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Researchers have cast doubt on the theory that tropical forests can store the greenhouse gas carbon dioxide for decades or even centuries, but have allayed fears that tropical rivers might be releasing previously unaccounted for carbon dioxide into the atmosphere.

In a paper published today in Nature, the scientists say that within just five years of trees in the Amazon basin absorbing carbon dioxide, the Amazon river and its tributaries return much of the gas to the atmosphere.

Plants use the carbon dioxide they absorb to grow, and the carbon ends up in rivers when rain washes fallen leaves, dead plants or soil into them.

"Our results were surprising because those who have previously made measurements found that the carbon in rivers that came from the surrounding forests was 40 to 1,000 years old," says Anthony Aufdenkampe, of the Stroud Water Research Center in the United States, who led the research with Emilio Mayorga of the University of Washington.

Until recently, it was thought the Amazon basin stored large amounts of carbon dioxide in its forest and soil, and that its rivers acted as pipes, carrying some of this carbon to the Atlantic — another important 'sink' for carbon storage.

But in 2002, Aufdenkampe and colleagues showed that only a small fraction of the carbon entering rivers reached the ocean.

Aufdenkampe collecting data
from the Urupa river, Brazil
Credit: Stroud Water Research
Center / B. Dickson

This is because microbes and animals feed on the carbon-rich soil, leaves and wood washed into the rivers by rainfall, and breathe out carbon dioxide.

"What that means," explains Aufdenkampe, "is that a lot of carbon is going into the forest [and rivers] but not staying there because it is leaking out [into the atmosphere]."

Aufdenkampe's latest findings show that the forest carbon is stored for only a very short period of time.

"The time from carbon dioxide being taken in by a leaf, dropped into a river where it is consumed, and then released back into atmosphere is five years or less," he says.

Carbon dioxide is the main greenhouse gas responsible for climate change. Scientists had hoped that large areas of forest — such as the Amazon — could act as huge carbon 'sinks', absorbing the gas from the atmosphere and preventing it from contributing to global warming.

Even a short storage period, on the scale of 100 years or so, says Aufdenkampe, would give us "breathing room" to deal with global greenhouse gas emissions. "We are showing that this whole process is so rapid it does not even give a temporary respite."

In an article accompanying the team's paper in Nature, Peter Raymond of Yale University, United States, says the good news is that rivers are not a source of carbon dioxide to the atmosphere that climate scientists had previously not known about.

"The carbon dioxide [released by tropical rivers] simply represents the cyclical movement of the gas from the atmosphere, through land and rivers and then back to the atmosphere, and does not represent an additional input of greenhouse gas," writes Raymond.

Aufdenkampe agrees, but adds that it also suggests there is no system by which forest carbon gets stored in forests or rivers for decades or even centuries before returning to the atmosphere.

Because rivers 'breathe out' the carbon within five years of receiving it, they are highly sensitive to changes such as deforestation, say the researchers. They found that in one region the carbon emitted by the rivers had come from farmland that had replaced the forest.

Because the Amazonian rivers get almost all of their carbon from the surrounding forests, deforestation can affect the biodiversity of rivers by reducing the availability of carbon for aquatic life to feed on.

Aufdenkampe and Mayorga's findings suggest that deforesting an area means cutting off a river's food supply almost immediately.

Link to a letter to the editor by E. Mayorga and A. Aufdenkampe, clarifying aspects of their research

Link to full paper in Nature

Reference: Nature 436, 538 (2005)

Link to SciDev.Net policy brief 'The lowdown on carbon sinks'