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CC: Arctic Ice Retreating More Quickly Than Models Project

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    Message 1 of 1 , May 1, 2007
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      physorg.com / National Center for Atmospheric Research
      April 30, 2007


      [Visit the link above for a graphic that illustrates the discrepancy between
      actual melting and computer model projections. --DS]

      Arctic sea ice is melting at a significantly faster rate than projected by
      even the most advanced computer models, a new study concludes. The research,
      by scientists at the National Center for Atmospheric Research (NCAR) and the
      University of Colorado's National Snow and Ice Data Center (NSIDC), shows
      that the Arctic's ice cover is retreating more rapidly than estimated by any
      of the 18 computer models used by the Intergovernmental Panel on Climate
      Change in preparing its 2007 assessments.

      The study, "Arctic Sea Ice Decline: Faster Than Forecast?" will appear
      tomorrow in the online edition of Geophysical Research Letters. It was led
      by Julienne Stroeve of the NSIDC and funded by the National Science
      Foundation, which is NCAR's principal sponsor, and by NASA.

      "While the ice is disappearing faster than the computer models indicate,
      both observations and the models point in the same direction: the Arctic is
      losing ice at an increasingly rapid pace and the impact of greenhouse gases
      is growing," says NCAR scientist Marika Holland, one of the study┬╣s

      The authors compared model simulations of past climate with observations by
      satellites and other instruments. They found that, on average, the models
      simulated a loss in September ice cover of 2.5 percent per decade from 1953
      to 2006. The fastest rate of September retreat in any individual model was
      5.4 percent per decade. (September marks the yearly minimum of sea ice in
      the Arctic.) But newly available data sets, blending early aircraft and ship
      reports with more recent satellite measurements that are considered more
      reliable than the earlier records, show that the September ice actually
      declined at a rate of about 7.8 percent per decade during the 1953-2006

      "This suggests that current model projections may in fact provide a
      conservative estimate of future Arctic change, and that the summer Arctic
      sea ice may disappear considerably earlier than IPCC projections," says

      Thirty years ahead of schedule

      The study indicates that, because of the disparity between the computer
      models and actual observations, the shrinking of summertime ice is about 30
      years ahead of the climate model projections. As a result, the Arctic could
      be seasonally free of sea ice earlier than the IPCC- projected timeframe of
      any time from 2050 to well beyond 2100.

      The authors speculate that the computer models may fail to capture the full
      impact of increased carbon dioxide and other greenhouse gases in the
      atmosphere. Whereas the models indicate that about half of the ice loss from
      1979 to 2006 was due to increased greenhouse gases, and the other half due
      to natural variations in the climate system, the new study indicates that
      greenhouse gases may be playing a significantly greater role.

      There are a number of factors that may lead to the low rates of simulated
      sea ice loss. Several models overestimate the thickness of the present-day
      sea ice and the models may also fail to fully capture changes in atmospheric
      and oceanic circulation that transport heat to polar regions.

      March ice

      Although the loss of ice for March is far less dramatic than the September
      loss, the models underestimate it by a wide margin as well. The study
      concludes that the actual rate of sea ice loss in March, which averaged
      about 1.8 percent per decade in the 1953-2006 period, was three times larger
      than the mean from the computer models. March is typically the month when
      Arctic sea ice is at its most extensive.

      The Arctic is especially sensitive to climate change partly because regions
      of sea ice, which reflect sunlight back into space and provide a cooling
      impact, are disappearing. In contrast, darker areas of open water, which are
      expanding, absorb sunlight and increase temperatures. This feedback loop has
      played a role in the increasingly rapid loss of ice in recent years, which
      accelerated to 9.1 percent per decade from 1979 to 2006 according to
      satellite observations.


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      Published by David Sunfellow
      NewHeavenNewEarth (NHNE)
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