A. Khazendar

A damage mechanics assessment of the Larsen B ice shelf prior to collapse: Toward a physically-based calving law

Calving is a primary process of mass ablation for glaciers and ice sheets, though it still eludes a general physical law. Here, we propose a calving framework based on continuum damage mechanics coupled with the equations of viscous deformation of glacier ice. We introduce a scalar damage variable that quantifies the loss of load-bearing surface area due to fractures and that feeds back with ice viscosity to represent fracture-induced softening. The calving law is a standard failure criterion for viscous damaging materials and represents a macroscopic brittle instability quantified by a critical or threshold damage. We constrain this threshold using the Ice Sheet System Model (ISSM) by inverting for damage on the Larsen B ice shelf prior to its 2002 collapse. By analyzing the damage distribution in areas that subsequently calved, we conclude that calving occurs after fractures have reduced the load-bearing capacity of the ice by 60 ± 10%.

Fast retreat of Zachariæ Isstrøm, northeast Greenland

Shrinking shelf and faster flow Zachariæ Isstrøm, a large glacier in northeast Greenland, began a rapid retreat after detaching from a stabilizing sill in the late 1990s. Mouginot et al. report that between 2002 and 2014, the area covered by the glaciers ice shelf shrank by 95%; since 1999, the glaciers flow rate has nearly doubled; and its acceleration increased threefold in the fall of 2012. These dramatic changes appear to be the result of a combination of warmer air and ocean temperatures and the topography of the ocean floor at the head of the glacier. Rising sea levels should continue to destabilize the marine portion of Zachariæ Isstrøm for decades. Science, this issue p. 1357 A large glacier in northeast Greenland is retreating rapidly as air and ocean warm. After 8 years of decay of its ice shelf, Zachariæ Isstrøm, a major glacier of northeast Greenland that holds a 0.5-meter sea-level rise equivalent, entered a phase of accelerated retreat in fall 2012. The acceleration rate of its ice velocity tripled, melting of its residual ice shelf and thinning of its grounded portion doubled, and calving is now occurring at its grounding line. Warmer air and ocean temperatures have caused the glacier to detach from a stabilizing sill and retreat rapidly along a downward-sloping, marine-based bed. Its equal-ice-volume neighbor, Nioghalvfjerdsfjorden, is also melting rapidly but retreating slowly along an upward-sloping bed. The destabilization of this marine-based sector will increase sea-level rise from the Greenland Ice Sheet for decades to come.

Observed thinning of Totten Glacier is linked to coastal polynya variability

Analysis of ICESat-1 data (2003-2008) shows significant surface lowering of Totten Glacier, the glacier discharging the largest volume of ice in East Antarctica, and less change on nearby Moscow University Glacier. After accounting for firn compaction anomalies, the thinning appears to coincide with fast-flowing ice indicating a dynamical origin. Here, to elucidate these observations, we apply high-resolution ice-ocean modelling. Totten Ice Shelf is simulated to have higher, more variable basal melting rates. We link this variability to the volume of cold water, originating in polynyas upon sea ice formation, reaching the sub-ice-shelf cavity. Hence, we propose that the observed increased thinning of Totten Glacier is due to enhanced basal melting caused by a decrease in cold polynya water reaching its cavity. We support this hypothesis with passive microwave data of polynya extent variability. Considering the widespread changes in sea ice conditions, this mechanism could be contributing extensively to ice-shelf instability.

Roles of marine ice, rheology, and fracture in the flow and stability of the Brunt/Stancomb‐Wills Ice Shelf

Marine ice, sometimes as part of an ice melange, significantly affects ice shelf flow and ice fracture. The highly heterogeneous structure of the Brunt/Stancomb-Wills Ice Shelf (BSW) system in the east Weddell Sea offers a rare setting for uncovering the difference in rheology between meteoric and marine ice. Here, we use data assimilation to infer the rheology of the Brunt/Stancomb-Wills Ice Shelf by an inverse control method that combines interferometric synthetic aperture radar measurements with numerical modeling. We then apply the inferred rheology to support the hypothesis attributing the observed 1970s ice shelf flow acceleration to a change in the stiffness of the ice melange area connecting Brunt proper with Stancomb-Wills and to examine the consequences of frontal rift propagation. We conclude that while the Brunt/Stancomb-Wills system is currently not susceptible to extreme fragmentation similar to that of the Larsen B Ice Shelf in 2002, our inverse and forward modeling results emphasize its vulnerability to destabilization by relatively rapid changes in the ice melange properties, resulting from the interaction of its marine ice component with ocean water, or by the further propagation of a frontal rift. Copyright 2009 by the American Geophysical Union.

Acceleration and spatial rheology of Larsen C Ice Shelf, Antarctic Peninsula

The disintegration of several Antarctic Peninsula ice shelves has focused attention on the state of the Larsen C Ice Shelf. Here, we use satellite observations to map ice shelf speed from the years 2000, 2006 and 2008 and apply inverse modeling to examine the spatial pattern of ice-shelf stiffness. Results show that the northern half of the ice shelf has been accelerating since 2000, speeding up by 15% between 2000 and 2006 alone. The distribution of ice stiffness exhibits large spatial variations that we link to tributary glacier flow and fractures. Our results reveal that ice down-flow from promontories is consistently softer, with the exception of Churchill Peninsula where we infer a stabilizing role for marine ice. We conclude that although Larsen C is not facing imminent collapse, it is undergoing significant change in the form of flow acceleration that is spatially related to thinning and fracture. Copyright

Continued retreat of Thwaites Glacier, West Antarctica, controlled by bed topography and ocean circulation

The Amundsen Sea sector is experiencing the largest mass loss, glacier acceleration, and grounding line retreat in Antarctica. Enhanced intrusion of Circumpolar Deep Water onto the continental shelf has been proposed as the primary forcing mechanism for the retreat. Here we investigate the dynamics and evolution of Thwaites Glacier with a novel, fully coupled, ice-ocean numerical model. We obtain a significantly improved agreement with the observed pattern of glacial retreat using the coupled model. Coupled simulations over the coming decades indicate a continued mass loss at a sustained rate. Uncoupled simulations using a depth-dependent parameterization of sub-ice-shelf melt significantly overestimate the rate of grounding line retreat compared to the coupled model, as the parameterization does not capture the complexity of the ocean circulation associated with the formation of confined cavities during the retreat. Bed topography controls the pattern of grounding line retreat, while oceanic thermal forcing impacts the rate of grounding line retreat. The importance of oceanic forcing increases with time as Thwaites grounding line retreats farther inland.

Rapid submarine ice melting in the grounding zones of ice shelves in West Antarctica

Enhanced submarine ice-shelf melting strongly controls ice loss in the Amundsen Sea embayment (ASE) of West Antarctica, but its magnitude is not well known in the critical grounding zones of the ASEs major glaciers. Here we directly quantify bottom ice losses along tens of kilometres with airborne radar sounding of the Dotson and Crosson ice shelves, which buttress the rapidly changing Smith, Pope and Kohler glaciers. Melting in the grounding zones is found to be much higher than steady-state levels, removing 300–490 m of solid ice between 2002 and 2009 beneath the retreating Smith Glacier. The vigorous, unbalanced melting supports the hypothesis that a significant increase in ocean heat influx into ASE sub-ice-shelf cavities took place in the mid-2000s. The synchronous but diverse evolutions of these glaciers illustrate how combinations of oceanography and topography modulate rapid submarine melting to hasten mass loss and glacier retreat from West Antarctica.

A constitutive framework for predicting weakening and reduced buttressing of ice shelves based on observations of the progressive deterioration of the remnant Larsen B Ice Shelf

Geophysical Research Letters RESEARCH LETTER 10.1002/2015GL067365 Key Points: • Assimilated observations indicate pro- gressive weakening of ice shelf from 2000 to 2015 • New framework introduced for vis- cous ice deformation with analytical solution for damage • New framework reproduces observed weakening and is generalizable to any ice shelf Supporting Information: • Supporting Information S1 Correspondence to: C. Borstad, chris.borstad@unis.no Citation: Borstad, C., A. Khazendar, B. Scheuchl, M. Morlighem, E. Larour, and E. Rignot (2016), A constitutive frame- work for predicting weakening and reduced buttressing of ice shelves based on observations of the progressive deterioration of the remnant Larsen B Ice Shelf, Geophys. Res. Lett., 43, 2027–2035, doi:10.1002/2015GL067365. Received 9 DEC 2015 Accepted 9 FEB 2016 Accepted article online 11 FEB 2016 Published online 4 MAR 2016 A constitutive framework for predicting weakening and reduced buttressing of ice shelves based on observations of the progressive deterioration of the remnant Larsen B Ice Shelf Chris Borstad 1 , Ala Khazendar 2 , Bernd Scheuchl 3 , Mathieu Morlighem 3 , Eric Larour 2 , and Eric Rignot 2,3 1 Department of Arctic Geophysics, University Centre in Svalbard, Longyearbyen, Norway, 2 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA, 3 Department of Earth System Science, University of California, Irvine, California, USA Abstract The increasing contribution of the Antarctic Ice Sheet to sea level rise is linked to reductions in ice shelf buttressing, driven in large part by basal melting of ice shelves. These ocean-driven buttressing losses are being compounded as ice shelves weaken and fracture. To date, model projections of ice sheet evolution have not accounted for weakening ice shelves. Here we present the first constitutive framework for ice deformation that explicitly includes mechanical weakening, based on observations of the progressive degradation of the remnant Larsen B Ice Shelf from 2000 to 2015. We implement this framework in an ice sheet model and are able to reproduce most of the observed weakening of the ice shelf. In addition to predicting ice shelf weakening and reduced buttressing, this new framework opens the door for improved understanding and predictions of iceberg calving, meltwater routing and hydrofracture, and ice shelf collapse. 1. Introduction Many of the largest and fastest changes to the Antarctic ice sheet over the last decade have been linked to the thinning and loss of ice shelves in a manner that is consistent, at least qualitatively, with notions of ice shelf buttressing [Intergovernmental Panel on Climate Change, 2013]. To predict the fate of the ice sheet, therefore, the dominant physical mechanisms of ice shelf evolution must be accurately represented in mod- els. Ocean-driven basal melting of ice shelves [Pritchard et al., 2012; Rignot et al., 2013] is believed to be the predominant cause of ice shelf buttressing losses. However, as ice shelves thin they also become more suscep- tible to fracture [Shepherd et al., 2003]. In West Antarctica, fracturing and weakening of ice shelf shear margins appears to be compounding the buttressing losses associated with ice shelf thinning [MacGregor et al., 2012]. The irreversible collapse of the West Antarctic Ice Sheet, which is speculated to already be underway [Rignot et al., 2014; Joughin et al., 2014], may have been hastened by the combined effects of thinning and mechanical weakening of buttressing ice shelves. Yet the mechanisms of fracture-induced weakening are poorly under- stood and still absent in projections of ice shelf evolution. Although advances in ice-ocean model coupling [Goldberg et al., 2012; Hellmer et al., 2012] are providing insight into feedbacks driven by warming oceans, ice sheet models are still failing to capture associated changes in bulk ice rheology and buttressing due to mechanical weakening of ice shelves. To address this need, we first assemble the longest available time series to date of ice shelf weakening. We then devise a new constitutive formalism that is consistent with the observations and generalizable to represent other glaciological processes involving fractures. We focus here on the remnant Larsen B Ice Shelf (RLBIS, Figure 1a), the surviving portion of the ice shelf that filled the Larsen B embayment prior to its partial collapse in 2002. The buttressing provided by RLBIS diminished over the period 2000 to 2010 [Khazendar et al., 2015], which has facilitated the thinning and acceleration of its tributary glaciers [Scambos et al., 2014; Khazendar et al., 2015]. ©2016. American Geophysical Union. All Rights Reserved. BORSTAD ET AL. Using remote sensing observations assimilated in the Ice Sheet System Model (ISSM) [Larour et al., 2012], we calculate the spatial pattern of ice damage for the years 2000, 2006, 2010, and 2015 (Figure 1). We then analyze CONSTITUTIVE FRAMEWORK FOR ICE WEAKENING

Representation of sharp rifts and faults mechanics in modeling ice shelf flow dynamics: Application to Brunt/Stancomb‐Wills Ice Shelf, Antarctica

Ice shelves play a major role in buttressing ice sheet flow into the ocean, hence the importance of accurate numerical modeling of their stress regime. Commonly used ice flow models assume a continuous medium and are therefore complicated by the presence of rupture features (crevasses, rifts, and faults) that significantly affect the overall flow patterns. Here we apply contact mechanics and penalty methods to develop a new ice shelf flow model that captures the impact of rifts and faults on the rheology and stress distribution of ice shelves. The model achieves a best fit solution to satellite observations of ice shelf velocities to infer the following: (1) a spatial distribution of contact and friction points along detected faults and rifts, (2) a more realistic spatial pattern of ice shelf rheology, and (3) a better representation of the stress balance in the immediate vicinity of faults and rifts. Thus, applying the model to the Brunt/Stancomb-Wills Ice Shelf, Antarctica, we quantify the state of friction inside faults and the opening rates of rifts and obtain an ice shelf rheology that remains relatively constant everywhere else on the ice shelf. We further demonstrate that better stress representation has widespread application in examining aspects affecting ice shelf structure and dynamics including the extent of ice melange in rifts and the change in fracture configurations. All are major applications for better insight into the important question of ice shelf stability.