peed 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 39 glaciers studied in Box and Decker (2011) ice sheet wide dataset.
Several studies have identified differences between the northern and southern sections of Humboldt Glacier's terminus. Ice flow velocities vary spatially across the glacier front (Rignot et al., 2001; Carr et al., 2015), with up to four times faster flow, and increased glacier thinning in the north section compared to the south (Joughin et al., 1996a; Abdalati et al., 2001; Rignot et al., 2001; Figure 5A). Joughin et al. (1996a) first hypothesized that this was due to a bedrock channel beneath this northern section and later work by Carr et al. (2015) confirmed the presence of a large deep basal trough (>300 m) that extends 72 km into the ice sheet interior (Figure 2B).
Figure 5. North Greenland region. (A) Sub-region North NW including Humboldt and Petermann. (B) Sub-region North Central including eight outlet glaciers between Steensby Glacier and Henson Glacier. (C) Sub-region North NE, including Marie-Sophie, Academy and Hagen 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).
Recent modeling suggests that both reduced sea-ice buttressing, particularly in the northern sector, and enhanced meltwater availability derived from increased surface temperatures are responsible for the recent retreat of Humboldt Glacier's terminus (Carr et al., 2015). Observations and modeling also suggest regional differences in glacier response to external forcing may occur along the calving front, largely controlled by underlying topography (Rignot et al., 2001; Carr et al., 2015). This glacier is significant in terms of its wide terminus and catchment area. It is hypothesized that if it retreats past a potential pinning point, which is located close to the northern portion of the terminus, into a deep trough extending ~70 km inland, rapid retreat and acceleration and 