Hurricane Katrina boosted greenhouse gases-report

By Deborah Zabarenko, Environment Correspondent

WASHINGTON, Nov 15 (Reuters) - Hurricane Katrina left a big "carbon footprint" along the U.S. Gulf Coast, where the destruction of large trees cut into the amount of greenhouse gases the area can absorb, researchers reported on Thursday.

The monster storm that tore across Louisiana and Mississippi in 2005 killed or severely damaged approximately 320 million big trees, the scientists wrote in the journal Science.

Trees, like other vegetation, soak up carbon dioxide, a potent global-warming gas, creating what climate scientists call a carbon sink. In a carbon sink, the carbon stays locked up in living trees instead of being released into the atmosphere to create greenhouse warming.

But when trees die, they stop taking in carbon dioxide and the process of decay starts emitting this gas, and that is what happened after Katrina roared through, said Jeffrey Q. Chambers of Tulane University.

All those dead and damaged trees that had been reducing carbon emissions suddenly became emitters. Altogether, they gave off about 100 million tons of carbon, enough to offset the total amount of carbon that forest trees in the United States suck up in a year.

"It was astonishing that a single storm could be of that magnitude," Chambers said by telephone from Manaus, Brazil, where he was doing field work.

A carbon footprint is a measure of the impact that activities have on the environment in terms of the amount of greenhouse gases produced, measured in units of carbon dioxide.

Compared with total carbon emissions from fossil fuel burning, emissions from the Katrina-ravaged forests are a tiny fraction, he said.

To figure out how many trees were hit by the big storm, Chambers and his colleagues reviewed satellite images of the area from 2003 and compared them with images from 2006. The research was published in the current edition of the journal Science.

He noted that some climate scientists, including most of those on the U.N. Intergovernmental Panel on Climate Change, have predicted a greater number of powerful storms as the planet warms. If this is the case, the dead-tree effect could add another dimension to storm disasters.

And while the impact of one storm, even one as strong as Katrina, is small by comparison with other sources of greenhouse emissions, the impact could climb if more storms each year are intense, he said.

"What if you look at a year when you've got 25 big tropical cyclones all over the world?" Chamber said. "It starts adding up."