Repeated laser‐altimeter surveys and modelled snowfall/summer melt show average ice loss from Greenland between 1997 and 2003 was 80 ± 12 km3 yr−1, compared to about 60 km3 yr−1 for 1993/4–1998/9. Half of the increase was from higher summer melting, with the rest caused by velocities of some glaciers exceeding those needed to balance upstream snow accumulation. Velocities of one large glacier almost doubled between 1997 and 2003, resulting in net loss from its drainage basin by about 20 km3 of ice between 2002 and 2003.

Present destabilization of marine-based sectors in Greenland may increase sea level for centuries to come.

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ABSTRACT. Rapid thinning and velocity increase on major Greenland outlet glaciers during the last two decades may indicate that these glaciers became unstable as a consequence of the Jakobshavn effect (Hughes, 1986), with terminus retreat leading to increased discharge from the interior and consequent further thinning and retreat. To assess whether recent trends deviate from longer-term behavior, we measured glacier surface elevations and terminus positions for Jakobshavn Isbrae, West Greenland, using historical photographs acquired in 1944, 1953, 1959, 1964

Comparison of ice discharge from higher elevation areas of the entire Greenland Ice Sheet with total snow accumulation gives estimates of ice thickening rates over the past few decades. On average, the region has been in balance, but with thickening of 21 centimeters per year in the southwest and thinning of 30 centimeters per year in the southeast. The north of the ice sheet shows less variability, with average thickening of 2 centimeters per year in the northeast and thinning of about 5 centimeters per year in the northwest. These results agree well with those from repeated altimeter surveys, except in the extreme south, where we find substantially higher rates of both thickening and thinning.During 1993 through 1997, ice motion was inferred from repeat GPS (Global Positioning System) measurements at stations (Fig. 1) completely circumnavigating the Greenland Ice Sheet (1), with a 1-or 2-year interval between repeat surveys. Stations were about 30 km apart, close to the 2000-m contour, apart from several in the southeast, which were substantially higher because of high mountains, crevasses, and nunataks. We estimated ice discharge (Q) through gates between adjacent stations as the product of surface ice velocity, ice thickness, and a correction factor, R, equal to column-averaged velocity divided by surface velocity. This product was integrated across the gate width normal to ice motion, assuming linear variation of velocity across the gate (2). Airborne ice-thickness measurements were made along the stake line with a coherent radar depth sounder operating at a center frequency of 150 MHz (3). Values of R were derived from a model simulation of the ice sheet that takes account of basal sliding and a variable temperature with depth (4). We then compared Q with the total flux (V ) of ice accumulated as snow over the catchment region, with area S, corresponding to the gate, and estimated an average ice thickening rate T ϭ (V Ϫ Q)/S (Fig. 2). Accumulation rates (A) are from published estimates (5) updated with information from other investigations (6). Catchment areas were estimated by reconstructing flow lines passing through all velocity stations, assuming the ice to move in the direction of maximum regional surface slope (2).Errors are large for individual gates (2), mainly because of large percentage errors in S, A, and R for the small associated catchment regions. Consequently, we present our results as values of T calculated for several adjacent traverse stations, such that their collective catchment area is about 30,000 km 2 (Fig. 2). The group of gates was shifted, one traverse station at a time, to give values of T plotted in Fig. 2 at positions corresponding to the centers of the groups of gates. Errors here are dominated by uncertainty in local values of A and R, which we assume to be Ϯ10% and Ϯ5%, respectively (2). The resulting error in T is about 0.11A, with A Ͻ 40 cm of ice per year for about 80% of the ice sheet. Estimates of T derived from satellite radar altimetry are correlated over distances le...