land research. Nevertheless, they collectively drain a similar area to Tracy Glacier (~3800 km2), and thus represent an important component of the mass budget of the region.
North NE Greenland
At the far north-eastern margin of the GrIS lie several major outlet glaciers (Figure 1) which are Academy, Marie Sophie and Hagen Bræ Glaciers.
Academy and Marie Sophie Glaciers
Academy and Marie Sophie Glaciers collectively drain an area of ~9000 km2 into Independence Fjord (Figure 5C). Academy Glacier has a much wider terminus (8.4 km), compared to Marie Sophie (3.9 km; Table 1), and neither glacier has a floating ice tongue (Figure 2A; Higgins, 1991; Rignot et al., 2001).
Relatively few records of terminus change exist for either glacier. At Marie Sophie, early records showed minimal retreat of 0.06–0.09 km between 1921 and 1956 and sketches by Peary (1892) suggest that Academy Glacier retreated 12 km between 1892 and 1956 (Davies and Krinsley, 1962). More recently, terminus changes showed substantial inter-annual variability between 2000 and 2010 (Murray et al., 2015). Academy Glacier, in particular, advanced (0.59 km) between 2008 and 2009, and subsequently retreated by a similar magnitude (0.49 km) in the following year (2009–2010; Murray et al., 2015). Overall, between 2000 and 2010, both Academy and Marie Sophie Glaciers underwent retreat of 0.9 km and 0.2 km, respectively (Murray et al., 2015).
Early work in the 1970s estimated velocities of 220 m a−1 at Marie Sophie Glacier (Higgins, 1991). This contrasts markedly with more recent values of only 40 m a−1 in 1996 (Rignot et al., 1997) and <100 m a−1 in 2006 (Joughin et al., 2010a). Initial velocity estimates (1970s) at Academy Glacier found values of 256 to 290 m a−1 at the terminus (Higgins, 1991; Rignot et al., 1997). It then maintained a steady velocity of 270 m a−1 from the 1970s to the mid-1990s (Rignot et al., 2001), followed by deceleration between 1996 and 2000/01 (Rignot and Kanagaratnam, 2006). More recently, both Marie Sophie and Academy Glaciers accelerated between 2000/01 and 2005/06 (from 200 to 600 m a−1 at Academy; Joughin et al., 2010a).
It has been suggested that recent increases in glacier velocity could reflect surge behavior on Academy Glacier (Rignot and Kanagaratnam, 2006). However, as the glacier has not shown multi-year advance (despite reduced retreat rates between 2004/05 and 2005/06 of ~500 m; Murray et al., 2015), this is questionable. No record of surging exists for Marie Sophie Glacier.
Both glaciers are significant within northern Greenland in terms of their drainage areas, and because they have both shown variable ice velocities and retreat rates throughout the historical record. Whether variable terminus positions and flow speeds are representative of a cyclic process or surge activity remains unknown. Thus they require future consideration, particularly in terms of deciphering the impacts of external forcing vs. internal glacier dynamics.
Hagen Bræ is a large outlet glacier that is 9.4 km wide at the grounding line (Table 1) and drains ~31,000 km2 of ice area into Hagen Fjord (Figure 5C). Previously, the terminus was pinned on two islands (Higgins, 1989, 1991). Early studies also documented a floating ice tongue ~18 km in length between 1947 and 1978 (Davies and Krinsley, 1962; Higgins, 1989, 1991), but recent imagery (2015) suggests the floating section is only 2.1 km long (Table 1). Academy and Hagen Bræ Glaciers lie within two basal troughs that deepen inland, ~10 km wide that extend ~100 km inland (Morlighem et al., 2014; Figure 2B).
Relatively few studies have considered terminus changes at Hagen Bræ. Early work by Higgins (1989) found the glacier terminus advanced at 0.5 km a−1 during the 1970s. In 2008/09, Hagen Bræ underwent the largest retreat in a single year out of 199 outlet glaciers across Greenland during the period 2000–2010 (15 km) (Murray et al., 2015). However, the glacier also experienced 3.8 km of total advance between 2001 and 2007 (Murray et al., 2015).
In 1996, velocities at the terminus were 94 m a−1 (Rignot et al