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Global Warming Topped Natural Cycles in Fueling 2005 Hurricanes

Hurricane Katrina approches landfall, Aug. 28, 2005 (Credit: NASA/Jeff Schmaltz, MODIS Land Rapid Response Team)

Washington -- Global warming accounted for about half of the extra hurricane-fueling warmth in the waters of the tropical North Atlantic in 2005, and natural cycles were a minor factor, according to a new analysis by researchers at the National Center for Atmospheric Research (NCAR).

The research was supported by the U.S. National Science Foundation (NSF), NCAR's primary sponsor, according to a joint June 22 press release by NCAR, NSF and the American Geophysical Union.

"The global warming influence provides a new background level that increases the risk of future enhancements in hurricane activity," said NCAR’s Kevin Trenberth.

The study runs counter to recent claims that natural cycles are responsible for the upturn in Atlantic hurricane activity since 1995. It also supports the premise that hurricane seasons will become even more active as global temperatures rise.

In 2005, there were a record number of 28 tropical storms and hurricanes in the Atlantic. Hurricanes Katrina, Rita and Wilma all reached Category 5 strength. (See related article.)

The research by Trenberth and Dennis Shea, also from NCAR, focuses on an increase in ocean temperatures.

During much of the 2005 hurricane season, sea-surface temperatures across the tropical Atlantic between 10 degrees and 20 degrees north, where many Atlantic hurricanes originate, were a record 0.94 degrees Celsius above the 1901-1970 average.

Researchers agree that the warming waters fueled hurricane intensity, but they have been uncertain about whether Atlantic waters have heated up because of a natural decadeslong cycle or because of global warming.

Analyzing worldwide data on sea-surface temperatures since the early 20th century, Trenberth and Shea calculated that global warming accounted for about 0.4 degrees Celsius of this rise.

Aftereffects from the 2004-2005 El Nino -- a warming of the ocean surface off the western coast of South America that occurs every 4-12 years -- accounted for about 0.2 degrees Celsius increase.

The Atlantic multidecadal oscillation, a 60- to 80-year natural cycle in sea-surface temperatures, explained less than 0.1 degrees Celsius of the rise, Trenberth said. The rest is due to yearly temperature variation.

Previous studies have attributed the warming and cooling patterns in the North Atlantic in the 20th century -- and associated hurricane activity -- to the Atlantic multidecadal oscillation.

Trenberth, suspecting that global warming also played a role, looked beyond the Atlantic to temperature patterns throughout Earth's tropical and midlatitude waters.

He subtracted the global trend from the irregular Atlantic temperatures -- in effect, separating global warming from the Atlantic natural cycle.

The results show that the Atlantic multidecadal oscillation is actually much weaker now than it was in the 1950s, when Atlantic hurricanes also were quite active. But it did contribute to the lull in hurricane activity from about 1970 to 1990 in the Atlantic.

But global warming does not guarantee that each year will set records for hurricanes, Trenberth said. The 2005 activity was related to very favorable upper-level winds, he added, and extremely warm sea-surface temperatures.

Each year will bring ups and downs in tropical Atlantic sea-surface temperatures due to natural variations, such as the presence or absence of El Nino, he said. But the long-term ocean warming should raise the baseline of hurricane activity.

Text of the press release is available at the NCAR Web site.