BioEd Online

The United States is bracing itself for a bruising encounter with Hurricane Ivan, currently storming across the Gulf of Mexico. The damage caused by the hurricanes Charley and Frances is still fresh, and the tropical storm Jeanne looks set to intensify to hurricane strength shortly. Surely there must be a reason behind this battering?

But climatologists are insisting it is pure chance. Global warming is probably having little or no effect on the number of hurricanes, they say, and there have been no more hurricanes this year than last. In 2003, however, most of the storms blew themselves out over the sea. This year, many of them have headed straight for populated areas, which is simply bad luck.

"They just happened to be aligned towards land this year," says Robert Weisberg, an expert in hurricane prediction at the University of South Florida in St Petersburg. "There is no apparent underlying trend."

Ivan the Terrible

That is little consolation to those in the path of the devastation, however. Ivan has already killed at least 68 people on its week-long tour of the Caribbean, causing huge amounts of damage in Grenada, Jamaica and the Cayman Islands.

And Florida suffered over $14 billion of property damage in the past month after encounters with the hurricanes Charley and Frances. Oil rigs in the Gulf of Mexico were evacuated earlier this week, and onshore refineries were closed, disrupting about a quarter of US oil and natural gas production.

Forecasters say that when Ivan reaches the United States, it will hammer the states of Florida, Alabama, Mississippi and Louisiana, with the Mississippi coast taking the brunt of the storm.

Ivan's winds are now gusting up to 225 kilometres per hour. This is slower than the peak of 260 kilometres per hour that they reached on 11 September, but still enough to make it an "extremely dangerous hurricane" when it next hits land. The hurricane-force winds cover an area roughly 340 kilometres across, and the whole body of swirling air is moving north at about 20 kilometres per hour.

The US Census Bureau has estimated that Ivan could affect more than 6.1 million people when it reaches the Gulf Coast. "It looks like it's going to be a major disaster," says Weisberg. Residents of New Orleans in Louisiana have been advised to flee by their mayor, Ray Nagin, who says that the city could be flooded with over five metres of water. The Miami-based National Hurricane Centre reported that, at 10:00 GMT today, water levels along the north Gulf Coast were 35 centimetres higher than normal.

Rampant Jeanne

Puerto Rico and the US Virgin Islands are preparing for a new assault, as they are in the path of the tropical storm Jeanne, which is expected to attain hurricane strength within the next 24 hours.

It is impossible to say for certain where Jeanne will be this time next week, however. "Predicting where a hurricane will go is just as hard as predicting the weather," says Weisberg.

Most North Atlantic hurricanes start over the middle of the ocean, and move west towards the Caribbean. Some continue west through the Gulf of Mexico, but others move up the east coast of Florida. It is extremely difficult to say whether the storms will move over land or back out over the sea.

However, forecasters can usually predict a hurricane's path about three days in advance, with enough accuracy to give residents time to evacuate if necessary. This means hurricanes cause far fewer deaths today compared with 50 years ago.