Is Climate Change Causing More Powerful Tornadoes?

April 25, 2014; 4:00 AM ET
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Cleanup crews work in the aftermath of the tornado that struck Moore, Okla., last year. See larger image below. (AP Photo/Sue Ogrocki)

The extreme weather, including the notable outbreaks in El Reno, Okla.; Moore, Okla.; and Washington, Ind.; last year forced the debate of the impact of climate change on severe weather to resurface.

While there has been research into the subject, there are still many unknowns.

Is a Warming Climate Causing More Active Severe Weather Already?
Before examining how climate change may affect severe weather in the future, it is important to analyze whether the frequency or strength of severe weather has changed already with rising temperatures.

There is no strong evidence to support severe weather becoming stronger, more frequent or more widespread during the past 50 years in the United States. One of the reasons that the change in severe weather is hard to track is the fact that the reporting systems have changed so much over time.

David Cabral, of Demolition and Debris Removal, helps to clear a lot in Moore, Okla., Wednesday, June 19, 2013. City officials said that more than 56,000 tons of debris have been removed from the 19 neighborhoods in Moore that were affected by the deadly tornado that ripped through the area last year. (AP Photo/Sue Ogrocki)

Ted Fujita developed the Fujita Scale, which measures the intensity of tornadoes by examining damage and estimated winds, in 1971. Meteorologists did not start rating twisters using the Fujita Scale until 1973. The National Oceanic and Atmospheric Administration (NOAA) went back through tornado reports from 1950 through 1972, retroactively rating tornadoes based on the damage information provided in the reports.

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"Our best effort is to try to take care of those changes in reporting and indicate that we really haven't seen any changes [in severe weather]," Harold Brooks, research meteorologist at NOAA's National Severe Storms Laboratory in Norman, Okla., said.

However, with advancements in technology and social media, news of extreme and severe weather spread very quickly to all corners of the U.S.

An SUV was tossed onto a tree in Tushka, Okla., after a 2011 tornado outbreak.

"The big reason why we think that severe weather has gotten worse is our ability to communicate information about it. If you think back 100 years ago, a tornado that happened 10 or 20 miles away, you might not even be aware of it, if it didn't affect where you live directly. Now, you can watch people chasing tornadoes online live," Brooks said. "So it's the fact that we are more aware and able to communicate that information about events so much better than we used to be able to that it makes us think severe weather has increased."

Future Climate Change Impacts on Severe Weather
There are still many limits to our knowledge of how severe weather may change over time, but some preliminary conclusions can be made from research so far.

"As the planet warms with more greenhouse gases, we really don't have very strong evidence as to what will happen with severe thunderstorms," Brooks said.

Key ingredients for severe thunderstorms include warm, moist air to fuel thunderstorm initiation and growth and winds that change with altitude, or wind shear, to help organize a thunderstorm and create rotation. Big changes of wind with height, or high wind shear, are especially important for tornado and hail formation.

Global Land-Ocean Temperature Index Graph from NASA's Goddard Institute for Space Studies.

"As the planet warms, the moisture content of the atmosphere will also increase. And that's the basic fuel that drives thunderstorms. It's where the storms get their energy from... as we warm the planet that will increase the energy available for producing storms," Brooks said. "The other primary ingredient, the shear that organizes the storm, is likely going to decrease."

The wind shear will likely decrease due to a lower temperature contrast from pole to pole.

Since one major factor favors a more conducive environment for severe thunderstorms to spark with a warming climate and another is less conducive for severe thunderstorm organization, it is very difficult to determine how severe weather will change in the future.

"We may see a shift toward non-tornadic wind storms in the future, but that's still a preliminary result," Brooks added. Straight-line winds may increase since high wind shear is not as much of an influence, while the frequency and strength of tornadoes may not change very much.

It is difficult to conclude confidently whether the regions that get the most severe weather and tornadoes will shift as the climate warms.

"The balance of the ingredients that we need [for severe weather and tornadoes]... are tied to really large-scale features on the planet," Brooks said. "Like the presence of the Rocky Mountains and the Gulf of Mexico. Since they aren't going to move in the future, the regions where tornadoes occur are likely to be tied to their relationship to those two features."

How Do We Research Severe Weather in the Future?
Researchers use a collaboration of climate models and basic concepts to examine the potential frequency and strength of severe weather in the future.

"Our primary understanding of what will happen in the future with severe weather is actually based on our current understanding... Then we look in the future from climate models and from basic physical understanding of how the atmosphere works to understand how those basic environments will change," Brooks said. "So, we essentially try to approach the problem of future climate changes based on thinking about them as the day-to-day weather forecaster."

There are still limits to research on severe weather changing with a warming climate due to the fact that the exact balance of changing thunderstorm ingredients is an unknown.

Furthermore, there has been limited research on how inhibiting factors of thunderstorms would change due to climate change.


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