While southern Europe has almost no longer known winters in 2100, undergoing the effects of climate change hard, imagine London to shive under temperatures worthy of Canada – or Scandinavian countries, as desired. The frozen rang, transport to slow motion, including the famous Tube of the capital. Sacro-Sainte football matches Premier league are postponed due to weather conditions. Like the first meetings of the Rugby VI Nations Tournament. Supporters are assigned at home – even beer ice outside.
Delirium of English climatosceptics? Science fiction scenario? Not for Tim Lenton, director of the Global Systems Institute of the University of Exeter, holder of a chair on climate change and the science of the terrestrial system. The scientist participated in the development of an alarming report published last October across the Channel. The authors criticize “the blind spot” of the authorities concerning “the climatic tilting points”, and in particular that of the AMOC (English acronym for “meridian circulation of the Atlantic”), a set of ocean ocean currents which contributes, among other things, to maintain a mild climate in Europe.
It would suffice that it is disturbing so that the whole United Kingdom and Ireland are strongly impacted. And the consequences would not be limited to disaggregates as trivial as the postponement of sports meetings. The cooling produced by the modification of ocean currents “would fundamentally eliminate arable crops in the United Kingdom, would cause a water crisis in the Southeast and require major changes in terms of infrastructure, in particular transport,” warns Tim Lenton .
This scenario remains, for the moment, only one of many. But the threat grows, warn several scientists, now anxious to alert the general public. “The United Kingdom could be even less well prepared to deal with the main threats related to climate security than it was to face a pandemic. The government does not even assess these risks in an adequate manner, and Even less prepares to face their consequences.
Roland Emmerich used hers. The German-American director has made catastrophe films his trademark: Independence Day, 2012and of course The day afterrebroadcast on the small screen at regular intervals. Released in 2004, the film describes the journey of an American paleoclimatologist – not really the usual hero of big budget films – embodied by Dennis Quaid, on a planet returned to the ice age. Los Angeles is devastated there by tornadoes, Tokyo lacerated by hailstones, New York drowned and then transformed into a ice floe. Hollywood obliges, the film favors the spectacle to coherence, temporal and physical, not hesitating to stage a cyclone eye of several tens of kilometers wide or people joining instantly in the streets. Nevertheless, the blockbuster is based on a real scientific hypothesis: the collapse of the Amoc.
“A double penalty”
Its influence on the climate is indeed planetary. It is, in a simplified way, a set of marine currents and vortices which transport heat, oxygen and carbon across the Atlantic. The famous Gulf Stream is only a component of this complex circulation, often described as a treadmill: warm and tropical salty waters flow to the surface to the north, then cool at the level of Greenland, where they densify And dive on several thousand meters before leaving deep south.
These movements, which carry the energy equivalent of a million nuclear power plants, allow Western Europe to benefit from a temperate climate. They also play a crucial role in the regulation of monsoons or for the capture of CO2. Climate change would hit this mechanics. Surface waters, warming, would become less dense. The melting of ice in Greenland as well as more precipitation would increase the amount of fresh water, also less dense, in the North Atlantic. Which would slow the treadmill … until stopping it.
“If the Amoc stops, even if Elon Musk finds us an incredible technique for removing all the greenhouse gases from the planet, there is a good chance for this circulation does not leave. And that we do not have So never again the same climate. In northern and western Europe, France included, temperatures could seriously fall. “Perhaps eight to ten degrees on an annual average. But we must take into account the level of global warming, which could alleviate this cooling, and the speed of collapse of the Amoc, which we do not know” , specifies Henk Dijkstra, professor of physical oceanography at the University of Utrecht (Netherlands), and co -author of a recent study on the subject.
Since the Amoc plays mainly on winter at our latitudes, the seasonal differences would be amplified. “A double penalty”, analyzes Didier Swingedouw: massive heat waves on one side, even more rigorous cold waves on the other. A cocktail that would multiply extreme climatic events. The repercussions on agriculture or fishing would be considerable, while elevation of sea level could reach one meter.
So much for Europe. Regarding the rest of the world, the effects would be just as devastating. “The displacement towards the south of the tropical precipitation zones would result in major consequences in terms of droughts and floods according to the region,” adds climatologist Stephan Rahmstorf, of the Potsdam Institute (Germany) for research on impact climate. West Africa, above all, “would lose up to 30 % of precipitation, continues the French researcher. This would have a catastrophic effect on food crops, and would plunge tens of millions of food insecurity. would then be absolutely huge. ” Asian monsoon, vital for densely populated areas, would also experience important disturbances.
Models and their biases
Scientists, aware of the dark painting, are increasingly raising the voice to warn against a risk that they consider largely underestimated. At the end of October, 44 of them addressed An open letter to the leaders of the Nordic Council of Ministerswarning that “such a change in ocean circulation would have devastating and irreversible effects”. This evokes the “inexpensive” conclusion of the intergovernmental group of experts on climate evolution (IPCC). In their latest report, published in 2021, experts give “moderate confidence” in the fact that there will be no collapse of the Amoc by 2100. Which “is clearly open the possibility” that ‘He arrives during this century.
The position of the IPCC has however slightly informed on the issue in recent years. She excluded this scenario more firmly in the 2000s and 2010s. The fault, interpreted Didier Swingedouw… in the film The day after. “He provoked discomfort in the scientific community: it was impression that we were talking about it too much. The reports of the time, not necessarily written by AMOC experts, then made statements too prudent.
Climatologists sail in the world of uncertain, especially on such a complex subject. But all go on one point: the Amoc will slow down. How fast? From when? This is the subject of the controversy of the moment, with numerous studies which are confirmed or infirm. Reason: diverging methodologies.
Because data on these sea currents are still too recent to make it possible to identify a clear dynamic. “We have continuous observations of the AMOC only since 2004 thanks to the deployment of dedicated moorings [NDLR : instruments mesurant les propriétés des courants]exhibits Damien Desbruyères, researcher at the physical and spatial oceanography laboratory at Ifremer. Before, our means were quite limited to monitor, describe and explain its variability “. In order to expand the study scale, scientists therefore rely on” proxys “, indirect indicators, which they integrate into Their climatic models. Are they robust? Are they plausible? Do we really know the underlying physics? “
“Nothing should be overlooked”
In July 2023, a study published in Nature Communications has reacted a lot. Two Danes, Peter and Susanne Ditlevsen, a brother and a sister, estimated that without drastic greenhouse gases, the Amoc could collapse between 2025 and 2095, with a central estimate of 2057. A surprisingly precise, and No doubt too alarmist, estimate many experts.
This reproach is also regularly addressed to Stephan Rahmstorf. “It is the duty of scientists to warn against risks. This is to say that a doctor is alarmist because he warns his patient that cigarettes can cause lung cancer,” sweeps the German researcher. According to him, the AMOC has already weakened about 15 % since the 1950s. The proof? “THE ‘Cold Blob ‘a vast area west of Great Britain and in the south of Iceland and Greenland, is the only part of the planet that has cooled up since the 19th century, while everything else was Warmled “. Didier Swingedouw strongly disapprove of:” It is controversial because indirect. The usual allocation detection procedure has not been applied. “In the state of knowledge, and for lack of solid arguments, he explains this”Cold Blob“By the natural variability of the climate – these fluctuations caused by processes other than human influence.
Despite their dissensions, the two climatologists have initialed the open letter warning governments against drastic changes which could now happen in the time of a human life. “Some are starting to pay attention,” appreciates Tim Lenton, another signatory. Essentially the first concerned by this cooling. The United Kingdom, for example, launched, at the end of last year, a program of 81 million pounds in order to create an early alert system to closely monitor the evolution of a component of the AMOC which Particularly worried: the “subpolar gyre”.
Several months earlier, the Irish senator Malcolm Byrne had, before the parliament, arrested his colleagues : “The collapse of the Amoc, even if it may not happen in our lifetime, would have serious consequences for future Irish generations. Our climate could change and become similar to that of Iceland, this Who would have deep implications … As a state, nothing should be overlooked in terms of preparation. “
In France, the hypothesis has not yet infused among the political class. “The scenario of a collapse was however taken for the first time seriously by the Ministry of the Armed Forces in the early 2000s”, rewinding Didier Swingedouw, who himself had some exchanges with the Grande Muette on the subject. But he seems to remain, since then, circumscribed military projections and anticipations. This “dead angle”, to use the title of the British report, questions the country’s adaptation trajectory, necessarily incomplete without considering this possibility. “It is a bet that is made,” continues the CNRS researcher. But when you make a bet, it is better to bet on several horses to have more chance of winning. “
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