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Posted at 10:00 AM ET, 06/02/2008

Freedman: Is Climate Change Twisting Tornadoes?

The 2008 tornado season is off to such an abnormally active and deadly start that even typically storm-hardy residents of tornado-prone areas of the country have begun asking: what is going on?

For example, on May 21, the New York Times ran a story that told the woeful tale of Mr. John E. Hill of Clinton, Arkansas, who has been befallen by two tornadoes this year, three months apart. One tornado destroyed his workplace in February, and another twister in May took out his house, cars and cash savings. "I don't know what this is," Mr. Hill told the paper. "I've lived in Arkansas most of my life, and I've never see this many tornadoes. They're all over the place."

Continue reading to understand the range of opinions on global warming and tornadoes. See Jason's post for this week's full forecast.

Some in the meteorological community, who are seeing the signs of a record-setting year in the works, share Mr. Hill's view that something may be amiss. They are asking the question of whether the frequency and distribution of tornadoes this year - they have occurred earlier and farther north than would be expected thus far in a normal season - have something to do with global climate change.

Stu Ostro, who is the senior director of weather communications at The Weather Channel, has noticed some odd trends in tornado climatology in recent years, with more significant fall outbreaks and wintertime tornadoes, and many twisters striking outside of traditionally tornado prone areas. Ostro said that changes in the timing and distribution of tornadoes could be part of the atmosphere's response to warming temperatures, since warming alters the availability of moisture and influences large-scale atmospheric circulation patterns.

Ostro pointed to this year's unusually violent January and February skies as evidence that human forcing may be playing a role in addition to natural variability. In total, eighty-four tornadoes were recorded in January (although this number has not yet undergone final review by NOAA), which far exceeded the three-year average of 34, and 148 struck in February, which was well above the three-year average of 25. These two winter months also were above normal when compared to the longer-term averages as well.

Add to that recent trends in fall severe weather outbreaks and Ostro says there may be a human hand in the overall patterns, although global warming cannot be pinpointed as the cause of an individual severe weather outbreak or specific tornado, just as it can't be blamed for causing a particular hurricane. However, overall trends in such storms can be evaluated for their links to climate change.

"While there's no clear indication of an increase in tornado frequency or intensity as a result of global warming, there is evidence that it's affecting the seasonality and geographic distribution of tornadoes and severe thunderstorms, with many outbreaks in recent years that have been unusually far north for the autumn or winter," Ostro said in an email conversation. "And it's logical that this would happen as the climate warms."

Ostro's Weather Channel colleague, Severe Weather Expert Greg Forbes, wrote on the station's weather blog in January that recent trends justify raising the question of whether "global warming has already begun to affect tornado and severe thunderstorm climatology."

"The rarity of tornado events makes it hard to prove that there's a trend - and to disprove that there isn't one," Forbes wrote.

Other meteorologists aren't willing to go quite as far as Ostro and Forbes, however. For example, in a Washington Post article on May 21 that was discussed on this blog, tornado researcher Howard Bluestein of the University of Oklahoma was quoted as saying that "We absolutely don't know" whether climate change is influencing tornadoes. "And I would not even . . . venture a guess," Bluestein told the paper.

According to Greg Carbin, who serves as the warning coordination meteorologist at NOAA's Storm Prediction Center in Norman, Oklahoma, it will take several more years to sort out whether climate change is altering tornado frequency and distribution.

"We saw a pretty remarkable outbreak back in February coming roughly a month ahead of when you'd normally expect something like that, but one month does not make a trend," Carbin said in a phone interview.

Carbin cautioned that while research indicates that warming may increase atmospheric instability and that "tornado alley" may be pushed northward, tornadoes need a variety of ingredients in order to form, and they all must be carefully calibrated in ways that scientists still don't fully understand. "The complex interaction of the variables make it difficult to draw any conclusions as to what we might see in a warmer world," Carbin said.

Moisture, instability, upper level disturbances, and atmospheric wind shear are some of the necessary ingredients for tornado formation. Regarding moisture, Carbin said that because increased distance from the Gulf of Mexico tends to limit the available moisture for severe thunderstorms to form, it may be difficult for tornado alley to simply shift northward. In addition, climate change may shift the area of stronger upper atmospheric winds that are vital for instigating powerful tornadoes.

Wind shear, which is the difference in wind speed or direction with increasing height, may also be in shorter supply over the U.S. if warming trends continue, Carbin said. "By all accounts the shear is going to be a real problem with a warmer continental United States," he said.

Interestingly, hurricane researchers are also wrestling with the wind shear issue, since climate change is suspected of playing a role in changing the frequency and/or intensity of hurricanes. While some shear is essential for tornado formation, it is the death knell of many incipient hurricanes, and some research indicates that shear could increase in the Atlantic Basin if the ocean and atmosphere continue to warm.

As tornado season barrels on, it may be little comfort to people like Mr. Hill of Arkansas that in time it may be possible to tie global climate change to the tornadoes that tore through his life. But if he gets struck again, whether or not is warming related, perhaps storm chasers should just start chasing him?

By  |  10:00 AM ET, 06/02/2008

Categories:  Climate Change

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