( The rivers of South America's Amazon ba...)Amazon source of 5-year-old river breath

Most of the carbon being exhaled ?or outgassed ?as carbon dioxide from Amazonian rivers and wetlands has spent a mere 5 years sequestered in the trees, other plants and soils of the surrounding landscape, U.S. and Brazilian researchers report in the July 28 issue of Nature.

It had been hoped that regions such as the nearly 2.4 million-square-mile Amazon River basin ?where tropical forests rapidly gulp carbon dioxide during photosynthesis ?were holding onto that carbon for decades, even centuries, says Emilio Mayorga, University of Washington oceanographer and lead author of the Nature piece with Anthony Aufdenkampe of the Stroud Water Research Center in Pennsylvania.

As policy makers turn increasingly to carbon-credit trading as a means of grappling with the impacts of human-induced climate change, knowing how much carbon can be stored ?and where and for how long ?is critical, the authors say.

"Our results were surprising because those who've previously made measurements found carbon in the rivers that came from the surrounding forests to be 40 to more than 1,000 years old," Aufdenkampe says. "They assumed that the return of this forest carbon to the atmosphere must be a slow process that offered at least temporary respite from greenhouse effects.

"As part of the largest radiocarbon age survey ever for a single watershed, we show that the enormous amount of carbon dioxide silently being returned to the atmosphere is far younger than carbon being carried downstream," he said. "Previous studies failed to detect the rapid recycling of forest carbon because they never dated the invisible greenhouse gas as it is literally exhaled by the river organisms."

"River breath is much deeper and faster than anyone realized," says Jeff Richey, UW oceanographer and another co-author.