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Upper atmosphere and greenhouse gases

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    ... Subject: [CCG] Upper atmosphere and greenhouse gases Source: Naval Research Laboratory Date: 2004-02-04 URL:
    Message 1 of 1 , Feb 5, 2004
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      Subject: [CCG] Upper atmosphere and greenhouse gases

      Source: Naval Research Laboratory Date: 2004-02-04
      URL: http://www.sciencedaily.com/releases/2004/02/040203234243.htm
      ------------------------------------------------------------------------

      Increasing Greenhouse Gases Lead To Dramatic Thinning Of The Upper
      Atmosphere

      Washington, D.C. (February 2, 2004) -- The highest layers of the
      Earth's atmosphere are cooling and contracting, most likely in
      response to increasing levels of greenhouse gases, according to a new
      study by scientists at the Naval Research Laboratory (NRL). This
      contraction could result in longer orbital lifetimes for both
      satellites and hazardous space debris.

      In a paper that will appear in the Journal of Geophysical Research -
      Space Physics, Dr. John Emmert and his colleagues, Drs. Michael
      Picone, Judith Lean, and Stephen Knowles, report that the average
      density of the thermosphere has decreased by about 10% during the
      past 35 years. The thermosphere is the highest layer in the
      atmosphere, and begins at an altitude of about 90 km (56 mi).

      The study utilized orbital tracking data on 27 space objects that
      have been aloft for over 30 years and whose closest approach to the
      Earth ranges from 200-800 km (124-497 mi). The Space Shuttle
      typically orbits at 300-450 km (186-279 mi), and the International
      Space Station is at an altitude of about 400 km (248 mi). Although
      the atmosphere is extremely thin in this region (the air at the
      Earth's surface is a trillion times thicker), it is enough to exert a
      drag force on satellites, causing their orbits to decay slowly and
      ultimately resulting in a fiery disintegration at lower altitudes. By
      analyzing changes in the orbits of the selected objects, the
      scientists derived the yearly average density encountered by each
      object. After adjusting for other factors, the data from every object
      indicated a long-term decline in the density of the thermosphere.

      This decrease in density had been predicted by theoretical
      simulations of the upper atmosphere's response to increasing carbon
      dioxide and other greenhouse gases. In the troposphere (the lowest
      layer of the atmosphere) greenhouse gases trap infrared radiation,
      causing the well-known "global warming" effect. Higher in the
      atmosphere (above about 12 km (7.5 mi)), however, these gases
      actually enhance the ability of the atmosphere to radiate heat out to
      space, thereby causing a cooling effect. As the amount of carbon
      dioxide increases, the upper atmosphere becomes cooler and contracts,
      bringing lower-density gas to lower heights. Consequently, at a given
      height, the average density will decrease. Because each layer of the
      atmosphere rests on the layers below it, small changes at lower
      altitudes become amplified as one moves upwards. The NRL study found
      that the observed decrease in density depends on height in the same
      way as predicted by the theoretical simulations, indicating that
      greenhouse gases are a likely source of the change.

      An extreme example of the greenhouse gas effect can be found on
      Venus, whose atmosphere is 96% carbon dioxide (compared to trace
      amounts in the Earth's atmosphere), resulting in a very hot lower
      atmosphere (8008F, 4278C) and a very cold and compact upper
      atmosphere.

      These new results verify and significantly expand a limited earlier
      investigation, by scientists at The George Washington University,
      which also used orbital data to derive a long-term decrease in
      thermospheric density. The new NRL study utilizes more orbital data
      over a longer period of time and employs more precise analysis
      methods. By carefully examining all potential sources of error, the
      NRL team has provided solid evidence that the trend is neither
      artificial nor the result of physical processes other than internal
      atmospheric cooling.

      Based on the NRL analysis and projections of carbon dioxide levels in
      the atmosphere, the density at thermospheric heights could be cut in
      half by the year 2100. This change may present mixed blessings: while
      operational satellites will be able to stay aloft longer, using less
      fuel, so will damaging spacecraft debris, potentially increasing the
      frequency of collisions.

      ###

      This research was funded by the Office of Naval Research. Dr. Emmert
      conducted the study as a National Research Council Postdoctoral
      Research Associate at NRL. Drs. Picone and Lean are members of NRL's
      Space Science Division, and Dr. Knowles is a former Navy employee,
      now with SAIC.

      These results appear in the Journal of Geophysical Research (In
      Press, doi:10.1029/2003JA010176).

      NRL is the Department of the Navy's corporate laboratory and conducts
      a broad program of scientific research, technology and advanced
      development. The Laboratory, with a total complement of nearly 2,500
      personnel, is located in southwest Washington, DC, with other major
      sites at the Stennis Space Center, MS; and Monterey, CA.

      Editor's Note: The original news release can be found here.
      ------------------------------------------------------------------------

      This story has been adapted from a news release issued by Naval
      Research Laboratory.

      --


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