a result of both increased ice discharge and increased surface melt (van den Broeke et al., 2016). Together, these processes currently contribute ~0.6 mm per year to global sea level rise (Fürst et al., 2015). The increased ice discharge is associated with marine-terminating outlet glaciers that have undergone thinning, retreat and acceleration since the mid-1990s (Rignot and Kanagaratnam, 2006; Moon and Joughin, 2008; Joughin et al., 2010a). Their retreat between 2000 and 2010 was considered exceptional over the past half century (Howat and Eddy, 2011), and dynamic discharge from outlet glaciers was thought to be responsible for ~40% of mass loss from the ice sheet between 1991 and 2015 (van den Broeke et al., 2016). The recent rapid outlet glacier retreat and flow acceleration is understood to be in response to ocean-climate forcing (McFadden et al., 2011; Cook et al., 2014). Potential mechanisms by which atmospheric temperatures may promote retreat through glacier calving are the drainage of supraglacial lakes or water-filled crevasses fracturing through the full ice thickness (van der Veen, 2007; Das et al., 2008). Alongside this, ocean warming may increase rates of submarine melting at marine-terminating glaciers (Holland et al., 2008), which may be further enhanced by submarine meltwater plume discharge (Motyka et al., 2003; Jenkins, 2011). In addition, sea ice removal or decline may promote calving and a longer ice-free season may allow greater volumes of ice to be lost during the year (e.g., Carr et al., 2013b, 2014; Moon et al., 2015).
However, the magnitude of individual glacier responses to these forcings is known to be modulated by local topographic factors (Howat et al., 2007; Moon et al., 2012; Carr et al., 2013b). For example, fjord width is a key local control on glacier retreat, where a narrow fjord can delay the removal of icebergs from the terminus (e.g., Warren and Glasser, 1992; Jamieson et al., 2012; Carr et al., 2014). Another important glacier-specific factor is basal topography, whereby a reverse inland bed slope can make a glacier vulnerable to feedbacks between rapid thinning, acceleration and retreat (e.g., Thomas et al., 2009). The relative contribution of external factors (oceanic and climatic) vs. localized glacier-specific factors (most notably fjord geometry and basal topography) remains poorly understood and identifying their respective influence on outlet glacier retreat is of paramount importance for estimating future glacier response to climate change and sea level rise (Nick et al., 2013; Porter et al., 2014; Carr et al., 2015).
Over the last two decades, several areas of the ice sheet have been the focus of regional to local scale studies of glacier change, particularly Jakobshavn Isbræ in west Greenland (Joughin et al., 2008b, 2012; Podrasky et al., 2012), and Kangerdlugssuaq and Helheim Glaciers in south east Greenland (Howat et al., 2007; Joughin et al., 2008a). However, with the possible exception of Petermann Glacier and the northeast Greenland Ice Stream (NEGIS), major outlet glaciers in northern Greenland have received much less attention. This is despite several studies documenting large calving events from northern Greenland ice tongues during the past decade (Moon and Joughin, 2008; Johannessen et al., 2013; Murray et al., 2015), the most notable of which was the 270 km2 retreat of Petermann Glacier's floating tongue in 2010 (Nick et al., 2012; Johannessen et al., 2013). Thus, there is a paucity of data from northern Greenland, compared to other areas. It remains unclear how these glaciers are responding to climate change compared to other areas of Greenland and in the context of their longer-term behavior over the last 100 years. There is also uncertainty about the forcing of glacier retreat (atmospheric vs. oceanic warming) and how these glaciers might respond to future changes at their terminus (such as ice tongue losses) and whether these changes have the potential to trigger substantial inland ice loss and glacier acceleration, similar to that experienced at Jakobshav