July 2007
Sea level rise this century may
be greater than previously thought, posing risks to hundreds of millions
of people who live close to the world's oceans, concludes a new study of
ice loss from glaciers and ice caps. The researchers say that in the near
future, the giant Greenland and Antarctic ice sheets will contribute less
to sea level rise than glaciers and ice caps.
Scientists with the University of Colorado-Boulder's Institute of Arctic
and Alpine Research, INSTAAR, and the Russian Academy of Sciences conclude
that glaciers and ice caps now contribute about 60 percent of the ice
melting into the oceans and the rate has been accelerating over the past
decade.
"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," says
lead author Emeritus Professor Mark Meier, former INSTAAR director and
CU-Boulder 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, he says.
Alaska's Columbia Glacier, now discharging about two cubic miles of ice
into Prince William Sound every year, is a good example says study
co-author Robert Anderson, a CU-Boulder geology professor and INSTAAR
researcher.
The Columbia Glacier has thinned up to 1,300 feet in places. It has shrunk
by about nine miles since 1980 and is expected to shrink by another nine
miles in the next two decades.
The team estimates the accelerating melt of glaciers and ice caps could
add from four 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 rise in sea level rise typically causes a shoreline retreat of
100 feet or more, and about 100 million people now live within about three
feet of the world's shorelines.
Anderson says that although the volume of ice locked up in Greenland is
equal to roughly 23 feet in sea level rise, only a small fraction of that
amount is likely to be "pulled out" during the next century, most of it
through outlet glaciers.
The glaciers and ice caps are presently contributing about 100 cubic miles
of ice annually to sea level rise - a volume nearly equal to the water in
Lake Erie. This volume is rising by about three cubic miles per year, the
study shows.
By 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 contribute as much to sea level rise as
glaciers and ice caps until the end of the century.
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, says Anderson.
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, he says.
Water controls how rapidly glaciers slide along their beds, he explains.
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," he says.
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 half a 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 and 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 says.
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," says Anderson. "They will continue to play a key role in
the sea level story."
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.
He says scientists need to start gathering benchmark information on some
of the larger glaciers that are unlikely to disappear, for a long-term
record of their behavior.
"Since the world is becoming increasingly aware that sea-level rise is a
very real problem," he said, "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."
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.
Funded by NASA and the National Science Foundation, the research appears
in the July 19 issue of "Science Express," the online edition of the
journal "Science."
"At the very least," wrote the researchers in "Science Express," "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."
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