The link between hurricanes and global warming

This year's hurricane season shattered records for frequency and ferocity. And it's going to get worse, some scientists say. But others aren't so sure. Steve Connor reports

It was the year of the hurricane. It was also the year when scientists said that global warming can increase the intensity of hurricanes. But it was not the year when everyone could agree that hurricane Katrina, which devastated New Orleans in August, was at least in part the result of global warming brought on by emissions of man-made greenhouse gases.

The 2005 hurricane season is now officially rated as the busiest on record by the US National Oceanic and Atmospheric Administration (NOAA). The 2005 season included 26 named storms, of which 13 grew big enough to be classified as hurricanes. Of these, seven turned out to be major hurricanes of category three or higher on the five-point Saffir-Simpson scale. This was five more major hurricanes than average for a typical Atlantic season. Hurricane Wilma in October became the strongest Atlantic storm ever recorded, breaking the previous record set in 1988.

It wasn't just strength, it was frequency. For the first time since 1953, when scientists started to name tropical Atlantic storms, letters of the Greek alphabet had to be used because meteorologists had exhausted the original list of 21 alphabetically ordered names. The final tropical storm, Epsilon, formed over the central Atlantic Ocean on the penultimate day of the hurricane season.

"This hurricane season shattered records that have stood for decades - most named storms, most hurricanes and most category-five storms," says NOAA administrator Conrad Lautenbacher. "Arguably, it was the most devastating hurricane season the country has experienced in modern times. I'd like to foretell that next year will be calmer, but I can't. Historical trends say the atmosphere patterns and water temperatures are likely to force another active season upon us."

The real record-breaker was Katrina, the most destructive hurricane to strike the United States in living memory. More than 1,000 people died and some estimates put the cost of repairing New Orleans and its flood defences as high as $200bn (£117bn).

Some environmentalists were quick to point the finger of guilt at global warming, but not all scientists were convinced - not until some of them began to look at the data more carefully, that was. Until recently there was a fairly broad consensus among meteorologists that global warming could not explain the general upsurge in hurricanes recorded over the past couple of decades. People thought this current increase was just part of the normal long-term cycle driven by changes to the Atlantic circulation system. Few believed that it was directly related to an increase in sea-surface temperatures caused by climate change.

But then came a study by Kerry Emanuel of the Massachusetts Institute of Technology in Cambridge who, in a letter published in the journal Nature, described how he had formulated a new measure of hurricane destructiveness. He compared changes in this measure of destructiveness, called "total dissipation of power", with the rise in sea-surface temperatures recorded in the Atlantic over the past 30 years.

Emanuel found that he could link an increase in hurricane activity with rises in ocean temperature caused by global warming and climate change. "My results suggest that future warming may lead to an upward trend in tropical cyclone destructive potential, and - taking into account an increasing coastal population - a substantial increase in hurricane-related losses in the 21st century," he says.

The study concluded that tropical storms are lasting 60 per cent longer and their wind speeds are up to 15 per cent higher. "This work implies that global tropical cyclone activity is responding in a rather large way to global warming," Emanuel says.

It is not unreasonable to pose a link between hurricanes and global warming given that sea-surface temperatures play a critical role in their formation. A hurricane is a tropical cyclone, an area of intense low pressure in the tropics surrounded by a violent, rotating storm. In the Atlantic, such phenomena are called hurricanes, in the eastern Pacific they are known as typhoons, and in the Indian Ocean they are known as cyclones.

"On the face of it, global warming can only make things worse," says science author Fred Pearce in the latest issue of New Scientist. "The initial pillar of humid air generally forms only when the temperature of the sea surface exceeds 26C. As the oceans warm, larger areas will exceed the threshold. And every degree above the threshold seems to encourage stronger hurricanes," he says.

When Katrina formed, the temperature of the Gulf of Mexico was about 30C, which encouraged the view that in a warmer world we can expect more violent hurricanes. Real life is more complex, however. A warmer world could also lead to a warmer atmosphere and this could diminish the critical difference in the temperatures between air and ocean that helps hurricanes to form.

Another way global warming could kill hurricanes is by generating stronger winds, which could stir up the sea, thereby cooling surface temperatures. Stronger winds could also disrupt the tall pillar of humid air needed for hurricane formation. "With all these uncertainties and contradictions, it is not surprising that different computer models have predicted everything from fewer hurricanes to more hurricanes as global warming kicks in," Pearce says.

More fuel was added to the fire of the debate in September when meteorologist Peter Webster of the Georgia Institute of Technology in Atlanta and his colleagues published data showing that the frequency of category four and five cyclones - the two strongest types of hurricanes - had increased significantly over the past 35 years. Globally, category four and five storms climbed 57 per cent from the first half of the period to the second, they found. With the characteristic understatement of scientists treading in a controversial area, they conclude: "This trend is not inconsistent with recent climate model simulations that a doubling of carbon dioxide may increase the frequency of the most intense cyclones, although attribution of the 30-year trend to global warming would require a longer global data record and, especially, a deeper understanding of the role of hurricanes in the general circulation of the atmosphere and ocean, even in the present climate state."

In other words, the evidence is mounting that global warming can indeed affect the formation of hurricanes but as yet no one can say for sure whether this is happening now. And equally no one can say for sure that Katrina was made worse by greenhouse emissions. Previous hurricanes have after all killed far more people - the East Pakistan hurricane of 1970 probably killed half a million.

Most climate scientists believe that it will take far longer to assess whether global warming is having a real impact on the frequency or intensity of hurricanes. Emanuel, for example, estimates that it could take a further 50 years to detect an unambiguous trend in the intensity of hurricanes hitting the US. But whatever the long-term trend, it is clear that more hurricanes than usual are expected next year. In the words of one NOAA scientist: "We are in an active hurricane era."

How hurricanes form

All types of hurricanes start out as a cluster of thunderstorms which form as columns of warm, moist air rise above the surface of a tropical ocean where the temperature of the sea surface is above a critical threshold of 26C. If the thunderstorms become concentrated they can form a tall pillar of humid air extending from the sea surface to the high-altitude boundary of the stratosphere.

The low pressure at the base of the pillar of air sucks in more humid air. As the air rises, it cools and condenses, releasing heat that drives the engine of the tropical storm. Wind speeds pick up, and the entire weather system begins to rotate in a violent vortex centred around the eye of the storm.