Peter Nienow

Melt-induced speed-up of Greenland ice sheet offset by efficient subglacial drainage

Fluctuations in surface melting are known to affect the speed of glaciers and ice sheets, but their impact on the Greenland ice sheet in a warming climate remains uncertain. Although some studies suggest that greater melting produces greater ice-sheet acceleration, others have identified a long-term decrease in Greenland’s flow despite increased melting. Here we use satellite observations of ice motion recorded in a land-terminating sector of southwest Greenland to investigate the manner in which ice flow develops during years of markedly different melting. Although peak rates of ice speed-up are positively correlated with the degree of melting, mean summer flow rates are not, because glacier slowdown occurs, on average, when a critical run-off threshold of about 1.4 centimetres a day is exceeded. In contrast to the first half of summer, when flow is similar in all years, speed-up during the latter half is 62 ± 16 per cent less in warmer years. Consequently, in warmer years, the period of fast ice flow is three times shorter and, overall, summer ice flow is slower. This behaviour is at odds with that expected from basal lubrication alone. Instead, it mirrors that of mountain glaciers, where melt-induced acceleration of flow ceases during years of high melting once subglacial drainage becomes efficient. A model of ice-sheet flow that captures switching between cavity and channel drainage modes is consistent with the run-off threshold, fast-flow periods, and later-summer speeds we have observed. Simulations of the Greenland ice-sheet flow under climate warming scenarios should account for the dynamic evolution of subglacial drainage; a simple model of basal lubrication alone misses key aspects of the ice sheet’s response to climate warming.

Seasonal changes in the morphology of the subglacial drainage system, Haut Glacier d'Arolla, Switzerland

A spreader for feeding and uniformly distributing grain in a bin. The spreader has a hopper to receive the incoming grain. A screw mounted in the hopper evenly spreads the grain and propels it into a chute whose discharge mouth is normally closed by a spring pressed gate, or valve. A rotary thrower blade, mounted below the chute, aids the chute to distribute the grain generally radially and downwardly uniformly in the bin. The screw, the discharge chute and the thrower are driven as a unit by a single shaft, mounted coaxially in the hopper, and rotated by a motor mounted externally of the hopper. The pressure of grain movement propelled by the screw opens the chute gate allowing grain to pass to the spreader.

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.

Ice sheets as a significant source of highly reactive nanoparticulate iron to the oceans

The Greenland and Antarctic Ice Sheets cover ~\n10% of global land surface, but are rarely considered as active components of the global iron cycle. The ocean waters around both ice sheets harbour highly productive coastal ecosystems, many of which are iron limited. Measurements of iron concentrations in subglacial runoff from a large Greenland Ice Sheet catchment reveal the potential for globally significant export of labile iron fractions to the near-coastal euphotic zone. We estimate that the flux of bioavailable iron associated with glacial runoff is 0.40–2.54 Tg per year in Greenland and 0.06–0.17 Tg per year in Antarctica. Iron fluxes are dominated by a highly reactive and potentially bioavailable nanoparticulate suspended sediment fraction, similar to that identified in Antarctic icebergs. Estimates of labile iron fluxes in meltwater are comparable with aeolian dust fluxes to the oceans surrounding Greenland and Antarctica, and are similarly expected to increase in a warming climate with enhanced melting.

AN INTEGRATED APPROACH TO MODELLING HYDROLOGY AND WATER QUALITY IN GLACIERIZED CATCHMENTS

The results are summarized of an integrated investigation of glacier geometry, ablation patterns, water balance, meltwater routing, hydrochemistry and suspended sediment yield. The ultimate objective is to evaluate the assumptions of lumped, two-component mixing models as descriptors of glacier hydrology, and to develop a semi-distributed physically based model as an alternative. The results of the study demonstrate that a reconstruction of probable subglacial drainage alignments can be achieved through a combination of terrain modelling based on estimated potential surface and dye tracing experiments. Recession curve analysis, evidence of the seasonal instability of the englacial and subglacial electrical conductivities assumed in a mixing model, evidence of the non-conservative behaviour of water chemistry in the presence of suspended sediment, and evidence of the seasonal evolution of the subglacial drainage system based on dye tracing all indicate that an alternative to a lumped, static model of the hydrology is necessary. The alternative presented in this paper is based on the combination of an energy balance model for surface melt which operates on an hourly time step and accounts for the changing spatial distribution of melt through the day as shading patterns change, and routing procedures that transfer surface melt to moulins on the basis of glacier surface gradients, then route water through reconstructed conduit systems using a hydraulic sewer-flow routing procedure.

Hydrological controls on patterns of surface, internal and basal motion during three "spring events" : Haut Glacier d'Arolla, Switzerland

Three early-melt-season high-velocity events (or “spring events”) occurred on Haut Glacier d’Arolla, Switzerland, during the melt seasons of 1998 and 1999. The events involve enhanced glacier velocity during periods of rapidly increasing bulk discharge in the proglacial stream and high subglacial water pressures. However, differences in spatial patterns of surface velocity, internal ice deformation rates, the spatial extent of high subglacial water pressures and in rates of subglacial sediment deformation suggest different hydrological and mechanical controls. The data from two of the events suggest widespread ice–bed decoupling, particularly along a subglacial drainage axis creating the highest rates of basal motion and “plug flow” in the overlying ice. The other event showed evidence of less extensive ice–bed decoupling and sliding along the drainage axis with more mechanical support for ice overburden transferred to areas adjacent to decoupled areas. We suggest that: (1) plug flow may be a common feature on glaciers experiencing locally induced reductions in basal drag; (2) under certain circumstances, enhanced surface motion may be due in part to non-locally forced enhanced bed deformation; and (3) subglacial sediment deformation is confined to a depth of the order of centimetres to decimetres.

Spatial patterns of glacier motion during a high-velocity event : Haut Glacier d'Arolla, Switzerland

The surface motion of Haut Glacier d’Arolla, Switzerland, was monitored at a high spatial and temporal resolution. Data are analyzed to calculate surface velocities, surface strain rates and the components of the glacier force budget before, during and after an early melt season speed-up or “spring event”. We investigate the extent to which variations in glacier motion can be attributed to hydrologically induced local forcing or to non-local forcing transmitted via horizontal stress gradients. Enhanced glacier motion is dependent on a change in the spatial distribution of areas of high drag across the glacier.

Links between short-term velocity variations and the subglacial hydrology of a predominantly cold polythermal glacier

The surface velocity of a predominantly cold polythermal glacier (John Evans Glacier, Ellesmere Island, Canada) varies significantly on both seasonal and shorter time-scales. Seasonal variations reflect the penetration of supraglacial water to the glacier bed through significant thicknesses of cold ice. Shorter-term events are associated with periods of rapidly increasing water inputs to the subglacial drainage system. Earlyseason short-term events immediately follow the establishment of a drainage connection between glacier surface and glacier bed, and coincide with the onset of subglacial outflow at the terminus. A mid-season short-term event occurred as surface melting resumed following cold weather, and may have been facilitated by partial closure of subglacial channels during this cold period. There is a close association between the timing and spatial distribution of horizontal and vertical velocity anomalies, the temporal pattern of surface water input to the glacier, and the formation, seasonal evolution and distribution of subglacial drainage pathways. These factors presumably control the occurrence of highwater-pressure events and water storage at the glacier bed. The observed coupling between surface water inputs and glacier velocity may allow predominantly cold polythermal glaciers to respond rapidly to climate-induced changes in surface melting.

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.

Decadal slowdown of a land-terminating sector of the Greenland Ice Sheet despite warming

Ice flow along land-terminating margins of the Greenland Ice Sheet (GIS) varies considerably in response to fluctuating inputs of surface meltwater to the bed of the ice sheet. Such inputs lubricate the ice–bed interface, transiently speeding up the flow of ice. Greater melting results in faster ice motion during summer, but slower motion over the subsequent winter, owing to the evolution of an efficient drainage system that enables water to drain from regions of the ice-sheet bed that have a high basal water pressure. However, the impact of hydrodynamic coupling on ice motion over decadal timescales remains poorly constrained. Here we show that annual ice motion across an 8,000-km2 land-terminating region of the west GIS margin, extending to 1,100 m above sea level, was 12% slower in 2007–14 compared with 1985–94, despite a 50% increase in surface meltwater production. Our findings suggest that, over these three decades, hydrodynamic coupling in this section of the ablation zone resulted in a net slowdown of ice motion (not a speed-up, as previously postulated). Increases in meltwater production from projected climate warming may therefore further reduce the motion of land-terminating margins of the GIS. Our findings suggest that these sectors of the ice sheet are more resilient to the dynamic impacts of enhanced meltwater production than previously thought.