halvfjerdsfjorden found 20 km of retreat between 1950 and 1963 (Thomsen et al., 1997). Subsequently, the terminus retreated 5–7 km during 1978–2003 (Khan et al., 2014). Zachariae Isstrøm, lost 1400 km2 of its ice shelf between 2000 and 2006 (Moon and Joughin, 2008) and its grounding line has begun to rapidly retreat downslope (Mouginot et al., 2015). Both glaciers underwent similar ice loss in 2004/05, with 60 km2 of ice lost at Nioghalvfjerdsfjorden and 67 km2 at Zachariae Isstrøm (Box and Decker, 2011). Two years later, during 2006/07, both glaciers advanced, although of differing magnitudes: 8.4 km2 at Nioghalvfjerdsfjorden and 45.8 km2 at Zachariae Isstrøm (Box and Decker, 2011). Subsequently, between 2009 and 2012, sections of the Nioghalvfjerdsfjorden retreated by a further 2–3 km (Khan et al., 2014).
Nioghalvfjerdsfjorden and Zachariae Isstrøm were considered slow-moving in the early 1950s (Helk and Dunbar, 1953), although no specific velocity values were given. However, more recent data show that they are relatively fast-flowing reaching speeds of >1 km a−1 at their termini (Khan et al., 2014; Figure 6). No significant acceleration or deceleration was detected between 2000 and 2006 on Nioghalvfjerdsfjorden (Rignot and Kanagaratnam, 2006; Joughin et al., 2010a). However, a more recent study observed an acceleration of 100 m a−1 between 2000 and 2011 (Khan et al., 2014). Zachariae Isstrøm has been accelerating since the early 2000s, increasing by up to 200 m a−2, following the disintegration of part of the ice shelf in 2004/05 (Rignot and Kanagaratnam, 2006; Moon and Joughin, 2008; Joughin et al., 2010a). Acceleration then continued, although more steadily, until 2012 (Khan et al., 2014; Mouginot et al., 2015). Since 2012, ice velocities on Zachariae Isstrøm have increased by 25%, accompanied by accelerated frontal retreat (Mouginot et al., 2015).
Figure 6. Northeast Greenland region that includes Nioghalvfjerdsfjorden (79North), Zachariae Isstrøm, Kofoed-Hansen Bræ, Storstrømmen, and L. Bistrup Bræ. Grounded ice is shown in a black outline. Velocity is shown on each glacier in m a−1. Velocity data was acquired from the 2008/2009 MEaSUREs v2 Greenland velocity (Joughin et al., 2010b). Background imagery is from Landsat 8 (late summer 2015).
Estimates suggest that these glaciers must have experienced submarine melt rates of between 6 and 8 m a−1 to explain their 1996 ice flux (Rignot et al., 1997). This is similar to submarine melt rates recorded at other northern Greenland outlet glaciers e.g., Petermann Glacier (Rignot and Steffen, 2008). The break-up of fast ice offshore of Nioghalvfjerdsfjorden in 1997 was followed by a large glacier calving event and is thus considered an important control on the rates of calving (Reeh et al., 2001).
More generally, the NEGIS is thought to be undergoing dynamic thinning as a result of climate change, losing mass at a rate of >10 Gt yr−1 (Khan et al., 2014). As the NEGIS extends far into the ice sheet interior (Khan et al., 2014; Figure 2C) and sits on a reverse bed slope, these glaciers have the potential to discharge large volumes of ice. Should retreat continue at the present or increased rates (particularly at Zachariae Isstrøm; Khan et al., 2014), subsequent ice flow speed-ups could cause significant mass loss from a large inland area of the GrIS (Csatho et al., 2014). This could further destabilize this region and increase its contribution to twenty first century sea level rise.
Storstrømmen, Kofoed-Hansen Bræ, and L. Bistrup Bræ
Storstrømmen is another large outlet of the NEGIS that, along with Kofoed-Hansen Bræ, drains ~120,000 km2 (Table 1). The catchment extends to the summit of the GrIS (Reeh et al., 2003; Figure 2C). Kofoed-Hansen Bræ is the northeastern branch of Storstrømmen which discharges ~25% of the Storstrømmen ice flux (Mohr et al., 1998). Storstrømmen has a two-lobed calving front, one of which drains directly into the ocean and the other of which joins with L. Bistrup Bræ to the south (Figure 6; Khan et al., 2014). L. Bistrup Bræ terminates alongside Storst