Mass balance of the northeast sector of the Greenland ice sheet: a remote-sensing perspective

Synthetic-aperture radar interferometry data and airborne ice-sounding rada r (ISR) data are employed to obtain modern estimates of the inland ice prod uction from Nioghalvfjerdsbrae (NB) and Zachariae Isstrom (ZI), the two lar gest glaciers draining the northeast sector of the Greenland ice sheet. Ice fluxes are measured at the grounding line (14.2 +/- 1 km(3) ice a(-1) for NB and 10.8 +/- 1 km(3) ice a(-1) for ZI) with an ice thickness deduced fro m ice-shelf hydrostatic equilibrium, and along an ISR profile collected ups tream of the grounding line (14.3 +/- 0.7 km(3) ice a(-1) for NB and 11.6 /- 0.6 km(3) ice a(-1) for ZI). Balance fluxes calculated from a map of sno w accumulation and model predictions of surface melt are 11.9 +/- 2 km(3) i ce a(-1) for NB and 10.0 +/- 2 km(3) ice a(-1) for ZI at the grounding line , and 12.2 and 10.3 km(3) ice a(-1), respectively, at the ISR line. The two glaciers therefore exhibit a negative mass balance equivalent to 14% of th eir balance flux, with a +/-12% uncertainty. Independently, we detect a ret reat of the grounding line of NB between 1992 and 1996 which is larger at t he glacier center (920 +/- 250 m) than on the sides (240 +/- 50 m). The cor responding ice-thinning rates (2 +/- 1 m a(-1) at the glacier center and 0. 6 +/- 0.3 m a(-1) on the sides) are too large to be accommodated by tempora l changes in ablation or accumulation, and must be due to dynamic thinning.