. That displaced water starts to flow south along the ocean bottom. Once it returns to the tropics, the water is drawn back to the surface through a process called upwelling, and the cycle begins again.

The Gulf Stream plays a role in the AMOC, but the two are not synonymous. Whereas the AMOC is a complex system of many currents, the Gulf Stream is a single fast-moving current that carries warm water from the Gulf of Mexico, along the east coast of the United States and across to northern Europe. It is driven primarily by winds and the rotation of the Earth, which means that even if climate change disrupts the deep ocean portion of the AMOC, the Gulf Stream won’t necessarily shut down.

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The AMOC has shut down before

The AMOC is considered a “tipping element” in the Earth system, which means it has the potential to flip abruptly between two fundamentally different states – and once it flips, it’s hard to switch back again.

Think of it like someone leaning back in a chair. As long as they don’t lean too far, they are able to return to their original stable state, with all four legs of the chair on the floor. But if they tilt past a certain threshold – the tipping point – the chair will topple over, and they will be in a new stable state: lying on the floor.

Right now, the AMOC is self-reinforcing. A difference in water density drives circulation, which creates differences in density, which keeps the cycle going.

But evidence from Earth’s past shows how an influx of freshwater could throw a wrench into the system. By studying sediments at the bottom of the ocean, scientists know that huge pulses of meltwater entered the North Atlantic at the end of the last ice age, as the massive ice sheets that covered Europe and North America started to disintegrate. The fossilized shells of tiny ocean organisms show how that pulse diluted the salty water that accumulated just below Greenland. That dilution made the water less dense, which prevented it from sinking, which meant there was nothing to push the deep ocean portion of the AMOC.

This in turn meant there was less warm, salty water flowing north along the surface, which further weakened the density gradient that drives the circulation. Eventually, the feedback became so profound that the AMOC couldn’t sustain itself.

“There’s evidence from a number of different types of ocean circulation records that you have a strong decline in AMOC – something that approximates a collapse state,” said Sophie Hines, a paleoceanographer at the Woods Hole Oceanographic Institution who was not involved in Van Westen’s study.

Other geologic records – ice cores from Greenland, stalagmites from deep within caves – indicate that the shutdown of the AMOC led to major changes in weather around the globe. Europe got much colder, and rain stopped falling in parts of the tropics.

“Information about what actually did happen in the past is really important for understanding … exactly what conditions are necessary to drive the AMOC to switch from on to off,” Hines said. “That is really going to be crucial for then applying to what might be happening today.”

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The growing signs that the AMOC is in trouble

For the past few decades, researchers have used networks of floating robotic sensors to observe the ocean in real time. As these sensors drift with the currents and swim through the water column, they collect information about the ocean’s temperature and salinity and beam it up to satellites, which then transmit the information to scientists around the globe.

These observations have revealed a strange cooling spot at the southern tip of Greenland – one of the only places on the planet where the ocean isn’t getting hotter. This suggests that the AMOC isn’t delivering as much warm water to the North Atlantic and hints that the system is slowing down. Other studies have combined direct observations with computer simulations to conclude that the AMOC has already weakened by about 15 percent since the 1950s.

In their Science Advances study, Van Westen and his colleagues found that the amount of freshwater being moved around the southernmost portion of the Atlantic Ocean was a good indicator of the strength of the feedback system powering the AMOC. When that metric was a positive number, it meant that the system was self-reinforcing. But when Van Westen’s team looked at real-world data, it found that its measure of freshwater transport was negative.

“The AMOC is destabilizing,” he said.

But there is a significant difference between the linear process of becoming less stable and the abrupt transition of crossing a tipping point.

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An AMOC collapse would have devastating consequences

Van Westen’s study modeled how quickly the impacts of an AMOC collapse could unfold. Once the system passed its tipping point, he said, Western Europe would start to cool down by as much as 3 degrees Celsius (5.4 degrees Fahrenheit) every decade. Sea levels in the North Atlantic would surge by up to a meter as water piled up in the region, and the bottom of the ocean would run short on oxygen – killing the creatures that live in its depths. Meanwhile, the rest of the world would continue to get warmer, thanks to humanity’s heat-trapping greenhouse gas pollution.

“When the AMOC tips, it goes very fast,” Van Westen said. “And the responses are very severe.”

Right now, he added, the world is warming by about 0.2 degrees Celsius (0.36 degrees Fahrenheit) per decade – and society is already struggling to cope. If the temperature swings induced by an AMOC shutdown are an order of magnitude greater, “it will be very hard to adapt,” Van Westen said.

Climate scientist Tim Lenton, director of the Global Systems Institute at the University of Exeter, has also used computer models to assess how an AMOC shutdown could affect the world’s food supply. The dramatic cooling in the Northern Hemisphere would cause a shift in the band of clouds and rainfall that encircle the globe at the tropics. The monsoons that typically deliver rain to West Africa and South Asia would become unreliable, and huge swaths of Europe and Russia would plunge into drought. As much as half of the world’s viable area for growing corn and wheat could dry out.

“In simple terms [it] would be a combined food and water security crisis on a global scale,” Lenton said.

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Scientists aren’t certain if a shutdown is close

Despite the signs that the AMOC is getting weaker, scientists still can’t say how soon a shutdown might occur. The AMOC is huge and slow-moving – it would take a drop of water hundreds of years to cycle through the whole ocean – which means that a mere 30 years of observations is not enough to assess how much the system is changing.

So researchers turn to computer models to simulate how the planet might respond to a given perturbation – for example, dumping Arctic meltwater into the North Atlantic – and see what it would take to push systems over the edge.

Many of these simulations don’t predict that the AMOC will cross its tipping point in the near future. In its most recent assessment report on the state of Earth’s climate, the U.N. Intergovernmental Panel on Climate Change drew on multiple climate models and concluded with “medium confidence” that the AMOC would not fully collapse this century.

But Lenton, who was not involved in the new Science Advances study, believes the models are too conservative and don’t fully capture the complex interactions that shape our planet. He said it was “significant” that Van Westen and his colleagues were able to re-create an AMOC collapse using one of the world’s most sophisticated climate models.

Getting a definitive answer about the AMOC’s vulnerability will require more direct observations of the ocean and improvements in the scientists’ computer simulations. Further research on past collapses can also help identify triggers and early warning signs that scientists should be looking for today, Hines said.

And even if the probability of crossing the AMOC tipping point this century is low, researchers agreed, the potentially devastating consequences are reason enough to invest in better research and start thinking about how the world would adapt.

“We’re not taking this seriously enough,” Lenton said.