The gathering storms: How man is making the wind blow

As tourists flee the path of Hurricane Florence, scientists are warning that climate change will unleash regular and worsening devastation on the world. By Michael McCarthy and Abigail Townsend

It's hard at first to get your head around the idea, indeed it seems outlandish: that by switching on the light, or stamping on the car accelerator, you're helping to pulverise a great city such as New Orleans.

But that's the inescapable implication of a piece of research published yesterday by a group of the world's most distinguished climate scientists. Freak storms such as Hurricane Katrina, which devastated the Big Easy a year ago, are not just freaks, they suggest. They are down to us.

Warmer seas causing more violent hurricanes and typhoons are almost certainly the result of greenhouse gas emissions, they conclude; they are caused, ultimately, by the carbon dioxide from the power station that provides your electricity, from the exhaust of the car you drove to work this morning.

Ironically, the study was published just as holidaymakers in Bermuda were battening down the hatches in their luxury hotels to escape the effects of Hurricane Florence, the second storm of the 2006 Atlantic hurricane season, which was pounding the island with winds topping 100mph, although the worst of the storm passed Bermuda by.

The 19 scientists, from America, Britain and Germany, include James Hansen of Nasa, the doyen of American climate change researchers, and Professor Phil Jones from the Climatic Research Unit of the University of East Anglia at Norwich.

They said that, in a comprehensive investigation, they had found an 84 per cent probability that human triggers accounted for most of the observed increases in sea surface temperatures (SSTs), during the past century, in the breeding grounds for hurricanes (as they are called in the Atlantic) and cyclones (as they are known in the Pacific).

Warm temperatures at the surface are known to be a primary force behind the formation of violent tropical storms, in which moist air is sucked up by powerful convection currents and spun to form a cyclone.

Research published in the past year has shown a link between rising ocean temperatures and increases in hurricane intensity, a fact that seems intuitively true: more heat means more energy.

The thrust of the new study is this: the extra heat in the ocean is human-induced.

The American-based team, whose work is reported in the US journal Proceedings of the National Academy of Sciences, used 22 different computer climate models to look at the warming of oceans during the 20th century, which in the hurricane breeding grounds of the tropics range from 0.32C to 0.67C. Unlike previous investigations, the new research has focused on small cyclone- forming regions rather than entire ocean basins.

"The important conclusion is that the observed SST increases in these hurricane breeding grounds cannot be explained by natural processes alone," said one of the scientists involved, Dr Tom Wigley, from the National Centre for Atmospheric Research in Boulder, Colorado. "The best explanation for these changes has to include a large human influence."

He added: "It is important to note we expect global temperatures and SSTs to increase even more rapidly in the next century.

Dr Benjamin Santer, another member of the research team, from the Lawrence Livermore National Laboratory in California, said: "In a post-Katrina world, we need to do the best job we possibly can to understand the complex influences on hurricane intensity and how our actions are changing those influences."

Hurricane Katrina, which broke the Mississippi levees in August last year, allowing New Orleans to flood, killed 1,500 people along the US Gulf Coast and caused $80bn (£42bn) in damage in Louisiana and elsewhere.

It was the peak of the 2005 hurricane season, which broke records with 28 tropical storms, of which 15 became hurricanes. (By contrast, the six-month hurricane season that began on 1 June this year has produced only two hurricanes so far. Tropical Storm Ernesto briefly reached hurricane strength near Haiti last month but weakened before drenching the US east coast.)

In the wake of Katrina and the other storms of last year, there has been a fierce debate among researchers in the US as to whether hurricanes have indeed increased in frequency and power over recent decades.

Professor Kerry Emmanuel of the Massachusetts Institute of Technology caused a stir with a paper published in Nature just three weeks before Katrina hit, in which he said that hurricanes had doubled in power since 1950, with most of the increase in force occurring in the past 30 years.

Another paper published in September 2005 reported that the number of category 4 and 5 hurricanes - the strongest - had almost doubled in number over the same period.

But not all researchers accept those conclusions (which, incidentally, are very unwelcome to the Bush administration with its continuing stance of de-emphasising the potential effects of global warming). Some doubt has been cast on the accuracy of earlier hurricane data, which might make comparisons invalid. One observer who is following the debate closely is the authoritative British science writer Fred Pearce, who has just published a study of the latest climate change research entitled The Last Generation. "Not every hurricane researcher is convinced," he said. "Some think we may be seeing no more than natural fluctuations in hurricane activity. And they say that we can't be sure how many hurricanes there were, and how strong they were, 30 or 40 years ago.

"But the evidence is growing ever stronger that hurricanes are becoming more intense, and maybe more frequent too, as a result of global warming.

"The science is simple. A warmer atmosphere makes for warmer seas. Hurricanes form only when temperatures at the ocean surface rise above 26C. The heat in the water drives the strong winds and updrafts. So the more often that critical temperature is exceeded, the more hurricanes will form. And the warmer the water gets, the more intense the hurricanes will become."

Mr Pearce added: "The theory seems to be backed up in the real world. As oceans warm, the statistical evidence is growing there have been more hurricanes in the past decade or so - not just in the Atlantic but also the Pacific.

"And the most recent study, published in the prestigious Geophysical Research Letters in June, found global warming accounted for about half of the extra warmth in the oceans last year. A year that saw a record 28 tropical storms and hurricanes in the Atlantic, including Katrina. That makes climate change the likely cause of the destruction of New Orleans."

Meanwhile, back in Bermuda yesterday, all 122 guests at Cambridge Beaches, a luxury resort on the island's western tip (bungalows up to $1,695 per night) had to fight gale-force winds in the dark as they were moved from cottages to the main reception and dining area at 6.30am. As the winds thrashed outside, whipping palm trees back and forth and churning up the normally placid Mango Bay, food and entertainments from bingo to cookery demonstrations were laid on to keep the mainly English and American guests occupied.

Only one row flared up as the morning progressed, momentarily drowning out the sounds of the wind battering the shutters and boarded-up windows.

A British couple, Colin and Elizabeth Sanderson from Fareham in Hampshire, got married on the island four days ago but had to spend their honeymoon with friends and family as none of them could get off the island.

Hurricane Katrina it was not. But do not doubt it, Katrina, or her like, will be back. And thinking about why, is a sobering thought indeed.