Quantifying forces controlling the 2004-2005 retreat, mass loss, and speed-up of Kangerlussuaq Glacier, Greenland from remotely sensed data
McCormick, David Patrick, II
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Kangerlussuaq Glacier, in SE Greenland, is the largest outlet glacier on the east coast of Greenland, draining approximately 3% of the Greenland Ice Sheet (GrIS). In 2004/05 this glacier underwent a dramatic retreat, as well as acceleration and mass loss, indicating a significant change in ice dynamics. During this time, the ice velocity increased from 6-8 km/yr to 14 km/yr, resulting in a peak mass loss of 40 Gt/yr by 2005, approximately 20% of the mass loss of the whole SE GrIS. Other SE Greenland outlet glaciers exhibited synchronous acceleration, retreat and thinning, and thus in 2004/05 the mass loss from SE Greenland dominated the overall mass balance of the GrIS. My study investigated the possible causes of increased outlet glacier mass loss in this sector by reconstructing the surface history and using the force budget technique to quantify the forces that control the flow of Kangerlussuaq Glacier before and after its major acceleration event. I used multiple sets of remotely sensed data, including repeat stereo imagery from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) instrument on the Terra satellite and from the Satellite Pour l`Observation de la Terre (SPOT) satellite, as well as a bedrock DEM from radar observations provided by Center for Remote Sensing of Ice Sheets (CReSIS) to reconstruct the ice sheet surface topography and velocity in 2003 and 2006. These input data were then used to generate 2D force balance models. Previous studies have suggested that speed-up and thinning of Kangerlussuaq Glacier was caused by a collapse of the calving front in 2004/05 resulting in a loss of back-stress. However, my surface reconstruction revealed that thinning began in the summer of 2002; at least two years before the start of the rapid thinning and retreat of the calving front. This discovery was made possible by using the Surface Elevation Reconstruction And Change Detection (SERAC) method to combine the laser altimetry data and the stereo-image DEMs to improve the accuracy of the DEMs and to generate a high-resolution, accurate elevation change record. The force balance analysis showed only small changes in driving and resisting stresses between 2003 and 2006 despite the significant retreat. Therefore, I reject the hypothesis that the speed-up was the result of a collapsed calving front. My results suggest the speed-up was in part due to a change in the subglacial hydrology that caused a change in effective basal pressure. The 2003-2004 period showed below-average meltwater runoff that may have reduced water entering the subglacial drainage system. If subglacial drainage is through a network of tunnels, a reduction in the subglacial water flux would lower the effective basal pressure. Because the sliding velocity is inversely proportional to the effective pressure, this would increase the sliding speed. The increase in surface melt and runoff starting in 2005 would have increased the subglacial water flux again, and the resulting increase in effective pressure would have caused the glacier to slow down.