We present a new record of ice thickness change, reconstructed at nearly 100,000 sites on the Greenland Ice Sheet (GrIS) from laser altimetry measurements spanning the period 1993-2012, partitioned into changes due to surface mass balance (SMB) and ice dynamics. We estimate a mean annual GrIS mass loss of 243 ± 18 Gt·y The spatial pattern of dynamic mass loss changed over this time as dynamic thinning rapidly decreased in southeast Greenland but slowly increased in the southwest, north, and northeast regions. Most outlet glaciers have been thinning during the last two decades, interrupted by episodes of decreasing thinning or even thickening. Dynamics of the major outlet glaciers dominated the mass loss from larger drainage basins, and simultaneous changes over distances up to 500 km are detected, indicating climate control. However, the intricate spatiotemporal pattern of dynamic thickness change suggests that, regardless of the forcing responsible for initial glacier acceleration and thinning, the response of individual glaciers is modulated by local conditions. Recent projections of dynamic contributions from the entire GrIS to SLR have been based on the extrapolation of four major outlet glaciers. Considering the observed complexity, we question how well these four glaciers represent all of Greenland's outlet glaciers.Greenland Ice Sheet | laser altimetry | mass balance | ice dynamics

Abstract. We present a new data assimilation method within the Ice Sheet System Model (ISSM) framework that is capable of assimilating surface altimetry data from missions such as ICESat (Ice Cloud and land Elevation Satellite) into reconstructions of transient ice flow. The new method relies on algorithmic differentiation to compute gradients of objective functions with respect to model forcings. It is applied to the Northeast Greenland Ice Stream, where surface mass balance and basal friction forcings are temporally inverted, resulting in adjusted modeled surface heights that best fit existing altimetry. This new approach allows for a better quantification of basal and surface processes and a better understanding of the physical processes currently missing in transient ice-flow models to better capture the important intra-and interannual variability in surface altimetry. It also demonstrates that large spatial and temporal variability is required in model forcings such as surface mass balance and basal friction, variability that can only be explained by including more complex processes such as snowpack compaction at the surface and basal hydrology at the bottom of the ice sheet. This approach is indeed a first step towards assimilating the wealth of high spatial resolution altimetry data available from EnviSat, ICESat, Operation IceBridge and CryoSat-2, and that which will be available in the near future with the launch of ICESat-2.

Abstract. We present a new data assimilation method within the ISSM framework that is capable of assimilating surface altimetry data from missions such as ICESat-1 into reconstructions of transient ice flow. The new method relies on algorithmic differentiation to compute gradients of diagnostics with respect to model forcings. It is applied to the North East Greenland Ice Stream where surface mass balance and basal friction forcings are temporally inverted, resulting in significantly improved modeled surface heights that match existing altimetry. This new approach allows for a better quantification of basal and surface processes, and a better understanding of the physical processes currently missing in transient ice flow models to better capture the important intra and inter-annual variability in surface altimetry. It also demonstrates that large spatial and temporal variability is required in model forcings such as surface mass balance and basal friction, variability that can only be explained by including more complex processes such as snowpack compaction at the surface and basal hydrology at the bottom of the ice sheet. This approach is indeed a first step towards assimilating the wealth of high spatial resolution altimetry data available from EnviSat, ICESat-1, Operation IceBridge and CryoSat-2, and that will be available in the near future with the launch of ICESat-2.

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Nioghalvfjerdsfjorden Glacier (N79) is one of the two main outlets for Greenland’s largest ice stream, the Northeast Greenland Ice Stream (NEGIS), and is the more stable of the two, with no calving front retreat expected in the near future. Using a novel elevation reconstruction approach combining digital elevation models (DEMs) and laser altimetry, previously undetected local phenomena are identified complicating this assessment. N79 is found to have a complex network of basal channels that were largely stable between 1978 and 2012. Since then, an along-flow central basal channel has been growing rapidly, likely due to increased runoff and ocean temperatures, and possibly threatening to decouple the glacier’s northwestern and southeastern halves.