Global Warming and Tornadoes
Diffenbaugh, N.S., Trapp, R.J. and Brooks, H. 2008. Does global warming influence tornado activity? EOS, Transactions, American Geophysical Union 89: 553-554.
On the theoretical side of the issue, Diffenbaugh et al. indicate there are competing ideas with regard to whether tornadoes might become more or less frequent and/or severe as the planet warms. On the observational side, there is also much uncertainty about the matter. They write, for example, that "the number of tornadoes reported in the United States per year has been increasing steadily (~14 per year) over the past half century." However, they say "determining whether this is a robust trend in tornado occurrence is difficult," because "the historical record is both relatively short and non-uniform in space and time." In addition, the increase in yearly tornado numbers runs parallel with the concurrent increase in the country's population, which makes for just that much better geographical coverage and more complete (i.e., numerous) observations.
On the other hand, the three researchers report that the number of tornadoes classified as the most damaging (F2-F5 on the Fujita scale) may well have truly decreased over the past five decades (1954-2003), as their frequency of occurrence actually runs counter to the trend of the country's population. In fact, the graphs they present show yearly F2-F5 tornado numbers in the latter half of the record period dropping to only about a half of what they were during its first half, while corresponding data from the U.S. Southern Great Plains show damaging tornado numbers dropping to only about a third of what they were initially. Nevertheless, when all is said and done, Diffenbaugh et al. consider the question posed in the title of their paper to be unresolved, stating that "determining the actual background occurrence and trend in tornado activity over recent decades will certainly require further development of other analysis approaches."