rated 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., 2001). More recently, the glacier has accelerated substantially close to the grounding line: relatively low velocities in 2000/01 (200 m a−1 inland and 60 m a−1 at the grounding line) increased to over 600 m a−1 by 2005/06 and 2007 (Joughin et al., 2010a; Moon et al., 2012). Moon et al. (2012) hypothesized that these large velocity increases may have been due to glacier surging. Several other studies have noted potential surge-type behavior at Hagen Bræ (Abdalati et al., 2001; Rignot et al., 2001). Ice velocities were higher in the 1970s (Higgins, 1991) than in 1996 (Rignot et al., 2001). In 1996, velocity decreased between the equilibrium line altitude and the glacier terminus (Rignot et al., 2001). In addition, the grounding line retreated 400 m (Rignot et al., 2001) between 1992 and 1996. Based on these observations, Rignot et al. (2001) suggested surging occurred in the 1970s and that the glacier was then in quiescence between 1992 and 1996. Between 2000 and 2007, large increases in velocity (Joughin et al., 2010a) coincided with 3.8 km of advance (Murray et al., 2015). This may suggest a second surge of Hagen Bræ. Following this potential surge event, however, the glacier retreated substantially (15 km; Murray et al., 2015). However, velocities remained high (2008/09) (Figure 5C). This behavior is not usually associated with the quiescent phase of a surge-cycle (Meier and Post, 1969). Thus, it could be that external forcing, or retreat from a stable position in the fjord, as opposed to internal, surge-related changes have become the primary control on terminus position at Hagen Bræ in more recent years.
Recent large retreat at Hagen Bræ and its location in a deep basal trough below sea level, might suggest the glacier is vulnerable to rapid retreat in the near-future. This glacier is significant in terms of discharge, draining 6% of northern Greenland by area and it is surprising that this glacier has not been studied in more detail.
The northeast region (Figure 1) consists of five outlet glaciers which a