Cities wiped off the map, houses ripped open, vehicles overturned, at least 78 people killed … The United States was hit, Friday, December 10, by tornadoes “historical”, which devastated everything in their path in parts of Kentucky, Illinois, Tennessee, Arkansas and Missouri. The violence of the phenomenon quickly raised the question of a cause and effect link with the ongoing global warming. “This is going to be our new standard”, the head of the Federal Emergency Situations Agency (Fema) told CNN, before adding: “The effects we are seeing from climate change are the crisis of our generation.” “Everything is more intense when the climate warms”, for his part estimated the US President, Joe Biden, without however making a direct causal link between climate change and the disaster.
And for good reason: this link has not been established in the state of scientific knowledge, as we can read in the last report of the Intergovernmental Panel on Climate Change (IPCC), published in August. “This is a subject on which the link with global warming is still quite uncertain, explains to franceinfo Sonia Seneviratne, Swiss climatologist and co-author of one of the chapters of the report, where tornadoes are discussed. It’s quite difficult to assess because we don’t have a lot of data on past tornadoes and it’s very localized. Unlike heat waves which occur globally and for which it is easier to establish a statistical link. “
To do their job, climatologists need “of time series, measured in the same way, over many decades because climate change is assessed over decades”, reminds franceinfo Robert Vautard, director of the Pierre-Simon Laplace Institute for Climate Sciences and co-author of another chapter of the IPCC report where tornadoes are mentioned. Gold, “Before radar and satellite observations, we depended on observations made by humans, which are not everywhere and can miss events”, continues the climatologist, specifying that researchers do not have serious data on these swirling winds that “for forty years”.
Another difficulty is the level of “zoom” climate models, supercomputers capable of simulating a world with and without global warming to compare them and identify the effects of the latter. “At best, the models have a resolution of a few kilometers. The diameter of a tornado is a few tens of meters, so we’re not at the right scale.”, summarizes Robert Vautard.
These limits being established, the climatologist specifies that “The fact that the research tools do not allow a direct answer to the question does not mean that there is no link between tornadoes and global warming”. For him, the “signals from different studies tend to say that there is a link that is not going in the right direction”. He points out that, in the future, “atmospheric conditions favorable to tornadoes [aux Etats-Unis] should, with a moderate degree of confidence, be more frequent, due to the increase in temperature in the Gulf of Mexico “.
In the latest IPCC report, a new trend drawn from the observation of tornadoes across the Atlantic is emerging, with a moderate level of confidence. “Lhe number of tornado days has not really increased and has even fallen. But there are more tornadoes these days, reports Sonia Seneviratne. The event which has just taken place in the United States corresponds to this trend. There are not necessarily more, but more at the same time. ” About thirty of these storms swept across the United States on Friday evening.