Abstract
Coupling between runoff, hydrology, basal motion, and mass loss (“hydrology-dynamics”) is a critical component of the Greenland Ice Sheet system. Despite considerable research effort, the mechanisms by which runoff influences ice dynamics and the net long-term (decadal and longer) dynamical effect of variations in the timing and magnitude of runoff delivery to the bed remain a subject of debate. We synthesise key research into land-terminating ice sheet hydrology-dynamics, in order to reconcile several apparent contradictions that have recently arisen as understanding of the topic has developed. We suggest that meltwater interaction with subglacial channels, cavities, and deforming subglacial sediment modulates ice flow variability. Increasing surface runoff supply to the bed induces cavity expansion and sediment deformation, leading to early-melt season ice flow acceleration. In the ablation area, drainage of water at times of low runoff from high-pressure subglacial environments toward more efficient drainage pathways is thought to result in reductions in water pressure, ice-bed separation and sediment deformation, causing net slow-down on annual to decadal time-scales (ice flow self-regulation), despite increasing surface melt. Further inland, thicker ice, small surface gradients and reduced runoff suppress efficient drainage development, and a small net increase in both summer and winter ice flow is observed. Predicting ice motion across land-terminating sectors of the ice sheet over the twenty-first century is confounded by inadequate understanding of the processes and feedbacks between runoff and subglacial motion. However, if runoff supply increases, we suggest that ice flow in marginal regions will continue to decrease on annual and longer timescales, principally due to (i) increasing drainage system efficiency in marginal areas, (ii) progressive depression of basal water pressure, and (iii) thinning-induced lowering of driving stresses. At higher elevations, we suggest that minor year-on-year ice flow acceleration will continue and extend further into the interior where self-regulation mechanisms cannot operate and if surface-to-bed meltwater connections form. Based on current understanding, we expect that ice flow deceleration due to the seasonal development of efficient drainage beneath the land-terminating margins of the Greenland Ice Sheet will continue to regulate its future mass loss.
Highlights
Future variations in the dynamics and mass balance of the Greenland Ice Sheet will influence rates of sea level rise, ocean circulation and climate (Cazenave and Llovel, 2010; Rignot et al, 2011; Gillet-Chaulet et al, 2012; Bamber and Aspinall, 2013; Nick et al, 2013)
This review aims to reconcile the apparent contradictions of recent research and develop a conceptual model of the coupling between runoff, hydrology, basal motion and mass loss (“hydrology-dynamics”) of the land-terminating Greenland Ice Sheet
At times in this review we draw on this earlier body of literature to provide relevant context, we focus on the latter; the reader is directed toward Hubbard and Nienow (1997), Fountain and Walder (1998), and Irvine-Fynn et al (2011) for reviews of mountain glacier hydrology
Summary
Future variations in the dynamics and mass balance of the Greenland Ice Sheet will influence rates of sea level rise, ocean circulation and climate (Cazenave and Llovel, 2010; Rignot et al, 2011; Gillet-Chaulet et al, 2012; Bamber and Aspinall, 2013; Nick et al, 2013). Observations of the subglacial environment and constraints on crucial model parameters are sparse, leading to uncertainty in inferences from models: modelled R-channels can form under conditions representative of the upper ablation area if a large (∼1 m2) initial conduit size is used (e.g., Hewitt, 2011; Gulley et al, 2012) Such conditions may occur due to uplift during rapid lake drainage (Das et al, 2008; Pimentel and Flowers, 2010; Andrews et al, 2018) or may be facilitated if persistent surface-to-bed hydrological connections (Catania and Neumann, 2010) enable erosion of preferential flow pathways into the substrate (Gulley et al, 2012; Beaud et al, 2018). Observations suggest that ice flow during the early-melt season is typically fastest overlying deep troughs (Joughin et al, 2013) and spatial variations in ice flow relate to bed topography via its effect on the routing of surface water (Bartholomew et al, 2011b; Palmer et al, 2011), relative annual and inter-annual ice flow variability is broadly consistent over scales characterised by spatially varying bed topography (Tedstone et al, 2014, 2015). Lindbäck et al (2015) observed a negative correlation between bed roughness and ice velocity in west Greenland, but the effect of roughness on modulating the dynamic variability of the ice sheet has not yet been investigated
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