Comprehensive altimetry monitoring of the Greenland and Antarctic ice sheets revealed a complex pattern of elevation changes and mass loss since the 1990s. However, the determination of detailed and accurate surface changes from the various altimetry data sets and DEMs remained a challenging problem. We developed a novel method, Surface Elevation Reconstruction And Change detection (SERAC) to reconstruct the spatio-temporal evolution of the Greenland and Antarctic ice sheet elevations, mass balance and dynamic changes from airborne and satellite laser altimetry.

I present a detailed record of Greenland Ice Sheet (GrIS) elevation change from fusing ICESat, ATM and LVIS laser altimetry measurements, spanning the period of 1993-2011. Using elevation histories of small regions at more than 30,000 locations, ice sheet mass loss was investigated at different spatial scales, ranging from single outlet glaciers to the entire GrIS. Moreover, elevation changes were partitioned into a climate-induced component associated with changes in the Surface Mass Balance (SMB), and an ice dynamics component.  For the ice sheet as a whole, mass loss from ice dynamics remained constant, but the spatial pattern of dynamic thinning changed in important ways. Dynamic thinning decreased in SE Greenland as outlet glaciers slowed down after 2005, while at the same time NW Greenland ice loss accelerated as outlet glacier speed increased along the coast. Most GrIS outlet glaciers exhibited intermittent dynamic thinning during the period of 1993-2011, interrupted  by a slow-down of thinning, standstill or even short-term thickening.  However, dynamic thinning usually resumed after a few years, thus indicating that considerable GrIS dynamic mass losses will continue in the future.

Annual elevation changes of the Antarctic Ice Sheet (AIS), reconstructed from ICESat altimetry, are also being presented. The spatial pattern of elevation change depicts the interplay of several processes, including (1) rapid thinning caused by ice dynamics over most West Antarctic Ice Sheet outlet glaciers, (2) changes in SMB, (3) short-term, large amplitude elevation changes caused by subglacial hydrological processes, and (4) wind-driven redistribution of snow.

These results show the complexity of the ice sheet response to external forcings. However, our 20-year long GrIS elevation record also revealed a simple classification of outlet glaciers based on the evolution of elevation changes. Furtheinvestigation of the different glacier types and their spatial distribution will shed light on how their response is controlled by atmospheric and oceanic forcings, as well as local conditions such as bed topography and size of the drainage basin. It will ultimately further  our understanding of dynamically induced mass loss and related sea level rise.