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'Earthshine' fall heats global warming debate

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    ... From: Sonya To: Paleontology_and_Climate@yahoogroups.com Date: Fri, 28 May 2004 00:44:29 -0400 Subject: [P&C] Earthshine fall
    Message 1 of 1 , May 28, 2004
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      From: "Sonya" <msredsonya@...>
      To: Paleontology_and_Climate@yahoogroups.com
      Date: Fri, 28 May 2004 00:44:29 -0400
      Subject: [P&C] 'Earthshine' fall heats global warming debate
      Message-ID: <3A68560EF6414FC45BEBE26647F9C7FB@...>

      'Earthshine' fall heats global warming debate

      19:00 27 May 04 NewScientist.com news service

      A new study of earthshine, the sunlight reflected back onto the Moon
      from our planet, suggests that falling cloud cover could explain the
      warming of the Earth's lower atmosphere seen over the last 20 years.

      The idea presents a highly controversial alternative to most
      scientists' prime suspect for the warming - rising levels of
      greenhouse gases. However, although other researchers say the
      technique could produce useful data in the future, they argue the
      current study is simply not strong enough to draw meaningful
      conclusions.

      The new study, published in Science, was conducted by Enric Palle and
      colleagues at the Big Bear Solar Observatory in California and the
      California Institute of Technology in Pasadena. They report that the
      brightness of earthshine decreased steadily during the period 1984 to
      2000, although that trend appears to have reversed since.

      The decline suggests fewer clouds, which reflect sunlight, and
      therefore that more sunlight has been making it into the lower
      atmosphere (troposphere). That change is "consistent with the large
      tropospheric warming that has occurred over the most recent decades",
      they write.

      Why cloud cover should have thinned is "the big question that no-one
      knows how to answer," says Palle. "We associate it with natural
      climate variability, but that's really another way of saying we don't
      know."


      Pinch of salt


      However, Bruce Wielicki from the NASA Langley Research Centre in
      Hampton, Virginia, says that a link between earthshine and
      tropospheric warming "cannot be concluded or even implied from these
      results". And John Harries, an expert in earth observation at
      Imperial College in London, UK, recommends taking the study's
      conclusions "with a pinch of salt".

      Wielicki leads the science team of a mission called CERES - the
      Clouds and the Earth's Radiant Energy System - which has instruments
      on three satellites measuring both the amount of visible light
      reflected by the Earth and the amount of infrared radiation it is
      emitting.

      Without measuring both, he explains, you cannot infer anything about
      warming. CERES data, for example, show that clouds not only reflect
      lots of visible light, they also act like a blanket and trap infrared
      radiation in the lower atmosphere. The two effects almost cancel out,
      so there is little overall change in the energy the atmosphere
      retains.


      A further objection is that direct telescope measurements of
      earthshine in Palle's study only start in 1998. The data for the
      previous 14 years is based on satellite measurements of clouds which
      were used to calculate an estimate of earthshine.

      Kevin Trenberth, a climate modeller at NCAR in Boulder, Colorado,
      point out another possible problem. He says satellites are very
      difficult to calibrate against one another, meaning that systematic
      errors are a real risk. "I don't believe the downward earthshine
      trend is real," he says.

      But Trenberth adds that future earthshine measurements could be
      an "innovative" way of probing the effect of clouds on
      climate. "Climate models do not do clouds well - they are perhaps the
      biggest problem we have in using climate models to make predictions
      about global warming."

      Journal reference: Science (vol 304, p 1299)


      Jenny Hogan

      Sent by Medscape Mail: Free Portable E-mail for Professionals on the Move

      http://www.medscape.com



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