(Bloomberg) – Tornadoes kill more people when they occur in "outbreaks," storm systems that spin out more than a half dozen or so funnels in a limited time and area. Such twisters killed 49 people in 2015, and almost 80 percent of tornado fatalities from 1972 to 2010 occurred during outbreaks.
That's an increasing concern for citizens of "tornado alley" and the insurers and reinsurers who cover them, as outbreaks in the U.S. are becoming more extreme, according to a study published Thursday in the journal Science. The work, building on previous research, has left researchers with some unexpected questions. For one, the data are inconsistent with how global warming is expected to change our world.
The study looks at the number of tornadoes and outbreaks, as well as trends in environmental conditions that encourage thunderstorms and tornadoes. The surprise in the work comes from the latter part.
Storm fuel
As the planet warms, scientists project an increase in energy and moisture near the ground, something meteorologists call "convective available potential energy" or CAPE. Think of it as storm fuel. Greenhouse gases trap more energy in the atmosphere, which heats things up, which allows air to hold an exponential amount more water for every degree rise in temperature. Researchers generally expect that CAPE may mean more extreme storms.
The other major ingredient in these violent storms is vertical wind shear, the phenomenon of wind direction and intensity shifting with altitude. Unlike CAPE, shear hasn't been projected to change much with global warming.
What they found — "the wrinkle," as lead author Michael Tippett put it — was wind shear tracked the tornado-outbreak trends more closely than CAPE. "Who do I blame for the trend, is it the CAPE or is it the wind shear? I think a lot of people, including us, would have expected CAPE. That's not what we see. So that's why it's a wrinkle."
Tippett, an applied mathematician at Columbia University, offered a couple of hypotheses. One was that the tornado outbreak trend may not be related to climate change, "or, it's climate change but we don't understand it," he said.
The research was funded in part by the Willis Research Network, an organization that underwrites investigations into scientific questions of interest to the reinsurance industry.
Questions unanswered
Tornado experts who reviewed the paper split on the value of the findings.
James Elsner teaches applied spatial statistics and climatology at Florida State University and has written about the increasing number of tornadoes in outbreaks. He thinks the trend is in part a function of increased ground-level energy leading to fewer days with tornadoes, and more active, concentrated activity on days when they do occur. "The current study offers nothing new in this regard," he said, and suggests that a faster rise in the most extreme outbreaks can be explained just as well by general statistical behavior.
Harold Brooks, a senior scientist at the National Oceanic and Atmospheric Administration's Severe Storms Laboratory, led a 2014 study showing that tornadoes are bunching up even as their absolute number has held relatively constant. Brooks said he tried to explain why the trend was occurring by looking at overall environmental variables like CAPE and wind shear, but he couldn't reach a satisfying answer. He called the authors' lack of correlation between CAPE and tornadoes "a significant result."
"The work is well done and intriguing," Brooks said, in part because it raises the question of why large-scale atmospheric patterns or wind shear might or might not be related to warming. "Trying to sort that out is the next big research question."
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