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The Greenland ice sheet is losing mass to the ocean at an increasing rate (Thomas et al. 2006). During the 1980s the ice sheet was believed to be in near-equilibrium (van den Broeke et al. 2009). Within the first decade of the 21st century, however, a net negative balance was observed. Greenland’s present rate of ice loss is c. 250 Gt yr–1, equivalent to a sea-level rise contribution of c. 0.69 mm yr–1. The rate of ice loss has increased over the post 1992 observation period (Shepherd et al. 2012). The ice-sheet mass budget can be partitioned into two main components: (1) surface mass balance (SMB; the net difference between accumulation and surface ablation) and (2) marine ice loss (D; iceberg discharge via glacier dynamics plus subsurface melt at the glacier terminus). Over the past decade, the surface mass-balance proportion has accelerated relative to the D component, changing from c. 50% in 2000–2008 (van den Broeke et al. 2009) to more than two thirds (68%) in 2009–2012 (Enderlin et al. 2014). Whereas modern climate models appear to capture the surface mass-balance response to climate change, the physical processes driving variability in glacier discharge are more complex. Recent increases in D may be due to: (1) changing force-balance at the ice-ocean interface as suggested by model simulations (e.g. Nick et al. 2009, 2013), (2) changing basal lubrication at the ice-bed interface due to increased meltwater availability (Zwally et al. 2002; Andersen et al. 2010), and/or (3) decreasing ice viscosity due to increasing ice temperature (van der Veen et al. 2011). The high spatial variability in these forcing mechanisms and a large sensitivity to local fjord geometry (Nick et al. 2013) require basin scale studies of glacier dynamics to elucidate local causes of glacier acceleration. In 2007 the Programme for Monitoring the Greenland Ice Sheet (PROMICE) was initiated to gain insight into the changing mass balance of the Greenland ice sheet using quantitative meteorological observations, as well as airborne surveys of ice thickness and flow-velocity observations (Ahlstrom et al. 2008). Here we present the first calculations of ice discharge using PROMICE observations, with focus on a West Greenland ice-sheet drainage basin previously defined as ‘Basin 7’ (Zwally et al. 2012; Fig. 1). The c. 400 km long ice-sheet margin within Basin 7 includes the 6 km wide Jakobshavn Isbrae, and several other marine-terminating outlet glaciers, such as Store Gletscher and Rink Isbrae (Figs 1, 2). We combine satellite-derived, ice-surface velocities, airborne ice-thickness measurements, and modelled surface mass balance to assess the dynamic discharge from Basin 7.

Mass loss from an ice-sheet drainage basin in West Greenland