Bill McKibben credits climate change for the recent series of tornadoes decimating the Mid-West. This might be climate change related. On the other hand, it might just be random. I don't have the background to answer this question definitively, but nerdy curiosity led me to gather some data from the Tornado History Project.
THP keeps track of every single recorded tornado in the United States since 1950: where it was, its intensity, and even the path it took. I gathered tornado data from the past sixty years to see if the number of tornadoes have been trending upward.
"But wait!" you might say, "What if the number of tornadoes is the same but they are getting more intense? You can't just look at the number of tornadoes." Indeed I cannot. The way THP records tornado intensity is with the Fujita scale, ranging from F0 (min 40 mph) and F5 (min 261 mph). So I created an alternative time series of total tornadoes weighted by intensity. Admittedly, this approach is very simple but sufficient. Each F0 counts as one tornado; F1s each count as 2 two tornadoes, etc. Then I added them together. For example, in 1955 there were 116 F0s, 200 F1s, 163 F2s, 29 F3s, 8 F4s, and 2 F5s, or 518 total tornadoes and 1,173 weighted tornadoes. Below is the graph Excel spit out for me:
There's a pretty clear upward trend in that diagram, consistent with concerns over global warming. BUT this is assuming that we're just as good detecting tornadoes today as we were in the 1950s. If an F5 came through, I doubt that would get past anyone's radar regardless of where it touched down. But an F1 or F0 (i.e. the ones that are barely tornadoes) could slip past undetected without satellite imagery or other modern equipment. So what do the graphs look like without the small tornadoes? Much less frightening. Here's total tornadoes only counting at various thresholds of intensity:
And here's weighted tornadoes with the same thresholds:
The F0s seems to be doing most of the work, which does not fit with the global warming story. For completeness, here's the F0s over time. Most of the jump occurs in the early 1990s; I'm not sure why.
Again, this doesn't mean that the GW explanation isn't true, but it doesn't suggest it either. This data tells us that tornado activity varies widely. The ten-year low for all intensities is 934 tornadoes (2002) and the high is almost twice that: 1817 (2004, just two years later!).