Has dynamic thinning switched off in south-east Greenland?

Following a relatively stable period during the 1990s, dramatic changes have been reported for many tidewater outlets in SE Greenland. Some of the most important results come from measurements using the GRACE (Gravity Recovery and Climate Experiment) mission (1, 2). These data clearly identified the SE of the Greenland Ice Sheet (GrIS) as having the highest rates of mass loss. Two of the major outlet glaciers in this area, Helheim and Kangerdlugssuaq accelerated by about 100% and 40%, respectively, and their calving fronts retreated by several km (3, 4). Retreat and acceleration occurred in two phases during summer 2003 and 2005 at Helheim, and in a single period between late 2004 and early 2005 at Kangerdlugssuaq. Further south, widespread glacier acceleration between 1996 and 2005 affected most of the outlet glaciers, and Greenland's mass loss doubled in the period (5). Increased discharge due to thinning in the marginal areas, coupled to rapid changes in ice dynamics and synchronous retreat of calving front positions, led to speculations that the GrIS had crossed a "tipping point" induced by global warming. However, subsequent studies in summer 2006 showed that Helheim and Kangerdlugssuaq had simultaneously slowed down again and thinning stopped (6). In summer 2007, we collected lidar data over Helheim and Kangerdlugssuaq flown by the NERC Airborne Research and Survey Facility. Data collected were single swaths over mountain areas, as well as centerline profiles. In cooperation with NASA Goddard Space Flight Center, we conducted a similar but extended campaign in 2008, collecting lidar and radar data for the 16 largest outlet glaciers in SE Greenland, targeting the full extent of the major GRACE anomaly. We used lidar swaths from bedrock as ground-control for extracting DEMs from ASTER satellite images covering the period with major changes in 2004 to 2006, and compared them to lidar and SPOT 5 DEMs to produce the most recent volume change and velocity estimates. Velocity estimates for the glaciers are derived from the ASTER and SPOT data, as well as repeated lidar profiles. To place the contemporary findings in a broader temporal context we use aerial photographs from the 1980s to extract the calving front positions of all targeted glaciers. The position of the calving front of marine terminating outlets appears a good indicator for their behaviour, as all recent dynamic changes were accompanied by large fluctuations in terminus position. For the first time, we will characterise multi-decadal glacier changes for SE part of Greenland, aiming to answer the questions: (i) Has mass loss slowed in the whole SE of Greenland and dynamic thinning switched off? (ii) Do recent changes in SE Greenland outlet glaciers represent profound alterations in the ice sheet, or are they simply expected short- term variability? (1) Chen JL et al. Science 313 (2006). (2) Velicogna I Wahr J, Science 311 (2006). (3) Howat IM et al. Geophys. Res. Lett. 32 (2005). (4) Luckman A et al. Geophys. Res. Lett. 33 (2006). (5) Rignot E, Kanagaratnam P, Science 311 (2006). (6) Howat IM et al. Science 315 (2007).