The mass-budget method was applied by Thomas et al. (2000b) to estimate the mass balance of the interior region of the Greenland Ice Sheet (Plate 44.1a). They compared the total snow accumulation over Greenland ice-sheet drainage basins with the flux out of them. The study included determination of surface ice motion by using repeated GPS measurements at 30 km intervals along the 2000-m elevation contour around the whole ice sheet. The ice thickness along the same line was measured with airborne ice radar. The ice flux across the 2000-m elevation contour was calculated with due account of the difference between surface velocity and depth-averaged velocity. This method should measure the dynamic response of the ice sheet on a longer time-scale, because flow velocity is believed to be relatively unaffected by moderate changes of surface slope, ice thickness, internal ice temperature and basal conditions that might occur on short time-scales. Fast-flowing ice streams embedded in the ice sheet, for example the ice stream feeding the Jacobshavn Glacier in West Greenland (Echelmeyer & Harrison, 1990) and the Northeast Greenland ice stream (Fahnestock et al., 1993), however, may undergo significant short-term changes of ice dynamics. Apart from problems caused by possible unsteady ice-stream motion, the method should provide a reasonably accurate estimate of the long-term average mass flux out of the central Greenland region. In order to assess the mass balance of the region, however, this mass flux must be compared with the long-term average input to the ice-sheet area above the 2000-m contour line. As mentioned in the following section, a reliable dataset providing such information is still missing, causing some uncertainty of the massbalance estimate. The derived average thickness changes for different regions of the ice sheet are shown in Plate 44.1c. Whereas the total interior ice sheet region appears to be nearly in balance (thickness change = 0 ± 7mmyr-1), some individual regions appear to be undergoing large changes. In particular, a large area of the ice sheet in the southeast has thinned significantly (261 ± 52mmyr-1) over the past few decades in accordance with the short-term changes inferred from radar- and laser-altimeter measurements.
The individual terms of the mass budget of the Greenland Ice Sheet: accumulation rate (520 ± 26), runoff (329 ± 32), and iceberg calving (235 ± 33) are also encumbered with a large uncertainty. Numbers are from Houghton et al. (2001) in 1012kgyr-1.
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