rømmen and is ~11 km wide with a catchment of ~20,000 km2 (Table 1). Within this embayment, both Storstrømmen and L. Bistrup Bræ have floating ice sections (Figure 2A) that are 8.4 and 6.2 km long, respectively, whereas data suggests Kofoed-Hansen Bræ is grounded (Morlighem et al., 2015).
Relatively little is known about frontal position changes at these glaciers. Available data show that all three glaciers retreated between 2001 and 2005 (Seale et al. (2011). In total, this resulted in retreat of 0.3 km at Storstrømmen, and 0.76 km at both Kofoed-Hansen Bræ and L. Bistrup Bræ (Seale et al., 2011). However, during this period Storstrømmen underwent a short term advance of 0.4 km2 in 2001/02 (Box and Decker, 2011). Later, between 2005 and 2008, only Storstrømmen glacier continued to retreat, while L. Bistrup Bræ and Kofoed-Hansen Bræ both advanced by 0.29 km (Seale et al., 2011).
The velocity of Storstrømmen was thought to be ~230 m a−1 in 1996 (Rignot et al., 2001). L. Bistrup Bræ is considered slow moving, although specific values were not documented in the literature (Rignot et al., 2001), with almost zero velocity between 2000/01 and 2009/10 at the terminus (Joughin et al., 2010a). However, further inland the glacier flows at ~100 m a−1 in 2008/09 (Figure 6). During this same period (2000/01–2009/10) Storstrømmen Glacier decelerated by 60 m a−2 (Joughin et al., 2010a).
Evidence of surging exists for all three glaciers. A surge was documented at Storstrømmen between 1978 and 1984 (Reeh et al., 1994), when the glacier advanced ~12 km. By the 1990s it was considered to be quiescent (Mohr et al., 1998), and has remained so since that time (Reeh et al., 2003; Rignot and Kanagaratnam, 2006). L. Bistrup Bræ is also likely to be a surge-type glacier, based on thickening at the grounding line, although there is no direct evidence of an actual surge event. The glacier is instead hypothesized to have surged in the past and now be quiescent (Rignot et al., 2001). Both L. Bistrup Bræ and Storstrømmen have experienced very low flow speeds over the past decade, potentially indicative of quiescence (Moon et al., 2012), as well as a characteristic pattern of thickening inland, where the surge may have initiated, and thinning toward the terminus (Abdalati et al., 2001; Csatho et al., 2014). Surveys between 1995 and 1999 found thinning rates of 2 m a−1 in the lower reaches of both glaciers and thickening up to 3 m a−1 further inland (Thomas et al. (2009). Following this, however, thickening rates on the upper reaches of Storstrømmen began to decrease (Thomas et al., 2009).
Despite these three outlet glaciers collectively draining an area (27%) of the GrIS, which is far greater than the well-studied Petermann Glacier (Table 1), little previous work has focused on their dynamics. Considerable variability in terminus positions have taken place over the last two decades, particularly at Storstrømmen and L. Bistrup Bræ, which has been linked to surge activity. These glaciers are important in terms of draining a large proportion of the NEGIS and it is necessary for further work to better understand their surge-nature and their implications for increased ice discharge from potentially unstable regions.
Summary of Northern Greenland Outlet Glacier Changes
Overall, substantial changes have taken place in northern Greenland, particularly during the past two decades, and there has been considerable variability between sub-regions and individual glaciers. Figure 7 summarizes the events recorded from the literature at the selected outlet glaciers in northern Greenland between 1880 and 2015. For all glaciers where records exist (17 out of 21 in Figure 7), retreat has occurred at some stage between 2000 and 2015, with the most substantial at Humboldt, Tracy, Hagen Bræ, C. H. Ostenfeld, and Petermann Glaciers. In the case of the latter three glaciers, this retreat resulted in substantial loss of their floating ice tongues. For example, Petermann Glacier lost 27 km of its floating ice tongue in 2010 (Falkner et al., 2011), and Hagen Bræ lost a 15 km flo