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The Greenland Ice Sheet: What It's Been Doing Lately
Reference
Kjaer, K.H., Khan, S.A., Korsgaard, N.J., Wahr, J., Bamber, J.L., Hurkmans, R., van den Broeke, M., Timm, L.H., Kjeldsen, K.K., Bjork, A.A., Larsen, N.K., Jorgensen, L.T., Faerch-Jensen, A. and Willerslev, E. 2012. Aerial photographs reveal late-20th-century dynamic ice loss in northwestern Greenland. Science 337: 569-573.

Background
The authors write that "mass loss from the Greenland Ice Sheet is a complex function of processes related to surface mass balance (SMB) and dynamic ice loss (DIL)," the former of which is defined by them as "the difference between accumulation from solid precipitation (snow) and mass loss from ablation (ice melt and sublimation)," while the latter is said to be related to "marine-terminating outlets due to the marginal breakup of floating ice tongues and to subsequent accelerated flow caused by decreased buttressing and reduced basal drag, resulting in thinning." Unfortunately, as they go on to state, "only limited observational evidence of ice mass changes exists before the 21st century," after which "space-based observations from interferometric synthetic aperture radar (InSAR), intensity tracking on SAR images, GRACE (Gravity Recovery and Climate Experiment) and ICESat (Ice, Cloud and land Elevation Satellite) became available." Nevertheless, they indicate that data from these more recent advanced systems "together with data from the Global Positioning System (GPS)," suggest "potentially severe consequences for global sea-level rise." However - and this is a very big "however" - they rightly acknowledge that "current records suffer from the lack of multidecadal observational data, making any such interpretation tentative."

What was done
In light of this serious data-based limitation, Kjaer et al., as they describe it, "extended the record of DIL back almost threefold (1985-2010) for northwestern Greenland, using a digital elevation model derived from aerial photographs."

What was learned
The fourteen researchers were able to identify two independent dynamic ice loss events on the northwestern Greenland Ice Sheet margin, one of which extended from 1985 to 1993 and the other of which extended from 2005 to 2010, in between which periods there were what they describe as "limited mass changes." And these findings, in their estimation, "suggest that the ice mass changes in this sector were primarily caused by short-lived dynamic ice loss events rather than changes in the surface mass balance."

What it means
In the concluding words of the international team of scientists (hailing from Denmark, the Netherlands, the United Kingdom and the United State), "this finding challenges predictions about the future response of the Greenland Ice Sheet to increasing global temperatures." And we all know of what those predictions generally consist.

Reviewed 19 December 2012