In the 2004 movie "The Day After Tomorrow," a climatologist played by Dennis Quaid holds global warming to world leaders. He warns that man-made climate change could trigger a catastrophic ice age.
Although the concept of global warming anticipates a cooling trend, the Hollywood disaster strip does not necessarily seem to be wrong.
The real star of the show (sorry Dennis Quaid) is a sea current called Atlantic Meridional Overturning Circulation (AMOC), which carries warm water from the equatorial tropics to Europe and the North Atlantic. This inflow of warmer water contributes to a mild, temperate climate in Western Europe.
Read more: Scientists could seriously underestimate the risk of a major freeze in Europe
In the film, this flow ceases and causes an almost ice-age in Europe and North America. Temperatures drop to minus 1
These effects (and the speed with which they occur) were hyperbolized in the movie for moviegoers, but the idea that water circulation in the Atlantic could be closed is not out of the question. In fact, such a shift has already begun. According to a study from 2018, the circulation is the weakest in the last 1,600 years.
"We are definitely entering a world where AMOC is weakening," said Francesco Muschitiello, author of a new study on AMOC, to Business Insider.
Muschitiello's research, published earlier this month in the journal Nature Communications, points to a potential cause-and-effect timeline of this weakening flow. According to the model of the new study, AMOC changes reveal significant climatic fluctuations, which will take around 400 years in the future.
The results show that changes in the strength of the water cycle in the Atlantic are actually preceded by abrupt climate changes – a kind of proverbial canary in the climate coal mine.
The conveyor belt of the Atlantic
The AMOC moves ocean waters in the Atlantic to the north and south and also circulates them from the surface to the water depth. Scientists have compared the system to a conveyor belt.
Once the warmer water reaches the area around Britain, it cools and sinks to the bottom of the Labrador and the Nordic Sea. Then this cold water makes a U-turn and meanders along the seabed all the way down to the Antarctic South Pole.
The strength of this conveyor belt is partly responsible for the climate in the northern hemisphere. When the AMOC is flowing fast, there is a humid and warm climate in Western Europe. However, when it is sluggish and weak, warm tropical waters are not moved up and the North Atlantic cools off.
The speed of AMOC depends on a delicate balance of salt and fresh water. Salt water is dense, so it drops slightly. But as the Greenland ice sheet and glaciers continue to melt, more fresh water enters the AMOC. The meltdown is happening fast: Greenland lost more than 400 billion tons of ice in 2012, nearly quadrupling in 2003.
Read More: Greenland is approaching the threshold of an irreversible melt and its consequences for coastal cities could be bleak
The addition of this fresh water makes the salty surface water lighter and sinks less, reducing the flow the circulation is clogged.
A change in the AMOC can trigger a climate change 400 years later
The new study gives us a sense of how fast AMOC-controlled cooling has taken place during climate change in the past – information that serves as a model for that Can serve up to date.
For their analysis, Muschitiello and his team investigated core samples from the Norwegian Sea, a lake in the south of Scandinavia and ice in Greenland. Their results showed that about 400 years ago, AMOC eased off a major cold spell some 13,000 years ago. The AMOC also became stronger around 400 years before a sudden warming 11,000 years ago (with temperatures rising 14 degrees Celsius).
The authors found that after these 400-year delays, a sudden warming or cooling occurred within a few decades.
"It's crazy," said Muschitiello. "If you look at the ice cores, some climate indicators point to a shift of two to five years."
Muschitiello's research is the first to calculate how rapidly an AMOC change leads to a change in the global climate.
But he does not believe that today's melting ice is enough to trigger the dramatic slowdown in AMOC that the world experienced 13,000 years ago.
"Still, it's really scary," he said. "You can see that the AMOC has slowed down in the last few centuries."
Could the stream ever stop altogether?
According to Muschitiello, "there are reconstructions that suggest that AMOC has completely ceased in the past and that these major AMOC distortions have led to the coldest events ever recorded."
However, such an event would require a significant amount of meltwater. The dislocations of the past referred to by Muschitiello have occurred after large iceberg swarms have broken off glaciers and floated into the North Atlantic. As the iceberg armadas melted, the seawater added too much water and destroyed the AMOC.
However, this type of inflow of fresh water was several orders of magnitude higher than today's melt rates.
"If the Greenland ice sheet melted in the course of a few days, that would probably be bad," Muschitiello said.
Other studies in recent years have also investigated possible effects of attenuation of AMOC.
A government report from 2008 suggested that although the AMOC is significantly weaker, there is no indication that it would collapse abruptly in the 21st century. However, this report noted that AMOC would cause an additional sea level rise of almost 3 feet if sea level were broken.
A 2017 study found that the AMOC could collapse in 300 years if the atmospheric carbon dioxide concentration doubled from 1990 immediately. However, greenhouse gas emissions are unlikely to increase dramatically.
Higher seas and extreme weather
Even if the AMOC came to a complete standstill, an ice age would not immediately arise as in "The Day After Tomorrow".
But the slowdown that is already underway is likely to continue to effect changes in our global climate.
"We will certainly see even more extreme weather patterns," said Muschitiello. "Europe will become colder and drier in the long term, and there will be a surplus of heat in subtropics, which is important for hurricane production."
When subtropical waters are warmer, it contributes to more frequent and intense hurricanes in the Atlantic, as warm air contains more water vapor – and this extra moisture supplies the hurricanes with fuel.
A weakening of the AMOC would also increase the sea level along the east coast of the US, as Grist has reported. And in parts of Central and West Africa drought conditions prevail, as these areas also benefit from the spread of AMOC.
Muschitiello noted that while "The Day After Tomorrow" shows an exaggerated scenario, there were historical turning points after which the climate system responded fairly quickly ,
"It used to be very fast in the old days," he said. "For example, everything has changed within a lifetime."