ick et al., 2012; Johannessen et al., 2013). However, the terminus now resides at its furthest position inland since 1953 (Johannessen et al., 2013). This retreat is likely due to increased submarine melt under the floating portion of its terminus as a result of recent ocean warming (Nick et al., 2012).
North Central Greenland
The northernmost region of Greenland in our study (North Central, Figure 1) consists of eight marine-terminating outlet glaciers. The region was first documented in studies by early explorers (Peary, 1892; Rasmussen, 1919), but the majority of these glaciers have had little scientific attention in recent years.
Steensby Glacier is a 4.8 km wide glacier that has a catchment area of 4700 km2 and has a 5.1 km long floating tongue (Table 1). Previous observations suggested this floating ice tongue was formerly between 48 and 62 km long (Ahnert, 1963). Some of the first observations were made by Ahnert (1963) and terminus changes were later recorded by Higgins (1991). Aerial photographs in 1947 showed the terminus to be floating (Ahnert, 1963), and later oblique photographs from 1953 suggested that it advanced between 1947 and 1953 (Higgins, 1991). By 1996, the grounding line had advanced slightly, and the glacier thickened between these two studies (Rignot et al., 2001). Grounding line data between 1993 and 2013 (Morlighem et al., 2014) showed a more extensive, 16 km-long floating tongue at Steensby Glacier (Figure 2A). However, recent satellite imagery shows 15 km of retreat between 1999 and 2015 (Figures 4E,F).
Few records of ice velocities at Steensby Glacier exist. Ice velocities showed little fluctuation between estimates made in the 1970s (430 m a−1: (Higgins, 1991)) and in 1996 (Rignot et al., 2001). More recently, velocities decreased by 10–15% between 2000/01 and 2005/06 (Joughin et al., 2010a). Steensby Glacier has often been absent from regional to ice-sheet-wide studies of glacier retreat and flow acceleration, despite having retreated a substantial 1 km a−1 over the last 15 years (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