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Planet Ark World Environment News Global Warming May Spur Wind Shear, Sap Hurricanes

Date: 18-Apr-07
Country: US
Author: Jim Loney

The study, to be published on Wednesday in Geophysical Research Letters, found that climate model simulations show a "robust increase" in wind shear in the tropical Atlantic during the 21st century from global warming.

Wind shear, a difference in wind speed or direction at different altitudes, tends to tear apart tropical cyclones, preventing nascent ones from growing and already-formed hurricanes from becoming the monster storms that cause the most damage.

The effect of global warming on wind shear is similar to the impact of El Nino, the periodic eastern Pacific warm-water phenomenon that tends to put a damper on Atlantic storms. The sudden development of El Nino was credited for an unexpectedly mild Atlantic season last year, when only 10 storms formed.

Debate on the likelihood that human-generated climate change contributes to hurricane development has raged since the 2005 Atlantic season, which produced a record-shattering 28 tropical storms and hurricanes.

That season saw some of the most powerful hurricanes in history and produced Katrina, which killed 1,500 people and caused US$80 billion damage on the US Gulf Coast. The hurricane threat roiled global oil and gas markets.

HEATING UP SEA WATER

In recent years some scientists have suggested that human-induced greenhouse warming may be increasing the intensity of Atlantic hurricanes by heating up the sea water from which they draw their energy.

In February a report by the Intergovernmental Panel on Climate Change said it was "more likely than not" that humans contribute to a trend of increasingly intense hurricanes.

But researchers in the new study said increased wind shear could counter the effect of warming waters in the Atlantic.

"The environmental changes found in the study do not suggest a big increase in tropical Atlantic hurricane activity during the 21st century," said Brian Soden, a co-author of the report.

The increase in wind shear was only seen in the Atlantic and eastern Pacific. In the western Pacific, global warming appeared to cause both increased water temperatures and a reduction in wind shear, Soden said.

The study, by scientists at the University of Miami's Rosenstiel School and a National Oceanic and Atmospheric Administration lab in Princeton, New Jersey, is the first to identify changes in wind shear that could counteract the other impacts of global warming on hurricanes, the researchers said.

Soden said as the climate warms, atmospheric circulation tends to weaken and upper level westerlies shift further eastward into the Atlantic, increasing wind shear there. The effect is similar to an El Nino impact.

"The difference is that El Nino is a natural cycle, whereas the results from global warming are much smaller year-to-year but they accumulate over time," he said.

Hurricane researchers believe the Atlantic-Caribbean hurricane zone is in a period of heightened activity that began around 1995 and could last between 25 and 40 years.

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