In this file photo, lightning strikes the Willis Tower (formerly Sears Tower) in downtown Chicago on June 12, 2013.
As the Earth continues to warm during this century, atmospheric conditions ripe for severe thunderstorms and tornadoes will increase in the U.S., according to a new study.
Given the amount of damage caused by the straight-line winds, golf-ball-sized hail or flash floods associated with any given severe thunderstorm, understanding whether they will increase in frequency or intensity on a warming planet is a key question in climate science.
Answering it, however, has "been a bit of a conundrum," lead author Noah Diffenbaugh, an associate professor of earth science at Stanford University, told NBC News. That's because the two main atmospheric ingredients for thunderstorm activity are expected to change in contradictory ways.
One ingredient that climate change theory suggests will become more abundant is known as the convective available potential energy, or CAPE, that is created as the air in the low atmosphere warms. Warm air rises, carrying with it moisture to higher altitudes.
The second ingredient is to transform into a severe thunderstorm, CAPE must also interact with strong vertical wind shear, which is essentially the change of wind speed with height. Theoretically, vertical wind shear is expected to diminish as the planet warms.
While prior studies have indicated that the increasing CAPE is the stronger signal, thus leading to more thunderstorms, "expectations of decreasing shear continue to create uncertainty," Diffenbaugh and colleagues note today in the Proceedings of the National Academy of Sciences.
Theory holds up
He and Martin Scherer, also at Stanford, and Robert Trapp at Purdue University in Indiana, used a suite of global climate model experiments to examine the atmospheric conditions present over the U.S. each day in boxes that were roughly 60 miles on each side. Prior studies have used just one model run.
"We find in fact the theoretical predictions hold up, meaning there is increasing CAPE with global warming and there is decreasing shear," Diffenbaugh said. "But what's new is we found that the decreases in mean shear are most concentrated on days that have low CAPE."
In other words, when CAPE is high, vertical wind shear is more likely to be high as well, which means "the total frequency of occurrence of severe thunderstorm environments actually increases as a result of global warming."
The team also took a stab at connecting the dots between global warming and twisters that "occur in a subset of severe thunderstorms," noted Diffenbaugh, even though the atmospheric dynamics that produce tornadoes are too complex to resolve in the climate models used in the new study.
The models "do show an increase in the occurrence of the combination of high CAPE and high low-level shear which has been identified as a potential indicator of tornadic storms," Diffenbaugh said. "But it is important to keep in mind that we are not resolving tornadoes."
The biggest increase in the conditions conducive for severe thunderstorms occurs in the spring season and the greatest agreement among the models studied is also found during the spring, the researchers note.
"The really interesting science result is that (the increase thunderstorm conditions) ties into that one season," Harold Brooks, a meteorologist and expert on climate change and severe storms at the National Oceanic and Atmospheric Administration's National Severe Storms Laboratory in Norman, Okla., told NBC News.
Prior research may have missed the springtime uptick in storm conditions because they looked at the whole year instead of seasons, added Brooks, who was not involved with the new research.
Will the storms form?
One caveat, Brooks noted, is that even though the conditions conducive for severe thunderstorms increase in the spring as the climate warms, "there may be a lot of these environments in which a storm just never forms."
Other research, he said, has overestimated the frequency of thunderstorms in southern Texas, which is one of the regions the new study indicates will see a surge in springtime thunderstorm activity, simply because the storms never appeared even though conditions were ripe.
"If you could ever get a storm to develop in that environment, it would be a violent severe storm," Brooks said. "Things just don't initiate."
Another issue, he added, is placing the 25 to 30 percent increase in thunderstorm conditions predicted by the new research into context with natural variability, which can swing wildly. He noted that in the past three years, records were set for the most and fewest tornadoes in a 12 month period.
"How well do the models capture that current level of variability?" he said. "And then, what is the future level of variability? Does it change or does it stay the same."
Brooks as well as Diffenbaugh's team aim to dig further into these questions. One more area of potential concern from the new study, Brooks added, is the apparent decrease in summertime thunderstorm activity over the central U.S.
"The existence of severe storms in the central part of North America is tied heavily to where people live," he noted. "Because if you get severe storms, the one thing you get out of them is rain and people live where enough rain exists to grow crops."
Should summertime thunderstorm activity drop as indicated by the models, the nation's breadbasket could be dealing with more drought.
John Roach is a contributing writer for NBC News. To learn more about him, visit his website.
First published September 23 2013, 12:02 PM