John Christy on US-tornadoes

Observational evidence for changes in small-scale severe weather phenomena (such as tornadoes, hail and thunderstorms) is mostly local and too scattered to draw general conclusions; increases in many areas arise because of increased public awareness and improved efforts to collect reports of these phenomena.

Evidence for changes in the number or intensity of tornadoes relies entirely on local reports. In the USA, databases for tornado reporting are well established, although changes in procedures for evaluating the intensity of tornadoes introduced significant discontinuities in the record. In particular, the apparent decrease in strong tornadoes in the USA from the early period of the official record (1950s–1970s) to the more recent period is, in large part, a result of the way damage from the earlier events was evaluated. Trapp et al. (2005) also questioned the completeness of the tornado record and argued that about 12% of squall-line tornadoes remain unreported. In many European countries, the number of tornado reports has increased considerably over the last decade (Snow, 2003), leading to a much higher estimate of tornado activity (Dotzek, 2003). Bissolli et al. (2007) showed that the increase in Germany between 1950 and 2003 mainly concerns weak tornadoes (F0 and F1 on the Fujita scale), thus paralleling the evolution of tornado reports in the USA after 1950 (see, e.g., Dotzek et al., 2005) and making it likely that the increase in reports in Europe is at least dominated (if not solely caused) by enhanced detection and reporting efficiency.

This open letter was written by six, leading tornado experts from research institutions across the United States. Their brief bios follow below. The authors contributed this article to LiveScience's Expert Voices: Op-Ed & Insights.

Already this year, nearly 1,200 tornadoes have crisscrossed Tornado Alley, killing nearly 500 people and leaving thousands more homeless. Meanwhile, twice as much rain fell in several states in the Ohio Valley this April than during any other April on record. It produced extreme floods in May, swelling the Mississippi River to a record depth of 60 feet.

Many people are wondering: Is climate change to blame for the extreme weather?

Several studies have investigated the relationship between global warming and the vicious tornado outbreaks. They've all given the same answer: "The data is inconclusive."