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Jan. 20, 2000

Impact from rainforest greenhouse gasses may not be as great as thought, study shows

EAST LANSING, Mich. - The Amazon rainforest is striking a balance - albeit a precarious one - in terms of the greenhouse gasses that are released as its rainforests are consumed, a study shows.

The study, published in today's edition of the British science journal Nature, indicates that the carbon released into the atmosphere by deforestation offsets that absorbed by new forests growing.

According to David Skole, director of MSU's Basic Science and Remote Sensing Initiative and one of the paper's authors, data that suggests that the tropical rainforest both giveth and taketh away creates new questions about the world's carbon cycle.

"It appears that tropical forests that have been thought of as large sources of greenhouse gasses may be neutral," Skole said. "This may change the way we look at the significance of fossil fuel emissions."

Skole co-authored the Nature paper "Annual Fluxes of Carbon from Deforestation and Regrowth in the Brazilian Amazon" with Walter Chomentowski from MSU; Richard Houghton, J.L. Jackler and K.T. Lawrence of Woods Hole Research Center in Massachusetts; and Carlos Nobre in Brazil. The group analyzed satellite data that charts deforestation each year, calculated carbon emissions caused by land-use changes, such as farmland, and estimated the biomass of forests and farmlands in Brazil from 1988 to 1998.

The result: The first year-to-date analysis not only of the carbon that is released into the atmosphere by deforestation and natural decay, but also of how much of that carbon is reused as rainforests grow back.

Carbon - the building block of life - weaves through all of life on earth in a delicate balance. It is released into the atmosphere from the burning of fossil fuels and wood, and from the decay of plant and animal material. These are known as "sources."

Likewise, atmospheric carbon re-enters the life cycle as trees draw upon carbon dioxide to grow and as the oceans absorb it. The sites of this reabsorption are known as "sinks."

Ultimately, scientists theorize, source and sink must be equal. Imbalances such as too much carbon gas released into the air result in global warming. It also appears that atmosphere rich in carbon gas can act as a fertilizer, spurring quicker growth of plant life.

Signs of quicker forest growth are a classic problem of seeing the forest for the trees. The change in growth rates is too small to measure on a single tree. "You wouldn't find anything if you went into a wood lot with calipers," Skole said.

Yet satellite data plus computer models can indicate changes that are small on a single tree, but significant in an entire forest - and how much carbon gas is absorbed.

One of the most difficult problems is that the balance of source and sink in the Amazon can fluctuate from year to year, and satellite monitoring is needed to capture these variations over large areas such as the Amazon with very high accuracy, Skole said.

The study was funded by the Landsat Program in NASA's Office of Earth Science, the Terrestrial Ecology Program, and the Land Cover and Land Use Change Program.

The Basic Science and Remote Sensing Initiative is a global change research program in the Department of Geography at Michigan State University. The goal is to develop an interdisciplinary approach to understanding global change, at both regional and global scales, through the integration of both physical and social sciences.