In the wake of the deadly twister outbreak last week, Sen. Bernard Sanders declared that climate change is making tornadoes worse, to which researchers say: Not so fast.

Purdue University professor Ernest Agee, who has studied tornadoes for 50 years, said his research and that of other scientists shows that the number of violent U.S. tornadoes has in fact tapered off in recent decades.

“My opinion is that strong and violent tornadoes have actually leveled off,” said Mr. Agee. “They’re definitely not increasing with time over the last few decades. In fact, there’s a slight tendency, just a very slight tendency, of the decline in the number of violent and strong tornadoes.”

What’s more, 2018 was the first year since record-keeping began in 1950 without an EF4 or EF5 tornado, the most devastating twisters, as rated on the Enhanced Fujita Scale from EF0 to EF5, according to the NOAA/NWS Storm Prediction Center.

“We’re definitely not seeing a trend of increase. If anything, we’re seeing a decrease in the number of strong and violent tornadoes,” Mr. Agee said, adding “and that’s in papers that I’ve published and my students and other colleagues that are prominent in the field.”

Ten deaths were reported last year from U.S. tornadoes, the fewest since unofficial tabulations started in 1875, breaking the previous low of 12 deaths in 1910. About 70 deaths per year are attributed on average to U.S. tornadoes.

“Last year was, for a lack of a better term, a tornado drought over the United States,” said Victor Gensini, a meteorology professor at Northern Illinois University.

None of that should reassure those living in storm-prone regions, as demonstrated by the destructive March 3 outbreak in Lee County, Alabama, that left 23 dead after a tornado registering winds of up to 170 mph, enough for an EF4 rating, swept through the area.

More tornadoes touched down over the weekend from Texas to Tennessee. The strongest was a twister with EF1 wind speeds in Logan County, Arkansas. The storms produced damage but no reported deaths, according to AccuWeather.

Climate change is inevitably blamed for any natural disaster, and Mr. Sanders led the charge after the deadly tornado, saying in a Facebook post, “The science is clear, climate change is making extreme weather events, including tornadoes, worse. We must prepare for the impacts of climate change that we know are coming.”

The Vermont independent linked to an article in EcoWatch about an October study by Mr. Gensini showing that Tornado Alley is gravitating from the Great Plains to the Midwest and Southeast, but the professor said the shift cannot be attributed conclusively to human-caused climate change.

“[Mr. Sanders] references our study, which says that climate change is shifting eastward. We just don’t know for sure if it’s precisely climate change that’s causing it, and certainly we cannot say at all that climate change caused the Lee County, Alabama, tornado,” Mr. Gensini said. “We’re not there as a science to be able to do that.”

Are tornadoes getting worse because of climate change? “It’s a bit of a premature statement,” said Stephen Strader, Villanova University assistant professor of geography and the environment. “In terms of noticeable trends due to climate change, we don’t see much there yet.”

Tornado damage up, deaths down

According to the Center for Climate and Energy Solutions, the “link between tornadoes and climate change is currently unclear.”

“It is likely that a warmer, moister world would allow for more frequent instability,” the center said on its website. “However, it is also likely that a warmer world would lessen chances for wind shear. Climate change also could shift the timing of tornadoes or the regions that are most likely to be hit, with less of an impact on the total number of tornadoes.”

A December study led by Florida State University professor James B. Elsner that found tornado “power” increasing from 1994 to 2016 cited research showing that tornadoes are increasingly occurring in bunches; that EF3 or stronger tornadoes are more likely to occur on days with more tornadoes, and that “damage paths are getting longer.”

The study multiplied path area, wind density and air speed as measured on the EF scale to determine “total energy dissipated by a tornado” and found an “upward trend in tornado power.”

Any findings on tornado behavior inevitably come with the disclaimer that the science is still young — researchers still don’t know exactly how tornadoes form — while EF4 and EF5 events are so rare that one or two can swing the trend.

“Tornadoes are one of the things we know and understand the least about, of all these meteorological events,” said Mr. Strader. “We understand a lot about drought and rain and precipitation and all kinds of stuff, but severe weather is way down the list.”

He noted that the Enhanced Fujita scale measures damage, meaning that an enormous twister could receive a low rating if it doesn’t hit buildings, fences or other structures.

One example: the deadly 2013 El Reno tornado, which was the widest on record and killed three storm-chasers, but was initially rated EF3 because it didn’t cause sufficient destruction. The disaster was later upgraded to EF5.

“When we’re looking at the ratings of these tornado events, they’re very fickle,” Mr. Strader said. “A tornado could go across the entire surface of the Earth, not touch anything, and then hit one home causing EF5 damage, and it’s rated as an EF5. We know nothing about the event before that or after that in terms of intensity.”

That’s why he takes issue with reports of last year’s “tornado drought.” He argues that it wasn’t the first year without a violent tornado, but the first on record.

“I am confident from a statistical standpoint as a scientist that there has been a decrease, but where we have to be careful is why there’s a decrease,” said Mr. Strader, adding that explanations could shift from “climate change to how we rate tornadoes to random chance.”

What he can say is that tornado damage has increased, which he and others attribute in large part to greater exposure from development. The southeastern shift to more densely populated regions also creates the potential for more destruction.

“I have other colleagues here at Northern Illinois and Villanova who will argue — and I would agree with them — these type of tornado disasters are going to continue to happen in the future and perhaps get more frequent, and it has nothing to do with climate change necessarily,” Mr. Gensini said. “It has everything to do with the fact that our cities continue to grow larger and the population density continues to increase.”

He added, “We’re essentially putting more targets on the dartboard. It’s an exposure and vulnerability issue.”

Even with more people, tornado deaths are on the decline. Fatalities increased faster than population growth from 1808 to 1915, but the trend reversed starting in 1916 thanks to advances in technology, communication and building codes, according to a paper published last month by Purdue’s Mr. Agee and Lindsey Taylor.

“People’s perceptions can be greatly skewed by a singular event,” said Patrick Marsh, NOAA/NWS Storm Prediction Center warning coordination meteorologist. “For example, 2013 was actually a below-normal tornado year, but if you talk to people in Oklahoma, they think it’s one of the worst tornado years we’ve ever had.”

That was the year the EF5 tornado hit El Reno.

“In terms of the severity of a year, if a tornado hits you,” said Mr. Marsh, “it’s probably going to be the worst year of your life.”

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