tions absent from both glaciers, which recent grounding line data confirms (Morlighem et al., 2014; Figure 2A). Basal topographic data from these glaciers (Figure 2B), shows they both lie below sea level for a distance of 36–42 km inland of the terminus (Morlighem et al., 2014).
Both Heilprin and Tracy Glaciers have undergone net retreat in the twentieth Century (Kollmeyer, 1980). Tracy Glacier retreated by 7 km between 1892 and 1959 (Davies and Krinsley, 1962) and between 1949 and 2009 the glacier retreated a further 15 km away from Josephine Peary Øer Island (Figure 3C; Dawes and van As, 2010). In contrast, Heilprin Glacier, retreated only 4 km between 1892 and 2009 (Porter et al., 2014). The retreat of Tracy Glacier has increased over the last two decades: between 2000 and 2005, Tracy Glacier lost 8 km of its terminus (Figures 4A,B) followed by 2 km retreat between 2005 and 2013 (Porter et al., 2014). These differing rates of retreat were attributed at least partly to fjord geometry, and primarily deeper basal topography below Tracy Glacier which could allow warm water intrusion (Porter et al., 2014). Surprisingly, despite an inland-sloping bed at Heilprin, it is undergoing slower dynamic change than Tracy Glacier (Porter et al., 2014).
Figure 4. Several dramatic retreat events observed at northern Greenland outlet glaciers. Panels (A,B) show Tracy Glacier retreat between 2000 and 2006. Petermann Glacier's large calving event in 2010 is shown in panels (C/D). Steensby Glacier retreat in panels (E/F). A significant disintegration of C. H. Ostenfeld floating ice tongue is shown in panels (G/H). All background Landsat imagery was derived from USGS Earth Explorer from the years shown in the panels.
Estimates of ice discharge from European Remote Sensing (ERS) data in the mid-1990s at these glaciers are given in Table 1. Porter et al. (2014) recorded a doubling in thinning rates between 2011 and 2012 at Tracy Glacier. In terms of ice velocity fluctuations, both Heilprin and Tracy Glaciers experienced increases in speed between 2000/01 and 2005/06 of 20 and 40%, respectively (Joughin et al., 2010a), and coincided with the large retreat observed at Tracy Glacier (Figure 4). There is no record of surging at these glaciers.
Despite retreat taking place at both glaciers, and aside from study by Porter et al. (2014), Tracy and Heilprin Glaciers have been subject to little in-depth research in comparison to other northern Greenland areas. The differing responses of these neighboring glaciers, potentially attributed to fjord geometry, suggests uncertainty remains regarding the controls on these outlet glaciers.
The northern sub-region extends from Humboldt Glacier (79°22′N, 64°57′W) to Hagen Bræ (81°17′N, 28°30′W; Figure 1) and contains 13 outlet glaciers. A further three sub-regions (North NW, North Central, and North NE) within North Greenland (Figure 1) form the following sections.
The north NW region (Figure 1) consists of two of the largest outlet glaciers in Greenland: Petermann and Humboldt. These glaciers have received the most recent research attention in northern Greenland.
Humboldt Glacier drains ~5% of the GrIS by area (Rignot and Kanagaratnam, 2006) and has a ~91 km wide calving front (Table 1), making it the widest outlet glacier in Greenland. The majority of the terminus is thought to be grounded (Higgins, 1989; Joughin et al., 1999), but the northern bay possesses a floating section (Rignot et al., 2001; Carr et al., 2015). The glacier terminus rests significantly below sea level, extending ~100 km distance inland (Figure 2B).
Early work by Davies and Krinsley (1962) using early expedition maps (Koch, 1928), suggested that the frontal position changed little between 1922 and 1960. A more recent synthesis of 2000–2010 calving positions with results from Rignot et al. (2001), concluded that Humboldt Glacier has been retreating since the 1990s (Box and Decker, 2011). Carr et al. (2015) confirmed accelerated retreat since 1999. Between 2000 and 2010, Humboldt Glacier underwent the largest area change (−311 km2) of the 