Global warming cited in storms

Boulder scientists claim phenomenon led to hurricanes

Jim Erickson, Rocky Mountain News

Published June 26, 2006 at midnight

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Global warming provided much of the ocean heat that fueled last year's record-setting hurricane season, two Boulder climate scientists contend.

Kevin Trenberth and Dennis Shea say global warming accounted for nearly half of the above-normal ocean heating that fed 15 Atlantic hurricanes, four of which struck the U.S. coastline.

Natural climate cycles were only a minor factor in the excess warming, they contend.

But other researchers reject the Boulder team's conclusions.

Trenberth and Shea work at the National Center for Atmospheric Research in Boulder. Their research paper, to be published in the June 27 edition of the journal Geophysical Research Letters, is the latest volley fired in an ongoing skirmish over global warming's role - if any - in recent powerful hurricanes.

One side in the debate says the human-caused buildup of heat-trapping "greenhouse" gases, such as carbon dioxide, is warming the tropics and has helped strengthen recent hurricanes.

The other side argues that recent warming in the tropical Atlantic is part of a natural cycle and has little, if anything, to do with greenhouse gases emitted by the burning of fossil fuels.

In their new study, Trenberth and Shea attempt to separate human contributions to the 2005 Atlantic warming from natural climate variability.

During much of last year's hurricane season, sea surface temperatures in a swath of the tropical Atlantic between 10 and 20 degrees north latitude were a record 1.66 degrees Fahrenheit above normal. That's a region where many Atlantic hurricanes form.

Trenberth and Shea calculated that global warming accounted for 48.9 percent of the above-normal 2005 water temperatures.

The after-effects of the 2004-2005 El Niño accounted for 21.7 percent of the 2005 increase, according to Trenberth and Shea. El Niño is a periodic warming of the tropical Pacific that affects climate worldwide.

A natural, long-term fluctuation in Atlantic sea-surface temperatures, called the Atlantic Multidecadal Oscillation, or AMO, was responsible for just 10.9 percent of the increase.

Previous studies have attributed a greater share of the Atlantic warming to the AMO. But to get an accurate view of the AMO's contribution to tropical Atlantic warming, you must first subtract global warming's effects on the oscillation, Trenberth and Shea contend.

But Colorado State University hurricane forecaster William Gray said it's not that simple.

"You can't get at the AMO by subtracting out global temperature from it," he said. "Salinity is the main driver for the AMO, not temperature."

Gray also said the NCAR researchers placed too much emphasis on warm sea waters and ignored other factors that have a greater influence on the formation and intensification of hurricanes: vertical and horizontal wind shear, mid-level moisture, and sea-surface air pressure.

"I'm not at all impressed with this paper," Gray said Thursday. "It just doesn't get the job done, in my view."

Roger Pielke Jr. of the University of Colorado's Center for Science and Technology Policy Research said the Trenberth and Shea study is solid "but does not provide any smoking gun or final word on the hurricane issue."

"The debate among the climate community will still be ongoing, I'm sure, after this paper's digested," Pielke said.

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