The investigators estimate that 320 million large trees were killed or severely damaged by the intense hurricane that made landfall on August 29, 2005.
The Interstate 10 bridges that cross Lake Pontchartrain east of New Orleans are seen here pre- and post-Katrina. Bayou Sauvage National Wildlife Refuge is the large patch of forest that shows green in the left image. After Katrina inflicted heavy tree mortality it shows red in the right image. (Satellite images courtesy NASA)
Young forests are valued as carbon sinks, which remove carbon dioxide from the atmosphere and store it in growing vegetation and soils. In the aftermath of a storm as intense as Katrina, vegetation killed by the storm decomposes over time, reversing the carbon storage process, making the forest a carbon source. "The loss of so many trees will cause these forests to be a net source of carbon dioxide to the atmosphere for years to come," said Chambers. "If, as many believe, a warming climate causes a rise in the intensity of extreme events like Hurricane Katrina, we're likely to see an increase in tree mortality, resulting in an elevated release of carbon by impacted forest ecosystems."
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. "The carbon cycle is intimately linked to just about everything we do, from energy use to food and timber production and consumption," said Chambers.
"As more and more carbon is released to the atmosphere by human activities, the climate warms, triggering an intensification of the global water cycle that produces more powerful storms, leading to destruction of more trees, which then act to amplify climate warming," he explained.
Study co-author George Hurtt of the University of New Hampshire Institute for the Study of Earth, Oceans, and Space says, "This could potentially escalate the problem of global warming." Hurtt, an ecologist who specializes in mathematical modeling, says the next step is to put the data into sophisticated computer models to better gauge the magnitude of the potential positive feedback mechanism.
"We now need to figure out how strong of a feedback this might be, how much more severe these storms might become for every unit of global warming, and how the forests will be affected by the storms." Hurtt says. "Until that work is done, it's not clear if current projections are overly alarmist or too conservative." 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.
"It is surprising to learn that one extreme event can release nearly as much carbon to the atmosphere as all U.S. forests can store in an average year," said Diane Wickland, manager of the Terrestrial Ecology Program at NASA Headquarters in Washington. "Satellite data enabled Chambers' research team to pin down the extent of tree damage so that we now know how these kinds of severe storms affect the carbon cycle and our atmosphere," she said. The study was funded by the U.S. Department of Energy's National Institute of Climatic Change Research.