rs (Koch, 1928; Higgins, 1991). In this respect, they are unique in comparison to other regions of the ice sheet, where floating ice tongues are generally much shorter or absent. Historically, a significant proportion of northern Greenland glaciers are documented to have retreated, particularly between 1894 and 1962 (Davies and Krinsley, 1962). More recently, estimates of ice discharge in the late 1990s indicated widespread thinning (Rignot et al., 1997). Studies by Higgins (1991), Rignot et al. (1997), and Rignot et al. (2001) provided more comprehensive observations of glacier width, length, the presence of floating tongues, as well as initial velocity measurements and grounding line positions. Since then, a comprehensive analysis of northern Greenland glacier retreat and velocity fluctuations has not been conducted, although outlet glaciers from this region are often incorporated into GrIS wide studies (Moon and Joughin, 2008; Joughin et al., 2010a; Box and Decker, 2011; Murray et al., 2015). These syntheses reported significant increases in outlet glacier retreat rates across Greenland (for 1992–2006; Moon and Joughin, 2008) and that the largest cumulative area changes (during 2000–2010) occurred in northern Greenland, particularly at glaciers with the largest floating portions (e.g., Petermann, Humboldt, and Zachariae Isstrøm; Box and Decker, 2011).
An outline of the characteristics of each of the outlet glaciers in our study region is provided in Table 1, which also includes glacier catchment sizes delineated using bedrock topography and ice thickness data (Morlighem et al., 2014) input into the Shreve hydropotential formula (Shreve, 1972) to determine subglacial water routing and thus glacier drainage catchments. The following sections are a synthesis of previous work on each of the glaciers by region (Figure 1), with a focus on describing the key characteristics of each glacier and setting recent observations in a broader historical context. For comparison between sub-sections, we focus on terminus length changes, but in some cases where length is not reported in the literature, we refer to terminus change as it is reported in area.
Northwest Greenland (Figure 1), has recently undergone glacier retreat, particularly between 2000 and 2010 (Murray et al., 2015). Most outlets in this region have undergone long-term thinning (1994–2014; Csatho et al., 2014) and have accelerated by 28% between 2000 and 2010 (Moon et al., 2012). Three of the largest outlet glaciers in the far northwestern region are Harald Moltke Bræ, which drains into Wolstenholme Fjord, and Heilprin and Tracy Glaciers, which terminate in the neighboring Inglefield Bay (Figure 3).
Figure 3. (A) Location of studied glaciers in Northwest Greenland, including Harald Moltke Bræ, Heilprin, and Tracy Glaciers (green circles) and grounded ice (black line). Velocity data were acquired from the 2005/2006 MEaSUREs v2 Greenland velocity (Joughin et al., 2010b). Background imagery is from Landsat 8 (late summer 2015). (B) Estimated terminus positions from Wright (1939), showing changes between 1916 and 1932, and its position in 2015 (green). (C) Previous terminus positions for Tracy and Heilprin Glaciers (Dawes and van As, 2010) and the 2015 position (light blue).
Harald Moltke Bræ
Harald Moltke Bræ is an outlet glacier in northwest Greenland that is 6.3 km wide at its grounded terminus (Koch, 1928; Wright, 1939; Davies and Krinsley, 1962; Rignot et al., 2001; Figure 2). Early observations suggested that the terminus had two calving lobes (Wright, 1939) shown in Figure 3B. However, recent imagery shows this is no longer the case (Figure 3B). The catchment area of Harald Moltke Bræ is smaller than most other glaciers in the study area, draining only 1400 km2 (Table 1).
Early studies provided a detailed historical account of terminus change for the period 1916–1965 (Wright, 1939; Davies and Krinsley, 1962; Mock, 1966; Figure 3B). Between 1916 and 1926 the glacier retreated, which was followed by advance until 1932 when it reached a similar position as in 1916 (Wrig