September 29, 2005
On Sept. 21, 2005, sea ice extent dropped to 2.05 million sq. miles, the lowest extent yet recorded in the satellite record. Incorporating the 2005 minimum using satellite data going back to 1978, with a projection for ice growth in the last few days of this September, brings the estimated decline in Arctic sea ice to 8.5 percent per decade over the 27 year satellite record.
Scientists involved in this research are from NASA's Goddard Space Flight Center, Greenbelt, Md., NASA's Jet Propulsion Laboratory, Pasadena, Calif., the National Snow and Ice Data Center at the University of Colorado, Boulder, and the University of Washington, Seattle.
Satellites have made continual observations of Arctic sea ice extent since 1978, recording a general decline throughout that period. Since 2002, satellite records have revealed early onsets of springtime melting in the areas north of Alaska and Siberia. In addition, the 2004-2005 winter season showed a smaller recovery of sea ice extent than any previous winter in the satellite record and the earliest onset of melt throughout the Arctic.
Arctic sea ice typically reaches its minimum in September, at the end of the summer melt season. The last four Septembers (2002-2005) have seen sea ice extents 20 percent below the mean September sea ice extent for 1979-2000.
Perennial ice cover is ice that survives the summer melt, consisting mainly of thick multiyear ice floes that are the mainstay of the Arctic sea ice cover. "Since 1979, by using passive microwave satellite data, we've seen that the area of Arctic perennial sea ice cover has been declining at 9.8 percent per decade," said Joey Comiso, senior scientist at Goddard.
For the perennial ice to recover, sustained cooling is needed, especially during the summer period. This has not been the case over the past 20 years, as the satellite data show a warming trend in the Arctic, and it is not expected to be the case in the future, as climate models project continued Arctic warming. If ice were to grow back in these areas, the new ice would likely be thinner and more susceptible to future melt than the thick perennial ice that it replaces.
Scientists are working to understand the extent to which these decreases in sea ice are due to naturally occurring climate variability or longer-term human influenced climate changes.
Scientists believe that the Arctic Oscillation, a major atmospheric circulation pattern that can push sea ice out of the Arctic, may have contributed to the reduction of sea ice in the mid-1990s by making the sea ice more vulnerable to summertime melt.
Sea ice decline could also affect future temperatures in the region. Ice reflects much of the sun's radiation back into space. As sea ice melts, more exposed ocean water reduces the amount of energy reflected away from the Earth. "Feedbacks in the system are starting to take hold," says the National Snow and Ice Data Center's lead scientist Ted Scambos.
Claire Parkinson, senior scientist at Goddard, cautions against thinking that Arctic sea ice is gone for good, especially with such limited data. "The reduced sea ice coverage will lead to more wintertime heat loss from the ocean to the atmosphere and perhaps therefore to colder water temperatures and further ice growth," said Parkinson.
There are many factors beyond warmer temperatures that drive changes in the Arctic. A longer data record, combined with observations from additional environmental parameters now available from NASA satellites, will help scientists better understand the changes they are now seeing.
The study used data from the Defense Meteorological Satellite Program Special Sensor/ Microwave Imager and data from NASA's Scanning Multi-channel Microwave Radiometer (SMMR) on the NIMBUS-7 satellite.
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