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.