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 subsequent increased mass loss may be expected in future (Carr et al., 2015). Thus, Humboldt Glacier may be particularly susceptible to external forcing.
Petermann Glacier was first documented during the US Polaris Expedition by Hall in 1871 (Kollmeyer, 1980) and has since become one of the most studied glaciers in northern Greenland (Johannessen et al., 2013). Its terminus is ~21 km wide at the grounding line (Table 1), narrowing down-fjord to between 15 and 20 km at the current floating terminus (Rignot, 1996; Johannessen et al., 2013; Figure 5A). It's floating ice tongue is one of the most extensive in northern Greenland, previously up to 70 km long (Rignot et al., 2001; Nick et al., 2012), and now 48 km in length (Table 1).
The glacier drains ~6% of the GrIS by area into Hall Basin (Rignot and Kanagaratnam, 2006; Table 1). Large sections of this catchment are grounded well below sea level (Rignot and Steffen, 2008; Johnson et al., 2011), and a deep subglacial trough extends far (100 km) into the ice sheet interior (Morlighem et al., 2014). The trough is ~200 to 400 m deep and coincides with the fastest ice flow (Joughin et al., 1999; Figure 2).
At the grounding line the ice is ~600 m thick (Johannessen et al., 2013), thinning considerably to 200 m toward the ice tongue's terminus (Falkner et al., 2011). There are large differences in estimated ice discharge, depending on whether calculations are made at the glacier front (Higgins, 1991) or the grounding line (Rignot et al., 1997). Estimates at the grounding line, give a value of 13.2 km3 a−1 (Rignot et al. (1997), much higher than estimated calving fluxes at the glacier terminus (0.59 km3 a−1; Higgins, 1991). These different estimates are likely to be due to increasing rates of mass loss through extreme melting beneath the floating ice tongue (Rignot et al., 1997). However, it could also be in part attributed to different measurement accuracy between using aerial photographs (Higgins, 1991) and radar satellite imagery accompanied by digital elevation models (