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An island in the Arctic Ocean - Source: Reuters -
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Arctic sea ice has thinned dramatically since 2004, with the
older, thicker ice giving way to a younger, thinner kind that melts
in the northern summer, NASA scientists reported.
Researchers have known for years that ice covering in the Arctic
Sea has been shrinking in area, but new satellite data that measure
the thickness of ice show that the volume of sea ice is declining
as well.
That is important because thicker ice is more resilient and can
last from summer to summer. Without ice cover, the Arctic Sea's
dark waters absorb the sun's heat more readily instead of
reflecting it as the light-coloured ice does, accelerating the
heating effect.
Using NASA's ICESat spacecraft, scientists figured that overall
Arctic sea ice thinned about seven inches a year since 2004, for a
total of 0.67 metres over four winters.
Their findings were reported in the Journal of Geophysical
Research-Oceans.
The total area covered by thicker, older ice that has survived at
least one summer shrank by 42%.
Beyond that, the new satellite data showed that the proportion of
tough old ice is decreasing at the same time as the amount of young
fragile ice is increasing, information that was hard to discern
from earlier data.
Losing the old ice
In 2003, 62% of the Arctic's total ice volume was stored in
multi-year ice and 38% in first-year seasonal ice.
By last year, 68% was first-year ice and 32% the tougher
multi-year ice.
The research team blamed these changes on recent warming and
anomalies in sea ice circulation.
"We're losing a lot more of the old ice, and that's significant,"
said Ron Kwok of the Jet Propulsion Laboratory in Pasadena,
California.
"Basically we knew how much the area (of ice) was shrinking, but
we didn't know how thick it was."
To find the volume of ice, NASA's ICESat spacecraft measured how
high the ice rose above sea level in the Arctic, Kwok said in a
telephone interview.
"If we know how much is floating on top, we can use that to compute
the rest of the ice thickness," Kwok said in a telephone
interview.
About nine-tenths of the ice is beneath the water, he
said.
The ICESat measurements cover virtually the entire Arctic, and they
tally with ice volume measurements made by submarines, which cover
only a few passes across the area.
Arctic sea ice melted to its second-lowest level last year, rising
slightly from its all-time low in 2007, according to the US
National Snow and Ice Data Center.
Arctic ice is a factor in global climate and weather patterns,
because the difference between the cool air at the poles and the
warm air around the Equator drives air and water currents,
including the jet stream.
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