0 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 (Nick et al., 2012; Johannessen et al., 2013). However, the terminus now resides at its furthest position inland since 1953 (Johannessen et al., 2013). This retreat is likely due to increased submarine melt under the floating portion of its terminus as a result of recent ocean warming (Nick et al., 2012).
North Central Greenland
The northernmost region of Greenland in our study (North Central, Figure 1) consists of eight marine-terminating outlet glaciers. The region was first documented in studies by early explorers (Peary, 1892; Rasmussen, 1919), but the majority of these glaciers have had little scientific attention in recent years.
Steensby Glacier is a 4.8 km wide glacier that has a catchment area of 4700 km2 and has a 5.1 km long floating tongue (Table 1). Previous observations suggested this floating ice tongue was formerly between 48 and 62 km long (Ahnert, 1963). Some of the first observations were made by Ahnert (1963) and terminus changes were later recorded by Higgins (1991). Aerial photographs in 1947 showed the terminus to be floating (Ahnert, 1963), and later oblique photographs from 1953 suggested that it advanced between 1947 and 1953 (Higgins, 1991). By 1996, the grounding line had advanced slightly, and the glacier thickened between these two studies (Rignot et al., 2001). Grounding line data between 1993 and 2013 (Morlighem et al., 2014) showed a more extensive, 16 km-long floating tongue at Steensby Glacier (Figure 2A). However, recent satellite imagery shows 15 km of retreat between 1999 and 2015 (Figures 4E,F).
Few records of ice velocities at Steensby Glacier exist. Ice velocities showed little fluctuation between estimates made in the 1970s (430 m a−1: (Higgins, 1991)) and in 1996 (Rignot et al., 2001). More recently, velocities decreased by 10–15% between 2000/01 and 2005/06 (Joughin et al., 2010a). Steensby Glacier has often been absent from regional to ice-sheet-wide studies of glacier retreat and flow acceleration, despite having retreated a substantial 1 km a−1 over the last 15 years (