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Want to blame climate change for killer twisters? Not so fast

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Climate Cast
Every Thursday, MPR meteorologist Paul Huttner joins The Daily Circuit to talk about the latest research on our changing climate and the consequences we're seeing here in Minnesota and worldwide.
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Tempting as it is to chalk up a severe weather event to climate change, the killer tornado that hit Oklahoma has no clear link to global warming, according to the chief meteorologist for MPR News.

"To be honest, we still don't know the triggering mechanism" for tornadoes, Paul Huttner said on Thursday's edition of Climate Cast. And concerning the relationship between tornadoes and climate change generally, he said, "we don't know a lot, and the trends don't seem to show much. There's really no clear connection between an increase in the most violent tornadoes, these EF4 and EF5 monsters that we saw in Moore, and climate change. If you look at the trends from 1950 on, or even back before that, you don't see a real uptick in these most violent tornadoes."

  Even though social media were quick to see a link, "It's one of those things we can't connect the dots on," he said. "There doesn't seem to be any conclusive evidence that it's changing."

Kerri Miller, host of The Daily Circuit, asked whether it was true that climate change is producing more humid, unstable air, one of the conditions that promote tornado activity. "Might there be a connection there?" she asked. 

Oklahoma tornado
A woman carries her child through a field near the collapsed Plaza Towers Elementary School in Moore, Okla., Monday, May 20, 2013. A tornado as much as a mile (1.6 kilometers) wide with winds up to 200 mph roared through the Oklahoma City suburbs Monday, flattening entire neighborhoods, setting buildings on fire and landing a direct blow on an elementary school.
AP Photo/Sue Ogrocki

"There are two things going on that you really need to create tornadoes," Huttner replied. "One is, you need that warm, humid, moist air mass to generate thunderstorms. And with the climate change we're seeing, warmer temperatures and more water vapor in the atmosphere, that is happening. So as you model this out ... it looks like we are seeing an increase in thunderstorms.

"The second thing you need is wind shear. You need these rotating, spinning winds with height to create these rotating, super-cell thunderstorms, these tornadic super-cells, that produce tornadoes. And the other part of climate change is, as we reduce the contrast between the poles and the Equator, we're getting less wind shear. So they appear to be counteracting each other, and overall it seems to look like a wash in terms of more tornadoes or more violent tornadoes."

If we're seeing more urban tornadoes, he said, it may be because we have more urban development. Huttner pointed out that as urban areas spread into what used to be farmland — as is happening in the area around Oklahoma City, including suburban Moore — tornadoes that have been hitting the region all along are now hitting cities that weren't there before. 

"It's worth noting that if there is an epicenter in Tornado Alley, Moore, Okla., has to be it. It's had three EF4, EF5 tornadoes in the past 14 years. This one looks like over $2 billion in damages, could be the third-costliest in U.S. history."

According to the Climate Central website, Oklahoma City is the one most likely American city to be hit by a tornado.

"Based on data from 1982-2011, Oklahoma City was the likeliest spot in the country for seeing severe thunderstorms on May 20," says Climate Central. "Tornado statistics show that the Oklahoma City metro area has had the most direct tornado hits of any American city, with at least 100 since 1890. That's according to the Storm Prediction Center in Norman, Okla., which is situated just down the road from Moore, and whose forecasters were forced to take shelter as the storm moved through."