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This study revisited 158 documented active subglacial lakes across Antarctica, applying a height‐change anomaly delineation algorithm to CryoSat‐2 and ICESat‐2 satellite altimetry data to assemble a 14.75‐year (2010.5–2025.25) time series of active subglacial lake dynamics. We generated time‐evolving lake outlines at quarter‐year intervals that revealed spatiotemporal variability, including lake expansions and migrations. Evolving outlines enabled improved estimates of subglacial water volume change. We found that the previous method of using stationary outlines underestimated cumulative volume change between ∼${\\sim} $ 4.8–7.4 km3${\\text{km}}^{3}$(65–100%$\\%$of the volume change estimated using evolving outlines) compared to evolving outlines when integrated across Antarctica. Evolving lake outlines also enabled the generation of a novel time series of subglacial lakebed active area, which we leveraged to estimate time‐varying dissolved inorganic carbon production.

Subglacial lakes have been mapped across Antarctica with two methods, radio‐echo sounding (RES) and ice‐surface deformation. At sites where both are coincident, these methods typically provide conflicting interpretations about the ice‐bed interface. With a single exception, active subglacial lakes identified by surface deformation do not display the expected flat, bright, and specular bed reflection in RES data, characteristic of non‐active lakes. This observational conundrum suggests that our understanding of Antarctic subglacial hydrology, especially beneath important fast‐moving ice streams, remains incomplete. Here, we use an airborne RES campaign that surveyed a well‐characterized group of active subglacial lakes on lower Mercer and Whillans ice streams, West Antarctica, to explore inconsistency between the two observational techniques. We test hypotheses of increased scattering and attenuation due to the presence of an active subglacial lake system that could suppress reflected bed‐echo power for RES observations in these locations, finding that entrained water is most plausible. Plain Language Summary The bottom of an ice sheet is insulated from cold air temperatures, often warm enough to melt and pond liquid water into lakes. These lakes beneath the ice sheet have been identified by two independent measurements, first with radar methods and second with changes in height of the ice surface (altimetry). Interestingly, the two methods rarely identify the same lakes: radar generally detects lakes in the ice‐sheet interior, whereas altimetry detects active lakes near the ice‐sheet margins that fill and drain within the time series of repeated measurements (∼years). In this study, we investigate a group of active subglacial lakes at which both radar and altimetry data sets are available. We demonstrate that the radar returns from active lake reflections are much dimmer than expected based on non‐active lake signatures and investigate the physical processes controlling those dim reflections. We argue that water moves into the ice when the lake fills or drains and that is the most plausible explanation for the observational discrepancy. Key Points Active subglacial lakes, identified by surface deformation, do not create the expected bright and specular radar reflection Entrained water in basal ice suppresses radar power by scattering and attenuation, and it also likely alters the basal ice mechanics Understanding the radar expression of subglacial water on Earth provides context for investigations of subsurface water on planetary bodies

Recent observations of dynamic water systems beneath the Greenland and Antarctic ice sheets have sparked renewed interest in modelling subglacial drainage. The foundations of today's models were laid decades ago, inspired by measurements from mountain glaciers, discovery of the modern ice streams and the study of landscapes evacuated by former ice sheets. Models have progressed from strict adherence to the principles of groundwater flow, to the incorporation of flow 'elements' specific to the subglacial environment, to sophisticated two-dimensional representations of interacting distributed and channelized drainage. Although presently in a state of rapid development, subglacial drainage models, when coupled to models of ice flow, are now able to reproduce many of the canonical phenomena that characterize this coupled system. Model calibration remains generally out of reach, whereas widespread application of these models to large problems and real geometries awaits the next level of development.

Subglacial water flow strongly modulates glacier basal motion, which itself strongly influences the contributions of glaciers and ice sheets to sea level rise. However, our understanding of when and where subglacial water flow enhances or impedes glacier flow is limited due to the paucity of direct observations of subglacial drainage characteristics. Here, we demonstrate that dense seismic array observations combined with an innovative systematic seismic source location technique allows the retrieval of a two-dimensional map of a subglacial drainage system, as well as its day-to-day temporal evolution. We observe with unprecedented detail when and where subglacial water flows through a cavity-like system that enhances glacier flow versus when and where water mainly flows through a channel-like system that impedes glacier flow. Most importantly, we are able to identify regions of high hydraulic connectivity within and across the cavity and channel systems, which have been identified as having a major impact on the long-term glacier response to climate warming. Applying a similar seismic monitoring strategy in other glacier settings, including for ice sheets, may help to diagnose the susceptibility of their dynamics to increased meltwater input due to climate warming.

Subglacial water beneath the Antarctic Ice Sheet is often funneled via subglacial channels, which inject freshwater into ice‐shelf cavities where it interacts with ocean water. The temporal variability of this system has been poorly observed, but its importance for ice dynamics is well recognized. Airborne radar data show a subglacial channel evolving within a decade near of the grounding zone of the Roi Baudouin Ice Shelf (East Antarctica), while topographic signatures on the ice shelf indicate prior inactivity for 60 years. Combining our observations with subglacial hydrological modeling, we suggest that the interplay between episodic subglacial water pulses and ocean water intrusion drive the opening and closing of the channels. Our findings illuminate the short‐term transient nature of subglacial channel activity. This impacts ice‐shelf–ocean processes, which are important for constraining increasing ocean warming onto ice‐shelf basal mass balance, but pose significant challenges for subglacial hydrological modeling at the grounding zone. Plain Language Summary This study explores how water moves under the ice in Antarctica and how it interacts at the ice–ocean boundary. Using radar, we studied a part of the Roi Baudouin Ice Shelf in East Antarctica and found that water channels under the ice have changed significantly over the last decades. The radar images from 2011 to 2019 showed that these channels open and close within a decade, likely due to changes in water supply from the inland and intrusion of ocean water. This means that the water flow under the ice is not steady, influencing melting at the underside and affecting its movement. These discoveries show that the subglacial water system under Antarctic Ice Sheet is more complicated, making it harder to project future changes in sea‐level rise and the effects of climate change on the polar regions. Key Points Subglacial channels near the grounding zone are stable in space but toggle activity within decades Ice‐shelf channel morphology identifies one water outlet as active for hundreds of years before it shut down 60 years ago Radio‐echo sounding documents the reactivation of a subglacial channel within eight years near the grounding zone

Extreme melt and rainfall events can induce temporary acceleration of Greenland Ice Sheet motion, leading to increased advection of ice to lower elevations where melt rates are higher. In a warmer climate, these events are likely to become more frequent. In September 2022, seasonally unprecedented air temperatures caused multiple melt events over the Greenland Ice Sheet, generating the highest melt rates of the year. The scale and timing of the largest event overwhelmed the subglacial drainage system, enhancing basal sliding and increasing ice velocities by up to ∼240% relative to pre‐event velocities. However, ice motion returned rapidly to pre‐event levels, and the speed‐ups caused a regional increase in annual ice discharge of only ∼2% compared to when the effects of the speed‐ups were excluded. Therefore, although late melt‐season events are forecast to become more frequent and drive significant runoff, their impact on net mass loss via ice discharge is minimal. Plain Language Summary Extreme melt and rainfall events can cause the flow of ice on the Greenland Ice Sheet to accelerate, potentially causing more ice to move to lower elevations, where temperatures are warmer and melt rates are higher. In September 2022, there were multiple unprecedented melt events. Their intensity caused some glaciers on the ice sheet to speed up by 240% relative to pre‐event speeds. Despite these accelerations, our analyses show that these events had only a minimal long‐term impact on how much ice was moved to lower elevations due to the short duration of the speed‐ups. As a result, while these melt‐induced speed‐ups are expected to become more common in a warmer climate, their effect on the amount of ice transported toward the ice margins is minimal. Key Points September 2022 saw multiple melt events over the west Greenland Ice Sheet, with the largest daily runoff of any late melt‐season since 1950 Large quantities of surface‐generated meltwater overwhelmed the subglacial drainage system causing brief increases in ice velocity Late‐season runoff‐induced speed‐ups have only minimal impact on the mass balance of the Greenland Ice Sheet via dynamical processes

High resolution measurements of ice motion along a ∼120 km transect in a land‐terminating section of the GrIS reveal short‐term velocity variations (<1 day), which are forced by rapid variations in meltwater input to the subglacial drainage system from the ice sheet surface. The seasonal changes in ice velocity at low elevations (<1000 m) are dominated by events lasting from 1 day to 1 week, although daily cycles are largely absent at higher elevations, reflecting different patterns of meltwater input. Using a simple model of subglacial conduit behavior we show that the seasonal record of ice velocity can be understood in terms of a time‐varying water input to a channelized subglacial drainage system. Our investigation substantiates arguments that variability in theduration and rate, rather than absolute volume, of meltwater delivery to the subglacial drainage system are important controls on seasonal patterns of subglacial water pressure, and therefore ice velocity. We suggest that interpretations of hydro‐dynamic behavior in land‐terminating sections of the GrIS margin which rely on steady state drainage theories are unsuitable for making predictions about the effect of increased summer ablation on future rates of ice motion. Key Points High‐resolution GPS data show short‐term variations in Greenland ice motion Velocity changes are driven by meltwater input to the subglacial drainage system Time‐varying behavior of the drainage system explains patterns in ice velocity

Two outstanding questions for the future of the Greenland Ice Sheet are (a) how enhanced meltwater draining beneath the ice will impact the behavior of large tidewater glaciers, and (b) to what extent tidewater glacier velocity is driven by changes at the terminus versus changes in sliding velocity due to meltwater. We present a two‐way coupled framework to simulate the nonlinear feedbacks of evolving subglacial hydrology and ice dynamics using the Subglacial Hydrology And Kinetic, Transient Interactions (SHAKTI) model within the Ice‐sheet and Sea‐level System Model (ISSM). Through coupled simulations of Helheim Glacier, we find that terminus effects dominate the seasonal velocity pattern up to 15 km from the terminus, while hydrology drives the velocity response upstream. With increased melt, the hydrology influence yields seasonal acceleration of several hundred meters per year in the interior, suggesting that hydrology will play an important role in future mass balance of tidewater glaciers. Plain Language Summary Water draining under glaciers and ice sheets affects the friction between the ice and the bed, and controls how fast the ice can slide into the ocean, contributing to sea‐level rise. We present a framework for simulating the feedbacks between hydrology and ice flow. We investigate the relative influence of changes at the terminus of the glacier where it meets the ocean, versus changes in meltwater drainage, in determining how fast the glacier moves. Our modeling of Helheim Glacier in southeast Greenland highlights the importance of terminus effects up to 15 km from the terminus, and hydrology farther upstream, with increased melt yielding higher inland acceleration. These results suggest that meltwater will play an increasingly important role in the future behavior of glaciers. Key Points We couple a subglacial hydrology model with an ice flow model to simulate the relationship between sliding velocity and effective pressure Terminus effects at Helheim Glacier drive velocity up to 15 km upstream, but seasonal hydrology controls velocity patterns further inland Increased melt accelerates ice inland of the main trunk, implying importance of hydrology in tidewater glacier future mass balance

Subglacial drainage networks regulate the response of ice sheet flow to surface meltwater input to the subglacial environment. Simulating subglacial hydrology evolution is critical to projecting ice sheet sensitivity to climate, and contribution to sea‐level change. However, current numerical subglacial hydrology models are computationally expensive, and, consequently, evolving subglacial hydrology is neglected in large‐scale ice sheet simulations. We present a deep learning emulator of a state‐of‐the‐art subglacial hydrology model, trained at multiple Greenland glaciers. Our emulator performs strongly in both temporal (R2 > 0.99) and spatial (R2 > 0.95) generalization, offers high computational savings, and can be used to force numerical ice sheet models. This will enable century‐ and large‐scale ice sheet model simulations, including interactions between ice flow and increased meltwater input to the subglacial environment. Generally, our work demonstrates that machine learning can further improve ice sheet models, reduce computational bottlenecks, and exploit information from high‐fidelity models and novel observational platforms. Plain Language Summary Meltwater at the surface of ice sheets can drain to the subglacial environment, lubricate the bed, and influence ice sheet flow. Complex numerical subglacial hydrology models represent the subglacial drainage system, but are too computationally expensive to be included in large‐scale and long‐term ice sheet simulations. Consequently, model predictions of future ice sheet contribution to sea‐level rise ignore ice flow modulation by evolving subglacial hydrology. Here, we use deep learning to emulate a state‐of‐the‐art subglacial hydrology model. The emulator can directly force large‐scale ice sheet models to capture ice flow sensitivity to subglacial hydrology. The computational speed and accuracy of our emulator show the potential to use machine learning to efficiently incorporate previously neglected processes into ice sheet models. Key Points We develop a deep learning emulator to simulate evolving subglacial hydrology in response to meltwater input for ice sheet simulations The emulator shows generalization capabilities, large computational savings, and can be used to force numerical ice sheet models We demonstrate that machine learning has substantial potential in improving ice sheet models, through using information‐rich data sets

The distribution and drainage of meltwater at the base of glaciers sensitively affects fast ice flow. Previous studies suggest that thin meltwater films between the overlying ice and a hard-rock bed channelize into efficient drainage elements by melting the overlying ice. However, these studies do not account for the presence of soft deformable sediment observed underneath many West Antarctic ice streams, and the inextricable coupling that sediment exhibits with meltwater drainage. Our work presents an alternate mechanism for initiating drainage elements such as canals where meltwater films grow by eroding the sediment beneath. We conduct a linearized stability analysis on a meltwater film flowing over an erodible bed. We solve the Orr–Sommerfeld equation for the film flow, and we compute bed evolution with the Exner equation. We identify a regime where the coupled dynamics of hydrology and sediment transport drives a morphological instability that generates spatial heterogeneity at the bed. We show that this film instability operates at much faster time scales than the classical thermal instability proposed by Walder. We discuss the physics of the instability using the framework of ripple formation on erodible beds.