Figures 4E,F).
Ryder Glacier is a 10 km wide outlet glacier that drains ~3.5% of the ice sheet by area (Table 1) into Sherard Osborn Fjord (Figure 5B). The glacier comprises two tributaries that combine at 1000 m elevation (Joughin et al., 1996b, 1999), and it currently has a 29 km long floating tongue (Table 1). An early estimate (1978) of discharge at the terminus was 0.66 km3 a−1, making it one of the more important northern Greenland glaciers (Higgins, 1991). Later work found a substantially larger grounding line flux of 3.88 km3 a−1, confirming its high discharge (Rignot et al. (2001).
Relatively few records of terminus change are available for Ryder Glacier. Some of the first observations, from 1917, suggested that the floating tongue extended further north than at present (Koch, 1928), but then retreated by 5 km between 1947 and 1956 (Davies and Krinsley, 1962). The position of the grounding line also showed retreat during 1992–1996, along with 4 m a−1 of ice surface thinning (Rignot et al., 2001). Following this period, the glacier thinned by 2–4 m a−1 between 1997 and 1999 (Abdalati et al., 2001). More recent observations of terminus change are limited, although Murray et al. (2015) documented 0.43–0.55 km a−1 of glacier advance between 2002 and 2006, followed by a substantial retreat of 3 km in 2006/07. This was followed by advance during 2007–2010 (Box and Decker, 2011). Should observed thinning continue at Ryder Glacier, large areas of ice may become ungrounded (Thomas et al., 2009; Csatho et al., 2014) making it more susceptible to retreat and further large ice losses. Recent work by Joughin et al. (2010a), however, found no notable changes in velocity at Ryder Glacier between the winters of 2000/01 and 2005/06, with flow speeds similar to those of earlier studies (Joughin et al., 1999; Rignot et al., 2001).
The majority of reported velocity changes recorded at Ryder Glacier focused on a postulated mini-surge event in 1995 during which velocity increased three-fold (Joughin et al., 1996b, 1999). This suggested event occurred between September and October 1995, when ice velocity in the slower upstream areas of the glacier was recorded to have increased from 20 to 150 m a−1 and then returned to normal in just a 7-week period (Joughin et al., 1996b). However, as velocity change for the faster main trunk of the glacier was not available during this period, uncertainty remains as to the true magnitude of this mini-surge. It was also unclear if the glacier simultaneously advanced during this interval (Joughin et al., 1996b, 1999; Rignot et al., 2001), although it was hypothesized that this acceleration may have caused a substantial increase in ice discharge (Joughin et al., 1996b; Abdalati et al., 2001). Whether this “mini-surge” reflects true surge-behavior at Ryder Glacier is ambiguous and is discussed in more detail in Section Glacier Surging.
C. H. Ostenfeld Glacier
C. H. Ostenfeld Glacier is ~7.9 km wide (Table 1) and has a drainage area of ~14,000 km2. Of the three outlet glaciers draining into Victoria Fjord, it is the largest and has the highest ice discharge (Higgins, 1989; Rignot et al., 2001; Figure 5B).
Limited information is available on past terminus changes at C. H. Ostenfeld Glacier. Over the past two decades, the terminus shows variable periods of advance and retreat (Box and Decker, 2011; Murray et al., 2015). The floating glacier tongue previously extended ~25 km down fjord of the grounding line (Higgins, 1989, 1991). It still has a floating ice tongue, but it is now only 1.5 km long (Table 1). During 1992–1996, the grounding line retreated 500 ± 200 m (Rignot et al., 2001). More recently, an advance of 5.6 km2 occurred in 2001/02, followed by an annual retreat of 20.6 km2 in 2002/03 (Box and Decker, 2011). Between the years 2000 and 2006, the majority of the ice tongue disintegrated (Figures 4G,H), removing an estimated total area of 350 km2 (Moon and Joughin, 2008). Following this, a further 1.2 km retreat occurred (2006–2007; Murray et al., 2015).
Ice velocities have shown little increase bet