There have beenmany discussions amongst experts and on the press about the relationship between the high US tornado frequency of 2011 and climate change. Many climate change proponents, Al Gore among them, have attributed the trend to climate change, and there is a constant tendency of attributing disasters to climate change. Others have denied this relationship, as with hurricanes for example. The debate is discussed here, here and here.
With so much confusion, discussion and opinion it is worth running the numbers to sort things out. Here is a neat post with some numbers where you can draw conclusions for yourself.