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'Remarkable' Drop
in Arctic Sea Ice Raises Questions
September 25, 2007 NASA-Melting
Arctic sea ice has shrunk to a 29-year low, significantly below the minimum set
in 2005, according to preliminary figures from the National Snow and Ice Data
Center, part of the University of Colorado at Boulder. NASA scientists, who have
been observing the declining Arctic sea ice cover since the earliest
measurements in 1979, are working to understand this sudden speed-up of sea ice
decline and what it means for the future of Earth's northern polar region.
At the end of each
summer, the sea ice cover reaches its minimum extent and the ice that remains is
called the perennial ice cover, which consists mainly of thick multi-year ice
floes. The area of the perennial ice has been steadily decreasing since the
satellite record began in 1979, at a rate of about 10% per decade. But the 2007
minimum, reached around Sept. 14, is far below the previous record made in 2005
and is about 38% lower than the climatological average. This data visualization
shows the annual sea ice minimum from 1979 through 2007.
"The decline in
the amount of thick ice that survives the summer melt season this year is quite
remarkable," said Josefino C. Comiso, senior scientist at NASA's Goddard
Space Flight Center, Greenbelt, Md. "The extent of this 'perennial' sea ice
and the area it covers are both nearly 38 percent lower than average. Compared
to the record low in 2005, the extent and area are 24 percent and nearly 26
percent lower this year, respectively."
"From what we know of how Arctic sea ice behaves after nearly 30 years of
continuous satellite observations, this kind of drop in sea ice usually takes
more than three years to happen. The rapid trend of the perennial ice previously
reported in 2002 appears now to be in an accelerated mode," Comiso
observed.
The Advanced Microwave Scanning
Radiometer (AMSR-E) is a high-resolution passive microwave Instrument on NASA's
Aqua satellite. AMSR-E provides a remarkably clear view of sea ice dynamics in
greater detail than has ever been seen before. Researchers use this information
to study polar bear habitats, plan expeditions to the ice, and to study the
interactions between the ocean and sea ice from season to season. This data
visualization shows Arctic sea ice from Jan. 1, 2007 to Sept. 16, 2007.
Because
Arctic ice cover varies so much year to year, it can be dangerous to look at any
one year and draw too much of a conclusion from it," said Waleed Abdalati,
head of Goddard's Cryospheric Sciences Branch. "But this year, the amount
of ice is so far below that of previous years that it really is cause for
concern. The trend in decreasing ice cover seems to be getting stronger and
stronger as time goes on."
NASA developed the original capability to observe the extent and concentration
of sea ice from space using passive microwave sensors. More recently, NASA
launched an advanced microwave instrument in 2002 -- the Advanced Microwave
Scanning Radiometer (AMSR-E) on the Aqua satellite -- that provides a view of
sea ice dynamics in greater detail than has ever been seen before. Researchers
use this information to study polar bear habitats and the unique movements of
sea ice from season to season. AMSR-E is a joint project of NASA and the
National Space Development Agency of Japan.
credit:NASA
Polar
ice reflects light from the sun. As this ice begins to melt, less sunlight gets
reflected into space. It is instead absorbed into the oceans and land, raising
the overall temperature, and fueling further melting. This results in a positive
feedback loop called ice albedo feedback, which causes the loss of the sea ice
to be self-compounding. The more it disappears, the more likely it is to
continue to disappear.
The
accelerating decline in sea ice may be due to changes in climate brought on by
the lack of sea ice itself, Comiso believes. "When there is less sea ice in
the summer, the Arctic Ocean receives more heat. The warmer water makes it
harder for the ice to recover in the winter, and, therefore, there is a higher
likelihood that sea ice will retreat farther during the summer. This process
repeats itself year after year," Comiso said.
"The longer this process continues, the less likely recovery becomes,"
Abdalati believes. "The implications on global climate are not well known,
but they have the potential to be quite large, since the Arctic ice cover
exhibits a tremendous influence on our climate. It really is imperative that we
try to understand the interactions between the ice, ocean and atmosphere. And
satellites hold the key to developing this understanding."
In
September 2007, the Northwest Passage was ice-free for the first time since
satellite records began. The passage is a direct route from Europe to Asia for
ships traveling through the Arctic. The Moderate Resolution Imaging
Spectroradiometer (MODIS) instrument on NASA's Terra satellite captured this
image of the ice-free Northwest Passage on Sept. 15, 2007.
Current satellites, however, can
map sea ice in two dimensions, but it is much more difficult to find out how the
thickness of the ice contributes to the change in the total volume of the ice.
NASA's ICESat spacecraft (Ice, Cloud, and land Elevation Satellite), launched in
2003, with the primary goal of determining how much ice sheets are contributing
to sea-level rise. ICESat is also collecting data that enables scientists to
make estimates of sea ice thickness with unprecedented detail.
"What we need to truly understand the interaction of the ice, ocean and
atmosphere in the Arctic is sea ice thickness information," said Abdalati.
"The new capability we have with ICESat is expected to be extended into the
next decade based on recent recommendations by the National Research Council for
a follow-on mission. Ultimately, like the 29-year record we have now of sea ice
cover, a long-term ice thickness record will help scientists understand these
complex interactions and what the changes in the ice cover will mean to the
ecology of the Arctic and to life on Earth."
NASA has been observing sea ice from space since the 1970s, beginning with the
Electricallly Scanning Microwave Radiometer (ESMR), Scanning Multichannel
Microwave Radiometer (SSMR) and Special Sensor Microwave/Imager (SSM/I) sensors
on the US Defense Meteorological Space Program (DMSP) satellites, and now with
the AMSR-E instrument on NASA's Earth Observing System/Aqua satellite. Data
collected by these instruments have been instrumental in shaping public policy
and international perspectives on the Arctic.
Stephen Cole
Goddard Space Flight Center
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