HURRICANES of the intensity of Katrina have become almost twice as common over the past 35 years, according to research suggesting that global warming could be worsening severe storms.

The overall frequency of tropical storms worldwide has remained broadly static since 1970, but the number of extreme Category 4 and 5 events has risen sharply, satellite measurements have shown.

Since 1990 an average of 18 Category 4 and 5 storms, of similar strength to Hurricane Katrina, have occurred every year, compared with an average of 10 in the 1970s, US scientists have found.

Ocean surface temperatures — one of the key drivers of hurricane intensity — have increased by an average of 0.5C (0.9F) over the same period, indicating a potential connection to global warming.

Researchers said that it was too early to be certain that climate change is fuelling stronger hurricanes, but such a link would be consistent with the best predictions of the likely effects of warmer seas.

Tropical storms, which always form over water, are known as hurricanes when they occur over the Atlantic, and as typhoons or cyclones in the Pacific and Indian Oceans. The storms are heat engines that build intensity by sucking up more and more water vapour, generating winds of more than 100mph (161km/h).

Category 4 hurricanes sustain winds of between 131mph and 155mph, and the biggest Category 5 storms blow at 156mph or more. Hurricane Katrina reached Category 5 at its peak over the Gulf of Mexico and stood at Category 4 when it devastated New Orleans and Mississippi.

In the latest study, published today in the journal Science, a team led by Peter Webster, of the Georgia Institute of Technology, analysed all the satellite records of hurricanes and typhoons since 1970.

“What we found was rather astonishing,” he said. “In the 1970s, there was an average of about 10 Category 4 and 5 hurricanes per year. Since 1990, the number of Category 4 and 5 hurricanes has almost doubled, averaging 18 per year.”

Judith Curry, his colleague, said: “Category 4 and 5 storms are also making up a larger share of the total number of hurricanes. Category 4 and 5 hurricanes made up about 20 per cent of all hurricanes in the 1970s, but over the past decade they have accounted for about 35 per cent of these storms.”

The findings, Dr Webster said, could be a result of global warming, though he said much more research was needed.

“Our work is consistent with the concept that there is a relationship between increasing sea surface temperature and hurricane intensity,” he said. “However, it’s not a simple relationship.” Several recent studies indicate that intense storms can be expected to become more common with climate change. In August, research by Kerry Emmanuel, of the Massachusetts Institute of Technology, published in Nature, found that the destructive energy of hurricanes had increased in line with rising ocean temperatures. “We are clearly seeing the same signal in the data,” Dr Emmanuel said yesterday.

Another Science paper, by Kevin Trenberth, of the US National Centre for Atmospheric Research, found in July that there is 2 per cent more water vapour above the oceans today than there was in 1988. This suggests that more water will be drawn into swirling tropical storms, generating higher wind speeds and greater rainfall.

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