Andrew Sole

Seasonal speedup of a Greenland marine‐terminating outlet glacier forced by surface melt–induced changes in subglacial hydrology

We present subdaily ice flow measurements at four GPS sites between 36 and 72 km from the margin of a marine‐terminating Greenland outlet glacier spanning the 2009 melt season. Our data show that >35 km from the margin, seasonal and shorter–time scale ice flow variations are controlled by surface melt–induced changes in subglacial hydrology. Following the onset of melting at each site, ice motion increased above background for up to 2 months with resultant up‐glacier migration of both the onset and peak of acceleration. Later in our survey, ice flow at all sites decreased to below background. Multiple 1 to 15 day speedups increased ice motion by up to 40% above background. These events were typically accompanied by uplift and coincided with enhanced surface melt or lake drainage. Our results indicate that the subglacial drainage system evolved through the season with efficient drainage extending to at least 48 km inland during the melt season. While we can explain our observations with reference to evolution of the glacier drainage system, the net effect of the summer speed variations on annual motion is small (∼1%). This, in part, is because the speedups are compensated for by slowdowns beneath background associated with the establishment of an efficient subglacial drainage system. In addition, the speedups are less pronounced in comparison to land‐terminating systems. Our results reveal similarities between the inland ice flow response of Greenland marine‐ and land‐terminating outlet glaciers.

Enhanced basal lubrication and the contribution of the Greenland ice sheet to future sea-level rise

We assess the effect of enhanced basal sliding on the flow and mass budget of the Greenland ice sheet, using a newly developed parameterization of the relation between meltwater runoff and ice flow. A wide range of observations suggest that water generated by melt at the surface of the ice sheet reaches its bed by both fracture and drainage through moulins. Once at the bed, this water is likely to affect lubrication, although current observations are insufficient to determine whether changes in subglacial hydraulics will limit the potential for the speedup of flow. An uncertainty analysis based on our best-fit parameterization admits both possibilities: continuously increasing or bounded lubrication. We apply the parameterization to four higher-order ice-sheet models in a series of experiments forced by changes in both lubrication and surface mass budget and determine the additional mass loss brought about by lubrication in comparison with experiments forced only by changes in surface mass balance. We use forcing from a regional climate model, itself forced by output from the European Centre Hamburg Model (ECHAM5) global climate model run under scenario A1B. Although changes in lubrication generate widespread effects on the flow and form of the ice sheet, they do not affect substantial net mass loss; increase in the ice sheet’s contribution to sea-level rise from basal lubrication is projected by all models to be no more than 5% of the contribution from surface mass budget forcing alone.

Rapid erosion beneath the Greenland ice sheet

The Pleistocene ice sheets left a clear signature of erosion, but the rate at which ice sheets erode is difficult to determine from either paleolandscapes or observations of contemporary processes. Here we use two years of sediment flux data, derived from meltwaters emerging from an outlet glacier in west Greenland, to calculate an average rate of subglacial erosion across a catchment extending >50 km inland from the ice margin. Erosion in this zone occurs at 4.8 ± 2.6 mm a −1 , a rate 1–2 orders of magnitude greater than previous estimates of erosion rate beneath the Greenland Ice Sheet. Our results suggest that where surface meltwaters are able to access the bed, the rate of erosion by ice sheets is in keeping with the rapid erosion observed at temperate alpine glaciers. During deglacial phases, when meltwater was abundant, ice sheet margins should therefore have acted as highly efficient agents of erosion.

Effect of near‐terminus subglacial hydrology on tidewater glacier submarine melt rates

Submarine melting of Greenlandic tidewater glacier termini is proposed as a possible mechanism driving their recent thinning and retreat. We use a general circulation model, MITgcm, to simulate water circulation driven by subglacial discharge at the terminus of an idealized tidewater glacier. We vary the spatial distribution of subglacial discharge emerging at the grounding line of the glacier and examine the effect on submarine melt volume and distribution. We find that subglacial hydrology exerts an important control on submarine melting; under certain conditions a distributed system can induce a factor 5 more melt than a channelized system, with plumes from a single channel inducing melt over only a localized area. Subglacial hydrology also controls the spatial distribution of melt, which has the potential to control terminus morphology and calving style. Our results highlight the need to constrain near-terminus subglacial hydrology at tidewater glaciers if we are to represent ocean forcing accurately.

Greenland ice sheet motion insensitive to exceptional meltwater forcing

Significance During summer, meltwater generated on the Greenland ice sheet surface accesses the ice sheet bed, lubricating basal motion and resulting in periods of faster ice flow. However, the net impact of varying meltwater volumes upon seasonal and annual ice flow, and thus sea level rise, remains unclear. In 2012, despite record ice sheet runoff, including two extreme melt events, ice at a land-terminating margin flowed more slowly than in the average melt year of 2009, due principally to slower winter flow following faster summer flow. Our findings suggest that annual motion of land-terminating margins of the ice sheet, and thus the projected dynamic contribution of these margins to sea level rise, is insensitive to melt volumes commensurate with temperature projections for 2100. Changes to the dynamics of the Greenland ice sheet can be forced by various mechanisms including surface-melt–induced ice acceleration and oceanic forcing of marine-terminating glaciers. We use observations of ice motion to examine the surface melt–induced dynamic response of a land-terminating outlet glacier in southwest Greenland to the exceptional melting observed in 2012. During summer, meltwater generated on the Greenland ice sheet surface accesses the ice sheet bed, lubricating basal motion and resulting in periods of faster ice flow. However, the net impact of varying meltwater volumes upon seasonal and annual ice flow, and thus sea level rise, remains unclear. We show that two extreme melt events (98.6% of the Greenland ice sheet surface experienced melting on July 12, the most significant melt event since 1889, and 79.2% on July 29) and summer ice sheet runoff ∼3.9σ above the 1958–2011 mean resulted in enhanced summer ice motion relative to the average melt year of 2009. However, despite record summer melting, subsequent reduced winter ice motion resulted in 6% less net annual ice motion in 2012 than in 2009. Our findings suggest that surface melt–induced acceleration of land-terminating regions of the ice sheet will remain insignificant even under extreme melting scenarios.

Modeling the impact of glacial runoff on fjord circulation and submarine melt rate using a new subgrid‐scale parameterization for glacial plumes

The injection at depth of ice sheet runoff into fjords may be an important control on the frontal melt rate of tidewater glaciers. Here we develop a new parameterization for ice marginal plumes within the Massachusetts Institute of Technology General Circulation Model (MITgcm), allowing three-dimensional simulation of large (500 km2) glacial fjords on annual (or longer) time scales. We find that for an idealized fjord (without shelf-driven circulation), subglacial runoff produces a thin, strong, and warm down-fjord current in the upper part of the water column, balanced by a thick and slow up-fjord current at greater depth. Although submarine melt rates increase with runoff due to higher melt rates where the plume is in contact with the ice front, we find that annual submarine melt rate across the ice front is relatively insensitive to variability in annual runoff. Better knowledge of the spatial distribution of runoff, controls on melt rate in those areas not directly in contact with plumes, and feedback mechanisms linking submarine melting and iceberg calving are necessary to more fully understand the sensitivity of glacier mass balance to runoff-driven fjord circulation.

Northeast sector of the Greenland Ice Sheet to undergo the greatest inland expansion of supraglacial lakes during the 21st century

The formation and rapid drainage of supraglacial lakes (SGL) influences the mass balance and dynamics of the Greenland Ice Sheet (GrIS). Although SGLs are expected to spread inland during the 21st century due to atmospheric warming, less is known about their future spatial distribution and volume. We use GrIS surface elevation model and regional climate model outputs to show that at the end of the 21st century (2070–2099) approximately 9.8 ± 3.9 km3 (+113% compared to 1980-2009) and 12.6 ± 5 km3 (+174%) of meltwater could be stored in SGLs under moderate and high representative concentration pathways (RCP 4.5 and 8.5), respectively. The largest increase is expected in the northeastern sector of the GrIS (191% in RCP 4.5 and 320% in RCP 8.5), whereas in west Greenland, where the most SGLs are currently observed, the future increase will be relatively moderate (55% in RCP 4.5 and 68% in RCP 8.5).

Greenland ice sheet annual motion insensitive to spatial variations in subglacial hydraulic structure

We present ice velocities observed with global positioning systems and TerraSAR-X/TanDEM-X in a land-terminating region of the southwest Greenland ice sheet (GrIS) during the melt year 2012-2013, to examine the spatial pattern of seasonal and annual ice motion. We find that while spatial variability in the configuration of the subglacial drainage system controls ice motion at short timescales, this configuration has negligible impact on the spatial pattern of the proportion of annual motion which occurs during summer. While absolute annual velocities vary substantially, the proportional contribution of summer motion to annual motion does not. These observations suggest that in land-terminating margins of the GrIS, subglacial hydrology does not significantly influence spatial variations in net summer speedup. Furthermore, our findings imply that not every feature of the subglacial drainage system needs to be resolved in ice sheet models.

A model for tidewater glacier undercutting by submarine melting

Dynamic change at the marine-terminating margins of the Greenland Ice Sheet may be initiated by the ocean, particularly where subglacial runoff drives vigorous ice-marginal plumes and rapid submarine melting. Here we model submarine melt-driven undercutting of tidewater glacier termini, simulating a process which is key to understanding ice-ocean coupling. Where runoff emerges from broad subglacial channels we find that undercutting has only a weak impact on local submarine melt rate but increases total ablation by submarine melting due to the larger submerged ice surface area. Thus, the impact of melting is determined not only by the melt rate magnitude but also by the slope of the ice-ocean interface. We suggest that the most severe undercutting occurs at the maximum height in the fjord reached by the plume, likely promoting calving of ice above. It remains unclear, however, whether undercutting proceeds sufficiently rapidly to influence calving at Greenlands fastest-flowing glaciers.

Recent advances in our understanding of the role of meltwater in the Greenland ice sheet system

Purpose of the ReviewThis review discusses the role that meltwater plays within the Greenland ice sheet system. The ice sheet’s hydrology is important because it affects mass balance through its impact on meltwater runoff processes and ice dynamics. The review considers recent advances in our understanding of the storage and routing of water through the supraglacial, englacial, and subglacial components of the system and their implications for the ice sheet.Recent FindingsThere have been dramatic increases in surface meltwater generation and runoff since the early 1990s, both due to increased air temperatures and decreasing surface albedo. Processes in the subglacial drainage system have similarities to valley glaciers and in a warming climate, the efficiency of meltwater routing to the ice sheet margin is likely to increase. The behaviour of the subglacial drainage system appears to limit the impact of increased surface melt on annual rates of ice motion, in sections of the ice sheet that terminate on land, while the large volumes of meltwater routed subglacially deliver significant volumes of sediment and nutrients to downstream ecosystems.SummaryConsiderable advances have been made recently in our understanding of Greenland ice sheet hydrology and its wider influences. Nevertheless, critical gaps persist both in our understanding of hydrology-dynamics coupling, notably at tidewater glaciers, and in runoff processes which ensure that projecting Greenland’s future mass balance remains challenging.