The planet is burning, but most French people are in thin air. Such is the paradox of a summer 2023 of all contrasts. On the one hand, a month of July marked by scorching temperatures around the Mediterranean, and spectacular fires in Sicily and Greece, leading the UN to declare the beginning of “the era of global boiling”; on the other, gloomy, rainy weather over the northern half of the country.
Disconcerting? Yes, to hear many conversations in everyday life. On social media, some climate change skeptics have even seized on the paradox, to quip what some call an “asymptomatic heat wave”. Yet this contrast does not contradict scientific knowledge, nor does it minimize the seriousness of the situation. Explanations.
Due to the jet stream. This “wind tube”, as Météo-France calls it, crosses the globe from west to east in the upper atmospheres and brings winds and precipitation. It usually evolves in summer at the level of Scandinavia, but not this year. “It is currently located in the north of France, which explains this coolness”, explains François Gourand, forecaster at Météo-France.
Result: the mercury plummeted during the second half of July, France being largely protected by this veil of low pressure. The French situation is however singular: on a global level, it is on the contrary expected that the month of July 2023 will become the hottest on record.
In June, scientists were worried about a scorching summer, particularly due to the El Niño phenomenon. This powerful anticyclone, the source of some of the warmest years, was to add to the continuous increase in summer temperatures. But the basic dynamics of the climate, which are established over the long term, can be blurred by numerous short-term meteorological phenomena, linked to atmospheric flows of air and wind – this is the eternal opposition between climate and weather report.
The movement of the jet stream was “not really anticipated”, admits Mr. Gourand, and “is part of the variability of the weather”. It is expected to move up towards northern Europe from the second week of August, leading to a return to seasonal normals throughout France.
It is too early to tell: for that, complex attribution studies would be needed, and the phenomenon would be repeated over several summers. This is the whole problem that currently arises for the study of the climate: without hindsight, it is difficult to sort out temporary meteorological phenomena from new climatic phenomena.
The idea that the jet stream is influenced by climate change cannot in any case be ruled out, since it moves according to the evolution of pressure and temperature contrasts. A study by the University of Pennsylvania showed in 2022 that climate change caused a change in behavior, by increasing its propensity to change latitude – which was the case this summer.
“France is located at the interface of the Mediterranean, hit by an exceptionally large heat dome, and northern Europe, subject to low pressure conditions, relocates François Gourand. The heat has probably accentuated the contrast, which feeds the famous jet stream. »
No. Firstly because, if we move away from the north of France, the temperatures were (very) high in the South. Corsica experienced its second longest heat wave in July since 1947, reports Météo-France. In addition, several records were recorded in Provence-Alpes-Côte d’Azur and Pyrénées-Occidentales, with readings of 39.2°C in Cannes on July 19, and 40.4°C in Serralongue. “This month of July is all the same the fifteenth hottest since 1900 on the scale of France”, recalls the climatologist Christine Berne in Liberation.
June had already ranked as the second hottest month over the period 1900-2023, according to Météo-France records. And if France should not experience a heat wave by August 15, temperatures are expected to rise thereafter.
No, quite the contrary. In all the forecast models, the rain does not disappear, but falls differently, more spaced out and more intense. “In regions like France, the total amount of precipitation will fluctuate little, confirms CNRS researcher Julie Deshayes. But it’s the concentration of precipitation in a more limited window of time that changes: what we had in one month, we will have in two days. That too is a signature of climate change. »
This is how the northern half of France was hit by violent storms and torrential rains, for example in the Doubs. “We imagine that as soon as there is no more heat wave there is no global warming, but it is a bad association”, deplores Ms. Deshayes. It is actually accompanied by a more general disruption, which takes the form of extreme episodes, in one direction (heat wave, drought) as in the other (cold wave, storms, torrential rains). In Beijing, record rains have fallen in three days since the end of July, causing dozens of deaths and missing.
Conversely, Italy, Greece, Turkey, Tunisia and Algeria were confronted, in the second part of July, with extreme temperatures, with peaks at 48.2 oC in Jerzu ( Sardinia), 47.8 oC in Syracuse (Sicily), 48.7 oC in Algiers, 49 oC in Tunis, with fires in Croatia, Portugal or Bulgaria.
Certainly, it is expected to be hot in summer in this region of the world, and in isolation, very high temperatures have already been recorded there. “But the fact that they are happening more and more frequently, with more and more amplitude, is characteristic of climate change,” says oceanographer and climatologist Julie Deshaye, who sees it as “a glimpse of the climate of the future. “, with increasingly intense and frequent heat waves.
The heat dome that marked the month of July in the Mediterranean rim took the forecasters by surprise. “If we look at the heat wave in Southern Europe, the duration and intensity of which promise to be exceptional, we are facing an event that we were rather expecting around 2050,” climatologist Davide Faranda recently explained to Le Monde. .
Not to the local weather, because it is by definition changeable, and many short-term phenomena mask long-term upheavals. However, one indicator seems to provide indisputable evidence of global warming: ocean temperatures. Indeed, the oceans capture a large part of the atmospheric heat, and unlike the air, they are less versatile, less exposed to contradictory atmospheric dynamics, and retain this heat longer. “It’s a better indicator of global warming,” says Julie Deshayes. However, at the end of July, the historic records of the Mediterranean Sea and the North Atlantic were broken.
