James Taylor writes at Forbes:

Global Warming Taking The Wind Out Of Tornadoes

An excerpt:

To the average reader who does not feel compelled to fact-check these claims, it seems pretty clear that global warming is at least somewhat to blame for each and every tornado that touches down, destroys property, and takes human life. But the facts show just the opposite; that global warming is reducing the threat from tornadoes.
National Oceanic and Atmospheric Administration (NOAA) data, for example, show the number of strong to violent (F3-to-F5) tornadoes have been sharply declining since the mid-1970s. From 1967 through 1977 at least 30 F3-to-F5 tornadoes assaulted the U.S. each year, with an annual average of 50. Nearly 120 F3-to-F5 tornadoes struck the U.S. in 1974 alone. By comparison, merely 26 F3-to-F5 tornadoes have struck the U.S. on average each year since the turn of the century. The frequency of strong-to-violent tornadoes has been cut in half during the past 40 years.
These numbers are very important, as F3-to-F5 tornadoes produce the vast majority of damage and inflict the overwhelming majority of human casualties each year. Moreover, because modern radar technology can detect far more minimal (F1-to-F2) tornadoes than was the case in previous decades, meteorologists do not have reliable data on F1-to-F2 tornado trends. It should be noted, however, that even with modern radar technology detecting more minor tornadoes that could be detected in prior decades, the number of F2-to-F5 tornadoes (in other words, all but the very weakest of tornadoes) has similarly been declining since the mid-1970s.
A very striking aspect of the declining tornado activity is turning points when tornado activity has accelerated and declined. Global temperatures were cooling from the late 1940s through the late 1970s, yet NOAA records show strong tornadoes became more prevalent as global temperatures cooled. When global cooling turned to global warming in the late 1970s, tornado activity began its abrupt decline.