Explore the vast range of titles available.

Understanding ice sheet behaviour in the geological past is essential for evaluating the role of the cryosphere in the climate system and for projecting rates and magnitudes of sea level rise in future warming scenarios 1 – 4 . Although both geological data 5 – 7 and ice sheet models 3 , 8 indicate that marine-based sectors of the East Antarctic Ice Sheet were unstable during Pliocene warm intervals, the ice sheet dynamics during late Pleistocene interglacial intervals are highly uncertain 3 , 9 , 10 . Here we provide evidence from marine sedimentological and geochemical records for ice margin retreat or thinning in the vicinity of the Wilkes Subglacial Basin of East Antarctica during warm late Pleistocene interglacial intervals. The most extreme changes in sediment provenance, recording changes in the locus of glacial erosion, occurred during marine isotope stages 5, 9, and 11, when Antarctic air temperatures 11 were at least two degrees Celsius warmer than pre-industrial temperatures for 2,500 years or more. Hence, our study indicates a close link between extended Antarctic warmth and ice loss from the Wilkes Subglacial Basin, providing ice-proximal data to support a contribution to sea level from a reduced East Antarctic Ice Sheet during warm interglacial intervals. While the behaviour of other regions of the East Antarctic Ice Sheet remains to be assessed, it appears that modest future warming may be sufficient to cause ice loss from the Wilkes Subglacial Basin. Studies of an Antarctic marine sediment core suggest that the East Antarctic Ice Sheet retreated in the vicinity of the Wilkes Subglacial Basin during extended warm periods of the late Pleistocene, when temperatures were similar to those predicted to occur within this century.

Proglacial lakes along the retreating margin of the Laurentide ice sheet (LIS) significantly influenced the ice sheet's dynamics. This study investigates the interaction between proglacial lake drainage events and ice sheet evolution during deglaciation. Using a flowline ice sheet model, we demonstrate that abrupt lake drainage caused by the opening of spillways during the retreat of the ice sheet can temporarily reverse ice retreat and trigger rapid grounding line advance despite ongoing climate warming. We also show that ice shelf regrounding on a retrograde lake bed can follow lake drainage and further amplify ice sheet advance. These processes can decouple ice dynamics from climate forcing, offering a non‐climatic mechanism to explain the observed highly irregular ice margin fluctuations of the LIS. Our findings suggest that proglacial lakes might play an important role in modulating ice sheet evolution in warming climates. Plain Language Summary When the Laurentide ice sheet (LIS) retreated approximately 20,000 years ago, large lakes formed at its southern margin with the ice sheet blocking some of the lake outlets. These were opened during retreat, leading to sudden drainage that rapidly lowered the lake level. We examine how the sudden drainage of these lakes influenced ice sheet dynamics. Using a numerical ice sheet model, we show that abrupt lake drainage events could temporarily halt or even reverse ice retreat, leading to rapid ice margin advance despite ongoing climate warming. Additionally, if ice shelves existed in the proglacial lake, they could become regrounded in the aftermath of lake drainage, amplifying ice sheet advance. These findings suggest that ice sheet extent was not solely driven by climate forcing, and that proglacial lake drainage may explain the irregular retreat patterns of the southern margin of the LIS. Key Points Rapid proglacial lake drainage can temporarily reverse ice sheet retreat and drive grounding line advance in a warming climate Ice shelf regrounding after lake drainage amplifies ice sheet advance and stabilizes grounding lines Proglacial lake interactions explain the irregular ice retreat observed during the deglaciation of the Laurentide ice sheet

Warm water of open ocean origin on the continental shelf of the Amundsen and Bellingshausen Seas causes the highest basal melt rates reported for Antarctic ice shelves with severe consequences for the ice shelf/ice sheet dynamics. Ice shelves fringing the broad continental shelf in theWeddell and Ross Seas melt at rates orders ofmagnitude smaller. However, simulations using coupled ice–ocean models forced with the atmospheric output of the HadCM3 SRES-A1B scenario run (CO₂ concentration in the atmosphere reaches 700 ppmv by the year 2100 and stays at that level for an additional 100 years) show that the circulation in the southern Weddell Sea changes during the twenty-first century. Derivatives of Circumpolar Deep Water are directed southward underneath the Filchner–Ronne Ice Shelf, warming the cavity and dramatically increasing basal melting. To find out whether the open ocean will always continue to power the melting, the authors extend their simulations, applying twentieth-century atmospheric forcing, both alone and together with prescribed basal mass flux at the end of (or during) the SRES-A1B scenario run. The results identify a tipping point in the southern Weddell Sea: once warm water flushes the ice shelf cavity a positive meltwater feedback enhances the shelf circulation and the onshore transport of open ocean heat. The process is irreversible with a recurrence to twentieth-century atmospheric forcing and can only be halted through prescribing a return to twentieth-century basal melt rates. This finding might have strong implications for the stability of the Antarctic ice sheet.

Constraining past West Antarctic Ice Sheet (WAIS) change helps validate numerical models simulating future ice sheet dynamics. Following rapid deglaciation during the mid‐Holocene, ice near Thwaites Glacier was ∼35 m thinner than present; however, the timing of ice regrowth to its present configuration remains unknown. To fill this knowledge gap, we present cosmogenic nuclide exposure ages of cobbles from the surface of a moraine situated between Thwaites and Pope glaciers. We infer that the moraine formed and stabilized in the Late Holocene (∼1.4 ka) when a small glacier thickened. We also present a novel reconstruction of WAIS volume constrained by sea‐level data, which demonstrates that moraine formation coincided with a large‐scale WAIS readvance. Our new geologic constraints will help inform models of the solid Earth response to surface mass loading, improving our understanding of ice sheet dynamics in a vulnerable part of WAIS. Plain Language Summary The Antarctic ice sheets are melting and adding to sea‐level rise, with the rate at which they lose mass expected to increase in the coming decades to centuries. However, recent studies have shown that, only a few thousand years ago, the Antarctic ice sheets were smaller than they are now, and subsequently regrew to their present size. Understanding how the Antarctic ice sheets regrew to their present size, as well as the rate it happened, helps us understand whether ongoing loss of ice sheet mass is reversible. In this study, we identify a glacial landform (a moraine) that was deposited as an Antarctic glacier grew in size after the Antarctic ice sheets were smaller than they are today. We collected rocks from the surface of the glacial landform and measured isotopes in them. These isotopes tell us the landform was created by the glacier about 1,400 years ago. The new information from this study on the past of the Antarctic ice sheets can be used to improve our understanding of how the Antarctic ice sheets will change, and add to sea‐level rise, in the future. Key Points Direct evidence is found for a Late Holocene episode of glacier thickening in the Amundsen Sea Embayment, West Antarctica Ice thickening is evidenced by a moraine between Thwaites and Pope glaciers and is dated with surface exposure dating of erratic cobbles The moraine may help to constrain the end of a mid‐Holocene contracted ice sheet configuration in the Amundsen Sea sector

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.

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.

Supraglacial lakes can have considerable impact on ice sheet mass balance and global sea-level-rise through ice shelf fracturing and subsequent glacier speedup. In Antarctica, the distribution and temporal development of supraglacial lakes as well as their potential contribution to increased ice mass loss remains largely unknown, requiring a detailed mapping of the Antarctic surface hydrological network. In this study, we employ a Machine Learning algorithm trained on Sentinel-2 and auxiliary TanDEM-X topographic data for automated mapping of Antarctic supraglacial lakes. To ensure the spatio-temporal transferability of our method, a Random Forest was trained on 14 training regions and applied over eight spatially independent test regions distributed across the whole Antarctic continent. In addition, we employed our workflow for large-scale application over Amery Ice Shelf where we calculated interannual supraglacial lake dynamics between 2017 and 2020 at full ice shelf coverage. To validate our supraglacial lake detection algorithm, we randomly created point samples over our classification results and compared them to Sentinel-2 imagery. The point comparisons were evaluated using a confusion matrix for calculation of selected accuracy metrics. Our analysis revealed wide-spread supraglacial lake occurrence in all three Antarctic regions. For the first time, we identified supraglacial meltwater features on Abbott, Hull and Cosgrove Ice Shelves in West Antarctica as well as for the entire Amery Ice Shelf for years 2017–2020. Over Amery Ice Shelf, maximum lake extent varied strongly between the years with the 2019 melt season characterized by the largest areal coverage of supraglacial lakes (~763 km2). The accuracy assessment over the test regions revealed an average Kappa coefficient of 0.86 where the largest value of Kappa reached 0.98 over George VI Ice Shelf. Future developments will involve the generation of circum-Antarctic supraglacial lake mapping products as well as their use for further methodological developments using Sentinel-1 SAR data in order to characterize intraannual supraglacial meltwater dynamics also during polar night and independent of meteorological conditions. In summary, the implementation of the Random Forest classifier enabled the development of the first automated mapping method applied to Sentinel-2 data distributed across all three Antarctic regions.

Iceberg calving is a major source of ice loss from the Antarctic and Greenland ice sheets. However, it is still one of the most poorly understood aspects of ice sheet dynamics, in part due to its variability at a wide range of spatial and temporal scales. Despite this variability, most current large-scale ice sheet models assume that calving can be represented as a deterministic flux. In this study, we describe an approach to modeling calving as a stochastic process, using a one-dimensional depth-integrated marine-terminating glacier model as a demonstration. We show that for glaciers where calving occurs more frequently than the typical model time steps (days-months), stochastic calving schemes sampling a binomial distribution accurately simulate the probabilistic distribution of glacier state. We also find that incorporating stochastic calving into simulations of a glacier with a buttressing ice shelf changes the simulated mean glacier state, due to nonlinearities in ice shelf dynamics. Relatedly, we find that changes in calving frequency, without changes in the mean calving flux, can cause ice shelf retreat. This new stochastic approach can be implemented in large-scale ice sheet models, which should improve our capability to quantify uncertainty in predictions of future ice sheet change.

Geothermal heat flow (GHF) is a key basal boundary condition for Antarctic ice‐sheet flow. Large‐scale variations are resolved by several recent models but knowledge of the smaller‐scale variations, crucial for ice sheet dynamics, is limited by unresolved variations in crustal radiogenic heat production. To define this at continent‐scale we use 3D gravity inversion constrained by seismic Moho estimates to identify variations in crustal composition and geometry beneath thick ice. Geochemically‐defined empirical relationships between density and heat production capture the global average trend and its variability, and allow to estimate from upper‐crust density spatial variations in radiogenic heat production. Significant variations are observed typically 1.2–1.6 μW/m3, and as high as 2 μW/m3 in West Antarctica. The contribution to GHF from these heat‐production variations is similarly variable, typically 16–24 mW/m2 and up to 60 mW/m2. The mapped variations are significant for correctly representing GHF in Antarctica. Plain Language Summary Antarctica's crustal structure ‐ including sedimentary basins, the igneous and metamorphic crust, and the interface between the crust and mantle ‐ dictates the delivery of heat from depth to the ice sheet's base, with capacity to influence ice sheet flow. Crustal structure is not well‐understood due to the extensive and thick ice cover combined with limited geophysical observations. We investigate the variations in crustal geometry and density, by examining anomalies in the Earth's gravity field and using independent depth constraints from seismic studies. Our findings indicate substantial variations in heat production characterized by heterogeneous crustal structure, influencing the heating of the ice sheet's base to a significant degree. Key Points A new Antarctic crustal model is derived by seismic‐constrained gravity inversion Variations in crustal radiogenic heat production are inferred from upper‐crust density and geochemical data The potential impact of heterogeneity in crustal heat production for geothermal heat flow is quantified

We address the numerical solution of infinite-dimensional inverse problems in the framework of Bayesian inference. In Part I of this paper [T. Bui-Thanh, O. Ghattas, J. Martin, and G. Stadler, SIAM J. Sci. Comput. , 35 (2013), pp. A2494--A2523] we considered the linearized infinite-dimensional inverse problem. In Part II, we relax the linearization assumption and consider the fully nonlinear infinite-dimensional inverse problem using a Markov chain Monte Carlo (MCMC) sampling method. To address the challenges of sampling high-dimensional probability density functions (pdfs) arising upon discretization of Bayesian inverse problems governed by PDEs, we build upon the stochastic Newton MCMC method. This method exploits problem structure by taking as a proposal density a local Gaussian approximation of the posterior pdf, whose covariance operator is given by the inverse of the local Hessian of the negative log posterior pdf. The construction of the covariance is made tractable by invoking a low-rank approximation of the data misfit component of the Hessian. Here we introduce an approximation of the stochastic Newton proposal in which we compute the low-rank-based Hessian at just the maximum a posteriori (MAP) point, and then reuse this Hessian at each MCMC step. We compare the performance of the proposed method to the original stochastic Newton MCMC method and to an independence sampler. The comparison of the three methods is conducted on a synthetic ice sheet inverse problem. For this problem, the stochastic Newton MCMC method with a MAP-based Hessian converges at least as rapidly as the original stochastic Newton MCMC method, but is far cheaper since it avoids recomputing the Hessian at each step. On the other hand, it is more expensive per sample than the independence sampler; however, its convergence is significantly more rapid, and thus overall it is much cheaper. Finally, we present extensive analysis and interpretation of the posterior distribution and classify directions in parameter space based on the extent to which they are informed by the prior or the observations.