Radar images of northern Greenland glaciers collected by the European Space Agency's Earth Remote Sensing Satellites (ERS-1 and 2) were employed to
measure the vector velocity, surface topography, and grounding-line position of the major outlet glaciers of the northern sector of the Greenland Ice Sheet.
The first figure shows the grounding-line position of the glaciers (green), and the extent of their drainage basin (black), and topography (blue). In a number
of cases, vector velocity mapping was not possible because no ascending data were collected. Elsewhere a combination of ascending and descending tracks
were available to map ice motion in vector form. Surface topography was either taken from the digital elevation model of Greenland distributed by S. Ekholm
from the KMS Institute in Denmark, and from topography derived from differential interferometric SAR. This goes on a case per case basis.

Using these ERS data, we calculated ice discharge at both the grounding line (assuming hydrostatic equilibrium of ice) and along a cross-glacier ice sounding
radar profile located upstream of the grounding line. Radar profiles available from the University of Kansas are shown in red in the top figure.

The main results of the study were:

1) the grounding line ice discharge is, on average, 3.5 times larger than prior-estimated calf-ice production. The decrease in ice discharge between the grounding line and the ice front is attributed to basal melting at the underside of the glacier floating tongues. Prior estimates of the mass balance of northern Greenland
(very positive budget) made the erroneous assumption that discharge is controlled solely by surface melt and iceberg calving. In the north of Greenland, the
dominant form of mass loss is basal melting at the underside of floating ice shelves.

2) Basal melt rates inferred from steady state conditions average 5-8 m/yr, but reach 20 m/yr in the first 10 km of floating ice. These rates are one order of magnitude or two larger than those inferred for the large ice shelves in Antarctica, and in the higher end of the spectrum of model calculations.

3) Overall, the northern part of the Greenland Ice Sheet is not grossly out of balance, but probably thinning. Thinning is indicated from its slightly negative
mass budget at the grounding line, and by the systematic retreat of its grounding lines, except in the case of surge-type glaciers in
a quiescent phase. The advantage of measuring grounding line retreat over other methods is that it is not dependent on accumulation and ablation.

4) The mass budget of the large glaciers (Petermann, Niohalvfjerdbrae and Zachariae Isstrom) is more strongly into the negative. Ice thinning is not likely to
be due to an increase in summer melt alone. Glacier thinning must include a dynamic component as well. This is consistent with the concept of enhanced
thinning in areas of concentrated flow. This trend is emphasized at lower latitudes, e.g. along the east coast of Greenland.

Other details are summarized in a paper in press by Rignot et al., Contribution to the glaciology of northern Greenland from satellite radar interferometry,
J. Geophys. Res., Special Issue on PARCA, along with additional references on prior work. Also see references.

The main purpose of this web page is to introduce the ERS survey of this region to potential users. A number of example vector map of ice velocity are shown
below, yet access to the digital form of these maps requires that you contact us at JPL. The data are available at a nominal posting of 50 m, in a polar
stereographic projection with a 70 degree secant plane. Each velocity map is typically about 100 km x 100 km wide. The precision of the velocity estimates
varies on a case per case basis.

This research was performed at the Jet Propulsion Laboratory, California Institute of Technology under a contract with the Polar Program of the National Aeronautics and Space Administration. The ERS data employed in this study were provided by the European Space Agency under several data grants.
 

Grounding-line position and migration of northern Greenland glaciers between 1992 and 1996 inferred from ERS InSAR data.  (a) Humboldt  Gletscher (b) Petermann  Gletscher; (c) Steensby  Gletscher; (d) Ryder Gletscher; (e) C.  H.  Ostenfeld  Gletscher;  (f) Hagen  Brae; (g)  Nioghalvfjerdsbrae;  (h)  Zachariae Isstrom (no 1992 InSAR); and (i)  Storstrommen and L.  Bistrup  Brae.  The location of the  grounding  line is shown in black for 1992 and  white  for  1996,  and  marked  with a pointing  arrow.  ISR lines are shown in white for 1995, and  black  for  1999.  Intercepts between  grounding lines and ISR are marked as diamonds.  Thick, continuous lines in (b, c, d, f, g, h, i) denote portions of the ISR track for which ice is in hydrostatic equilibrium based on a  comparison  with ATM,  represented  in white for 1995 and black for  1999.  (b) includes a 1999  transverse  ISR line used to estimate the grounding  line ice flux; no ATM elevation  was  available for that flight due to cloud cover.  All plots are overlay on the radar brightness of the scene from ERS.  copyright ESA 1996.