NCPA - National Center for Policy Analysis


July 26, 2007

Some scientists, journalists and activists see a direct link between the post-1995 upswing in Atlantic hurricanes and global warming brought on by human-induced greenhouse gas increases.  This belief, however, is unsupported by long-term Atlantic and global observations, says William M. Gray, professor emeritus in the Department of Atmospheric Science at Colorado State University and a research fellow at the Independent Institute.

Consider, for example, the intensity of U.S. land-falling hurricanes over time -- keeping in mind that the periods must be long enough to reveal long-term trends:

  • During the most recent 50-year period, 1957 to 2006, 83 hurricanes hit the United States, 34 of them major.
  • In contrast, during the 50-year period from 1900 to 1949, 101 hurricanes (22 percent more) made U.S. landfall, including 39 (or 15 percent more) major hurricanes.

The hypothesis that increasing carbon dioxide in the atmosphere increases the number of hurricanes fails by an even wider margin when we compare two other multi-decade periods: 1925-1965 and 1966-2006.

  • In the 41 years from 1925-1965, there were 39 U.S. land-falling major hurricanes. In the 1966-2006 period there were 22 such storms -- only 56 percent as many.
  • Even though global mean temperatures have risen by an estimated 0.4 Celsius and CO2 by 20 percent, the number of major hurricanes hitting the United States declined.

If global warming isn't the cause of the increased Atlantic hurricane activity seen over the past dozen years, what is?

According to Gray and other researchers at Colorado State University, the increase in hurricane activity is due to the speed-up of water circulating in the Atlantic Ocean.  This circulation began to strengthen in 1995 -- at exactly the same time that Atlantic hurricane activity showed a large upswing.

Source: William M. Gray, "Hurricanes and Hot Air," July 26, 2007.


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