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Surface melting on the Antarctic Ice Sheet has been monitored by satellite microwave radiometry for over 40 years. Despite this long perspective, our understanding of the microwave emission from wet snow is still limited, preventing the full exploitation of these observations to study supraglacial hydrology. Using the Snow Microwave Radiative Transfer (SMRT) model, this study investigates the sensitivity of microwave brightness temperature to snow liquid water content at frequencies from 1.4 to 37 GHz. We first determine the snowpack properties for eight selected coastal sites by retrieving profiles of density, grain size and ice layers from microwave observations when the snowpack is dry during wintertime. Second, a series of brightness temperature simulations is run with added water. The results show that (i) a small quantity of liquid water (≈0.5 kg m−2) can be detected, but the actual quantity cannot be retrieved out of the full range of possible water quantities; (ii) the detection of a buried wet layer is possible up to a maximum depth of 1 to 6 m depending on the frequency (6–37 GHz) and on the snow properties (grain size, density) at each site; (iii) surface ponds and water-saturated areas may prevent melt detection, but the current coverage of these waterbodies in the large satellite field of view is presently too small in Antarctica to have noticeable effects; and (iv) at 1.4 GHz, while the simulations are less reliable, we found a weaker sensitivity to liquid water and the maximal depth of detection is relatively shallow ( m) compared to the typical radiation penetration depth in dry firn (≈1000 m) at this low frequency. These numerical results pave the way for the development of improved multi-frequency algorithms to detect melt intensity and the depth of liquid water below the surface in the Antarctic snowpack.

The Greenland Ice Sheet (GrIS) is losing mass as the climate warms through both increased meltwater runoff and ice discharge at marine terminating sectors. At the ice sheet surface, meltwater runoff forms a dynamic supraglacial hydrological system which includes stream/river networks and large supraglacial lakes (SGLs). Streams/rivers can route water into crevasses, or into supraglacial lakes with crevasses underneath, both of which can then hydrofracture to the ice sheet base, providing a mechanism for the surface meltwater to access the bed. Understanding where, when and how much meltwater is transferred to the bed is important because variability in meltwater supply to the bed can increase ice flow speeds, potentially impacting the hypsometry of the ice sheet in grounded sectors, and iceberg discharge to the ocean. Here we present a new, physically-based, supraglacial hydrology model for the GrIS that is able to simulate a) surface meltwater routing and SGL filling, b) rapid meltwater drainage to the ice-sheet bed via the hydrofracture of surface crevasses both in, and outside of, SGLs, c) slow SGL drainage via overflow in supraglacial meltwater channels and, by offline coupling with a second model, d) the freezing and unfreezing of SGLs from autumn to spring. We call the model Supraglacial Hydrology Evolution and Drainage (or SHED). We apply the model to three study regions in South West Greenland between 2015 and 2019 inclusive and evaluate its performance with respect to observed supraglacial lake extents, and proglacial discharge measurements. We show that the model reproduces 80 % of observed lake locations, and provides good agreement with observations in terms of the temporal evolution of lake extent. Modelled moulin density values are in keeping with those previously published and seasonal and inter-annual variability in proglacial discharge agrees well with that observed, though the observations lag the model by a few days since they include transit time through the subglacial system and the model does not. Our simulations suggest that lake drainage behaviours may be more complex than traditional models suggest, with lakes in our model draining through a combination of both overflow and hydrofracture, and some lakes draining only partially and then refreezing. This suggests that in order to simulate the evolution of Greenland’s surface hydrological system with fidelity, then a model that includes all of these processes needs to be used. In future work we will couple our model to a subglacial model and an ice flow model, and thus use our estimates of where, when and how much meltwater gets to the bed to understand the consequences for ice flow.

Surface meltwater is becoming increasingly widespread on Antarctic ice shelves. It is stored within surface ponds and streams, or within firn pore spaces, which may saturate to form slush. Slush can reduce firn air content, increasing an ice-shelf's vulnerability to break-up. To date, no study has mapped the changing extent of slush across ice shelves. Here, we use Google Earth Engine and Landsat 8 images from six ice shelves to generate training classes using a k-means clustering algorithm, which are used to train a random forest classifier to identify both slush and ponded water. Validation using expert elicitation gives accuracies of 84% and 82% for the ponded water and slush classes, respectively. Errors result from subjectivity in identifying the ponded water/slush boundary, and from inclusion of cloud and shadows. We apply our classifier to the Roi Baudouin Ice Shelf for the entire 2013–20 Landsat 8 record. On average, 64% of all surface meltwater is classified as slush and 36% as ponded water. Total meltwater areal extent is greatest between late January and mid-February. This highlights the importance of mapping slush when studying surface meltwater on ice shelves. Future research will apply the classifier across all Antarctic ice shelves.

Increasing surface melt has been implicated in the collapse of several Antarctic ice shelves over the last few decades, including the collapse of Larsen B Ice Shelf over a period of just a few weeks in 2002. The speed at which an ice shelf disintegrates strongly determines the subsequent loss of grounded ice and sea level rise, but the controls on collapse speed are not well understood. Here we show, using a novel cellular automaton model, that there is an intrinsic speed limit on ice shelf collapse through cascades of interacting melt pond hydrofracture events. Though collapse speed increases with the area of hydrofracture influence, the typical flexural length scales of Antarctic ice shelves ensure that hydrofracture interactions remain localized. We argue that the speed at which Larsen B Ice Shelf collapsed was caused by a season of anomalously high surface meltwater production.

Supraglacial melt is observed across the majority of Antarctic ice shelves and is expected to increase in line with rising air temperatures. Surface meltwater may run off the ice shelf edge and into the ocean, or be stored within firn pore spaces (slush) and supraglacial water bodies (ponds, lakes or streams). When stored either as slush or supraglacial water bodies, the water can indirectly impact ice shelf dynamics, and potentially facilitate ice shelf collapse. Numerous studies have quantified ice shelf meltwater in supraglacial water bodies, however, despite its importance, no studies exist that quantify the extent of slush on a pan-Antarctic scale.Here, we develop a supervised classifier in Google Earth Engine capable of identifying both slush and ponded water on a pan-Antarctic scale using Landsat 8 imagery. We train and test our classifier on six ice shelves: (1) Nivlisen, (2) Roi Baudouin, (3) Amery, (4) Shackleton, (5) Nansen, (6) George VI. A k-means clustering algorithm is applied to selected Landsat 8 training scenes, and the output clusters are manually interpreted to form training classes (i.e. slush, water, and other surface types (e.g. blue ice, dirty ice)). These training classes are then used to train a Random Forest Classifier, and the accuracy of the outputs are assessed using expert elicitation. Overall, the classifier accuracy for water and slush is 78 % and 70 % respectively. The validated classifier is then applied to numerous ice shelves across Antarctica, in order to produce estimates of slush and water extent from 2013 to the present day.

The Greenland Ice Sheet (GrIS) rapid mass loss is primarily driven by an increase in meltwater runoff, which highlights the importance of understanding the formation, evolution and impact of meltwater features on the ice sheet. Buried lakes are meltwater features that contain liquid water and exist under layers of snow, firn, and/or ice. These lakes are invisible in optical imagery, challenging the analysis of their evolution and implication for larger GrIS dynamics and mass change. Here, we present a method that uses a convolutional neural network, a deep learning method, to automatically detect buried lakes across the GrIS. For the years 2018 and 2019, we compare total areal extent of both buried and surface lakes across six regions, and use a regional climate model to explain the spatial and temporal differences. We find that the total buried lake extent after the 2019 melt season is 56 % larger than after the 2018 melt season across the entire ice sheet. Northern Greenland observes the largest increase in buried lake extent after the 2019 melt season, which we attribute to late-summer surface melt and high autumn temperatures. We also provide evidence that different processes are responsible for buried lake formation in different regions of the ice sheet. For example, in western Greenland, buried lakes often appear on the surface during the previous melt season, indicating that these features form when surface lakes partially freeze and become insulated as snowfall buries them. In contrast, in southeast Greenland, most buried lakes never appear on the surface, signifying that these features may form due to subsurface penetration of shortwave radiation and/or downward percolation of meltwater. This study helps to provide additional perspective on the potential role of meltwater on GrIS dynamics and mass loss.

In the 2019/2020 austral summer, the surface melt duration and extent on the northern George VI Ice Shelf (GVIIS) was exceptional compared to the 31 previous summers of dramatically lower melt. This finding is based on analysis of near-continuous 41-year satellite microwave radiometer (and scatterometer) data, which are sensitive to meltwater on the ice-shelf surface and in the near-surface snow. Using optical satellite imagery from Landsat 8 (since 2013) and Sentinel-2 (since 2017), record volumes of surface meltwater ponding are also observed on north GVIIS in 2019/2020, with 23 % of the surface area covered by 0.62 km3 of meltwater on January 19. These exceptional melt and surface ponding conditions in 2019/2020 were driven by sustained air temperatures ≥ 0 °C for anomalously long periods (55–90 hours) from late November onwards, likely driven by warmer northwesterly and northeasterly low-speed winds. Increased surface ponding on ice shelves may threaten their stability through increased potential for hydrofracture initiation; a risk that may increase due to firn air content depletion in response to near-surface melting.

Between 1992 and 2017, the Antarctic Ice Sheet (AIS) lost ice equivalent to 7.6 ± 3.9 mm of sea level rise. AIS mass loss is mitigated by ice shelves that provide a buttress by regulating ice flow from tributary glaciers. However, ice‐shelf stability is threatened by meltwater ponding, which may initiate, or reactivate preexisting, fractures, currently poorly understood processes. Here, through ground penetrating radar (GPR) analysis over a buried lake in the grounding zone of an East Antarctic ice shelf, we present the first field observations of a lake drainage event in Antarctica via vertical fractures. Concurrent with the lake drainage event, we observe a decrease in surface elevation and an increase in Sentinel‐1 backscatter. Finally, we suggest that fractures that are initiated or reactivated by lake drainage events in a grounding zone will propagate with ice flow onto the ice shelf itself, where they may have implications for its stability., Key Points We present the first field observations of the rapid drainage of a buried lake via hydrofracture in AntarcticaLake‐bed fractures are detected from ground penetrating radar prior to drainage and suggest the lake may have been preconditioned to drainRemote sensing analysis of digital elevation model and Sentinel‐1 microwave image differences provide additional lake evolution information

Surface meltwater on ice shelves can exist as slush, it can pond in lakes or crevasses, or it can flow in surface streams and rivers. The collapse of the Larsen B Ice Shelf in 2002 has been attributed to the sudden drainage of ∼3000 surface lakes and has highlighted the potential for surface water to cause ice-shelf instability. Surface meltwater systems have been identified across numerous Antarctic ice shelves, although the extent to which these systems impact ice-shelf instability is poorly constrained. To better understand the role of surface meltwater systems on ice shelves, it is important to track their seasonal development, monitoring the fluctuations in surface water volume and the transfer of water across ice-shelf surfaces. Here, we use Landsat 8 and Sentinel-2 imagery to track surface meltwater across the Nivlisen Ice Shelf in the 2016–2017 melt season. We develop the Fully Automated Supraglacial-Water Tracking algorithm for Ice Shelves (FASTISh) and use it to identify and track the development of 1598 water bodies, which we classify as either circular or linear. The total volume of surface meltwater peaks on 26 January 2017 at 5.5×107 m3. At this time, 63 % of the total volume is held within two linear surface meltwater systems, which are up to 27 km long, are orientated along the ice shelf's north–south axis, and follow the surface slope. Over the course of the melt season, they appear to migrate away from the grounding line, while growing in size and enveloping smaller water bodies. This suggests there is large-scale lateral water transfer through the surface meltwater system and the firn pack towards the ice-shelf front during the summer.

There is growing interest in surface and shallow subsurface water bodies across Antarctic ice shelves as they impact the ice shelf mass balance. Additionally, the filling and draining of lakes has the potential to flex and fracture ice shelves, which may even lead to their catastrophic break up. The study of lakes on ice shelf surfaces typically uses optical satellite imagery to delineate their area and a parameterised physically-based light attenuation theory to calculate their depths. The approach has been developed and validated using various data sets collected on the Greenland Ice Sheet, but so far the approach has not been validated for Antarctic ice shelves. Here we use simultaneous field measurements of lake water depth and surface spectral properties (red, blue, green, panchromatic), to parameterise the light attenuation theory for use during the filling and draining of shallow lakes on the McMurdo Ice Shelf during the 2016/2017 austral summer. We then apply the approach to calculate lake areas, depths and volumes across several water bodies observed in high resolution Worldview imagery, which helps validate the approach to calculating water volumes across a larger part of the ice shelf using Landsat 8 imagery. Results suggest that using parameter values relevant to the Greenland Ice Sheet may bias the calculation of water volumes when applied to Antarctic ice shelves, and we suggest more appropriate values.

The evolution of surface and shallow subsurface meltwater bodies across Antarctic ice shelves has important implicationsfor their (in)stability, as demonstrated by the 2002 rapid collapse of the Larsen B Ice Shelf. Ice-shelf break-up may be triggered by stress variations associated with meltwater movement, ponding and drainage, causing ice-shelf flexure and fracture. We have recently begun a four year, jointly-funded US-NSF / UK-NERC project that will provide important geophysical insights into the stability of the George VI Ice Shelf on the Antarctic Peninsula, where hundreds of surface lakes form each summer.In November 2019, we deployed global positioning systems, pressure transducers, automatic weather stations, and in-ice thermistor strings to record ice-shelf flexure, surface water depths, and surface and subsurface melting, respectively, in and around several surface lakes. Next austral summer (November 2020), we also plan to record fracture seismicity with a passive seismometer deployment, and to conduct ground penetrating radar surveys to detect subsurface water. Instruments, which are all within ~30 km of BAS's Fossil Bluff Station, will remain on the ice shelf until January 2022, resulting in a 27-month observational record in total.Here, we report results of satellite image analysis of surface and shallow subsurface meltwater bodies, together with preliminary field and modelling results associated with our project. Using NDWIice thresholds applied to Landsat 8, Sentinel-2 and WorldView optical imagery, we show how patterns of surface meltwater evolve within and between summer melt seasons. Using Sentinel-1 SAR imagery and a convolutional neural network technique, we detect and track bodies of shallow subsurface water and show how they relate to patterns of surface water. We also report on field reconnaissance surveys made to two dolines (drained lake basins) on the ice shelf, and present a simple model to describe the process of doline formation. Throughout the project, we will combine field and remotely sensed data to extend and validate our existing approach to modelling ice-shelf flexure and stress, and possible ‘Larsen-B style’ ice-shelf instability and break-up at less geographically confined ice shelves.

Widespread surface meltwater systems have been identified across numerous Antarctic ice shelves and have been implicated in their possible instability and eventual breakup. It is crucial to better understand the seasonal and year-to-year development of these surface meltwater systems, which comprise saturated firn (slush) as well as distinct water bodies (lakes and streams). It has been suggested that repeated melting and re-freezing of the surface firn pack over successive years reduces the firn air content, and therefore its porosity, encouraging the formation of surface water bodies over time. Firn air depletion and the formation of surface water bodies may contribute to ice shelf instability, as the ice becomes increasingly susceptible to hydrofracture.Here, we use Google Earth Engine to investigate the distributions of slush and deeper water bodies across all Antarctic ice shelves known to have surface melt, to quantify how surface meltwater systems evolve both seasonally and over successive summers. To do this, we use supervised classification of Sentinel-2 and Landsat 7/8 imagery to guide the selection of suitable NDWIice thresholds for both the detection of slush and deep surface meltwater. Preliminary results for the George VI Ice Shelf between 2000 and 2017 reveal seasonal patterns in the overall extent of surface meltwater, and the overall meltwater extent typically peaks between January and March each year. The 2009-2010 melt season was characterised by significant melt, and over the course of the melt season the proportion of the overall surface meltwater extent that was held within deep water bodies varied between 0 % (November) and 60 % (January). An increase in the proportion of deep water vs. slush typically aligns with warmer air surface temperatures and, therefore periods of more intense melt.

Melting is pervasive along the ice surrounding Antarctica. On the surface of the grounded ice sheet and floating ice shelves, extensive networks of lakes, streams and rivers both store and transport water. As melting increases with a warming climate, the surface hydrology of Antarctica in some regions could resemble Greenland’s present-day ablation and percolation zones. Drawing on observations of widespread surface water in Antarctica and decades of study in Greenland, we consider three modes by which meltwater could impact Antarctic mass balance: increased runoff, meltwater injection to the bed and meltwater-induced ice-shelf fracture — all of which may contribute to future ice-sheet mass loss from Antarctica. With warming, meltwater will play an increasingly important role in driving ice loss from Antarctica, raising global sea levels. This Perspective discusses the key process through which Antarctic surface hydrology impacts mass balance.

We present a newly developed 1-D numerical energy-balance and phase transition supraglacial lake model: GlacierLake. GlacierLake incorporates snowfall, in situ snow and ice melt, incoming water from the surrounding catchment, ice lid formation, basal freeze-up and thermal stratification. Snow cover and temperature are varied to test lake development through winter and the maximum lid thickness is recorded. Average wintertime temperatures of −2 to$-30^{\circ }{\rm C}$and total snowfall of 0 to 3.45 m lead to a range of the maximum lid thickness from 1.2 to 2.8 m after${\sim }250$days, with snow cover exerting the dominant control. An initial ice temperature of$-15^{\circ }{\rm C}$with simulated advection of cold ice from upstream results in 0.6 m of basal freeze-up. This suggests that lakes with water depths above 1.3 to 3.4 m (dependent on winter snowfall and temperature) upon lid formation will persist through winter. These buried lakes can provide a sizeable water store at the start of the melt season, expedite future lake formation and warm underlying ice even in winter.

Warming of the Antarctic Peninsula in the latter half of the twentieth century was greater than any other terrestrial environment in the Southern Hemisphere, and clear cryospheric and biological consequences have been observed. Under a global 1.5◦C scenario, warming in the Antarctic Peninsula is likely to increase the number of days above 0◦C, with up to 130 of such days each year in the northern Peninsula. Ocean turbulence will increase, making the circumpolar deep water (CDW) both warmer and shallower, delivering heat to the sea surface and to coastal margins. Thinning and recession of marine margins of glaciers and ice caps is expected to accelerate to terrestrial limits, increasing iceberg production, after which glacier retreat may slow on land. Ice shelves will experience continued increase in meltwater production and consequent structural change, but not imminent regional collapses. Marine biota can respond in multiple ways to climatic changes, with effects complicated by past resource extraction activities. Southward distribution shifts have been observed in multiple taxa during the last century and these are likely to continue. Exposed (ice free) terrestrial areas will expand, providing new habitats for native and non-native organisms, but with a potential loss of genetic diversity. While native terrestrial biota are likely to benefit from modest warming, the greatest threat to native biodiversity is from non-native terrestrial species.

Surface debris covers much of the western portion of the McMurdo Ice Shelf and has a strong influence on the local surface albedo and energy balance. Differential ablation between debris-covered and debris-free areas creates an unusual heterogeneous surface of topographically low, high-ablation, and topographically raised (‘pedestalled’), low-ablation areas. Analysis of Landsat and MODIS satellite imagery from 1999 to 2018, alongside field observations from the 2016/2017 austral summer, shows that pedestalled relict lakes (‘pedestals’) form when an active surface meltwater lake that develops in the summer, freezes-over in winter, resulting in the lake-bottom debris being masked by a high-albedo, superimposed, ice surface. If this ice surface fails to melt during a subsequent melt season, it experiences reduced surface ablation relative to the surrounding debris-covered areas of the ice shelf. We propose that this differential ablation, and resultant hydrostatic and flexural readjustments of the ice shelf, causes the former supraglacial lake surface to become increasingly pedestalled above the lower topography of the surrounding ice shelf. Consequently, meltwater streams cannot flow onto these pedestalled features, and instead divert around them. We suggest that the development of pedestals has a significant influence on the surface-energy balance, hydrology and flexure of the ice shelf.

Remote sensing is commonly used to monitor supraglacial lakes on the Greenland Ice Sheet (GrIS); however, most satellite records must trade off higher spatial resolution for higher temporal resolution (e.g. MODIS) or vice versa (e.g. Landsat). Here, we overcome this issue by developing and applying a dual-sensor method that can monitor changes to lake areas and volumes at high spatial resolution (10–30 m) with a frequent revisit time (∼3 days). We achieve this by mosaicking imagery from the Landsat 8 Operational Land Imager (OLI) with imagery from the recently launched Sentinel-2 Multispectral Instrument (MSI) for a ∼12 000 km2 area of West Greenland in the 2016 melt season. First, we validate a physically based method for calculating lake depths with Sentinel-2 by comparing measurements against those derived from the available contemporaneous Landsat 8 imagery; we find close correspondence between the two sets of values (R2=0.841; RMSE = 0.555 m). This provides us with the methodological basis for automatically calculating lake areas, depths, and volumes from all available Landsat 8 and Sentinel-2 images. These automatic methods are incorporated into an algorithm for Fully Automated Supraglacial lake Tracking at Enhanced Resolution (FASTER). The FASTER algorithm produces time series showing lake evolution during the 2016 melt season, including automated rapid (≤4 day) lake-drainage identification. With the dual Sentinel-2–Landsat 8 record, we identify 184 rapidly draining lakes, many more than identified with either imagery collection alone (93 with Sentinel-2; 66 with Landsat 8), due to their inferior temporal resolution, or would be possible with MODIS, due to its omission of small lakes  km2. Finally, we estimate the water volumes drained into the GrIS during rapid-lake-drainage events and, by analysing downscaled regional climate-model (RACMO2.3p2) run-off data, the water quantity that enters the GrIS via the moulins opened by such events. We find that during the lake-drainage events alone, the water drained by small lakes ( km2) is only 5.1 % of the total water volume drained by all lakes. However, considering the total water volume entering the GrIS after lake drainage, the moulins opened by small lakes deliver 61.5 % of the total water volume delivered via the moulins opened by large and small lakes; this is because there are more small lakes, allowing more moulins to open, and because small lakes are found at lower elevations than large lakes, where run-off is higher. These findings suggest that small lakes should be included in future remote-sensing and modelling work.

Supraglacial lakes are known to trigger Antarctic ice-shelf instability and break-up. However, to date, no study has focused on lakes on Greenland's floating termini. Here, we apply lake boundary/area and depth algorithms to Landsat 8 imagery to analyse the inter- and intraseasonal evolution of supraglacial lakes across Petermann Glacier's (81°N) floating tongue from 2014 to 2016, while also comparing these lakes to those on the grounded ice. Lakes start to fill in June and quickly peak in total number, volume and area in late June/early July in response to increases in air temperatures. However, through July and August, total lake number, volume and area all decline, despite sustained high temperatures. These observations may be explained by the transportation of meltwater into the ocean by a river, and by lake drainage events on the floating tongue. Further, as mean lake depth remains relatively constant during this time, we suggest that a large proportion of the lakes that drain, do so completely, likely by rapid hydrofracture. The mean areas of lakes on the tongue are only ~20% of those on the grounded ice and exhibit lower variability in maximum and mean depth, differences likely attributable to the contrasting formation processes of lakes in each environment.

On 2 March 2016, several small en échelon tabular icebergs calved from the seaward front of the McMurdo Ice Shelf, and a previously inactive rift widened and propagated by ~3 km, ~25% of its previous length, setting the stage for the future calving of a ~14 km2 iceberg. Within 24 h of these events, all remaining land-fast sea ice that had been stabilizing the ice shelf broke-up. The events were witnessed by time-lapse cameras at nearby Scott Base, and put into context using nearby seismic and automatic weather station data, satellite imagery and subsequent ground observation. Although the exact trigger of calving and rifting cannot be identified definitively, seismic records reveal superimposed sets of both long-period (>10 s) sea swell propagating into McMurdo Sound from storm sources beyond Antarctica, and high-energy, locally-sourced, short-period (

Although the Greenland Ice Sheet (GrIS) is losing mass at an accelerating rate, much uncertainty remains about how surface runoff interacts with the subglacial drainage system and affects water pressures and ice velocities, both currently, and into the future. Here, we apply a physically based, subglacial hydrological model to the Paakitsoq region, west Greenland, and run it into the future to calculate patterns of daily subglacial water pressure fluctuations in response to climatic warming. The model is driven with moulin input hydrographs calculated by a surface routing model, forced with distributed runoff. Surface runoff and routing are simulated for a baseline year (2000), before the model is forced with future climate scenarios for the years 2025, 2050, and 2095, based on the IPCC's Representative Concentration Pathways (RCPs). Our results show that as runoff increases throughout the 21st century, and/or as RCP scenarios become more extreme, the subglacial drainage system makes an earlier transition from a less efficient network operating at high water pressures, to a more efficient network with lower pressures. This will likely cause an overall decrease in ice velocities for marginal areas of the GrIS. However, short-term variations in runoff, and therefore subglacial pressure, can still cause localized speedups, even after the system has become more efficient. If these short-term pressure fluctuations become more pronounced as future runoff increases, the associated late-season speedups may help to compensate for the drop in overall summer velocities, associated with earlier transitioning from a high to a low pressure system.

Ka Roimata o Hine Hukatere (Franz Josef Glacier) is a fast-flowing maritime glacier and its climatological and hydrological drivers are different from those of many previously studied alpine glaciers. The glacier tongue has recently advanced as well as retreated, remains largely snow free, has significant volumes of melt and rainwater inputs throughout the year, and experiences small radiation and air temperature fluctuations over diurnal to seasonal time scales. We discuss measurements of surface velocity made between 2000 and 2012 at annual, seasonal, weekly, and daily time scales together with measurements of glacier geometry change, and calculations of surface water inputs and subglacial water pressure variations derived from a distributed surface mass balance model and a one-dimensional conduit hydrology model, respectively. Annual velocity variations are linked to changes in glacier geometry and advance/retreat cycles with accelerations during thickening and advance and decelerations during...

Seasonal meltwater lakes on the Greenland Ice Sheet form when surface runoff is temporarily trapped in surface topographic depressions. The development of such lakes affects both the surface energy balance and dynamics of the ice sheet. Although areal extents, depths and lifespan of lakes can be inferred from satellite imagery, such observational studies have a limited temporal resolution. Here, we adopt a modelling-based strategy to estimate the seasonal evolution of surface water storage for the ~ 3600 km2 Paakitsoq region of W. Greenland. We use a high-resolution time-dependent surface mass balance model to calculate surface melt, a supraglacial water routing model to calculate lake filling and a prescribed water-volume-based threshold to predict rapid lake drainage events. This threshold assumes that drainage will occur through a fracture if V = Fa ⋅ H, where V is lake volume, H is the local ice thickness and Fa is the potential fracture area. The model shows good agreement between modelled lake locations and volumes and those observed in nine Landsat 7 ETM images from 2001, 2002 and 2005. We use the model to investigate the lake water volume required to trigger drainage, and the impact that varying this threshold volume has on the proportion of meltwater that is stored in surface lakes and enters the subglacial drainage system. Model performance is maximised with values of Fa between 4000 and 7500 m2. For these thresholds, lakes transiently store < 40% of available meltwater at the beginning of the melt season, decreasing to ~ 5 to 10% by the middle of the melt season; over the course of a melt season, 40 to 50% of total meltwater production enters the subglacial drainage system through moulins at the bottom of drained lakes.

Supraglacial meltwater lakes trigger ice-shelf break-up and modulate seasonal ice-sheet flow, and are thus agents by which warming is transmitted to the Antarctic and Greenland ice sheets. To characterize supraglacial lake variability we perform a comparative analysis of lake geometry and depth in two distinct regions, one on the pre-collapse (2002) Larsen B ice shelf, Antarctica, and the other in the ablation zone of Paakitsoq, a land-terminating region of the Greenland ice sheet. Compared to Paakitsoq, lakes on the Larsen B ice shelf cover a greater proportion of surface area (5.3% cf. 1%), but are shallower and more uniform in area. Other aspects of lake geometry (e.g. eccentricity, degree of convexity (solidity) and orientation) are relatively similar between the two regions. We attribute the notable difference in lake density and depth between ice-shelf and grounded ice to the fact that ice shelves have flatter surfaces and less distinct drainage basins. Ice shelves also possess more stimuli to small-scale, localized surface elevation variability, due to the various structural features that yield small variations in thickness and which float at different levels by Archimedes’ principle.

This research is supported by the U.S. National Science Foundation under grant GEOP/ANT – 0944248 awarded to D.R.M and grants ANT-0838811 and ARC-0934534 awarded to O.V.S.

[1] The functioning of the Greenland Ice Sheet's subglacial drainage system and its effect on ice dynamics have been inferred largely from hypothetical hydrology dynamics models and from analysis of satellite data and in situ GPS measurements. Despite this, there is still uncertainty about how the surface hydrology interacts with the subglacial drainage system and affects basal water pressures and ice flow, especially over annual time scales. To address this, we developed a high spatial (100 m) and temporal (1 h) resolution, distributed, physically based, subglacial hydrological model, and applied it to the Paakitsoq region, western Greenland. The model is driven with moulin input hydrographs calculated by a surface routing and lake filling/draining model, forced ultimately with distributed hourly runoff calculated by a surface mass balance model. Key outputs from the model are spatially and temporally varying subglacial water pressures and proglacial stream hydrographs. Early in the melt season, short spikes in water pressure lasting less than a day occur as a result of lake drainage events. During midsummer, there are sustained periods of high water pressure lasting days to weeks, even at times when the subglacial system is inferred to be predominantly efficient. Later in the summer, large diurnal fluctuations in water pressure occur with peaks regularly exceeding ice overburden pressure superimposed on a gradually declining trend. These phenomena support the results of previous hypothetical modeling efforts and inferences drawn from GPS measurements.

Uncertainty remains about how the surface hydrology of the Greenland Ice Sheet influences its subglacial drainage system, affecting basal water pressures and ice velocities, particularly over intra- and inter-seasonal timescales. Here, we apply a high spatial (200 m) and temporal (1 h) resolution subglacial hydrological model to a marginal (extending ~25 km inland), land-terminating, ~200 km^2 domain in the Paakitsoq region, West Greenland. The model is based on that by Hewitt [2013], but adapted for use with both real topographic boundary conditions and calibrated modeled water inputs [Banwell et al. 2013]. The inputs consist of moulin hydrographs, calculated by a surface routing and lake-filling/draining model, which is forced with distributed runoff from a surface energy-balance model. Results suggest that the areal density of lake-bottom moulins, and their timing of opening during the melt season, strongly affects subglacial drainage system development. A higher moulin density causes an earlier onset of subglacial channelization (i.e. water transport through channels rather than the distributed sheet), that becomes relatively widespread across the bed, whereas a lower moulin density results in a later onset of channelization that becomes less widespread across the bed. In turn, moulin density has a strong control on spatial and temporal variations in subglacial water pressures, which will influence basal sliding rates, and thus ice motion. The density of active surface-to-bed connections should be considered alongside surface melt intensity and extent in future predictions of the ice sheet’s dynamics.

Dye-trace breakthrough curves (BTCs) that increase in velocity and decrease in dispersivity through a melt season have been interpreted as indicating a switch from a distributed to a conduit subglacial drainage system, but this interpretation has not been validated in glaciers where the drainage system configuration was independently known. To test if processes other than a change in the configuration of the subglacial drainage system could produce similar BTCs, we measured BTCs from a persistent, mapped subglacial conduit beneath Rieperbreen, Svalbard, which lacks a distributed system because it is frozen to its bed. This conduit produced slow and highly dispersed BTCs early in the melt season when meltwater delivery rates were low, and fast and sharply peaked BTCs after the snowpack had retreated past the injection moulin. At Rieperbreen, the seasonal evolution of BTCs was controlled by decreases in conduit roughness as increased rates of meltwater delivery increased the relative submergence depths of rocks on the conduit floor. Because seasonal changes in roughness can produce slow and highly dispersed BTCs, dye-tracing studies may not be capable of uniquely identifying subglacial drainage system configurations. As a result, conduits may form earlier in melt seasons than previously recognized.

Summary The configuration of glacier hydrological systems is often inferred from proxy data, such as hydrographs, that are collected in proglacial streams. Seasonal changes in the peakedness of hydrographs are thought to reflect changes in the configuration of the subglacial drainage system. However, the amount of information that proglacial hydrographs contain about drainage system configurations depends critically on the degree to which the drainage systems modify recharge hydrographs. If the drainage system does not modify recharge hydrographs, then proglacial hydrographs primarily reflect the recharge conditions produced by supraglacial inputs. Here, we develop a theoretical framework to determine the circumstances under which glacier drainage systems can modify recharge hydrographs and the circumstances under which recharge pulses pass through glaciers unchanged. We address the capability of single conduits, simple arborescent conduit networks, and linked cavity systems to modify diurnal recharge pulses. Simulations of discharge through large sets of such systems demonstrate that, unless large reservoirs or significant constrictions are present, the discharge hydrographs of simple glacial conduit systems are nearly identical to their recharge hydrographs. Conduit systems tend not to modify hydrographs because the changes in storage within englacial and subglacial conduit networks on short time scales are typically small compared to their ability to transmit water. This finding suggests that proglacial hydrographs reflect a variety of factors, including surface melt rate, surface water transfer, and subglacial water transfer. In many cases the influence of suglacial processes may be relatively minor. As a result, the evolution of proglacial hydrographs cannot be used unambiguously to infer changes in the structure or efficiency of englacial or subglacial hydrological systems, without accurate knowledge of the nature of the recharge hydrograph driving the flow.

Modelling the hydrology of the Greenland ice sheet, including the filling and drainage of supraglacial lakes, requires melt inputs generated at high spatial and temporal resolution. Here we apply a high spatial (100 m) and temporal (1 hour) mass-balance model to a 450 km2 subset of the Paakitsoq region, West Greenland. The model is calibrated by adjusting the values for parameters of fresh snow density, threshold temperature for solid/liquid precipitation and elevation-dependent precipitation gradient to minimize the error between modelled output and surface height and albedo measurements from three Greenland Climate Network stations for the mass-balance years 2000/01 and 2004/05. Bestfit parameter values are consistent between the two years at 400 kg m-3, 2°C and +14% (100 m)-1, respectively. Model performance is evaluated, first, by comparing modelled snow and ice distribution with that derived from Landsat-7 ETM+ satellite imagery using normalized-difference snow index classification and supervised image thresholding; and second, by comparing modelled albedo with that retrieved from the MODIS sensor M0D10A1 product. Calculation of mass-balance components indicates that 6% of surface meltwater and rainwater refreezes in the snowpack and does not become runoff, such that refreezing accounts for 31% of the net accumulation.

Surface lakes and streams are forming on Antarctica's ice shelves, making them susceptible to instability and possible collapse. But rivers could mitigate this effect by efficiently exporting meltwater to the ocean. See Letters p.344 & p.349 The collapse of ice shelves can be hastened by surface meltwater that can seep into crevasses and refreeze, causing hydrofracturing. Such a mechanism was observed in the collapse of the Larsen B Ice Shelf on the Antarctic Peninsula. Robin Bell et al. show that in the Nansen Ice Shelf, Antarctica, a surface river has been in operation for over a century. The river terminates in a waterfall that can drain all of the surface meltwater produced annually by the ice shelf in one week. Thus, for the Nansen it seems that this surface drainage mechanism might be preventing meltwater from destabilizing the ice shelf. It isn't clear at present why meltwater is retained in some systems and drained in others, but the finding suggests that meltwater may not be a universally destructive force. Elsewhere in this issue, Jonathan Kingslake et al. report on the extent of meltwater across Antarctica and find that surface drainage systems have existed for decades as far as 85° S. Melt ponds and streams form across Antarctica, but their full extent and the duration of their existence have remained unclear. Now, Jonathan Kingslake et al. show that surface drainage systems, including interconnected ponds and streams, have existed for decades as far as 85° S and up to 1,300 metres elevation. They show that surface water can be transported more than 100 kilometres, creating pond networks almost 80 kilometres long. Although the implications of the findings for ice-sheet dynamics are not explored, the study demonstrates that surface water drainage is more extensive than previously thought, and is probably a persistent feature of the Antarctic Ice Sheet. Elsewhere in this issue, Robin Bell et al. report on the influence of surface meltwater on Antarctic ice-shelf stability and find that it might not be a universally destructive force as was previously thought.

We develop a formal thin-plate treatment of the viscoelastic flexure of floating ice shelves as an initial step in treating various problems relevant to ice-shelf response to sudden changes of surface loads and applied bending moments (e.g. draining supraglacial lakes, iceberg calving, surface and basal crevassing). Our analysis is based on the assumption that total deformation is the sum of elastic and viscous (or power-law creep) deformations (i.e. akin to a Maxwell model of viscoelasticity, having a spring and dashpot in series). The treatment follows the assumptions of well-known thin-plate approximation, but is presented in a manner familiar to glaciologists and with Glen’s flow law. We present an analysis of the viscoelastic evolution of an ice shelf subject to a filling and draining supraglacial lake. This demonstration is motivated by the proposition that flexure in response to the filling/drainage of meltwater features on the Larsen B ice shelf, Antarctica, contributed to the fragmentation process that accompanied its collapse in 2002.

Using a previously derived treatment of viscoelastic flexure of floating ice shelves, we simulated multiple years of evolution of a single, axisymmetric supraglacial lake when it is subjected to annual fill/drain cycles. Our viscoelastic treatment follows the assumptions of the well-known thin-beam and thin-plate analysis but, crucially, also covers power-law creep rheology. As the ice-shelf surface does not completely return to its un-flexed position after a 1-year fill/drain cycle, the lake basin deepens with each successive cycle. This deepening process is significantly amplified when lake-bottom ablation is taken into account. We evaluate the timescale over which a typical lake reaches a sufficient depth such that ice-shelf fracture can occur well beyond the lake itself in response to lake filling/drainage. We show that, although this is unlikely during one fill/drain cycle, fracture is possible after multiple years assuming surface meltwater availability is unlimited. This extended zone of potential fracture implies that flexural stresses in response to a single lake filling/drainage event can cause neighbouring lakes to drain, which, in turn, can cause lakes farther afield to drain. Such self-stimulating behaviour may have accounted for the sudden, widespread appearance of a fracture system that drove the Larsen B Ice Shelf to break-up in 2002.