SHRINKING OF GREENLAND'S GLACIERS

David E. Steitz
Headquarters, Washington, DC March 4, 1999
(Phone: 202/358-1730)

Keith Koehler
Wallops Flight Facility, Wallops Island, VA
(Phone: 757/824-1579)

RELEASE: 99-33

NASA RESEARCHERS DOCUMENT
SHRINKING OF GREENLAND'S GLACIERS

Greenland's southeastern glaciers are rapidly thinning and
their lower elevations may be particularly sensitive to potential
climate changes, a NASA study suggests.

"The results of this study are important in that they could
represent the first indication of an increase in the speed of
outlet glaciers," said Bill Krabill, principal investigator at
NASA Goddard Space Flight Center's Wallops Flight Facility,
Wallops Island, VA. An outlet glacier acts as a major ice
drainage region for an ice sheet.

"The excess volume of ice transported by these glaciers has
had a negligible effect on global sea level thus far, but if it
accelerates or becomes more widespread, it would begin to have a
detectable impact on sea level," Krabill said.

In the March 5 issue of SCIENCE, researchers report the
glacial thinning is too large to have resulted from increased ice-
surface melting or decreased snowfall. The researchers believe
the thinning, as much as 30 feet over five years in some
locations, is the result of increasing discharge speeds of
glaciers flowing into the Atlantic Ocean.

Krabill said surface-melt water might be seeping to the
bottom of glaciers. Such seepage may be reducing the friction
between the ice and the rock below it, enabling the glaciers to
slide with less friction across the bedrock and thus allow more
ice to slip off into the ocean, according to Krabill.

"The results of this study are significant because they
provide the first evidence of widespread thinning of low-elevation
parts of one of the great polar ice sheets. The results also
suggest that the thinning outlet glaciers must be flowing faster
than necessary to remove the annual accumulation of snow within
their basins," said Krabill.

"Why they are behaving like this is a mystery," said Krabill,
"but it might indicate that the coastal margins of ice sheets are
capable of responding quite rapidly to external changes, such as a
potential warming of the climate."

Researchers noted that while some internal areas of Greenland
that were surveyed showed ice thickening, areas along the coast
showed ice thinning. "Taken as a whole, the surveyed region is in
negative balance," Krabill said.

In 1993 and 1994, NASA researchers surveyed the Greenland ice
sheet using an airborne laser altimeter flown on a NASA P-3
aircraft and measured the thickness of the entire ice sheet. Ten
flight lines flown in 1993 in Southern Greenland were resurveyed
in 1998. The flight lines in Northern Greenland flown in 1994
will be resurveyed in May 1999. Throughout the study, pilots have
used the Global Positioning System and other navigational
equipment to fly the same flight path some 400 meters above the
icy surface.

The results showed three areas in the South accumulating at
rates up to ten inches per year. These areas located in the
internal sections of Greenland are in regions of high snowfall.

In the outer regions of the ice sheets, the researchers
reported large areas of thinning, with the rate of thinning
increasing rapidly towards the ocean. Most-rapid thinning rates
(more than three feet per year) were observed in the lower depths
of East-coast outlet glaciers, the researchers reported.

The researchers noted that the areas of thinning in the East
also saw warmer than normal temperatures for 1993 to 1998.
"However, we also observe areas of thinning near the West coast,
where many locations were cooler than normal," the researchers
reported.

These surveys have established baseline data sets that will
be extended with information from NASA's ICESAT spacecraft. The
ICESAT satellite laser altimeter will be launched in 2001 to
measure ice-surface elevations in Greenland and Antarctica.

Further information on the Greenland mapping project,
including the technology behind the science, can be found on the
web at:

http://aol.wff.nasa.gov/aoltm.html

The Office of Earth Science enterprise, NASA Headquarters,
Washington, DC, sponsors the Greenland and Antarctic ice mapping
projects as part of NASA's ongoing efforts to understand the total
Earth system and the effects of natural and human-induced changes
on the global environment.

-end-