ating 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 velocity increases were often accompanied by glacier retreat (Figure 7), namely at Hagen Bræ, Academy, Marie Sophie, Tracy, and Heilprin Glaciers. Apart from Hagen Bræ, the retreat of large floating ice tongues (e.g., Petermann and C.H. Ostenfeld) did not appear to coincide with increased velocities. The impact of major ice tongue losses on ice velocities is therefore complex.
Figure 8. Velocity change between winters 2000/01 and 2008/09 using MEaSUREs v2 Greenland velocity data (Joughin et al., 2010b). Orange and red colors show velocity increase, and green and blue show velocity decrease.
Figure 7 also shows that there are several glaciers in northern Greenland for which measurements of retreat, advance, and other glacier changes have not been made, particularly at the smaller glaciers such as Henson, Naravana Fjord, Jungersen, Brikkerne, and Harder. Several other glaciers have very few measurements. Further research into these large and potentially important outlet glaciers including frontal retreat and ice velocity measurements would help to improve understanding of region wide drivers on glacier retreat.
In the following sections we discuss the potential factors which may have driven recent changes in the dynamics of outlet glaciers in northern Greenland. This begins with external forcing via increasing atmospheric and oceanic temperatures, and the impact of reductions in sea ice. Following that we consider the role of glacier-specific factors as well as the potential surge-dynamics of several of the study glaciers in this region. We also assess the future implications of mass loss in northern Greenland.
Atmospheric and Oceanic Forcing of Glacier Change in Northern Greenland
Recently observed changes at northern Greenland outlet glaciers may have been driven by changes in atmospheric and oceanic temperatures. Here we discuss these potential external controls on surface ice and submarine melting and their links to observed outlet glacier change across northern Greenland.
Subaerial Ice Melt
Atmospheric temperature