The previous two posts have discussed the occurrence of tornadoes in Missouri and surrounding states, as well as national and Missouri trends in tornado frequency. This post concerns the most violent and destructive tornadoes, the F 3-5 tornadoes.
The amount of destruction a tornado causes depends not only on the strength of the tornado, but also on where it occurs. If it occurs in unpopulated, undeveloped land, the destruction may be less. If it occurs in a heavily populated area, and it if catches people unprepared, the destruction and loss of life may be greater. The last 50+ years have seen a significant increase in population, and a significant expansion of the amount of developed land. Thus, tornadoes are more likely to strike populated and developed areas.
The graph at right, copied from the U.S. Tornado Climatology web page of the National Climatic Data Center shows the annual number of strong to violent tornadoes in the United States. As has been the case with all of the annual tornado data we have looked at so far, this data varies greatly from year-to-year. However, it seems pretty clear that there is no overall trend toward increasing frequency. In fact, if anything, the trend has been towards decreased frequency – there are fewer highly destructive tornadoes now than there were during the mid-1960s. Who’da thunk it!
Some may take this as a disconfirmation of the theory of global warming. It turns out, however, that an increase in the number of tornadoes was not forecast by the IPCC Fourth Assessment Report or in the most recent two assessments of Global Climate Change Impacts in the United States. These documents all conclude that it is “…difficult to know if and how such events have changed as climate has warmed, and how they might change in the future.” (Global Climate Change Impacts in the United States, Thomas R. Karl, Jerry M. Melillo, and Thomas C. Peterson, (eds.). Cambridge University Press, 2009, p. 11.)
The confusion may arise because these documents did forecast an increase in some kinds of extreme weather events (the intensity of hurricanes, for instance). While all kinds of extreme weather may seem similar, a tornado is a very unique and specific weather phenomenon that may or may not be related to other kinds of extreme weather.
Thus, in summary, the total number of tornadoes reported annually has increased, but almost entirely from an increase in the number of small, weak tornadoes reported. If you eliminate the F-0 tornadoes from the count, the trend is basically flat. And the trend in the most powerful tornadoes seems to be towards decreased frequency.
U.S. Tornado Climatology, National Climate Data Center, http://www.ncdc.noaa.gov/oa/climate/severeweather/tornadoes.html#history.
Global Climate Change Impacts in the United States, Thomas R. Karl, Jerry M. Melillo, and Thomas C. Peterson, (eds.). Cambridge University Press, 2009.
IPCC, 2007: Summary for Policymakers. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M.Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA..