Surface Elevation Changes of the Greenland Ice Sheet - Results from ESA's Ice Sheet CCI
Levinsen, Joanna Fredenslund1; Khvorostovsky, Kirill2; Meister, Rakia1; Ticconi, Francesca3; Forsberg, René1; Shepherd, Andrew3

For more than two decades, satellite altimetry observations have been used to monitor changes occurring on the Earth's surface. The high temporal and spatial resolution of such measurements provides us with unique possibilities for monitoring the effects of climate changes on glaciers and ice sheets. In order to ensure long-term climate records, ESA has launched the Climate Change Initiative (ESA CCI), which emphasizes 13 different Essential Climate Variables, one of them being Ice Sheets. In this program, four selected key parameters are going to be determined: Surface elevation changes (SEC), surface velocities, calving front locations, and grounding line locations. This work focuses on the derivation of SEC for the entire Greenland Ice Sheet (GIS). The methodology is based on the conclusions from our recently finished ''Round Robin exercise'' in which researchers from various European and US institutions contributed with their best estimates of SEC in the area of Jakobshavn Isbrae's drainage basin. This was done using either ICESat or Envisat data, and the results were validated against airborne lidar data from NASA's IceBridge and ESA's CryoVex campaigns. We found that both radar and laser altimetry resolve the surface changes well, and that a combination of the cross-over and repeat-track techniques provides the highest spatial resolution and accuracy. This is due to the possibility of correcting the large amount of repeat-track data with the cross-overs as slope effects in the latter can be ignored. This is important particularly along the ice margin. Using radar altimeter data from Envisat and, at a later stage ERS-1 and ERS-2, we apply geostatistical interpolation techniques such as kriging or collocation to merge the results from the two methods. This allows us to correct for data errors and to find the best method for estimating the surface elevation changes occurring throughout GIS. Here, we present the results of the Ice Sheet CCI SEC parameter in its most up-to-date stage.