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April 3, 2007
NASA FINDS ARCTIC
REPLENISHED
VERY LITTLE THICK SEA ICE IN 2005
A
new
NASA study has found that in 2005 the Arctic
replaced very little of the thick sea ice it normally loses and
replenishes
each year. Replenishment of this thick, perennial sea ice each year is
essential to the maintenance and stability of the Arctic summer ice
cover.
The
findings complement a NASA study released in fall 2006 that found a
14-percent
drop in this perennial ice between 2004 and 2005. The lack of
replenishment
suggests that the decline may continue in the near future.
Perennial
ice coverage fluctuates seasonally for two reasons: summer melting and
the
transport of ice out of the Arctic.
When
perennial ice, which is three or more meters (10 or more feet) thick,
is lost
in these ways, new, thinner, first-year seasonal ice typically replaces
it.
Some of this seasonal ice melts in the following summer, and some is
thick
enough to survive and replenish the perennial ice cover.
"Recent studies indicate Arctic perennial ice is declining seven to 10
percent each decade," explained Ron Kwok of NASA's Jet Propulsion
Laboratory, Pasadena,
Calif.
"Our study gives the first
reliable estimates of how perennial ice replenishment varies each year
at the
end of summer. The amount of first-year ice that survives the summer
directly
influences how thick the ice cover will be at the start of the next
melt
season."
Using satellite data from NASA's QuikScat and other data, Kwok studied
six
annual cycles of Arctic perennial ice coverage from 2000 to 2006. The
scatterometer instrument on QuikScat sends radar pulses to the surface
of the
ice and measures the echoed radar pulses bounced back to the satellite.
These
measurements allow scientists to differentiate the seasonal ice from
the older,
perennial ice.
Kwok found that after the 2005 summer melt, only about four percent of
the
nearly 2.5 million square kilometers (965,000 square miles) of thin,
seasonal
ice that formed the previous winter survived the summer and replenished
the
perennial ice cover. That was the smallest replenishment seen in the
study. As
a result, perennial ice coverage in January 2006 was about 14 percent
smaller
than the previous January.
Kwok examined how movement of ice out of the Arctic
affected the replenishment of perennial sea ice in 2005. That year, the
typically small amount of ice that moves out of the Arctic
in summer was unusually high -- about seven percent of the perennial
ice
coverage area. Kwok said the high amount was due to unusual wind
conditions at Fram
Strait,
an Arctic passage between Antarctic
Bay in
Greenland and Svalbard,
Norway.
Troughs of low atmospheric pressure in the Greenland and
Barents/Norwegian Seas
on both sides of Fram
Strait
created winds that pushed ice out of the Arctic
at an increased rate.
The effects of ice movement out of the Arctic
depend on the season. When ice moves out of the Arctic
in the summer, it leaves behind an ocean that does not refreeze. This,
in turn,
increases ocean heating and leads to additional thinning of the ice
cover.
These findings suggest that the greater the number of freezing
temperature days
during the prior season, the thicker the ice cover, and the better its
chances
of surviving the next summer's melt. "The winters and summers before
fall
2005 were unusually warm," Kwok said. "The low replenishment seen in
2005 is potentially a cumulative effect of these trends."
Kwok also examined the 2000-2006 temperature records within the context
of
longer-term temperature records dating back to 1958. He found a gradual
warming
trend in the first 30 years, which accelerated after the mid-1980s.
"The
record doesn't show any hint of recovery from these trends," he stated.
"If the correlations between replenishment area and numbers of freezing
and melting temperature days hold long-term, its expected the perennial
ice
coverage will continue to decline."
Kwok points to a possible trigger for the declining perennial ice
cover. In the
early 1990s, variations in the North Atlantic Oscillation, a
large-scale
atmospheric seesaw that affects how air circulates over the Atlantic Ocean, were linked to a
large increase in Arctic ice export. It
appears the ice cover has not yet recovered from these variations.
"We're seeing a decreasing trend in perennial ice coverage," he said.
"Our study suggests that, on average, the area of seasonal ice that
survives the summer may no longer be large enough to sustain a stable
perennial
ice cover, especially in the face of accelerating climate warming and
Arctic
sea ice thinning."
Data from the 2005-06 season have not yet been analyzed.
The study appeared March 2 in Geophysical
Research Letters.
For
more information and images,
visit:
http://www.nasa.gov/vision/earth/lookingatearth/arctic-20070403.html
QuikScat
data:
http://winds.jpl.nasa.gov/index.cfm
##
Contact:
Alan Buis
NASA Jet Propulsion Laboratory
818-354-0474
This text is
derived from:
http://www.nasa.gov/vision/earth/lookingatearth/arctic-20070403.html
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