Tornadic thunderstorms do not require tropical-type warmth. In fact, tornadoes are almost unheard of in the tropics, despite frequent thunderstorm activity.
Instead, tornadoes require strong wind shear (wind speed and direction changing rapidly with height in the lower atmosphere), the kind which develops when cold and warm air masses “collide”....
It is well known that strong to violent tornado activity in the U.S. has decreased markedly since statistics began in the 1950s, which has also been a period of average warming. So, if anything, global warming causes FEWER tornado outbreaks…not more. In other words, more violent tornadoes would, if anything, be a sign of “global cooling”, not “global warming”....
But then there's this: Global Warming Will Bring Violent Storms And Tornadoes, NASA Predicts:
NASA scientists have developed a new climate model that indicates that the most violent severe storms and tornadoes may become more common as Earth's climate warms....
The model developed at NASA's Goddard Institute for Space Studies by researchers Tony Del Genio, Mao-Sung Yao, and Jeff Jonas is the first to successfully simulate the observed difference in strength between land and ocean storms and is the first to estimate how the strength will change in a warming climate, including "severe thunderstorms" that also occur with significant wind shear and produce damaging winds at the ground.
This information can be derived from the temperatures and humidities predicted by a climate computer model, according to the new study published on August 17 in the American Geophysical Union's Geophysical Research Letters. It predicts that in a warmer climate, stronger and more severe storms can be expected, but with fewer storms overall.
(This paper won the "GISS Best Publication" award in 2007.) From their abstract:
For the central-eastern United States, stronger updrafts combined with weaker wind shear suggest little change in severe storm occurrence with warming, but the most severe storms occur more often.
But Steve Goddard points out this NOAA data (though I doubt the authors are "delusional"), which indicates fewer strong+ tornadoes in recent decades:
though the chart seems to be for the U.S. as a whole and not just the central-eastern U.S., which the paper talks about (is there a substantial difference?). But NOAA also gives this chart for tornadoes of all types, from their 2010 Annual Report on Tornadoes:
But the Washington Post weather blog says this may be due to more measurements:
There is no clear indication that severe thunderstorms and tornadoes have become more common due to climate change, in part because of major limitations in relying on the historical record of severe weather reports. While the number of tornadoes recorded in the U.S. has just about doubled during the past 50 years, the number of strong tornadoes (EF2 and above) has actually been decreasing. It may be the case that more tornadoes are being noticed today, given a network of trained storm spotters and a national Doppler radar network, both of which didn’t exist as recently as the early 1980s.
and an author of this 2008 EOS paper (from the AGU) on tornado statistics says we just don't know:
“The changes in reporting practices make it impossible to tell anything about frequency and strength changes” of tornadoes to date.
which is basically what the IPCC says that I quoted the other day.
More (via AFP): "If you look at the past 60 years of data, the number of tornadoes is increasing significantly, but it's agreed upon by the tornado community that it's not a real increase," said Grady Dixon, assistant professor of meteorology and climatology at Mississippi State University.