Calving of icebergs

Iceberg calving is the term for the Greenland Ice Sheet mass budget that is still encumbered with the largest uncertainty. Within the past few decades, Satellite Synthetic Aperture Radar (SAR) Interferomery has revolutionized measurement of icesheet flow, providing area distributions of surface velocity which, when combined with ice-radar measurement of calving-front thickness, allows derivation of iceberg calving fluxes. However, until now the new technique has been applied only to north and northeast Greenland (Rignot et al., 2000, 2001). Observations of calving rates by traditional methods, i.e. repeated surveying from fixed rock points and repeated aerial photography, are summarized by Reeh (1994) and Weidick (1995). Table 44.1 shows measured calving rates from Greenland glaciers. The total measured calving flux amounts to ca. 211km-3yr-1 of ice equivalent corresponding to ca. 193 1012kgyr-1. Still, however, many calving glaciers, particularly in northwest and southeast Greenland, have not been studied. Hence, only about 80% of the estimated total calving flux of 235 1012kgyr-1 from Greenland is based on observations of calving-front thickness and velocity. Moreover, most of the calving-front velocities used for calculating the calving fluxes shown in Table 44.1 are 'snap shot' measurements over short periods (a few days or at best a few weeks) performed at different times during the past 50 yr. Summing up, application of SAR interferometry and other remote-sensing techniques has, until now, not substantially reduced the uncertainty of the estimate of the total iceberg calving loss from the Greenland Ice Sheet.

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