Background
The authors write that "basal lubrication by surface meltwater penetrating the Greenland Ice Sheet generates summer speedups of 50-200% for the regions of the ablation zone experiencing sheet flow," but they say that this seasonal acceleration has led to greater uncertainty "in the dynamic sensitivity of the Greenland Ice Sheet mass balance to increased atmospheric temperatures," citing Bamber et al. (2007) and Shepherd et al. (2009). They note, for example, that "if surface melting translates directly to increased sliding, rising temperatures could generate a positive feedback to mass loss as faster flow causes the ice sheet to lower into warmer elevations," but they add that "if the subglacial hydrologic system of the ice sheet adapts to variable meltwater input, as observed on mountain glaciers (e.g., Iken and Bindschadler, 1986; Mair et al., 2002; Bartholomaus et al., 2008), increased melt could generate a limited, or even decelerating, effect on seasonally-averaged sliding and long-term dynamic response to a warming climate (Truffer et al., 2005; van de Wal et al., 2008; Sundal et al., 2011; Schoof, 2010)."

What was done
In further exploring this subject, Hoffman et al. compared melt intensity variations (based on hourly temperature measurements made at two weather stations) and episodic supraglacial lake drainage events (based on manual inspection of Landsat images made at approximately biweekly time intervals between early June and late August during summer 2007) to ice velocity (based on measurements made at nine GPS stations across a study area spanning ~50 km of the western Greenland Ice Sheet), from which data they calculated strain rates and bed separation.

What was learned
Similar to the previous studies of Zwalley et al. (2002), Joughin et al. (2008), van de Wal et al. (2008), Shepherd et al. (2009), Bartholomew et al. (2010), Sundal et al. (2011) and Palmer et al. (2011), the five U.S. scientists say they observed "an early summer background period of constant ice velocity in west Greenland, followed by a speedup that lasted most of the summer and was associated with surface melt," along with "uplift and diurnal velocity variations that occurred throughout most of the summer." They also report, however, that "areas in the ablation zone typically experienced [only] one to two velocity events that are inferred to result from supraglacial lake drainage," and they say that "in all cases the effects are short-lived (less than one day) and local (less than 10 km)," throwing a monkey wrench into the hypothesis that rising temperatures may generate a positive feedback to mass loss.

What it means
In the words of Hoffman et al., "episodic pulses of water are key for generating enhanced sliding (Bartholomaus et al., 2008; Schoof, 2010)," but they say that "these daily increases in velocity are superimposed on a nighttime velocity that generally decreases over the season," concluding, therefore, that their observations "support the idea that rising air temperatures in Greenland may not translate directly into increased sliding at the seasonal scale," once again citing Truffer et al. (2005), van de Wal et al. (2008), Bartholomew et al. (2010), Schoof (2010) and Sundal et al. (2011) in support of the view they express.

References
Bamber, J.L., Alley, R.B. and Joughin, I. 2007. Rapid response of modern day ice sheets to external forcing. Earth and Planetary Science Letters 257: 1-13.

Bartholomaus, T., Anderson, R.S. and Anderson, S. 2008. Response of glacier basal motion to transient water storage. Nature Geoscience 1: 33-37.

Bartholomew, I., Nienow, P., Mair, D., Hubbard, A., King, M.A. and Sole, A. 2010. Seasonal evolution of subglacial drainage and acceleration in a Greenland outlet glacier. Nature Geoscience 3: 408-411.

Iken, A. and Bindschadler, R. 1986. Combined measurements of subglacial water pressure and surface velocity of Findelengletscher, Switzerland: Conclusions about drainage system and sliding mechanism. Journal of Glaciology 32: 101-119.

Joughin, I., Das, S.B., King, M.A., Smith, B.E., Howat, I.M. and Moonb, T. 2008. Seasonal speedup along the western flank of the Greenland Ice Sheet. Science 320: 781-783.

Mair, D., Nienow, P., Sharp, M., Wohlleben, T. and Willis, I. 2002. Influence of subglacial drainage system evolution on glacier surface motion: Haut Glacier d'Arolla, Switzerland. Journal of Geophysical Research 107: 10.1029/2001JB000514.

Palmer, S., Shepherd, A., Nienow, P. and Joughin, I. 2011. Seasonal speedup of the Greenland Ice Sheet linked to routing of surface water. Earth and Planetary Science Letters 302: 423-428.

Schoof, C. 2010. Ice-sheet acceleration driven by melt supply variability. Nature 468: 803-806.

Shepherd, A., Hubbard, A.L., Nienow, P., King, M.A., Mcmillan, M. and Joughin, I. 2009. Greenland ice sheet motion coupled with daily melting in late summer. Geophysical Research Letters 36: 10.1029/2008GL035758.

Sundal, A.V., Shepherd, A., Nienow, P., Hanna, E., Palmer, S. and Huybrechts, P. 2011. Melt-induced speed-up of Greenland ice sheet offset by efficient subglacial drainage. Nature 469: 521-524.

Truffer, M., Harrisson, W.D. and March, R. 2005. Record negative glacier balances and low velocities during the 2004 heatwave in Alaska, USA: Implications for the interpretation of observations by Zwally and others in Greenland. Journal of Glaciology 51: 663-664.

van de Wal, R., Boot, W., van den Broeke, M.R., Smeets, C.J.P.P., Reijmer, C.H., Donker, J.J.A. and Oerlemans, J. 2008. Large and rapid melt-induced velocity changes in the ablation zone of the Greenland ice Sheet. Science 321: 111-113.

Zwally, H.J., Abdalati, W., Herring, T., Larson, K.M., Saba, J. and Steffen, K. 2002. Surface melt-induced acceleration of Greenland Ice-Sheet flow. Science 297: 218-222.