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In the 2004 film "The Day After Tomorrow", a climate scientist played by Dennis Quaid gives lectures to world leaders on global warming. A man-made climate change, he warned, could trigger a cataclysmic glaciation.
Although the concept of global warming leading to a cooling trend seems counterintuitive, the Hollywood disaster film has not necessarily been deceived.
The real star of the show (sorry, Dennis Quaid) is an ocean current called AMOC (Atlantic Meridional Inverting Circulation) that moves the warm waters of the equatorial tropics towards Europe and the North Atlantic. This rush of warmer water contributes to the mild temperate climate of Western Europe.
Read more: Scientists could seriously underestimate the risk of a major freeze in Europe
In the film, this current stops, causing a glacial period almost overnight in Europe and North America. Temperatures drop negatively to 150 degrees Fahrenheit, people are dying of cold in the streets, helicopters are falling from the sky and a massive tidal wave is invading New York.
These effects (and the speed with which they occur) have been hyperbolised in the film for the enjoyment of the audience, but the idea that the circulation of Atlantic water can be closed is not likely to be excluded. In fact, such a change has already begun. According to a study conducted in 2018, the draw is the lowest in at least 1,600 years.
"We are definitely going into a world where AMOC is weakening," said Business Insider Francesco Muschitiello, the author of a new study on AMOC.
Muschitiello's research, published earlier this month in the journal Nature Communications, suggests a possible chronology of the causes and effects of this slowing of the current. According to the model of the new study, changes in AMOC predict important climatic fluctuations that will occur in about 400 years.
The results show that changes in the strength of water circulation in the Atlantic actually precede sudden changes in climate – a kind of proverbial canary in the climate coal mine.
The & # 39; treadmill & # 39; from the Atlantic
The AMOC moves ocean water north and south of the Atlantic while flowing it from the surface to the watery depths. Scientists compared 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 seas and northern seas. Then this cold water turns around and makes its way from the bottom of the ocean to the Southern Ocean.
The strength of this conveyor belt is partly responsible for the climate of the northern hemisphere. When AMOC flows rapidly, Western Europe enjoys a warm and humid climate. But though it is slow and weak, the warm tropical waters have not risen and the North Atlantic is cooling down.
The speed of AMOC depends on a delicate balance between seawater and freshwater. The salt water is dense, so it flows easily. But as the ice cap and glaciers of Greenland continue to melt, more and more fresh water joins AMOC. This melting is fast: in 2012, Greenland lost more than 400 billion tonnes of ice, almost four times the loss of 2003.
Read more: Greenland is approaching the threshold of an irreversible meltdown and the consequences for coastal cities could be disastrous
The addition of this fresh water makes the salty surface water clearer and less likely to flow, which obstructs the flow of traffic.
A change in AMOC can trigger a climate change 400 years later
The new study gives us an idea of how quickly climate-induced cooling has occurred during past climate change – information that could serve as a model for the future.
To perform their analysis, Muschitiello and his team examined core samples taken from the bottom of the Norwegian Sea, a lake in southern Scandinavia and ice in Greenland. Their results showed that AMOC began to weaken about 400 years before a major cold spell 13,000 years ago. AMOC also began to strengthen about 400 years before a brutal warming 11,000 years ago (during which the temperature soared 14 degrees Fahrenheit).
The authors noted that after 400 years, warming or rapid cooling occurred within a few decades or less.
"It's crazy," Muschitiello said. "If you look at ice cores, some climate indicators suggest a lag of 2 to 5 years."
Muschitiello's research is the first to calculate how rapidly AMOC changes lead to global climate change.
But he does not think that the current melting ice is enough to trigger the dramatic slowdown of AMOC observed in the world 13,000 years ago.
"Nevertheless, it's really scary," he says. "You can see that AMOC has slowed down in the last two centuries."
Could the current ever stop completely?
According to Muschitiello, "some reconstructions suggest that AMOC has completely stopped in the past and that these major distortions of AMOC have led to the coldest events ever recorded".
But such an event would require a significant amount of melting water. Earlier distortions referred to by Muschitiello occurred after large swarms of icebergs broke away from glaciers and floated in the North Atlantic Ocean. When the armadas of the iceberg melted, an excess of fresh water in the ocean was created, wreaking havoc on the AMOC.
But this type of freshwater influx was several orders of magnitude higher than current melting rates.
"If Greenland's icecap were to melt in a few days, it would probably be bad," said Muschitiello.
Other studies in recent years have also focused on the potential effects of a weakening of AMOC.
A 2008 government report suggested that, while AMOC was clearly weakening, there was no evidence that it would collapse sharply in the 21st century. This report indicated, however, that if the UHC stops, this would cause an additional rise in sea level of nearly 3 feet.
A study conducted in 2017 found that if the concentration of carbon dioxide in the atmosphere doubled instantly from the 1990 level, AMOC could collapse in 300 years. But it is unlikely that greenhouse gas emissions will increase significantly.
Higher seas and more extreme weather conditions
Even if the AMOC stopped completely, an ice age would not be triggered quite like in "The day after tomorrow".
But the weakening already underway is still likely to change our global climate.
"We will certainly see more extreme weather conditions," said Muschitiello. "Europe will become colder and dryer in the long run.There will be a surplus of heat in the subtropics, which is important for the formation of a hurricane."
When subtropical waters are warmer, this creates more frequent and intense hurricanes in the Atlantic, because hot air retains more water vapor – and this extra moisture feeds hurricanes.
A weakening of AMOC would also cause sea level rise along the US east coast, as reported by Grist. And some parts of West and Central Africa will experience periods of drought as they also benefit from AMOC circulation.
Muschitiello noted that although the "Day after tomorrow" scenario shows an exaggerated scenario, there were historic tipping points after which the climate system reacted rather quickly.
"In the past, it really happened very quickly," he said. "As, in the space of a lifetime, everything has changed."
