From: Timothy Gardner, Reuters
Published May 24, 2007 12:00 AM

Killer Hurricanes Thrived in Cool Seas

NEW YORK -- Hurricanes over the past 5,000 years appear to have been controlled more by El Nino and an African monsoon than warm sea surface temperatures, such as those caused by global warming, researchers said Wednesday.


The study, published in the journal Nature, adds to the debate on whether seas warmed by greenhouse gas emissions lead to more hurricanes, such as those that bashed the Gulf of Mexico in 2005.


Some researchers say warmer seas appear to have contributed to more intense hurricanes, while others disagree. The U.N. International Panel on Climate Change said this year it was more likely than not that humans contribute to a trend of increasingly intense hurricanes.


Frequent strong hurricanes thrived in the Western Atlantic during times of weak El Ninos, or warming of surface waters of the Eastern Pacific Ocean, and strong West African monsoons even when local seas were cooler than now, the study said.


"Tropical sea surface temperatures as warm as at present are apparently not a requisite condition for increased intense hurricane activity," Jeffrey Donnelly, the lead author and researcher at the Woods Hole Oceanographic Institution, said in the study.


Intense hurricanes made landfall during the latter half of the Little Ice Age, a period of cooling that occurred approximately from the 14th to mid-19th centuries, he said.


Donnelly took core sediment samples from coastal lagoons in Puerto Rico to determine the frequency and strength of hurricanes that hit the Caribbean island over thousands of years. The storms whipped up sand and other coarse grains that were deposited in the lagoons.


He compared the deposits with historic paleoclimatology records to determine that the storms hit during periods when El Ninos were weak and when Western African monsoons were strong.


Intense hurricanes hit when local sea surface temperatures were warm or cool. In fact, "the Caribbean experienced a relatively active interval of intense hurricanes for more than a millennium when local sea surface temperatures were on average cooler than modern," the study said.


Changes in intense hurricane activity should be better predicted with more study of the Eastern Pacific and West African climate patterns, it said.


Source: Reuters


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