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Mass Balance of the Greenland Ice Sheet at High Elevations

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Science  21 Jul 2000:
Vol. 289, Issue 5478, pp. 426-428
DOI: 10.1126/science.289.5478.426

Abstract

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 rateT = (VQ)/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).

Figure 1

Greenland, showing ice velocities at traverse stations where ice motion was inferred from repeated GPS measurements. Elevation contours and ice flow lines corresponding to the velocity stations are also shown. A, B, C, etc., mark 1000-km intervals along the traverse.

Figure 2

Rates of ice-thickness change (T) around the perimeter of the Greenland Ice Sheet inferred from our work (solid curves) and from the results of Krabillet al. (11, 12) (dashed curves). Distance is measured clockwise around the ice sheet from the northwest corner (A in Fig. 1). Values of the thickening rate (T) are averages for catchment areas of about 30,000 km2. The data gap near C on the east side of the ice sheet lies to the west of a coastal ice dome, where our observations were insufficient to provide an estimate of the mass balance. The other data gap is at the southeast tip of the ice sheet (G in Fig. 3), where ice flow lines are poorly defined.

Errors are large for individual gates (2), mainly because of large percentage errors in S, A, andR 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 km2 (Fig. 2). The group of gates was shifted, one traverse station at a time, to give values of Tplotted 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 ±10% and ±5%, respectively (2). The resulting error in Tis about 0.11A, with A < 40 cm of ice per year for about 80% of the ice sheet. Estimates of Tderived from satellite radar altimetry are correlated over distances less then about 170 km (7), suggesting that accumulation rates and velocities are correlated over similar distances. Consequently, we assume that errors in A andR are independent over distances greater than 170 km so that errors should be less than 0.11A. For areas larger than 100,000 km2 (Fig. 3), errors reduce to about 0.07A, or less than 3 cm/year for most of the ice sheet, apart from the southeast corner, where accumulation rates increase to about 80 cm of ice per year.

Figure 3

Regions with distinctive patterns of ice thickening rate shown, in millimeters per year, by the numbers in boxes ± our estimated errors. These error estimates should be reliable for most of the ice sheet but may be an underestimate in the southeast, where data are sparse and accumulation rates are high and have large spatial gradients.

Our results are a comparison between current ice discharge and total accumulation based on measurements for time periods ranging from a few years to centuries and for different time windows in the past. Taken as a whole, they refer to conditions averaged over the past few decades, and our estimated thickening rates are appropriate to the same period, assuming that velocities close to the 2000-m contour line change slowly with time. The inferred values of T are spatially variable in the south. Here the western part of the ice sheet is thickening by 66 ± 27 mm/year, increasing to 211 ± 52 mm/year at the southern tip, whereas the eastern part is thinning at an average of 108 ± 62 mm/year, rising to 295 ± 79 mm/year over a 30,000-km2 area near the southeast tip (Fig. 3). South of about 69°N, our study area totals 240,000 km2, with an overall thickening rate of 22 ± 23 mm/year. North of 69°N, the ice sheet has been thinning in the west (8) at an average of 41 ± 14 mm/year, thickening in the east at 21 ± 6 mm/year, and thinning by 2 ± 13 mm/year in the northernmost zone. There may have been thinning at latitude 70°N on the east side of the ice sheet. Overall, the northern region, with a total area of 737,000 km2, thinned by 11 ± 7 mm/year. The almost 1 million km2 of the ice sheet within our area of study thinned by 2 ± 7 mm/year during the past few decades. Consequently, within the errors of our measurements, the higher elevation parts of the ice sheet have been almost exactly in balance when considered as a whole and as northern and southern parts. However, major changes have been occurring within the southern part of the ice sheet, with a remarkable contrast between rapid thinning in the east and thickening in the west. The north also exhibits bimodal behavior, but at more subdued rates, and a reversal to thickening in the east and thinning in the west.

Thickening in the southwest is consistent with a similar pattern of long-term thickening from a model simulation of the ice-sheet evolution, forced by the past temperature record (9). This suggests that the observed thickening in this area represents a long-term dynamic response of the ice sheet rather than the effects of recent changes in accumulation rates. Our results show close agreement with estimates of Greenland ice thickening rates, for 1978–88 up to latitude 72°N from comparison of satellite radar-altimeter data (10) and for 1993/4–1998/9 for the entire region from comparison of aircraft laser-altimeter data (11,12). These estimates of T are based on changes in measured surface elevation where repeat surveys pass over the same locations and include vertical motion of a few mm/year of underlying rock. Figure 2 includes results from the laser-altimeter survey (12), averaged over the regions of our study. The greater spatial variability in our results is probably indicative of larger errors (about 5 cm/year). However, the two results diverge in the south (between C and D in Fig. 2), where our inferred rates of thickening in the west and thinning in the east are both far larger than the laser-derived estimates. This is an area of high accumulation rates (70 to 90 cm of ice per year) with high spatial variability, so our observations could simply result from overestimation in the west and underestimation in the east of local accumulation rates (by about 25%). We believe that this is unlikely, so earlier thickening and thinning rates in this area appear to have been substantially larger than those observed (12) between 1993 and 1998. The most likely explanation is recent changes in local snow-accumulation rates, as suggested by the analysis of shallow ice cores (13).

Finally, we stress that these results apply only to average conditions over higher elevation parts of the ice sheet. At lower elevations, the repeat laser-altimeter measurements show that thinning predominates (12).

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