least three-fold over a 7-week period (Joughin et al., 1996b). However, as no further or larger surge events have been recorded, we deem it “Possibly” surge-type. We assign the same classification to Hagen Bræ (Figure 1). Here, past acceleration has been attributed to surge behavior, but it is unclear whether surging persists today and recent velocity increases may instead be attributed to reduced resistive stresses at the terminus, due to retreat from basal pinning points (Joughin et al., 2010a). The neighboring Academy Glacier also experienced an order of magnitude increase in ice velocity between 2000 and 2006 (Joughin et al., 2010a), which is suggestive of surge behavior (Rignot and Kanagaratnam, 2006). However, the glacier continued to retreat during this period, albeit at a reduced rate (Murray et al., 2015).
As previously stated, glacier surges may either by thermally or hydrologically controlled (Murray et al., 2003). At Ryder Glacier, it is likely the mini-surge was hydrologically induced due to its underlying topography. Two transverse subglacial ridges beneath the glacier were suggested to have allowed water ponding upstream and, once it reached a critical pressure threshold, could have initiated the surge (Joughin et al., 1996b, 1999; Rignot et al., 2001). However, the drainage of supraglacial lakes and water-filled crevasses may also have been at least partly responsible for the surge event (Joughin et al., 1996b). Given recent advances in our understanding of GrIS dynamics (Das et al., 2008; Sole et al., 2013; Bougamont et al., 2014), we hypothesize that it is unlikely the “mini-surge” event at Ryder Glacier (Joughin et al., 1996b) satisfies a strict definition (Meier and Post, 1969; Sharp, 1988) of surging. Instead it may reflect a seasonal speed up event, similar to which has been seen on the west coast of Greenland (Palmer et al., 2011; Doyle et al., 2015). Such acceleration events appear to be followed by an extra slowdown (Meier et al., 1994), which offsets the annual average (Sole et al., 2013). However, more recent work recorded substantial glacial advance during several years between 2000 and 2010 (Box and Decker, 2011; Murray et al., 2015), which may suggest surging is continuing at Ryder Glacier. Thus it remains unclear whether these advances were internally or externally controlled and whether Ryder Glacier is of true surge-type requires further study.
Generally, surge type glaciers have not been systematically identified across Greenland, apart from in eastern Greenland (Jiskoot et al., 2003; Pritchard et al., 2005). As such, large uncertainties remain as to the nature of surge-type glaciers in northern Greenland, and the possible surge mechanisms have been little-studied. Thus, with several glaciers in northern Greenland having been referred to as “surge type” there is an important need for further research to provide a comprehensive account of surge behavior in northern Greenland and to separate this behavior from changes driven by external forcing.
An important consideration in northern Greenland is the region's sensitivity to future climate change. During the period 2081-2100, average Arctic air temperatures are expected to be 4.2°C warmer than present under Representative Concentration Pathway (RCP) 4.5 and 8.3°C warmer under RCP8.5 (Collins et al., 2013).
As northern Greenland experiences the lowest accumulation rates across the ice sheet (Goelzer et al., 2013) and warming in these high latitudes is expected to be greatest (Gregory and Huybrechts, 2006), it is likely the northern regions of the ice sheet could be more sensitive to future climate change. However, this will also be dependent on how much additional precipitation may be delivered by a warmer atmosphere. Of particular concern to northern Greenland is that simulations of surface melt show the largest amplification in northern Greenland (Fettweis et al., 2012), due to reduced sea ice cover as a result of increased air temperatures (Mernild et al., 2010; Franco et al., 2011). Recent work has also shown an exceptional atmospheric r