The weakening of water circulation in the Atlantic Ocean could cause climate change

In the 2004 film “The Day After Tomorrow”, a climatologist played by Dennis Quaid warns world leaders against rapid climate change.

The key factor is an ocean current system called the Atlantic Meridional Reversing Circulation (AMOC), which moves warm water from the equatorial tropics to Europe and the North Atlantic. This influx of warmer water contributes to the mild and temperate climate of Western Europe.

In the movie, the AMOC comes to a complete halt, causing an ice age to begin almost overnight. While the speed and intensity of this cold snap is hyperbolized in the film, AMOC is very real, and research suggests that a slowing of its circulation is a likely consequence of climate change.

In an article published last week, climatologist Niklas Boers concluded that AMOC was approaching a tipping point. If enough fresh water from melting polar ice enters the ocean, the current system will experience “sharp weakening” and destabilize, he told Insider.

A new climate report from the UN Intergovernmental Panel on Climate Change (IPCC) also suggests that AMOC will most likely weaken by the end of the century.

According to Boers, a researcher at the Potsdam Climate Impact Research Institute in Germany, this weakening would cause temperatures in Europe to drop and “the cooling effect would be stronger the further you head north.” .

On the east coast of the United States, meanwhile, sea levels would rise. Parts of Central and West Africa are said to experience persistent drought conditions as these areas also benefit from AMOC circulation.

An on / off switch for Atlantic currents

When AMOC sinks rapidly, Western Europe enjoys a humid and hot climate. Scientists compared the system to a conveyor belt: Once the warmer water reaches the UK area, it cools and sinks to the bottom of the Labrador and North Seas. Then this cold water turns around and winds along the ocean floor, to the Southern Ocean of Antarctica.

But if this circulation is slow and weak, the warm tropical waters do not rise and the North Atlantic cools.

The speed of the AMOC is determined by a delicate balance of salt water and fresh water. Salt water is dense, so it flows easily. But as Greenland’s ice cap and glaciers continue to melt, more and more fresh water is joining the salty surface waters of AMOC, making it lighter and less likely to sink. This obstructs the flow of traffic.

Previous research suggests that a change in the strength of water circulation in the Atlantic really precedes abrupt climate changes. By examining ice cores dated to Earth’s last ice age, scientists found that AMOC alternates between two states – a strong “on” state, where the current system is working fast, and a weak “off” state. where this circulation slows down.

“An AMOC shutdown is the easiest and most effective way to disrupt the climate system,” Francesco Muschitiello, a paleoclimatology geographer at Cambridge University, told Insider, adding: “95% of the time when we’re talking about rapid climate change, it’s associated with AMOC. “

Still, Boers said, any cooling related to AMOC wouldn’t look like “The Day After” – it “would take a few decades” and North America “won’t get as cold as the movie suggests.”

It would take at least a few hundred years for AMOC to grow stronger

In the past, when AMOC reached a tipping point, the transition from strong to weak took a few decades, Boers said. But the system takes much longer to go back.

“It has generally taken a few hundred to a few thousand years for AMOC to return to strong mode,” he said, adding, “if AMOC were to collapse into weak mode at some point in the year. the future, it would indeed be very difficult to bring it back to strong mode. “

According to the IPCC report, it is possible that AMOC will experience a sudden collapse by 2100. This collapse, according to the report’s authors, could be triggered by unexpected additional melted ice from the Greenland ice cap, which has thawed. at an unprecedented rate. . A 2019 study found that the ice cap was melting six times faster than 40 years ago.

It is unlikely, however, that the Greenland ice sheet will ever melt quickly enough to completely stop AMOC.

A full shutdown like the one described in “The Day After” would only happen “if the Greenland ice sheet melted within days,” Muschitiello told Insider.

That said, some studies suggest that AMOC stopped altogether in the past.

“These major distortions of AMOC led to the coldest events on record,” said Muschitiello.

In these cases, the cold events may have lasted for up to 1,000 years. If AMOC were to shut down completely again, Boers said, the only way to reverse it would be “to reverse the global temperature trend and return to pre-industrial conditions.”