By Jim Loney
MIAMI, Jan 23 (Reuters) - Rising ocean temperatures linked to global warming could decrease the number of hurricanes hitting the United States, according to new research released on Wednesday.
The study, published in Geophysical Research Letters, challenges recent research that suggests global warming could be contributing to an increase in the frequency and the intensity of Atlantic hurricanes.
At the same time, it reaffirmed earlier views that warmer sea waters might result in atmospheric instabilities that could prevent tropical storms from forming.
Atlantic storms play a pivotal role in the global energy, insurance and commodities markets, particularly since the devastating 2004 and 2005 hurricane seasons, which hammered U.S. oil and gas production in the Gulf of Mexico.
The new study suggests that warmer seas, caused by greenhouse gases blamed for a rise in global temperatures, are linked to an increase in vertical wind shear, a difference in wind speeds at different altitudes that can tear apart nascent cyclones.
Hurricanes feed on warm water, leading to conventional wisdom supported by some recent research that global warming could be revving up more powerful storms.
But the new study, by oceanographer Chunzai Wang of the National Oceanic and Atmospheric Administration, and Sang-Ki Lee, a scientist at the University of Miami, examined 150 years of hurricane records and found a small decline in hurricanes making landfall in the United States as the oceans warmed.
"The attribution of the recent increase in Atlantic hurricane activity to global warming is premature. ... Global warming may decrease the likelihood of hurricanes making landfall in the United States," the researchers wrote.
Much of the recent research focused on the total number of tropical storms and hurricanes recorded in the Atlantic Ocean and Caribbean, but Wang said the number of those hurricanes actually hitting the United States is a much better indicator.
Prior to the mid-1960s when satellites and other technology made it easier to spot cyclones, some tropical storms and hurricanes lived and died far out at sea, undetected.
As a result, scientists trying to track long-term trends in the frequency of Atlantic storms work with uncertain data.
"We believe U.S. landings for hurricanes are most reliable measurements over the long term," Wang said.
The study found that warming of the tropical Pacific and Indian oceans increases Atlantic wind shear while rising sea temperatures in the tropical North Atlantic decrease shear.
The two effects compete, but the net impact is an increase in wind shear in the main Atlantic hurricane development zone, from the west coast of Africa to Central America.
"The Pacific and Indian warming wins and the result is a decrease in landfalling U.S. hurricanes," Wang said.
In 2004, four strong hurricanes hit Florida, causing billions of dollars in damage across the state. In 2005, a record-breaking 28 tropical storms formed, including Katrina, which killed 1,500 people and caused $80 billion damage.
The back-to-back years of unusually intense hurricane activity fueled debate about the impact of global warming. (Editing by Michael Christie and Todd Eastham)