Amazon basin stores carbon dioxide 'for just five years'
28 July 2005
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
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
"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
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.
This is because microbes and animals feed on the carbon-rich soil,
leaves and wood washed into the rivers by rainfall, and breathe out
"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
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
Aufdenkampe and Mayorga's findings suggest that deforesting an area
means cutting off a river's food supply almost immediately.
Link to full paper in Nature
Reference: Nature 436, 538 (2005)