ectively, around 40% of the ice sheet area is drained by 21 glaciers), of which a large proportion sits below sea level. This paper reviewed previously published work focusing on 21 major outlet glaciers in northern Greenland to provide a synthesis of changes in their dynamics between the late nineteenth century and 2015, and potential links to changes in the ocean-climate system. A clear conclusion from this analysis is that all glaciers have retreated over the last century and that this retreat has accelerated in the last two decades. Indeed, several glaciers have shown kilometer-scale retreat (>10 km) over the last two decades, in particular at Petermann, Hagen Bræ, Tracy, Zachariae Isstrøm, and C. H. Ostenfeld Glaciers. The flow velocity of a number of outlets has also accelerated during this period (e.g., Academy and Hagen Bræ). Despite an overall pattern of retreat, however, we also note variability in glacier response, which likely results from differing sensitivity to various forcings (e.g., air vs. ocean temperatures) and/or local factors, such as fjord geometry or the presence of floating ice tongues. Indeed, the impact of ice tongue retreat on glacier velocity remains uncertain because some glaciers experienced enhanced velocities (e.g., Hagen Bræ), but others showed only a limited response (Petermann and C. H. Ostenfeld). There is also some confusion surrounding the possibility of surge-type glaciers in this region, which add further complexity when attempting to elucidate the precise drivers of glacier change.
Given the above, large uncertainty surrounding glacier responses to external factors in northern Greenland remain. While several studies have focused on the major calving event at Petermann Glacier in 2010, it remains unclear whether this was exceptional or part of a long-term cyclical trend. Studies of a similar nature, comprising detailed measurements of frontal retreat and ice velocity are needed for surrounding outlet glaciers in northern Greenland to improve our understanding of the factors that are forcing recent outlet glacier retreat in the region. Future work could usefully focus on improving high resolution data, in particular fjord bathymetry and ocean temperatures, alongside assessing the role of glacier specific factors (e.g., through numerical modeling), to better understand the links between climatic-oceanic forcing and local topographic factors. Further work is also required to systematically classify surge-type glaciers in the region, and help distinguish externally-driven retreat from internally-driven surge cycles that may not be related to changes in the ocean-climate system.
EH designed and led the writing of the manuscript. RC and CS contributed ideas and provided editorial input on both the manuscript and the figures.
This project was funded by a Doctoral Studentship awarded to EH for 3.5 years by the Natural Environment Research Council and through the IAPETUS Doctoral Training Partnership (grant number NE/L002590/1).
Conflict of Interest Statement
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
EH was funded by the IAPETUS Natural Environment Research Council Doctoral Training Partnership (grant number NE/L002590/1). We acknowledge the availability of several free datasets, which we have used to produce the Figures in this review. Version 2 of the IceBridge Bed Machine dataset was acquired from the National Snow and Ice Data Center (NSIDC) produced by Morlighem et al. (2015). We also acknowledge the use of several years of MEaSUREs Greenland Ice Velocity dataset also acquired from the NSIDC and produced by Joughin et al. (2010b). Several Landsat 8 scenes were sourced from the United States Geological Survey's Earth Explorer platform. We would like to thank two reviewers and the Editor for their comments on an earlier version of the manuscript.
Abdalati, W., Krabill, W., Frederick, E., Manizade, S., Martin, C., Sonntag, J., et al. (2001). Outlet glacier an