Unexceptional dynamic response of the Greenland ice sheet to exceptional climate forcing

Surface melting and runoff from the Greenland ice sheet (GrIS) has increased during the last 30 years, coincident with northern hemisphere warming and with widespread dynamic thinning which has penetrated 120 km into the ice sheet interior. One potential dynamic thinning mechanism is surface melt-induced acceleration of ice sheet motion (termed hydro-dynamic coupling) during summer, but recent observations suggest that the net effect of summer speed upon annual motion may be limited. Here we use observations of ice motion to examine the dynamic response of a land-terminating outlet glacier in south-west Greenland to the exceptional melting observed during 2012. We show that two extreme melt events - 98.6% of the GrIS surface experienced melting on 12 July (unprecedented since 1889) and 79.2% on 29 July - and summer ice sheet runoff ~3.9σ above the 1958-2011 mean resulted in enhanced summer ice flow relative to the average melt year of 2009. However, despite record summer melting, subsequent reduced winter ice flow resulted in 6% less net annual ice motion in 2012 than in 2009. Our findings suggest that surface melt-induced acceleration of land-terminating regions of the ice sheet will remain insignificant even under extreme melting scenarios.