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By Peter Rejcek
Antarctic sea ice remains relatively healthy while the
sea ice in the Arctic continues a precipitous decline
in overall volume.
That’s the latest state of the union report from
NASA, which held a teleconference for the media on March
18, with three scientists presenting the latest analysis
from satellite data. NASA satellite records on sea ice
extent go back about 40 years.
“The area of Antarctic ice has been relatively stable
in the past 35 years,” said Seelye Martin, program
manager of Cryospheric Sciences in the Earth Science Division
at NASA headquarters in Washington, D.C.
Martin said sea ice extent in the Antarctic has increased
about 1 percent per decade during that time. “In
other words, it’s basically staying constant,”
he said.
The news is far more grim in the Arctic, despite a slight
increase in overall winter sea ice extent in March, when
the sea ice pack peaks in the northern hemisphere. The
concern is that the perennial sea ice — the thick,
multi-year ice that survives through more than one summer
— is rapidly declining in the Arctic.
“The area of the thicker, perennial ice has reached
an all-time minimum,” Martin said. “In other
words, the volume of the Arctic pack ice is continuing
to decrease.”
Perennial ice used to cover 50 to 60 percent of the Arctic,
but this year it covers less than 30 percent, according
to NASA data from the satellites. Very old ice that remains
in the Arctic for at least six years comprised over 20
percent of the Arctic area in the mid to late 1980s, but
this winter it decreased to just six percent.
In 2007, perennial sea ice was 38 percent below the 28-year
average, said Josefino Comiso, senior research scientist
at the Cryospheric Sciences Branch at NASA’s Goddard
Space Flight Center in Greenbelt, Md.
“Considering that the perennial sea ice cover has
been persistently low in recent years, this unusual drop
in perennial ice cover in 2007 was a big event, and may
be a turning point in the Arctic sea ice,” Comiso
said. However, he advised caution in interpreting the
anomaly, particularly because of other climatic events
that would have induced unusual melting that year.
Polar sea ice is important for several reasons. The ice
reflects light from the sun, but as it begins to melt,
it reflects less sunlight back into space. Instead, the
oceans and land absorb the light and heat, raising the
overall temperature, and fueling further melting. This
creates a positive feedback loop called an ice albedo
feedback, which causes the loss of the sea ice to be self-perpetuating:
The more it disappears, the more likely it is to continue
to disappear.
In addition to helping regulate heat — serving as
a sort of air conditioner for the planet — sea ice
serves as a vital habitat for the Arctic’s polar
bears. In the Antarctic, winter sea ice is important to
links all across the food chain, from the shrimp-like
krill at the bottom to Adélie penguins at the top.
Sea ice operates differently in Antarctica, where historically
little perennial sea ice survives each summer season. During
the winter, sea ice covers up to 18 million square kilometers
of ocean, but by the end of summer, only about 3 million
square kilometers of sea ice remain.
The difference is that Antarctica is land surrounded
by ocean, while the Arctic is ocean surrounded by land.
The Southern Ocean around Antarctica allows the sea ice
to move more freely than it does in the northern hemisphere,
and so the sea ice floats northward into warmer waters
where it eventually melts. The surrounding ocean and the
Antarctic Circumpolar Current also help isolate and buffer
the continent from warmer weather.
The scientists at the NASA press conference noted there
is some strong regional variability in sea ice extent
around Antarctica.
“Even though the Antarctic ice has remained constant
in overall extent, it is changing from region to region,
and one of the regions where we’re losing ice extent
is around the peninsula,” Martin said in response
to a question about the changes in melting of Antarctic
winter sea ice.
U.S. Antarctic Program scientists have said the peninsula,
particularly the northern tip, is heating up faster than
just about anywhere else on the planet. And winter sea
ice is forming later and melting earlier — with the
season declining by about three months since the late
1970s. (See story: Getting warmer.)
“It is a source of concern for us,” Martin
said of the retreat in sea ice along West Antarctica.
But the focus of most concern is the Arctic, which appears
to be growing dangerously thinner each year.
“[Depth is] a key parameter in determining the long-term
health of the sea ice, because the thicker ice is more
resilient to any short-term changes in weather …
It’s tough as nails. It’s the Gen. Patton of
sea ice, so to speak. It is resistant to short-term melting,”
explained Walter Meier, with the National Snow and Ice
Data Center (NSIDC) at the University of Colorado, Boulder,
Colo. The NSIDC receives part of its funding from the
National Science Foundation.
Meier used an analogy of a Hollywood set to make his
point: On the surface, the buildings of the set look real,
but when you look behind the façade, there is nothing
there.
“What we’re seeing with the ice cover is that
it’s becoming more and more empty,” Meier said.
“We’re getting thinner and thinner ice, and
that ice is much more susceptible to melting during the
summer of any type, and it’s most likely to melt
away.”
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Antarctic
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