The Atlantic Ocean has experienced the highest level of hurricane activity since the medieval era, circa 1000 A.D., when cyclone activity might have been even more intense.
But the reasons for elevated hurricane activity today differ from a millennium ago, said Michael E. Mann, a Penn State University professor of meteorology and geosciences and director of the Penn State Earth System Science Center.
His team's study detailing the history of hurricanes in the Atlantic Ocean appears in the current edition of Nature.
Current-day hurricane activity, he said, can be explained by higher sea-surface temperatures in the Atlantic, possibly but not definitively linked to global warming. Activity 1,000 years ago combined elevated sea-surface temperatures in the North Atlantic with La Nina conditions in the Pacific Ocean to generate similar hurricane activity.
Fortunately, the current era has experienced more El Nino rather than La Nina conditions -- two distinct temperature and wind conditions in the Pacific Ocean that produce opposite weather effects for North, Central and South America with an impact on the Atlantic Ocean.
Mann said La Nina conditions, marked by cooler water temperatures in the Pacific, are more conducive to hurricane activity in the Atlantic. Had the North Atlantic experienced La Nina conditions in recent years, hurricane activity could have been even worse.
But the last 15 years, he said, didn't involve La Nina conditions. That leaves higher sea-surface temperatures to explain increased activity, including 28 named hurricanes in 2005.
"Does that mean global warming leads to more cyclones? That's an active area of debate," he said. "I wouldn't pretend that our study ends that."
But Mann said the study does add evidence to the debate that warmer temperatures favor more hurricane activity.
The study, "Atlantic hurricanes and climate over the past 1,500 years," gathered results from existing sediment studies from lagoons from the Caribbean and Gulf Coast to New England to determine land-falling hurricane activity over the past 1,500 years. The studies measured fine layers of sand that overwashed beaches into lagoons during hurricanes.
The study also used a computer model of tropical cyclones and factors driven by year-to-year variations in cyclone behavior and El Nino and La Nina activity, along with the history of sea-surface temperatures in the tropical Atlantic. The computer model and lagoon study produced similar results.
"There has to be an asterisk next to the conclusion that future warming will lead to more tropical cyclones," Mann said, noting assumptions in the model predict hurricane activity based on projections of La Nina and El Nino activity. "But all things being equal, warming oceans lead to more tropical cyclones."
Although ocean temperatures in recent decades have risen only 1 degree Fahrenheit, that's enough to spawn more hurricanes.
Warm sea-surface temperatures with cooler air above it create temperature instability that feeds hurricane activity. This year has been relatively quiet. But hurricanes in 2005, including Katrina that slammed New Orleans and the Gulf Coast, raised the specter of global warming's impact on hurricane activity.
"What's interesting about the Atlantic, it needs 27 degrees Celsius (or 80.6 degrees Fahrenheit) to produce tropical cyclones," Mann said. "A large part of the ocean is right at that threshold. A little bit of warming will expand the area where global warming could produce hurricane activity."
(David Templeton can be reached at dtempleton(at)post-gazette.com.)
(Distributed by Scripps Howard News Service, www.scrippsnews.com.)
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