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. …