Search

The Antarctic ice sheet (AIS) lies on a solid Earth that displays large spatial variations in rheological properties, with a thin lithosphere and low‐viscosity upper mantle (weak Earth structure) beneath West Antarctica and an opposing structure beneath East Antarctica. This contrast is known to have a significant impact on the ice‐sheet grounding‐line stability. Here, we embed within an ice‐sheet model a modified glacial‐isostatic Elastic Lithosphere‐Relaxing Asthenosphere model that considers a dual pattern for the Earth structure beneath West and East Antarctica supplemented with an approximation of gravitationally consistent geoid changes, allowing to approximate near‐field relative sea‐level changes. We show that this elementary GIA model captures the essence of global Self‐Gravitating Viscoelastic solid‐Earth Models (SGVEMs) and compares well with both SGVEM outputs and geodetic observations, allowing to capture the essential features and processes influencing Antarctic grounding‐line stability in a computationally efficient way. In this framework, we perform a probabilistic assessment of the impact of uncertainties in solid‐Earth rheological properties on the response of the AIS to future warming. Results show that on multicentennial‐to‐millennial timescales, spatial variability in solid‐Earth deformation plays a significant role in promoting the stability of the West Antarctic ice sheet (WAIS). However, WAIS collapse cannot be prevented under high‐emissions climate scenarios. On longer timescales and for unmitigated climate scenarios, continent‐wide mass loss projections may be underestimated because spatially uniform Earth models, as typically considered in numerical ice sheet models, will overestimate the stabilizing effect of GIA across East Antarctica, which is characterized by thick lithosphere and high upper‐mantle viscosity. Plain Language Summary: When an ice sheet loses mass, the pressure it exerts on the underlying solid Earth decreases and the Earth surface rebounds. This process, called glacial isostatic adjustment, is important to consider in ice sheet models because it can stabilize an ice sheet undergoing unstable retreat. Most models consider that the solid Earth response to ice mass changes is uniform. However, because of the weak mantle observed beneath West Antarctica, the isostatic rebound in this region is much faster than previously thought. Oppositely, a slow Earth response is observed in East Antarctica, characterized by a more rigid mantle. In this study, we consider this contrast in the isostatic response of the Antarctic solid Earth and show that it plays a crucial role on the future evolution of the Antarctic ice sheet. More specifically, we find that the rapid Earth response in West Antarctica significantly stabilizes the ice sheet on multicentennial‐to‐millennial timescales, even though a collapse of the West Antarctic ice sheet cannot be avoided under high‐emissions climate scenarios. We also find that the slow Earth response in East Antarctica has an influence on long timescales, potentially leading to an underestimation of future mass loss in the East Antarctic ice sheet. Key Points: We developed an elementary glacial isostatic adjustment (GIA) model featuring a spatially varying Elastic Lithosphere‐Relaxing Asthenosphere model and an approximation to geoid changesOn multicentennial‐to‐millennial timescales, GIA feedbacks significantly promote the stability of the West Antarctic ice sheetOn millennial timescales, considering a uniform Antarctic Earth structure may overestimate the stabilizing role of GIA feedbacks in the East Antarctic ice sheet [ABSTRACT FROM AUTHOR]

We use updated drainage inventory, ice thickness, and ice velocity data to calculate the grounding line ice discharge of 176 basins draining the Antarctic Ice Sheet from 1979 to 2017. We compare the results with a surface mass balance model to deduce the ice sheet mass balance. The total mass loss increased from 40 ± 9 Gt/y in 1979-1990 to 50 ± 14 Gt/y in 1989-2000, 166 ± 18 Gt/y in 1999-2009, and 252 ± 26 Gt/y in 2009-2017. In 2009-2017, the mass loss was dominated by the Amundsen/Bellingshausen Sea sectors, in West Antarctica (159 ± 8 Gt/y), Wilkes Land, in East Antarctica (51 ± 13 Gt/y), and West and Northeast Peninsula (42 ± 5 Gt/y). The contribution to sea-level rise from Antarctica averaged 3.6 ± 0.5 mm per decade with a cumulative 14.0 ± 2.0 mm since 1979, including 6.9 ± 0.6 mm from West Antarctica, 4.4 ± 0.9 mm from East Antarctica, and 2.5 ± 0.4 mm from the Peninsula (i.e., East Antarctica is a major participant in the mass loss). During the entire period, the mass loss concentrated in areas closest to warm, salty, subsurface, circumpolar deep water (CDW), that is, consistent with enhanced polar westerlies pushing CDW toward Antarctica to melt its floating ice shelves, destabilize the glaciers, and raise sea level. [ABSTRACT FROM AUTHOR]

We present a new digital elevation model (DEM) of the Antarctic ice sheet and ice shelves based on 2.5×108 observations recorded by the CryoSat-2 satellite radar altimeter between July 2010 and July 2016. The DEM is formed from spatio-temporal fits to elevation measurements accumulated within 1, 2, and 5 km grid cells, and is posted at the modal resolution of 1 km. Altogether, 94% of the grounded ice sheet and 98% of the floating ice shelves are observed, and the remaining grid cells north of 88° S are interpolated using ordinary kriging. The median and root mean square difference between the DEM and 2:3×107 airborne laser altimeter measurements acquired during NASA Operation IceBridge campaigns are -0.30 and 13.50 m, respectively. The DEM uncertainty rises in regions of high slope, especially where elevation measurements were acquired in low-resolution mode; taking this into account, we estimate the average accuracy to be 9.5m - a value that is comparable to or better than that of other models derived from satellite radar and laser altimetry. [ABSTRACT FROM AUTHOR]

The response of the Antarctic ice sheet (AIS) to changing global temperatures is a key component of sea-level projections. Current projections of the AIS contribution to sea-level changes are deeply uncertain. This deep uncertainty stems, in part, from (i) the inability of current models to fully resolve key processes and scales, (ii) the relatively sparse available data, and (iii) divergent expert assessments. One promising approach to characterizing the deep uncertainty stemming from divergent expert assessments is to combine expert assessments, observations, and simple models by coupling probabilistic inversion and Bayesian inversion. Here, we present a proof-of-concept study that uses probabilistic inversion to fuse a simple AIS model and diverse expert assessments. We demonstrate the ability of probabilistic inversion to infer joint prior probability distributions of model parameters that are consistent with expert assessments. We then confront these inferred expert priors with instrumental and paleoclimatic observational data in a Bayesian inversion. These additional constraints yield tighter hindcasts and projections. We use this approach to quantify how the deep uncertainty surrounding expert assessments affects the joint probability distributions of model parameters and future projections. [ABSTRACT FROM AUTHOR]

Ice-shelf basal melting is the largest contributor to the negative mass balance of the Antarctic ice sheet. However, current implementations of ice/ocean interactions in ice-sheet models disagree with the distribution of sub-shelf melt and freezing rates revealed by recent observational studies. Here we present a novel combination of a continental-scale ice flow model and a calibration technique to derive the spatial distribution of basal melting and freezing rates for the whole Antarctic ice-shelf system. The modelled ice-sheet equilibrium state is evaluated against topographic and velocity observations. Our high-resolution (10-km spacing) simulation predicts an equilibrium ice-shelf basal mass balance of −1648.7 Gt a−1 that increases to −1917.0 Gt a−1 when the observed ice-shelf thinning rates are taken into account. Our estimates reproduce the complexity of the basal mass balance of Antarctic ice shelves, providing a reference for parameterisations of sub-shelf ocean/ice interactions in continental ice-sheet models. We perform a sensitivity analysis to assess the effects of variations in the model set-up, showing that the retrieved estimates of basal melting and freezing rates are largely insensitive to changes in the internal model parameters, but respond strongly to a reduction of model resolution and the uncertainty in the input datasets. [ABSTRACT FROM PUBLISHER]

Understanding the climate response of the Antarctic Peninsula ice sheet is vital for accurate predictions of sea-level rise. However, since climate models are typically too coarse to capture spatial variability in local scale meteorological processes, our ability to study specific sectors has been limited by the local fidelity of such models and the (often sparse) availability of observations. We show that a high-resolution (5.5 km × 5.5 km) version of a regional climate model (RACMO2.3) can reproduce observed interannual variability in the Larsen B embayment sufficiently to enable its use in investigating long-term changes in this sector. Using the model, together with automatic weather station data, we confirm previous findings that the year of the Larsen B ice shelf collapse (2001/02) was a strong melt year, but discover that total annual melt production was in fact ~30% lower than 2 years prior. While the year before collapse exhibited the lowest melting and highest snowfall during 1980–2014, the ice shelf was likely pre-conditioned for collapse by a series of strong melt years in the 1990s. Melt energy has since returned to pre-1990s levels, which likely explains the lack of further significant collapse in the region (e.g. of SCAR Inlet). [ABSTRACT FROM PUBLISHER]

A Regional Antarctic and Global Ocean (RAnGO) model has been developed to study the interaction between the world ocean and the Antarctic ice sheet. The coupled model is based on a global implementation of the Finite Element Sea-ice Ocean Model (FESOM) with a mesh refinement in the Southern Ocean, particularly in its marginal seas and in the sub-ice shelf cavities. The cryosphere is represented by a regional setup of the ice flow model RIMBAY comprising the Filchner-Ronne Ice Shelf and the grounded ice in its catchment area up to the ice divides. At the base of the RIMBAY ice shelf, melt rates from FESOM's ice-shelf component are supplied. RIMBAY returns ice thickness and the position of the grounding line. The ocean model uses a pre-computed mesh to allow for an easy adjustment of the model domain to a varying cavity geometry.

RAnGO simulations with a 20th-century climate forcing yield realistic basal melt rates and a quasi-stable grounding line position close to the presently observed state. In a centennial-scale warm-water-inflow scenario, the model suggests a substantial thinning of the ice shelf and a local retreat of the grounding line. The potentially negative feedback from ice-shelf thinning through a rising in-situ freezing temperature is more than outweighed by the increasing water column thickness in the deepest parts of the cavity. Compared to a control simulation with fixed ice-shelf geometry, the coupled model thus yields a slightly stronger increase of ice-shelf basal melt rates. [ABSTRACT FROM AUTHOR]

The most rapid loss of ice from the Antarctic Ice Sheet is observed where ice streams flow into the ocean and begin to float, forming the great Antarctic ice shelves that surround much of the continent. Because these ice shelves are floating, their thinning does not greatly influence sea level. However, they also buttress the ice streams draining the ice sheet, and so ice shelf changes do significantly influence sea level by altering the discharge of grounded ice. Currently, the most significant loss of mass from the ice shelves is from melting at the base (although iceberg calving is a close second). Accessing the ocean beneath ice shelves is extremely difficult, so numerical models are invaluable for understanding the processes governing basal melting. This paper describes the different ways in which ice shelf/ocean interactions are modeled and discusses emerging directions that will enhance understanding of how the ice shelves are melting now and how this might change in the future. [ABSTRACT FROM AUTHOR]

In this article, spatio-temporal dynamics of snowmelt in Antarctica from 2001 to 2014 using OSCAT and QuikSCAT scatterometer data is presented. Melting over Antarctic ice sheet can influence shelf dynamics and stability. Here, we have utilized the sensitivity of scatterometer data to detect the presence of liquid water in the snow caused due to melt conditions. After analysing decadal data, a spatial and temporal variation in the average backscatter coefficient was observed over the shelf areas. An adaptive thresholdbased classification using austral winter mean and standard deviation of HH polarization is used which takes into account the spatial and temporal variability in backscatter from snow/ice. Significant spatiotemporal variability in melt area, duration and melt index was observed. Around 9.5% of the continent experienced melt over the study period. Larsen C and George VI shelves had maximum melt duration. The high correlation between melt duration obtained from satellite data and the positive degree day validates the efficacy of the melt algorithm used in the analysis and sensitivity of OSCAT data in detecting presence of water due to melt. There is seasonal and spatial variation in melt onset. Based on MI, 2004-05 was the warmest summer over the continent with 2011-12 being the coldest summer. Consistent and intensive melting was observed over Amery, Larsen C, George VI, Lazarev and Fimbul shelves. Melting of sporadic nature was observed over Ronne-Filchner, Ross and Riiser-Larsen shelves. The East Antarctic shelves experienced large melt during the study period. This article presents the suitability of OSCAT in melt identification and status of melt over the continent. [ABSTRACT FROM AUTHOR]

The floating ice shelves surrounding the Antarctic Ice Sheet restrain the grounded ice-sheet flow. Thinning of an ice shelf reduces this effect, leading to an increase in ice discharge to the ocean. Using 18 years of continuous satellite radar altimeter observations, we have computed decadal-scale changes in ice-shelf thickness around the Antarctic continent. Overall, average ice-shelf volume change accelerated from negligible loss at 25 ± 64 cubic kilometers per year for 1994-2003 to rapid loss of 310 ± 74 cubic kilometers per year for 2003-2012. West Antarctic losses increased by ~70% in the past decade, and earlier volume gain by East Antarctic ice shelves ceased. In the Amundsen and Bellingshausen regions, some ice shelves have lost up to 18% of their thickness in less than two decades. [ABSTRACT FROM AUTHOR]

Here we present the results of a comprehensive glaciological investigation of Union Glacier (79°46' S/83°24' W) in the West Antarctic Ice Sheet (WAIS), a major outlet glacier within the Ellsworth Mountains. Union Glacier flows into the Ronne Ice Shelf, where recent models have indicated the potential for significant grounding line zone (GLZ) migrations in response to changing climate and ocean conditions. To elaborate a glaciological base line that can help to evaluate the potential impact of this GLZ change scenario, we installed an array of stakes on Union Glacier in 2007. The stake network has been surveyed repeatedly for elevation, velocity, and net surface mass balance. The region of the stake measurements is in near-equilibrium, and ice speeds are 10 to 33ma-1. Ground-penetrating radars (GPR) have been used to map the subglacial topography, internal structure, and crevasse frequency and depth along surveyed tracks in the stake site area. The bedrock in this area has a minimum elevation of -858ma.s.l., significantly deeper than shown by BEDMAP2 data. However, between this deeper area and the local GLZ, there is a threshold where the subglacial topography shows a maximum altitude of 190 m. This subglacial condition implies that an upstream migration of the GLZ will not have strong effects on Union Glacier until it passes beyond this shallow ice pinning point. [ABSTRACT FROM AUTHOR]