Numerical modeling is crucial for quantifying the evolution of cryospheric processes. At the same time, uncertainties hamper process understanding and predictive accuracy. Here, we suggest improving glacier surface mass balance simulations for the Kongsvegen glacier in Svalbard through the application of Bayesian data assimilation techniques in a set of large ensemble twin experiments. Noisy synthetic observations of albedo and snow depth, generated using the multilayer CryoGrid community model with a full energy balance, are assimilated using two ensemble-based data assimilation schemes: the particle batch smoother and the ensemble smoother. A comprehensive evaluation exercise demonstrates that the joint assimilation of albedo and snow depth improves the simulation skill by up to 86% relative to the prior in specific glacier regions. The particle batch smoother excels in representing albedo dynamics, while the ensemble smoother is marginally more effective for snow depth under low snowfall conditions in the ablation area. By combining the strengths of both observations, the joint assimilation achieves improved surface mass balance simulations across different glacier zones using either assimilation scheme. This work underscores the potential of ensemble-based data assimilation methods for refining glacier models by offering a robust framework to enhance predictive accuracy and reduce uncertainties in cryospheric simulations. Further advances in glacier data assimilation research with both synthetic and real observations will be critical to better understanding the fate and role of Arctic glaciers in a changing climate

Sea ice outflow through the Transpolar Drift (TPD) is essential in Arctic sea ice loss. Twenty-four buoys deployed in the Arctic Ocean during the summer of 2021 were used to analyse sea ice kinematics and deformation across the pack ice zone (PIZ) and marginal ice zone (MIZ), mainly focusing on the TPD region. Three stages were identified as sea ice transitions from melt to growth and to melt again. In Stage 1, sea ice exhibited active internal motion, with a high deformation rate (5.7 d−1) determined using the buoy trajectory-stretching exponents. In Stage 2, ice consolidation reduced wind response and deformation rates (2.3 d−1), but still with intermittently enhanced ice deformation over 6.0 d−1 caused by severe storms. In Stage 3, the combined impacts of a super cyclone, MIZ ice and oceanic conditions, and tidal dynamics north of Svalbard remarkably altered the ice kinematic regime. Variations in sea ice kinematics along the TPD region support the MIZ definition by the threshold of certain sea ice concentration variability. This study demonstrates how seasonal transitions, spatial heterogeneities of sea ice conditions, atmospheric or oceanic forcings, and extreme cyclones collectively shape sea ice dynamics in the TPD region, amplifying its seasonal changes relative to those in the central Arctic Ocean.

Ice sheets and glaciers flow through basal sliding and internal deformation, each governed by physical laws commonly expressed as power-law relations. These formulations include coefficients—the sliding coefficient and rate factor—whose values and units depend on the respective exponents. This dependency complicates the systematic exploration of parameter space, especially in ensemble simulations. To address this, we propose dimensionless formulations of both sliding and flow laws, in which the coefficients are of order unity and decoupled from the exponents. This separation simplifies sensitivity studies and parameter variations. The dimensionless laws are straightforward to implement in existing models; we demonstrate this with the SICOPOLIS ice-sheet model using three test simulations in an idealized set-up. These simulations illustrate that independent variation of exponents and coefficients is feasible and practical, supporting the use of dimensionless laws in efforts to better constrain ice dynamics in past and future climate scenarios.

Understanding the formation and evolution of fast flowing ice streams is essential to projecting the response of ice sheets to climate forcings. Slow-flowing or stagnant ice streams can slide at low velocities over a bed that is below the bulk melting point of ice, dissipating small amounts of frictional heat. This subtemperate sliding complicates the assumed dichotomy between frozen and thawed beds embedded in many large-scale ice sheet models. In this study, we allow for subtemperate sliding in a simple ice stream box model. This leads to non-negligible frictional heat dissipation, which can accelerate sliding and lead to runaway acceleration of stagnant ice streams. These results suggest that subtemperate sliding and the associated thermo-frictional feedback are important control on ice stream temporal variability over longer timescales characterizing Heinrich events. In addition, subtemperate sliding is likely an important physical process to consider in modeling the potential reactivation of stagnant ice streams like Kamb Ice Stream, which evolve on shorter centennial timescales.

Sequential photographic records of glacier termini go back to the mid-20th century, revealing climate-driven retreat. More recently, time-lapse imagery has enabled detailed analysis of glacier dynamics, including calving. Here, we use long-term, high-frequency (15 min) time-lapse images from May–October 2016 to manually investigate calving activity at Hansbreen, a marine-terminating glacier in Hornsund Fjord, Svalbard. We explore the spatio-temporal variability in calving frequency and styles along the glacier terminus and its relationship to environmental drivers. Average calving frequency was found to be 30 events d−1, and peaking at 60 d−1. The calving activity aligns well with air and ocean temperature at a seasonal scales, while some peaks in calving correspond to increased modeled meltwater runoff and rainfall. These links are inconsistent and reflect high complexity. Calving at Hansbreen varies greatly in time and across five delineated terminus zones. In two of the zones, over 60% of waterline events occur during ebb tide; these segments also experience regular ice cave formation. However, further efforts are needed to explore the high-frequency evolution of terminus morphology and its link to calving. Our dataset can also be used to train automated algorithms for calving detection from time-lapse imagery.

The Lunana region in Bhutan, which hosts four large glacial lakes with significant hazard potential, has undergone rapid changes over the past decade. Using PlanetScope satellite scenes, we mapped ice velocities at monthly intervals from 2017 to 2023. We reveal that the disintegration of Thorthormi Glacier’s terminus in 2022 coincided with year-on-year acceleration with mean surface velocities as high as 448 ± 10.0 m a−1 by 2021, and seasonal variability in surface velocity magnitude >144.6 ± 10.0 m a−1. This acceleration is attributed to a reduction in basal drag as the terminus reached flotation, evidenced by the calving of tabular icebergs. While Bechung, Raphstreng and Lugge exhibited a similar interannual velocity trend, the upper regions of Bechung and Raphstreng showed a higher seasonal range (31% and 19.9% from their mean) compared to Lugge (4.2%). In the upper regions, we also find a decelerating velocity trend (3.5–20.6% over the 6 years), which is attributed to surface thinning and reducing driving stresses. We show that accelerating trends in velocity can be a precursor to higher rates of retreat and rapid lake expansion, demonstrating the importance of continuous monitoring of lake-terminating glacier ice velocities in the Himalaya.

The basal thermal state of the Antarctic ice sheet (AIS)—whether the base is frozen or thawed—fundamentally underpins its flow and is an important factor in understanding its large-scale response to external forcings. Here, we present a first synthesis of the AIS basal thermal state combining two indirect and independent methods: (1) a compilation of nine three-dimensional thermomechanical simulations that calculate AIS basal temperature as part of the Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6) and (2) an estimate of the basal slip ratio, defined as the ratio of observed surface speed to deformational speed. This synthesis is evaluated against direct observations from deep boreholes and predicted flowpaths for water originating from subglacial lakes detected by altimetry and radar sounding. The synthesis predicts a thawed bed across most of West Antarctica and localized regions in East Antarctica. Most of the Antarctic Peninsula, the Transantarctic Mountains and several regions of East Antarctica are likely frozen at the bed. Overall, our synthesis suggests 46% of the AIS bed is likely thawed, 18% likely frozen and the remaining 36% is uncertain. Additional observations, particularly at the continental scale, are required to improve our understanding of Antarctica’s basal thermal state.

Our ability to accurately quantify the total ice volume in glaciers and the loss of glacier volume, discharge and freshwater in response to climate change is limited by a paucity of ice thickness and bed topography observations. Consequently, glacial ice thickness is often inferred indirectly from more easily obtained surface measurements. Here, we present a simple inversion building on the assumption of perfect plasticity. In the traditional perfect-plastic approximation, the ice thickness (or bed) can be inferred from the surface elevation and yield strength. Here, we extend this to demonstrate that, provided glaciers are changing, we can simultaneously determine the yield strength and bed topography from observations of surface elevation alone. We demonstrate that the ice thicknesses and bed topographies we infer perform comparably to other inversions documented in the Ice Thickness Models Intercomparison eXperiment. Unlike other inversions, we do not require surface mass balance or glacier velocities, which can be inaccurate and difficult to obtain. Given the increasing availability of high-resolution surface elevation data, it may be possible to apply this method to glaciers worldwide to better constrain the ice thickness, bed topography and volume of glaciers globally.

Baltic Sea ice coverage was modelled using a sea-ice thermodynamics and dynamics model coupled with a three-dimensional (3-D) PM3D hydrodynamic model. The validation for 1958–2007 showed the modelled maximum ice extents (MIEs) agree well with observations (r = 0.97) and the ice thickness less so, but satisfactory for most stations (r > 0.8). This enabled the production of cumulative ice thickness (CIT) maps and the determination of the spatial variation in sea-ice extent in the Baltic over the analysed period for four air temperature scenarios with a constant value reduction. This showed the spatial sensitivity of ice cover dynamics to temperature changes and allowed to distinct regions with different impact of change in temperature on CIT. The simulation for temperature of 2°C lower than 1958–2007 was consistent with the reconstruction of MIEs in the entire Baltic Sea for the end of the Little Ice Age (LIA) (1721–1860). For the western Baltic, the compliance was highest for temperature reduced by 3°C and 4°C. This indicates that climatic conditions may have differed between individual regions of the Baltic during the LIA, and the air temperature anomaly in the western Baltic may have been greater than indicated by previous studies

Ice cliffs and supraglacial ponds are key drivers of mass loss on debris-covered glaciers. However, the relationship between melt ponds and adjacent ice cliffs has not been fully explored. We investigated the seasonal drainage patterns of a melt pond on the debris-covered Zhuxi Glacier in southeast Tibet and estimated the mass loss of its adjacent ice cliff during 2023–24. Using hourly time-lapse photogrammetry, we built a series of high-resolution point clouds to quantify the evolution of the ice cliff-pond system. Our findings indicate that subaerial melting and undercutting were the primary mechanisms of ice cliff mass loss during summer. In winter when the pond water level dropped, ice cliff calving became the dominant mode of ice loss. As the water level rose in spring, calving and subaerial melting occurred simultaneously and ice loss from calving accounted for approximately 19.5% of total ice loss from February to July 2024. Our results reveal the transitional state of this ice cliff-pond system, exhibiting characteristics of both melt hotspots and lake-terminating calving fronts, and highlight the interplay between seasonal drainage-refill pond and differing modes of ice loss on adjacent ice cliff. Future research should focus on additional high-resolution monitoring of similar systems and incorporation of ice cliff-pond dynamics in glacier-scale numerical models.

Ice sheet mass loss is typically provided for grounded ice, because changes in floating ice are more difficult to measure and contribute minimally to sea level rise. However, gross freshwater mass flow rate across ice sheet boundaries, including floating ice, is a better metric of ice sheet health. Here, we present total mass flows across ice sheet boundaries for both the Greenlandic and Antarctic ice sheets and their peripheral glaciers from 2010 through 2019. In addition to total mass flow, we provide constituent terms and gross rather than net values, including components that combine to provide surface mass balance. Ice mass loss in Greenland is 330 $\pm$50 Gt yr-1 which is $\sim$30% larger than the 255 $\pm$40 Gt yr-1 grounded ice mass loss estimates that neglect floating ice changes. Ice mass loss in Antarctica is 450 $\pm$270 Gt yr-1 which is $\sim$2.4x the 190 $\pm$115 Gt yr-1 grounded ice mass loss estimates. Freshwater mass flow rate from Greenland is $\sim$1065 $\pm$120 Gt yr-1 or $\sim$3x mass loss ($\sim$4x grounded mass loss), and from Antarctica is $\sim$3110 $\pm$1515 Gt yr-1, or $\sim$7x mass loss ($\sim$16x grounded mass loss).

Tropical Andean glaciers provide an important flux of freshwater to communities living both in high-altitude Cordillera and population centres downstream in countries such as Peru and Bolivia. Glacier recession threatens the sustainability of these water resources, and accurate modelling of future glacier behaviour is required to manage water stress in the region. These models must capture all processes contributing significantly to overall glacier mass budgets. Here we examine supraglacial pond and ice cliff development on three clean-ice glaciers in the Cordillera Vilcanota, Peru and their overall contribution to glacier mass balance. Whilst such features are common and well-studied on debris-covered glaciers, their development on debris-free glaciers has not been examined in detail. We use high-resolution contemporary and historical satellite imagery and repeat drone surveys to examine surface structure and geometry change over three glaciers during 1977–2024. We show how cliff and pond formation is driven by aspect-dependent surface melt of crevasse walls. These features act as ice loss hotspots, which enhance glacier net mass loss by ∼10% despite accounting for <5% glacier surface area. Incorporation of such amplified ice loss processes should be a priority for glacier model advances to achieve more accurate projections of future tropical glacier recession.

Near-surface ice layers in the lower accumulation zone of Arctic glaciers and ice sheets may significantly affect deep meltwater percolation and runoff availability. This study presents a framework to assess three methods for characterizing near-surface ice layer permeability and its influence on runoff and mass balance on the Devon Ice Cap using field data. In the most effective method, ice layer permeability depends on temperature and thickness: they remain permeable above a threshold temperature (Tth = −0.15°C) and become impermeable once exceed a critical thickness (Himp = 1 m). Our modelling replicates ice layers that are typically thinner in the upper accumulation zone and thicker in the lower accumulation zone. Additionally, we simulate an observed increase in the number of ice layers in the upper accumulation zone after 2007. The evolution of thicker (>1 m) ice layers (or ice slab) in the lower accumulation zone reduces meltwater retention through refreezing, making surface mass balance (SMB) and runoff sensitive to climate changes. Simulated mean SMB ranged from −0.09 to 0.26 m w.e. a−1 from 1999 to 2022. Our model can be applied to simulate the long-term evolution of ice slab and project its impact on ice sheet runoff.