Global warming & Katrina 1

More strong Katrina-like hurricanes reported

By Deborah Zabarenko

WASHINGTON (Reuters) - The number of strong hurricanes -- like the devastating Katrina -- significantly increased in the last 35 years, fueled by hotter seas that have been linked to global warming, researchers reported on Thursday.

Twice as many of the most powerful hurricanes, those ranked Category 4 or 5, have been detected since 1990 as were seen in the period from 1970 to 1985, scientists found in a global survey.

But the overall number of hurricanes has decreased during the last decade, the researchers wrote in a study published in the journal Science.

The rise in intense, destructive hurricanes worldwide goes along with a rise in sea surface temperatures, said Judith Curry of the Georgia Institute of Technology.

"This trend in sea surface temperature that's sort of relentlessly rising and the hurricane intensity that's relentlessly rising (means that) it's with some confidence we can say that these two things are connected and that there's probably a substantial contribution from greenhouse warming," Curry said in a telephone news briefing.

Warm sea surfaces help fuel hurricanes, and the higher the temperature at the water's surface, the stronger the hurricane can become, Georgia Institute of Technology's Peter Webster explained.

Water vapor that evaporates from the sea's surface into the atmosphere eventually condenses as rain, releasing heat and driving a tropical cyclone -- the swirling pattern that can beget a hurricane.

FUEL FOR HURRICANES

The warmer the sea surface, the greater amount of potential evaporation and the greater the fuel for a possible hurricane, Webster said. And even small rises in sea surface temperature can cause rapid rises in evaporation.

The surface temperature in the Atlantic Ocean has risen about 1 degree Fahrenheit (.5 degree Celsius) since 1970, the researchers said.

They noted, however, that only 12 percent of the world's hurricanes form in the Atlantic , so they looked at global data dating back to 1970. They discovered that the number and duration of hurricanes has remained generally stable, but the intensity has increased.

Because the results were similar across the globe, the scientists discounted natural variability as the cause.

These findings were in line with research published recently in the journal Nature, reporting that hurricanes have become more destructive over the last 30 years.

Hurricane Katrina was considered a Category 5 storm by the researchers, even though it had weakened to Category 4 when it came onshore on the U.S. Gulf coast, on Aug 29.

The U.S.

 National Hurricane Center has forecast an extremely active Atlantic hurricane season, with 9 to 11 hurricanes, including 7 to 9 major hurricanes, from July to November. 

Category 5 is the highest rating on the Saffir-Simpson scale of hurricane intensity, meaning the storm has winds greater than 155 miles an hour.

The next-highest rating, Category 4, is a storm packing winds between 131 and 155 miles an hour.

To be classified as a hurricane, tropical storms must have winds above 74 miles an hour.