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 (Rignot et al., 1997).
The majority of mass loss (80%) at Petermann is via high rates of submarine melting beneath the floating ice tongue (Rignot et al., 2001; Rignot and Steffen, 2008). This explains relatively low iceberg calving rates (Higgins, 1991), despite its large grounding line flux (Rignot et al., 1997; Reeh et al., 1999). Melt rates vary spatially beneath the ice tongue, from 0 m a−1 at the grounding line, to a peak of 25 m a−1 at 10 km downstream of the grounding line (Rignot and Steffen, 2008). Ocean heat transported into the fjord is likely to account for these high rates of submarine melt (Johnson et al., 2011). Rignot and Steffen (2008) also observed several channels on the underside of the tongue, aligned in the direction of ice flow, which are believed to have formed from submarine melt and warm ocean water having been transported beneath the ice. Recently, it has been suggested that ice thinning in these channels may have weakened the ice shelf and been a precursor to recent calving events in 2010 and 2012 (Münchow et al., 2014).
Terminus retreat normally occurs via the calving of large, tabular icebergs (Johnson et al., 2011), and early studies observed sporadic calving of tabular icebergs up to 50 m thick and up to 120 km2 (Dunbar, 1978; Kollmeyer, 1980). The frontal position of Petermann remained relatively stationary between 1876 (Koch, 1928; Davies and Krinsley, 1962) and the 1980s, which suggests that iceberg calving is an important component of the longer-term mass balance of the glacier as opposed to solely losing mass via submarine melt (Higgins, 1989). A large calving event took place in August 2010 and attracted substantial scientific attention due to its size (Box and Decker, 2011; Falkner et al., 2011; Nick et al., 2012; Johannessen et al., 2013). This event removed 25% of the glacier tongue by area (Falkner et al., 2011), creating a tabular iceberg ~27 km in length and 270 km2 in area (Johannessen et al., 2013; Figures 4C,D). This was followed by another large retreat in 2012 of 1