cean could have important implications for submarine-melt induced ice tongue collapse elsewhere in northern Greenland. However, limited in situ measurements of submarine melt rates, ocean temperatures (Thomsen et al., 1997), fjord circulation, and meltwater plumes means the extent of this process across northern Greenland remains unknown.
Forecasts suggest that ocean temperatures and submarine melt rates are likely to increase with future climate warming (e.g., Collins et al., 2013). As submarine melt rates of between 6 and 25 m a−1 dominates mass loss at several northern Greenland outlet glaciers (e.g., Petermann, Nioghalvfjerdsfjorden, and Zachariae Isstrøm; Rignot et al., 2001), and has the potential to greatly influence glacier stability, there is need for improved fjord temperature data to better estimate submarine melt rates and the role of subglacial meltwater plumes (Nick et al., 2012). Systematic measurements of surface mass balance vs. submarine melting and calving are also required to better understand the importance of these processes of mass loss for many glaciers in northern Greenland. Northern Greenland glaciers are likely to be particularly vulnerable to ocean warming, due to the presence of extensive floating ice tongues, with large surface areas susceptible to submarine-melt induced collapse.
Sea Ice Influence
Sea ice has previously been identified as an important control on glacier stability and calving rates, both in Greenland (Joughin et al., 2008b; Amundson et al., 2010) and elsewhere (e.g., Miles et al., 2016). Northern Greenland glaciers may be particularly susceptible to this control, as they have long floating ice tongues, which are likely to be more sensitive to changes in buttressing provided by sea ice than grounded glaciers (Reeh et al., 2001). Indeed, this was first hypothesized in northern Greenland by Higgins (1989), who suggested that icebergs discharged from outlet glaciers in this region are held in place by semi-permanent sea ice for extended periods of time. Northern Greenland is characterized by multi-year sea ice, which undergoes periodic disintegration events. These are thought to allow the release of icebergs and to reduce back stress, thus promoting calving events and glacier retreat (Higgins, 1991; Reeh et al., 2001). Subsequent studies at Humboldt (Carr et al., 2015) and Petermann (Johannessen et al., 2013) have partly supported this theory, although the relationship appears to be more complex. At Petermann, the impact of sea ice buttressing appears to be less important than surface melt on seasonal velocity increases (Nick et al., 2012), whilst at Humboldt, icebergs were able to move away from the terminus, despite the formation of winter sea ice (Carr et al., 2015).
The outlets of the NEGIS are thought to be particularly susceptible to the effects of sea ice buttressing (Khan et al., 2014). At Nioghalvfjerdsfjorden, evidence suggests that sea ice holds icebergs in place at the calving front (Helk and Dunbar, 1953; Reeh et al., 2001), and that sea ice disintegration led to a large calving event in August 1997 (Reeh et al., 2001). Ten kilometers of frontal retreat occurred at Zachariae Isstrøm during 2002–2003, which led to the complete loss of its floating ice tongue (Mouginot et al., 2015). Khan et al. (2014) linked this to reduced sea ice concentration, due to high atmospheric temperatures, however, others have suggested it was primarily due to warmer subsurface temperatures (Mouginot et al., 2015). With Arctic sea ice predicted to decrease in the near-future (e.g., Collins et al., 2013), there is clear potential for reduced sea ice buttressing on glacier termini to allow for faster, enhanced ice discharge from the northern regions of the ice sheet. However, uncertainty remains as to the importance of sea ice buttressing on all outlet glaciers in northern Greenland. There is therefore a need for more detailed study of the impact of these processes on glacier retreat and inland ice velocities.
Across the GrIS, glacier-specific factors (basal topography and fjord geometry) ha