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Hurricane surge may be part of a long cycle

June 07, 2007|Alan Zarembo | Times Staff Writer

A surge in major Atlantic hurricanes over the last decade -- often cited as evidence of increasing global warming -- may not be a surge at all but a return to normal storm patterns, according to a new study.

Using nearly three centuries of hurricane history recorded in organic storm debris encased in coral reefs, researchers found that the frequency of major hurricanes today was about the same as it was during extended periods from the mid-1700s to the mid-1900s.

"There were periods that were just as active as we see now," said study coauthor Terrence M. Quinn, a paleoclimatologist at the University of Texas at Austin.

At the same time, the researchers found that the number of major hurricanes from the late 1960s to the early 1990s -- a period that our present cycle is often compared to -- was unusually low.

The authors of the study, published Wednesday in the journal Nature, said their findings did not refute theories that global warming was increasing the frequency of major hurricanes. Rather, they argue that there are more powerful forces at work than rising water temperatures. Chief among suspects is a poorly understood atmospheric condition known as vertical wind shear.

It is well established that when wind shear is high, it is more difficult for hurricanes to form and gain strength. Scientists do not understand why vertical wind shear increases and decreases with time.

Some research has suggested that further global warming could change atmospheric circulation in a way that increases vertical wind shear, creating conditions unfavorable for major hurricanes. That could counteract the hurricane-inducing effects of warmer water.

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alan.zarembo@latimes.com

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