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BU geographers map carbon dioxide absorption of world's forests

By David J. Craig

Scientists and environmental advocates for years have urged that nations be held accountable for the irresponsible destruction of forests. But until recently there existed no reliable way to measure the size or density of forests in remote areas.

 

Robert Kaufmann (center) and Ranga Myneni (right), CAS associate professors of geography, and Jiarui Dong (GRS'02), a postgraduate researcher in the geography department, were among an international team of scientists who showed recently that satellite images can be used to determine how much carbon dioxide is absorbed by sections of forest too remote to be monitored on the ground. Photo by Kalman Zabarsky

 
 

A new study by an international team of researchers that includes three BU scientists, however, finds that satellite images can be used to map forests anywhere in the world and to determine the rate at which particular sections of forest absorb carbon dioxide through photosynthesis. The gas is considered the chief cause of global warming. The study, "A Large Carbon Sink in the Woody Biomass of Northern Forests," appears in the December 18 issue of the Proceedings of the National Academy of Sciences of the United States of America.

Robert Kaufmann and Ranga Myneni, both CAS associate professors of geography, and Jiarui Dong (GRS'02), a doctoral candidate in the geography department, were among the study's nine authors, who include scientists from Austria, Finland, and Russia. Myneni and Dong were the lead authors of the study, which was funded by NASA's Earth Science Enterprise, a long-term research program dedicated to understanding changes in the global environment.

By analyzing images of the world's northernmost forests taken by U.S. National Oceanic and Atmospheric Administration (NOAA) satellites between 1981 and 1999, the researchers determined that trees in the United States, Europe, and Russia stored about 700 million metric tons of carbon, taken in by the trees as carbon dioxide, each year during that period. That means they absorbed about 12 percent of all the carbon dioxide created annually by the burning of fossil fuels and other industrial activities. American forests stored 140 million tons of carbon, representing 11 percent of the nation's carbon dioxide emissions.

Most forests absorbed carbon dioxide and stored carbon at a consistent rate between 1981 and 1999, with the exception of large areas of Canada's needle-leaf forests, which in the late 1990s were found to be losing carbon to the atmosphere, probably, according to the paper, because of the increased incidence of fires and deadly insect infestations. (Through photosynthesis, plants absorb carbon dioxide, turning it into sugar and oxygen, and store unused carbon molecules in a form that does not contribute to global warming. Carbon, which composes about 50 percent of all wood, is released back into the environment as carbon dioxide, however, when forests burn or when trees die and decompose.)

Forests in the Nordic countries and in some parts of Russia and the United States were found to be storing larger amounts of carbon dioxide in the late 1990s than in the early 1980s, probably because of declining harvests, forest cultivation, fire suppression, and ironically, longer growing seasons due to warming temperatures.

The study was the first to use remote sensing technology to successfully measure biomass, or roughly the size and density of forests, according to Myneni. "Scientists have looked at how the greenness of our planet changes over time," he says, "but we translated that greenness into biomass, and into carbon dioxide absorption."

Sophisticated ground-based forest inventories are routinely done in many parts of the world. Researchers also can calculate the rate at which a section of forest absorbs carbon dioxide through photosynthesis by chopping down a few trees and sampling their carbon content and by considering factors such as the average height, width, and age of the trees in the area. Still, vast tracts of forests in large countries such as Canada and Russia are never inventoried or measured for carbon content because of their remoteness.
Researchers in the NASA-funded study analyzed carbon measurements collected by traditional methods in a few sections of northern forests between 1981 and 1999 to determine how much carbon dioxide was absorbed each year. Using high-resolution NOAA satellite images, they then analyzed the changes in surface greenness in those areas over the same period, which indicated the change in biomass, and created an equation to estimate how much carbon dioxide was absorbed by every eight-square-kilometer tract of land north of the 30th parallel. They then extrapolated how much carbon dioxide was absorbed every year over the 18-year span in nearly 3.7 billion square kilometers of forest.

"My job was to see if our measurements of carbon absorption and storage, which were done by looking at satellite imagery, were consistent with other ground-based estimates," says Kaufmann. "It was fairly accurate. Our estimates neither over- nor under-predicted the amounts of carbon."
Previously, scientists knew that land and oceans store about half of the seven to eight billion tons of carbon dioxide emitted annually from the burning of fossil fuels and the cutting and burning of tropical forests, but details of the earth's so-called "carbon sinks" were unknown. A carbon sink refers to a geographical area that absorbs great amounts of carbon dioxide, such as a large, dense, and quickly growing forest.

"In a country such as Russia, scientists had a few observations of forest land and carbon levels from all over the country, but they were spread very far apart," says Kaufmann. "Interpolating spatially between different areas is hard to do on a matter like this. But there is no interpolation in what we did. It's as if we put one person in every eight square kilometers and had them measure the amount of biomass there."

The research tools developed in the study, Myneni says, could provide a means of testing compliance with the proposed Kyoto Protocol, an international treaty that would consider a nation's ability to absorb carbon dioxide naturally when determining the amount of climate-warming greenhouse gases it can emit. Nations would be credited only for those carbon sinks, or forests that absorb great amounts of carbon dioxide, cultivated specifically for that purpose.

       

14 December 2001
Boston University
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