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Ice
loss from glaciers and ice caps is expected to cause more
global sea rise during this century than the massive Greenland
and Antarctic ice sheets, according to a new University
of Colorado at Boulder study.
The researcher, primarily funded by the National Science
Foundation (NSF) and NASA, concluded that glaciers and
ice caps are currently contributing about 60 percent of
the world's ice to the oceans and the rate has been markedly
accelerating in the past decade, said Emeritus Professor
Mark Meier of CU-Boulder's Institute of Arctic and Alpine
Research, lead study author. The contribution is presently
about 100 cubic miles of ice annually -- a volume nearly
equal to the water in Lake Erie -- and is rising by about
three cubic miles per year.
In contrast, the CU-Boulder team estimated Greenland
is now contributing about 28 percent of the total global
sea rise from ice loss and Antarctica is contributing
about 12 percent. Greenland is not expected to catch up
to glaciers and ice caps in terms of sea level rise contributions
until the end of the century, according to the study.
A paper on the subject appears in the July 19 issue of
Science Express, the online edition of Science magazine.
Co-authors include CU-Boulder INSTAAR researchers Mark
Dyurgerov, Ursula Rick, Shad O'Neel, Tad Pfeffer, Robert
Anderson and Suzanne Anderson, as well as Russian Academy
of Sciences scientist Andrey Glazovsky.
"One reason for this study is the widely held view
that the Greenland and Antarctic ice sheets will be the
principal causes of sea-level rise," said Meier,
former INSTAAR director and professor in geological sciences.
"But we show that it is the glaciers and ice caps,
not the two large ice sheets, that will be the big players
in sea rise for at least the next few generations."
The accelerating contribution of glaciers and ice caps
is due in part to rapid changes in the flow of tidewater
glaciers that discharge icebergs directly into the ocean,
said the study. Many tidewater glaciers are undergoing
rapid thinning, stretching and retreat, which causes them
to speed up and deliver increased amounts of ice into
the world's oceans, said CU-Boulder geology Professor
Robert Anderson, study co-author.
Water controls how rapidly glaciers slide along their
beds, said Anderson. When a glacier with its "toe
in the water" thins, a larger fraction of its weight
is supported by water and it slides faster and calves
more ice into the ocean at the glacier terminus.
"While this is a dynamic, complex process and does
not seem to be a direct result of climate warming, it
is likely that climate acts as a trigger to set off this
dramatic response," said Anderson, also an INSTAAR
researcher.
The human impact of this accelerated sea level rise could
be dramatic. The team estimated accelerating melt of glaciers
and ice caps could add from 4 inches to 9.5 inches of
additional sea level rise globally by 2100. This does
not include the expansion of warming ocean water, which
could potentially double those numbers. A one-foot sea-level
rise typically causes a shoreline retreat of 100 feet
or more. The World Bank estimates that about 100 million
people now live within about three feet of sea level.
"At the very least, our projections indicate that
future sea-level rise may be larger than anticipated,
and that the component due to glaciers and ice caps will
continue to be substantial," wrote the researchers
in Science Express.
The team summarized satellite, aircraft and ground-based
data from glaciers, ice caps, the Greenland ice sheet,
the West Antarctic ice sheet and the East Antarctic ice
sheet to calculate present and future rates of ice loss
for the study.
Meier estimated there are several hundred thousand small
glaciers and small, pancake-shaped ice caps in polar and
temperate regions. They range from modest, high mountain
glaciers to huge glaciers like the Bering Glacier in Alaska,
which measures about 5,000 square miles in area and is
nearly one-half mile thick in places.
The researchers used a mathematical "scaling"
process to estimate more remote glacier volumes, thicknesses
and trends by factoring in data like altitude, climate
and geography. They used data gathered from around the
world, including cold regions in Russia, Europe, China,
Central Asia, Canada and South America.
While warming temperatures will likely cause many small
high mountain glaciers in North America Europe to disappear
by the end of the century, large ice fields and ice caps
will continue to produce large amounts of melt water,
Meier said. The scientists also believe many "cold"
polar glaciers and ice caps will soon warm up enough to
begin melting and contributing to sea rise.
The retreat of the Greenland and Antarctic ice sheets
also is giving birth to new, smaller glaciers that are
prime candidates for study by scientists. "It is
incorrect to assume that the small glaciers will simply
go away next century -- they will continue to play a key
role in the sea level story," said Anderson.
Anderson also said that although the volume of ice locked
up in Greenland is equal to roughly 23 feet in sea rise,
only a small fraction is likely to be "pulled out"
during the next century, most of it through outlet glaciers.
Many smaller "benchmark" glaciers around the
world that have been under study for decades are expected
to disappear by the end of the century, said Anderson.
"We need to start gathering benchmark information
on some of the larger glaciers that are unlikely to disappear,
so that we can have a long-term record of their behavior."
Anderson said outlet glaciers in Greenland behave much
like tidewater glaciers in Canada and Alaska, making them
very relevant for long-term study. "Since the world
is becoming increasingly aware that sea-level rise is
a very real problem, we need to acknowledge the role of
all of the ice masses and understand the physical mechanisms
by which they deliver water to the sea."
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NSF.gov
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