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Sunday, November 30, 2008

Is Global Warming Making Hurricanes Worse?

Hurricanes bring winds and slashing rains that flood streets, flatten homes, and leave survivors struggling to pick up the pieces. But has global warming given the storms an added punch, making the aftereffects more dreadful?

According to hurricane historian Jay Barnes of Pine Knoll Shores, North Carolina, ocean heat is the key ingredient for hurricane formation. More heat could "generate more storms and more intense hurricanes," he said.

Numerous studies in recent years have found no evidence that the number of hurricanes and their northwest Pacific Ocean cousins, typhoons, is increasing because of the rise in global temperatures.

But a new study in the journal Nature found that hurricanes and typhoons have become stronger and longer-lasting over the past 30 years. These upswings correlate with a rise in sea surface temperatures.

The duration and strength of hurricanes have increased by about 50 percent over the last three decades, according to study author Kerry Emanuel, a professor of atmospheric science at the Massachusetts Institute of Technology in Cambridge.

Emanuel's finding defies existing models for measuring storm strength. Current models suggest that the intensity of hurricanes and typhoons should increase by 5 percent for every 1ºC (1.8ºF) rise in sea surface temperature.

"We've had half a degree [Celsius] of warming, so that should have led to a 2.5 percent increase [in intensity], which is probably not detectable," Emanuel said. "What we've seen is somewhat bigger than that, and we don't really know why."

One possibility, Emanuel said, is that ocean temperatures may be increasing more quickly than atmospheric temperatures.

"When that happens we've shown theoretically you get an increase in the intensity of hurricanes," he said.

Anatomy of a Hurricane

According to Barnes, who has authored several books on U.S. hurricane history, the physics of hurricanes are complex and full of variables. "But the sun beating down on Earth is the primary thing that gets it going," he said.

Barnes explains in his book North Carolina's Hurricane History that the summer heat warms the ocean's surface and spurs evaporation. As heat and moisture rise into the atmosphere, billowing clouds, scattered showers, and thunderstorms form.

As the thunderstorms multiply, they can get picked up by low-pressure systems drifting through equatorial waters, forming a tropical depression. The spin of the Earth causes the winds within the storm to whirl around the center of the low pressure. This spinning can form an eye of a storm.

The strongest part of a hurricane is the eye wall, on the edge of the calm center. "The size of the eye wall can vary, and the intensity of the storm can vary depending on how much heat is available" and other factors such as high altitude winds, Barnes said.

According to Emanuel, if global temperatures continue to rise, it is reasonable to assume that hurricane activity will increase, as there is more heat to drive the storms.

Global Observation

Previous studies have tried to measure whether typhoons and hurricanes were becoming more frequent. Emanuel's research, however, focused on the total energy generated by the storms over their duration.

"They can have the same frequency, but if they get stronger or last longer this metric will show an increase," he said.

When Emanuel looked at the hurricane record in the North Atlantic, where the storms of most interest to U.S. residents form, he found that intensity fluctuated from decade to decade.

This fluctuation roughly corresponded with factors such as the El Niño weather phenomenon, which has been shown to influence hurricane formation.

However, North Atlantic hurricanes account for only 12 percent of the total number of hurricanes and typhoons that form globally each year, Emanuel said.

"If you look at a more global measure of this metric, you don't see these strong interdecadal swings. They cancel each other out between one ocean and the other," he said. "You see instead a large upward trend."

According to Emanuel, on a global scale, the strength of storms corresponds with ocean temperatures: It goes up when temperatures go up, down when temperatures goes down.

Most scientists say the rise in sea surface temperature in the last 30 to 50 years is a signal of global warming.

"That's their conclusion, not mine," Emanuel said. "[But] it would follow reasonably well from this metric that the upswing [in intensity] … is a result of global warming."

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