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There are a number of possible controls on outlet glacier velocity, including:

• Surface melt water draining to the bottom of the glaciers would lubricate the bed (Figure 7), facilitating faster flow (commonly referred to as the “Zwally effect” after Zwally, 2002). Evidence for this includes seasonal velocity variations of ice sheets (Joughin et al., 2008); correlation between ice velocity and measured melt rates (van de Wal et al., 2008); the drainage of supra glacial lakes in late summer (Das et al., 2008)
• Reduced resistive forces at the glacier grounding line (e.g. reduced thickness or retreat of the floating tongue of a glacier) would cause glacier acceleration (Dupont and Alley, 2005; Pfeffer, 2007). Evidence of glacier retreat synchronised with acceleration support this theory (Howat et al., 2008). Step-wise changes in glacier flow speed coincide with calving and glacial earthquakes at Helheim Glacier (Nettles et al., 2008)

These two possibilities suggest alternative environmental controls on the ice dynamics. In the case of the Zwally effect, there is likely to be a strong atmospheric control, as warmer temperatures result in greater melt intensities. In the case of reduced resistive forces at the glacier grounding line, oceanographic controls are more significant (e.g. Hanna et al., 2009; Holland et al., 2008; Nick et al., 2009)

The GLIMPSE project aims to measure the changes occurring in the Greenland ice sheet margins and identify the processes driving these changes. This will enable us to improve current models to better predict the contribution of Greenland ice to sea level rise and freshwater input to the North Atlantic.

Figure 7: The Zwally Effect - Surface melt water draining to the bed of the Greenland Ice Sheet may lubricate ice-rock interface, fasciliating fast ice flow. Source: Zwally, et al., (2002), Surface Melt-Induced Acceleration of Greenland Ice-Sheet Flow, Science, 297 (5579), 218 - 222 doi: 10.1126/science.1072708

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