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Amazon basin stores carbon dioxide 'for just five years'

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  • Mike Neuman
    Amazon basin stores carbon dioxide for just five years Catherine Brahic 28 July 2005 Source: SciDev.Net Researchers have cast doubt on the theory that
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      Amazon basin stores carbon dioxide 'for just five years'
      Catherine Brahic
      28 July 2005
      Source: SciDev.Net

      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.

      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.
      http://www.scidev.net/News/index.cfm?
      fuseaction=printarticle&itemid=2255&language=1
      Link to full paper in Nature

      Reference: Nature 436, 538 (2005)
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