hurricane john satellite (Courtesy of NASA/MODIS)

Key factorsin global climate change, like warming sea temperatures and shifting wind patterns,have prompteda sharp rise in hurricanes, according to a study out Monday.

Thereportby Greg Holland of the National Center for Atmospheric Research, and Peter Webster of Georgia Institute of Technology was published online Monday in Philosophical Transactions of the Royal Society of London.

The study found that thefirst 30 yearsof the20th centurywas relatively quiet, with an annual average ofsix observed hurricanes and tropical storms. The annual average increased to 10 after 1930, and then reached 15 from 1995 to 2005.

"These numbers are a strong indication that climate change is a major factor in the increasing number of Atlantic hurricanes," said Holland in a statement.

The researchers analyzed data from about the past 100 years,ending with 2005. They used systematic meteorologist's data derived from aircraft flights starting in 1944, satellite data from about 1970, and more sophisticated measuring methods for the subsequent years.

Also, they said that the years 2004 and 2005 had unusually active hurricane seasons, spurring an abundance of research into whether more intense tropical cyclones are correlated with natural cycles, global warming, or some other cause.

"The new study indicates that natural cycles are probably not the entire cause because the increase has happened across the last century rather than oscillating in tandem with a natural cycle. "

"While the number of storms has steadily increased, the proportion of hurricanes to all Atlantic tropical cyclones has remained steady. Hurricanes have generally accounted for roughly 55 per cent of all tropical cyclones. However, the proportion of major hurricanes (those with maximum sustained winds of at least [177kilometres per hour]) to less intense hurricanes and tropical storms has oscillated irregularly, and has increased significantly in recent years."

Although the study did not cover 2006, it did note that the 2006 hurricane season was far less active than the two preceding years— partly because of the emergence of an El Nino event in the Pacific Ocean. However, 2006 "would have ranked above average a century ago, with five hurricanes and four other named storms," it added.