Global warming is 'causing more hurricanes'

Scientists have found support for the controversial idea that global warming is causing more frequent and destructive hurricanes, a subject that has been hotly debated during the past decade.

Data gathered from tide gauges, which monitor the rapid changes to sea levels caused by storm surges, show a significant link between both the frequency and intensity of tropical storms and increases in annual temperatures since the tidal records began in 1923.

The study found that during the 90-year period, when the average global temperature has increased by 0.7C, extreme hurricanes similar to Katrina, which devastated New Orleans in 2005, were nearly twice as likely in warmer years as colder years.

Although scientists were not able to prove that climate change is causing more large hurricanes, they believe the study is consistent with the predictions that global warming and warmer seas could bring about more intense tropical storms.

Hurricanes form when the sea's surface temperature increases above 26C. However, they result from a chaotic interaction between the difference in sea and air temperatures, humidity and wind, so there is disagreement about how frequent they will become in a warmer world.

Studying the link between global warming and tropical storms has been hampered by the lack of data on hurricanes before the satellite age. Many hurricanes out at sea were missed before the first weather satellites were launched about 40 years ago.

However, a network of tide gauges around the south-east coast of the US has produced a reliable record of the rapid changes to sea level caused by storm surges resulting from tropical cyclones, said Aslak Grinsted of the Niels Bohr Institute at Copenhagen University.

"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. I have looked at every time there was a rapid change in sea level and I could see there was a close correlation between sudden changes in sea level and historical accounts of tropical storms," Dr Grinsted said.

Once the correlation between storm surges and tropical storms was established, the researchers analysed global temperature records to compare the number of storm surges in warm years with the number observed in cold years.

"We simply counted how many extreme cyclones with storm surges there were in warm years compared with cold years and we could see that there was a tendency for more cyclones in warmer years," Dr Grinsted said.

Storms of destruction: devastating weather

Wilma (2005)

The most intense Atlantic hurricane on record started in the Caribbean Sea near Jamaica, moving across the Gulf of Mexico to Cancun where it hit land with devastating consequences.

Katrina (2005)

The most costly hurricane in history caused damages of $85bn. The category-3 storm formed over the Bahamas crossed Florida and the Gulf of Mexico before striking New Orleans.

Gilbert (1988)

The second most intense hurricane observed in the Atlantic. It began to the east of Barbados before hitting Jamaica and the Gulf of Mexico. It raged for nine days, killing 433 people.