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Plants, Temperature and CO2 Uptake

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  • Mike Neuman
    Carbon Sink Or Carbon Source? Aerosols Play Significant Role In Shifts Raleigh NC (SPX) Dec 09, 2004 Researchers at North Carolina State University have shown
    Message 1 of 1 , Dec 15, 2004
      Carbon Sink Or Carbon Source? Aerosols Play Significant Role In Shifts

      Raleigh NC (SPX) Dec 09, 2004

      Researchers at North Carolina State University have shown that the
      amount of aerosols - dust particles, soot from automobile emissions
      and factories, and other airborne particles - in the atmosphere has a
      significant impact on whether the surface area below either absorbs
      or emits more carbon dioxide (CO2).

      The researchers discovered that changes in the levels of airborne
      aerosols resulted in changes to the terrestrial carbon cycle, or the
      cycle in which CO2 is absorbed by plant photosynthesis and then
      emitted by the soil.

      Besides documenting the effects of aerosols on the carbon cycle, the
      research also showed that the type of landscape also influenced
      whether a surface area served as a carbon sink, an area that absorbs
      more CO2 than it emits, or as a carbon source, an area that emits
      more CO2 than it absorbs.

      [Caption] Researchers used data from six sites within NASA's AERONET
      (AErosol RObotic NETwork). Sites represented a wide variety of
      landscapes, including forests, cropland and grassland. This site in
      Walker Branch, Tenn., shows a sun photometer over a broadleaf
      deciduous forest. The sun photometer measures radiation and aerosol
      properties that impact light.

      In the research project, six locations across the United States -
      encompassing forests, croplands and grasslands - were studied.
      Increased amounts of aerosols over forests and croplands resulted in
      surface areas below becoming carbon sinks, but increased amounts of
      aerosols over grasslands resulted in surface areas becoming carbon

      Dr. Dev Niyogi, research assistant professor of marine, earth and
      atmospheric sciences at NC State and lead author of the study,
      hypothesizes that the differences among landscapes can be attributed
      to the amount of shade provided by tree and plant leaves in forests
      and croplands.

      The lack of shading in grasslands changes the ground surface
      temperature, which alters the rate of photosynthesis in plants and
      the CO2 emissions by soil. Since plants want to take in CO2 but also
      preserve water at the same time, Niyogi believes the lack of shade
      and increased temperatures may cause plants to slow the rate of
      photosynthesis, causing less CO2 to be absorbed and thus more CO2 to
      be effectively emitted. That would make the surface area a carbon

      The research was published in Geophysical Research Letters, a journal
      of the American Geophysical Union. Niyogi's co-authors on the
      research paper include NC State graduate student Hsin-I Chang; Dr.
      Vinod Saxena, professor of marine, earth and atmospheric sciences at
      NC State; Dr. Randy Wells, professor of crop science at NC State; Dr.
      Fitzgerald Booker, associate professor of crop science at NC State
      and USDA-ARS plant physiologist; Dr. Teddy Holt, adjunct professor of
      marine, earth and atmospheric sciences at NC State and a scientist at
      Naval Research Laboratory-Monterey; and colleagues from across the

      Aerosols have been known to affect the climate by changing the
      radiation that reaches the earth surface. Increase in aerosols is
      often considered one possible reason that the earth's surface has not
      seen as much warming as previously projected by climate models.

      Previous studies have shown that many factors affect the carbon
      cycle, including rainfall and changes in land cover. But this study
      is believed to be the first multisite, observational analysis
      demonstrating that aerosols affect the carbon cycle. The study shows
      aerosols affect the earth's regional climate in an even more profound
      manner by affecting its biological and chemical exchanges of the
      greenhouse gases.
      The study examined six sites across the United States in the
      summertime; these locations were chosen because data on aerosols and
      carbon fluxes, or the changes in the carbon absorption and emission
      rates, were readily available. Sites ranged from grassland in Alaska
      to mixed forestland in Wisconsin to cropland in Oklahoma.

      Before showing the effects of aerosols on the carbon cycle, the paper
      first showed the effects of diffuse radiation - radiation that is not
      direct sunlight but radiation scattered by clouds, haze, or something
      else - on carbon fluxes. The research showed that higher levels of
      diffuse radiation resulted in higher rates of carbon sink.

      Although common sense would suggest that areas with plants receiving
      more constant direct sunlight would result in a surface becoming a
      carbon sink, that is not necessarily the case, Niyogi says. In fact,
      more radiation means plants more quickly reach a level of

      As Niyogi explains it, "Plants absorb CO2 very efficiently. At very
      high levels of radiation, as is the case with direct radiation,
      additional increases do not necessarily cause increased
      photosynthesis. It doesn't matter how much more radiation you add,
      the plant is not going to absorb more CO2. But at lower levels of
      radiation, as is the case with diffuse radiation, any increase in
      radiation translates to additional photosynthesis."

      The study then examined the effects of cloudiness on the carbon
      cycle. Cloudiness, which increased the amount of diffuse radiation,
      resulted in a greater amount of carbon sink in surface areas.
      The study team then linked aerosols and diffuse radiation, and showed
      strong relationships between high amounts of aerosols and high
      amounts of diffuse radiation and between low amounts of aerosols and
      low amounts of diffuse radiation.
      Finally, the study yielded its most important findings: Aerosols
      affect the carbon cycle in different types of landscapes, with
      forests and croplands serving as carbon sinks while grasslands served
      as carbon sources.
      "When you have more carbon being absorbed, it means that plants and
      forests there are going to grow faster," Niyogi said. "And so it has
      the potential to alter the landscape. And when you have a change in
      landscape, or a change in the biogeochemical properties - like the
      carbon cycle - you have a landscape that is actively vulnerable to
      climate change.

      "Studies like these can really start putting forward the right
      processes in trying to quantify the carbon sink more accurately. Once
      we start introducing these reality-based processes into our models,
      we'll get better estimates" of carbon budget, Niyogi said.

      Niyogi now plans to add other variables to studying the carbon cycle,
      such as the effects of different types of aerosols, and factors like
      soil moisture. He is also planning regional and global analyses -
      using satellite remote sensing and models - to see if results square
      with the field studies.

      The research was funded by NASA, the National Science Foundation, the
      Office of Naval Research, and an NC State Faculty Research and
      Professional Development Award.
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