Background
The authors write that "fluctuations in surface melting are known to affect the speed of glaciers and ice sheets," but they say that "their impact on the Greenland ice sheet in a warming climate remains uncertain," citing Meehl et al. (2007), while further noting, in this regard, that "although some studies suggest that greater melting produces greater ice-sheet acceleration (Zwally et al., 2002; Parizek and Alley, 2004)," others have identified a long-term decrease in Greenland's flow despite increased melting (van de Wal et al., 2008)."

What was done
In a study designed to further explore this important subject, and based on data for five different years (1993 and 1995-1998), Sundal et al. used "satellite observations of ice motion recorded in a land-terminating sector of southwest Greenland to investigate the manner in which ice flow develops during years of markedly different melting."

What was learned
The six scientists determined that "although peak rates of ice speed-up are positively correlated with the degree of melting, mean summer flow rates are not, because glacier slow-down occurs, on average, when a critical run-off threshold of about 1.4 centimeters a day is exceeded." Thus, and "in contrast to the first half of summer, when flow is similar in all years," they found that "speed-up during the latter half is 62 ± 16 per cent less in warmer years," so that "in warmer years, the period of fast ice flow is three times shorter and, overall, summer ice flow is slower." And so it can finally be understood how van de Wal et al. (2008) identified, in the words of Sundal et al., "a long-term (17-year) decrease in Greenland's flow during a period of increased melting."

What it means
Sundal et al. conclude that "simulations of the Greenland ice-sheet flow under climate warming scenarios should account for the dynamic evolution of subglacial drainage," because "a simple model of basal lubrication alone misses key aspects of the ice sheet's response to climate warming," which is something that was indeed missed in the IPCC's 2007 Fourth Assessment Report.

References
Meehl, G.A., Stocker, T.F., Collins, W.D., Friedlingstein, P., Gaye, A.T., Gregory, J.M., Kitoh, A., Knutti, R., Murphy, J.M., Noda, A., Raper, S.C.B., Watterson, I.G., Weaver, A.J. and Zhao, Z.-C. 2007. In: IPCC Climate Change 2007: The Physical Science Basis: Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (Solomon, S., Qin, D., Manning, M., Chen, Z., Marquis, M., Averyt, K.B., Tignor, M. and Miller, H.L., Eds.), Cambridge University Press, Cambridge, United Kingdom, pp. 747-846.

Parizek, B.R. and Alley, R.B. 2004. Implications of increased Greenland surface melt under global-warming scenarios: ice-sheet simulations. Quaternary Science Reviews 23: 1013-1027.

van de Wal, R.S.W., 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., Saba, J. and Steffen, K. 2002. Surface melt-induced acceleration of Greenland ice-sheet flow. Science 297: 218-222.