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The Past and Future of Tornado Occurrences in the United States

Paper Reviewed
Brooks, H.E., Carbin,G.W. and Marsh, P.T. 2014. Increased variability of tornado occurrence in the United States. Science 346: 349-352.

In a paper published in Science, Brooks et al. (2014) write that "whether or not climate change has had an impact on the occurrence of tornadoes in the United States has become a question of high public and scientific interest," but they say that "changes in how tornadoes are reported have made it difficult to answer." Nevertheless, they state that the mean occurrence of well-reported aspects of U.S. tornadoes, such as "the mean annual number of all but the weakest tornadoes and their locations, has appeared to show little or no consistent trend," citing the work of Verbout et al. (2006).

On the modeling side of the issue, the three researchers report that "recent results from analysis of the higher-resolution climate simulations have suggested that the frequency of environments supportive of tornadoes will increase in a warming climate, particularly in springtime." So where does all of this leave us?

Back in the real world, Brooks et al. report that stronger tornadoes from 1954 to 2013 exhibited "much inter-annual variability but no long-term linear trend." They note, for example, that "there are more extreme months in the most recent 15 years of the database than in the first 45 years," but they say that "those extremes are essentially equally balanced between minima and maxima." In addition, they say "there is a lower probability of a day having a tornado, but if a day does have a tornado, there is a much higher chance of having many tornadoes," such that "tornadoes are 'concentrated' into a smaller number of days in more recent years."

As for their final conclusion, the three U.S. scientists indicate that they cannot offer a physical hypothesis for the increased variability in tornado strength discerned over the years involved in their study, which leads them to conclude that "how such a change would relate to the increase in global temperature, if it relates at all, is unknown at this time."

Reference
Verbout, S.M., Brooks, H.E., Leslie, L.M. and Schultz, D.M. 2006. Evolution of the U.S. tornado database: 1954-2003. Weather Forecasting 21: 86-93.

Posted 28 January 2015