The complicated science of tornadoes and climate

4-27-11 Tornado Tuscaloosa, Al from Crimson Tide Productions on Vimeo.

Is the number of tornadoes that happens every year going up? And, if so, is it a climate change thing? A natural variation? A little of both? Over at Time, Ecocentric writer Bryan Walsh digs into the complicated reality behind the inevitable questions.

What we really want to know is: did climate change play a role in the monster tornadoes of April, and will a warming world see more destructive cyclones like these? (I know this because my editor, as he's wont to do after major weather events, walked down to my office and asked me, "Bryan, does climate change play a role in this?")

Scientists really don't know. It's true that the average number of April tornadoes has steadily increased from 74 a year in the 1950s to 163 a year in the 2000s. But most of that increase, as A.G. Sulzberger reports in the New York Times, comes from the least powerful tornadoes, the ones that touch down briefly without causing much damage. Those are exactly the kind of tornadoes that would have been missed by meteorologists in the days before the Weather Channel and Doppler radar--scientists today would almost never miss an actual tornado touchdown, no matter how brief or weak. That makes it very difficult for researchers to even be sure that the actual number of tornadoes is on the rise, let alone, if they are, what might be causing it. The number of severe tornadoes per year has actually been dropping over time.

It is true, however, that as the climate warms, more moisture will evaporate into the atmosphere. Warmer temperatures and more moisture will give storm systems that much more energy to play with, like adding nitroglycerin to the atmosphere. This month's possibly record-breaking tornadoes are due in part to an unusually warm Gulf of Mexico, where as Freedman reports, water surface temperatures are 1 to 2.5 C above the norm. The Gulf feeds moisture northward to storm systems as they move across the country, and that warm moist air from the south meeting cool, dry air from the Plains often results in some powerful weather. But at the same time, other studies have forecast that warmer temperatures will reduce the wind shear necessary to turn a routine thunderstorm into a powerful system that can give birth to tornadoes. So in a hotter world we could see more frequent destructive thunderstorms, but fewer tornadoes--although some researchers think we could still end up with both.

What does it all mean? For one thing, we should remember that tornadoes--and other severe weather systems--have always been with us, and almost certainly will be, whatever happens to greenhouse gas emissions and temperatures in the years to come.

If you read the full story, there's also some great information about the role prediction and warning systems have played in lowering the death toll of tornadoes. Just as with earthquakes and tsunamis, using science and public systems to make sure people know what's coming is an important part of keeping people safe, no matter the circumstances.

P.S.: The video above was filmed by Christopher England, a University of Alabama employee. In an interview with a local CBS news affiliate he tells the heart-stopping story of how the video was shot. England, it seems, was with several coworkers in a storm shelter, but the power was out and nobody was sure what was going on. He went upstairs, to his third floor office to look out a window. This video is what he saw.