Amazon not holding expected carbon, carbon-credit trading scheme at risk?
Rice University press release
July 28, 2005
The rivers of South America’s Amazon basin are “breathing” far harder cycling the greenhouse gas carbon dioxide more quickly than anyone realized.
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, said Emilio Mayorga, University of Washington oceanographer and lead author of the Nature piece with Anthony Aufdenkampe of the Stroud Water Research Center in Pennsylvania and Carrie Masiello of Rice University.
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.
Amazon rainforest in Peru. The Amazon River Basin is home to the largest rainforest on Earth. The basin — roughly the size of the forty-eight contiguous United States — covers some 40% of the South American continent and includes parts of eight South American countries: Brazil, Bolivia, Peru, Ecuador, Colombia, Venezuela, Guyana, and Suriname. The region is home to more species of plants and animals than any other terrestial ecosystem on the planet — perhaps 30% of the world’s species are found these forests.
“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 said. “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.”
The carbon age measurements were made by Masiello, Mayorga, and Aufdenkampe at the Department of Energy’s accelerator mass spectrometer at Lawrence Livermore National Laboratory.
“With this single study, we more than doubled the number of existing river radiocarbon measurements,” said Masiello, now an assistant professor of Earth science at Rice. “We were fortunate to have access to the DOE’s accelerator mass spectrometer at Lawrence Livermore because it’s one of only a few in the world that have the capability of making radiocarbon measurements this precisely.”
No previous tropical study has used both radioactive carbon-14 and stable carbon-13 isotopes to address these questions. Funding from the Center for Accelerator Mass Spectrometry at Lawrence Livermore National Laboratory made the analysis possible, and Masiello said the survey required hundreds of measurements.
“We had some sleepless nights generating this volume of data, but we needed to scale up to get a new view of how rivers interact with the atmosphere,” she said.
Carbon is carried by rains and groundwater into waterways from soils, decomposing woody debris, leaf litter and other organic matter. Once in waterways it is chewed up by microorganisms, insects and fish. The carbon dioxide they generate quickly returns to the atmosphere, some 500 million tons a year, an amount equal to what is absorbed each year by the Amazonian rainforest.
“River breath is much deeper and faster than anyone realized,” said co-author Jeff Richey, a University of Washington oceanographer.
Study samples were collected by Richey’s research group and Brazilian scientists on expeditions going back as far as 1991 that were funded by the National Science Foundation, National Aeronautics and Space Administration and the Research Support Foundation for the State of San Paulo (FAPESP), Brazil.
“Having established that the amount of carbon outgassing is much greater than anyone imagined, the issue then becomes, where does it come from,” Mayorga said. “If it’s young, that indicates the carbon pool is dynamic, which could make the system much more reactive to deforestation and climate change.”
For example, data from a region of active deforestation in the southern Amazon already shows that the carbon leaving rivers has an identifiable isotopic signature of pasture grasses.
“You’re changing the land use, changing vegetation and other conditions,” Mayorga said. “In terms of what’s being respired, the system is responding fairly quickly. Human and natural systems, in turn, will be impacted.”
Other co-authors are Paul Quay and the late John Hedges, both UW oceanographers; Alex Krusche of the University of São Paulo, Brazil; and Thomas Brown of the Center for Accelerator Mass Spectrometry at Lawrence Livermore National Laboratory.
The background image shows deforestation associated with the Tierras Bajas project in eastern Bolivia where people have been resettled from the Altiplano to cultivate soybeans. The photo is from NASA’s Earth Observatory.
Rice University press release