Researchers led by biologist Jeffrey Chambers of Tulane University have determined that the losses inflicted by Hurricane Katrina on Gulf Coast forest trees are enough to cancel out a year's worth of new tree biomass (trunks, branches and foliage) growth in other parts of the country.
"The carbon that will be released as these trees decompose is enough to cancel out an entire year's worth of net gain by all U.S. forests. And this is only from a single storm," says Chambers, lead author of an article detailing the team's findings, "Hurricane Katrina's Carbon Footprint on Gulf Coast Forests," published in the Nov. 16 issue of the journal Science.
The study was carried out by researchers at Tulane and the University of New Hampshire. Using NASA satellite sensing technology, ecological field investigations and statistical analysis, the investigators estimate that 320 million large trees were killed or severely damaged by the August 2005 storm.
As the Earth's climate warms, evidence is accumulating that hurricanes, tornados and frontal systems will gain in energy, producing more violent storms and stronger winds. Increased wind disturbance will cause more tree mortality and damage, and this dead wood will release additional carbon to the atmosphere, potentially amplifying global warming.
Young, healthy forests play a vital role in removing carbon, in the form of carbon dioxide, a greenhouse gas, from the atmosphere by photosynthesis, and are thus important in the battle against warming. These young forests are valued as "carbon sinks," removing carbon dioxide from the atmosphere and storing it as growing vegetation.
The total amount of carbon stored in a forest is the result of the growth of new and existing trees, and tree death from age and disturbance. Dead trees and downed wood decompose and release carbon to the atmosphere. Thus, an increase in disturbance frequency, for example from more powerful storms, can tilt this balance toward the loss side, reversing the storing process and becoming a source of atmospheric carbon dioxide.
This increase in carbon emissions can enhance global warming in what is termed by scientists a "positive feedback mechanism." Increased carbon dioxide warms the climate, causing more intense storms and elevated tree mortality, releasing yet more carbon dioxide and further warming the climate.
The study was funded by the U.S. Department of Energy's National Institute of Climatic Change Research. NASA provided satellite data for the study and many of the methods used were first developed as part of a NASA Large Scale Biosphere-Atmosphere Experiment in Amazonia, led by Chambers.