Chris Borstad

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

Fracture propagation and stability of ice shelves governed by ice shelf heterogeneity

Tabular iceberg calving and ice shelf retreat occurs after full-thickness fractures, known as rifts, propagate across an ice shelf. A quickly evolving rift signals a threat to the stability of Larsen C, the Antarctic Peninsulas largest ice shelf. Here, we reveal the influence of ice shelf heterogeneity on the growth of this rift, with implications that challenge existing notions of ice shelf stability. Most of the rift extension has occurred in bursts after overcoming the resistance of suture zones that bind together neighboring glacier inflows. We model the stresses in the ice shelf to determine potential rift trajectories. Calving perturbations to ice flow will likely reach the grounding line. The stability of Larsen C may hinge on a single suture zone that stabilizes numerous upstream rifts. Elevated fracture toughness of suture zones may be the most important property that allows ice shelves to modulate Antarcticas contribution to sea level rise.