Lack of documentation, difficult forecasts… Climatologist Fabio D’Andrea deplores this: “We don’t have enough elements to understand the link between tornadoes and global warming. » Impossible to say that the violent phenomena which hit Amiens (Somme) and Bihucourt (Pas-de-Calais) on Sunday October 23 are a consequence of global warming. Some hypotheses nevertheless suggest the formation of more intense vortices in the coming years.
How do these violent phenomena form? “Tornadoes are always associated with very intense thunderstorms, wind and flat geography,” explains Davide Faranda, researcher at the CNRS. Thunderstorms are formed by a temperature difference between sea level, the ground, and the atmosphere: “The availability of ground heat and humidity forms thunderstorms and, therefore, tornadoes”, explains the climatologist.
The intensity of these 100 km/h vortices – for the weakest in intensity – should then increase with global warming: “We expect more conditions that form thunderstorms of great intensity, which then makes the formation of intense tornadoes more likely. »
If it remains impossible for researchers to announce a greater frequency of tornadoes in France, global warming risks providing “more conducive to their development”, confirms Fabio D’Andrea, Deputy Director of the CNRS Dynamic Meteorology Laboratory.
Impossible predictions
A gray area persists around the forecasts of the phenomenon. The Somme, the North, the Pas-de-Calais and the Eure were placed on Sunday October 23 in “yellow vigilance” thunderstorms by Météo France, while 20 other departments were on the alert « orange ». “Tornado forecasting is a major limitation of weather models,” says Davide Faranda.
While it is possible, therefore, for forecasters to say that there will be fertile ground for tornadoes, it is impossible to predict their trajectory.“their formation being linked to incalculable conditions, such as the location of a field”. Another downside: tornadoes are not visible on satellite images.
Only direct witnesses or Doppler radars can observe them. Since these radars have only existed for about ten years, the documentation on the phenomenon is weak and makes it difficult to establish more in-depth studies on a possible link with global warming.
Lack of documentation
“On August 19, 1845, a tornado had already killed 70 people in the north of Rouen”, says Davide Faranda. If we know of the existence of this tornado, it is only “thanks” to its human toll. But these phenomena are often too localized and too small to list.
“Tornadoes have always existed in France and Europe, but today we pay more attention to their formation with global warming and they are more visible thanks to our smartphones”, explains the researcher. Impossible, then, without knowing the number of tornadoes in recent years in France, to measure the evolution of their frequency.