Several large outlet glaciers of the Greenland Ice Sheet have recently undergone a change in flow behavior, characterized by rapid accelerations in flow speed (by more than 200%), large terminus retreats (>10 km), and rapid surface lowering (up to 90 m/yr). The synchoneity of these changes at several sites in Greenland implies a common triggering mechanism, probably related to an observed climate warming in the region.
New satellite remote sensing data have allowed us to quantify these changes in remarkable detail. The ASTER sensor on board NASA?s Terra satellite collects stereo imagery which can be used to extract topographic information across extensive regions of large outlet glaciers. By differencing a time series of digital elevation models, we can examine the spatial pattern and temporal evolution of surface elevation changes, and compute rates of ice volume loss and mass unloading. Some of the largest changes have taken place at Helheim and Kangerdlugssuaq glaciers in southeastern Greenland where the coastal portions of these glaciers lost ~47 Gt/yr between 2001 and 2006. Extrapolation of the measured data to the ice divide of the ice sheet yields an estimated loss of ~112 Gt/yr over the same period. To give this estimate some perspective, it is the equivalent of about 10% of the observed rise in global sea level over the same period.
The rapid unloading of ice from Helheim and Kangerdlugssuaq glaciers between 2001 and 2006 contributed about 60% to an elastic uplift of ~35 mm detected at a nearby GPS site in Kulusuk. The remaining uplift signal is attributed to significant ice volume loss occurring elsewhere along the southeastern margin of the ice sheet between 62°N and 66°N. This unique data set consisting of observations of a very large unloading event, well-constrained in time and space, and the resulting geodetic adjustment, allows us to explore some of the characteristics of the crust in this part of Greenland.