Big Storms Double Since Last Century

July 30, 2007 — The average number of tropical cyclones per year has doubled since the early 1900s, report scientists.

What’s more, the increase in the average number of storms each year has happened in three distinct steps, rather than in a gradual fashion, and tracks right along with rising sea surface temperatures, says Greg Holland of the National Center for Atmospheric Research (NCAR). Holland and Georgia Tech’s Peter Webster have published their discovery in the July 30 issue of the Philosophical Transactions of the Royal Society A.

From 1900 to 1930 there were on average six Atlantic tropical cyclones: four hurricanes and two tropical storms. From 1930 to 1940 the average jumped up to ten: five hurricanes and five tropical storms. There was yet another step up in the mid-1990s, bringing the average to 15: eight hurricanes and seven tropical storms.

"It actually hasn’t settled down yet," Holland said of the latest up-tick in tropical cyclones.

As a result of these three increases in storms, even a slow cyclone year these days — say, with four hurricanes and three tropical storms — is higher than the average a century ago. What’s more, the increases do not disappear even if one to three storms per year were missed and never counted in the early 1900s, said Holland. After all, the ability to monitor storms was much more limited than it is today.

"That only makes it a 90 percent (increase) instead of 100 percent," Holland said of any likely undercount a century ago.

"There is a clear trend," agreed Michael Mann, director of the Earth System Science Center at Pennsylvania State University. "It’s highly correlated with sea surface temperatures. There has been this debate about whether the trend is real. I think they’ve made a pretty good case."