Increasingly earlier heat peaks in France: the proof in figures

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Increasingly earlier heat peaks in France the proof in figures

If the French crowd into parks or on terraces, they know it full well: temperatures are abnormally high for the season. The brief heat peak expected this Saturday, above 25°C in most of France and up to 30°C in the South-West, is not unprecedented for the beginning of April. But these early events are becoming less and less rare, given global warming caused by human activity.

The threshold of 25°C, which characterizes a “hot day” for climatologists, is usually reached for the first time in the season “between April 20 and May 10” in the northern half, says Météo-France . And the 30°C mark, synonymous with “strong heat”, is generally crossed “between mid-May and the end of June” throughout the metropolis. The cause of this anticipated peak? A mass of hot air coming from the Sahara, accompanied by a cloud of sand.

“30°C is something that sounds like a summer temperature but in a warming climate, it’s something that will arrive earlier and earlier in spring,” Julien Cattiaux, climatologist, explains to AFP at the National Meteorological Research Center (CNRM), in Toulouse. Observing these thresholds as early as April is rare, but already seen. 30°C has already been reached or approached in March 1990 in the South-West and near the Mediterranean; in 1955, 25°C was exceeded on March 25 in Paris and in Indre (28°C in Châteauroux).

“What has changed since 1955 is that the climate has warmed: these early heat episodes have become more likely in today’s climate, which can also be seen in the statistics since most similar events are after the years 2000 or even 2010”, contextualizes Julien Cattiaux. “In a warming climate, hot events will either be more frequent or more intense,” he explains. Not only are the days above these heat thresholds more numerous outside of summer, but they occur earlier and earlier in spring and later in autumn.

Expected records

Between 1950 and 1970, Parisians waited on average until May 6 to experience 25°C. Half a century later, between 2000 and 2020, it is April 19, almost three weeks earlier. The trend is also observed in the South, such as in Perpignan, where the first day of heat occurred on average from April 24 to 11 over these periods. In the north of the country, the people of Lille experience 25°C from May 7, on average, instead of May 19 when General de Gaulle was president.

This Saturday’s episode could, however, break records: the earliest 30°C in Agen dates back to April 30, 2005 and April 17, 2003 in Bordeaux, two cities where this bar is only reached on average around the June 10 or 11. This heat peak occurs thanks to a “fairly hollow depression in the Atlantic, a little north of the Azores” which leads to “a flow from the South which brings up rather warm air masses over Europe” since Africa, describes Julien Cattiaux. “This air mass is quite warm, it’s something that is remarkable. And so including at altitude, in the Pyrenees, we are perhaps going to have April records broken,” warns the climatologist.

More and more frequent heat waves

March 2024 constitutes the 26th month in a row above normal in France. In total, France has experienced forty-seven heat waves, including twenty-two since 2010. Note that the frequency of these phenomena is expected to double by 2050. “The occurrence of heat waves in France, which was on average one summer every 5 years before 1989, has become annual since the year 2000”, underlines Meteo France. According to the meteorological service, temperatures will however “start to drop from the West on Sunday”, and “this drop will be marked, with a drop of around 6 to 10°C compared to the day before”, except in certain eastern regions.

“We are at the beginning of April, at the beginning of spring, and with climate variability, we can have 10°C above seasonal norms as well as 10°C below,” underlines Julien Cattiaux. Certainly, “the last day of frost also tends to move forward in the calendar” because of global warming and “it is less likely to have episodes of frost in April, but they can still happen”, warns the scientist.

Recently, this scenario presented itself twice, at the beginning of April 2020 and above all, 2021, with serious consequences for plants and arborists. That year, explains the climatologist, “the frost episode followed heat records at the end of March and was hard on the vegetation whose growth had already started.”



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