The Heat Is Online

PNAS: More Tornadoes Likely (Dec. 2007)

Changes in severe thunderstorm environment frequency during the 21st century caused by anthropogenically enhanced global radiative forcing


Proceedings of the National Academy of Sciences, December 4, 2007


Robert J. Trapp*, , Noah S. Diffenbaugh*, Harold E. Brooks , Michael E. Baldwin*, Eric D. Robinson*, and Jeremy S. Pal


*Department of Earth and Atmospheric Sciences, and Purdue Climate Change Research Center, Purdue University, 550 Stadium Mall Drive, West Lafayette, IN 47907; National Severe Storms Laboratory, National Oceanic and Atmospheric Administration, 120 David L. Boren Boulevard, Norman, OK 73072; Department of Civil Engineering, Loyola Marymount University, 1 LMU Drive, Los Angeles, CA 90045; and Abdus Salam International Centre for Theoretical Physics, Strada Costiera 11, 34014 Trieste, Italy


Edited by Kerry A. Emanuel, Massachusetts Institute of Technology, Cambridge, MA, and approved October 25, 2007 (received for review June 13, 2007)


Severe thunderstorms comprise an extreme class of deep convective clouds and produce high-impact weather such as destructive surface winds, hail, and tornadoes. This study addresses the question of how severe thunderstorm frequency in the United States might change because of enhanced global radiative forcing associated with elevated greenhouse gas concentrations. We use global climate models and a high-resolution regional climate model to examine the larger-scale (or "environmental") meteorological conditions that foster severe thunderstorm formation. Across this model suite, we find a net increase during the late 21st century in the number of days in which these severe thunderstorm environmental conditions (NDSEV) occur. Attributed primarily to increases in atmospheric water vapor within the planetary boundary layer, the largest increases in NDSEV are shown during the summer season, in proximity to the Gulf of Mexico and Atlantic coastal regions. For example, this analysis suggests a future increase in NDSEV of 100% or more in locations such as Atlanta, GA, and New York, NY. Any direct application of these results to the frequency of actual storms also must consider the storm initiation.


Freely available online through the PNAS open access option.

Author contributions: R.J.T. and H.E.B. designed research; R.J.T., N.S.D., and J.S.P. performed research; R.J.T., N.S.D., M.E.B., and E.D.R. analyzed data; and R.J.T. and N.S.D. wrote the paper.

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.


|| Based on a compilation of data from the U.S. National Climatic Data Center and the U.S. National Weather Service Office of Climate, Water, and Weather Services


( from 2000 to 2004, these are 5-year, nonadjusted means that include the fatalities, injuries, and damage inflicted by lightning, tornadoes, severe wind, and hail.


** Based on the same U.S. data as above, these are 5-year, nonadjusted means that include the fatalities, injuries, and damage inflicted by tropical cyclones. Note that this averaging period excludes Hurricane Katrina and others during the 2005 season.


Santer B (2005) The IPCC Historical Forcing Runs: PCMDI Analyses of an Ensemble of Opportunity, 10th Annual CCSM Workshop, June 2123, 2005, Breckenridge, CO.

To whom correspondence should be addressed. Department of Earth and Atmospheric Sciences, Purdue University, 550 Stadium Mall Drive, West Lafayette, IN 47906. E-mail:


© 2007 by The National Academy of Sciences of the USA