Ice Sheet Changes Research Articles

Limited sensitivity of Antarctic GIA mass change estimates to lateral viscosity variations

The Gravity Recovery and Climate Experiment (GRACE) has revealed spatiotemporal mass changes in the Antarctic Ice Sheet. However, GRACE data must be corrected for the gravity changes due to glacial isostatic adjustment (GIA). Here we investigate the sensitivity of GIA-induced gravity changes in Antarctica to the lithospheric thickness and upper mantle viscosity using a one-dimensional (1-D) model that assumes a radially varying Earth structure. The sensitivity is assessed using several Antarctic ice-history models that have been widely used to correct GRACE data. The results indicate a trade-off between lithospheric thickness and upper mantle viscosity in evaluating the Antarctic GIA correction. This trade-off exists for all ice-history models; however, the reason for the trade-off differs among models. Furthermore, since there is a sharp contrast in the Earth structure between West and East Antarctica, the adopted ice histories are separated into West and East Antarctic components to examine their contributions to the Antarctic GIA correction. We consider 1-D Earth structures that are averaged from the seismically derived three-dimensional Earth structure for West and East Antarctica. These results indicate that the contributions of the East and West Antarctic loads do not significantly affect the GIA corrections for the West and East Antarctic regions, respectively, and that the trade-off between lithospheric thickness and upper mantle viscosity results in minimal divergence in the assessment of the Antarctic GIA correction between the averaged Earth models of West and East Antarctica. Therefore, the contrast in Earth structure beneath Antarctica may have a limited effect on the ice-mass change estimates for the entire Antarctic Ice Sheet.

A comparative study of mass balance in the Lambert Glacier and Amery Ice Shelf along the Chinese inland traverse during 2019–2023 using altimetry, gravity, and in-situ observations

Abstract. The new photon-counting laser altimetry satellite ICESat-2 was successfully launched on September 15, 2018 with an unprecedented ice surface elevation measurement accuracy of 2–4 cm. It is meaningful for accurately estimating volumetric changes in the Antarctic Ice Sheet. Cross-validation of different types of data, especially comparison with in-situ data, is important for the ice sheet mass balance results of a new satellite. This paper proposes an elevation-difference method of grid elevation change rate model, based on ICESat-2 ATL11 elevation time series data, to distinguish the linear change trend of ice sheet surface elevation from the elevation change resulting from periodic precipitation. In order to compare the estimated elevation-change rate with the flux computed from in-situ snow stake velocity measurements and GRACE-FO gravity survey data, we made corrections for firn air content, elastic, and glacier isostatic adjustment. Based on the ATL11 data from 2019 to 2023, our results show that the ice sheet change in Basin 11 along the CHINARE traverse is from ~0.019 m yr−1 to ~0.121 m yr−1, and the mass balance in the upstream of the traverse in Basin 11 is ~1.9±0.2 Gt yr−1. It is comparable to ~2.4±1.2 Gt yr−1 from GRACE-FO during the same time period. Furthermore, the flux across traverse-11.9±1.1 Gt yr−1 is comparable to that of ~-9.7±0.9 Gt yr−1 across the same flux gate during 1997–2009 which is calculated based on GNSS-derived ice velocity observations, considering the time period difference and uncertainties.

Orbital forcing of aridity/humidity fluctuations in the Hexi Corridor and Alxa Plateau, NW China, during the last 1.8 million years

The processes controlling aridity/humidity variations in mid-latitude Asia dust source regions on orbital timescales are unclear. Both insolation and polar ice sheets can affect the climate at mid-latitudes, but determining their individual contributions is challenging. Here we present a compilation of paleoclimate data (the content of smectite and aeolian sand, and the proportion of the hydraulic component) from sites in the Hexi Corridor and Alxa Plateau, located at the junction of the regions influenced by the mid-latitude East Asian Monsoon (EAM) and the Westerlies. Our objectives were to determine the links between aridity/humidity variations in this region and Earth orbital forcing and changes in high-latitude ice sheets. We found that the aridity/humidity variations in the Hexi Corridor and Alxa Plateau were paced mainly by the long eccentricity cycle (∼400 ka), facilitating roughly coherent beat in changes of the mass accumulation rate (MAR) and the excursions of thick/coarse loess layers L1, L5, L9, L15 and L24 on the Chinese Loess Plateau. Additionally, spectral analysis of the paleoclimate data revealed that the expression of the Mid-Pleistocene transition (MPT) was spatially variable. A transition between the dominant periodicities in the western Hexi Corridor and Alxa Plateau occurred at ∼0.9 Ma, but in the eastern Alxa Plateau this transition occurred at ∼0.7 Ma. Our data compilation also demonstrates an interval of wet climate in the Hexi Corridor and Alxa Plateau after ∼0.34 Ma, which is consistent with paleoclimate records from loess deposits in Westerlies-dominated Central Asia. We attribute this to insolation forcing via complex internal feedbacks in the Southern Ocean, involving the enhancement of the Atlantic deep-water circulation, and the interplay between Indian Monsoon and the Westerlies. Overall, our findings highlight the importance of the Earth orbital control of aridity/humidity cycles on the boundary of the EAM and the Westerlies in mid-latitude Asia.

A marine record of Patagonian ice sheet changes over the past 140,000 years

Terrestrial glacial records from the Patagonian Andes and New Zealand Alps document quasi-synchronous Southern Hemisphere-wide glacier advances during the late Quaternary. However, these records are inherently incomplete. Here, we provide a continuous marine record of western-central Patagonian ice sheet (PIS) extent over a complete glacial-interglacial cycle back into the penultimate glacial (~140 ka). Sediment core MR16-09 PC03, located at 46°S and ~150 km offshore Chile, received high terrestrial sediment and meltwater input when the central PIS extended westward. We use biomarkers, foraminiferal oxygen isotopes, and major elemental data to reconstruct terrestrial sediment and freshwater input related to PIS variations. Our sediment record documents three intervals of general PIS marginal fluctuations, during Marine Isotope Stage (MIS) 6 (140 to 135 ka), MIS 4 (~70 to 60 ka), and late MIS 3 to MIS 2 (~40 to 18 ka). These higher terrigenous input intervals occurred during sea-level low stands, when the western PIS covered most of the Chilean fjords, which today retain glaciofluvial sediments. During these intervals, high-amplitude phases of enhanced sediment supply occur at millennial timescales, reflecting increased ice discharge most likely due to a growing PIS. We assign the late MIS 3 to MIS 2 phases and, by inference, older advances to Antarctic cold stages. We conclude that the increased sediment/meltwater release during Southern Hemisphere millennial-scale cold phases was likely related to higher precipitation caused by enhanced westerly winds at the northwestern margin of the PIS. Our records complement terrestrial archives and provide evidence for PIS climate sensitivity.

Mechanism Design and Motion Planning of a Hexapod Curling Robot Exhibited During the Beijing 2022 Winter Olympics Games

When a curling rock slides on an ice sheet with an initial rotation, a lateral movement occurs, which is known as the curling phenomenon. The force of friction between the curling rock and the ice sheet changes continually with changes in the environment; thus, the sport of curling requires great skill and experience. The throwing of the curling rock is a great challenge in robot design and control, and existing curling robots usually adopt a combination scheme of a wheel chassis and gripper that differs significantly from human throwing movements. A hexapod curling robot that imitates human kicking, sliding, pushing, and curling rock rotating was designed and manufactured by our group, and completed a perfect show during the Beijing 2022 Winter Olympics Games. Smooth switching between the walking and throwing tasks is realized by the robot’s morphology transformation based on leg configuration switching. The robot’s controlling parameters, which include the kicking velocity vk, pushing velocity vp, orientation angle θc, and rotation velocity ω, are determined by aiming and sliding models according to the estimated equivalent friction coefficient μequ and ratio e of the front and back frictions. The stable errors between the target and actual stopping points converge to 0.2 and 1.105 m in the simulations and experiments, respectively, and the error shown in the experiments is close to that of a well-trained wheelchair curling athlete. This robot holds promise for helping ice-makers rectify ice sheet friction or assisting in athlete training.

Local forcing mechanisms challenge parameterizations of ocean thermal forcing for Greenland tidewater glaciers

Abstract. Frontal ablation has caused 32 %–66 % of Greenland Ice Sheet mass loss since 1972, and despite its importance in driving terminus change, ocean thermal forcing remains crudely incorporated into large-scale ice sheet models. In Greenland, local fjord-scale processes modify the magnitude of thermal forcing at the ice–ocean boundary but are too small scale to be resolved in current global climate models. For example, simulations used in the Ice Sheet Intercomparison Project for CMIP6 (ISMIP6) to predict future ice sheet change rely on the extrapolation of regional ocean water properties into fjords to drive terminus ablation. However, the accuracy of this approach has not previously been tested due to the scarcity of observations in Greenland fjords, as well as the inability of fjord-scale models to realistically incorporate icebergs. By employing the recently developed IceBerg package within the Massachusetts Institute of Technology general circulation model (MITgcm), we here evaluate the ability of ocean thermal forcing parameterizations to predict thermal forcing at tidewater glacier termini. This is accomplished through sensitivity experiments using a set of idealized Greenland fjords, each forced with equivalent ocean boundary conditions but with varying tidal amplitudes, subglacial discharge, iceberg coverage, and bathymetry. Our results indicate that the bathymetric obstruction of external water is the primary control on near-glacier thermal forcing, followed by iceberg submarine melting. Despite identical ocean boundary conditions, we find that the simulated fjord processes can modify grounding line thermal forcing by as much as 3 °C, the magnitude of which is largely controlled by the relative depth of bathymetric sills to the Polar Water–Atlantic Water thermocline. However, using a common adjustment for fjord bathymetry we can still predict grounding line thermal forcing within 0.2 °C in our simulations. Finally, we introduce new parameterizations that additionally account for iceberg-driven cooling that can accurately predict interior fjord thermal forcing profiles both in iceberg-laden simulations and in observations from Kangiata Sullua (Ilulissat Icefjord).

Holocene southwest Greenland ice sheet behavior constrained by sea-level modeling

The melting of the Greenland ice sheet (GrIS) is a major contributor to past and future global sea-level rise. Understanding the response of the GrIS to times in the past when temperatures were as warm or warmer than today offers insights into its current and future response to climate change. In the southwest sector, the GrIS retreated inland beyond its current margin during the (at least regionally) warmer-than-present mid-Holocene, before it readvanced to the historical maximum position during the Little Ice Age. This was then followed by a slight retreat to its current position. To investigate the timing and magnitude of southwest GrIS retreat and readvance in response to Holocene warmth, we model the response of the solid Earth and local relative sea level (RSL) to past ice sheet change. We test a suite of eleven ice sheet scenarios that are based on ICE-6G_C but are modified in the timing and magnitude of ice retreat and readvance and pair them with four different viscoelastic Earth structures. We compare model predictions to observations of paleo sea level, present-day sea-level change, and present-day vertical land motion (VLM) around Nuuk, Greenland. We find that the modeled timing and magnitude of the Holocene retreat and readvance have a significant impact on modern sea-level change and VLM in Nuuk. Models that assume a readvance approaching the southwest GrIS’ historical maximum between 2 and 1 ka are most consistent with observations. The RSL response, however, is less sensitive to the timing of the minimum GrIS extent. Nonetheless, better data-model fits are generally obtained when the minimum ice sheet extent is reached between 5 and 3 ka, within the tested range of 6-3 ka. Comparing this timing to local and regional records of temperature and ice-sheet change suggest that the evolution of the southwestern GrIS presented here was in-phase with the likely evolution of southwestern GrIS mass balance through the Holocene. Our results have implications for future ice sheet modeling studies targeting southwestern Greenland by providing additional constraints and strengthening existing ones. Moreover, this work provides a deeper understanding of the interactions between the climate and the cryosphere and thus of future ice sheet change.

Unusual weakening trend of the East Asian winter monsoon during MIS 8 revealed by Chinese loess deposits and its implications for ice age dynamics

A fundamental aspect of marine benthic foraminiferal δ18O records of the last one million years is their “saw-tooth” pattern, characterized by gradual cooling to full glacial conditions followed by very rapid glacial terminations. Understanding the mechanism of this pattern is crucial for understanding ice age dynamics. We investigated the climatic trend within each glacial of the last 880 kyr by comparing the signal of the East Asian winter monsoon (EAWM) with global terrestrial and marine paleoclimate records. Our new grain size record from the Huining loess-paleosol section in the western Chinese Loess Plateau, together with published climate records from the Yimaguan and Luochuan sections in the central Chinese Loess Plateau, demonstrates a strengthening trend of the EAWM within most glacials, which largely mirrors the “saw-tooth” pattern of the benthic foraminiferal δ18O record. However, the EAWM record during Marine Isotope Stage (MIS) 8 shows an unusual weakening trend which is the exception over the last 880 kyr. Based on the linkages between Arctic ice sheets, the Siberian High Pressure system, and the EAWM, we propose that the weakening trend of the EAWM throughout MIS 8 reflects the overall shrinking of Arctic ice sheets towards the glacial termination, in contrast to the increasing trend of the benthic foraminiferal δ18O values. This conclusion is supported by a compilation of global paleoclimate records suggesting that MIS 8 was a unique glacial period over the last 900 kyr, characterized by a mild climate in middle to high northern latitudes and a cold climate in high southern latitudes, during late MIS 8. The inferred changes in Arctic ice sheets would alone cause a decreasing trend in benthic foraminiferal δ18O values during MIS 8, which is the opposite to the observed trend. We suggest that an increased ice volume in high southern latitudes led to the cooling of Antarctic Bottom Water and hence to a decrease in ocean bottom water temperature with an effect to increase benthic foraminiferal δ18O during MIS 8 as observed. Our results highlight the complexity of using the benthic δ18O signal alone as a paleoclimate proxy, and that MIS 8 is an important time window for better understanding glacial dynamics driven by changes in the polar regions of both hemispheres.

A stochastic parameterization of ice sheet surface mass balance for the Stochastic Ice-Sheet and Sea-Level System Model (StISSM v1.0)

Abstract. Many scientific and societal questions that draw on ice sheet modeling necessitate sampling a wide range of potential climatic changes and realizations of internal climate variability. For example, coastal planning literature demonstrates a demand for probabilistic sea level projections with quantified uncertainty. Further, robust attribution of past and future ice sheet change to specific processes or forcings requires a full understanding of the space of possible ice sheet behaviors. The wide sampling required to address such questions is computationally infeasible with sophisticated numerical climate models at the resolution required to accurately force ice sheet models. Stochastic generation of climate forcing of ice sheets offers a complementary alternative. Here, we describe a method to construct a stochastic generator for ice sheet surface mass balance varying in time and space. We demonstrate the method with an application to Greenland Ice Sheet surface mass balance for 1980–2012. We account for spatial correlations among glacier catchments using sparse covariance techniques, and we apply an elevation-dependent downscaling to recover gridded surface mass balance fields suitable for forcing an ice sheet model while including feedback from changing ice sheet surface elevation. The efficiency gained in the stochastic method supports large-ensemble simulations of ice sheet change in a new stochastic ice sheet model. We provide open source Python workflows to support use of our stochastic approach for a broad range of applications.

Miocene Antarctic Ice Sheet area adapts significantly faster than volume to CO2-induced climate change

Abstract. The strongly varying benthic δ18O levels of the early and mid-Miocene (23 to 14 Myr ago) are primarily caused by a combination of changes in Antarctic Ice Sheet (AIS) volume and deep-ocean temperatures. These factors are coupled since AIS changes affect deep-ocean temperatures. It has recently been argued that this is due to changes in ice sheet area rather than volume because area changes affect the surface albedo. This finding would be important when the transient AIS grows relatively faster in extent than in thickness, which we test here. We analyse simulations of Miocene AIS variability carried out using the three-dimensional ice sheet model IMAU-ICE forced by warm (high CO2, no ice) and cold (low CO2, large East AIS) climate snapshots. These simulations comprise equilibrium and idealized quasi-orbital transient runs with strongly varying CO2 levels (280 to 840 ppm). Our simulations show a limited direct effect of East AIS changes on Miocene orbital-timescale benthic δ18O variability because of the slow build-up of volume. However, we find that relative to the equilibrium ice sheet size, the AIS area adapts significantly faster and more strongly than volume to the applied forcing variability. Consequently, during certain intervals the ice sheet is receding at the margins, while ice is still building up in the interior. That means the AIS does not adapt to a changing equilibrium size at the same rate or with the same sign everywhere. Our results indicate that the Miocene Antarctic Ice Sheet affects deep-ocean temperatures more than its volume suggests.

The discovery of the southernmost ultra-high-resolution Holocene paleoclimate sedimentary record in Antarctica

The response of the Antarctic ice sheet to climate warming is the main source of uncertainty regarding future global sea level rise, since little is known about its present and past dynamics. The last deglaciation is the most recent interval of large-scale climate warming, during which the Northern and Southern Hemisphere ice sheets retreated, and sea level rose globally, although at a non-uniform rate. Geologic records from the polar regions are fundamental in determining the factors that caused the major changes in ice sheets during the last deglacial under different boundary conditions. Here, we combine morpho-bathymetric and seismic data with sediment cores and oceanographic measurements to reconstruct the processes that influenced the deposition of the southernmost, most extensive, ultrahigh-resolution record of the Holocene in Edisto Inlet fjord (Ross Sea, Antarctica). We find that post-glacial sedimentation resulted in a layered diatom mud up to 110 m thick that was locally redistributed by bottom currents over confined drifts-moats in the central part of the fjord. After the Holocene climatic optimum, the fjord was not carved by ground ice, and there continued to be internal fjord water circulation associated with Ross Sea circulation. These results support a retreat of coastal glaciers by about 11 kiloyears ago (ka) from the continental shelf of North Victoria Land.

Quantifying the uncertainty in the Eurasian ice-sheet geometry at the Penultimate Glacial Maximum (Marine Isotope Stage 6)

Abstract. The North Sea Last Interglacial sea level is sensitive to the fingerprint of mass loss from polar ice sheets. However, the signal is complicated by the influence of glacial isostatic adjustment driven by Penultimate Glacial Period ice-sheet changes, and yet these ice-sheet geometries remain significantly uncertain. Here, we produce new reconstructions of the Eurasian ice sheet during the Penultimate Glacial Maximum (PGM) by employing large ensemble experiments from a simple ice-sheet model that depends solely on basal shear stress, ice extent, and topography. To explore the range of uncertainty in possible ice geometries, we use a parameterised shear-stress map as input that has been developed to incorporate bedrock characteristics and the influence of ice-sheet basal processes. We perform Bayesian uncertainty quantification, utilising Gaussian process emulation, to calibrate against global ice-sheet reconstructions of the Last Deglaciation and rule out combinations of input parameters that produce unrealistic ice sheets. The refined parameter space is then applied to the PGM to create an ensemble of constrained 3D Eurasian ice-sheet geometries. Our reconstructed PGM Eurasian ice-sheet volume is 48±8 m sea-level equivalent (SLE). We find that the Barents–Kara Sea region displays both the largest mean volume and volume uncertainty of 24±8 m SLE while the British–Irish sector volume of 1.7±0.2 m SLE is the smallest. Our new workflow may be applied to other locations and periods where ice-sheet histories have limited empirical data.

Inland thinning of Byrd Glacier, Antarctica, during Ross Ice Shelf formation

AbstractGeomorphological records of past ice sheet change offer the opportunity to examine their centennial‐scale response to changing boundary conditions, which are not adequately captured in the satellite record. Here, we present the first reconstruction of ice surface lowering at Byrd Glacier, the largest outlet glacier of the Transantarctic Mountains. Using surface exposure ages from glacial erratic cobbles collected in two vertical transects along the Lonewolf Nunataks, we find the initial emergence of this set of nunataks occurred at ~15 ka, with a rapid pulse of thinning at ~8 ka. We compare our glacier thinning profiles with modelled ice sheet thickness and grounding line histories from two model ensembles to identify key processes responsible for ice sheet change. All model runs from the two ensembles predict grounding line retreat and inland thinning to occur in one rapid step from Last Glacial Maximum to present, in line with marine geology records, our exposure age data and derived glacier thinning rates. Experiments best matching the glacial thickness constraints, reconstructed from the surface exposure data, have faster basal sliding (i.e., promote greater sliding rates resulting in thinner ice). However, experiments best matching the timing and rapid rate of ice thinning derived from the same surface exposure data have higher basal friction. This apparent change in the modelled basal sliding regime, from when the ice surface is at maximum thickness, to the rapid thinning at ~8 ka, occurs as the grounding line retreats towards the Byrd Glacier and Ross Ice Shelf forms during the Holocene. This past context has implications for the stability of the modern grounding line of Byrd Glacier, which is characterised by high basal melt rates at the terminus—a process that has the potential to propagate glacier thinning far inland, impacting the overall (in)stability of the Byrd Glacier and Ross Ice Shelf.

Orbitally-paced coastal sedimentary records and global sea-level changes in the early Permian

Orbital forcing affects climate system by regulating the amount of solar radiation received at the Earth's surface, which is vital for global hydrological cycles. However, a systematic understanding of the global water cycle with synergistic changes in ice sheets and sea levels under orbital forcing during the late Paleozoic ice age (LPIA) remains elusive. Here, we perform cyclostratigraphic analysis and sedimentary noise modeling for coastal sedimentary records (Well Greer #2 in the Permian Basin) from the early Permian and two sets of high-resolution paleoclimate simulation experiments using the Community Earth System Model. We provide the first high-resolution absolute astronomical time scale for the Permian Basin, which suggests that the timing of the early Permian Lower Leonard and Upper Wolfcamp Formations range from 286.29 ± 0.55 Ma to 289.05 ± 0.55 Ma, and the Lower Wolfcamp Formation ranges from 292.97 ± 0.16 Ma to 298.9 ± 0.15 Ma, respectively. In addition, we find a coupled phase relationship between high sea levels and eccentricity maxima, suggesting that eccentricity may have played a critical role in the early Permian sea-level oscillations. Paleoclimate simulations show that eccentricity and obliquity maxima lead to an overall increase in global mean temperature. Significant changes in surface temperature at middle to high latitudes are manifested as the polar amplification effect during the LPIA. Compared with obliquity, eccentricity leads to higher annual and summer variability in global surface temperature, and enhanced polar amplification results in ice retreat and sea-level rise. Evidence from Earth system modeling elucidates the phase relationship between orbital cycles and sea levels. Therefore, eccentricity, as the leading driving force in low latitudes, may dominate climate changes in the Earth's climate system and regulates global hydrological cycles during the LPIA.

Characterising the ice sheet surface in Northeast Greenland using Sentinel-1 SAR data

AbstractOver half of the recent mass loss from the Greenland ice sheet, and its associated contribution to global sea level rise, can be attributed to increased surface meltwater runoff, with the remainder a result of dynamical processes such as calving and ice discharge. It is therefore important to quantify the distribution of melting on the ice sheet if we are to adequately understand past ice sheet change and make predictions for the future. In this article, we present a novel semi-empirical approach for characterising ice sheet surface conditions using high-resolution synthetic aperture radar (SAR) backscatter data from the Sentinel-1 satellite. We apply a state-space model to nine sites within North-East Greenland to identify changes in SAR backscatter, and we attribute these to different surface types with reference to optical satellite imagery and meteorological data. A set of decision-making rules for labelling ice sheet melting states are determined based on this analysis and subsequently applied to previously unseen sites. We show that our method performs well in (1) recognising some of the ice sheet surface types such as snow and dark ice and (2) determining whether the surface is melting or not melting. Sentinel-1 SAR data are of high spatial resolution; thus, in developing a method to identify the state of the surface from these data, we improve our capability to understand the variation of ice sheet melting across time and space.

AutoTerm: an automated pipeline for glacier terminus extraction using machine learning and a “big data” repository of Greenland glacier termini

Abstract. Ice sheet marine margins via outlet glaciers are susceptible to climate change and are expected to respond through retreat, steepening, and acceleration, although with significant spatial heterogeneity. However, research on ice–ocean interactions has continued to rely on decentralized, manual mapping of features at the ice–ocean interface, impeding progress in understanding the response of glaciers and ice sheets to climate change. The proliferation of remote-sensing images lays the foundation for a better understanding of ice–ocean interactions and also necessitates the automation of terminus delineation. While deep learning (DL) techniques have already been applied to automate the terminus delineation, none involve sufficient quality control and automation to enable DL applications to “big data” problems in glaciology. Here, we build on established methods to create a fully automated pipeline for terminus delineation that makes several advances over prior studies. First, we leverage existing manually picked terminus traces (16 440) as training data to significantly improve the generalization of the DL algorithm. Second, we employ a rigorous automated screening module to enhance the data product quality. Third, we perform a thoroughly automated uncertainty quantification on the resulting data. Finally, we automate several steps in the pipeline allowing data to be regularly delivered to public databases with increased frequency. The automation level of our method ensures the sustainability of terminus data production. Altogether, these improvements produce the most complete and high-quality record of terminus data that exists for the Greenland Ice Sheet (GrIS). Our pipeline has successfully picked 278 239 termini for 295 glaciers in Greenland from Landsat 5, 7, 8 and Sentinel-1 and Sentinel-2 images, spanning the period from 1984 to 2021. The pipeline has been tested on glaciers in Greenland with an error of 79 m. The high sampling frequency and the controlled quality of our terminus data will enable better quantification of ice sheet change and model-based parameterizations of ice–ocean interactions.

Antarctic Ice Sheet paleo-constraint database

Abstract. We present a database of observational constraints on past Antarctic Ice Sheet changes during the last glacial cycle intended to consolidate the observations that represent our understanding of past Antarctic changes and for state-space estimation and paleo-model calibrations. The database is a major expansion of the initial work of Briggs and Tarasov (2013). It includes new data types and multi-tier data quality assessment. The updated constraint database, AntICE2 (https://theghub.org/resources/4884, Lecavalier et al., 2022), consists of observations of past grounded- and floating-ice-sheet extent, past ice thickness, past relative sea level, borehole temperature profiles, and present-day bedrock displacement rates. In addition to paleo-observations, the present-day ice sheet geometry and surface ice velocities are incorporated to constrain the present-day ice sheet configuration. The method by which the data are curated using explicitly defined criteria is detailed. Moreover, the observational uncertainties are specified. The methodology by which the constraint database can be applied to evaluate a given ice sheet reconstruction is discussed. The implementation of the AntICE2 database for Antarctic Ice Sheet model calibrations will improve Antarctic Ice Sheet predictions during past warm and cold periods and yield more robust paleo-model spin ups for forecasting future ice sheet changes.

Geological sketch map and implications for ice flow of Thwaites Glacier, West Antarctica, from integrated aerogeophysical observations.

The geology beneath Thwaites Glacier, the Antarctic glacial catchment most vulnerable to climate change, is unknown. Thwaites Glacier lies within the West Antarctic Rift System, but details of the subglacial geology relevant to glacial flow, including sediment availability, underlying lithology, and heat flux, are lacking. We present the first sketch map of the subglacial geology of Thwaites Glacier, interpreted from maps of airborne gravity, magnetic and radar data, supported by 2D models and 3D inversion of subsurface properties, and the regional geological context. A zone of Cretaceous mafic magmatism extending ~200 km inland from the coast is interpreted, while sedimentary basins are restricted to a region 150 to 200 km inboard of the coast, underlying just 20% of the catchment. Several granitic subglacial highlands are identified, forming long-lived topographic highs. Our geological interpretation places constraints on the basal properties of Thwaites Glacier, laying the foundation for both improved predictions of ice sheet change and studies of West Antarctic tectonics.

Spatial and temporal variability of 21st century sea level changes

SUMMARY Mass loss from polar ice sheets is becoming the dominant contributor to current sea level changes, as well as one of the largest sources of uncertainty in sea level projections. The spatial pattern of sea level change is sensitive to the geometry of ice sheet mass changes, and local sea level changes can deviate from the global mean sea level change due to gravitational, Earth rotational and deformational (GRD) effects. The pattern of GRD sea level change associated with the melting of an ice sheet is often considered to remain relatively constant in time outside the vicinity of the ice sheet. For example, in the sea level projections from the most recent IPCC sixth assessment report (AR6), the geometry of ice sheet mass loss was treated as constant during the 21st century. However, ice sheet simulations predict that the geometry of ice mass changes across a given ice sheet and the relative mass loss from each ice sheet will vary during the coming century, producing patters of global sea level changes that are spatiotemporally variable. We adopt a sea level model that includes GRD effects and shoreline migration to calculate time-varying sea level patterns associated with projections of the Greenland and Antarctic Ice Sheets during the coming century. We find that in some cases, sea level changes can be substantially amplified above the global mean early in the century, with this amplification diminishing by 2100. We explain these differences by calculating the contributions of Earth rotation as well as gravitational and deformational effects to the projected sea level changes separately. We find in one case, for example, that ice gain on the Antarctic Peninsula can cause an amplification of up to 2.9 times the global mean sea level equivalent along South American coastlines due to positive interference of GRD effects. To explore the uncertainty introduced by differences in predicted ice mass geometry, we predict the sea level changes following end-member mass loss scenarios for various regions of the Antarctic Ice Sheet from the ISMIP6 model ensemblely, and find that sea level amplification above the global mean sea level equivalent differ by up to 1.9 times between different ice mass projections along global coastlines outside of Greenland and Antarctica. This work suggests that assessments of future sea level hazard should consider not only the integrated mass changes of ice sheets, but also temporal variations in the geometry of the ice mass changes across the ice sheets. As well, this study highlights the importance of constraining the relative timing of ice mass changes between the Greenland and Antarctic Ice Sheets.

Deglacial climate changes as forced by different ice sheet reconstructions

Abstract. During the last deglaciation, the climate evolves from a cold state at the Last Glacial Maximum (LGM) at 21 ka (thousand years ago) with large ice sheets to the warm Holocene at ∼9 ka with reduced ice sheets. The deglacial ice sheet melt can impact the climate through multiple ways: changes of topography and albedo, bathymetry and coastlines, and freshwater fluxes (FWFs). In the PMIP4 (Paleoclimate Modelling Intercomparison Project – Phase 4) protocol for deglacial simulations, these changes can be accounted for or not depending on the modelling group choices. In addition, two ice sheet reconstructions are available (ICE-6G_C and GLAC-1D). In this study, we evaluate all these effects related to ice sheet changes on the climate using the iLOVECLIM model of intermediate complexity. We show that the two reconstructions yield the same warming to a first order but with a different amplitude (global mean temperature of 3.9 ∘C with ICE-6G_C and 3.8 ∘C with GLAC-1D) and evolution. We obtain a stalling of temperature rise during the Antarctic Cold Reversal (ACR, from ∼14 to ∼12 ka) similar to proxy data only with the GLAC-1D ice sheet reconstruction. Accounting for changes in bathymetry in the simulations results in a cooling due to a larger sea ice extent and higher surface albedo. Finally, freshwater fluxes result in Atlantic meridional overturning circulation (AMOC) drawdown, but the timing in the simulations disagrees with proxy data of ocean circulation changes. This questions the causal link between reconstructed freshwater fluxes from ice sheet melt and recorded AMOC weakening.