Velocities of Outlet Glaciers, Ice streams, and Ice shelves, Antarctica, From Satellite Images

Changes in global climate and sea level are intricately linked to changes in
the area and volume of polar ice sheets. Thus, melting of the ice sheets may
severely impact the densely populated coastal regions on Earth. Melting of the
West Antarctic ice sheet alone could raise sea level by approximately 5m. In
spite of their importance, the current mass balances (the net gains or losses)
of the Antarctic ice sheets are not known. Because of difficult logistic
problems in Antarctica, field research has focused on only a few major ice
streams and outlet glaciers. Yet, to understand the ice sheet dynamics fully,
we must carefully document all of the coastal changes associated with advance
and retreat of ice shelves, outlet glaciers, and ice streams. A critical
parameter of ice sheets is their velocity field, which, together with ice
thickness, allows the determination of discharge rates. Remote sensing, using
moderate- to high-resolution satellite images, permits glacier movement to be
measured on sequential images covering the same area; the velocities can be
measured quickly and relatively inexpensively by tracking crevasses or other
patterns that move with the ice. Especially important are velocities where the
ice crosses the glaciers' grounding lines (locations along the coast where the
ice is no longer ground supported and begins to float).

Landsat images are particularly useful because they provide synoptic views
covering as much as 185 square km. Thus several fixed points in the scenes,
needed for geometric corrections and coregistration of images, are likely to be
found. On the other hand, Landsat images have disadvantages: the early
Multispectral Scanner (MSS) images have moderate resolution (about 80 m),
permitting tracking of only the larger patterns in the floating part of ice
tongues or shelves. Thematic Mapper (TM) images have high resolution (about 30
m), but digital TM data are very expensive. Also, the long polar winter night
reduces acquisition opportunities, and cloud cover may impede recognition of
features. An alternative is ERS SAR images (European Remote Sensing Satellite,
Synthetic Aperture Radar), which have 30-m resolution and similar viewing
conditions regardless of season or cloud cover. Thus they permit the tracking
of small crevasses and other patterns above or at the grounding line.

An extensive set of Landsat images covering Antarctica was acquired in the
early to middle 1970s. Since 1984, new Landsat images of Antarctica's coastal
regions have been obtained largely through a program sponsored by an
international consortium of nations belonging to the Scientific Committee on
Antarctic Research (SCAR). A period of 20 years between acquisitions of some of
the Landsat images makes them an invaluable resource.

ERS-1 images have been available since mid-1991 in both ascending and
descending orbits. They have repeat orbital cycles varying from 3 days to 35
days, and they cover 100 square km on the ground. We herewith acknowledge the
support of the European Space Agency (ESA), which makes the images (and tapes)
available at no cost to researchers of accepted projects.

Examination of the image pairs showed that many glaciers do not have suitable
floating tongues. Tongues on coastlines where ice shelves are narrow or absent
tend to be short, perhaps due to vigorous ocean-current and wind regimes. Also,
short tongues having distinctive crevasse patterns may break off in a time
frame shorter than that between image acquisitions. For these regions, only
methods that employ high-resolution images that permit recognition of features
near and above the grounding line can be used.

This report summarizes the results of velocity measurements of outlet glaciers,
ice streams, and ice shelves around the Antarctic periphery. For some regions,
where suitable images were available, the same area was measured repeatedly to
validate the data or register changes in velocity with time. The results given
here are a compendium of published papers and work in progress. The results
constitute a data base that will be added to and amended as more velocity
measurements become available.