al., 2014), and deserves further research.
Differences in bed topography, fjord geometry, and the presence of floating ice tongues at outlet glaciers across northern Greenland are likely to partly explain the varying responses in glacier dynamics observed. However, little examination of glacier specific factors on outlet glacier retreat have been conducted in northern Greenland and we therefore identify this as a key area for future research.
Some changes in northern Greenland glacier dynamics may not be driven by climatic forcing, and instead relate to surge behavior. Whilst the majority of northern Greenland glaciers have retreated over the last two decades, a number have undergone periods of advance (Figure 7).
Several glaciers within the study region have been previously identified as surge-type and, based on the evidence presented in this review, we have classified the study glaciers according to their potential surge likelihood (Figure 1). (i) Glaciers where surge-type cycles have been observed are defined as “Likely;” (ii) Glaciers which have shown surge characteristics but either have not been referred to as surge-type or have not undergone a large surge event (e.g., Ryder) are defined as “Possibly;” and (iii) glaciers at which no evidence has been recorded in the literature about surging are classed as “No Evidence.”
Eight of the twenty-one glaciers reviewed have been referred to as surge-type within the literature (Table 1; Figure 1), although the evidence for surging varies from glacier to glacier. Harald Moltke Bræ, Brikkerne, Storstrømmen, and L. Bistrup Bræ have all undergone periods of advance, alongside an order of magnitude increase in glacier velocity (Mock, 1966; Higgins, 1991; Reeh et al., 1994; Seale et al., 2011), and we therefore consider it likely these are true surge-type glaciers, which fit the above definition (Figure 1). A potential surge event in northern Greenland that received notable research attention was a “mini-surge” at Ryder Glacier in 1995, during which velocity increased at 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