Abstract
The Greenland ice sheet is losing mass to the ocean at an increasing rate (Thomas et al. 2006). During the 1980s the ice sheet was believed to be in near-equilibrium (van den Broeke et al. 2009). Within the first decade of the 21st century, however, a net negative balance was observed. Greenland’s present rate of ice loss is c. 250 Gt yr–1, equivalent to a sea-level rise contribution of c. 0.69 mm yr–1. The rate of ice loss has increased over the post 1992 observation period (Shepherd et al. 2012).
Highlights
Rignot & Kanagaratnam (2006) invoked an assumption of negligible changes in ice geometry between their flux gates and the grounding line
A preliminary assessment of such a correction for Basin 7 suggests that the rate of change in downstream ice volume is equivalent to c. 25% of D, which would further decrease the total mass balance by up to 15 Gt yr–1
The plug-flow assumption adds a negative bias to the massloss estimates by assuming that all flow is caused by sliding at the bed, i.e., the surface speed is equal to the mean flow velocity of the ice column. This may be valid in the fast flowing coastal areas, but higher up on the ice sheet the assumption is less valid, where the surface velocity is a mix of sliding and deformation, and the vertically averaged flow speed can be as low as 80% of the observed surface speed
Summary
Rignot & Kanagaratnam (2006) invoked an assumption of negligible changes in ice geometry between their flux gates and the grounding line. As their flux gates are located at the c.
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