In a major review of "temporal fluctuations in weather and climate extremes that cause economic and human health impacts" -- as they titled their study of the subject -- Kunkel et al. (1999) analyzed empirical data related to historical trends of several different types of extreme weather events and their societal impacts. This work revealed, in their words, that "most measures of the economic impacts of weather and climate extremes over the past several decades reveal increasing losses." However, they found that "trends in most related weather and climate extremes do not show comparable increases with time," suggesting that "increasing losses are primarily due to increasing vulnerability arising from a variety of societal changes, including a growing population in higher risk coastal areas and large cities, more property subject to damage, and lifestyle and demographic changes subjecting lives and property to greater exposure."

With respect to hurricane losses (which have shown a steady increase with time), the three researchers stated that "when changes in population, inflation, and wealth are considered, there is instead a downward trend [italics added]." They also found that "increasing property losses due to thunderstorm-related phenomena (winds, hail, tornadoes) are explained entirely [italics added] by changes in societal factors." In addition, they say there has been "no apparent trend in climatic drought frequency" and "no evidence of changes in the frequency of intense heat or cold waves."

Balling and Cerveny (2003) also reviewed the scientific literature to determine what had been learned about severe storms in the United States during the modern era of greenhouse gas buildup in the atmosphere, paying particular attention to thunderstorms, hail events, intense precipitation, tornadoes, hurricanes and winter storm activity. In doing so, they found that several scientists had identified an increase in heavy precipitation, but that "in other severe storm categories, the trends are downward [italics added]," which, of course, is just the opposite of what climate alarmists contend is the case.

Noting that "media reports in recent years have left the public with the distinct impression that global warming has resulted, and continues to result, in changes in the frequencies and intensities of severe weather events," which implied changes are mostly for the worse, Hage (2003) attempted to get to the truth of the matter by using "previously unexploited written resources such as daily and weekly newspapers and community histories" to establish a database adequate for determining long-term trends of all destructive windstorms (primarily thunderstorm-based tornadoes and downbursts) in the prairie provinces of Alberta and Saskatchewan in western Canada over the period 1882 to 2001. And because "sampling of small-scale events such as destructive windstorms in the prairie provinces of Canada depends strongly on the human influences of time and space changes in rural settlement patterns," Hage says that "extensive use was made of Statistics Canada data on farm numbers by census years and census areas, and on farm sizes by census years in attempts to correct for sampling errors." The results of these operations were stated quite simply: "all intense storms showed no discernible changes in frequency after 1940," while prior to that time they had actually exhibited minor maxima.

Changnon (2003) investigated trends in both severe weather events and changes in societal and economic factors over the last half of the 20th century in the United States. In doing so, he found that trends in various weather extremes were mixed, noting that "one trend is upwards (heavy rains-floods), others are downward (hail, hurricanes, tornadoes, and severe thunderstorms), and others are unchanging flat trends (winter storms and wind storms)." It should noted, however, that had the analysis of heavy rains-floods been extended back to the beginning of the 20th century, the longer-term behavior of this phenomenon would have been found to be indicative of no net change over the past hundred years, as demonstrated by Kunkel (2003).

So why did insurance losses rise so rapidly over the past several decades? Changnon reports that "the primary reason for the large losses [was] a series of societal shifts (demographic movements, increasing wealth, poor construction practices, population growth, etc.) that collectively had increased society's vulnerability." When properly adjusted for societal and economic trends over the past half-century, therefore, monetary loss values associated with damages inflicted by extreme weather events, as Changnon describes the situation, "do not exhibit an upward trend." Consequently, as he emphasizes, "the adjusted loss values for these extremes [do] not indicate a shift due to global warming." And to make this point perfectly clear, he reiterates that these real-world observations "do not fit the predictions, based on GCM simulations under a warmer world resulting from increased CO2 levels, that call for weather extremes and storms to increase in frequency and intensity."

Khandekar (2003) briefly reviewed what he had learned about extreme weather events in Canada in the course of conducting a study of the subject for the government of Alberta. In doing so, he noted how his research had led him to conclude that "extreme weather events such as heat waves, rain storms, tornadoes, winter blizzards, etc., [were] not increasing anywhere in Canada at [that] time." In addition, he noted that a recent special issue of Natural Hazards (Vol. 29, No. 2) had concluded much the same thing about other parts of the world; and in this context he cited a survey article by Robert Balling that concluded "there is no significant increase in overall severe storm activity (hurricanes, thunderstorms/tornadoes, winter blizzards) across the conterminous United States," as well as an article by Stanley Changnon, which concluded that "increasing economic loss due to weather extremes in the conterminous United States is a result of societal change and not global warming."

Focusing solely on tornados, Daoust (2003) catalogued daily tornado frequencies for each county of Missouri (USA) for the period 1950-2002, after which he transformed the results into monthly time series of tornado days for each of the state's 115 counties, its six climatic divisions, and the entire state. This work revealed the presence of positive trends in tornado-day time series for five of the six climatic divisions of Missouri; however, none of these trends was statistically significant. For the sixth climatic division, on the other hand, the trend was significant; but it was negative. Most importantly, at the level of the entire state, Daoust reported that "for the last 53 years, no long-term trend in tornado days can be found."

Last of all, Diffenbaugh et al. (2008) briefly reviewed what is known about responses of U.S. tornadoes to rising temperatures. On the theoretical side of the issue, they indicate there are competing ideas with regard to whether tornadoes might become more or less frequent and/or severe as the planet warms. On the observational side, there is also much uncertainty about the matter. They write, for example, that "the number of tornadoes reported in the United States per year has been increasing steadily (~14 per year) over the past half century." However, they say that "determining whether this is a robust trend in tornado occurrence is difficult," because "the historical record is both relatively short and non-uniform in space and time." In addition, the increase in yearly tornado numbers runs parallel with the concurrent increase in the country's population, which makes for better geographical coverage and more complete (i.e., numerous) observations.

On the other hand, the three researchers report that the number of tornadoes classified as the most damaging (F2-F5 on the Fujita scale) may well have truly decreased over the past five decades (1954-2003), as their frequency of occurrence actually runs counter to the trend of the country's population. In fact, the graphs they present show yearly F2-F5 tornado numbers in the latter half of the record period dropping to only about half of what they were during the first half of the record, while corresponding data from the U.S. Southern Great Plains show damaging tornado numbers dropping to only about a third of what they were initially. Nevertheless, Diffenbaugh et al. considered the question posed in the title of their paper -- Does global warming influence tornado activity? -- to be unresolved, stating that "determining the actual background occurrence and trend in tornado activity over recent decades will certainly require further development of other analysis approaches."

In conclusion, therefore, and in spite of all of the media hype surrounding the rising economic impact of each new year's weather extremes, there is really nothing unusual about the weather extremes themselves, including tornados. Consequently, since the earth did indeed warm significantly over the 20th century, it must be concluded, on the basis of this empirical evidence, that global warming does not bring about an increase in extreme weather events. Like "the emperor's new clothes," which some pretend to see, they're just not there.

References
Balling Jr., R.C. and Cerveny, R.S. 2003. Compilation and discussion of trends in severe storms in the United States: Popular perception vs. climate reality. Natural Hazards 29: 103-112.

Changnon, S.A. 2003. Shifting economic impacts from weather extremes in the United States: A result of societal changes, not global warming. Natural Hazards 29: 273-290.

Daoust, M. 2003. An analysis of tornado days in Missouri for the period 1950-2002. Physical Geography 24: 467-487.

Diffenbaugh, N.S., Trapp, R.J. and Brooks, H. 2008. Does global warming influence tornado activity? EOS, Transactions, American Geophysical Union 89: 553-554.

Hage, K. 2003. On destructive Canadian prairie windstorms and severe winters. Natural Hazards 29: 207-228.

Khandekar, L. 2003. Comment on WMO statement on extreme weather events. EOS, Transactions, American Geophysical Union 84: 428.

Kunkel, K.E. 2003. North American trends in extreme precipitation. Natural Hazards 29: 291-305.

Kunkel, K.E., Pielke Jr., R.A. and Changnon, S.A. 1999. Temporal fluctuations in weather and climate extremes that cause economic and human health impacts: A review. Bulletin of the American Meteorological Society 80: 1077-1098.