slopes upwards inland (Rignot et al., 2001; Carr et al., 2015). Petermann Glacier also has a deep channel that extends to the interior of the ice sheet (Bamber et al., 2013; Morlighem et al., 2014) which could have a substantial impact on ice dynamics.
At some of the smaller glaciers in northern Greenland, basal topography has also been identified as a potentially important control on dynamics. In particular, contrasting basal topography may have been responsible for the differing rates of retreat at Heilprin and Tracy Glaciers, where a deeper bed beneath Tracy Glacier allows a greater ice area to be subject to warmer ocean waters and associated increased submarine melt rates (Porter et al., 2014). Similarly, at Hagen Bræ and Academy Glaciers, deep basal troughs could be susceptible to ocean warming and linked to glacier instability (Morlighem et al., 2014). However, an alternative explanation for increased flow velocities at Hagen Bræ, may be the loss resistance as the glacier retreated from being in contact with an island pinning point (Joughin et al., 2010a). Basal topography is also important at Ryder Glacier, through its impact on water storage and short-term velocity variations (Joughin et al., 1996b; Abdalati et al., 2001). Thus, basal topography is likely to be an important control on observed glacier retreat and could have important implications for future instability in northern Greenland.
Of further consideration is the nature of the bed, which can have a key influence on ice-sheet dynamics. In particular, beneath the NEGIS, a weak, deforming bed has been suggested responsible for its streaming flow (Joughin et al., 2001; Layberry and Bamber, 2001), similar to that observed at ice streams in Antarctica (e.g., Bindschadler et al., 2001). More recent work supports this hypothesis that water-saturated till contributes to the flow speed on the NEGIS (Christianson et al., 2014). However, this, alongside the effect of subglacial geology, remain under-studied in Greenland in contrast to Antarctica (Walter et 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 