A new NASA study revealed that the oldest and thickest Arctic sea ice is
disappearing at a faster rate than the younger and thinner ice at the
edges of the Arctic Ocean's floating ice cap.
The thicker ice, known as multi-year ice, survives through the
cyclical summer melt season, when young ice that has formed over winter
just as quickly melts again. The rapid disappearance of older ice makes
Arctic sea ice even more vulnerable to further decline in the summer,
said Joey Comiso, senior scientist at NASA Goddard Space Flight Center,
Greenbelt, Md., and author of the study, which was recently published in
Journal of Climate.
The new research takes a closer look at how multi-year ice, ice that
has made it through at least two summers, has diminished with each
passing winter over the last three decades. Multi-year ice "extent" --
which includes all areas of the Arctic Ocean where multi-year ice covers
at least 15 percent of the ocean surface -- is diminishing at a rate of
-15.1 percent per decade, the study found.
There's another measurement that allows researchers to analyze how
the ice cap evolves: multi-year ice "area," which discards areas of open
water among ice floes and focuses exclusively on the regions of the
Arctic Ocean that are completely covered by multi-year ice. Sea ice area
is always smaller than sea ice extent, and it gives scientists the
information needed to estimate the total volume of ice in the Arctic
Ocean. Comiso found that multi-year ice area is shrinking even faster
than multi-year ice extent, by -17.2 percent per decade.
"The average thickness of the Arctic sea ice cover is declining
because it is rapidly losing its thick component, the multi-year ice. At
the same time, the surface temperature in the Arctic is going up, which
results in a shorter ice-forming season," Comiso said. "It would take a
persistent cold spell for most multi-year sea ice and other ice types
to grow thick enough in the winter to survive the summer melt season and
reverse the trend."
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