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Greenland Ice Sheet: High-Elevation Balance and Peripheral Thinning

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Science  21 Jul 2000:
Vol. 289, Issue 5478, pp. 428-430
DOI: 10.1126/science.289.5478.428

Abstract

Aircraft laser-altimeter surveys over northern Greenland in 1994 and 1999 have been coupled with previously reported data from southern Greenland to analyze the recent mass-balance of the Greenland Ice Sheet. Above 2000 meters elevation, the ice sheet is in balance on average but has some regions of local thickening or thinning. Thinning predominates at lower elevations, with rates exceeding 1 meter per year close to the coast. Interpolation of our results between flight lines indicates a net loss of about 51 cubic kilometers of ice per year from the entire ice sheet, sufficient to raise sea level by 0.13 millimeter per year—approximately 7% of the observed rise.

The mass balance of the Greenland Ice Sheet is of considerable importance to global sea level, yet there is uncertainty as to whether the ice sheet as a whole is increasing or decreasing in size. Recent advances in airborne laser altimetry and global positioning system (GPS) technology have made possible large-scale assessment of elevation change characteristics of the entire ice sheet through repeat surveys separated in time by several years. Such repeat surveys in 1993 and 1998 (1) showed that the southeast margin of the Greenland Ice Sheet has been thinning. Here, we report results from similar measurements in the north of Greenland (1994–99) and provide an assessment of the mass balance of the entire ice sheet.

In 1993 and 1994, NASA's Airborne Topographic Mapper (ATM) measured ice-surface elevations with root mean square (rms) accuracy of 10 cm or better (1–3), within a 140-m swath beneath the aircraft, along flight lines crossing all the major ice drainage basins. Ten flight lines from June and July 1993 were resurveyed in June and July 1998 (1), and 12 from May and June 1994 were resurveyed in May 1999. For computational efficiency, data from each survey were resampled to 70-m planes (or platelets) that best fit the data acquired on each side of the aircraft (1). Elevation changes (dH/dt) for most of the ice sheet were determined by comparing elevation differences at the midpoints between platelet centers from the different years, accounting for the elevation slopes in each platelet (Fig. 1). The comparisons were made only for platelets located within 100 m of each other. Nearer the coast, where the surface becomes too rough to be well fit by planes, the elevation of each laser footprint from the second survey was compared with elevations of all footprints from the first survey lying within a 1-m horizontal radius.

Figure 1

Greenland, showing flight tracks (outlined in black) of laser-altimeter surveys color-coded according to the rate of change in surface elevation (dH/dt). Pale gray segments are in balance within the survey errors (±2 cm/year). Regional values ofdH/dt were obtained over most of the ice sheet by interpolating between flight-track data. In areas near the coast (outside the pink boundary) that were not bounded by survey data, we interpolated between flight-track data and hypothetical values of dH/dt derived from PDD anomalies at coastal weather stations (10). The 13 coastal stations are shown in green along with the dH/dt (cm/year) values derived from the PDD anomalies. The lines of major ice-sheet ridges are shown in violet, and the 2000-m elevation contour is marked by a violet dashed line.

Above 2000 m surface elevation, most of the northern ice sheet lies above the region of summer melting; in the south, there is melting over much of the ice sheet above 2000 m, but most of the meltwater percolates into underlying snow and refreezes. North of 70°N, ∣dH/dt∣ is less than 10 cm/year, and spatial variability is low (Fig. 2). By contrast, the area to the south has high spatial gradients, and ∣dH/dt∣ reaches 20 cm/year or more. This difference may be associated with lower snow-accumulation rates in the north and comparatively low temporal variability, compared to high snowfall and high temporal variability in the south (4). However, the large areas of significant thickening in the south lie in areas where both ice cores (5) and model predictions (4) show reduced snowfall during the 1990s. This is consistent with results from satellite radar measurements showing higher rates of thickening between latitudes 65° and 68°N from 1978 to 1988 (6) and suggests longer term thickening in this area.

Figure 2

Histograms of interpolateddH/dt above 2000-m surface elevation. (A) North of 70°N. (B) South of 70°N.

The effects of thickening are closely balanced by those of thinning to yield average thickening rates for the ice sheet above 2000 m of 14 ± 7 mm/year in the north and −11 ± 7 mm/year in the south, and 5 ± 5 mm/year for the entire region. Bedrock uplift, estimated to average 4 mm/year in the south and 5 mm/year in the north (7) with unknown errors, decreases the average thickening rate to zero. The resulting estimate of 1 ± >5 mm/year average thickening for the entire region above 2000 m is close to the estimate of −2 ± 7 mm/year for approximately the same region derived independently by comparing total snow accumulation within the region with total ice discharge from it (8).

Below 2000 m surface elevation, the coastal regions are more sparsely covered by flight lines. However, it appears that thinning predominates along approximately 70% of the coast. This applies both to flight lines along and across the direction of ice flow. Thickening regions also exist, but generally at lower rates than areas that are thinning. One exception is the isolated ice cap in the extreme northeast, which is thickening by about 0.5 m/year. Snow accumulation here is strongly influenced by the North East Water polynya, an area of open water surrounded by sea ice. The period between our surveys included 2 years with exceptionally large polynyas, in contrast to the 2 years before with smaller than normal polynyas (9). Consequently, the ice-cap thickening is probably a response to locally increased snowfall.

In order to extend our estimates to the edge of the ice sheet in areas not bounded by our surveys, we calculated a hypothetical thinning rate at the coast on the basis of the coastal positive degree day (PDD) anomalies (Fig. 1) (10), using a factor of 9 mm per PDD (11). We then interpolated between this calculated coastal thinning rate and nearest observed elevation changes to yield thinning rates within the ice-covered coastal regions shown in Fig. 1(12). This approach only considers melt near the coast and neglects the contribution of dynamic thinning. As such, it is a minimum estimate. The total net reduction in ice volume associated with the interpolated values of dH/dt was 51 km3/year, which is equivalent to 0.13 mm/year sea-level rise, or about 7% of the observed rate of sea-level increase (13). Although we are unable to assign errors to this estimate, we believe that it represents a lower bound for the reasons stated above.

We do not have a satisfactory explanation for the observed, widespread thinning at elevations below 2000 m. Although conditions between 1993–94 and 1998–99 were warmer than the 20-year average (14), increased melting and/or reduced snow accumulation cannot explain more than about 50 cm/year of the observed thinning (less in most areas), unless recent summer temperatures were far higher than those that provided equilibrium conditions for the low-elevation parts of the ice sheet. This would require that these equilibrium temperatures were significantly lower than those measured since 1979. However, the most recent colder period ended in the late 1800s (15), and Greenland temperature records from 1900–95 (16) show highest summer temperatures in the 1930s, followed by a steady decline until the early 1970s and a slow increase since. The 1980s and early 1990s were about half a degree cooler than the 96-year mean. Consequently, if present-day thinning is attributable to warmer temperatures, thinning must have been even higher earlier this century, with total near-coastal thinning of 100 m or more along most of the coast. To some extent, this scenario is supported by historical data (17) indicating widespread glacier retreat since the 1800s. However, thinning rates exceeding 1 m/year on many of the glaciers during the survey period are probably too large to be explained in this way, leaving a change in ice dynamics as the most likely cause. Increased creep rates in the lower reaches of the glaciers, and therefore increased discharge velocities, would cause the ice to thin. For example, a typical glacier with thickness of 1000 m would require an increase in longitudinal creep rates by 0.001/year to cause a thinning rate of 1 m/year. If sustained over a distance of 50 km, this would increase discharge velocities by 50 m/year. We have no evidence for such changes, and we cannot explain why they should apply to many glaciers in different parts of Greenland.

  • * To whom correspondence should be addressed. E-mail: krabill{at}osb1.wff.nasa.gov

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