| Summary: | Synthetic-aperture radar interferometry data and airborne ice-sounding radar (ISR) data are employed to obtain modern estimates of the inland ice production from Nioghalvfjerdsbrae (NB) and Zachariae Isstrom (ZI), the two largest glaciers draining the northeast sector of the Greenland ice sheet. Ice fluxes are measured at the grounding line (14.2 ± 1 km3ice a-1for NB and 10.8 ± 1 km3ice a-1for ZI) with an ice thickness deduced from ice-shelf hydrostatic equilibrium, and along an ISR profile collected upstream of the grounding line (14.3 ± 0.7 km3ice a-1for NB and 11.6 ± 0.6 km3ice a-1for ZI). Balance fluxes calculated from a map of snow accumulation and model predictions of surface melt are 11.9 ± 2 km3ice a-1for NB and 10.0 ± 2 km3ice a-1for ZI at the grounding line, and 12.2 and 10.3 km3ice a-1, respectively, at the ISR line. The two glaciers therefore exhibit a negative mass balance equivalent to 14% of their balance flux, with a ±12% uncertainty. Independently, we detect a retreat of the grounding line of NB between 1992 and 1996 which is larger at the glacier center (920 ± 250 m) than on the sides (240 ± 50 m). The corresponding ice-thinning rates (2±1 m a-1at the glacier center and 0.6±0.3 m a-1on the sides) are too large to be accommodated by temporal changes in ablation or accumulation, and must be due to dynamic thinning.
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