Dynamic response of the Greenland ice sheet to recent cooling

Abstract

Gourmelen & peter nienow the subglacial hydrological system critically controls ice motion at the margins of the Greenland ice Sheet. However, over multi-annual timescales, the net impact of hydro-dynamic coupling on ice motion remains poorly understood. Here, we present annual ice velocities from 1992-2019 across a ~10,600 km 2 land-terminating area of southwest Greenland. From the early-2000s through to ~2012, we observe a slowdown in ice motion in response to increased surface melt, consistent with previous research. from 2013 to 2019 however, we observe an acceleration in ice motion coincident with atmospheric cooling and a ~15% reduction in mean surface melt production relative to 2003-2012. We find that ice velocity speed-up is greater in marginal areas, and is strongly correlated with ice thickness. We hypothesise that under thinner ice, increases in basal water pressure offset a larger proportion of the ice overburden pressure, leading to reduced effective pressure and thus greater acceleration when compared to thicker ice further inland. Our findings indicate that hydro-dynamic coupling provides the major control on changes in ice motion across the ablation zone of land terminating margins of the Greenland ice Sheet over multi-annual timescales. The Greenland Ice Sheet (GrIS) has lost mass at an accelerating rate over the past two decades, with persistent mass loss observed since 1998 1-5. Approximately 52% of this mass loss can be attributed to surface melt 6 , which increased in the late 2000s and early 2010s to levels unprecedented since at least 1900 7. Increases in surface melt have been driven by increasing air temperatures over Greenland since the mid-1980s 7,8 and variability in cloud cover, both of which are forced by larger scale circulation patterns 9-13. Increased cloud-cover warms the ice sheet interior through the trapping of longwave radiation 14,15 , whereas a reduction in summer cloud cover since 1995 has driven enhanced melt in the ablation zone through increasing the shortwave flux 10,16. Moreover, the seasonal migration of the snowline causes the exposure of dark bare-ice, decreasing the albedo of the ice surface and reducing meltwater re-freezing, further driving surface melt and runoff 17. Alongside changes in surface mass balance, roughly 48% of mass loss is due to increases in ice discharge through Greenland's marine terminating outlet glaciers 6. However, the dynamic response of the ice sheet to variability in surface mass balance and ocean conditions remains a large source of uncertainty in projecting future sea level rise 18. Land-terminating margins are isolated from processes acting at the ice/ocean boundary, and thus provide ideal study sites for investigating how the ice-sheet responds to atmospheric, and thus surface melt forcing 19,20. This is particularly prescient as the largely land-terminating margin in South West Greenland exhibits a strong and sustained negative mass balance 21,22 , and is projected to make a greater contribution to sea level rise with continued atm...

Calving at Ryder Glacier, Northern Greenland

“…Unlike Ryder, the velocities at Petermann glacier show a seasonal signal with defined minima during summer. These minima may be related to the development of a channelized subglacial hydrological system as a result of increased meltwater inputs from the surface (Nienow et al., 2017; Williams et al., 2020). A channelized basal hydrological system is more efficient, reducing water pressures at the bed of the glacier and so leading to a reduction in sliding (Hewitt, 2013).…”

A model for interaction between conduits and surrounding hydraulically connected distributed drainage based on geomorphological evidence from Keewatin, Canada

“…14a and c). During this phase, the geomorphic work in the hydraulically connected distributed drainage system is likely limited by the small cross-sectional area of passage and slow water movement (Willis et al, 1990;Alley et al, 1997). However, there could be migration of finer sediments into the central conduit contributing to gradual lateral channel growth over time; this has been invoked to explain steadystate growth of tunnel valleys for example (e.g.…”

“…Water flow through the distributed system is slow and inefficient with limited sediment mobilisation and restricted transport (e.g. Willis et al, 1990;Alley et al, 1997). Faster and more turbulent water flow within conduits is more efficient at eroding and transporting sediment, and this capability increases rapidly with increased discharge (Alley et al, 1997).…”

Supraglacial River Forcing of Subglacial Water Storage and Diurnal Ice Sheet Motion

“…On the Greenland Ice Sheet (GrIS) ablation zone, surface melting activates a perennial hydrologic system of supraglacial streams, rivers, and lakes (Irvine‐Fynn et al., 2011; Lampkin & VanderBerg, 2014; Pitcher & Smith, 2019; Rennermalm et al., 2013), which commonly drain into moulins forming a dynamic subglacial drainage system that modifies basal pressures and ice motion (e.g., Bartholomew et al., 2012; Meierbachtol et al., 2013; Van de Wal et al., ; Zwally et al., 2002). While early concerns about warming‐induced runaway sliding now seem unfounded (e.g., Flowers, 2018; Tedstone et al., 2015, 2013; van de Wal et al., 2015), physical processes linking GrIS supraglacial meltwater runoff, ice sheet basal pressures, and ice sliding remain under intense study (Davison et al., 2019; Nienow et al., 2017; Williams et al., 2020), particularly processes governing englacial connectivity and subglacial evolution due to surface melting (e.g., Christoffersen et al., 2018; Poinar et al., 2015; Stevens et al., 2015).…”