The Courier Mail had quite a good piece. If you want pics the BBC has a gallery or enter Oklahoma tornado at Google images. There are some videos collected here. The Courier Mail has a large photo gallery.
Climate Progress revisits tornadoes, extreme weather and climate change. The bottom line is that the jury is out.
In the US tornado alley warm moist air flows north from the Gulf of Mexico in the lower atmosphere and becomes unstable. Dry, cold air comes from the west over the Rockies in the upper atmosphere, with a shear effect to create the top of the column. The warm air becomes unstable and lifts. The shear effect of the upper wind spins the rising column. More air is sucked in by the spinning, rising column. That’s simplistic but those are the basic elements as I understand them.
With climate change the lower atmosphere warm air flow and instability are likely to be enhanced, but there could actually be less wind shear. We don’t know what the result of those factors will be over time, but the suggestion is that if anything there have been fewer severe tornadoes over recent decades.
Kevin Trenberth says:
Tornadoes come from thunderstorms in a wind shear environment. This occurs east of the Rockies more than anywhere else in the world. The wind shear is from southerly (SE, S or SW) flow from the Gulf overlaid by westerlies aloft that have come over the Rockies. That wind shear can be converted to rotation. The basic driver of thunderstorms is the instability in the atmosphere: warm moist air at low levels with drier air aloft. With global warming the low level air is warm and moister and there is more energy available to fuel all of these storms and increase the buoyancy of the air so that thunderstorms are strong. There is no clear research on changes in shear related to global warming. On average the low level air is 1 deg F [warmer] and 4 percent moister than in the 1970s.
There are also problems in counting. Graphs like this one look impressive:
On closer inspection the increase is mainly in the weaker events, which could be due to counting, and even if accurate do not represent a huge concern.
Increased tornado numbers tend to be associated with La Niña years.
Jeff Masters summed up in 2012:
The tornado data base going back to 1950 doesn’t show an increasing trend in strong tornadoes in recent decades. While climate change could potentially lead to an increase in tornadoes, by increasing instability, it could also decrease them, by decreasing wind shear. I’d need to see a lot more bad tornado years before blaming climate change for the severe tornado seasons of the past two years. One thing that climate change may be doing, though, is shifting the season earlier in the year. The 5-day total of tornadoes from February 28 – March 3 will probably break the record of 131 set in 1999 for the largest tornado outbreak so early in the year. Warmer winters, and an earlier arrival of spring due to a warming climate, will allow tornado season to start earlier–and end earlier.
This year the tornado season was actually delayed and the previous 12 months had seen a ‘tornado drought’.
The scale now used in the US since 2007 is the Enhanced Fujita Scale (EF). The scale is based on damage rather than wind speed, since damage provides the evidence for the strength of the event and wind speeds are not almeasured. However, that does translate into bands of wind speed. Essentially EF with winds of 320 kph or more will reduce a house to the concrete slab, so this image is relevant:
The Oklahoma tornado has been classified as EF5.
Moore has been hit by severe tornadoes four times in the last 15 years. In 1999 the winds were recorded at 486 kmh, the highest ever recorded near the earth’s surface.
There’s a simple account of the climate change issue, together with a video explaining tornadoes at The Carbon Brief.
Wikipedia has a comprehensive article on tornadoes.
I didn’t look at the denialist blogs.
While the climate change issue needs to be monitored, the more urgent issues relate to building codes, warning systems (this time about 16 minutes), availability of shelters in public venues like schools (tornadoes typically come late in the day, mercifully), rescue, retrieval, insurance, rebuilding and how the poor get on.
Of the links in the comments thread Dr Jeff Masters’ Tornadoes and Climate Change: Huge Stakes, Huge Unknowns is a ‘must read’.
A point well made is that severe thunderstorms can be just as dangerous and deadly as tornadoes.