The Greenland ice sheet loses mass due to changing surface mass balance, direct melting on the surface, ice flow through the numerous outlet glaciers, and basal melt. This Ph.D. thesis focuses on the outlet glaciers terminating in Godthåbsfjord near Nuuk in West Greenland, with Kangiata Nunaata Sermia (KNS) being the main contributing glacier. The mass loss of this glacier forms a small contribution to the total mass loss of the Greenland ice sheet, but it will have a large impact on the local ecosystem when there is a large freshwater flux into the fjord. Here, two independent methods are used to estimate the total mass loss by KNS and its neighboring outlets, namely the ice sheet model PISM applied to KNS and stable oxygen isotope measurements in the fjord. Both studies show that KNS is currently losing mass. Prognostic studies show that KNS will likely continue to lose mass.