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Subglacial hydrology is an important component of the ice dynamic system in Antarctica but is challenging to investigate due to the large spatial scales of the catchment systems, the ice thickness, and remote location. Here I discuss key discoveries about Antarctic subglacial hydrology from the Glacier Drainage System (GlaDS) model, including the presence of long, often high-pressure, subglacial channels. These channels pump tens of cubic metres per second of freshwater into ice-shelf cavities and directly affect melt rates at the critical grounding zone regions. Future ice dynamics and ice-shelf cavity models should take subglacial hydrology into account if they are to accurately predict future behaviour of the Antarctic Ice Sheet.

Petermann Glacier is a major outlet glacier of northern Greenland that drains a marine‐based basin vulnerable to destabilization from enhanced oceanic and atmospheric forcings. Using satellite radar interferometry data from the Sentinel‐1a/b missions, we observe a seasonal glacier acceleration of 15% in the summer, from 1,250 to 1,500 m/yr near the grounding line, but the physical drivers of this seasonality have not been elucidated. Here, we use a subglacial hydrology model coupled one‐way to an ice sheet model to evaluate the role of subglacial hydrology as a physical mechanism explaining the seasonal acceleration. We find excellent agreement between the observed and predicted velocity in terms of timing and magnitude with the addition of an applied lower limit on effective pressure of 6% of ice overburden pressure. We conclude that seasonal changes in subglacial hydrology are sufficient to explain the observed seasonal speed up of Petermann Glacier. Plain Language Summary Petermann Glacier is one of the largest glaciers in northern Greenland with a sea‐level rise potential of about half a meter. In recent years, extensive satellite data have become available which provide frequent and widespread observations over much of the Greenland ice sheet, including Petermann. Access to such records provides the opportunity to investigate changes in glacier dynamics taking place over much smaller time periods. We see that in the summer Petermann's speed increases by about 15% from 1,250 to 1,500 m/year near the grounding line. We are able to reproduce this observed pattern in speed by predicting how friction at the base of the glacier will change with the addition of summer surface melt to the bed, and using those changes in friction to calculate seasonal speedup of the ice. Key Points Petermann Glacier in northern Greenland accelerates by 15% from 1,250 to 1,500 m/yr during the summer months near the grounding line A subglacial hydrology model quantifies seasonal changes in effective pressure at the bed in response to meltwater runoff input Simulated effective pressure drives velocity behavior that matches observations in magnitude and timing over three consecutive years

Hydrologic pathways beneath ice sheets and glaciers play an important role in regulating ice flow. Antarctica has experienced, and will continue to experience, changes in ice dynamics and geometry, but the associated changes in subglacial hydrology have received less attention. Here, we use the GlaDS subglacial hydrology model to examine drainage evolution beneath an idealised Antarctic glacier in response to steepening ice surface slopes, accelerating ice velocities and subglacial lake drainages. Ice surface slope changes exerted a dominant influence, redirecting basal water to different outlet locations and substantially increasing channelised discharge crossing the grounding line. Faster ice velocities had comparatively negligible effects. Subglacial lake drainage results indicated that lake refilling times play a key role in drainage system evolution, with lake flux more readily accommodated following shorter refilling times. Our findings are significant for vulnerable Antarctic regions currently experiencing dynamic thinning since subglacial water re-routing could destabilise ice shelves through enhanced sub-shelf melting, potentially hastening irreversible retreat. These changes could also affect subglacial lake activity. We, therefore, emphasise that including a nuanced and complex representation of subglacial hydrology in ice-sheet models could provide critical information on the timing and magnitude of sea-level change contributions from Antarctica.

Aurora Subglacial Basin (ASB) in East Antarctica is among the most rapidly changing regions in Antarctica. Beneath the ASB, subglacial drainage networks allow for fast ice flow and facilitate mass losses by eroding the coastal ice shelves. The ASB underwent advance-retreat cycles since its inception 34 million years ago, and projections rely on these findings to predict future ice behaviour. Yet, these hindcasts and forecasts seldom consider the effects of subglacial drainage. Here, we model subglacial drainage networks and ice shelf basal melting of the ASB from 34 million years ago to 2100 CE. Our results indicate continual reorganisation of ASB drainage systems and ice shelf melt patterns, suggesting that the past does not fully serve as an analogue for the future. Since subglacial drainage accounts for up to 70% of ice shelf melt, our results imply that subglacial hydrology could trigger higher rates of mass loss than previously suggested. The authors model subglacial water flow in East Antarctica and show significant drainage reorganization from 34 Mya to the present and forward to 2100 CE. These drainage networks affect ice dynamics, ice shelf melt, and resulting sea level change.

Subglacial freshwater discharge from beneath Antarctic glaciers likely has a strong impact on ice shelf basal melting. However, the difficulty in directly observing subglacial flow highlights the importance of modeling these processes. We use an ocean model of the Totten Ice Shelf cavity into which we inject subglacial discharge derived from a hydrology model applied to Aurora Subglacial Basin. Our results show (a) discharge increases melting in the vicinity of the outflow region, which correlates with features observed in surface elevation maps and satellite‐derived melt maps, with implications for ice shelf stability; (b) the change in melting is driven by the formation of a buoyant plume rather than the addition of heat; and (c) the buoyant plume originating from subglacial discharge‐driven melting is far‐reaching. Basal melting induced by subglacial hydrology is thus important for ice shelf stability, but is absent from almost all ice‐ocean models. Plain Language Summary Grounding line subglacial discharge is the outflow of freshwater at the region where Antarctic ice shelves begin to float. This process is difficult to observe as the outflow region is typically below ice up to several kilometers thick. Using a computer model, we show how the outflow of this fresh water melts the underside of an Antarctic ice shelf. We demonstrate that the outflow leads to a strong increase in melting, which is driven not by the heat in the outflow, but rather by the low salinity water's buoyancy. This buoyancy leads to the formation of a plume that rapidly ascends the underside of the ice shelf and can carry even warmer water to the ice, leading to increased melting. Given that melting of ice shelves can destabilize the grounded ice upstream of the floating region, we show that the relationship between subglacial discharge and ice shelf melt is important for understanding the stability of Antarctic ice. Key Points Subglacial discharge has a strong local effect on melting beneath the Totten Ice Shelf Addition of buoyancy explains the change in melting Discharge has far‐reaching impacts due to the extent of the buoyant plume

The Drygalski Ice Tongue in East Antarctica stretches 90 km into the Ross Sea and influences the local ocean circulation, and persistence of the Terra Nova Bay Polynya. We examine the controls on the size of this floating ice body by comparing the propagation of six large fractures on the ice tongue's northern side using 21 years of Landsat imagery with hydrostatic ice thickness maps and strain rate calculations. We also apply a subglacial hydrology model to estimate the location and discharge from subglacial channels over the grounding line and compare these with basal channels identified along the ice tongue using remote sensing and airborne radar data. Our results suggest that large fractures are inhibited from full-width propagation by thicker ice between basal channels. We hypothesize that only once the ice tongue thins towards the terminus, can fractures propagate and cause large calving events. This suggests an important relationship between the melting of floating ice from subglacial and ocean sources and the expansion of fractures that lead to ice tongue calving.

Liquid water can exist at temperatures well below freezing beneath glaciers and ice sheets, where subglacial water systems, fresh and saline, have been shown to host unique microbial ecosystems. Geophysical techniques sensitive to fluid-content contrasts, e.g. electromagnetics, can characterize subglacial water and its salinity. Here, we assess the ground-based transient electromagnetic (TEM) method for deriving the resistivity and salinity of subglacial water. We adapt an existing open-source Bayesian inversion algorithm, which uses independent depth constraints, to output posterior distributions of resistivity and pore fluid salinity with depth. A variety of synthetic models, including a thin (5 m), conductive (0.16 Ωm), hypersaline (147 psu) subglacial lake, are used to evaluate the TEM method for imaging under 800 m-thick ice. The study demonstrates that TEM methods can resolve conductive, saline bodies accurately using external depth constraints, for example, from radar or seismic data. The depth resolution of TEM can be limited beneath deep (>800 m), thick (>50 m) conductive, water bodies and additional constraints from passive electromagnetic (EM) methods could be used to reduce ambiguities in the TEM results. Subsequently, non-invasive active and passive EM methods could provide profound insights into remote aqueous systems under glaciers and ice sheets.

This study presents the first light detection and ranging (LiDAR)-based morphometric description of a water blister from a past ice-sheet bed caused by rapid supraglacial drainage. The blister formed during the rapid early Holocene deglaciation of the Fennoscandian Ice Sheet (FIS). It is located in southwest Finland within a subglacial meltwater route interpreted to represent the transition from a distributed to a channelized drainage system. A LiDAR digital elevation model was supplemented with sedimentological and ground-penetrating radar data on blister outflow channels and sedimentology of downflow polymorphous mounds and ridges (PMRs). Unlike the water blisters recorded from the rapid drainage of supraglacial lakes on the Greenland Ice Sheet, the smaller blister size here was either due to crevasse or moulin drainage, or was a supraglacial lake drainage that tapped into a pre-existing, relatively efficient drainage system and related semi-sorted sediments, promoting rapid drainage and reworking of PMRs along the meltwater route. The preservation potential or exposure probability of blister marks is presumably low but they can provide important information about evolution of subglacial drainage systems that is of value to modern interpretations of glacial hydrology.

Remotely sensed datasets indicate that Fisher Glacier underwent two surges since 1948: during approximately 1969–72 and 2013–16. These were characterized by an advanced terminus position (terminus-wide average advance 571 ± 143 m from 1963 to 1972 and 868 ± 8 m from 2014 to 2017), intense surface crevassing (up to >30 km up-glacier from the terminus during both surges), high surface velocities and a down-glacier transfer of mass. The intervening quiescent phase lasted for 40 years, during which velocities were generally low (<50 m a−1), but underwent a slow multidecadal increase starting around 1985, spreading from the middle of the glacier. A pre-surge buildup phase beginning around 2008 resulted in velocities of up to ∼200 m a−1. The active phase of the surge initiated in winter 2013/14, with velocities of up to 1500 m a−1 propagating both up- and down-glacier from the mid-glacier region. In July 2016, the surge rapidly terminated within a period of ∼1 month. Characterized by a rapid onset and termination, but also displaying a multidecadal acceleration prior to the surge, the cause of Fisher Glacier’s surges may be best explained by a unifying framework such as the enthalpy balance theory.

Glacier surficial melt rates are commonly modelled using surface energy balance (SEB) models, with outputs applied to extend point-based mass-balance measurements to regional scales, assess water resource availability, examine supraglacial hydrology and to investigate the relationship between surface melt and ice dynamics. We present an improved SEB model that addresses the primary limitations of existing models by: (1) deriving high-resolution (30 m) surface albedo from Landsat 8 imagery, (2) calculating shadows cast onto the glacier surface by high-relief topography to model incident shortwave radiation, (3) developing an algorithm to map debris sufficiently thick to insulate the glacier surface and (4) presenting a formulation of the SEB model coupled to a subsurface heat conduction model. We drive the model with 6 years of in situ meteorological data from Kaskawulsh Glacier and Nàłùdäy (Lowell) Glacier in the St. Elias Mountains, Yukon, Canada, and validate outputs against in situ measurements. Modelled seasonal melt agrees with observations within 9% across a range of elevations on both glaciers in years with high-quality in situ observations. We recommend applying the model to investigate the impacts of surface melt for individual glaciers when sufficient input data are available.