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 being 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. 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, Colorado.
"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.
"What did surprise me when we first found it in 2005 was that the increases had developed for so long without us noticing it," he said in an interview via e-mail.
Holland said about half the U.S. population and "a large slice" of business are "directly vulnerable" to hurricanes.
"Our urban and industrial planning and building codes are based on past history," he said. If the future is different, "then we run the very real risk of these being found inadequate, as was so graphically displayed by (Hurricane) Katrina in New Orleans."
Hurricanes derive their energy from warm ocean water. North Atlantic surface temperature increased about 1.3 degrees Fahrenheit during the 100-year period studied. Other researchers have calculated that at least two-thirds of that warming can be attributed to human and industrial activities.
Some experts have sought to blame changes in the sun. But a recent study by British and Swiss experts concluded that "over the past 20 years, all the trends in the sun that could have had an influence on the Earth's climate have been in the opposite direction to that required to explain the observed rise in global mean temperatures."
As the sea surface temperatures warm, they cause changes in atmospheric wind fields and circulations, and these changes are responsible for the changes in storm frequency, Holland said.
Chris Landsea, science and operations officer at the National Oceanic and Atmospheric Administration's National Hurricane Center, said the study is inconsistent in its use of data.
The work, he said, is "sloppy science that neglects the fact that better monitoring by satellites allows us to observe storms and hurricanes that were simply missed earlier. The doubling in the number of storms and hurricanes in 100 years that they found in their paper is just an artifact of technology, not climate change."
But Kerry Emanuel, a hurricane expert at Massachusetts Institute of Technology, said the study was significant. "It refutes recent suggestions that the upward trend in Atlantic hurricane activity is an artifact of changing measurement systems," said Emanuel, who was not part of the research team.
Improvements in observation began with aircraft flights into storms in 1944 and satellite observations in 1970. The transitions in hurricane activity that were noted in the paper occurred around 1930 and 1995.
"We are of the strong and considered opinion that data errors alone cannot explain the sharp, high-amplitude transitions between the climatic regimes, each with an increase of around 50 percent in cyclone and hurricane numbers," wrote Webster, of the Georgia Institute of Technology, and Holland.
The research was funded by the National Science Foundation.