... First, they influence surface melt rates, particularly through their effect on lowering albedo (Lüthje et al., 2006;Tedesco and Steiner, 2011;Tedesco et al., 2012). Second, they affect ice-dynamic processes, since their rapid drainage by hydrofracture allows large pulses of surface meltwater to reach the GrIS's bed, which may impact subglacial effective pressures, raising basal water pressures and hence surface ice velocities (Zwally et al., 2002;Alley et al., 2005;Shepherd et al., 2009;Bartholomew et al., 2010Bartholomew et al., , 2011aBartholomew et al., , 2011bBartholomew et al., , 2012Schoof, 2010;Sundal et al., 2011;Hoffman et al., 2011;Colgan et al., 2011a;Cowton et al., 2013;Joughin et al., 2013;Tedstone et al., 2013;Andrews et al., 2014;Bougamont et al., 2014;Dow et al., 2015;Stevens et al., 2015). This process is of particular concern since over 200 rapid SGL drainages are thought to occur annually across the GrIS, affecting~10% of the total SGL population (Selmes et al., 2011). ...