In 2014, the disaster film "The Day After" depicted a world that had become difficult to live in because of the transformations linked to climate change. Its director Roland Emmerich took as the starting point of his catastrophic scenario the collapse of an oceanic and climatic phenomenon with an abstruse name: the "Atlantic meridional overturning circulation" (Amoc). A crucial phenomenon – including the famous Gulf stream – that brings warm water up from the equator to the north of the Atlantic and acts as a thermostat for temperatures everywhere on Earth.

It could be that this cinematic "Day After" is not so far away in reality. At least according to a study published Tuesday, July 26 in the very prestigious magazine Nature and which has since been the subject of intense scientific debate. This article may, indeed, seem very alarmist: it ensures that due to global warming the end of the Amoc could be imminent, and occur as early as 2024 in the worst scenario envisaged by the two Danish climatologists who wrote it.

"95% certainty" of a collapse before 2100

Less catastrophic assumptions are not much more reassuring. These climatologists are "95% convinced that this collapse will take place at some point between 2025 and 2095. The most likely date is 2057, which is 34 years from now," said a statement from the Niels Bohr Institute at the University of Copenhagen, where the authors of the study work.

This is much earlier and radical than the forecasts of the experts of the IPCC, which does not envisage a collapse of this phenomenon in the next 100 years, says the American channel CNN.

"It's really scary, and we wouldn't make that kind of prediction lightly," Peter Ditlevsen, one of the study's authors, told CNN.

To understand what is at stake in this scientific article, it must be borne in mind that "the collapse of the Amoc is considered one of the main tipping points of global warming with profound effects for humanity," says Andrew Watson, director of the Atmospheric and Maritime Research Group at the University of Exeter.

Rollover circulation is a complex system of currents, often compared to a huge conveyor belt. It carries warm water from the equator to the North Atlantic where currents cool. The water then becomes heavier and denser, and sinks to the depths of the ocean where contrary currents bring it south.

This warm water allows, in particular, to have milder winters in the northern hemisphere and these currents also have a significant impact on the functioning of monsoons and the power of certain extreme climatic events such as hurricanes.

Colder winters in the north, end of rains in the Sahel

If the Amoc stopped working – that is, there would no longer be this movement of warm water going up to the north and cold water going down to the south – the consequences would be brutal. Winters would become much colder in the north, and entire regions, such as West Africa and India, would be at risk of being almost entirely deprived of precipitation.

"Some densely populated areas – particularly in India – would be profoundly impacted by this drop in rainfall," said Jon Robson, a climatologist and oceanologist at the University of Reading. Without rainfall or monsoons, the food security of millions of people would be jeopardized.

The changes would mostly be very sudden. Such a collapse occurred about 20,000 years ago during the last ice age, and temperatures had changed by more than 10°C on average in just a decade. Humanity would have the greatest difficulty adapting to such a profound climate transformation in such a short period of time.

To reach their conclusion, the authors of the Nature paper compared ocean surface temperatures in the North Atlantic – where the reversal phenomenon occurs – over the past 150 years. They observed that temperatures varied more and more from year to year. An instability that would be, according to them, the prelude to this tipping point. "It's a bit like with the financial markets. When they become very unstable, that is to say very volatile, the collapse of prices is usually close, "says the American geologist Jeffrey Kargel, in a contribution to the popular science site Science Media Center.

"The great novelty of this article is that it offers a temporal assessment of this tipping point. An estimate that, it must be recognized, is daring," summarizes Didier Swingedouw, Amoc specialist at the Laboratory of Oceanic and Continental Environments and Paleoenvironments (EPOC) of the University of Bordeaux.

Unnecessarily alarmist conclusion?

More than daring, this would be "an unnecessarily alarmist conclusion," says Penny Holliday, oceanologist and Amoc specialist at the British National Oceanography Centre. She recalls that there have only been scientific observations of the functioning of AMOC for 20 years. If we stick to these data alone, "there is no evidence of a rapid decline in the health of Amoc," she said. Since 2014, after ten years of slowdown that could have made us fear the worst, it has even recovered.

All the experts interviewed believe that this study should be taken with a grain of salt. Another problem identified comes from the choice of ocean surface temperature as a standard meter to assess the good health of the Amoc. "The authors do not take into account at all other factors that could also explain these temperature variations," Robson said.

Not to mention that the consequence of the instability of Amoc is not necessarily a collapse of the system. "There may be something else less dramatic on the other side of this instability," Watson said. "There is no evidence that there are only two choices: either Amoc works or it doesn't," adds Jonathan Bamber, director of the Bristol Glaciology Centre. Perhaps the worst that can happen is a sharp slowdown of the Amoc.

This would not be reassuring. If the power of Amoc decreases, "there will still be significant effects on the global climate," says Andrew Watson. The only difference is "that it will happen slower, which could give us time to react," concludes Jon Robson. That is, trusting governments to reduce our greenhouse gas emissions as quickly as possible, because in the opinion of all the experts interviewed, the longer global warming lasts or worsens, the greater the risk of an Amoc collapse.

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