In Glasgow, the COP26 ended on Saturday 13 November with a hammer blow – the adoption of a “climate pact” – and a big blow of the blues. Despite the promises of renewed, or even increased, efforts by the 197 countries present, in terms of reducing greenhouse gases, the global temperature is expected to increase, by the end of the century, by 2.7 ° C. compared to the pre-industrial era, according to calculations by the United Nations Program (UNEP). A bitter failure, since under the Paris Agreement, signed in 2015, this increase should rather be maintained “well below 2 ° C”, preferably at +1.5 ° C.
This scenario which awaits the planet has been described as “climate catastrophe “ by UN Secretary General Antonio Guterres. Drought, floods, cyclones, famines, mass extinctions… What threats weigh on our planet if we continue to follow this path?
For climatologist Christophe Cassou, research director at the CNRS and member of the international group of experts on climate change (IPCC), no need to look so far ahead to understand the challenges of this announced rise in temperatures. “Rather than dwelling on this final figure of 2.7 ° C in 2100, a distant date, we can look at what awaits us on the trajectory that leads us to this warming because it implies that the threshold of 2 ° C will be crossed in the middle of the century, maybe even a little before “, he explains.
Two advantages at this time: closer, it is more palpable and leaves less room for uncertainty, necessarily increasing as soon as we look far ahead. In addition, the scenario of an increase of 2 ° C compared to the pre-industrial era has been the subject of numerous studies, synthesized by scientists from the IPCC.
Thus, the first part of their latest report, published in August, established a direct link between the rise in temperatures and the occurrence of extreme weather events. “Including phenomena of unprecedented magnitude”, explains the climatologist, citing the episode of the “heat dome” which hit the southwest of Canada and the northwest of the United States this summer, with record heat spraying 3 to 4 degrees the peak previous. An increase of 1.1 ° C compared to the pre-industrial era was enough to approach the 50 ° C mark.
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With an increase of 2 ° C by the middle of the century – and +2.4 ° C, or even +2.7 ° C towards its end – the frequency of events such as heatwaves or torrential rains will increase accordingly. , warns the climatologist. And for good reason, an increase of only 1 ° C already increases the capacity of the air to retain water by 7%, promoting heavy precipitation.
Because “if we are talking about an increase to +1.5 ° C, +2.7 ° C or +3 ° C, you should know that it is a continuum and not a breaking point, where everything would be fine at + 1.9 ° C to suddenly collapse at + 2.1 ° C “, emphasizes the scientist. Similarly, the calculation of an increase of + 2.7 ° C at the end of the century, put forward by the UN, corresponds “to the best estimator” for a given scenario. “There is an interval around this most probable estimator: if we take into account the range of climatic possibilities, this same scenario can take us up to 3.5 ° C. The message, concludes Christophe Cassou, is that to avoid disasters, +1.5 ° C is better than +1.9 ° C, which is better than 2 ° C … “
In June 2019, the heat record was broken in France: 46 ° C, in Vérargues (Hérault). Climatologists have studied the risk of such an event occurring depending on the level of global warming. “We are today at +1 ° C compared to the pre-industrial level and such a heat wave has a one in 50 chance of occurring, illustrates Christophe Cassou. It’s like throwing a 50-sided die at the start of the summer. With a heating of +1.5 ° C, this die has only 10 sides. At +2 ° C, it has four. By going from one to two degrees, we go from one chance in 50 to one chance in 4 “. And the climatologist to insist: “This is a good illustration of the fact that every fraction of a degree counts. Rising global temperature paves the way for more frequent, intense and longer heatwaves.”
“With +2 ° C, the threshold of 50 ° C could be crossed in the summer in France. It will be a rare event, of course, but possible. Just as after a while, the temperatures of June 2019 in France would be that of a normal summer. “
Christophe Cassou, climatologist and research director at CNRSto Franceinfo
In its latest report, the High Council for the Climate projects a rise of 2.6 ° C at the end of the century. Result ? Around 2071-2100, in France, there would be on average between 65 and 105 nights per year with minimum temperatures above 20 ° C. (PDF) on the Mediterranean rim and the entire Corsican coast. Under these conditions, the risk of forest fires would titillate the Loire.
A 2 ° C rise in global temperatures would already have serious consequences for health, agriculture and fire risks, detailed a study published in the journal International environment. Globally, one billion people could be exposed to life-threatening heat by mid-century, a report from the Met Office, Britain’s national meteorological service, reveals. Today, these conditions of thermal stress (meeting high heat and high humidity) endanger 68 million people. Finally, according to the IPCC, 400 million people will face water shortages with such a scenario.
Still according to scientists, this increase would hit biodiversity hard: out of 105,000 species studied, 18% of insects, 16% of plants and 8% of vertebrates would lose more than half of the area where they live. If the rise in temperature in the atmosphere kills, so too is that of the ocean. The warming of the seas caused by a rise of 2 ° C would condemn 99% of corals to death.
In the Arctic, a global increase limited to 1.5 ° C would lead to the possibility of having an ice-free summer once in a century, against once a decade if the planet takes 2 ° C, estimates the IPCC. However, the melting of ice and glaciers, as well as the expansion of water under the effect of heat, condemn many territories, like the Maldives. “We remind the world that we have 98 months to halve global emissions. The difference between 1.5 and 2 ° C is a death sentence for us,” Island authorities recalled to Glasgow.
In its latest report, the IPCC gives a forecast interval going from 0.28 cm to 1.01 m in 2100, but the current rate of our emissions is pushing us towards the top of this range. Since the rise in temperatures and the rapidity of the rise in water levels are directly correlated, a global warming of 2 ° C would already plunge several regions under water, illustrates Climate Central: the south of Bangladesh or Vietnam, the Japanese megalopolis of Nagoya, the surroundings. from Miami or New Orleans, in the United States, Alexandria and Port Said in Egypt …
In Europe, the coastline will be eaten away from Calais (Pas-de-Calais) and Dunkirk (North) to the border with Denmark, swallowing up a large part of the Netherlands. In France, a large part of the Poitevin marsh, part of La Rochelle (Charente-Maritime) or the neighboring municipality of Aytré, the surroundings of the municipalities of Redon (Ille-et-Vilaine) or Libourne (Gironde), districts of Cannes or even the airport of Nice would be found under water around the middle of the century.
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Underwater … or threatened by submersion in stormy weather, notes Christophe Cassou, taking the example of storm Xynthia, which killed 47 people in 2010. “The probability that this event will happen again is of the order of a chance in 200. So it really remains an exceptional event. With a rise in sea level of 50 cm, the centennial event becomes almost multi-decade. , with a chance in 50 “, he warns.