WASHINGTON – The number of tropical storms developing annually in the Atlantic Ocean more than doubled over the past century, with the increase taking place in two jumps, researchers say. The increases coincided with rising sea surface temperature, largely the byproduct of human-induced climate warming, researchers Greg J. Holland and Peter J. Webster concluded. Their findings were published online Sunday by Philosophical Transactions of the Royal Society of London. An official at the National Hurricane Center called the research “sloppy science” and said technological improvements in observing storms accounted for the increase. From 1905 to 1930, the Atlantic-Gulf Coast area averaged six tropical cyclones per year, with four of those storms growing to become hurricanes. Holland said about half the U.S. population and “a large slice” of business are “directly vulnerable” to hurricanes.160Want local news?Sign up for the Localist and stay informed Something went wrong. Please try again.subscribeCongratulations! You’re all set! The annual average jumped to 10 tropical storms and five hurricanes from 1931 to 1994. From 1995 to 2005, the average was 15 tropical storms and eight hurricanes annually. Even in 2006, widely reported as a mild year, there were 10 tropical storms. “We are currently in an upward swing in frequency of named storms and hurricanes that has not stabilized,” said Holland, director of mesoscale and microscale meteorology at the National Center for Atmospheric Research in Boulder, Colo. “I really do not know how much further, if any, that it will go, but my sense is that we shall see a stabilization in frequencies for a while, followed by potentially another upward swing if global warming continues unabated,” Holland said. It is normal for chaotic systems such as weather and climate to move in sharp steps rather than gradual trends, he said.