By Matt McGrath

BBC environmental reporter



Hurricanes have become more frequent over the past century

The study says that warmer sea surface temperatures and changes in wind patterns caused by climate change are fuelling much of the increase.

Some researchers say hurricanes are cyclical and the increase is just a reflection of a natural pattern.

But the authors of this study say it is not just nature - they say the frequency has risen across the century.

Two-decade rise

Hurricanes are a spinning vortex of winds that swirl around an eye of low pressure.

Thunder clouds surround the edges of these storms and they can wreak devastation on people and property when they hit land - most famously in the case of Hurricane Katrina in New Orleans in 2005.

Scientific analyses in recent years suggest hurricane numbers have increased since the mid-1980s.

This new study, published in Philosophical Transactions of the Royal Society in London, looks at the frequency of these storms from 1900 to the present and it says about twice as many form each year now compared to 100 years ago.

The authors say that man-made climate change, which has increased the temperature of the sea surface, is the major factor behind the increase in numbers.

"Over the period we've had natural variability in the frequency of storms, which has contributed less than 50% of the actual increase in our view," said Dr Greg Holland from the United States National Centre for Atmospheric Research in Colorado, who authored the report.

"Approximately 60%, and possibly even 70% of what we are seeing in the last decade can be attributed directly to greenhouse warming," he said.

Experts say that 2007 will be a very active season with nine hurricanes forecast, of which five are expected to be intense.