A heat wave is setting in these days in the south-east of France and in the Mediterranean. And climate skeptical accounts multiply misleading posts to minimize heat waves and dismiss any influence of global warming. First example: a live broadcast from BFMTV in Nice on Sunday July 16 became very viral on Twitter. “Olala, we are in Nice in the middle of July, it is 30 degrees”, mocks a post shared more than 2,000 times.
Another mocks these 30 degrees “which are unheard of on July 16 in the middle of summer” in the capital of the Alpes-Maritimes department. In reality, in the commentary, the journalist does not indicate at all that this heat wave episode is “unheard of”. Since July 9, the Alpes-Maritimes have remained on heatwave orange vigilance, an alert to populations issued by Météo-France and built on temperature thresholds. This alert extended to three departments on July 18 and is valid until July 19.
Another variant: while several heat waves are taking place around the world and the media are reporting them, a viral post, shared more than 400 times, estimates that there is “absolutely nothing special in the northern hemisphere”, denouncing “fakes that are linked” and “an instruction to talk about an international heat wave”. As proof are mentioned the temperatures in Quebec and, for example, an article from the Quebec daily, The duty. The latter takes up an AFP dispatch on the northern hemisphere hit by heat waves.
The little music of these posts is to affirm that it is hot in a completely normal way, taking an example in Nice or Quebec, and that the media exaggerate when they publish information on heat wave alerts, provided by meteorological services.
FAKE OFF
But these posts are misleading in several ways. First of all, the heat wave announced in the south-east of France is a “non-exceptional” episode, indicates Meteo France on its site, although its “persistence requires particular vigilance”. The orange vigilance level corresponds to a period of intense heat for three consecutive days and three nights.
In the eastern Pyrenees and in Andorra, a peak of heat is expected, that is to say a brief episode of 24 to 48 hours during which are higher than expected normals. The 40 ° C have already been reached locally this Tuesday, July 18 in the Pyrénées-Orientales, the Var and in Corsica. Since 2020, 40 ° C have been crossed 25 times a year in France, still indicates Meteo France. Previously, this happened once or twice every ten years.
A heat dome that affects Italy
This episode is part of a heat wave, which affects more intensely the Mediterranean countries, in particular Italy. This wave should take “even larger proportions, with a considerable geographical extent, an extreme intensity, and a duration still unknown” in the country, indicates the meteorological service.
For what ? Quite simply, because a heat dome has settled over the Mediterranean. It is “rather centered on Italy, whereas if it were indeed a little further west, France would probably be affected”, notes Christine Berne, climatologist at Météo-France. This closed zone of high pressure traps hot air at all levels of the atmosphere and acts as “a pressure cooker”, she underlines. This creates conditions for expected temperatures “around 10 degrees above seasonal norms” in Italy, she adds. Mercury spikes are likely to reach 42°C in Rome and more than 48 in Sardinia on Tuesday.
“Separate and concurrent” heat waves
As AFP wrote in a dispatch, several phenomena are taking place at the same time: a heat dome is also over the United States, where the weather services are predicting record temperatures. The capital of Arizona, Phoenix, chained Monday July 17 an 18th day above 43 ° C. China, for its part, broke a record for mid-July on July 16, with 52.2 ° C in the arid region of Xinjiang. If the appearance of these domes “is not correlated, it alerts anyway”, believes Christine Berne.
This is also what the climatologist and member of the IPCC, Christophe Cassou, explained in a thread on Twitter : “It is wrong to speak of heat waves / planetary heat waves, he explains. No physical mechanism supports this claim. The heat waves are “distinct” and “occur simultaneously and in several places in the northern hemisphere”. And this “concomitance in temporalities and regions of extremes is expected”, he specifies, it is one of the conclusions of the IPCC report on the state of knowledge on the climate, published in 2021. “Is it a surprise ? No. A runaway? No. Is that bad ? Yes,” he warns.
The attribution of a meteorological phenomenon to climate change
Another point to clarify: yes, obviously, heat waves in themselves are not a consequence of global warming, they existed before the industrial era. But “the intensity and level of temperatures [de ces vagues de chaleur] can only be achieved with the global warming at work at the moment,” emphasizes Christine Berne.
In the background, there is the question of the attribution of a meteorological phenomenon to climate change. This cannot cause a meteorological event on its own, because these phenomena have multiple causes, but it can “influence the probability and intensity of an event”, explains the World Weather Attribution (WWA), an international network of scientists that publishes attribution studies.
An “extremely robust” link
These are carried out a posteriori, using climate simulation models. “Scientists take all the atmospheric measurement data from an event, explains Christine Berne, and imagine that there is no global warming, so they lower temperatures by two degrees, for example. They randomly run thousands of scenarios to see if an event can occur with slightly cooler temperatures. When a phenomenon is attributed, the result indicates that without global warming, such an event would have for example 20 times less chance of occurring or 50 times less, etc. »
The link between global warming and the increase in the intensity and frequency of heat waves is “extremely robust, underlines the WWA, this applies to both large-scale heat waves and hot days outside. ‘local scale’. In France, since 1947, the census of heat waves indicates “clearly that the frequency and intensity of these events have increased”, according to Météo-France. While the country experienced an average of 1.7 heat wave days per year before 1989, it has suffered 8 per year since 2000 and 9.4 over the last decade.