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 floating section in 2008/09 (Murray et al., 2015). However, glacier retreat is not uniform across the region, and several glaciers underwent advance between 2000 and 2010 (e.g., L. Bistrup Bræ, Kofoed-Hansen Bræ, and Harald Moltke Bræ). This could either reflect surge behavior (which has been hypothesized at all three of these glaciers) or a differing response to external environmental forcing, perhaps due to local topographic controls. Nevertheless, over the last 20 years, all glaciers have experienced some retreat, which would suggest a common response to external forcing.
Figure 7. A summary of recorded changes at northern Greenland outlet glaciers based on a review of the literature. Key events of terminus change (circles, red for retreat, blue for advance), velocity change (triangle), grounding line retreat (cross), and glacier thinning (square), that have occurred at all northern Greenland focus glaciers between 1880 and present (2015) are recorded. Data points are shown in the middle of study periods and the gray lines show the duration over which the change refers to. All data points are converted to m a−1 across the study period and the size of all data points are based on m a−1 magnitude. The legend shows the symbol sizes and their corresponding values for each category of data shown.
Alongside terminus changes, our review has shown that several of the study glaciers have accelerated, particularly over the last two decades (Figure 7). However, this has not been the case at all study glaciers. This is evident in Figure 8, which uses MEaSUREs velocity data (Joughin et al., 2010b) from 2000/01 and 2008/09 to show changes in velocity between these two periods across northern Greenland. These two datasets are winter velocities, between 3 September and 24 January 2000–2001 and 1 December and 28 February 2008–2009. Instead, velocity change is highly variable, with several glaciers accelerating substantially (e.g., Hagen Bræ and Academy Glacier) and others slowing (e.g., Petermann and Ryder Glaciers'). Recent velo