Growing tornado impact puzzles scientists

The US seems to be experiencing more and worse tornado outbreaks – groups of twisters in quick succession. But climate change may not be the culprit.

Tornado outbreaks – those sudden, multiple whirlwinds that can randomly destroy whole townships or pass by and do little more than ruffle the prairie grass – may be getting more frequent and more powerful in the United States. And nobody can be sure why.

Climate change driven by global warming which is in turn the consequence of greenhouse gas emissions from fossil fuel combustion could be a candidate cause. But meteorologists cannot rule out some other potential explanation, such as some natural cycle in climate behaviour involving oceans and atmosphere.

But they do know that tornado outbreaks are becoming potentially more destructive. Two years ago, researchers checked the data and found that the tornado “season” in the US was now two weeks earlier than it had been in the early 20^th century.

In the spring of 2016 a team from Columbia University checked the records since 1954 and found that the number of individual tornadoes during any single episode of tornado outbreaks has been rising for the past six decades.

Worse winds

And now Michael Tippett, a physicist at Columbia Engineering, has returned to the challenge. He and two colleagues report in the journal Science that not only are the numbers of twisters in each outbreak growing, but the overall severity of the whirlwinds is on the increase. And the fastest increase is in the most extreme range of the phenomenon.

For four decades climatologists have been warning that global warming will be accompanied by a greater frequency of extreme events – hurricanes, heat waves, flooding, drought and so on – but that may not mean that climate change is behind the multiplying tornado hazard. A meteorological trend is visible, but it isn’t the one expected under climate change.

“This study raises new questions about what climate change will do to severe thunderstorms and what is responsible for recent trends,” said Dr Tippett, a member of the Data Science Institute and the Columbia Initiative on Extreme Weather and Climate.

“The fact that we don’t see the presently understood meteorological signature of global warming in changing outbreak statistics leaves two possibilities: either the recent increases are not due to a warming climate, or a warming climate has implications for tornado activity that we don’t understand. This is an unexpected finding.”

Unexpected increase

In 2015, tornadoes killed 49 people in the US. In the first half of 2016, tornadoes and severe thunderstorms caused $8.5bn worth of insured losses in the US. The Columbia study involved one set of tornado reports from the US government’s National Oceanic and Atmospheric Administration, and a second set based on observations of weather data linked with tornado outbreaks.

A tornado outbreak is six or more of the destructive monsters in close succession. Between 1972 and 2010, these rapid-fire whirlwinds have caused almost eight out of 10 tornado deaths in the US. And over five-year periods, the numbers of tornadoes in the most extreme outbreaks have more or less doubled. But the researchers cannot be sure why this should happen.

They looked at a factor called convective available potential energy, which theory says should increase in a warming world. But that wasn’t the factor behind the observed meteorological trends.

Instead what was at work was another factor called storm relative helicity – very crudely, the whirl in a whirlwind – which nobody expected to increase in a world of climate change.

“We’ve used new statistical tools that haven’t been used before to put tornadoes under the microscope. The findings are surprising”

So the head-scratching continues. It could be some factor connected with an oscillation in the surface temperatures of the Atlantic Ocean, an oscillation that takes many decades and certainly affects the North American climate. In a phrase scientists use rather often, more research is necessary.

“Tornadoes blow people away, and their houses and cars and a lot else,” said Joel Cohen, co-author of the paper and director of the Laboratory of Populations, jointly at Rockefeller University and Columbia’s Earth Institute.

“We’ve used new statistical tools that haven’t been used before to put tornadoes under the microscope. The findings are surprising. We found that, over the last half century or so, the more extreme the tornado outbreaks, the faster the numbers of such extreme outbreaks have been increasing.

“What’s pushing this rise in extreme outbreaks is far from obvious in the present state of climate science. Viewing the thousands of tornadoes that have been reliably recorded in the US over the past half-century or so as a population has permitted us to ask new questions and discover new, important changes in outbreaks of these tornadoes.”