The record number of hurricanes and tropical storms in the North Atlantic over the past decade can be linked directly to rising temperatures caused by global warming, a study has found.
There are now about twice as many Atlantic hurricanes forming each year compared with a century ago and the rise has generated an intense debate over whether this is due to natural variability or man-made global warming.
A study by scientists at the US National Centre for Atmospheric Research in Boulder Colorado has found that natural variability cannot explain the increase. Instead, they have attributed the rise to warmer sea-surface temperatures caused by greenhouse gases.
The conclusion runs directly counter to the official position of the US National Oceanic and Atmospheric Administration (NOAA) - the US government agency charged with monitoring hurricane activity - which stated in 2005 that the increase in Atlantic hurricanes and tropical storms is due to "natural occurring cycles". However in a study published in the Philosophical Transactions of the Royal Society, Greg Holland and Peter Webster point out that the unequivocal NOAA statement was made without any supporting references to the peer-reviewed scientific literature.
When the two scientists did their own analysis of tropical cyclones - hurricanes and storms - over the past 100 years they found no convincing evidence that natural cycles could account for the dramatic increase seen in recent decades.
"We are led to the confident conclusion that the recent upsurge in tropical cyclone frequency is due in part to greenhouse warming, and this is most likely the dominant effect," the scientists say.
Their statistical analysis of hurricanes and tropical storms in the North Atlantic found that there were two abrupt increases in intensity over the past century - and each time the level remained elevated and relatively steady.
Between 1900 and 1930, there was an annual average of six tropical cyclones, of which four were hurricanes and two were storms. Between 1930 and 1940, the annual average increased to 10, with five hurricanes and five tropical storms.
Finally, in the period from 1995 to 2005, the yearly average rose to 15 tropical cyclones, of which eight were hurricanes and seven were tropical storms.
At the same time, the average temperature of the sea surface in the North Atlantic has increased by more than 0.7C. Sea surface temperatures are understood to be a critical factor determining whether or not a tropical storm or a more powerful hurricane forms over the ocean.
"These numbers are a strong indication that climate change is a major factor in the increasing number of Atlantic hurricanes," Dr Holland said.
Hurricanes were relatively frequent in 2004, but the number and intensity of those occurring in 2005 was unprecedented, with 28 named tropical cyclones and 14 hurricanes - of which seven were major. In July 2005 there were three category 5 hurricanes - the most extreme with windspeeds greater than 155mph - and two category 4 hurricanes.
By comparison, 2006 was far less active but the scientists believe this was due in part to the emergence of an El Niño event in the Pacific Ocean that year. Nevertheless, 2006 - with five hurricanes and four named storms - would still have ranked above average a century ago. And, since 2002, a total of 29 tropical storms and hurricanes have struck the United States - an average of seven named storms to hit land per year.
"Even a quiet year by today's standards would be considered normal or slightly active compared to an average year in the early part of the 20th century," Dr Holland said.
The five costliest storms
* KATRINA (2005)
The most costly hurricane in history - estimated damage $85bn. It formed over the Bahamas on 23 August and crossed southern Florida and the Gulf of Mexico, before striking the US on 29 August as a category 3 hurricane.
* ANDREW (1992)
The second most destructive hurricane in American history in terms of cost. Struck land as category 5, the top category, with windspeeds greater than 155mph, killing 23 people in the US and three more in the Bahamas.
* WILMA (2005)
Started life in the Caribbean Sea, eventually moving across the Gulf of Mexico to Cancun (pictured left), where it hit land with devastating consequences. Wilma was the third category 5 hurricane to develop in 2005.
* CHARLEY (2004)
Developed into a category 4 hurricane with windspeeds up to 150mph. Hit southwestern Florida just 22 hours after the northwestern part of the state had been struck by tropical storm Bonnie - the first time a US state had been hit by two tropical cyclones in a 24-hour period.
* IVAN (2004)
The strongest hurricane of the 2004 season, reaching category 5. Formed around Cape Verde and hit the island of Grenada, causing widespread destruction. Jamaica and Grand Cayman, as well as the western tip of Cuba, were also badly affected.
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