 The number of Atlantic hurricanes in an average season has doubled in the last century due in part to warmer seas and changing wind patterns caused by global warming, according to a study released on Sunday. Hurricane researchers have debated for years whether climate change caused by greenhouse gases from cars, factories and other human activity is resulting in more, and more intense, tropical storms and hurricanes. The new study, published online in Philosophical Transactions of the Royal Society of London, said the increased numbers of tropical storms and hurricanes in the last 100 years is closely related to a 1.3-degree Fahrenheit rise in sea surface temperatures. The influential UN Intergovernmental Panel on Climate Change, in a report this year warning that humans contribute to global warming, said it was "more likely than not" that people also contribute to a trend of increasingly intense hurricanes. In the new study, conducted by Greg Holland of the National Center for Atmospheric Research and Peter Webster of Georgia Institute of Technology, researchers found three periods since 1900 when the average number of Atlantic tropical storms and hurricanes increased sharply, and then leveled off and remained steady. From 1900 to 1930, Atlantic hurricane seasons saw six storms on average, with four hurricanes and two tropical storms. From 1930 to 1940, the annual average rose to ten, including five hurricanes. From 1995 to 2005, the average rose to 15, with eight hurricanes and seven tropical storms, the researchers said. Changes in sea surface temperatures occurred before the periods of increased cyclones, with a rise of 0.7 degrees Fahrenheit before the 1930 period and a similar increase before the 1995 period, they said. "These numbers are a strong indication that climate change is a major factor in the increasing number of Atlantic hurricanes," Holland said in a statement. Skeptics say hurricane data from the early decades of the 20th century are not reliable because cyclones likely formed and died in mid-ocean, where no one knew they existed. More reliable data became available in 1944 when researchers had airplane observations, and from 1970 when satellites came into use. But Holland and Webster said the improved data from the last half of the century cannot be solely responsible for the increase. "We are led to the confident conclusion that the recent upsurge in the tropical cyclone frequency is due in part to greenhouse warming, and this is most likely the dominant effect," the authors wrote. In 2004, four powerful hurricanes, Charley, Frances, Ivan and Jeanne, hit Florida. All four placed in the top ten costliest storms in US history. The record-shattering 2005 season produced 28 storms, 15 of which became hurricanes including Katrina, which caused $80 billion damage and killed 1,500 people. The 2006 season was relatively mild, with ten storms.