Global Warming Kicked 2005 Hurricanes Up A Notch

BOULDER, Colorado, June 26, 2006 (ENS) - Global warming created about half the extra warmth in the waters of the tropical North Atlantic that stimulated hurricane formation in 2005, while natural cycles were a minor factor, a new study from the National Center for Atmospheric Research demonstrates.

The research by world leading climate scientists contradicts recent claims that natural cycles are responsible for the increase in Atlantic hurricane activity since 1995 and adds support to the theory that hurricane seasons will become more active as global temperatures rise.

Hurricane Wilma was the most intense hurricane ever recorded in the Atlantic basin. The storm is shown here north and east of Mexico's Yucatan Peninsula October 19, 2005. (Photo courtesy NASA)

The new analysis by lead author Dr. Kevin Trenberth and associate scientist Dennis Shea of the National Center for Atmospheric Research (NCAR) will appear in the June 27 issue of "Geophysical Research Letters," published by the American Geophysical Union.

"The global warming influence provides a new background level that increases the risk of future enhancements in hurricane activity," says Trenberth, who heads NCAR's Climate Analysis Section.

Last year produced a record 28 tropical storms and hurricanes in the Atlantic. Hurricanes Katrina, Rita, and Wilma all reached Category 5 strength, the highest level on the Saffir-Simpson Hurricane Scale. Category 5 hurricanes carry winds greater than 155 mph (249 km/hr). The storm surge is greater than 18 feet (5.5 meters) above normal.

This year the National Weather Service Climate Prediction Center forecasts a "very active" season, with 13-16 named storms, 8-10 hurricanes, and 4-6 major hurricanes.

The 2006 prediction indicates a continuation of above-normal Atlantic activity that began in 1995, but forecasters say they do not currently expect a repeat of last year’s record season.

Dr. Kevin Trenberth, a New Zealand national, was a lead author of the 1995 and 2001 Scientific Assessments of Climate Change issued by the Intergovernmental Panel on Climate Change IPCC). He is now a coordinating lead author of the IPCC's 2007 Fourth Assessment Report. (Photo courtesy NCAR)

During much of last year's hurricane season, sea-surface temperatures across the tropical Atlantic between 10 and 20 degrees north, where many Atlantic hurricanes originate, were a record 1.7 degrees Fahrenheit above the 1901-1970 average.

By analyzing worldwide data on sea-surface temperatures since the early 20th century, Trenberth and Shea were able to calculate the causes of the increased temperatures in the tropical North Atlantic.

Their calculations show that global warming explained about 0.8 degrees F of this temperature rise.

Aftereffects from the 2004-05 El Nino accounted for about 0.4 degrees F.

The Atlantic multidecadal oscillation (AMO), a 60 to 80-year natural cycle in sea-surface temperatures, explained less than 0.2 degrees F of the rise, Trenberth says.

The remainder is due to year-to-year variability in temperatures.

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

Stronger hurricanes cause greater damage. Destroyed tractor trailers along gulf coast Highway I-90 near the port of Gulfport, Mississippi as a result of winds and tidal surge from Hurrican Katrina. October 5, 2005. (Photo by John Fleck courtesy FEMA)

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

The results show that the AMO is weaker now than it was in the 1950s, when Atlantic hurricanes were also active.

However, the AMO did contribute to the lull in hurricane activity from about 1970 to 1990 in the Atlantic.

Global warming does not guarantee that each year will set records for hurricanes, according to Trenberth. He notes that last year's activity was related to very favorable upper-level winds as well as the extremely warm sea-surface temperatures.

Trenberth says 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, a warming pattern in the tropical Pacific Ocean.

Still, the researchers conclude that over the long-term ocean warming will raise the baseline of hurricane activity.