Did climate change cause catastrophic tornadoes that struck the United States the last weekend? Although the conditions for their formation may be fostered by warming, scientists are very cautious about a possible direct link.

This year a connection was made between climate change and a heat wave in the northwestern United States, or even flooding in Germany and Belgium. However, the specific phenomenon of tornadoes is one of the most difficult to study.

“In recent decades, we have seen a trend of more favorable conditions” for tornado formation “in the Midwest and Southeast” of the United States, John Allen, a climatologist from Central Michigan, told AFP.

“And that signal is stronger in winter.”

In any case, he clarified that “it is misleading to attribute this event to climate change.”

James Elsner, a professor of climatology at Florida State University, makes an eloquent comparison: Although fog tends to increase the number of car accidents, the cause of a specific accident in foggy weather may be something else.

To determine this cause, research is needed: The science of “attributing” extreme events to climate change is really on the rise, but such a study will take time if it is ever done.

Meanwhile, 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 definitively yet,” Allen acknowledges.

The most recent report by UN climate experts (IPCC), dated August, highlighted “a low degree of confidence” regarding a link between climate change and localized phenomena such as tornadoes. And this applies to both “observed trends” and “projections.”

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What changes do we observe?

The annual average of tornadoes in the United States, most of which occur in the spring, has not increased in recent years: the number remains around 1,300.

“Most of the months are even,” says Jeff Trapp, head of the atmospheric sciences department at the University of Illinois. “The exception is the months of December and January, which have seen an increase in tornadoes in the last 30 to 40 years,” he says. Particularly in the southern United States, which is “consistent” with a “potentially linked explanation for climate change.”

This is because the two ingredients necessary for tornado formation are warm, humid air near the ground, and winds that blow in opposite directions at different altitudes.

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

On the other hand, tornadoes seem to concentrate in fewer days. When they form, they “tend to contain more” at the same time, explains Chiara Lepore, a researcher at Columbia University. And “this has consequences in terms of damage,” he stresses.

Finally, scientists note a geographical shift to the east compared to the area of ​​the United States nicknamed Tornado Alley or “Tornado Alley”, moving to the states of Arkansas, Mississippi or Tennessee, all three affected this week.

What to expect in the future?

The problem for researchers in studying tornadoes is that they are too transient and small to show up in commonly used climate models. Therefore, scientists are limited to studying only the evolution of the conditions potentially favorable for their formation.

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A study published in early November estimated that for each degree Celsius of additional warming, the probability of favorable conditions for a severe storm (with hailstorms, hurricanes, etc.) increased between 14% and 25% in the United States.

However, that does not mean that hurricanes will strike every time these conditions are right, it is even highly unlikely.

“This is kind of a ceiling on what we can achieve with each degree of global warming,” said Chiara Lepore, lead author of the study.

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

Still, very violent tornadoes will remain “unusual events,” he predicts.

“We are still in the early stages of our understanding of the link between climate change and what we call severe localized storms,” ​​estimates James Elsner. “But in the next five to ten years we will see some real progress.”


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