Retreat of
the West Antarctic Ice Sheet
Scientists say that the West
Antarctic Ice Sheet is retreating more slowly than
they thought. In fact, it may have been growing just
8,000 years ago -- long after the end of the most
recent Ice Age.
NASA
Science News
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December 27, 2000 -- New evidence
suggests that the West Antarctic Ice Sheet is
retreating more slowly and contributing less to rising
global sea levels than scientists once thought. In
fact, said researchers at a recent meeting, the sheet
was still growing as recently as 8,000 years ago --
thousands of years after the most recent Ice Age.
"Our previous best estimates
that the ice sheet was adding 1 millimeter per year to
global sea level are almost certainly too high,"
says Robert Bindschadler, a glaciologist at NASA's Goddard
Space Flight Center in Greenbelt, Maryland.
Right: Antarctica
is divided by the Transantarctic Mountains into East
Antarctica, or "Greater Antarctica," and
West Antarctica, or "Lesser Antarctica."
Most of Antarctica is covered by ice, with an average
thickness of nearly a mile -- constituting roughly 90
percent of the Earth's total amount of ice. The
Antarctic ice sheet is the largest body of fresh water
on our planet, amounting to 70 percent of the total.
This revised assessment is based on
a synthesis of new data including past sea-level rise
estimates presented at a workshop this fall on the
West Antarctic Ice Sheet. Bindschadler, who organized
the fall workshop, discussed the latest research
results and changing views of the West Antarctic Ice
Sheet's history at the American Geophysical Union's annual
meeting in San Francisco on December 16.
Calculations of how much and how fast
the ice sheet has thinned and retreated since the peak
of the Earth's last major ice age 20,000 years ago are
based in large part on a recent reconstruction of how
big the ice sheet was during that last glacial
maximum. That reconstruction included a West Antarctic
Ice Sheet three times as large as it is now.
Currently, the ice sheet averages 2000 meters thick,
covers an area the size of Mexico, and contains enough
water to raise global sea level 5 meters.
But analysis of a 30-by-50-mile rise
in the ice sheet near the Ross Ice Shelf called Siple
Dome suggests that this feature was not overrun by a
massive ice sheet in the past, which is what the
reconstruction suggests. A team of glaciologists from
the University of Washington led by Charles Raymond
used an ice-penetrating radar to study the subsurface
layering of Siple Dome.
Another line of evidence that throws
the ice sheet's ancient bulk into question is the
discovery that the ice sheet was still growing as
recently as 8,000 years ago. The reconstruction
assumed that the ice sheet reached its maximum growth
20,000 years ago and has only been in retreat since
then.
Above: These two figures
show former temperatures with major periods of
glaciation labeled. The dashed lines are the present
global average temperature of about 15° C (59° F).
The solid curves show small changes from this average.
Note that the temperature drops only about 5° C
during a glaciation. This has occurred about every
100,000 years, with smaller wiggles in between. The
most recent glaciation, 20,000 years ago, is called
the Laurentide, and Earth is still recovering from it.
According to a new reconstruction of
historic sea level around the world by W. R. Peltier
of the University of Toronto, a major jump in sea
level occurred before the West Antarctic Ice Sheet
began its current retreat, but there is no sign of a
subsequent rise large enough to account for melting of
so much West Antarctic ice.
The
question of how fast the ice sheet retreated still
challenges scientists. Recent work, however, leads
Bindschadler to conclude that the ice sheet
experienced a rapid retreat phase some 7,000 years ago
that was preceded and followed by a slower retreat
that continues today. Bindschadler points to the
geologic record of dated stages in the retreat of the
ice sheet's continental base as evidence that it has
shrunk in fits and starts. Such episodic retreats may
be controlled more by the varying depth of the
underlying surface and water than by the changing
climate.
Above: Sea level rise is one
of the greatest threats of climate change. In New
York, the Battery tide gauge measured an increase in
sea level of roughly .25 meters since 1920. (Graph
based on data from the Goddard Institute for Space
Studies) [more
information]
"The portion of the West
Antarctic Ice Sheet we have focused on for the past
ten years appears to be in a stage of near-zero
retreat now," says Bindschadler, "but what
it will do in the future is still uncertain."
"If you extend the new evidence
and the new line of reasoning into the future, the
behavior of the ice sheet is more difficult to
predict," he says. "It suggests, however,
that if the ice sheet loses its hold on the present
shallow bed it is resting on, the final retreat could
be very rapid."
Related Links
2000
Fall Meeting -- Home page for American
Geophysical Union meeting in San Francisco
NASA
Goddard Space Flight Center -- Home page
The
West Antarctic Ice Sheet Initiative -- Home page
Related Science@NASA Stories:
A
Disintegrating Glacier -- Recent satellite
images reveal two new icebergs floating off the
Antarctic coast. The icy behemoths are fragments of
the Ninnis Glacier.
Earth's
Fidgeting Climate -- Science@NASA article about
the vexing ambiguities of global climate change
research.
Source: NASA
GSFC Press Release
Production Editor: Dr.
Tony Phillips
Curator: Bryan
Walls
Media Relations: Steve
Roy
Responsible NASA official: Ron
Koczor
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