Calving laws and where to find them

Abstract. We calculate rates of ice thickness change and bottom melt for ice shelves in West Antarctica and the Antarctic Peninsula from a combination of elevation measurements from NASA–CECS Antarctic ice mapping campaigns and NASA Operation IceBridge corrected for oceanic processes from measurements and models, surface velocity measurements from synthetic aperture radar, and high-resolution outputs from regional climate models. The ice thickness change rates are calculated in a Lagrangian reference frame to reduce the effects from advection of sharp vertical features, such as cracks and crevasses, that can saturate Eulerian-derived estimates. We use our method over different ice shelves in Antarctica, which vary in terms of size, repeat coverage from airborne altimetry, and dominant processes governing their recent changes. We find that the Larsen-C Ice Shelf is close to steady state over our observation period with spatial variations in ice thickness largely due to the flux divergence of the shelf. Firn and surface processes are responsible for some short-term variability in ice thickness of the Larsen-C Ice Shelf over the time period. The Wilkins Ice Shelf is sensitive to short-timescale coastal and upper-ocean processes, and basal melt is the dominant contributor to the ice thickness change over the period. At the Pine Island Ice Shelf in the critical region near the grounding zone, we find that ice shelf thickness change rates exceed 40 m yr−1, with the change dominated by strong submarine melting. Regions near the grounding zones of the Dotson and Crosson ice shelves are decreasing in thickness at rates greater than 40 m yr−1, also due to intense basal melt. NASA–CECS Antarctic ice mapping and NASA Operation IceBridge campaigns provide validation datasets for floating ice shelves at moderately high resolution when coregistered using Lagrangian methods.

We calculate a regional surface “melt potential” index (MPI) over Antarctic ice shelves that describes the frequency (MPI-freq; %) and intensity (MPI-int; K) of daily maximum summer temperatures exceeding a melt threshold of 273.15 K. This is used to determine which ice shelves are vulnerable to melt-induced hydrofracture and is calculated using near-surface temperature output for each summer from 1979/80 to 2018/19 from two high-resolution regional atmospheric model hindcasts (using the MetUM and HIRHAM5). MPI is highest for Antarctic Peninsula ice shelves (MPI-freq 23%–35%, MPI-int 1.2–2.1 K), lowest (2%–3%, <0 K) for the Ronne–Filchner and Ross ice shelves, and around 10%–24% and 0.6–1.7 K for the other West and East Antarctic ice shelves. Hotspots of MPI are apparent over many ice shelves, and they also show a decreasing trend in MPI-freq. The regional circulation patterns associated with high MPI values over West and East Antarctic ice shelves are remarkably consistent for their respective region but tied to different large-scale climate forcings. The West Antarctic circulation resembles the central Pacific El Niño pattern with a stationary Rossby wave and a strong anticyclone over the high-latitude South Pacific. By contrast, the East Antarctic circulation comprises a zonally symmetric negative Southern Annular Mode pattern with a strong regional anticyclone on the plateau and enhanced coastal easterlies/weakened Southern Ocean westerlies. Values of MPI are 3–4 times larger for a lower temperature/melt threshold of 271.15 K used in a sensitivity test, as melting can occur at temperatures lower than 273.15 K depending on snowpack properties.

We calculate a regional surface “melt potential” index (MPI) over Antarctic ice shelves that describes the frequency (MPI-freq, %) and intensity (MPI-int, K) of daily maximum summer temperatures exceeding a melt threshold of 273.15 K. This is used to determine which ice shelves are vulnerable to melt-induced hydrofracture and is calculated using near-surface temperature output for each summer from 1979/80 to 2018/19 from two high-resolution regional atmospheric model hindcasts (using the MetUM and HIRHAM5). MPI is highest for Antarctic Peninsula ice shelves (MPI-freq 23-35%, MPI-int 1.2-2.1 K), lowest (2-3%, < 0 K) for Ronne-Filchner and Ross ice shelves, and around 10-24% and 0.6-1.7 K for the other West and East Antarctic ice shelves. Hotspots of MPI are apparent over many ice shelves, and they also show a decreasing trend in MPI-freq. The regional circulation patterns associated with high MPI values over West and East Antarctic ice shelves are remarkably consistent for their respective region but tied to different large-scale climate forcings. The West Antarctic circulation resembles the central Pacific El Niño pattern with a stationary Rossby wave and a strong anticyclone over the high-latitude South Pacific. By contrast, the East Antarctic circulation comprises a zonally symmetric negative Southern Annular Mode pattern with a strong regional anticyclone on the plateau and enhanced coastal easterlies/weakened Southern Ocean westerlies. Values of MPI are 3-4 times larger for a lower temperature/melt threshold of 271.15 K used in a sensitivity test, as melting can occur at temperatures lower than 273.15 K depending on snowpack properties.

Our ability to predict complex environmental fluid flow and transport hinges on accurate and efficient simulations of multiple physical phenomenon operating simultaneously over a wide range of spatial and temporal scales, including overbank floods, coastal storm surge events, drying and wetting bed conditions, and simultaneous bed form evolution. This research implements a fully coupled strategy for solving shallow water hydrodynamics, sediment transport, and morphological bed evolution in rivers and floodplains (PIHM_Hydro) and applies the model to field and laboratory experiments that cover a wide range of spatial and temporal scales. The model uses a standard upwind finite volume method and Roe's approximate Riemann solver for unstructured grids. A multidimensional linear reconstruction and slope limiter are implemented, achieving second‐order spatial accuracy. Model efficiency and stability are treated using an explicit‐implicit method for temporal discretization with operator splitting. Laboratory‐and field‐scale experiments were compiled where coupled processes across a range of scales were observed and where higher‐order spatial and temporal accuracy might be needed for accurate and efficient solutions. These experiments demonstrate the ability of the fully coupled strategy in capturing dynamics of field‐scale flood waves and small‐scale drying‐wetting processes.

Abstract. Basal melting at the bottom of Antarctic ice shelves is a major control on glacier dynamics, as it modulates the amount of buttressing that floating ice shelves exert onto the ice streams feeding them. Three-dimensional ocean circulation numerical models provide reliable estimates of basal melt rates but remain too computationally expensive for century-scale projections. Ice sheet modelers therefore routinely rely on simplified parameterizations based on either ice shelf depth or more sophisticated box models. However, existing parameterizations do not accurately resolve the complex spatial patterns of sub-shelf melt rates that have been observed over Antarctica's ice shelves, especially in the vicinity of the grounding line, where basal melting is one of the primary drivers of grounding line migration. In this study, we couple the Potsdam Ice-shelf Cavity mOdel (PICO, Reese et al., 2018) to a buoyant plume melt rate parameterization (Lazeroms et al., 2018) to create PICOP, a novel basal melt rate parameterization that is easy to implement in transient ice sheet numerical models and produces a melt rate field that is in excellent agreement with the spatial distribution and magnitude of observations for several ocean basins. We test PICOP on the Amundsen Sea sector of West Antarctica, Totten, and Moscow University ice shelves in East Antarctica and the Filchner-Ronne Ice Shelf and compare the results to PICO. We find that PICOP is able to reproduce inferred high melt rates beneath Pine Island, Thwaites, and Totten glaciers (on the order of 100 m yr−1) and removes the “banding” pattern observed in melt rates produced by PICO over the Filchner-Ronne Ice Shelf. PICOP resolves many of the issues contemporary basal melt rate parameterizations face and is therefore a valuable tool for those looking to make future projections of Antarctic glaciers.

Quantifying vulnerability of Antarctic ice shelves to hydrofracture using microwave scattering properties

Motivation. Ice melting in Greenland and Antarctica has increasingly contributed to rising sea levels. Yet, the exact speed of melting, existence of abrupt tipping points, and in-detail links to climate change remain uncertain. Ice shelves essentially prevent the ice sheet from slipping into the ocean and better prediction of collapses is needed. Meltwater at the surface of ice shelves indicates ice shelf collapse through destabilizing ice shelves via fracturing and flexural processes (Banwell et al., 2013) and is likely impacted by a warming climate ( Kingslake et al., 2017). Maps of meltwater have been created from in-situ and remote observations, but their low and irregular spatiotemporal resolution severely limits studies (Kingslake et al., 2019).Research Gap. In particular, there does not exist daily high-resolution (< 500m) maps of surface meltwater. We propose the first daily high-resolution surface meltwater maps by developing a deep learning-based downscaling method, called DailyMelt, that fuses observations and simulations of varying spatiotemporal resolution, as illustrated in Fig.1. The created maps will improve understanding of the origin, transport, and controlling physical processes of surface meltwater. Moreover, they will act as unified source to improve sea level rise and meltwater predictions in climate models. Data. To synthesize surface meltwater maps, we leverage observations from satellites (MODIS, Sen-1 SAR) which are high-resolution (500m, 10m), but have substantial temporal gaps due to repeat time and cloud coverage. We fuse them with simulations (MAR) and passive microwave observations (MEaSURE) that are daily, but low-resolution (6km, 3.125km). In a significant remote sensing effort, we have downloaded, reprojected, and regridded all products into daily observations for our study area over Greenland’s Helheim glacier. Approach and expected results. Within deep generative vision models, diffusion-based models promise sharp and probabilistic predictions. We have implemented SRDiff (Li H. et al., 2022) and tested it on spatially downscaling external data. As a baseline model, we have implemented a statistical downscaling model that is a local hybrid physics-linear regression model (Noel et al., 2016). In our planned benchmark, we expect a baseline UNet architecture that minimizes RMSE to create blurry maps and a generative adversarial network that minimizes adversarial loss to create sharp but deterministic maps. We have started with spatial downscaling and will include temporal downscaling. In summary, we will create the first daily high-resolution (500m) surface meltwater maps, have introduced the first diffusion-based model for downscaling Earth sciences data, and have created the first benchmark dataset for downscaling surface meltwater maps. References.Banwell, A. F., et al. (2013), Breakup of the Larsen B Ice Shelf triggered by chain reaction drainage of supraglacial lakes, Geophys. Res. Lett., 40 Kingslake J, et al. (2017), Widespread movement of meltwater onto and across Antarctic ice shelves, Nature, 544(7650)Kingslake J., et al. (2019), Antarctic Surface Hydrology and Ice Shelf Stability Workshop report, US Antarctic Program Data CenterLi H., et al. (2022), SRDiff: Single image super-resolution with diffusion probabilistic models, Neurocomputing, 479Noël, B., et al. (2016), A daily, 1 km resolution data set of downscaled Greenland ice sheet surface mass balance (1958–2015), The Cryosphere,

<p>Water-saturated firn layers, or firn aquifers, have recently been identified from satellite microwave image time-series data in some western Antarctic Peninsula ice shelves. Subsequent field work on the Wilkins Ice Shelf (this work) and Müller Ice Shelf (MacDonell et al., 2021) has proven the existence of these perennial ice shelf firn aquifers. Most of the aquifer areas are maintained by seasonal meltwater recharge. Brine aquifers have been known for many decades in certain slow-moving ice shelves with shorter or absent melt seasons (e.g., McMurdo Ice Shelf). We present both satellite evidence of meltwater firn aquifers in several areas of the Antarctic Peninsula, and radar profile evidence consistent with extensive brine infiltration in the Abbott, Nickerson, and Shackleton Ice Shelves. These latter ice shelves had been previously identified as likely sites of widespread brine infiltration (Cook et al., 2018).</p><p>The hydrofracture-driven disintegration of the Wilkins Ice Shelf in February-March of 2008, and subsequent rapid calving events extending into the winter season, justify a closer look at the relative potential for fresh-water aquifers, brine aquifers, and surface melt ponds for inducing hydrofracture in ice shelves. The destructive impact of surface or near-surface meltwater on floating ice is now well-established and is implicated in the loss of the Larsen A and Larsen B ice shelves, and rapid late-stage disintegrations of several tabular icebergs. Brine-aquifer-induced disintegration was suspected for the Wilkins breakup (Scambos et al., 2009), but now appears to be related to the effects of a freshwater system. A question remains regarding the vulnerability of ice shelves with significant brine infiltration in an aquifer.</p><p>We will present field measurements from the Wilkins Ice Shelf and discuss the relative hydrofracturing potential of fresh water and brines under various scenarios pertinent to ice shelf stability. The potential for future expansion of fresh-water aquifers under warming coastal conditions, and the characteristics of a hypothetical transitioning from a cold brine aquifer to a fresh-water aquifer will be discussed.</p><p> </p><p>Cook, S., Galton-Fenzi, B.K., Ligtenberg, S.R. and Coleman, R., 2018. Brief communication: widespread potential for seawater infiltration on Antarctic ice shelves. <em>The Cryosphere</em>, <em>12</em>(12), 3853-3859, doi: 10.5194/tc-12-3853-2018.</p><p>MacDonell, S., Fernandoy, F., Villar, P. and Hammann, A., 2021. Stratigraphic analysis of firn cores from an antarctic ice shelf firn aquifer. Water, 13(5), 731, doi:10.3390/w13050731.</p><p>Scambos, T., Fricker, H.A., Liu, C.C., Bohlander, J., Fastook, J., Sargent, A., Massom, R. and Wu, A.M., 2009. Ice shelf disintegration by plate bending and hydro-fracture: Satellite observations and model results of the 2008 Wilkins ice shelf break-ups. Earth and Planetary Science Letters, 280(1-4), 51-60, doi:10.1016/j.epsl.2008.12.027.</p>

Conserving biodiversity in the face of expanding human degradation of ecosystems is facilitated by understanding the natural state of communities prior to the impact of anthropogenic disruptions. Reconstructing communities and ecosystems as they existed in the past requires data from the fossil record on their species composition, richness, and abundance. Fossil data are potentially different from data collected from living communities in their spatial, temporal, and taxonomic scales and these differences must be understood so that accurate comparisons can be made between past and present states of living communities. Fifty-four long-term ecological studies of a wide range of taxon groups (mammals, invertebrates, plants, corals) and habitat types (marine, terrestrial, freshwater) were surveyed from the published ecological literature to determine the range of spatial, temporal and taxonomic scales at which data are commonly collected in ecological research. Long-term ecological studies encompass spatial scales from 50m2 to 100,000km2 and temporal scales from 5 to 100 years. Most studies resolve taxa to the species level and count individuals, although plant and coral studies sometimes quantify species by percent cover. All taxon groups and habitat types were studied across a wide range of spatial and temporal scales. Whether or not data from fossils can be collected and analysed at scales comparable to data from living communities depends on the type of organism, as well as the taphonomic circumstances of preservation, accumulation and deposition. Marine invertebrates can be sampled at comparable spatial and taxonomic scales to living invertebrates, but time averaging degrades the temporal resolution of the fossil deposits. Vertebrate fossils provide data at comparable taxonomic scales with some reduction in spatial and temporal resolution relative to live data. Plant fossils and pollen are capable of being sampled at temporal resolutions comparable to modern ecological studies, but pollen data are prone to spatial averaging and have much poorer taxonomic resolution than censuses of living communities. It is important to be mindful of the limitations that scale mismatches produce in the ability to use fossil data to resolve ecological events and to compare the details of ecological composition and structure between the present and the past.

Ice shelves are relatively thick ice masses that are afloat but attached to coastal land rather than adrift. They form by the seaward extension of glaciers or ice sheets or by build up of multiyear landfast sea ice. They thicken further by surface accumulation of snow and superimposed ice and by accretion of ice from water beneath. Composite ice shelves are composed of sea ice and glacier ice. Glacier tongues are floating ice margins that are narrow relative to their length. Ice shelves comprise 55% or 18,000 km of the Antarctic coast. ‘Classical’ Antarctic ice shelves are fed from glaciers or ice streams and are dynamically part of the parent ice sheet; the largest, the Ross and Ronne, are 105 km2 and hundreds of metres thick. Where they ground on isolated bedrock peaks, ‘ice rises’ are formed. Arctic ice shelves are restricted to several archipelagos fringing the Arctic Ocean and to a few Greenland fjords. The Ward Hunt Ice Shelf is the largest at about 400 km2. Arctic and Antarctic ice shelves have expanded and contracted during the Holocene. The Ellesmere Ice Shelf developed about 5500 years ago in response to Holocene cooling. In the warmer Twentieth century, calving events have broken this continuous ice-shelf into several remnants. Floating glacier tongues of the Greenland Ice Sheet have also broken up recently. The entire Arctic Ocean may have been covered by a huge ice shelf during the coldest Late Cenozoic glacial periods. Large, often tabular icebergs calve from ice shelves. Ice islands are a form of tabular iceberg in the Arctic Ocean which have a characteristic undulating surface. Icebergs drift mainly under the influence of currents and Arctic Ocean ice islands have been used occasionally as research stations.

Abstract. Ice shelf dynamics and morphology play an important role in the stability of floating bodies of ice by driving fracturing that can lead to calving, in turn impacting the ability of the ice shelf to buttress upstream grounded ice. Following a 2016 calving event at the Nansen Ice Shelf (NIS), East Antarctica, we collected airborne and ground-based radar data to map ice thickness across the shelf. We combine these data with published satellite-derived data to examine the spatial variations in ice shelf draft, the cause and effects of ice shelf strain rates, and the possibility that a suture zone may be channelizing ocean water and altering patterns of sub-ice-shelf melt and freeze-on. We also use our datasets to assess limitations that may arise from relying on hydrostatic-balance equations applied to ice surface elevation to determine ice draft morphology. We find that the Nansen Ice Shelf has a highly variable basal morphology driven primarily by the formation of basal fractures near the onset of the ice shelf suture zone. This morphology is reflected in the ice shelf strain rates but not in the calculated hydrostatic-balance thickness, which underestimates the scale of variability at the ice shelf base. Enhanced melt rates near the ice shelf terminus and in steep regions of the channelized suture zone, along with relatively thin ice in the suture zone, appear to represent vulnerable areas in the NIS. This morphology, combined with ice dynamics, induce strain that has led to the formation of transverse fractures within the suture zone, resulting in large-scale calving events. Similar transverse fractures at other Antarctic ice shelves may also be driven by highly variable morphology, and predicting their formation and evolution could aid projections of ice shelf stability.

Surface meltwater on Antarctic ice shelves is comprised of slush (saturated firn), and ponded water (lakes and streams). Often, slush forms as a precursor to ponded water, and its formation leads subsequently to water collecting in basins or flowing across ice shelf surfaces. Where slush and/or ponded water refreeze at the end of a melt season, the firn air content of ice shelves may be lowered. This can increase ice shelves’ susceptibility to future meltwater ponding, making them more vulnerable to potential hydrofracture and break-up. Slush and ponded water also have a lower albedo than snow or dry firn, further increasing ice-shelf surface melt under warmer climates. To date, most satellite-derived estimates of surface water on ice shelves have identified only ponded water, potentially underestimating the extent of surface meltwater. Here, we use a previously developed random forest classifier to produce a novel, continent-wide dataset of slush and ponded water extent across all Antarctic ice shelves between 2013 and 2021. Our dataset is comprised of monthly meltwater products for the austral summers (November-March where data availability allows), from which continent-wide, regional, and individual ice-shelf trends are investigated.The continent-wide total meltwater coverage (assessed between November and February) was greatest during January 2017, reaching 6078 km2. Notably, we find that including the slush extent in total meltwater calculations increases surface water extent by a mean of 56% during the melt-season peak (January). However, we identify marked inter-regional variation, with slush accounting for 71% of January’s total surface meltwater extent in Dronning Maud Land, but only 46% in the Antarctic Peninsula. This indicates that until now, the extent of surface meltwater across Antarctica’s ice shelves has been largely under-estimated on ice shelf, regional, and continent-wide scales, which has significant repercussions for calculations of the surface and sub-surface energy and mass balance of ice shelves, the long-term storage of meltwater on ice shelves, and predictions of future ice shelf stability.

West Antarctic ice shelves in the Amundsen and Bellingshausen seas are losing mass from basal melting. This is caused by the intrusion of circumpolar deep water underneath the ice shelves. The mechanisms driving circumpolar deep water towards the Western Antarctic ice shelves are still not fully understood. I will present time series (for the years from 2003 to 2023) of basal melt rate for the ice shelves in Amundsen and Bellingshausen seas along with time series for the upper end of Antarctic bottom water layer from underneath ice shelves. From these two data sets, we want to quantify the mass loss from the ice shelves and see how the freshwater flux that results from it will affect the volume of Antarctic bottom water. Our work requires estimating ice shelf height change from 2003 to 2023 in two different reference frames: Lagrangian and Eulerian from differential equations of height change where we can incorporate satellite observations of ice elevation. From the differential equation of Lagrangian reference frame, we determine the characteristics of ice flow of the ice shelf. This will help us to highlight the contribution of basal melt rate to the ice shelf height change. The height change from Eulerian reference frame will be used in a formula that produces a time series of basal melt rate. This time series will show the differences between the basal melt rate in the Eulerian reference frame and the steady state in the Lagrangian frame. For the Lagrangian ice shelf height where the ice shelf height is equal to ice elevation data from satellite altimetry minus ocean height data, we will create a unique data set of ocean surface height containing data produced by Southern Ocean State Estimate (SOSE) general circulation model where the data shows the distribution of water properties to a high depth.  This can be used to evaluate the ocean heat content. With this dataset, we aim to determine how many times the circumpolar deep water travels from the Antarctic circumpolar current to the ice shelves in the Amundsen and Bellingshausen seas (in the years between 2003 and 2023). We also want to ascertain whether changes in the distribution of water layers beneath the ice shelves contribute to propelling the circumpolar deep water closer to the surface. This could potentially aid in the improvement of ice shelf models by incorporating a function that represents the effect of ocean thermal forcing on ice shelves.                        

<p>Mapping the ice bed interface with radar is challenging in many alpine glaciers where the ice is temperate, and in-ice absorption is high. It is also difficult in selected regions of polar ice sheets such as near grounding zones and in ice streams where clutter and rough beds increase incoherent volume scattering. The lack of information for the ice geometry impedes our process understanding, e.g., basal sliding (requires knowledge about the basal roughness) and the routing of subglacial water flow (requires knowledge on basal smoothness). The lack of observations to constrain variations in ice thickness on the sub-kilometre scale is thus still a bottleneck to confidently predict ice dynamics and expected rates of sea-level rise.</p><p>A recent development in radioglaciology, namely the application of phase-coherent polarimetric radar, provides an excellent opportunity to overcome these limitations. Radar polarimetry has made significant strides in the last few years to constrain internal ice structure and their impact on the deformation of ice sheets, including the reconstruction of ice micro-structure parameters previously obtained from ice cores. Here, we suggest that the ice-bed interface can be identified in characteristic patterns of the polarimetric coherence phase. This new metric provides information in areas where the backscattered power amplitude does not show any signatures of the ice-bed interface. We provide examples for this across a wide range of glaciological settings, including cold (Colle Gnifetti, Switzerland) and temperate (Hintereisferner, Austria) alpine glaciers, thin grounding zones (Ekström Ice Shelf, East Antarctica) and thick ice domes (Dome C, East Antarctica). If this holds, then the ice thickness mapping in challenging glaciological settings should preferably be done using a quad-polarimetric acquisition geometry. For ground-based surveys, this can be done using an autonomous ice rover, for which we provide a proof-of-concept study on the Ekström Ice Shelf in Antarctica.</p>

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.