Efficient Subglacial Drainage Systems Reduce Velocity and Duration of Ice Flow at Warm Temperatures

Recent studies on the mass balance of the Greenland Ice Sheet (GrIS) have shown that the thickening of the sheet’s interior is offset by its mass loss near coastal regions due to basal lubrication (a process by which the surface meltwater penetrates to the base of the ice sheet and enhances basal sliding). To study this mechanism further, Sundal et al. (2011) observed the spatial and temporal variations in ice flow of six glaciers in the GrIS over a period of five years. They found that there was a significant increase in the speed of the ice flow in the summer in comparison to that in the winter. Peak rates of ice flow are known to be positively correlated with an increased degree of melting; however, this trend is not universally observed, as ice speeds slow down significantly in warm summers. The authors hypothesize that a more efficient subglacial drainage system contributes to the reduction in ice velocities in warm summers.—Sachi Singh
Sundal, A. V., Shepherd, A., Nienow, P., Hanna, E., Palmer, S., Huybrechts, P. 2011. Melt-induced speed-up of Greenland ice sheet offset by efficient subglacial drainage. doi:10.1038/nature09740.

The combined effect of increased surface melting and ice sheet flow is said to hasten the mass loss of the GrIS; however, recent data models on the mass loss of the GrIS have not been able to incorporate the effects of surface melting induced acceleration into their predictions because little is known about the hydraulic forces associated with the melting. To explore these hydraulic forces, the authors studied the satellite observations of ice flow recorded in the southwest corner of the GrIS to examine the development of the ice flow in years of markedly different melting. They found that the average winter speed of glaciers (122 m/year) was significantly lower than the speed in the summer (138 m/year); they also observed an increased variance of glacier speeds over the summer. Thus, the authors conclude that the seasonal variations in melting drives the seasonal variations in ice flow. The variations in the timing, extent and quantity of surface run-off, and variations in the routing of water at the base of the the ice-sheet could all contribute to the seasonal ice flow cycles. The authors investigated further to find that the ice flow in the late summers was three times shorter and significantly slower than that in the early summer. While some scientists believe that greater melting induces greater ice velocity, the authors believe that even though the peak rate of flow increases with high melting, an efficient subglacial drainage system leads to a reduction in the speed as well as the duration of the flow. Since basal lubrication alone cannot explain this phenomenon, the authors claim that the glacial drainage adjusts to accommodate an increase in ice flow; abundant meltwater could trigger a change from an inefficient cavity system to an efficient channelized system of drainage, which could lead to a reduction in subglacial water pressure and ice speeds. These patterns have been observed in the High Arctic and Alaskan valley glaciers.
Since the rates of surface melting of the GrIS are said to double over the 21st century, it would be useful to gain a deeper understanding of the mechanism that drives these changes in subglacial drainage patterns. 

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