- In the movie "The Day After Tomorrow," the Earth is thrown into an ice age after ocean currents in the Atlantic Ocean grind to a halt.
- That ocean current system, called the Atlantic Meridional Overturning Circulation (AMOC), is responsible for western Europe's warm temperatures.
- But recent research has found that the AMOC could slow significantly if enough fresh water from melting ice enters the ocean current.
- That weakened circulation could bring more extreme weather to the Northern Hemisphere and serve as a warning sign for an abrupt climate shift in the future.
In the 2004 film "The Day After Tomorrow," a climatologist played by Dennis Quaid lectures world leaders about global warming. Human-driven climate change, he warns, could trigger a cataclysmic ice age.
Although the concept of global warming driving a cooling trend seems counterintuitive, the Hollywood disaster flick didn't necessarily get that wrong.
The real star of the show — sorry, Mr. Quaid — is an ocean current called the Atlantic Meridional Overturning Circulation (AMOC) that moves warm water from the equatorial tropics up to Europe and the north Atlantic. This influx of warmer water contributes to western Europe's mild and temperate climate.
Read more: Scientists might be seriously underestimating the risk of a major freeze in Europe
In the movie, that current stops, causing an almost overnight ice age in Europe and North America. Temperatures plummet to negative 150 degrees Fahrenheit, people freeze to death in the streets, helicopters fall out of the sky, and a massive tidal wave engulfs New York City.
Those effects, and the speed at which they occur, were hyperbolized in the film for the sake of moviegoers, but the idea that Atlantic water circulation could shut down isn't outside the realm of possibility. Such a shift has already started. According to a 2018 study, the circulation is the weakest it's been in at least the past 1,600 years.
"We are definitely going into a world where AMOC is getting weaker," Francesco Muschitiello, the author of a new study about the AMOC, told Business Insider.
Muschitiello's research, published this month in the journal Nature Communications, suggests there could be a cause-and-effect timeline of this slowing current. According to the new study's model, changes in AMOC foretell major climate fluctuations that will happen some 400 years in the future.
The findings show that changes in the strength of water circulation in the Atlantic do really precede abrupt climate changes — sort of the proverbial canary in the climate coal mine.
The Atlantic's 'conveyor belt'
The AMOC moves ocean water north and south in the Atlantic while circulating it from the surface to the watery depths. Scientists have likened the system to a conveyor belt.
Once the warmer water reaches the area around the UK, it cools and sinks to the bottom of the Labrador and Nordic Seas. Then that cold water makes a U-turn and snakes its way along the ocean floor all the way down to Antarctica's Southern Ocean.
The strength of this conveyor belt is partially responsible for the climate in the Northern Hemisphere. When the AMOC is flowing quickly, western Europe enjoys a wet and warm climate. But if it is sluggish and weak, warm tropical waters don't get moved up, and the north Atlantic cools.
The AMOC's speed depends on a delicate balance of salt and fresh water. Salty water is dense, so it sinks easily. But as Greenland's ice sheet and glaciers continue to melt, more fresh water is joining the AMOC. That melting is happening quickly: In 2012, Greenland lost more than 400 billion tons of ice, almost quadruple the loss in 2003.
The addition of that fresh water makes the salty surface water lighter and less likely to sink, clogging up the circulation's flow.
A change in the AMOC could trigger a climate shift 400 years later
The new study gives us a sense of how quickly AMOC-driven cooling occurred during climate shifts in the past — information that could serve as a model for what's ahead.
To do their analysis, Muschitiello and his team looked at core samples drilled from the bottom of the Norwegian Sea, a lake in southern Scandinavia, and ice in Greenland. Their results showed that the AMOC started weakening 400 years before a major cold snap 13,000 years ago. The AMOC also began getting stronger about 400 years before an abrupt warming 11,000 years ago (during which temperatures climbed by 14 degrees Fahrenheit).
The authors said that following those 400-year lags, abrupt warming or cooling took place over just a few decades or less.
"It's insane," Muschitiello said. "If you look at the ice cores, there are some climate indicators that suggest a two- to five-year shift."
Muschitiello's research is the first to calculate how quickly a change in the AMOC begets a shift in global climate.
But he doesn't think today's melting ice is enough to trigger the dramatic AMOC slowdown that the world saw 13,000 years ago.
"Nevertheless, it's really scary," he said. "You can see that the AMOC has slowed down over last couple centuries."
Could the current ever completely stop?
According to Muschitiello, there are "reconstructions that suggest that the AMOC stopped entirely in the past, and that these major distortions of the AMOC led to the coldest events ever recorded."
But such an occurrence would require a significant amount of melt water. The past distortions Muschitiello is referring to happened after large swarms of icebergs broke off glaciers and floated into the north Atlantic ocean. When the iceberg armadas melted, that added an excess of fresh water into the ocean, wreaking havoc on the AMOC.
But this kind of influx of fresh water was several orders of magnitude higher than today's melting rates.
"If the Greenland ice sheet was to melt over the course of a few days, though, that would probably be bad," Muschitiello said.
Other studies over the past few years have also looked into effects of a weakening AMOC.
A government report suggested in 2008 that while the AMOC is clearly weakening, there's no evidence that it would abruptly collapse in the 21st century. That report noted, however, that if the AMOC did stop, it would cause almost 3 feet of additional sea-level rise.
A 2017 study found that if the concentration of carbon dioxide in the atmosphere instantaneously doubled from the 1990 level, the AMOC could collapse in 300 years. But it's unlikely that greenhouse-gas emissions would increase that dramatically.
Higher seas and more extreme weather
Even if the AMOC were to stop completely, an ice age would not spring up immediately the way it does in "The Day After Tomorrow."
But the weakening that's already underway is still likely to cause changes in our global climate.
"We'll see more extreme weather patterns for sure," Muschitiello said. "Europe will get colder and drier in the long run. There will be surplus of heat in subtropics, which is important for hurricane formation."
When subtropical waters are warmer, that contributes to more frequent and intense hurricanes in the Atlantic, since warm air holds more water vapor — and that additional moisture provides fuel for hurricanes.
A weakening in the AMOC would also cause sea levels to rise along the US's eastern seaboard, as Grist has reported. And parts of central and west Africa would experience drought conditions, since those areas also benefit from the AMOC's circulation.
Muschitiello said that even though the "The Day After Tomorrow" shows an exaggerated scenario, there have been historical tipping points after which the climate system reacted quite swiftly.
"In the past it happened really, really quick," he said. "Like, within the span of a lifetime, everything changed."
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