The past decade has seen major advances in our knowledge of the mass balance of the Greenland Ice Sheet (PARCA, 2001). Based on this new knowledge, Rignot & Thomas (2002) concluded that, presently, the Greenland Ice Sheet is losing mass at a rate sufficient to raise sea level by 0.13 mmyr-1 because of rapid near-coastal thinning. However, it is not known whether this change is part of a long-term trend, or whether it reflects short-term temporal variability of accumulation and melt rates. So, despite many decades of intensive field studies and more recent air- and satellite-borne remote sensing measurements, we still do not know whether, in a longer term view, the Greenland Ice Sheet is increasing or decreasing in mass.

Two main factors contribute to the difficulty ofmeasuring total ice-sheet mass balance:

1 short-term (interannual to decadal scale) fluctuations in accumulation and melt rate cause variations in surface elevation that mask the long-term trend;

2 climate changes that occurred hundreds or even thousands of years ago still influence ice flow and, therefore, current mass changes, as do more recent climate changes.

The total mass balance of the ice sheet can be determined by two entirely different methods: (i) direct measurement of the change in volume by monitoring surface elevation change, and (ii) the budget method, by which each term of the mass balance budget (accumulation, runoff, iceberg calving) is determined separately. The strengths and limitations of the two methods in the context of modern, remote-sensing observation techniques are discussed by Reeh (1999).

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