I didn't realize I was going to have the cover story of the latest New Scientist with this in-depth article I did about the climate-tornado relationship. Essentially, the bottom line is this--it's even more complicated than the climate-hurricane relationship. And so for all those politicians, environmentalists, and bloggers out there who want to use tornadoes (and especially this extremely active U.S. tornado year) as an excuse to talk about global warming...well, the science provides a slender foundation for them indeed. You can't read the full New Scientist article online, but let me lay out the core scientific reason that we can't say much about tornadoes right now. The data are way, way too bad to detect reliable trends, and as for the theory/modeling...well, that's not far enough along either. Scientists are just starting to be able to use climate models to predict environmental changes that could favor severe thunderstorms (which generate tornadoes). But because not all severe thunderstorms are tornadic, and because tornadoes are far too tiny to capture in models, and because the reasons for their genesis are incompletely understood, this line of inquiry can't really go any farther right now--at least not yet. Here's the relevant excerpt from the piece. Unfortunately, you really need more context than this brief slice, but at least the core scientific points are there:
What about tornadoes? Del Genio says it is "plausible" that their numbers or other attributes could change as well, but adds that his study couldn't directly predict this. "Telling the difference between severe storms in general, and the particular ones that produce tornadoes, that's the most difficult thing to do," he adds. A similar conclusion emerged from the second study, done by Jeff Trapp of Purdue University, Indiana in collaboration with Brooks and others. They also found that global warming should increase CAPE but decrease overall wind shear. Higher CAPE more than makes up for lower wind shear, though: they predict that by the end of the century there will be an increase in the number of days in the US when conditions favour severe thunderstorms. For Trapp, as for Del Genio, there is still not enough information to make a firm prediction. "What we did is sort of the low hanging fruit," Trapp says. "Trying to separate out this frequency of severe thunderstorms from the frequency of tornadoes explicitly is much higher in the tree."
So, environmentalists be warned...
