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Many glaciers along the southeast and northwest coast of Greenland have accelerated, increasing the ice sheet's contribution to global sea-level rise. Here, we map elevation changes on Upernavik Isstrøm (UI), West Greenland, during 2003-2009 using high-resolution Ice, Cloud and land Elevation Satellite (ICESat) laser altimeter data supplemented with altimeter surveys from NASA's Airborne Topographic Mapper (ATM) during 2002-2010. To assess thinning prior to 2002, we analyze aerial photographs from 1985. We document at least two distinct ice loss events characterized by rapid dynamic thinning, increased ice speed, and a retreat of the calving front. The most recent event coincides with the speedup of several glaciers along the northwest coast of Greenland in 2005, and with changes in the rate of mass loss observed using data from the Gravity Recovery and Climate Experiment (GRACE) satellite gravity mission. The first event of increased ice loss could have also taken place along the extended northwest coast. The total dynamic induced ice volume loss on the frontal portion of UI caused by the two events is 45.5 +/- 5.4 km3 (during 1985-2010), while the total melt induced ice volume loss during this same period is 7.4 +/- 1.3 km3.

Currently available Digital Elevation Models(DEMs) of the Greenland Ice Sheet (GrIS) were originally derived from radar altimetry data, e.g. Bamber (Bamber et al., 2001) and later improved by photoclinometry to fill the regions between orbits (Scambos and Haran, 2002). The elevation error of these DEMs is a few meters in the higher part (above 2000 m) of the ice sheet, but it can be as much as 50-100 meters in marginal regions. The relatively low resolution and accuracy poses a problem, especially for ice sheet modeling. Although accurate elevation data have been collected by airborne and spaceborne laser altimetry (airborne: Airborne Topographic Mapper (ATM) (1993-present), Laser Vegetation Imaging Sensor(LVIS) (2007,2009 and 2011); spaceborne: Ice, Cloud, and land Elevation Satellite (ICESat) (2003-2009)) and DEMs have been derived from stereo satellite imagery (e.g., SPOT (40 m), ASTER (15 m)), a high resolution, consistent DEM of GrIS is not yet available. This is due to various problems, such as different error sources in the data and different dates of data acquisition. In order to overcome these difficulties, we generated a multi-resolution DEM of GrIS, reflecting June 2008 conditions, by fusing a photoclinometry DEM, SPOT and ASTER DEMs as well as elevations from ICESat, ATM and LVIS laser altimetry. The new multi-resolution DEM has a resolution of 40 m x 40 m in the marginal ice sheet regions and 250 m elsewhere. The ice sheet margin is mapped from SPOT and Landsat imagery and SPOT DEMs are used to cover the complex topography of ice sheet marginal regions. The accuracy of SPOT DEMs is approximately $\pm 6$ m except in the areas covered by clouds regions, where the SPOT elevations were replaced by ASTER DEMs. The ASTER DEMs were checked and improved by the DEM derived from aerial photography from the 1980s. A new photoclinometry DEM, derived from Advanced Very High Resolution Radiometer (AVHRR) and Moderate Resolution Imaging Spectroradiom

Many glaciers along the southeast and northwest coast of Greenland have accelerated, increasing the ice sheet's contribution to global sea-level rise. Here, we map elevation changes on Upernavik Isstrøm (UI), West Greenland, during 2003-2009 using high-resolution Ice, Cloud and land Elevation Satellite (ICESat) laser altimeter data supplemented with altimeter surveys from NASA's Airborne Topographic Mapper (ATM) during 2002-2010. To assess thinning prior to 2002, we analyze aerial photographs from 1985. We document at least two distinct ice loss events characterized by rapid dynamic thinning, increased ice speed, and a retreat of the calving front. The most recent event coincides with the speedup of several glaciers along the northwest coast of Greenland in 2005, and with changes in the rate of mass loss observed using data from the Gravity Recovery and Climate Experiment (GRACE) satellite gravity mission. The first event of increased ice loss could have also taken place along the extended northwest coast. The total dynamic induced ice volume loss on the frontal portion of UI caused by the two events is 45.5 +/- 5.4 km3 (during 1985-2010), while the total melt induced ice volume loss during this same period is 7.4 +/- 1.3 km3.