Mapping Glacier Velocity in Greenland
by Eric Rignot, Jet Propulsion Laboratory
The Greenland Ice Sheet is a vast expanse of ice which accumulates mass in the interior from snowfall and flows slowly toward the ocean where it is discharged along narrow channels occupied by outlet glaciers and ice streams.
Until a decade ago, we did not know whether the ice sheet was gaining or losing mass in response to climate change. In the 1990s, the NASA Polar Program initiated a research plan to determine the mass balance of the ice sheet using a combination of satellite data, airborne surveys, and in-situ measurements. The results show that the ice sheet is overall in balance in the interior regions, but is losing mass along the coast, and that overall it is contributing to a rise in sea level. Synthetic aperture radar interferometry (InSAR) is one of the techniques employed to study the ice sheet. With ERS-1 and ERS-2 SAR data from ASF, we demonstrated that InSAR can map ice velocity and topography at an unprecedented level of spatial detail and accuracy. We also found that InSAR can detect the transition boundary between grounded ice and floating ice, and it can detect changes in flow speed with time.
Our results yielded a detailed view of the discharge coming from the northern and eastern sectors of the ice sheet and the discovery of ice streams reaching far inland. InSAR revealed that in the northern part of Greenland, glaciers were melting more from the bottom, as they reached the ocean, than from the top, and iceberg production was comparatively small. InSAR estimates of mass balance also showed that the north was slowly thinning, the east coast was close to balance but thinning, and the southeastern coast was largely out of balance and rapidly thinning.
Then in 2001, a team of researchers initiated a detailed study focusing on the melt regime of the floating tongue of the Petermann Glacier, a major glacier in northwestern Greenland. This project is funded by NASA’s Cryospheric Science Program and NSF’s Arctic Research Program. The study involved detailed airborne surveys of the tongue, ground penetrating radar surveys, GPS surveys, automated weather stations, phase sensitive radar surveys, ablation measurements, and more recently, ice drilling.
The InSAR figure on this page shows a velocity map of Petermann Glacier that was derived from RADARSAT-1 data acquired in fall 2000. This velocity map was combined with a map of ice thickness from NASA/University of Kansas’ ice sounding radar to calculate the glacier flux and its changes along flow. Those changes map directly into the bottom melting regime of the ice tongue. The bottom melt rates reach 25 m/yr close to the grounding line, so that half of the glacier ice is removed from the bottom by the time the glacier advances 10 km, in 10 years. In a more detailed analysis of the SAR signal, we have also shown that bottom melting is strongly channelized, with sub-glacial river channels slowly eroding the ice shelf.
Near the ice front, the floating tongue of Petermann Glacier is only a few meters above the ocean surface. An increase in summer melt from warmer air temperatures, or in bottom melt from a warmer ocean, will have a major impact on the survival of the floating ice shelf, and in turn on the land ice discharge from this sector of the Greenland Ice Sheet into the Arctic ocean.
We are now collecting RADARSAT-1 data over Petermann Glacier on a regular basis, with the help of ASF and the Canadian Space Agency, to monitor the delicate equilibrium of melt/accumulation/ice flow processes on the floating tongue in a warming climate.
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