United States | Is there a hyperlink between local weather change and tornadoes?

(Washington) Did climate change cause the catastrophic tornadoes that hit the United States this weekend? If the conditions of their formation can indeed be fueled by heating, scientists are very cautious about a possible direct link.

Posted on December 13, 2021

Lucie AUBOURG
French media agency

A connection was made this year between climate change and a heat wave in the American Northwest, or even with flooding in Germany and Belgium. But the specific phenomenon of tornadoes is one of the most difficult to study.

“In the last few decades, we’ve seen a trend towards more favorable conditions” for the formation of tornadoes in the Midwest and Southeast of the United States, explains to AFP John Allen, a climatologist at Central Michigan University. “And that signal is stronger in winter. »

However, “it is misleading to attribute this event to climate change.”

James Elsner, professor of climatology at Florida State University, makes a revealing comparison: while fog tends to increase the number of car accidents, the cause of a specific accident that occurs in foggy weather can be entirely different.

To determine this cause, an investigation is needed: the science of “attributing” extreme events to climate change is indeed in full swing. But such a study will take time, if at all.

In the meantime, can we at least say that climate change, by creating these favorable conditions, will increase the number of tornadoes in the future?

“The evidence seems to point in that direction. But I don’t think we can say that definitively yet,” says John Allen.

The latest report by UN climate experts (IPCC) in August highlighted “a low degree of confidence” regarding a link between climate change and phenomena as localized as tornadoes. This applies to both “observed trends” and “projections”.

What changes observed?

The average number of tornadoes per year in the United States, most of which occur in the spring, has not increased in recent years: about 1,300.

“Most months are down,” says Jeff Trapp, head of the atmospheric sciences department at the University of Illinois.

“The exception is December and January, which have seen an increase in tornadoes over the last 30 to 40 years,” he notes. Particularly in the southern United States, which is “consistent” with a “potentially climate change explanation”.

In fact, the two ingredients needed for tornado formation are warm, moist air close to the ground and winds blowing in opposite directions at different altitudes (called vertical shear).

However, today we see “a higher probability of hot days during the cold period, which can favor the formation of storms and tornadoes”, says Jeff Trapp.

On the other hand, tornadoes appear to be concentrated in a smaller number of days. When they graduate, “you tend to have more of them” at once, explains Chiara Lepore, a researcher at Columbia University. And “this has consequences in terms of damage”, he stresses.

Finally, scientists notice a geographic shift eastward from the area of ​​the United States dubbed “tornado alley”, deporting them to Arkansas, Mississippi or Tennessee – all three affected this weekend.

What to expect in the future?

The problem for researchers who study tornadoes is that they are too fleeting and small to show up in commonly used climate models.

Scientists are therefore reduced to studying only the evolution of conditions potentially favorable to their formation.

A study published in early November estimated that for every degree Celsius of additional warming, the likelihood of favorable conditions for severe weather (hail, hurricanes, etc.) increased by 14-25% in the United States.

However, this does not mean that hurricanes will occur whenever these conditions are met – it is even very unlikely. “This is in some ways the upper limit of what can be achieved per degree of global warming,” explains Chiara Lepore, lead author of the study.

According to another future study, “tornadoes may prove more powerful in future weather,” according to Jeff Trapp. To reach that conclusion, the researchers this time took an already observed event and analyzed how future weather conditions would affect it.

However, very violent tornadoes will remain “rare events”, he predicts.

“We are only at the beginning of our understanding of the link between climate change and what we call severe localized storms,” summarizes James Elsner. “But in the next five or ten years, we will see real progress. »

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