From: ENN Editor
Published May 20, 2013 03:14 PM
Melting Glaciers
Most of the world's frozen water is locked up at the poles. 99
percent of Earth’s land ice is located in the Greenland and Antarctic
ice sheets. Yet the remaining ice in the world’s glaciers contributed
just as much to sea rise as the two major ice sheets combined from 2003
to 2009, says a new study led by Clark University and involving the
University Colorado Boulder. The new research found that all glacial
regions lost significant mass from 2003 to 2009, with the biggest ice
losses occurring in Arctic Canada, Alaska, coastal Greenland, the
southern Andes and the Himalayas. The glaciers outside of the Greenland
and Antarctic sheets lost an average of roughly 260 billion metric tons
of ice annually during the study period, causing the oceans to rise 0.03
inches, or about 0.7 millimeters per year.
A glacier is a large persistent body of ice that forms where the
accumulation of snow exceeds its ablation (melting and sublimation) over
many years, often centuries. Glaciers slowly deform and flow due to
stresses induced by their weight, creating crevasses and other
distinguishing features. Glaciers form only on land and are distinct
from the much thinner sea ice and lake ice that form on the surface of
bodies of water.
Glaciers may be found in mountain ranges of every continent except
Australia, and on a few high-latitude oceanic islands. Between 35°N and
35°S, glaciers occur only in the Himalayas, Andes, a few high mountains
in East Africa, Mexico, New Guinea and Iran.
Crucial to the survival of a glacier is its mass balance, the difference
between accumulation and ablation (melting and sublimation). Climate
change may cause variations in both temperature and snowfall, causing
changes in mass balance. A glacier with a sustained negative balance is
out of equilibrium and will retreat. A glacier with sustained positive
balance is also out of equilibrium, and will advance to reestablish
equilibrium.
The new study compared traditional ground measurements to satellite data
from NASA’s Ice, Cloud and Land Elevation Satellite, or ICESat, and the
Gravity Recovery and Climate Experiment, or GRACE, missions to estimate
ice loss for glaciers in all regions of the planet.
"For the first time, we’ve been able to very precisely constrain how
much these glaciers as a whole are contributing to sea rise," said
geography Assistant Professor Alex Gardner of Clark University in
Worcester, Mass., lead study author. "These smaller ice bodies are
currently losing about as much mass as the ice sheets."
A paper on the subject is being published in the May 17 issue of the
journal Science.
"Because the global glacier ice mass is relatively small in comparison
with the huge ice sheets covering Greenland and Antarctica, people tend
to not worry about it," said CU-Boulder Professor Tad Pfeffer, a study
co-author. "But it’s like a little bucket with a huge hole in the
bottom: it may not last for very long, just a century or two, but while
there’s ice in those glaciers, it’s a major contributor to sea level
rise," said Pfeffer, a glaciologist at CU-Boulder’s Institute of Arctic
and Alpine Research
ICESat, which ceased operations in 2009, measured glacier changes using
laser altimetry, which bounces laser pulses off the ice surface to
determine changes in the height of ice cover. The GRACE satellite
system, still operational, detects variations in Earth’s gravity field
resulting from changes in the planet’s mass distribution, including ice
displacements.
GRACE does not have a fine enough resolution and ICESat does not have
sufficient sampling density to study small glaciers, but mass change
estimates by the two satellite systems for large glaciated regions agree
well, the scientists concluded.
"Because the two satellite techniques, ICESat and GRACE, are subject to
completely different types of errors, the fact that their results are in
such good agreement gives us increased confidence in those results,"
said CU-Boulder physics Professor John Wahr, a study co-author and
fellow at the university’s Cooperative Institute for Research in
Environmental Sciences.
Ground-based estimates of glacier mass changes include measurements
along a line from a glacier’s summit to its edge, which are extrapolated
over a glacier’s entire area. Such measurements, while fairly accurate
for individual glaciers, tend to cause scientists to overestimate ice
loss when extrapolated over larger regions, including individual
mountain ranges, according to the team.
Current estimates predict if all the glaciers in the world were to melt,
they would raise sea level by about two feet. In contrast, an entire
Greenland ice sheet melt would raise sea levels by about 20 feet, while
if Antarctica lost its ice cover, sea levels would rise nearly 200 feet.
For further information see
Melting Glaciers.
Glacier image via Wikipedia.
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