Arctic sea
ice younger, thinner as melt season begins
April 6, 2009The
National Snow and Ice Data Center (NSIDC)
Arctic sea ice
extent has begun its seasonal decline towards the September minimum. Ice extent
through the winter was similar to that of recent years, but lower than the 1979
to 2000 average. More importantly, the melt season has begun with a substantial
amount of thin first-year ice, which is vulnerable to summer melt.
Overview of conditions
Sea ice extent averaged over the month of
March 2009 was 15.16 million square kilometers (5.85 million square miles). This
was 730,000 square kilometers (282,000 square miles) above the record low of
2006, but 590,000 square kilometers (228,000 square miles) below the 1979 to
2000 average.
Arctic sea ice extent for
March, 2009, was 15.16 million square kilometers (5.85 million square miles).
The magenta line shows the 1979 to 2000 median extent for that month. The black
cross indicates the geographic North Pole.
Conditions in context
At the end of last summer's melt season,
extensive areas of open water froze up quickly, once air temperatures cooled in
the fall. By February 28, ice extent had reached its annual maximum. Although
the maximum ice extent occurred slightly earlier than usual, ice extent remained
close to the maximum level through much of March.
The graph above shows daily
sea ice extent. The solid blue line indicates 2008 to 2009; the dashed green
line shows 2006 to 2007 (the record-low summer minimum occurred in 2007); and
the solid gray line indicates average extent from 1979 to 2000.
March 2009 compared to past
Marches
Including March 2009, the past six years
have all had ice extent substantially lower than normal. The linear trend
indicates that for the month of March, ice extent is declining by 2.7% per
decade, an average of 43,000 square kilometers (16,000 square miles) of ice per
year.
Monthly
March ice extent for 1979 to 2009 shows a decline of 2.7% per decade.
Arctic winter warmer than average
Overall, it was a fairly warm winter in
the Arctic. Air temperatures over the Arctic Ocean were an average of 1 to 2
degrees Celsius (1.8 to 3.6 degrees Fahrenheit) above normal, with notable
regional variations. The Barents Sea region was over 4 degrees Celsius (7.2
degrees Fahrenheit) warmer than average this winter. This warmth probably
stemmed from unusually low sea ice extent in the region throughout much of the
winter, which allowed the ocean to pump heat into the atmosphere. The Bering
Sea, in contrast, experienced a cool winter, with temperatures 1 to 2 degrees
Celsius (1.8 to 3.6 degrees Fahrenheit) below average. The cooler conditions
were consistent with the above-average sea ice extent in the Bering Sea through
much of the winter.
The
map of air temperature anomalies for winter 2008 to 2009 at the 925 millibar
level (roughly 1,000 meters [3,000 feet] above the surface) shows
warmer-than-usual conditions over much of the Arctic Ocean. Areas in orange and
red correspond to strong positive (warm) anomalies. Areas in blue correspond to
negative (cool) anomalies.
—Credit: National Snow and Ice Data Center courtesy
NOAA/ESRL Physical Sciences Laboratory
Sea ice young and thin as melt
season begins
How vulnerable is the ice cover as we go
into the summer melt season? To answer this question, scientists also need
information about ice thickness. Indications of winter ice thickness, commonly
derived from ice age estimates, reveal that the ice is thinner than average,
suggesting that it is more susceptible to melting away during the coming summer.
As the melt season begins, the Arctic
Ocean is covered mostly by first-year ice, which formed this winter, and
second-year ice, which formed during the winter of 2007 to 2008. First-year ice
in particular is thinner and more prone to melting away than thicker, older,
multi-year ice. This year, ice older than two years accounted for less than 10%
of the ice cover at the end of February. From 1981 through 2000, such older ice
made up an average of 30% of the total sea ice cover at this time of the year.
While ice older than two years reached
record lows, the fraction of second-year sea ice increased compared to last
winter. Some of this second-year ice will survive the summer melt season to
replenish the Arctic's store of older ice; however, in recent years less young
ice has made it through the summer. To restore the amount of older ice to
pre-2000 levels, large amounts of this young ice would need to endure through
summer for several years in a row.
But conditions may not always favor the
survival of second-year and older ice. Each winter, winds and ocean currents
move some sea ice out of the Arctic ocean. This winter, some second-year ice
survived the 2008 melt season only to be pushed out of the Arctic by strong
winter winds. Based on sea ice age data from Jim Maslanik and Chuck Fowler at
the University of Colorado, since the end of September 2008, 390,000 square
kilometers (150,000 square miles) of second-year ice and 190,000 square
kilometers (73,000 square miles) of older (more than two years old) ice moved
out of the Arctic.
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