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MIAMI — Hurricanes are forming twice as often as they did a century ago, largely because of global warming caused by humans, according to a new scientific study.

Other scientists say the report draws improper conclusions from partial data.

The study, conducted by two respected researchers and scheduled to be released Monday in a peer-reviewed publication, found that four hurricanes and two tropical storms developed during an average year between 1900 and 1930.

Between 1995 and 2005, however, the average shot up to eight hurricanes and seven tropical storms, the report said.

The scientists attributed the sharp increases to warmer ocean temperatures and altered wind patterns linked to human-induced global warming.

"When you look at the numbers and the strong relationship to sea surface temperatures and the reality of global warming, you end up with a causal link that can't be denied," said Greg Holland, co-author of the report and a scientist at the National Center for Atmospheric Research in Boulder, Colo.

But other experts said the study by Holland and Peter Webster of the Georgia Institute of Technology is undermined by "sloppy" research that used overly selective data and fell victim to a problem called "observation error."

In short, it may not be that many more hurricanes are forming now than a century ago, said the critics, who agree that global warming is occurring but believe it has not significantly affected hurricane development.

They see a much simpler reason for the higher number of reported hurricanes: The storms, they say, are more easily and frequently detected since hurricane hunter flights began in the 1940s and weather satellites began providing data in the 1970s.

The two camps have been taking potshots at each other for years, generally through peer-reviewed studies.

"The new paper by Holland and Webster is sloppy science that neglects the fact that better monitoring by satellites allows us to observe storms and hurricanes that were simply missed earlier," said Chris Landsea, a noted researcher who serves as the National Hurricane Center's science and operations officer.

"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," he said.

In addition, he and some other scientists said, long-term natural cycles that involve wind and ocean currents — and are unrelated to global warming — are mostly responsible for the upswing in hurricane activity since 1995.

In their paper, scheduled to be published online Monday in Philosophical Transactions of the Royal Society of London, Holland and Webster acknowledged that their analysis was "broad brush" but insisted that natural cycles and observation errors cannot fully explain the increase in hurricane frequency.

"The reasonable estimates of data problems in the early years do not change any of the conclusions," Holland said in an interview.

The study said that ocean temperatures have warmed by 1.4 degrees Fahrenheit during the last 100 years and there is a clear link between warmer oceans and enhanced hurricane activity.

"Collectively, this causal chain leads to the strong conclusion that the current level of tropical cyclone activity in the North Atlantic is largely a response to climate change from human causes," the report said.

The researchers noted a particular increase in hurricanes since 1995, including the record year of 2005.

They said it could signal the beginning of "substantially higher tropical cyclone, hurricane and major hurricane frequency than has hitherto been experienced in the historical record."

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(c) 2007, The Miami Herald.

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