Geoffrey Lean: Rita's global warning

With impeccable timing, environment ministers this weekend are opening talks on a new treaty to combat global warming. As Hurricane Rita hits George Bush's home state of Texas, they gather in Ottawa to begin the long and difficult wrangling over what should succeed the Kyoto Protocol, the international treaty he has determinedly tried to sabotage.

As they meet, the winds may be dying down around Galveston, but the storm is still building over whether the hurricanes can be blamed on climate change. Last week, Sir John Lawton, chairman of the Royal Commission on Environmental Pollution, told The Independent that the increasing intensity of hurricanes was "very likely" due to global warming, a view echoed by the Government's chief scientist, Sir David King.

Both Germany's Green environment minister, and the Christian Democrat who hopes to succeed him, blamed global warming for the devastation of New Orleans, while the left-wing President Chavez of Venezuela could not resist fingering US "capitalist consumerism".

You can see their point. Even though Rita - the 18th named storm of the year - thankfully got less vicious as it neared the Lone Star state, 2005 has seen more damage than the past 35 years put together, with still more storms expected. Those who have long predicted such a consequence might perhaps be forgiven some schadenfreude: last year's hurricanes hit Florida, where disputed votes gave Dubya his original victory, while Katrina and Rita seemed targeted on a massive concentration of the oil industry.

But while it is becoming clear beyond reasonable doubt that global warming is increasingly melting polar ice, and causing more droughts and floods around the world, its contribution to the current spate of hurricanes is much less certain. While scientists agree that hurricanes are powered by warm water - sea temperatures above 26.5C - and that both the Atlantic and the Gulf of Mexico are hotter than they used to be, they debate why the warming is taking place.

A natural cycle of these devastating storms - named after Hurican, the Carib god of evil - is thought to be caused by seawater growing saltier, thus denser and warmer. This can be traced back for centuries. But after decades of relative calm, the cycle entered a stormy phase in 1995. This was widely predicted, including in The Independent on Sunday, and is expected to last for another 10 to 20 years.

But a growing number of scientists believe that, while global warming may not make hurricanes more common, it may make them fiercer and longer lasting. Two authoritative studies in the past three months have shown that they have got more intense over the past half-century, and point the finger at global warming.

This is not conclusive proof - for one thing, the studies don't go back far enough to discount entirely the natural variation - but such evidence is changing scientific minds. The principal author of one of the reports, Professor Kerry Emanuel of the Massachusetts Institute of Technology, says: "I was one of those sceptics myself - a year ago"; even some US government scientists now believe in a link.

Whatever is decided in Ottawa will not make any immediate difference. Even if global warming is responsible for the recent hurricanes, it is down to pollution from long ago. But there is growing agreement that - whatever the cause of the present hurricanes - increased global warming will make them worse in future.

Let Al Gore, the President's dubiously defeated rival, have the last word: "Katrina is the first taste of a bitter cup that will be proffered to us over and over again. It is up to us to tackle climate change."