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 10 km (~130 km2 in area; Johannessen et al., 2013). To put these events into context, Johannessen et al. (2013) found that five major calving events occurred over the past 50 years. A particularly large event occurred in 1991 (153 km2), but the magnitude of the 2010 event exceeds all others in this 50 year record (Johannessen et al., 2013). Alongside observed terminus changes, grounding line retreat of 450 m was observed between 1992 and 1996 (Rignot et al., 2001). Future grounding line retreat could allow warm water to be transported greater distances inland, enhancing submarine melt and increasing the instability of Petermann Glacier (Nick et al., 2012). That said, large uncertainty remains over whether large calving events in recent years were part of a natural cycle or in response to climate-induced forcing (Johannessen et al., 2013).
This glacier is one of the fastest flowing outlets in northern Greenland, with velocities of 1000 m a−1 close to the grounding line (Johnson et al., 2011; Nick et al., 2012; Johannessen et al., 2013; see Figure 5A). However, mean annual velocity has changed little since early estimates (950 m a−1; Higgins, 1991), and has been relatively stable over recent decades (1985–2011; Rignot and Steffen, 2008; Johannessen et al., 2013). Following the 2010 calving event, only marginal acceleration was observed at the glacier terminus, which could be due to weak attachment of the floating ice tongue to the fjord walls, and suggests that glacier velocities may be largely insensitive to ice tongue retreat (Nick et al., 2012). Warm ocean water, accompanied by the absence of sea ice in Hall Basin prior to the 2010 calving event could be responsible for the magnitude of the 2010 calving event (Johannessen et al., 2013).
In summary, Petermann Glacier is one of the major outlets in Greenland and has lost large portions of its floating tongue over the last two decades. The occurrence of several large calving events over the last 50 years suggest these recent changes may be part of a natural cycle (N