European
satellite confirms UW analysis: Arctic Ocean has lost more than a
third of summer sea-ice volume since a decade ago
By
Hannah HIckey
13
February, 2013
The
September 2012 record low in Arctic sea-ice extent was big news, but
a missing piece of the puzzle was lurking below the ocean’s
surface. What volume of ice floats on Arctic waters? And how does
that compare to previous summers? These are difficult but important
questions, because how much ice actually remains suggests how
vulnerable the ice pack will be to more warming.
New
satellite observations confirm a University of Washington analysis
that for the past three years has produced widely quoted estimates of
Arctic sea-ice volume. Findings based on observations from a European
Space Agency satellite, published online in Geophysical Research
Letters, show that the Arctic has lost more than a third of summer
sea-ice volume since a decade ago, when a U.S. satellite collected
similar data.
Combining
the UW model and the new satellite observations suggests the summer
minimum in Arctic sea ice is one-fifth of what it was in 1980, when
the model begins.
“Other
people had argued that 75 to 80 percent ice volume loss was too
aggressive,” said co-author Axel Schweiger, a polar scientist in
the UW Applied Physics Laboratory. “What this new paper shows is
that our ice loss estimates may have been too conservative, and that
the recent decline is possibly more rapid.”
The
system developed at the UW provides a 34-year monthly picture of
what’s happening to the total volume of Arctic sea ice. The
Pan-Arctic Ice Ocean Modeling and Assimilation System, or PIOMAS,
combines weather records, sea-surface temperature and satellite
pictures of ice coverage to compute ice volume. It then verifies the
results with actual thickness measurements from individual moorings
or submarines that cruise below the ice.
“Because
the ice is so variable, you don’t get a full picture of it from any
of those observations,” Schweiger said. “So this model is the
only way to reconstruct a time series that spans multiple decades.”
The
UW system also checks its results against five years of precise ice
thickness measurements collected by a specialized satellite launched
by NASA in 2003. The Ice, Cloud, and Land Elevation Satellite, or
ICESat, measured ice thickness across the Arctic to within 37
centimeters (15 inches) until spring of 2008.
The
U.K.’s CryoSat-2 satellite resumed complete ice thickness
measurements in 2010; this is the first scientific paper to share its
findings about the recent years of record-low sea ice.
Between
2008 and now, the widely cited UW figures have generated some
controversy because of the substantial ice loss they showed.
“The
reanalysis relies on a model, so some people have, justifiably,
questioned it,” Schweiger said. “These data essentially confirm
that in the last few years, for which we haven’t really had data,
the observations are very close to what we see in the model. So that
increases our confidence for the overall time series from 1979 to the
present.”
Arctic
sea ice is shrinking and thinning at the same time, Schweiger
explained, so it’s normal for the summer ice volume to drop faster
than the area covered, which today is about half of what it was in
1980.
Schweiger
cautioned that past trends may not necessarily continue at the same
rate, and predicting when the Arctic might be largely ice-free in
summer is a different question. But creating a reliable record of the
past helps to understand changes in the Arctic and ultimately helps
to better predict the future.
“One
question we now need to ask, and can ask, is what are the processes
that are driving these changes in the ice? To what degree is it ocean
processes, to what degree is this in the atmosphere?” Schweiger
said. “I don’t think we have a good handle on that yet.”
The
UW system was created by co-author Jinlun Zhang, an oceanographer at
the Applied Physics Laboratory. The UW portion of the research was
funded by NASA and the Office of Naval Research.
Other
co-authors are first author Seymour Laxon, Katharine Giles, Andy
Ridout, Duncan Wingham and Rosemary Willatt at University College
London; Robert Cullen and Malcolm Davidson at the European Space
Agency; Ron Kwok at NASA’s Jet Propulsion Laboratory; Christian
Haas at York University in Canada; Stefan Hendricks at the Alfred
Wegener Institute for Polar and Marine Research in Germany; Richard
Krishfield at Woods Hole Oceanographic Institution; Sinead Farrell at
the University of Maryland; and Nathan Kurtz at Morgan State
University in Baltimore.
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