..."not much," I'm afraid, is the answer to the question in the title of this post. In light of the recent, surprising tornado disasters in the South, I've done my latest Daily Green "Storm Pundit" item on the subject, and concluded the following: Given the data difficulties when it comes to studying such small scale and short lived phenomena as tornadoes, it is of course very hard to detect trends. However, there are theoretical reasons for expecting a change--more intense tornadoes, but perhaps less of them. Is that cautious and unprovocative enough for you? See here for more.