The world's gathering storm

It was the year of the hurricane. It was also the year when scientists said 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 in part the result of global warming brought on by emissions of man-made greenhouse gases.

This year's hurricane season is rated as the busiest on record by the United States National Oceanic and Atmospheric Administration (NOAA).

It included 26 named storms, of which 13 grew big enough to be classified as hurricanes.

Of these, seven became major hurricanes of category three or higher on the five-point Saffir-Simpson scale - five more than average for a typical Atlantic season.

Hurricane Wilma in October became the strongest Atlantic storm 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.

Tropical storm Epsilon, formed over the central Atlantic Ocean on the second to last 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. "It was the most devastating hurricane season 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."

Katrina was the most destructive hurricane to strike the United States in living memory. More than 1000 people died.

Some environmentalists were quick to point the finger at global warming, but not all scientists were convinced.

Meteorologists express a broad consensus that global warming could not explain the upsurge in hurricanes over the past 20 years.

People thought the increase was just part of the normal long-term cycle driven by changes in the Atlantic circulation system. Few believed it was directly related to an increase in sea-surface temperatures caused by climate change.

Then Kerry Emanuel, of the Massachusetts Institute of Technology, described in the journal Nature a new measure of hurricane destructiveness. He compared changes in the measure of destructiveness, called "total dissipation of power", with the rise in sea-surface temperatures in the Atlantic over 30 years.

He linked an increase in hurricanes with rises in ocean temperature caused by global warming and climate change. "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."

The study found that tropical storms are lasting 60 per cent longer and their wind speeds can be 15 per cent higher, implying global tropical cyclone activity is responding in a major way to global warming.

It is reasonable to link hurricanes and global warming given that sea-surface temperatures play an essential role in their formation.

A hurricane is a tropical cyclone, an area of intense low pressure 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 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."

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. A warmer world could also lead to a warmer atmosphere and diminish the difference in the temperatures between air and ocean. 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 those uncertainties and contradictions, it is not surprising that different computer models have predicted everything from fewer hurricanes to more hurricanes as related to global warming, Pearce says.

More fuel was added to the debate when meteorologist Peter Webster, of Georgia Institute of Technology in Atlanta, showed that the frequency of category four and five cyclones - the two strongest types - had increased markedly 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.

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 affect the formation of hurricanes.

No one can say for sure that Katrina was made worse by greenhouse emissions. Previous hurricanes have killed far more people - the East Pakistan hurricane of 1970 killed about 500,000.

Most climate scientists believe it will take a considerable time to assess whether global warming is having a real impact on hurricanes.

Emanuel, for one, estimates that it could take 50 years to detect an accurate trend.

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."

- INDEPENDENT