December 18, 2000

 

WEST ANTARCTIC ICE SHEET MAY BE A SMALLER SOURCE

OF CURRENT SEA-LEVEL RISE

The West Antarctic Ice Sheet's contribution to global sea-level
rise may be much slower today than it was in the past. New evidence
indicates that the size of the ice sheet thousands of years ago has
been overestimated and the ice sheet may not have been as big or as
steady a source of sea-level rise as scientists thought.

Glaciologist Robert Bindschadler from NASA's Goddard Space
Flight Center (Greenbelt, Md.) will discuss 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.

"Our previous best estimates that the ice sheet is adding 1
millimeter per year to global sea level are almost certainly too
high," says Bindschadler. 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
organized by Bindschadler.

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.

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.

"The portion of the West Antarctic Ice Sheet we have focused on
for the past ten years appears to be in 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. 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."