Downloaded by guest on September 27, 2021 u r o nlddi urn oeso hi era.These retreat. their of models current glaciers, in show both included of to ice future not TG the imagery and to are satellite crucial PIG but appear the of that on 1) series areas (Fig. damage shelves time of use development rapid we the study, this In Evolution 14). Damage (13, models known sheet poorly ice in are for conditions accounted the boundary not of or their many and and as timing difficult processes future remains consequences key the instabilities large quantifying these Yet, with of 12). and magnitude (11, rise shelves level the ice sea with their for instability and of sheet PIG ice loss these marine bed, potential to Under retrograde prone 10). a considered are with (9, TG combination retreated in have thinned and and lines conditions accelerated grounding have their glaciers upstream both and thinned the result, on shelves a exert ice As they their glaciers. effect and buttressing 7) the decreasing (6, (8), and retreated PIG fronts melting, enhanced calving this their TG to of Due (3–5). oceanic melting shelves and ocean-induced ice floating atmospheric enhanced changes cause changing distinct show that by glaciers conditions driven Both decades 2). (i.e., recent (1, rise in rise) level level sea sea global global to of contribution P rise level sea projections. glaciology rise level sea improve for to accounted models, sheet not ice currently most are incorporating in which for processes, need feedback the these from shelf underline contributions ice they level future Moreover, sea to Antarctica. and key retreat, are line grounding processes stability, feedback sug- study damage this that of results gest The retreat. line disintegration grounding for enhances feed- and shelves ice damage these preconditions This potentially development. back damage zones, hence weakening, additional shear and promoting their shearing, speedup, in further damage in weak- results of which shelf development ice the initial triggers where ening process the feedback that a moreover the over reveal initiates that weakened results damage model structurally signs Idealized have decade. first shelves past are ice the highly both and of of fractures zones consist open shear Island areas Pine and damage combine of areas These zones crevassed shelves. we shear ice the Here, Thwaites in and understood. areas rapid the damage well uncover of to development not modeling with are imagery ground- satellite and multisource retreat weakening of shelf line if many ice as their ing weaken control uncertainty will that major glaciers processes a the both remains fast continue how changes in these and Yet, glaciers much level. sea Sea outlet how global Amundsen for changing assessing consequences the large fastest with in the Antarctica West Glacier for among (received Thwaites 2020 are 29, and July Embayment Jouzel Glacier Jean Member Island Board Editorial Pine by accepted and CA, Pasadena, 2019) Laboratory, 29, Propulsion July Jet review NASA Greene, Chad by Edited d’ national Berthier Etienne mH 00Inbuk uti;and Austria; Innsbruck, 6020 GmbH, 20771; MD Greenbelt, Center, Belgium; Bruxelles, B-1050 Bruxelles, a Lhermitte Stef Embayment loss Sea mass Amundsen and in instability shelf ice accelerates Damage www.pnas.org/cgi/doi/10.1073/pnas.1912890117 eateto esine&Rmt esn,DltUieit fTcnlg,20G ef,Netherlands; Delft, 2600GA Technology, of Univeristy Delft Sensing, Remote & Geoscience of Department mnsnSaEbyetaersosbefrtelargest the the for in responsible (TG) are Glacier Embayment Thwaites Sea and Amundsen (PIG) Glacier Island ine tdssails(NS/NSIsiu ercecepu ed le pour recherche de (CNES)/CNRS/Institut spatiales etudes | ´ Antarctica a,1 f | annSun Sainan , n hmsNagler Thomas and , eoesensing remote d nttt o aieadAmshrcRsac teh,UrctUiest,38 CUrct h Netherlands; The Utrecht, CC 3584 University, Utrecht Utrecht, Research Atmospheric and Marine for Institute f c bevtieMidi-Pyr Observatoire nvriyo ayad atmr ony on etrfrErhSse ehooy AAGdadSaeFlight Space Goddard NASA Technology, System Earth for Center Joint County, Baltimore Maryland, of University b | c he modeling sheet ice hitpe Shuman Christopher , e en ´ e/aoaor ’tdse G en d’Etudes ees/Laboratoire ´ | %of ∼5% vlpeet(IRD)/Universit eveloppement ´ c etWouters Bert , vle oadterpddvlpeto pnfatrsna the near and ( fractures shelf line open grounding ice of development eastern rapid how- and the remaining tongue 2016, toward the evolved glacier Since in the upstream 7. between farther of zone ref. moved shear part by damage large described TG a as this the of ever, tongue and removal disin- glacier tongue subsequent gradual TG glacier the the the its and between with shelf in ice zone started shelf eastern shear damage ice the the of PIG TG, tegration northern For the (6). unprecedented from 2015 the disconnection after intact and largely retreat remained zone shear ern oisS1 (Movies shelf zone rapidly ice shear southern has PIG the damage of the initial apart of tearing the into how 2016 satellite shows since but evolved study (7), documented our previously in been imagery has as 1999 in line in shelves ice both on lack zones 1). shear areas from (Fig. in 2019 -damaged evolution and observa- growing line the grounding rapidly Satellite the show to near shelves. 1997 decades in m ice two ice both past of dense the of contain over zones that tions shear fractures the open ground- of within the and near areas line crevassed ing highly of consist areas damage doi:10.1073/pnas.1912890117/-/DCSupplemental at online information supporting contains article This 1 BY-NC-ND) (CC 4.0 NoDerivatives License distributed under is article access open This hsatcei NSDrc umsin ..i us dtrivtdb h Editorial the by invited editor guest a is C.G. Submission. Direct PNAS Board. a is article This interest.y competing with no paper declare the authors wrote The S.L. and data; authors.y analyzed all E.B. from and assistance J.W., performed B.W., E.B. C.S., and S.S., J.W., B.W., S.L., C.S., research; S.S., S.L., research; designed S.L. contributions: Author owo orsodnemyb drse.Eal [email protected] Email: addressed. be may correspondence whom To aigdmg rcse nmdl oipoesalvlrise level sea improve incorpo- to for models projections. in need processes the stability, damage underline rating shelf they ice from Moreover, contributions for level Antarctica. sea future damage and retreat, of line grounding importance mechanism disin- damage the for the shelves include highlight that ice results these Model structural precondition tegration. their of they signs ice as satel- first Thwaites weakening are use and areas Island damage we with Pine These areas on Here shelves. damage fractures rise. of open development and level the crevasses show sea to for imagery lite uncertainty remains major glaciers these a of in future glaciers the outlet projecting Yet, changing Sea Antarctica. fastest Amundsen the the among in are Glacier Embayment Thwaites and Glacier Island Pine Significance o I,ti aaeeouinsatdna h grounding the near started evolution damage this PIG, For y a,d ohsqee Oc et eophysique ´ rn Pattyn Frank , oisS1 Movies alSbte US,300Tuos,France Toulouse, 31000 (UPS), Paul-Sabatier e ´ b aoaor eGailge Universit Glaciologie, de Laboratoire aorpi ptae OPLGS,Centre (OMP/LEGOS), Spatiales eanographie ´ and . y b raieCmosAttribution-NonCommercial- Commons Creative S3). a Wuite Jan , and . y https://www.pnas.org/lookup/suppl/ ,weestenorth- the whereas S2), NSLts Articles Latest PNAS e , e NE IT ENVEO ir de Libre e ´ | ´ elange f7 of 1

EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES Fig. 1. Damage evolution in Embayment. (A) Sentinel-2 satellite overview of the Amundsen Sea Embayment area showing PIG and TG glaciers and maximum strain rate (in purple-green) since 2015 derived from Sentinel-1 velocity time series. Zoom boxes, transects [P1, T1], and grounding line evolution from ref. 40 are illustrated in black and spectral colors (1992, purple; 1994, blue; 2000, green; 2011, orange), respectively. (B–D) Evolution of damage areas in Landsat (1997 to 2018) satellite image time series.

The observed damage for both PIG and TG ice shelves occurs shear zones with the existing fractures (23–26) perpendicular to typically in the shear zones where the is thin (Fig. 2 A–E) flow in the center of PIG’s ice tongue. When these rifts connect and where high positive maximum strain rates occur (Fig. 1A). with the damaged shear zone, the ice front is no longer stabi- These high maximum strain rates promote the development of lized due to the structural weakening, resulting in large calving damage through the opening of crevasses and rifts (Movies S1 events. A similar condition happened in September and October and S3). This process can be clearly seen in the southern shear 2018 and February 2020 when a large rift from the damage zone zone of PIG and in the shear zone between the TG’s glacier developed across the PIG ice shelf (Movie S4), resulting in an tongue and eastern ice shelf, where high maximum strain rates unprecedented retreat of the ice shelf front (Movie S5). Due to are observed (Fig. 1A). In the northern shear zone of PIG, on this retreat, the ice shelf is no longer stabilized by the southwest- the other hand, the observed damage evolution is absent or lim- ern tributary ice inflow (6), resulting in a further destabilization ited due negative maximum strain rates (Fig. 1A) that result in of PIG’s ice shelf. closing of crevasses and rifts. On the other hand, the preconditioning is the result of a Simultaneously with the damage development satellite altime- damage feedback process, where damage enhances speedup, try (Fig. 2 F–H) shows an average elevation lowering of 0.3 m/y shearing, and weakening, hence promoting additional damage between 2010 and 2017 for PIG and TG glaciers and ice shelves, development, but where loss of buttressing, enhanced shearing, which locally can reach up to 13 m as described by ref. 15. and glacier speedup also enhances damage. Initially, this damage Additionally, the satellite altimetry data reveal local patches of feedback can be triggered by a variety of weakening processes positive elevation changes, which are the result of the advection that range from 1) oceanic melting that undermines the shear of patches of thicker ice (Fig. 2C) in an Eulerian reference frame zones by thinning the ice shelf from below (17, 27); 2) high- (16). Also the observed velocity gradients across the PIG and TG velocity gradients that result in surface troughs (28); and 3) ice glaciers have increased in these shear zones with increases up to shelf retreat that results in unpinning, reduced buttressing of 30% since 1992 as well as with widening of the shear zones (Fig. 2 existent pinning points (9, 29), grounding line retreat, and glacier K and L). The largest velocity increase occurred between 2000 speedup (30). Each of these weakening processes could result in and 2010 corresponding to a period of warmer ocean waters that the initial development of damage areas in the shear zones. Once initiated an ice-dynamical response (10, 16–18). the damage is initiated, however, the feedback process kicks in and the weakening of the shear zones results in further speedup Damage Preconditioning for Ice Shelf Disintegration and shearing, hence promoting additional damage development. We hypothesize that the combination of localized ice shelf thin- ning, enhanced velocity gradients, and the rapid development of the damage areas in the shear zones of PIG’s and TG’s ice Modeling of Damage Feedback shelves is a sign that these ice shelves are already preconditioned To assess the importance of this damage feedback in the shear for further disintegration. On the one hand this preconditioning zones, a continuum damage model (CDM) was coupled to the is the result of the fact that the integrity of both ice shelves is BISICLES ice sheet model (31) in an idealized setup to illustrate compromised as shown in the satellite imagery, similarly to the the impact of damage on ice sheet response. Earlier simula- preconditioning of the shear zones of Larsen B (19–21) prior to tions (31) with this model have already shown that inclusion of its collapse as a result of hydrofracturing (22). An example of this damage in the model corresponds to a flow enhancement fac- preconditioning can be seen in the interaction of these damaged tor of ∼10 or ∼20 K difference in ice temperature, whereas

2 of 7 | www.pnas.org/cgi/doi/10.1073/pnas.1912890117 Lhermitte et al. Downloaded by guest on September 27, 2021 Downloaded by guest on September 27, 2021 hrit tal. et Lhermitte ekniesefmrisa nrf 28. ref. in may as that margins melting and shelf channelized ice (Materials of weaken introduction meters expressed the in by is depth or damage Methods) introduction crevasse where the integrated via model, vertically line the as grounding in the at crevasses/damage zone of dam- shear enhancing the locally in the by several age in implemented framework, this were damage In scenarios melting. of damage ocean to importance relative damage zones the shear and assess melting to ocean-induced parameterizations different with simulations experi- this that—in real-world conditions constrained. a initial poorly it chosen case—are and the particular as from control, setup suffer experimental may idealized ment better an such a chose gives deliberately 3A We (Fig. has conditions Methods). which PIG to setup, similarities (MISMIP+) strong Sheet Project bed Ice Marine retrograde Intercomparison the with following Model stresses, geometry lateral sheet strong applied and ice slopes was marine modeled model idealized the typically an Therefore, is on scenarios. poten- the sheet which the ice to future melting, relative quantify for process ocean-induced to feedback of goal damage opted process the the we of with however, mechan- importance study, approach the tial this assess different shown In to a has shelves. models for ice damage 32) of use (21, weakening to model ical possible damage is it another that with work as Same earlier (L) datasets. different show types line the and years observation different the 1A). represent (Fig. colors T1 the where 1992, since imagery 07 eie rmCyst2stlieatmtydt hwn hnigoe h I n Ggair n c hle ncmiainwt h oa advection local the with combination (F in 1). shelves Fig. ice (see and respectively glaciers TG colors, and spectral PIG (H and the ice. black over thicker in thinning of illustrated showing data patches are altimetry (40) of satellite 1992 Cryosat-2 since from evolution derived line 2017) grounding and T1], [P1, (A, PIG 2. Fig. Velocity [km/yr] nti daie eu ecridotsvrltime-dependent several out carried we setup idealized this In 0 1 2 3 4 and C, k lvto n hnigrtsoe I ra (A–E area. PIG over rates thinning and Elevation 0 n G(B TG and D) 10 –J and omaescrepnigt oe in boxes to corresponding areas Zoom ) 20 E c hle n h aaeaesi h hne ra.Ga ausaemse odt aus ombxs transects boxes, Zoom values. data no masked are values Gray areas. thinner the in areas damage the and shelves ice ) Distance [km]Distance Pine IslandPine 30 and SE aelt iia lvto oe rmJnayadFbur 08soigteeeainof elevation the showing 2018 February and January from model elevation digital satellite ASTER ) 40 aeil and Materials 1995 2000 2005 2010 2015 50 F and (K G. oe n deto fpthso naae hce c cor- altimetry ice satellite thicker by observed undamaged changes of elevation the patches ice to of responds thick advection undamaged, and relatively zones result of can patches 3 zones of shear (Fig. the advection in the decoupling in and the thinning More- to upstream. the ice transport over, grounded ice of with thinning accelerated consequent filled and to be ocean leading could buttressing, and shelf m stream ice the or ice in water the gaps open decouple open even that and zones shelf shear ice the in and line grounding undisturbed 3F the to (Fig. relative simulations. rates fractions model damage strain zones overall shear of maximum weaker increase enhanced a of development has 3D), downstream. the zone (Fig. also farther shear with the shelf effect in ice weakening similar melting and the channelized in zone enhancing resulting shear Locally 3E), complete (Fig. the damage increase rates the of of with strain together expansion maximum 3G) and (Fig. of zone increase shear shear the entire the the causes in across point 3C) one (Fig. at these damage zones weakens enhancing further Locally that shelves. channelized damage or ice enhanced instability damage in shelf localized results ice to for due melting driver weakening a initial as the feedback as damage the of tance 0 1 2 3 h oe eut loidct nacdtinn tthe at thinning enhanced indicate also results model The h oe eut nFg and 3 Fig. in results model The eoiytascsaogtasc 1drvdfo utsuc satellite multisource from derived P1 transect along transects Velocity ) l 0 G–J .Ti oee atr ftinn nteshear the in thinning of pattern modeled This ). 5 ´ lne hstinn n eopigreduce decoupling and thinning This elange. 10 Distance [km]Distance Thwaites 15 oi S6 Movie –J lvto hne 21 to (2010 changes Elevation ) NSLts Articles Latest PNAS 20 ihih h impor- the highlight K u ln transect along but 25 ,adan and ), | f7 of 3 30

EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES AB

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Fig. 3. Ice sheet model with damage feedback result (see Movie S6 for animated version). (A) Initial ice thickness in BISICLES ice sheet model with initial grounding line (black dotted line). Grounding lines after 100 y model simulation for different model forcings are illustrated in colors corresponding to key in B. Dashed lines are used for no initial damage and solid lines for damage via introduction of crevasses at the location of star in C on both sides of the shear margin, while the red dotted-dashed line corresponds to channelized melting scenario. Black dashed-dotted lines illustrate location of profile in B.(B) Profile showing the geometries and grounding lines locations for different model forcing corresponding to different colors (see key) where dashed lines represent no initial damage, solid lines show runs for damage via introduction of crevasses at the location of the star in C, and the red dotted- dashed line corresponds to a channelized melting scenario. (C) Integrated damage D for 25 m/y oceanic melt at grounding line with 20 m enhanced damage. Upper half shows simulation without the local damage enhancement at the grounding line, whereas Lower half shows simulation with damage enhancement at the grounding (at location of star) via introduction of crevasses. (D) Integrated damage D for 25 m/y oceanic melt at grounding line with 20 m enhanced channelized melting in the lower shear zone. Gray values represent areas where the ice thickness is 0. (E) Maximum (Max) strain rate for simulation without/with (Upper/Lower half) enhanced damage at the grounding line. (F) Max strain rate for simulation with channelized melting. (G–L) Changes in damage fraction (G and H), thickness (I and J), and velocity (K and L) in 25 m/y melt run without/with 20 m vertically integrated crevasse depths implemented in the shear zone at grounding line (G, I, and K) and without/with channelized melting (H, J, and L), respectively, where negative/positive values represent thinning/thickening/increased D values or slowdown/speedup/decreased D values due to damage introduction and channelized melting, respectively.

(Fig. 2C) and the visual pattern of crevasses and rifts that resem- 20 m deep in the shear zone at the grounding line causes stronger ble the strongly thinned or open patches (Movies S1 and S3). grounding line retreat than 100 m/y oceanic melting without this Finally, the model shows a speedup of the glacier tongue as a enhanced damage. result of the weakening, which results in an increase in maxi- Although the results of idealized model output show simi- mum strain rate (Fig. 3 E and F) and in velocity gradients across larities with observed damage, thinning, and velocity evolution, the shear zone (Fig. 3 K and L) similar to the increasing veloc- it is important to stress that the idealized experiments do not ity gradients across PIG and TG observed by satellites (Fig. 2 K allow us to directly evaluate the observed changes at PIG and and L). TG. First, the idealized model may not include the potential The damage also has an important impact on the modeled feedbacks that might be important when interpreting the obser- grounding line retreat as the enhanced damage scenarios in the vations. For example, the rapid thinning near the grounding line model initiate an enhanced grounding line retreat. For both the as a result of reduced buttressing may result in a larger ice flux local initialized damage and the channelized melting this ground- from upstream that reduces grounding line thinning and slows ing line retreat is equivalent to quadrupling the oceanic melting down grounding line retreat. Second, the observed changes in near the grounding line from 25 to 100 m/y (Fig. 3 A and B). reality are the result of the interplay between oceanic forcing This indicates that weakening the shear zones and enhancing and bathymetry [e.g., the stagnation of grounding line retreat damage are powerful mechanisms for triggering strong ground- over PIG (10, 16) due to reduction of oceanic melting (5, 17) or ing line retreat as all enhanced damage model runs result in a the changes in individual pinning points (6, 29), sea water intru- stronger grounding line retreat (up to 200%) than the model runs sions (15), or tidal/wave-induced stress (33, 34)]. The observed with only strong oceanic melting and no enhanced crevassing; for changes in damage, thinning, and velocity gradients are therefore example, a 20 m/y oceanic melting combined with crevasses of not expected to be the result of damage only, but also include

4 of 7 | www.pnas.org/cgi/doi/10.1073/pnas.1912890117 Lhermitte et al. Downloaded by guest on September 27, 2021 Downloaded by guest on September 27, 2021 hrit tal. et Lhermitte the show all to including S1 processed 2014 (Movies since winter was TG austral m) and the PIG during 10 at images of areas damage resolution of spatial development recent with interferomet- [IW] of wide images [GRD] ric detected (SAR) range radar aperture (ground and synthetic imagery Sentinel-1 backscatter 1 Copernicus of (Fig. series dam- projection time the a stereographic Second, of polar development in the show areas to https://earthexplorer. age processed Explorer down- were Earth respectively; series Survey time 1997, Geological usgs.gov) after/of US images the for from m loaded opti- 15/30 Thermal annual with of Spaceborne band resolution First, (panchromatic/blue Advanced spatial (ASTER) areas. and Radiometer damage Reflection Landsat and the from Emission of composites development satellite the cal show Evolution. to Damage used of Imagery Satellite Methods and from Materials contributions level sea future the surface glaciers. assess Antarctic and major to velocity, crucial ice forc- is ocean topography, melt of bedrock knowledge bathymetry, accurate ing, with sheet ice combination sensitivity future in This in models 39). processes (27, damage channels incorporating basal that local- suggests in as thinning stability have shelf shelf zones ice ice shear ized for the consequences in far-reaching weakening processes, similar mechanical dam- oceanic and the weakening Therefore, process or (3–5). age shelf variability coun- atmospheric climatic to ice be prone in can are other these which changes as from melt by subshelf different or terbalanced is surface as it such shelves processes for ice such, for only limited As expected are (38). is which shelf rates, healing strain ice damage maximum the negative as at damage coun- that gradients the feedbacks frictional negative terbalance trigger of to expected loss not are to margins due gradients grounding ity and loss zones mass shear enhanced impor- these retreat. to have line in vulnerable could damage them observed lines makes these the grounding that as the imply implications to tant results close model areas damage damage our even collapse, Nevertheless, area. in a shelf ice changes without in TG result and to could PIG which and of lead (6), reductions (6) large inflow could ice patterns and weakening points calving pinning could The in stabilizing and (17). changes 37) to 36, response moreover 5, TG nonlinear atmo- (4, and future conditions a PIG extreme ice trigger sea and of and varying response oceanic, to spheric, future sensitive more the shelves makes region ice this damage in melt the surface be projected (35), may limited the Embayment to collapse Amundsen due a restricted the such in of of result hydrofracturing potential a through as the collapse Although its to (22). prior hydrofracturing (19–21) area the B future of Larsen preconditioning of for the zones shelves to shear similar ice collapse, TG possible and and losses PIG the shear precondition weakening hap- quickly extensively zones These already conditions. is structural current 11, under the ref. pening which by that margins, Thwaites and shear for Thwaites expanding simulated and was rapidly Island Pine now the in is of described weakening 28 as damage and initial 7 the refs. that show results satellite Our Conclusion and Implications Damage TG. there- and is PIG when of It account evolution into loss. the processes grounding mass modeling weakening these hence introducing take and to of crucial flux, satellite fore way ice the increased effective in retreat, very observed line vul- a currently most be is their can observations, at as glaciers locations, the these nerable assess weakening process. that to damage illustrates and us but margins This shear allow thinning, weakened however, the and of the do, importance speedup of experiments observed effect idealized and the our cause on the is feedback it between experiment damage distinguish idealized to the possible and observations not 2011 the since both forcing In ocean abated (17). the including drivers other these nteftr,ti ehnclwaeigadicesdveloc- increased and weakening mechanical this future, the In utsuc aelt mgr was imagery satellite Multisource and S4). oisS2 Movies and S3). rmATLAdt curdi aur n eray21 fel available (freely 2018 February and derived January at models in acquired elevation data digital AST-L1A ASTER from individual mosaicking Changes. by retrieved Elevation and Elevation Surface xeiet a mlmne hr eapidvrigoenforcing ocean numerical varying of applied set we a where retreat, implemented line was grounding and experiments strain, velocity, physical thickness, the comprehend features. to observed with us conjunction enables in it work such, at As the mechanism melt. over weaken- shelf subshelf control ice to greater to due adjacent a damage ing to the of leads impact obliterate the setup could delineate the to that shortcomings experiment such, shelf prevent As ice work. to the at at of setup front mechanisms conditions model calving initial idealized unknown and delib- to an We shelf due conditions. such ice PIG for an with similarities opted to strong end erately show one which and at other divide bedrock the submarine ice 80-km-wide an and from 800-km-long trough, an along flows the ice to where similar extent 32. full and 21 its refs. through by cracked used is damage that scalar ice divid- isotropic for after 1, (D) and thickness ice, thickness. to intact ice ice converted integrated the be vertically by can the ing of which the fraction meters, unitless upon in as a based depth expressed consequently law crevasse is flow integrated Damage Glen’s (55). vertically Nye modifying of by integrated formula opening and vertically during crevasse model mesh introducing evolving the by levels nonuniform, in a four introduced crevasses use with was to run framework allows Damage was which refinement simulations. km, model mesh 2 The adaptive to (31). 0.5 an dynamics from using shelf y and 100 sheet for dam- ice between large-scale interaction out and carrying the age allows of examining This development coupled. experiments the strongly numerical to are both idealized field way, due includes flow this (54–56) ice This In the law viscosity. and 56). ice damage flow (55, the Glen’s affects damage in that of damage factor sources enhancement local down- time-dependent to and a due advection (54) damage ice of the stream damage conservation where The the simulations. considers (SSA) model the approximation continuum throughout constant shelf assumed balance shallow ice is stress the temperature BISICLES-CDM integrated vertically al. to the the et in according solving velocity Sun equation, by flow [see computed is Ice feedback model description]. damage sheet complete the a of for importance (31) the assess to mented Modeling. Sheet Ice from (53). Line 2 Grounding Version Interferometry, Antarctic Radar MEaSUREs Satellite from Differential derived was line (40) Grounding evolution respectively. strain damage, maximum of healing/enhancement two- negative/positive in the where result from rates 1), derived (Fig. rates data strain velocity from dimensional component stress principal deriving first by calculated the were 2 rates Fig. strain in maximum Subsequently, lines acquisition. dashed Enveo as the shown are via and which resolution and 500-m (50–52) at website interferom- available Cryoportal tandem are 2 Satellite which Fig. sensing tracking, in Remote etry/offset European lines (49) from dashed-dotted portal data as data NSIDC’s velocity shown via resolution are 1-km which at 1996, 2012 for and to available 2006 are dotted and which 2002, (9), as 2000, Embayment shown Sea and Amundsen are the Snow which of National Velocity and 2 (48) via Fig. portal resolution in data 1-km lines (NSIDC)’s and at Center 2005 available Data between are Ice Environ- Maps which Research Velocity 2 in (47), Ice Use Fig. 2017 Antarctic for in Annual Records lines (MEaSUREs) Data System solid ments website Earth Cryoportal as Making Enveo shown from the are data via which resolution and 200-m (46) at (45), 1) 2014 available datasets: since are Sentinel-1 velocity Copernicus which tracking available feature from different data combining velocity Lines. ice by Grounding retrieved and were Rates, data Strain Velocity, Ice the following resolution (44). 500-m al. a eleva- et of Wouters at derive set in framework presented dense to Eulerian method resulting a used The in then changes (43). was tion al. measurements et elevation Gray the time-dependent of following approach processed synthetic processing were its in mode swath Euro- satellite (SARIn) the interferometry Cryosat-2. by radar retrieved from aperture waveforms 2017 L1b to measurements Agency 2010 interferometric Space for pean change from elevation of derived Rates were resolution. horizontal m 500 at 2). 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EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES rates ranging from 25 to 100 m/y and where we enhanced damage in the interplay between ocean forcing and damage feedback and by analyzing shear zone at the grounding line by the local introduction of crevasses or by the changes in ice sheet/shelf thickness, velocity, strain, and grounding line the introduction of enhanced channelized melting of 20 m/y in the southern retreat we quantified their relative importance. Additionally, a simulation shear zone. The choice for the local introduction of damage is motivated was performed where we combined the 25-m/y ocean forcing introduc- by the fact that the observed damage at the grounding line (e.g., due tion of enhanced channelized melting of 20 m/y in the southern shear localized melting) (15, 59) is not represented in the idealized setup of Sun zone. The choice for this channelized melting scenario is motivated by the et al. (31). In this framework we implemented different crevasse depths for results of ref. 28 that show that the shear zone might also be weakened by different damage scenarios. These scenarios varied from no enhanced dam- channelized melting. age to 100-m vertically integrated crevasse depths. The location of initiated damage is illustrated in Fig. 3B by the black star and is applied on both sides Data Availability. SI Appendix contains a table providing information and of the shear margin and this location evolves with grounding line migra- a download link for every dataset used. Data have been deposited in the tion (i.e., if the grounding line retreats, the location of enhanced damage 4TU.ResearchData repository (60–63). also retreats). These locations compare well to the locations of observed damage origin and correspond to the observations where the damage is ACKNOWLEDGMENTS. S.L. was funded by the Dutch Research Council constantly initiated locally close to the grounding line while the damaged (NWO)/Netherlands Space Office Grant ALWGO.2018.043. C.S. was funded by the NASA Cryospheric Science Program. E.B. acknowledges support from zones are subsequently advected downstream (Movie S1). As such, the setup the French Space Agency (CNES) through the Terre solide, Ocean,´ Sur- also differs from just advecting a single crevasse as we constantly reimple- faces Continentales, Atmosphere` (TOSCA) program. B.W. was funded by ment a crevasse at the grounding line, resulting in a damaged area that NWO VIDI Grant 016.Vidi.171.065. T.N. and J.W. acknowledge support from expands while being advected downstream (Movie S6). The set of varying the European Space Agency through the Climate Change Initiative (CCI) oceanic melting rates and enhanced damage values allows us to assess the program.

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6 of 7 | www.pnas.org/cgi/doi/10.1073/pnas.1912890117 Lhermitte et al. Downloaded by guest on September 27, 2021 Downloaded by guest on September 27, 2021 5 .F y,Tedsrbto fsrs n eoiyi lcesadice-sheets. and glaciers in velocity and model stress calving of based distribution The physically Nye, F. A J. Benn, I. 55. D. Vieli, A. Veen, Der Van J. C. Nick, M. F. Grounding 54. Antarctic “MEaSUREs from Data Scheuchl, B. Mouginot, J. Rignot, interferome- E. tandem ERS 53. from IV Embayment “Amundsen from interferom- Data tandem Space, DTU ERS from 52. IV Embayment “Amundsen from Data interferom- Space, tandem DTU ERS from 51. IV Embayment “Amundsen from Data Ice Space, DTU InSAR-Based 50. “MEaSUREs from Data Scheuchl, B. Mouginot, J. Rignot, E. 49. hrit tal. et Lhermitte A dynamics. glacier the (2010). for 781–794 implications 56, and glaciers outlet marine to applied National NASA https://doi.org/10.5067/ Center. 2020. 2.” Archive May 1 Active Version Accessed Distributed IKBWW4RYHF1Q. Interferometry, Center Data Radar Ice and Satellite Snow Differential from Line 2020. May 1 Accessed http://cryoportal.enveo.at/data/. Cryoportal. 1995/1196.” tracking, try/offset 1 Accessed http://cryoportal.enveo.at/data/. 2020. May Cryoportal. 1992.” tracking, Center. etry/offset Archive 1 Accessed Active http://cryoportal.enveo.at/data/. 2020. NASA May Cryoportal. Distributed 1994.” 1.” tracking, etry/offset Center Version May Data Antarctica, 1 Ice Embayment, 2020. Accessed Sea and Amundsen https://doi.org/10.5067/MEASURES/CRYOSPHERE/nsidc-0545.001. Snow the National of Velocity 1–3 (1957). 113–133 239, rc .Soc. R. Proc. .Glaciol. J. 3 .Lemte .Brhe,Atrdgtleeainmdloe mnsnSea Amundsen over model elevation digital Aster Berthier, E. Lhermitte, S. 63. ice accelerates “Damage for years 100 at output BISICLES-CDM Sun, com- S. stress Lhermitte, S. principal first 62. the from rates strain maximum Amundsen Lhermitte, S. 61. over change elevation showing satellite Cryosat-2 from Data Wouters, B. Lhermitte, S. 60. melt ocean of new stability a PICOP, The communication: Brief Gagliardini, Bondzio, H. O. J. Favier, Morlighem, M. L. Pelle, T. Durand, G. 59. Krug, J. Gudmundsson, H. G. 58. calving of Asay-Davis dynamics S. the and X. processes Calving 57. Mottram, H. R. Warren, R. C. Benn, I. D. 56. ca oe necmaio rjcs IMPv MSI ) SMPv IOI +) (ISOMIP 2 (MISOMIP1). v. 1 ISOMIP v. +), MISOMIP (MISMIP 3 and v. MISMIP projects: intercomparison model ocean glaciers. 0ab8f7c9.Dpstd2 uut2020. https://doi.org/10.4121/uuid:06d53bf5-d4cd-4ffa- August 21 Deposited b01a-b586f87dcb93. 4TU.ResearchData. Embayment. 4TU.ResearchData. Embayment.” Sea 2020. Amundsen August 21 in loss Deposited mass https://doi.org/10.4121/uuid:6ed00ab2-4848-41a9-b9fd-10576324af67. and instability shelf 2020. https://doi.org/10.4121/ August 4TU.ResearchData. 21 data. Deposited velocity uuid:78ef1983-19ba-4572-b2dc-6f262c34dfe8. Sentinel-1 2D from ponent https://doi.org/10.4121/ 2020. August 4TU.ResearchData. 21 Glacier. Deposited Thwaites uuid:da761d33-2e37-41e9-af6b-cd4b8d588e55. and Glacier Island Pine model. plume a (2019). and 1043–1049 PICO 13, combining shelves ice under parameterization slopes. retrograde on lines grounding at-c.Rev. Earth-Sci. xeietldsg o he nerltdmrn c he and sheet ice marine interrelated three for design Experimental al., et 4–7 (2007). 143–179 82, esi oe Dev. Model Geosci. Cryosphere 4710 (2012). 1497–1505 6, 4129 (2016). 2471–2497 9, NSLts Articles Latest PNAS Cryosphere | f7 of 7

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