., 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 are Nioghalvfjerdsfjorden (also known as 79 North), Zachariae Isstrøm, Storstrømmen, Kofoed-Hansen Bræ, and L. Bistrup Bræ. These glaciers are associated with the Northeast Greenland Ice Stream (NEGIS), which is a large, fast flowing portion of the GrIS that rests substantially below sea level (Figure 2B) and drains ice some 600 km into the interior of the ice sheet (Joughin et al., 2001; Reeh et al., 2001). The entire NEGIS is considered potentially unstable, having undergone substantial ice thinning since the beginning of the twenty-first century (Khan et al., 2014).
Nioghalvfjerdsfjorden (79 North Glacier) and Zachariae Isstrøm
Nioghalvfjerdsfjorden and Zachariae Isstrøm are the two main outlets of the NEGIS (Khan et al., 2014) and collectively drain around 30% of the northern GrIS by area (Rignot and Kanagaratnam, 2006; Table 1). Nioghalvfjerdsfjorden is 24 km wide, and has an extensive (~69 km long) floating ice tongue (Figure 2A), that widens down-fjord to 30 km at the terminus (Thomsen et al., 1997). Zachariae Isstrøm is 27 km wide and terminates in an embayment typically surrounded by calved icebergs (Box and Decker, 2011). Zachariae Isstrøm previously terminated as a floating ice tongue (Figure 2A), but this dramatically disintegrated between 2000 and 2006, meaning that the glacier terminus is currently grounded (Khan et al., 2014). Both glaciers lie above deep basal troughs, which rest significantly below sea level (Mayer et al., 2000), and have a reverse bed slope (Bamber et al., 2013; Figure 2B).
Information on the frontal positions of these two glaciers has been comparatively limited due to year-round ice mélange in the fjords, resulting in an ambiguous calving region and therefore making it difficult to accurately identify the true calving front (Bevan et al., 2012; Murray et al., 2015). In 1976, the two glaciers were thought to have coalescing ice tongues (Weidick et al., 1995), which suggests they have since retreated substantially and become separated (Rignot et al., 2001). Early records from Niog