In the crushing, cold depths of the oceans, something unimaginably huge flows inexorably, barely a few centimeters per second, along a path it has traveled for millennia. Dense, dark rivers of water toil ceaselessly around the world, making up around 40 percent of the total volume of the deep oceans. They are gigantic conveyor belts transporting heat, oxygen, carbon, and nutrients around the planet, and shaping climate and weather at a global, regional, and local scale.
But something has changed, and these rivers appear to be slowing down. Unsurprisingly, climate change is likely to blame.
The sting in the tail is that the slowing of this abyssal machinery could actually speed up climate change, while also reducing the productivity of fisheries upon which so many organisms—including humans—depend for food.
In 1990, when the Intergovernmental Panel on Climate Change (IPCC) released its first ground-breaking report, the complex interaction between climate and ocean barely figured, says oceanographer and climate scientist Matthew England from the University of New South Wales in Sydney, Australia. “The projections back then were really simple,” he says. “They just had an atmosphere coupled to a very simplified ocean that had no dynamics.” A bit like a bathtub, he says. Oceans were known to absorb carbon dioxide and heat, but otherwise the interactions between ocean and climate were described in simplistic terms.
Ocean science has come a long way since then, and brought with it a detailed understanding of the key role that these global ocean conveyor belts play in shaping climate.
“Water moves, just like wind, in a three-dimensional space; we have currents that are going say, from left to right, and we have currents that are going up and down,” says coastal oceanographer Ruth Reef, from Monash University in Melbourne, Australia.
The horizontal movement of water is because of drag from wind. “When you have wind blowing across the ocean, it drags the ocean along with it,” Reef says. The vertical movement is the result of changes in the water density. At the poles, when salty seawater freezes into freshwater ice, the concentration of salt in the remaining water increases, which makes it denser, and so it sinks.
This is the start of the conveyor belt engine. Those trillions of tons of denser, colder water descend to the deepest reaches of the polar regions, then move through the depths towards the tropics. There that water rises and warms, and those warmer currents—such as the Gulf Stream, which moves west to east across the North Atlantic and maintains the relatively temperate winters in the United Kingdom—circulate around the Pacific, Indian, and Atlantic Oceans, releasing heat, oxygen, and nutrients and absorbing carbon dioxide, before they arrive back at the poles and the cycle starts again.
The Antarctic is the most powerful engine of this overturning circulation, through the formation of what’s called Antarctic bottom water. But this engine is in trouble.
“We show that a deep portion of the overturning circulation is slowing down, and the amount of oxygen reaching the deep ocean is declining,” says Kathryn Gunn, a physical oceanographer and climate scientist at the University of Southampton in the UK. She and her colleagues have been assessing how the formation of Antarctic bottom water has been changing. In a recently published study, which measured oxygen levels as a proxy for cold-water movement (because cold water carries more dissolved oxygen than warm), they looked at a particular section of the Antarctic shelf that borders the Ross Sea and the Australian Antarctic Basin. Their results suggest that the volume of this cold, salty, oxygen-rich water descending to the ocean floor declined by 28 percent between 1994 and 2017.
The likely cause of that slowing is global heating, which is causing Antarctic ice to melt at a faster rate. “Meltwater from around Antarctica makes the waters fresher, less dense, and therefore less likely to sink,” Gunn says. “This puts the brakes on the overturning circulation.”
If this sounds a little familiar, it’s because Hollywood got onto this story back in 2004 with the blockbuster The Day After Tomorrow, based on a 1999 novel, The Coming Global Superstorm. The premise is that ocean circulation in the North Atlantic—which is known as the Atlantic Meridional Overturning Circulation, or AMOC—shuts down abruptly, plunging the northern hemisphere into a new ice age almost overnight.
While the film is fictional, and highly dramatized, England isn’t too critical of it. But the scenario of a global freeze isn’t what keeps him up at night. It’s sea level rise. The region of Antarctica that he and Gunn examined with their study appears to be warming faster than other regions. And that’s a problem, because there’s a huge quantity of ice there that has only a tenuous grip on the land.
“This machinery of forming cold, salty water around Antarctica is a bit of a protective shield for the ice behind it,” England says. The ice shelf around West Antarctica is already more exposed to currents that circle Antarctica because it sticks out from the main mass of the continent. “The concern I have with the overturning slowing down is that you cease this machinery of keeping the Antarctic margin cold and icy,” England says. If warmer water intrudes into the edges of Antarctica, ice attached to the continent may lose its grip on land and collapse into the sea. The consequences for sea levels are catastrophic, with potential rises of more than 3 meters.
There are also potential knock-on effects for the global and regional climate. One concern is that the slowdown of the Antarctic overturning circulation could contribute to—or be mirrored by—a slowdown in the AMOC in the northern hemisphere. England says there’s evidence to suggest that AMOC circulation has slowed by around 10 to 15 percent, although there’s some debate about whether this slowing is due to natural variability of the current.
That could have a significant impact on the United Kingdom and Western Europe, says Shenjie Zhou, a physical oceanographer at the British Antarctic Survey in Cambridge, UK. “If the AMOC slows down, there’s less heat delivered to the UK, which is going to cause even more miserable weather here in the wintertime,” Zhou says.
That’s just one example, but the effect of the slowdown on regional climate isn’t always as clear-cut. In Australia, for example, regional weather is likely to be more acutely affected by systems such as El Niño and La Niña, which are forecast to intensify with climate change, bringing more severe droughts, heat waves, and flooding.
There may also be some cooling effects, as was dramatized in The Day After Tomorrow. “For the North Atlantic, we’ve got a local cold blob there at the moment that people are saying, ‘Hey, look, there’s the North Atlantic slowing down,’” England says. But these regional cold effects will likely be overwhelmed by global temperature increases, he says.
It also depends on the interaction between the Antarctic overturning and the Atlantic overturning. England says modeling suggests that if the AMOC slows more than the Antarctic current, then it could shift a massive rain band that normally sits just north of the equator by about 5 degrees of latitude southwards. But if the AMOC current changes are balanced out by the Antarctic changes, then that band of rainfall stays where it is.
Another concern is that the decline of this global conveyor belt could accelerate global warming. “The Antarctic overturning circulation, it plays a significant role in terms of heat uptake and the carbon uptake from the atmosphere,” Zhou says. The conveyor belt essentially captures heat and carbon and stows it away in the depths of the ocean for the hundreds of years it takes for that water to complete the cycle and return to the surface. As that current slows, it’s slowing the absorption of heat and carbon dioxide at the surface and dissipation of both in the depths.
There’s also a potential impact on food security, because these deep currents move the nutrients that have drifted to the bottom of the oceans and shift them up to the surface. Where these upwellings occur, there’s a wealth of marine life feeding on those nutrients, which in turn sustains significant commercial fisheries. “Changes to ocean circulation and to that density separation of the ocean will lead to different parts of the world having that upwelling on their doorstep,” Reef says.
The problem is complex, but the solution is simple: stop global heating. “We can’t stop this without reducing our emissions,” England says. “There’s no way to geoengineer yourself out of that problem.” But the window of opportunity to reverse these changes is narrowing fast, and England’s not sure we’ll make it. “With drastic action, we could probably stop that slowdown from being a full collapse,” he says. “But it’s pretty tight.”