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Ice–ocean interactions at the bases of Antarctic ice shelves are rarely observed, yet have a profound influence on ice sheet evolution and stability. Ice sheet models are highly sensitive to assumed ice shelf basal melt rates; however, there are few direct observations of basal melting or the oceanographic processes that drive it, and consequently our understanding of these interactions remains limited. Here we use in situ observations from the Ross Ice Shelf to examine the oceanographic processes that drive basal ablation of the world’s largest ice shelf. We show that basal melt rates beneath a thin and structurally important part of the shelf are an order of magnitude higher than the shelf-wide average. This melting is strongly influenced by a seasonal inflow of solar-heated surface water from the adjacent Ross Sea Polynya that downwells into the ice shelf cavity, nearly tripling basal melt rates during summer. Melting driven by this frequently overlooked process is expected to increase with predicted surface warming. We infer that solar heat absorbed in ice-front polynyas can make an important contribution to the present-day mass balance of ice shelves, and potentially impact their future stability.High melt rates in a key location beneath the Ross Ice Shelf result from a seasonal inflow of water heated in the Ross Sea Polynya, according to in situ observations.

We explore the links between elevation variability of the Antarctic Ice Sheet (AIS) and large‐scale climate modes. Using multiple linear regression, we quantify the time‐cumulative effects of El Niño Southern Oscillation (ENSO) and the Southern Annular Mode (SAM) on gridded AIS elevations. Cumulative ENSO and SAM explain a median of 29% of the partial variance and up to 85% in some coastal areas. After spatial smoothing, these signals have high spatial correlation with those from GRACE gravimetry (r∼ = 0.65 each). Much of the signal is removed by a firn densification model but inter‐model differences exist especially for ENSO. At the lower parts of the Thwaites and Pine Island glaciers, near their grounding line, we find the Amundsen Sea Low (ASL) explains ∼90% of the observed elevation variability. There, modeled firn effects explain only a small fraction of the variability, suggesting significant height changes could be a response to climatological ice‐dynamics. Plain Language Summary This study investigates how variations in the height of the Antarctic Ice Sheet (AIS) are connected to large‐scale climate patterns. We used a statistical method to measure the effects of two climate phenomena: El Niño Southern Oscillation (ENSO) and the Southern Annular Mode (SAM). We found that the cumulative effects of these phenomena account for about 29% of the variations in AIS height on average, and up to 85% in some coastal areas. These patterns match well with independent data from the GRACE satellites over the same period. Applying a model that considers the accumulation of snow and its compaction into ice (firn densification) removes much of this signal, suggesting much, but not all, of the signals are related to snowfall variations. At the fronts of the rapidly changing Thwaites and Pine Island glaciers, the dominant climate phenomenon is the Amundsen Sea Low (ASL), which varies in strength and location. Here, the cumulative effects of the ASL changes explain about 90% of the variations in height of these glaciers, with only a small part explained by firn effects. We suggest the unexplained variability could be partly due to changes in ice flow. Key Points Cumulative effects of large‐scale climate modes dominate detrended altimeter time series of Antarctic ice elevation 2002–2020 These decadal signals have the same spatial pattern in altimeter ice height and GRACE mass time series These decadal signals are largely due to surface mass balance, but ice dynamic changes may play a role in the Amundsen Sea Embayment

We present the first fully coupled 3D full-Stokes model of a tidewater glacier, incorporating ice flow, subglacial hydrology, plume-induced frontal melting and calving. We apply the model to Store Glacier (Sermeq Kujalleq) in west Greenland to simulate a year of high melt (2012) and one of low melt (2017). In terms of modelled hydrology, we find perennial channels extending 5 km inland from the terminus and up to 41 and 29 km inland in summer 2012 and 2017, respectively. We also report a hydrodynamic feedback that suppresses channel growth under thicker ice inland and allows water to be stored in the distributed system. At the terminus, we find hydrodynamic feedbacks exert a major control on calving through their impact on velocity. We show that 2012 marked a year in which Store Glacier developed a fully channelised drainage system, unlike 2017, where it remained only partially developed. This contrast in modelled behaviour indicates that tidewater glaciers can experience a strong hydrological, as well as oceanic, control, which is consistent with observations showing glaciers switching between types of behaviour. The fully coupled nature of the model allows us to demonstrate the likely lack of any hydrological or ice-dynamic memory at Store Glacier.

Supraglacial lake drainage events influence Greenland Ice Sheet dynamics on hourly to interannual timescales. However, direct observations are rare, and, to date, no in situ studies exist from fast-flowing sectors of the ice sheet. Here, we present observations of a rapid lake drainage event at Store Glacier, west Greenland, in 2018. The drainage event transported 4.8 × 10⁶ m³ of meltwater to the glacier bed in ∼5 h, reducing the lake to a third of its original volume. During drainage, the local ice surface rose by 0.55 m, and surface velocity increased from 2.0 m·d−1 to 5.3 m·d−1. Dynamic responses were greatest ∼4 km downstream from the lake, which we interpret as an area of transient water storage constrained by basal topography. Drainage initiated, without any precursory trigger, when the lake expanded and reactivated a preexisting fracture that had been responsible for a drainage event 1 y earlier. Since formation, this fracture had advected ∼500 m from the lake’s deepest point, meaning the lake did not fully drain. Partial drainage events have previously been assumed to occur slowly via lake overtopping, with a comparatively small dynamic influence. In contrast, our findings show that partial drainage events can be caused by hydrofracture, producing new hydrological connections that continue to concentrate the supply of surface meltwater to the bed of the ice sheet throughout the melt season. Our findings therefore indicate that the quantity and resultant dynamic influence of rapid lake drainages are likely being underestimated.

Antarctica’s contribution to global sea-level rise is deeply uncertain, with subglacial water suspected to play a critical role, yet its impact remains unclear. We demonstrate that water at the base of ice sheets influences sliding behaviour and that its exclusion from models can underestimate sea-level rise projections and delay the predicted onset of tipping points. Here we use an Antarctic Ice Sheet model (Elmer/Ice) to explore how different assumptions about water pressure at the ice base affect sea-level rise projections from 2015 to 2300. Our results indicate that incorporating subglacial water can amplify ice discharge across the Antarctic Ice Sheet by up to threefold above the standard approach, potentially contributing an additional 2.2 metres to sea-level rise by 2300. Notably, a smoothly decreasing basal drag near the grounding line more than doubles grounding line flux by 2300 relative to scenarios where effective pressure is simplified into a spatially constant coefficient. Basin-specific responses vary significantly, with some scenarios advancing tipping points by up to 40 years. These findings underscore the critical need to integrate evolving subglacial hydrology into ice sheet models. Hidden water beneath Antarctica’s ice can accelerate ice loss, potentially raising sea levels by over 2 meters by 2300. These findings highlight the urgent need to incorporate evolving subglacial hydrology into ice sheet models for more accurate sea-level rise projections.

Supraglacial lakes on the Greenland Ice Sheet are expanding inland, but the impact on ice flow is equivocal because interior surface conditions may preclude the transfer of surface water to the bed. Here we use a well-constrained 3D model to demonstrate that supraglacial lakes in Greenland drain when tensile-stress perturbations propagate fractures in areas where fractures are normally absent or closed. These melt-induced perturbations escalate when lakes as far as 80 km apart form expansive networks and drain in rapid succession. The result is a tensile shock that establishes new surface-to-bed hydraulic pathways in areas where crevasses transiently open. We show evidence for open crevasses 135 km inland from the ice margin, which is much farther inland than previously considered possible. We hypothesise that inland expansion of lakes will deliver water and heat to isolated regions of the ice sheet’s interior where the impact on ice flow is potentially large. Lakes on the Greenland Ice Sheet transfer water to the bed when they drain, but the impact is unknown. Here, the authors use a 3D model to show that lakes drain when fractures form, causing a chain reaction in which cascading lake drainages extend inland and deliver water to previously isolated regions of the bed.

We have developed an automatic method to identify changes in the position of calving glacier margins using daily MODIS imagery. Application of the method to 32 ocean‐terminating glaciers in East Greenland produced 26,802 margin positions for a 10 year long period (2000–2009). We report these high‐resolution data and show that the glaciers exhibit seasonal cycles with magnitudes of advance and retreat proportional to glacier width. Despite similar seasonality there is a distinct difference between the interannual trends of calving front positions north and south of 69°N. All glaciers above this latitude showed very limited or no change when seasonality was excluded, while glaciers south of 69°N retreated significantly between 2001 and 2005 (∼2.3 km on average). Approximately 26% of the retreat of southern glaciers was regained by readvance from 2005 to 2009. To explain the latitudinal boundary of glacier dynamics, we review basic climatic factors, including summer and winter atmospheric forcing, sea ice conditions, and ocean temperature. We conclude that the southern retreats were strongly influenced by warm oceanic conditions associated with increased transport of subtropical waters to the Irminger Sea and to fjords and coastal regions south of 69°N. Northern glaciers remained stable despite significant increase in runoff in this region because fjords at latitudes higher than 69°N are less exposed to subtropical waters. The southern retreats illustrate sensitive behavior of calving glaciers, and we hypothesize that the calving fronts retreated because they were exposed to rapid ice‐front melting. Key Points Automatic monitoring of glacier margins Ice‐ocean interaction Seasonal and interannual glacier change

Subglacial hydrologic systems regulate ice sheet flow, causing acceleration or deceleration, depending on hydraulic efficiency and the rate at which surface meltwater is delivered to the bed. Because these systems are rarely observed, ice sheet basal drainage represents a poorly integrated and uncertain component of models used to predict sea level changes. Here, we report radar-derived basal melt rates and unexpectedly warm subglacial conditions beneath a large Greenlandic outlet glacier. The basal melt rates averaged 14 mm · d−1 over 4 months, peaking at 57 mm · d−1 when basal water temperature reached +0.88 ◦C in a nearby borehole. We attribute both observations to the conversion of potential energy of surface water to heat in the basal drainage system, which peaked during a period of rainfall and intense surface melting. Our findings reveal limitations in the theory of channel formation, and we show that viscous dissipation far surpasses other basal heat sources, even in a distributed, high-pressure system.

Tidewater glaciers are an important and difficult part of the cryosphere to study owing to their complex nature and often inaccessible and physically challenging environments. The interaction of glacier and fjord processes furthermore presents particular observational challenges. Modelling provides a possible solution to these issues, but, at the glacier scale, the processual complexities require a 3-D full-Stokes approach that is computationally expensive. Additionally, the lack of data for model validation or constraints imposes further obstacles. Despite this, progress on modelling such glaciers with explicit inclusion of all relevant processes is being made. The key remaining challenges are including more realistic representations of calving and coupling 3-D glacier modelling with 3-D fjord circulation modelling to allow inclusion of the effect of cross-fjord circulation. We are confident, however, that these issues can be resolved and will be resolved over the next decade.

We investigate the physical basis of the crevasse-depth (CD) calving law by analysing relationships between glaciological stresses and calving behaviour at Sermeq Kujalleq (Store Glacier), Greenland. Our observations and model simulations show that the glacier has a stable position defined by a compressive arch between lateral pinning points. Ice advance beyond the arch results in calving back to the stable position; conversely, if melt-undercutting forces the ice front behind the stable position, it readvances because ice velocities exceed subaqueous melt rates. This behaviour is typical of self-organising criticality, in which the stable ice-front position acts as an attractor between unstable super-critical and sub-critical regimes. This perspective provides strong support for a ‘position-law’ approach to modelling calving at Sermeq Kujalleq, because any calving ‘rate’ is simply a by-product of how quickly ice is delivered to the critical point. The CD calving law predicts ice-front position from the penetration of surface and basal crevasse fields, and accurately simulates super-critical calving back to the compressive arch and melt-driven calving into the sub-critical zone. The CD calving law reflects the glaciological controls on calving at Sermeq Kujalleq and exhibits considerable skill in simulating its mean position and seasonal fluctuations.