From: Michael Christie, Reuters
Published September 4, 2007 05:25 PM

Global Warming Link To Hurricanes Dean And Felix, Possible But Unknown

MIAMI - Despite growing consensus that global warming may spawn stronger tropical cyclones, weather experts believe it is too soon to blame climate change for the unprecedented punch of back-to-back monster hurricanes.

Hurricane Felix, a top-ranked storm on forecasters' Saffir-Simpson scale of hurricane intensity, slammed into Central America on Tuesday. Hurricane Dean, also a Category 5, battered Mexico's Yucatan Peninsula on August 21.

It was the first time on record that two Atlantic hurricanes had made landfall as Category 5 storms in the same season, and only the fourth time since records began in 1851 that more than one Category 5 had formed in a year.

But climatologists, including those who believe global warming is having a dangerous impact on the ferocity of tropical cyclones, cautioned against making assumptions.

"My guess is that the high intensities of Dean and Felix had more to do with when and where they formed and tracked than with global warming per se," said Kerry Emanuel, a Massachusetts Institute of Technology professor of meteorology who has published ground-breaking research on the subject.


"But it is true that the theoretical (wind) speed limit in the tropical Atlantic is about 10 percent higher now than it was 15 years ago, and that may indeed be a contributing factor."

Weather experts said the similar paths taken by Dean and Felix was the result of a persistent high pressure weather system, usually located further northeast around Bermuda but now positioned over Florida and the Gulf of Mexico.

The high has been protecting the United States and steering the storms into parts of the Caribbean where sea surface temperatures are highest and atmospheric conditions are ideal for strengthening. Warm, deep water provides tropical cyclones with the fuel they need to grow.

"While two maximum-strength hurricanes could be cited as evidence of climate change, both the tracks of these storms have been guided over a region known to be a great hurricane intensifier," said Robert Muir Wood, chief research officer at London-based Risk Management Solutions.


Category 5 hurricanes, which have top sustained winds in excess of 155 miles per hour (249 km per hour), are capable of causing catastrophic destruction and can push a wall of sea water more than 18 feet high onshore.

They had been considered rare.

Before the devastating Atlantic hurricane season of 2005 there had only been two years on record -- 1960 and 1961 -- with more than one Category 5 storm. In 2005 there were four, including Katrina, which later swamped New Orleans, and Wilma, the Atlantic's most intense hurricane.

"We can't attribute a Category 5 or any high intensity storm to global warming. But certainly the pattern is very disconcerting," said Peter Frumhoff, chief scientist of the Climate Campaign of the Union of Concerned Scientists.

"They are clearly not rare in the current context."

The debate over global warming and hurricanes heated up in 2005, when a record 28 tropical storms formed in the Atlantic region. Of those, 15 became hurricanes.

Many hurricane experts say the Atlantic entered a period of naturally heightened hurricane activity in 1995 that could last 20 years. They say the impact of climate change would be undetectable against the backdrop of the natural variation.

Some research, however, has found that while global warming may not result in more tropical cyclones, it could already be increasing their average intensity and rainfall.

"There is no doubt at all that warmer sea surface temperatures lead to more powerful hurricanes: we can see this every time a hurricane moves over warmer ocean waters," said Terry Joyce, director of the Oceans and Climate Change Institute at the Woods Hole Oceanographic Institution in Massachusetts.

"But to attribute both of these to global warming, after only two Category 5 storms, is ... premature."

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