France is about to experience an extraordinary heat wave at this time of year. Since Monday June 13, temperatures have started to climb, first in the south, then further north. The mercury has already approached, even beaten, records for late spring: 36.2°C in Marseilles, 37.4°C in Aubagne, 36°C in Toulouse… This heat stroke should gradually spread throughout France between Wednesday and Saturday, according to Météo France. For the institute, it could even be the earliest heat wave ever recorded in the country.
While there is no universal definition of a heat wave, Météo France classifies as such episodes during which the national thermal indicator – i.e. the average daily temperature – meets several criteria: exceed the threshold of 25.3°C for at least one day, stay above 23.4°C for at least three days and not fall below 22.4°C once.
Since 1947, 43 heat waves have been recorded by Météo France. Only 8 episodes have started in June in the past, i.e. nearly 20% of the phenomena recorded. But none had ever happened before June 18. Starting Wednesday, June 15, the heat wave that begins is therefore three days ahead of the previous record, set in 2005 and equaled in 2017.
This heat wave also stands out for its intensity. Météo France is counting on a national thermal indicator of 27°C. “It will never have been so hot, so early in the season in France”underlined during a press briefing Matthieu Sorel, climatologist at Météo France.
However, it should not exceed 27.95°C in June 2019. During this episode, the record for the highest temperature ever recorded in France was set, with 46°C measured in Vérargues in Hérault. But the new wave that is starting promises to be warmer than the seven other heat periods recorded in June, according to Météo France forecasts.
Monthly temperature records are also expected in the coming days, in Occitania, New Aquitaine, Pays de la Loire and Brittany. According to Meteo France, maximum temperatures should be above 30°C over the entire territory on Friday and could exceed 40°C locally in the South-East quarter. Since Wednesday, 23 departments have been placed on orange “heat wave” vigilance, mainly in the West and South-West.
These high temperatures should know their peak between Friday and Saturday and experience a “settling” sunday according to Meteo France, accompanied by the arrival of a stormy degradation coming from the West. The current episode therefore promises to be rather short: by comparison, the June 2005 heat wave lasted eleven days.
Global warming is no stranger to this unprecedented situation. This one “results in earlier, later, but also more intense heat waves”, detailed Matthieu Sorel, climatologist at Météo France, during a press briefing on Tuesday. Analysis of heat waves since 1947 shows that the three earliest and six latest episodes in the season were all recorded after the year 2000.
The frequency of these events, above all, has increased. Since 1990, there have been 33 heat waves in France, three times more than in the previous 43 years. The last decade has been particularly prone to such episodes of high heat: 17 have been recorded since 2010.
These phenomena have also gained in intensity. Since 1947, of the nine heat waves whose maximum national thermal indicator has exceeded 27°C (not counting the current episode), seven have occurred in the 21st century. During the waves of August 2003 and July 2019, the average national temperature even exceeded 29°C, well above the temperatures reached in the previous century. A World Weather Attribution study showed that temperatures recorded during the July 2019 episode, for example, would have been 1.5 to 3°C lower in the absence of climate change.
In the present case, it was not global warming that caused the depression located off the Azores, itself responsible for this upwelling of warm air and the “heat plume” that is forming above France. But it is the evolution of the climate which explains the intensity of the temperatures observed.
The frequency of such hot spells is expected to increase further. According to the climate simulations of the Drias project, carried out by several French laboratories, a scenario of low greenhouse gas emissions would be synonymous with doubling the number of days of heat waves by the end of the century. Median emissions would multiply this figure by 3 or 4, while high emissions would lead to a factor of 5 to 10.
Although the early heat wave that is beginning does not bode well for the rest of the summer, the trends still point to a rather hot quarter. Météo France estimates a 50% probability that seasonal norms will be exceeded in the north of the country, and 70% for the south.