Report

Increased variability of tornado occurrence in the United States

Science  17 Oct 2014:
Vol. 346, Issue 6207, pp. 349-352
DOI: 10.1126/science.1257460

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Abstract

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 changes in how tornadoes are reported have made it difficult to answer it convincingly. We show that, excluding the weakest tornadoes, the mean annual number of tornadoes has remained relatively constant, but their variability of occurrence has increased since the 1970s. This is due to a decrease in the number of days per year with tornadoes combined with an increase in days with many tornadoes, leading to greater variability on annual and monthly time scales and changes in the timing of the start of the tornado season.

Tornadoes clustering in greater numbers

Will global warming cause more tornadoes? If so, that has not happened yet. Brooks et al. compiled data on the occurrence of tornadoes in the United States between 1954 and 2013 to determine if and how tornado numbers have changed. Although the authors saw no clear trend in the annual number of tornadoes, they did see more clusters of tornadoes since the 1970s. In other words, there has been a decrease in the number of days per year with tornadoes but an increase in the number of days with multiple tornadoes. Why this clustering effect has occurred is not clear.

Science, this issue p. 349

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