Another bit of alarmist nonsense gets flushed. Violent (F3-F5) tornadoes peaked during the ice age scare of the 1970s, and have been declining ever since. The graph below plots the ten year running mean in red.
The next graph shows the ten year running mean of violent tornadoes vs. the GISS US anomaly. Warmer years have fewer tornadoes.
A warming planet means less violent weather. Which means we are in trouble, because temperatures are dropping precipitously. The spring of 2008 was very cold in the US, and it had a lot of tornadoes.