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NASA survey reveals dramatic Arctic sea ice thinning
NASA NEWS RELEASE Posted: July 8, 2009

WASHINGTON -- Arctic sea ice thinned dramatically between the winters of 2004 and 2008, with thin seasonal ice replacing thick older ice as the dominant type for the first time on record. The new results, based on data from a NASA Earth-orbiting spacecraft, provide further evidence for the rapid, ongoing transformation of the Arctic's ice cover.

ICESat measurements of the distribution of winter sea ice thickness over the Arctic Ocean in 2004 and 2008. Credit: Ron Kwok, NASA/JPL
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Scientists from NASA and the University of Washington in Seattle
conducted the most comprehensive survey to date using observations
from NASA's Ice, Cloud and land Elevation Satellite, known as ICESat,
to make the first basin-wide estimate of the thickness and volume of
the Arctic Ocean's ice cover. Ron Kwok of NASA's Jet Propulsion
Laboratory in Pasadena, Calif., led the research team, which
published its findings July 7 in the Journal of Geophysical
Research-Oceans.
The Arctic ice cap grows each winter as the sun sets for several
months and intense cold ensues. In the summer, wind and ocean
currents cause some of the ice naturally to flow out of the Arctic,
while much of it melts in place. But not all of the Arctic ice melts
each summer; the thicker, older ice is more likely to survive.
Seasonal sea ice usually reaches about 6 feet in thickness, while
multi-year ice averages 9 feet.
Using ICESat measurements, scientists found that overall Arctic sea
ice thinned about 7 inches a year, for a total of 2.2 feet over four
winters. The total area covered by the thicker, older "multi-year"
ice that has survived one or more summers shrank by 42 percent.
Previously, scientists relied only on measurements of area to
determine how much of the Arctic Ocean is covered in ice, but ICESat
makes it possible to monitor ice thickness and volume changes over
the entire Arctic Ocean for the first time. The results give
scientists a better understanding of the regional distribution of ice
and provide better insight into what is happening in the Arctic.
"Ice volume allows us to calculate annual ice production and gives us
an inventory of the freshwater and total ice mass stored in Arctic
sea ice," said Kwok. "Even in years when the overall extent of sea
ice remains stable or grows slightly, the thickness and volume of the
ice cover is continuing to decline, making the ice more vulnerable to
continued shrinkage. Our data will help scientists better understand
how fast the volume of Arctic ice is decreasing and how soon we might
see a nearly ice-free Arctic in the summer."
In recent years, the amount of ice replaced in the winter has not been
sufficient to offset summer ice losses. The result is more open water
in summer, which then absorbs more heat, warming the ocean and
further melting the ice. Between 2004 and 2008, multi-year ice cover
shrank 595,000 square miles -- nearly the size of Alaska's land area.
During the study period, the relative contributions of the two ice
types to the total volume of the Arctic's ice cover were reversed. In
2003, 62 percent of the Arctic's total ice volume was stored in
multi-year ice, with 38 percent stored in first-year seasonal ice. By
2008, 68 percent of the total ice volume was first-year ice, with 32
percent multi-year.
"One of the main things that has been missing from information about
what is happening with sea ice is comprehensive data about ice
thickness," said Jay Zwally, study co-author and ICESat project
scientist at NASA's Goddard Space Flight Center in Greenbelt, Md.
"U.S. Navy submarines provide a long-term, high-resolution record of
ice thickness over only parts of the Arctic. The submarine data agree
with the ICESat measurements, giving us great confidence in
satellites as a way of monitoring thickness across the whole Arctic
Basin."
The research team attributes the changes in the overall thickness and
volume of Arctic Ocean sea ice to the recent warming and anomalies in
patterns of sea ice circulation.
"The near-zero replenishment of the multi-year ice cover, combined
with unusual exports of ice out of the Arctic after the summers of
2005 and 2007, have both played significant roles in the loss of
Arctic sea ice volume over the ICESat record," said Kwok.
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