320 Million Trees Lost to Katrina, Fuels Global Warming

Major hurricanes may not only be fueled by global warming, they may also contribute to it, according to a new study that puts the Hurricane Katrina death toll for trees at 320 million.

Recent research suggests that in our warming world, devastating hurricanes, like Katrina, may become more common. The new study, detailed in the Nov. 16 issue of the journal Science, adds another element to this dilemma, suggesting that the damage these hurricanes cause may actually fuel global warming due to the loss of carbon-consuming trees.

Tulane University researchers estimated the number of trees felled by Katrina using satellite imagery taken before and after the storm. Forest trees act as a carbon sink, sucking up carbon dioxide from the atmosphere to provide themselves with food. Carbon dioxide, a greenhouse gas, is the major driver of global warming.

When trees are destroyed, they decay, returning the carbon stored inside them to the atmosphere.

The total amount of biomass lost by the destruction of the trees lost during Katrina was about 105 teragrams of carbon (for comparison, the Great Pyramid of Giza weighs 6 teragrams), or at least half of the net annual carbon sink in U.S. forest trees.

Trees accumulate carbon as they grow, year by year, storing that carbon as their wood and leaves. But when they die, fungi, bacteria, termites and other decomposers consume that biomass and release all the carbon that the tree has accumulated over its lifetime. So when a huge number of trees are killed off by an event like Katrina, they become a carbon source, releasing anywhere from half to 140 percent of the carbon that all the trees in the United States take up in a year.

While it will take a number of decades for all of the carbon from the lost Katrina trees to be released, that is still less time than it took to build up the biomass of the forest into a carbon sink, said study leader Jeffrey Chambers.

While these lost trees will eventually be replaced by other vegetation, it will be younger and smaller and therefore a smaller carbon sink than the Gulf Coast forests once were.

If hurricanes and other forces of disturbance become more frequent in the future, forests may never have a chance to fully recover, the researchers say, permanently eliminating the carbon-sink work they do and allowing all that previously stored carbon to stay in the atmosphere.

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