ht, 1939; Davies and Krinsley, 1962; Mock, 1966). Between 1932 and 1937, the calving of large tabular icebergs was observed (Wright, 1939) and, by 1959, the terminus had retreated 5.5 km from its 1932 position (Davies and Krinsley, 1962). It is also known that the glacier underwent net retreat of 2.9 km between 2000 and 2010 (Murray et al., 2015).
From 1916 to 1965 the velocity of Harald Moltke Bræ fluctuated greatly between 30 and 1000 m a−1 (Mock, 1966). More recently, in 2000/01, the glacier was flowing at 30–100 m a−1 at the terminus, reaching up to a maximum of 300 m a−1 further up glacier (Joughin et al., 2010a). By 2005, the velocity at the terminus increased to 2000 m a−1 (Joughin et al., 2010a). Several authors have suggested these recent changes in velocity reflect surge behavior at Harald Moltke Bræ (Rignot and Kanagaratnam, 2006; Moon et al., 2012). In particular, velocity increase during 2005 coincided with terminus advance (1.2 km) in 2004/2005 (Murray et al., 2015).
Since the detailed work of Mock (1966), few studies have specifically focused on Harald Moltke Bræ. Its surge-like behavior is unusual in comparison to nearby outlet glaciers. Despite showing marked increases in velocity and advance during proposed surge events (Rignot and Kanagaratnam, 2006; Joughin et al., 2010a; Murray et al., 2015), the glacier has undergone large net retreat following surge events (Murray et al., 2015). This has led to a retreat of ~12 km compared to its position in 1916 (Figure 3B) and hints that the glacier has been influenced by longer-term external environmental drivers.
Heilprin and Tracy Glaciers
Heilprin and Tracy Glaciers are two large outlet glaciers in northwest Greenland that are 6.7 and 5 km wide, respectively (Table 1), and collectively drain an area of ~12,000 km2 into Inglefield Bay (Figure 3A). Davies and Krinsley (1962) noted that both glaciers had floating ice tongues in 1892, although early observations with limited data make this difficult to verify. Rignot et al. (2001) later found floating sections absent from both glaciers, which recent grounding line data confirms (Morlighem et al., 2014; Figure 2A). Basal topographic data from these glaciers (Figure 2B), shows they both lie below sea level for a distance of 36–42 km inland of the terminus (Morlighem et al., 2014).
Both Heilprin and Tracy Glaciers have undergone net retreat in the twentieth Century (Kollmeyer, 1980). Tracy Glacier retreated by 7 km between 1892 and 1959 (Davies and Krinsley, 1962) and between 1949 and 2009 the glacier retreated a further 15 km away from Josephine Peary Øer Island (Figure 3C; Dawes and van As, 2010). In contrast, Heilprin Glacier, retreated only 4 km between 1892 and 2009 (Porter et al., 2014). The retreat of Tracy Glacier has increased over the last two decades: between 2000 and 2005, Tracy Glacier lost 8 km of its terminus (Figures 4A,B) followed by 2 km retreat between 2005 and 2013 (Porter et al., 2014). These differing rates of retreat were attributed at least partly to fjord geometry, and primarily deeper basal topography below Tracy Glacier which could allow warm water intrusion (Porter et al., 2014). Surprisingly, despite an inland-sloping bed at Heilprin, it is undergoing slower dynamic change than Tracy Glacier (Porter et al., 2014).
Figure 4. Several dramatic retreat events observed at northern Greenland outlet glaciers. Panels (A,B) show Tracy Glacier retreat between 2000 and 2006. Petermann Glacier's large calving event in 2010 is shown in panels (C/D). Steensby Glacier retreat in panels (E/F). A significant disintegration of C. H. Ostenfeld floating ice tongue is shown in panels (G/H). All background Landsat imagery was derived from USGS Earth Explorer from the years shown in the panels.
Estimates of ice discharge from European Remote Sensing (ERS) data in the mid-1990s at these glaciers are given in Table 1. Porter et al. (2014) recorded a doubling in thinning rates between 2011 and 2012 at Tracy Glacier. In terms of ice velocity fluctuations, both Heilprin and Tracy Glaciers experienced increases in s