From: Science Daily, Adapted from materials
provided by Northern Arizona University.
Published March 3, 2010 08:48 AM
Alaskan Glacier Ice Loss Overestimated?
The melting of glaciers is well documented, but when looking at the
rate at which they have been retreating, a team of international
researchers steps back and says not so fast.
Previous studies have largely overestimated mass loss from Alaskan
glaciers over the past 40-plus years, according to Erik Schiefer, a
Northern Arizona University geographer who coauthored a paper in the
February issue of Nature Geoscience that recalculates glacier melt in
Alaska.
Schiefer said melting glaciers in Alaska originally were thought to
contribute about .0067 inches to sea-level rise per year. The team's new
calculations put that number closer to .0047 inches per year. The
numbers sound small, but as Schiefer said, "It adds up over the
decades."
While the team looked at three-fourths of all the ice in Alaska,
Schiefer noted, "We're also talking about a small proportion of ice on
the planet. When massive ice sheets (such as in the Antarctic and
Greenland) are added in, you're looking at significantly greater rates
of sea-level rise."
Schiefer said the team plans to use the same methodologies from the
Alaskan study in other glacial regions to determine if further
recalibrations of ice melt are in order. These techniques use satellite
imagery that spans vast areas of ice cover.
Previous methods estimated melt for a smaller subset of individual
glaciers. The most comprehensive technique previously available used
planes that flew along the centerlines of selected glaciers to measure
ice surface elevations. These elevations were then compared to those
mapped in the 1950s and 1960s. From this, researchers inferred elevation
changes and then extrapolated this to other glaciers.
Two factors led to the original overestimation of ice loss with this
method, Schiefer said. One is the impact of thick deposits of rock
debris that offer protection from solar radiation and, thus, melting.
The other was not accounting for the thinner ice along the edges of
glaciers that also resulted in less ice melt.
Schiefer and his colleagues used data from the SPOT 5 French satellite
and the Japanese ASTER instrument on NASA's Terra satellite and
converted the optical imagery to elevation information. They then
compared this information to the topographical series maps of glacial
elevations dating back to the 1950s.
While the team determined a lower rate of glacial melt during a greater
than 40-year span, Schiefer said other studies have demonstrated the
rate of ice loss has more than doubled in just the last two decades.
"With current projections of climate change, we expect that acceleration
to continue," Schiefer said. This substantial increase in ice loss since
the 1990s is now pushing up the rise in sea level to between .0098
inches and .0118 inches per year -- more than double the average rate
for the last 40 years.
Working on the Alaskan glacial melt revision with Schiefer and Berthier
were Garry Clarke of the University of British Columbia, Brian Menounos
of the University of Northern British Columbia and Frédérique Rémy of
the Université de Toulouse.
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