Scientists say they've discovered clear evidence for the much-disupted theory that global warming leads to more hurricanes in the US.
The difficulty with this theory is that records only go back about 40 years, making it hard to look for long-term trends. However, climate scientist Aslak Grinsted of the University of Copenhagen says he's been able to find instruments that have registered measurements continuously over a long period of time.
"Tropical cyclones typically form out in the Atlantic Ocean and move towards the US East Coast and the Gulf of Mexico. I found that there were monitoring stations along the Eastern Seaboard of the United States where they had recorded the daily tide levels all the way back to 1923," he says.
"I have looked at every time there was a rapid change in sea level and I could see that there was a close correlation between sudden changes in sea level and historical accounts of tropical storms."
Together with colleagues in China and England, Grinsted then looked at the global temperatures over the period to see whether there was a trend for a higher frequency of cyclones in a warmer climate.
"We simply counted how many extreme cyclones with storm surges there were in warm years compared to cold years and we could see that there was a tendency for more cyclones in warmer years," he says.
Cyclones with a similar strength to Katrina, which hit the New Orleans area in 2005 and caused devastating floods and thousands of deaths, make landfall every 10-30 years on average.
"We have calculated that extreme hurricane surges like Katrina are twice as likely in warm years than in cold years. So when the global climate becomes three degrees warmer in the future, as predictions show, what happens then?" he says.