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Downloaded by guest on September 25, 2021 a An Lu northeast and , Isstrøm Nioghalvfjerdsfjorden Zachariae the of melting Ocean PNAS is Greenland it but retreat, sheet models. sheet ice of most bathymetric driver in by main incorporated modulated not a line, therefore grounding is the barriers, combined at Ocean-induced a ice observations. m/y) for of of 12% 99 removal m/y) within to 50 also km, to m/y 4.4 of m/y (53 retreat (27 narrower within removal retreat a ice observations via thinning-induced 79N grounded of and independent to lower retreat AIW matches of yields line access that passage grounding limited from the y combined contrast, increase 41 In a to 14%. in for line m/y, that km ice flotation 217 ocean 13 Observed its to the 1979–2019. of m/y by in retreat 105 m/y line a 185 grounding caused to the thinning m/y at 108 removal from ice increased of rate a warmed 1.3±0.5 has channel by that in temperature ocean Intermediate Subsurface depth. Atlantic warm, (+1.25 subsurface, chan- (AIW) broad allows 800-m-deep, an that reveals nel data inver- gravity three-dimensional airborne A of glaciers. sion the of impact on a the despite forcing analyze experienced thermal stable We forcing. ocean more and climate same been m/y, the has to 650 exposure 79N its at ice retreated loss. float- an up, its mass lost sped 5-gigaton/y ZI hold rise. shelf, level that ice sea global ing Greenland 1.1-m a northeast to Schaffer) equivalent Janin volume in and Forster R. glaciers Richard marine- by are terminating reviewed (79N) Nioghalvfjerdsfjorden 2020; and 22, (ZI) July Isstrøm review Zachariae for (sent 2020 2, November Rignot, Lyngby, Eric 2800 by Denmark, Contributed of University Technical Observation, and Geodesy Institute, Space D le pour Recherche 91109; CA h rttm ntesme f19 3,adte ntesum- the in then in and 3.0 temperature by Air (3), warmed (7). 1997 region 2010–2014 this of and 2008, summer 2001–2005, the of for mers in disintegrated time barrier ice first Øer warmer-than- the Norske on The out in (6). flushed summers stranded being usual get before could decades thermomechanical for (3). why from the explained barrier shelf and ice ice 5) (4, the Øer breakup protected shelf Norske barrier ice the ice detached as the The of known pieces of glued previously disappearance that the together, cover with ice coincides sea timing permanent its but cidated, same ice the 79N explanation. the to an Meanwhile, until for exposure 2). calls its up which (1, despite forcing, speed 2013 climate stable not early relatively warm- did 2012 remained regional late has glacier in of the collapsed com- years but shelf a decay, following shelf—of for and disintegrate, ice equivalent ing extension—or to floating level started the sea 2004, ZI In m northwest m). in 0.19 0.55 equivalent) bined system and level glacier sea (0.36 Humboldt Jakobshavn m Greenland and are (0.47 Petermann basins Greenland the marine west and sea major central submarine for two in potential major other Isbræ largest The three the rise. with the level (see Greenland and of in m m one basins 1.1 0.54 drainage represent by combined level a They sea for Methods). global respectively, raise m, to 0.57 ice enough contain and Z eateto at ytmSine nvriyo aiona rie A92617; CA Irvine, California, of University Science, System Earth of Department h as fteZ c hl rau a o enflyelu- fully been not has breakup shelf ice ZI the of cause The ot lce,7N ri 2 fteGenadIeSheet 79 Ice Greenland (or the of Nioghalvfjerdsfjorden 12% drain and 79N) Glacier, (ZI) North Isstrøm achariae 01Vl 1 o e2015483118 2 No. 118 Vol. 2021 a,1 c | eateto ii n niomna niern,Uiest fClfri,Ivn,C 92617; CA Irvine, California, of University Engineering, Environmental and Civil of Department rcRignot Eric , ◦ ◦ ic 99 sn noenmdl ecalculate we model, ocean an Using 1979. since C )t ec h rn fZ i w il t350-m at sills two via ZI of front the reach to C) vlpeet rnbeIsiueo niern,Isiueo niomna esine,301Geol,Fac;and France; Grenoble, 38031 Geosciences, Environmental of Institute Engineering, of Institute Grenoble eveloppement, ´ | e level sea a,b,c,1,2 ◦ rm17 o21,o 0.08 or 2017, to 1979 from C | c–ca interaction ice–ocean ihe Wood Michael , a,b | aeil and Materials climate ohK Willis K. Josh , ◦ / (8). C/y doi:10.1073/pnas.2015483118/-/DCSupplemental at online information supporting contains article This 2 1 ulse eebr2,2020. 28, December Published BY-NC-ND) (CC 4.0 NoDerivatives License distributed under is article access open This and interest.y Polar competing for no declare Institute authors Wegener The Alfred J.S., and paper. Research. y Utah; the Marine L.A., of wrote J.M. research; University and R.R.F., performed J.K.W., L.A., Reviewers: J.M. and data; and analyzed L.A. S.A.K. and research; J.M., M.W., designed E.R. contributions: Author uelre hnsraeml ae:5myt / naeaefor average on m/y 6 to m/y 5 rates: melt surface than larger of tude circulation the controls sills access Bathymetry Shallow the glaciers. m. heat. facilitate over- the 150 ocean troughs cavity depth, to top deep the while m the AIW AIW in in 250 of of present below access polar the is resides fresh limit AIW AIW cold, (12). that by shelf show laid ice shelf 79N ice tempera- of 79N a the with clockwise on 15) a +1 (14, in than greater carries (AIW) circulating ture NT Water and (11–13). northwest Intermediate NT the Atlantic to the southeast via the from south fashion sec- the this entering from Current, Greenland tor East the by of carried set Water, a by anchored (10). front and m ice homogeneous its ice been at an islands has by shelf ensem- glued now ice heterogenous blocks is 79N a and ice was shelf of shelf the toward ble The ZI of 1). southbound rest north (Fig. The the stagnant. Island heading from Ø severed branch Schnauders shelf northbound along slower-moving (NT) Trough a Norske with , destroyed (7). action, barrier wind ice with the combination accelerated in and which, growth melt, ice summer sea slowed temperatures air Warmer b owo orsodnemyb drse.Eal [email protected] Email: addressed. be may correspondence whom work.y To this to equally contributed E.R. and L.A. e rplinLbrtr,Clfri nttt fTcnlg,Pasadena, Technology, of Institute California Laboratory, Propulsion Jet nlddi ueia c he oest erdc h high the reproduce to retreat. models be of the sheet rates must on ice ice control numerical grounded in major of included a undercutting is and forcing evolution, glacier thermal ocean demon- The retreat that 1979. calculated strates since and to retreat observed thinning line between from agreement grounding retreat observed flotation the and explain of line because grounding ocean 79N the the by to at removal than ice reconstruct ZI We to barriers. of access under- bathymetric water easier but salty has equivalent origin warm, volume Atlantic Subsurface, ice evolutions. rise different level of went sea Green- front 1.1-m which a in (79N), northeastern hold Nioghalvfjerdsfjorden bathymetry and and (ZI) of Isstrøm temperature Zachariae data ocean reveal gravity land and Oceanography Significance b I et h c hle trtsta r reso magni- of orders are that rates at shelves ice the melts AIW Atlantic North is glaciers the to heat ocean of source The Jøkelbugten into southbound flow to used shelf ice ZI The J , er ´ meMouginot emie ´ y ◦ .Maueet odce narf zone rift a in conducted Measurements C. d https://doi.org/10.1073/pnas.2015483118 nvriyo rnbeAps NS ntttde Institut CNRS, Alpes, Grenoble of University a,d . y raieCmosAttribution-NonCommercial- Commons Creative n haa .Khan A. Shfaqat and , . y ´ lne() ncnrs,the contrast, In (9). elange https://www.pnas.org/lookup/suppl/ e National e | f8 of 1

EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES A B

Fig. 1. ZI and 79N glaciers, northeast Greenland with (A) OMG airborne gravity data (milligal) color coded from blue (−140 mGal) to red (+40 mGal) with 10-mGal contour levels, ice speed on the color coded from brown (low speed) to blue and red (fast speed) (46), and grounding line positions color coded from year 1979 to 2019 and (B) bathymetry inferred from gravity inversion at sea and BMv3 on land with 200-m contour levels. AXCTD 1 through 5 are magenta stars labeled 1 through 5 (black circle). CTDs used to reconstruct thermal forcing in NT are colored triangles labeled with years. The ice thickness control lines are in red. Ocean flux gates are white. Longitudinal profiles used in Fig. 3 are purple (profile A-C) for 79N and blue (profile B-C) for ZI.

the entire ice shelf, 25 m/y within 10 km of the grounding line mine whether we can match the rates of grounding line retreat (16), and 50 m/y to 60 m/y near the grounding line of 79N (17, observed independently with satellite data. We conclude on the 18) versus less than 1 m/y at the surface. As more ocean heat recent and future evolution of this major sector of Greenland. reaches the cavity, ice shelf melt increases. As air temperature warms up, the ice m´elange that glues together ice shelf pieces Results and Interpretation melts away, and the ice shelf breaks up (4). The breakup reduces Glaciological Setting. ZI is 20 km wide. Satellite imagery shows resistance to flow, the glacier speeds up, it thins as a result, and that the ice shelf area decreased from 706 km2 in 1985 to 616 ice reaches flotation sooner; that is, the grounding line retreats. km2 in 2002, and, subsequently, to 37 km2 in 2014; 377 km2 of Once a glacier loses its ice shelf, it terminates with a vertical the detached ice shelf remains in the fjord to the north of the calving cliff. Ocean models and multibeam echo sounding obser- glacier (Fig. 1). Between 2014 and 2019, we find that the ice vations show that, in that configuration, ocean waters undercut front retreated 1.6 km to lose another 32 km2, leaving virtually the glacier, as melt rates are largest near the ice cliff base (19, no floating section. In ref. 21, we reported that the glacier speed 20). When an ice shelf is present, we do not know how fast ice increased from 1.2 km/y in 1979–1992 to 2.1 km/y in 2019, or 81% melts at the grounding line or what the shape of the cavity looks (22); the grounding line flux increased from 9.7±1 Gt/y in 1979 to like. Here, we hypothesize that ice is removed at the grounding 16.2±2 Gt/y in 2019, or 81%, versus a balance flux of 8.9 Gt/y; the line of an ice shelf at nearly the same rate as in the case of a mass loss averaged 5 Gt/y in 2009–2019 (SI Appendix, Table S1). vertical wall (Fig. 2). Glacier 79N is 20 km wide, with a speed that increased from To quantify ice–ocean interactions and their impact on glacier 1.3 km/y in 1979 to 1.5 km/y in 2019, or 12%, and a grounding line evolution, we need information about ocean temperature and flux that increased from 11.7 km/y to 13.1 Gt/y versus a balance bathymetric controls on the delivery of ocean heat to the glaciers. flux of 9.6 Gt/y (21). The mass loss of 79N averaged 2 Gt/y in In this part of Greenland, glacial fjords have not been mapped 1979–1990 and 4 Gt/y in 2009–2019. completely. Here, we employ airborne gravity data collected in ZI lost mass from enhanced flow but also from increased 2016 by the NASA mission Oceans Melting Greenland (OMG) surface melt, which produces runoff. Runoff production to infer the bathymetry in front of ZI between Jøkelbugten and reconstructed by the Regional Atmospheric Climate Model NT on the eastern flank of Schnauders Ø Island. We use the (RACMO)2.3p2 regional atmospheric climate model (23) results in combination with a time series of ocean temperature increased from 1.5±0.3 Gt/y in the 1980s to 3.3±0.7 Gt/y in the data to reconstruct ocean thermal forcing over the last 41 y. 2010s, or 126%. A time series of surface elevation updated from We then use an ocean model to estimate the rate of grounded ref. 21 indicates that ZI thinned at 0.8 m/y in the 1980s and 1990s ice removal by the ocean and combine the results with the and peaked at 3 m/y in 2012–2013, before turning into slight rate of grounding line retreat caused by ice thinning to deter- thickening in the last couple of years (SI Appendix, Table S1). For

2 of 8 | PNAS An et al. https://doi.org/10.1073/pnas.2015483118 Ocean melting of the Zachariae Isstrøm and Nioghalvfjerdsfjorden glaciers, northeast Greenland Downloaded by guest on September 25, 2021 Downloaded by guest on September 25, 2021 ca etn fteZcaieIsrmadNohlfedfodngair,nrhatGreenland northeast glaciers, Nioghalvfjerdsfjorden and Isstrøm Zachariae the of melting Ocean al. et An 2. Fig. -mhl-aeeghfitrwt nrserro . mGal. 1.5 with of filtered error were rms an data with The grid filter lines. 1-km half-wavelength inner at 1-km for a gridded m 500 were versus lines size Outer cell receiver. spacing GPS 2-km receivers NovA- OEM4 a m, Onboard used tel station 1). 150 reference the (Fig. while of OEMV-3, conducted offshore NovAtel clearance were spacing was ground 4-km survey a and The with inshore, airports (25). knots, the Nord 110 from Station at 208B col- and Caravans mission Kulusuk Grand AIRGrav of Cessna OMG Ltd. ocean Geophysics a the Sander on the grounding the 2016, deployed using over the NT June data and of ZI 30 gravity south between overdeep- airborne to km is higher-resolution, May 75 lected Fjord about 28 depth Jøkelbugten From toward m line. that values 900 lower revealed with and ened, glacier 79N front in the and gravity ZI airborne of (OIB) of IceBridge Operation center margin. the the m 600 at of depth 1996 line in grounding a reveals (24) Greenland Machine in thinning Bathymetry. lower with 2010, 2018–2019. in in m/y thickening 1.0 slight and at 2011–2017 m/y peaking 0.1 1970s, at from thinned the glacier Gt/y in The 4.8 240%. to or Gt/y 2009–2019, to 1.4 1979–1989 from increased production runoff 79N, in line Dashed line. grounding the at draft ice respectively. the draft, of ice slope and steep elevation the Note arrow). (red location in line line) (red topography bed new shelf, undercutting, Northing (km) D Northing (km) C B A 79N -20140429 79N -20170403 q Easting (km) c Easting (km) ceai fiesefadgairml noteoenwt noigieflux, ice incoming with ocean the into melt glacier and shelf ice of Schematic a eoegone c lcs (C blocks. ice grounded remove may q h lce e oorpyrcntutdfo Bed- from reconstructed topography bed glacier The m n hnigidcdrtetrate, retreat thinning-induced and , -1000 -1500 -200 -400 -800 Depth (m) -600

200 Depth (m) -500 500 0 0 0 0 10.0 C 10.0 –E o 9 n in and 79N for 20.0 20.0 –H Distance (km) Distance (km) aa one corm olce yNS I 4)wt ufc n e ik rddt)o o fthe of top on dots) (red picks bed and surface with (47) OIB NASA by collected echograms sounder Radar ) F 004. 50.0 40.0 30.0 q 004. 50.0 40.0 30.0 –H s o lce iha c shelf, ice an with glacier a For (A) . o Iwt oaino h aa rcso h et(re o niae aasat,adgrounding and start), data indicates dot (green left the on tracks radar the of location with ZI for H G F E ae ihadniyo .2 g/ a 1.028 of 1) g/cm density layers: 0.917 a of horizontal density with observed three a layer and with has layer calculated domain ice between solid model misfit The the gravity. minimize to (26) IAppendix , (SI mGal 0.1 S3 than Fig. less is sedimentary gravity between misfit modeled thickness, the and model when observed crustal stops the inversion in The accounts across intrusions. or variations shift basins, geology by DC underlying model the caused in of the domain variations interpolation with natural The gaps elevation (28). for bed data known where field not the gravity with min- is resulting fill gravity the a invert shift, using observed and results, grid DC the regular interpolated correct a the soundings. algorithm, onto multibeam shift curvature or DC imum tem- the BMv3, gravity conductivity, current interpolate (CTD), from observed direct We known depth and is the and elevation modeled calculate perature, (10, bed between where 79N We offset areas of in S2). or front Fig. shift, in (DC) Appendix , (BMv3) measurement v3 (SI bathymetry BedMachine 27) new from elevation and bed (24) the using g/cm culated 2.67 of density a Northing (km) Northing (km) Northing (km) Northing (km) 79N -20100525 eepo h esf MSS3Dwt akrsmethod Parker’s with 3-D GM-SYS Geosoft the employ We ZI -20100525 ZI -20170403 ZI -20140429 Easting (km) Easting (km) Easting (km) Easting (km) q ). f cbr avn rate, calving , q c o lce ihn ice no with glacier a For (B) ice. grounding remove not does -1000 -1000 -1500 -1000 -1500 -1500

A Depth (m) Depth (m) Depth (m)

-700 Depth (m) -500 -500 -300 -100 -500 500 -500 100 0 0 0 0 and 0 0 0 0 B 3 niae h mato q of impact the indicates owr oe ftegaiyi cal- is gravity the of model forward A . 10.0 10.5 10.0 10.5 q c ruddiermvlb h ca or ocean the by removal ice grounded , https://doi.org/10.1073/pnas.2015483118 cm 21.0 20.0 20.9 20.0 3 n )arc ae with layer rock a 3) and , Distance (km) Distance (km) Distance (km) Distance (km) 3 144. 52.3 41.9 31.4 004. 50.0 40.0 30.0 134. 52.2 41.8 31.3 30.0 s )a ca water ocean an 2) , n q and m PNAS ntesurface the on 40.0 | f8 of 3 50.0

EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES The results reveal an 800-m-deep trough between Schnaud- at the front of ZI, we detect a strong runoff signal consistent with ers Ø Island and Norske ØER. Jøkelbugten Trough shallows to the presence of a plume of subglacial water (Fig. 3). We comple- the south near a set of islands. The new trough, named here ment the CTD data with Estimating the Climate and Circulation Schnauders Ø Trough (ST), connects with NT and the channels of the Ocean (ECCO) model annual averages between 1992 and leading to 79N. ST has a maximum depth of 800 m versus 1,100 m 2011 (see Materials and Methods) (30, 31). The rms error between for Jøkelbugten Fjord and a minimum depth of 350 m at two sills CTD and ocean model is 0.55 ◦C. (Fig. 3B). To visualize the influence of bathymetry on ocean temper- ature, we use the line of lowest elevation through the trough Oceanographic Data. In the summer of 2019, NASA’s OMG systems starting from the grounding line to the middle of NT mission deployed Airborne Expendable CTD (AXCTD) ocean (Fig. 3). Bed elevation rises to 350 m depth about 170 km from probes in front of ZI (29). Additional AXCTD data were col- ZI at the junction between ST and NT, followed by a second lected on a NASA Gulfstream III aircraft in September to sill at the same depth common to ZI and 79N at the junction October 2016, C-130 Hercules in October 2017, a Basler DC-3 with NT. For 79N, the bed elevation is deepest at the center Turbo Prop in September 2018, and a Basler BT-67 in Septem- of the calving front (480 m) but is limited by an upstream sill ber 2019. AXCTD accuracy is ±0.1 ◦C for temperature and ±2% with a depth of 325 m (10). In contrast, ST offers a broader, 25- for depth. The probe in front of ZI reached 627 m depth, with a m-deeper pathway for AIW to reach ZI. Water temperature of bottom temperature of 1.4 ◦C (Fig. 3). The probe in the deepest 1.5 ◦C in NT below 300 m depth is blocked by the first sill at part of 79N reached 453 m depth, with a bottom temperature of AXCTD 3. Water at 1.25 ◦C reaches only the second sill in front 1.36 ◦C. of 79N but reaches the ZI grounding line. The temperature of At AXCTD 1 to 5, the temperature/salinity relationship indi- AIW changes little from AXCTD 1 to 5; that is, subsurface ocean cates a mixture of warm AIW at 1.5 ◦C and 35 psu with ice shelf waters are transferred with relatively low heat loss toward the melt water along the Gades mixing line. At AXCTD 4 glacier fronts.

1 3 5 2 A C 1 2 3 4 5

1 RUNOFF MIXING LINE C) o C) o

0 Temperature ( Temperature

TER MIXING LINE

Depth (m) -1 TWA

MEL Potential Temperature ( Potential Temperature

TF -2 79N 31 32 33 34 35 Salinity (psu) A C D Distance along profile (km) E B 4 3 5 100 C) o

200

300 Depth (m) Depth (m) Potential Temperature ( Potential Temperature 400 1979 1984 1993 1997 2000 2002 2005 2008 ZI 2016 500 2018 B C 2019 -2 -1 0 1 2 30 32 34 36 Distance along profile (km) Temp ( oC) Salinity (psu)

Fig. 3. Longitudinal profiles of ocean temperature color coded from blue (–2 ◦C) to red (+2 ◦C) with 0.25 ◦C contour level for (A) 79N (profile A-C, purple in Fig. 1) and (B) ZI (profile B-C, blue in Fig. 1) with sea floor in black, and areas with no water temperature data in white. AXCTD 1 to 5 locations are indicated with a triangle and number (red). Temperature data (C) from AXCTD 1 to 5 with Gades melt water line and runoff line, and (D) from historical CTD data (SI Appendix, Fig. S1) in NT for years 1965–2019 with (E) salinity in practical salinity units (psu).

4 of 8 | PNAS An et al. https://doi.org/10.1073/pnas.2015483118 Ocean melting of the Zachariae Isstrøm and Nioghalvfjerdsfjorden glaciers, northeast Greenland Downloaded by guest on September 25, 2021 Downloaded by guest on September 25, 2021 a . / n17,wihi infiat h ae temperature water The significant. is which 0.2 1979, was in m/y calculate 0.6 We was reference. a as 1979 cumulative was year glacier a use the we and so m/y), up, (0.1 speeding 1979 not in small was thinning 79N, For state, reference a use we near reported averages that (17). are to 79N of which comparable zone are grounding values, the width, These glacier the 2009–2019. across in m/y 185±48 100±26 to 79N, 1979–1989 For ZI. in m/y the 108±28 a of of retreat loss 1-km complete a the shelf. to balance, ice equivalent force 300-m- force long therefore buttressing of 50-km-long, is the terms line a reduce In grounding of to GN. necessary removal 3,000 be force total by will buttressing The shelf the ice MN. reduce thick will 200 shelf only thin- ice 20-m reduce by 50-km-long a will comparison, a glacier For of 20-km-wide GN. ning 2,000 a by of of force sides retreat the buttressing 1-km along the kPa A kN/m 100 shelf. of 100 ice drag or lateral an kPa a similarly, 100 and, of let line importance, drag ing its basal provide illustrate a to To assume thin flow. us to too resistance be directly basal therefore will ice reduces removal to grounded of resistance of Undercutting no area resistance. basal offer the not will below will hence Ice removal friction, of flow. basal area by the supported above Ice be balance. force glacier the radar . because penetrate not however, Radar do 2). shapes, signals (Fig. radar cavity locations reveal that loca- multiple cannot that at note echograms and at we years, thickness multiple ice but on in tion, lines, transitions grounding sharp reveal been shelf echograms not ice has Undercutting at (34). observed Alaska west The and in data 33) above 2). sounder (20, echo m (Fig. Greenland multibeam 50 with surface has situ to which the in undercutting confirmed m toward glacier been yields 30 rates melt of rates lower by distribution peak and predicted uneven with floor is uneven, melt sea be Ice the to line. grounding model the the from kilo- away several depth meters ocean versus grounding the salinity for and the temperature conditions include Boundary at model freshwater. of imposed source a is as discharged line (see water (32) Subglacial dimensions (MITgcm) ods). three Massachusetts Model in the Circulation model by m) General wall ocean vertical Technology (1 a of high-resolution along Institute undercutting, melt from ice ice derived of grounded modeling parameterization of rate a a using calculate we charge, Interaction. Ice–Ocean 2016–2019. thermocline to 1965–1979 The from m between. to 100±50 in column rose water variability the interannual of significant 40% esti- 1.3 lower by We the increased 1). have (Fig. in ST shelf forcing of ice thermal entrance 79N northern of mate the mouth at the located at one located one and gates, flux integrate ocean We two seawater. at of and point pressure-dependent the freezing and salinity-dependent ocean the of temperature situ in forcing, main mal the above and and 3 surface (Fig. the sills below evolu- temperature the ocean document (13) of circulates tion AIW where NT of shelf inner ca etn fteZcaieIsrmadNohlfedfodngair,nrhatGreenland northeast glaciers, Nioghalvfjerdsfjorden and Isstrøm Zachariae the of melting Ocean al. et An cumulative a calculate the and we Using 1979–1980) ZI, (13). for winter, 1979 reference to in prior 1965 collected conditions was ocean CTD colder express the or since differences, seasonal to oqatf h mato nectigo h lce retreat, glacier the on undercutting of impact the quantify To that calculate we model, ocean our Using affects directly ocean the by ice grounded of Undercutting the in collected 2019 to 1965 spanning data CTD Historical ◦ olri 95cmae o17,wihcudb due be could which 1979, to compared 1965 in cooler C TF Q q .W aclt ca ther- ocean calculate We S2). Table Appendix, SI m m stedphitgae ifrnebtenthe between difference depth-integrated the as , f17k n17–09 o I c thinning ice ZI, For 1979–2019. in km 1.7 of nrae rm52±13 from increased ±0.5 From q m ref ◦ o ohgair n16–09 with 1965–2019, in glaciers both for C hntegairwsi taystate. steady in was glacier the when , TF Q m ae et,adsblca dis- subglacial and depth, water , f35k n1979–2019. in km 3.5 of q m ref f5 / v.8 / in m/y 80 (vs. m/y 59 of / n20–09for 2009–2019 in m/y / n17–99to 1979–1989 in m/y aeil n Meth- and Materials q m 2 nrae from increased tteground- the at TF q m , n—fapial—h uuaiecligo ruddblocks, grounded of Q calving cumulative applicable—the and—if undercutting, in yteoen h rudn iertetwudb significantly be would retreat removal ice observed line grounded grounding the of underestimated. the of absence ocean, the half the In the explain ignored, ZI. by only and were 79N would ocean on thinning retreat the ice by of undercutting effect for If excellent glaciers. therefore is retreat both observations the and of calculations 39% between calculations, to our total due a with for is Compared 2009–2019 in km. m/y 4.4 149 and of 1979–1989 in m/y 131 by at caused or is error a retreat 30% for thinning, the and 2009–2019, undercutting of of in estimates the m/y with Compared 550 and 1979–1989 retreat. line grounding a forcing when positive are retreat, line grounding blocks, tive calving 3) ice and speed; grounded flow in undercutting, retreat, of increases and thinning-induced ice melt surface 2) by grounded caused conditions; 1) equilibrium processes: above retreat, line physical grounding three observed the that tates Conservation. Mass 4). (Fig. m 500 retreat line grounding linear a Q into translates which estimated 2019, to loss 1979 ice 217 totals grounded locations data The these line to respectively. from grounding and 2019, the (37) and derive 1978 1978 to from in DEM 2019 from a then from DEM to We data a correction ICESat-2 2014. firn from of same line results the the grounding apply of the SAR-derived reliability the comparing with the by 2015 verify correction InSAR- We to the firn 2001. m) and 7-m in line (7 line grounding grounding correction DEM-derived derived the depth fits firn best thickness a equivalent that solid-ice into select elevation surface first We ice transform ice position. ground- where the line for calculate proxy a ing to is (36) which elevation equilibrium, 2001 digital hydrostatic for a reached year retreat use from slow we a (DEM) record, versus this model both ZI complement on for 2014 To retreat and (21). 2011, rapid 79N 2000, a and reveal (35), to ZI glaciers on since 1996 (InSAR) 79N, on interferometry 1992 radar differential using observed Retreat. more Line its that Grounding with balance; consistent mass aver- surface dominates, from thinning thinning balance flow. m steady total mass the 8±1 lower 79N, surface of versus For times is, Most m origin. 10 S4 ). 12±2 dynamic or aged Fig. of m, Appendix, therefore 5±1 mass is (SI of surface for in observed used in resulted lowering line 1961–1990 than changes surface period thickness location, cumulative reference the that the a at At to m relative 1). 49±7 balance (Fig. by acceleration. thinned fluxes flow ZI ice from y, thinning 41 dynamic In and direction) temperature inland air the in ZI. for a rises along slopes retrograde retreated elevation yielding of glacier instead (bed 2009–2019, the because slopes in 1979–2019, prograde for m/y km 52±15 2.1 only to 1979–1989 a in yielding ZI, (q for 1979–2019 2009–2019 in calculated, m/y 217±56 and The thinning Appendix, S4). (SI BMv3 Fig. glacier from slopes surface observed and bed time-dependent the from deduced Retreat. Thinning-Induced c gl nZ,w bev rudn ierteto 0 / in m/y 200 of retreat line grounding a observe we ZI, On warmer from melt surface enhanced includes thinning Ice . Q f1 mfrZ n . mfr7N iha netit of uncertainty an with 79N, for km 4.4 and ZI for km 13 of s and Q Q c q m s fe 04 o 9,tegonigln retreated line grounding the 79N, For 2014. after s 7 to 47% , ref r nmtr n eesprdy epciey and respectively, day, per meters and meters in are q s ) o 79N, For 0). = Q 7myi 9918 to 1979–1989 in m/y 104±27 from increased m ascnevto ttegonigln dic- line grounding the at conservation Mass uuaietinn-nue retreat, thinning-induced cumulative , Q rudn ieretreat, line Grounding q s c km n 4 oerr.Teagreement The errors. to 14% and , Q hnigidcdretreat, Thinning-induced fapial Fg ) h cumula- The 2). (Fig. applicable if , gl Q 2 aacstecmltv anomaly cumulative the balances , o Iad130 and ZI for m https://doi.org/10.1073/pnas.2015483118 8 by 58% , q s m/y 27±7 from increased Q s km Q n 2 san is 12% and , Q q s gl gl 2 f76k for km 7.6 of q eut from results , o 9 from 79N for gl PNAS f1. km. 13.2 of a been has , q | Q s is , f8 of 5 s Q q Q q m of gl s s , , ,

EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES 10 14 A ZI TF (oC) 3 C ZI 3 dh/dt (m/yr) /yr) 12 Qm (km) Q (km) 8 2 S Q (km) * gl 10 1 6 8 0 4 6 -1 4 -2 2 2 /yr) 3 -3

0 dh/dt (m/yr) 0

10 C) 14 B 79N TF (oC) 3 o D 79N 3/yr) dh/dt (m/yr)

12 (km)Distance Qm (km) Q (km) 8 2 S Q (km) * gl 10 1 6 Thermal forcing ( forcing Thermal 8 0 6 4 -1 4 2 -2 2 -3 0 0 1979 1987 19952003 2011 2019 1979 1987 19952003 2011 2019 Year Year

Fig. 4. Evaluation of the components of the grounding line retreat of (A and C) ZI and (B and D) 79N, Greenland. (A and B) Time series of runoff production, qsg, (cubic kilometers per year, blue, left scale), thermal forcing, TF (degrees Celsius, green, right scale) and change in surface elevation, dh/dt, (meters per year, red, right scale) for (A) ZI and (B) 79N. (C and D) Cumulative anomaly in glacier undercutting, Qm, (kilometers, blue), cumulative thinning-induced retreat, Qs, (kilometers, red) versus the observed grounding line retreat, Qgl, (kilometers, black) with SE in light color for (C) ZI and (D) 79N for the years 1979–2019. If the components are correct, Qm + Qs should balance Qgl.

Discussion ice. Icebergs calving farther south in warmer parts of Greenland The impact of AIW in Greenland has been documented on are narrower and shorter, and tend to roll over when they detach other glaciers, for example, Jakobshavn Isbræ (38), mostly qual- from the ice front and remove some grounded ice. itatively and not including undercutting of grounded ice as a We have no direct evidence for undercutting at the grounded forcing mechanism. At Jakobshavn, a 250-m-deep sill at the fjord line of an ice shelf. Radar echograms reveal steep ice draft entrance limits the access of AIW to the cavity. A similar bathy- slopes at the grounding line (Fig. 2), but imaging cavities requires metric barrier protects the full access to AIW into the 79N cavity. multibeam acoustic echo sounding techniques. If the ice shelf In addition, the grounding line of 79N stands on prograde bed draft is almost horizontal, water circulation will be impaired, slopes (2), which decreases the sensitivity to ice thinning. In and the entrainment speed of the melt water plume will be the next decade, the grounding line of 79N will start retreating lower than along a vertical wall (39). Here, we assume that the along retrograde bed slopes, which will accelerate the retreat. process is as efficient as for a vertical wall. While we are able The grounding line of ZI already retreats along retrograde bed to match the glaciers’ retreat, this could be viewed as a coin- slopes, which extend another 10 km to 20 km, so ZI will continue cidence. Yet, if we do not include grounded ice removal by retreating for several decades. the ocean, glacier thinning only explains half of the observed For 79N, the agreement (14%) between observed Qgl and retreat. calculated Qs and Qm provides confidence in the component The buttressing force exerted by the ice shelf on ZI was prob- approach despite uncertainties in observed thinning, bed and ably small prior to 2014, because the ice shelf was located at surface slopes, ocean temperature, modeled undercutting, and the mouth of a bay with diverging walls. Indeed, the ice shelf ref reference state qm . For ZI, the 12% error is small and attributed started to disintegrate around 2004, but, for many years, the due to uncertainties. It is unlikely that Qc contributed to the glacier accelerated only slowly (2). We conclude that undercut- retreat, due to the nature of calving mechanisms on ZI. Most ting has been the main driver of the grounding line retreat, with a detached blocks are tabular icebergs, recognizable as much wider modulation from bathymetric barriers between ZI and 79N. The and longer than thick. Tabular icebergs detach from an ice front difference in bathymetric barriers between ZI and 79N explains, when already afloat. As such, they should not remove grounded in part, their different evolution.

6 of 8 | PNAS An et al. https://doi.org/10.1073/pnas.2015483118 Ocean melting of the Zachariae Isstrøm and Nioghalvfjerdsfjorden glaciers, northeast Greenland Downloaded by guest on September 25, 2021 Downloaded by guest on September 25, 2021 0 .Schaffer J. 10. 3 .Schaffer J. cav- the 13. and shelf continental the between exchange Water Straneo, F. Wilson, J. N. subglacial The 12. Oerter, H. Huybrechts, P. Jung-Rothenhuassler, F. Reeh, N. Mayer, C. 11. ca etn fteZcaieIsrmadNohlfedfodngair,nrhatGreenland northeast glaciers, Nioghalvfjerdsfjorden and Isstrøm Zachariae the of melting Ocean al. et An 9)t il nae vrg pe fsblca discharge, subglacial of speed the average by area flux an yield annual and the to the day m over 79N) (8,986,662 per divide gate meters production We flux ocean cubic year. the runoff in of per cross-section flux integrate gigatons a We in obtain production 2019. to annual glacier to each of 1958 basin from drainage (43) km 1 Runoff. rn o 9 1)ada h otenetac fS o I Fand TF ZI. for shelf ST ice of the entrance of mouth northern the the trans- at at this equivalent located and use temperature is We (10) gate a ZI. 79N ocean to for for The 90% temperature front fronts. esti- and NT ice we 79N the the for 2019, scale at 91.6% In to NT be S2). from coefficient Table to transmitted fer fronts m and 300 ice S1 below the Fig. temperature to ocean Appendix, of (SI fraction NT the mate in 2019 to 1965 Forcing. ). S1 Thermal Table Appendix, Ocean (SI 79N for m/d 3.74 to m/d 0.98 from and The day. per height, freeboard and the dilution) from for is m Flotation (−0.002 dilution). ZI f for for (−0.004 m 79N 0.54 for m find ice 0.57 We of number. volume smaller (V the a seawater minus Gt, to Gt), 362 ice (362 from rise level diluted sea flotation of below mm equiv- 1 to level equivalent sea water from a density into of kg/m converted 1,028 change is a flotation after above alent glacier each of basins Equivalent. Level Sea Methods and Materials even is ocean the of Greenland. in role than the critical where more obser- , same The com- to salinity. apply to and vations necessary temperature water is ocean it quantify in better trends goal, and Greenland this around mapping pro- achieve bathymetry plete To observed including the rates. replicate recommend to retreat order We high in removal retreat. global ice matching grounded of of difficulties from cesses have rates available models observed currently sheet the Ice than (41). modeling models ocean ocean higher-resolution forcing with km) of thermal combined undercutting (1 ocean bathymetry include Realistic precise (40). a not requires glaciers do for Greenland ice from grounded rise level sea ae hti stelwr4%o h ae ounta otosthermal controls that column water the of 40% lower indi- the water interactions is the ice–ocean it of 40% that of lower cates modeling the high-resolution over gates because flux column, ocean the across calculated are .A edc,“aelt mg ta fgair ftewrd–Genad in Greenland” – world the of glaciers of atlas image “Satellite Weidick, A. coastal 9. barrier, ice Øer Norske the M T. in Turton, V. changes J. production. Recent 8. ice Hamilton, calf G. and Sneed, velocities and A. W. glacier melange 7. iceberg Greenland of North force Higgins, buttressing K. weakens A. north ice 6. of stability sea the Thinning and ice Robel, Sea A. Weidick, A. A. Higgins, 5. K. A. Thomsen, H. H. fast-ice Reeh, N. of analysis 4. sensor Multi-satellite Wadhams, P. Wilkinson, P. J. Hughes, E. N. 3. Mouginot J. 2. Khan A. S. 1. b = tpeet otiesetnmrclmdl sdt project to used models numerical sheet ice most present, At lce tongue. glacier 141. p. 1386-C Paper Professional Survey logical Greenland). (Northeast glacier (2019). 79N the of istics Greenland. northeast forschung calving. promotes glaciers. floating Greenland northeast and Greenland. northeast barrier, ice Øer (2011). 151–160 Norske the in development (2015). 1357–1361 warming. regional by otes Greenland. northeast Glacier). (2015). North (79 Nioghalvfjerdsbræ beneath NE-Greenland. ity glacier, Nioghalvfjerdsfjorden under Lett. dynamics Res. Geophys. implied and cavity (1 − Runoff, ρ w 3 –3(1991). 1–23 60, /ρ saae est)(i.e., density) (seawater utie asls ftenrhatGenadiesettriggered sheet ice Greenland northeast the of loss mass Sustained al., et ahmtycntan ca etspl oGenadslargest Greenland’s to supply heat ocean constrains Bathymetry al. , et q amwtrptwy oadNohlfedfodnglacier, Nioghalvfjerdsfjorden toward pathways water Warm al., et atrteto ahræIsrm otes Greenland. northeast Isstrøm, Zachariæ of retreat Fast al., et i sg ) a.Geosci. Nat. q where b, aisfo .0mdt .4mdfrZ uig1979–2019 during ZI for m/d 1.64 to m/d 0.60 from varies l,D a s topei rcse n lmtlgclcharacter- climatological and processes Atmospheric As, Van D. olg, ¨ sg a.Commun. Nat. srcntutdwt h AM232dwsae to downscaled RACMO2.3p2 the with reconstructed is , 2929 (2000). 2289–2292 27, n.Glaciol. Ann. .Gohs e.Oceans Res. Geophys. J. a.Ci.Change Clim. Nat. sn M3 h oueo ice, of volume the BMv3, Using ca hra forcing, thermal Ocean b 2–3 (2020). 227–231 13, sbdeeain(42). elevation bed is 49 (2016). 14596 8, 75 (2016). 47–55 57, V .S ilas r,Es J .Frin,1995), Ferrigno, G. (J. Eds. Jr., Williams, S. R. , × 9–9 (2014). 292–299 4, ρ i n.Glaciol. Ann. ρ 1 kg/m 917 = i /ρ 0442 (2017). 4004–4020 122, w o.WahrRev. Weather Mon. iie ytems focean of mass the by divided ) epy.Rs Lett. Res. Geophys. 2 × o Iad60105m 6,081,095 and ZI for TF (ρ 7–8 (2001). 474–480 33, sfo 9CD from CTDs 19 from is , 3 w iedniy to density) (ice − V ρ ntedrainage the in , i )/ρ n.Glaciol. Ann. q i 1375–1394 147, ) iie by divided )), sg 7648–7654 42, nmeters in , Science ..Geo- U.S. Polar- ρ 2 w 350, for q 52, m = 5 M iso,Dt rm“ahmtydt rmteOGMsindrvdfo the from derived Mission OMG the from data “Bathymetry from Data Mission, OMG 25. 4 .Morlighem M. 24. 3 .No B. 23. Sheet Ice Greenland “MEaSUREs from Data Scambos, T. Howat, I. Smith, B. Joughin, I. 22. Mouginot J. 21. 0 .J Fried J. M. 20. Rignot E. 19. Mayer C. mass 18. and rates melt submarine Satellite-derived Heimbach, P. Straneo, F. Wilson, northern N. of glaciology 17. the to Contribution K., W. Joughin, I. Gogineni, S. Rignot, E. water 16. and Circulation Bud Tunnicliffe, G. D. M. Schneider, Paquette, W. G. R. 15. Newton, L. J. Bourke, H. R. 14. ehooysJtPouso aoaoyudracnrc ihNASA. with of contract Institute a California under at Laboratory and Propulsion Irvine, Jet Technology’s California, of University at Science, Dryad System in available ACKNOWLEDGMENTS. are CTD) Ice gate, and ocean at front, available National is (ice https://doi.org/10.7280/D19987. Greenland the data BedMachine at Other CO. available Boulder, NSIDC. in are (NSIDC) MEaSUREs Center the Data Remote veloc- publicly from Ice for (data.cresis.ku.edu). are data Kansas Center of ity the study University the at Sheets, this Laboratory Ice available of Propulsion for Sensing are Jet profiles used Technology’s Radar of AXCTD (omg.jpl.nasa.gov). Institute and California at data available gravity airborne The in noise Availability. Data The in 15%. error hence an peak 80-m yielding m/y, over 10%, 79N. 10 level is for noise is 25% slopes 2-m surface thinning to and Average 1- bed a 2%. have or DEM 2019. thinning, to 1979 from 79N thinning, ice of bed rate width, retreat, the Appendix , glacier convert (SI the to (21) across nesses velocity averaged ice thinning of OIB ice series slope, NASA’s calculate time We from a S4). altimetry and Fig. (36), 2019 airborne 2006 to with and 1993 2001 combined (37), spanning 1978 (44), from 2015–2016 DEMs on and based elevation of series time 79N. for Retreat. km Thinning-Induced 1.7 undercutting, and ZI cumulative for of in km series 3.5 uncertainty to time up nominal a adds derive a which We yielding years. (19), of dis- 15% number subglacial is of face distribution ice width-averaged spatial the the across with charge associated uncertainty in the uncertainty The data. perature depth, water melt varying TF with a model of ocean discharge, simulations glacial MITgcm three-dimensional the m), using (1 plume high-resolution on based Undercutting. Ice in Grounded trend linear a and bias absolute are an solutions remove km) two temperature. to (13.5 The data (31). medium-resolution CTD (LLC270) a using 2001–2017 calibrated 2) for initial and an solution (30) from domain 1992–2011 Arctic global the period of the out- model for model forward project km) state ECCO (4 two high-resolution use a we 1) data, puts: CTD the complement To forcing. aietinn-nue retreat, thinning-induced lative 1.18 IGa ntuet”Clfri nttt fTcnlg e rplinLaboratory. Propulsion Jet 2019. Technology January 23 of Accessed https://dx.doi.org/10.5067/OMGEV-BTYAG. Institute California Instrument.” AIRGrav with combined sounding echo multibeam conservation. from mass Greenland of mapping bathymetry (1958–2015). balance mass surface eoiyMpfo nA aa eso . AANtoa nwadIeDt Center Data 2019. Ice November Accessed and 1 https://doi.org/10.5067/OC7B04ZM9G6Q. Snow Center. National Archive NASA Active 2.” Version Distributed Data, InSAR from Map Velocity 2018. glacier. tidewater Greenland a adgair vrteps w decades. two past the over (2016). glaciers land Greenland. tongues. ice remaining (2017). largest 2773–2782 Greenland’s for (2011-2015) balance interferometry. (2001). radar satellite from Greenland Res. Geophys. shelf. Greenland East the of masses 1) eintegrate We (19). β el ¨ rc al cd c.U.S.A. Sci. Acad. Natl. Proc. n ufc slope, surface and , q s al,1k eouindt e fdwsae reln c sheet ice Greenland downscaled of set data resolution km 1 daily, A al., et = ag c osvraiiya igavjrsjre lce,northeast- glacier, Nioghalvfjerdsfjorden at variability loss ice Large al., et itiue ugaildshredie infiatsbaieml at melt submarine significant drives discharge subglacial Distributed al., et oeigo ca-nue c etrtso v etGreen- west five of rates melt ice ocean-induced of Modeling al., et a.Commun. Nat. dh/dt ot-i er fGenadiesetms aac rm17 to 1972 from balance mass sheet ice Greenland of years Forty-six al., et 2948 (1995). 4269–4286 100, q eMciev:Cmlt e oorpyadocean and topography bed Complete v3: BedMachine al., et q m l td aaaeicue nteatceand article the in included are data study All sg epy.Rs Lett. Res. Geophys. f2% ro in Error 26%. of [ – ([1 / n hra forcing, thermal and , u,O h eeaino h otes ae polynya. water northeast the of generation the On eus, ´ hswr a efre nteDprmn fEarth of Department the in performed was work This q 78(2018). 2768 9, m hnigidcdretreat, Thinning-induced ρ w costeoenflxgtsuigtesm tem- same the using gates flux ocean the across ruddieundercutting, ice Grounded ttegaircne vrsvrliethick- ice several over center glacier the at α, /ρ epy.Rs Lett. Res. Geophys. i 2994 (2019). 9239–9244 116, ] β Q .Gohs Res. Geophys. J. s q – oas76k o Iad21k for km 2.1 and ZI for km 7.6 totals , Q 15–16 (2017). 11051–11061 44, sg 4)i eesprya.Tecumu- The year. per meters in (45) α) m https://doi.org/10.1073/pnas.2015483118 s20%, is rgessa h qaero fthe of root square the as progresses TF 3127 (2016). 2361–2377 10, epy.Rs Lett. Res. Geophys. dh/dt .Gohs Res. Geophys. J. as , 3893 (2015). 9328–9336 42, b 7964 (1987). 6729–6740 92, q s5%, is m noart fflotation of rate a into , 000 b (0.0003 = q Q s s sddcdfo a from deduced is , f1%frZ and ZI for 18% of TF q 34,007–34,019 106, m s0.55 is PNAS scalculated is , 6374–6382 43, Cryosphere IAppendix SI q sg 0.33 ◦ | sub- b, 0.15) + ,and C, f8 of 7 Q 11, m J. . ,

EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES 26. R. Parker, The rapid calculation of potential anomalies. Geophys. J. 31, 447–455 37. NJ. Korsgaard et al., Digital elevation model and orthophotographs of Green- (1972). land based on aerial photographs from 1978–1987. Scientific Data 3, 160032 27. L. An et al., The tale of two ice shelves: Zachariae Isstrøm and Nioghalvfjerdsfjorden, (2016). Northeast Greenland. Dryad. https://doi.org/10.7280/D19987. Deposited 6 November 38. D. M. Holland, R. H. Thomas, B. de Young, M. H. Ribergaard, B. Lyberth, Acceleration 2020. of Jakobshavn Isbræ triggered by warm subsurface ocean waters. Nat. Geosci. 1, 659– 28. L. An, E. Rignot, R. Millan, K. Tinto, J. Willis, Bathymetry of northwest Greenland 664 (2008). using “Ocean Melting Greenland” (OMG) high-resolution airborne gravity and other 39. P. Holland, A model of tidally dominated ocean processes near ice shelf grounding data. Rem. Sens. 11, 131 (2019). lines. J. Geophys. Res. Oceans 113, C11002 (2008). 29. OMG Mission, Data from “Conductivity, Temperature, and Depth (CTD) data from 40. M. Morlighem, M. Wood, H. Seroussi, Y. Choi, E. Rignot, Modeling the response of the OMG Mission.” California Institute of Technology Jet Propulsion Laboratory. northwest Greenland to enhanced ocean thermal forcing and subglacial discharge. https://dx.doi.org/10.5067/OMGEV-AXCTD. Accessed 1 November 2019. Cryosphere 13, 723–734 (2019). 30. E. Rignot, I. Fenty, D. Menemenlis, Y. Xu, Spreading of warm ocean waters around 41. H. Goelzer et al., The future sea-level contribution of the : A Greenland as a possible cause for glacier acceleration. Ann. Glaciol. 53, 257–266 multi-model ensemble study of ISMIP6. Cryosphere 14, 3071–3096 (2020). (2012). 42. H. Goelzer, W. Coulon, F. Pattyn, B. Boer, R. van de Wal, Brief communication: On 31. H. Zhang, D. Menemenlis, I. G. Fenty, ECCO LLC270 ocean-ice state estimate. calculating the sea-level contribution in marine ice-sheet models. Cryosphere 14, 833– http://hdl.handle.net/1721.1/119821. Accessed 21 December 2018. 840 (2020). 32. Y. Xu, E. Rignot, I. Fenty, D. Menemenlis, M. M. Flexas, Subaqueous melting of Store 43. B. Noel,¨ W. J. van de Berg, S. Lhermitte, M. R. van den Broeke, Rapid ablation glacier, west Greenland from three-dimensional, high-resolution numerical modeling zone expansion amplifies north Greenland mass loss. Sci. Adv. 5, eaaw0123 and ocean observations. Geophys. Res. Lett. 40, 4648–4653 (2013). (2019). 33. E. Rignot, I. Fenty, Y. Xu, C. Cai, C. Kemp, Undercutting of marine-terminating glaciers 44. C. Porter et al., Data from “ArcticDEM.” Harvard Dataverse. http://doi.org/10.7910/ in west Greenland. Geophys. Res. Lett. 42, 5909–5917 (2015). DVN/OHHUKH. Accessed 20 April 2019. 34. D. Sutherland et al., Direct observations of submarine melt and subsurface geometry 45. R. H. Thomas, C. R. Bentley, A model for Holocene retreat of the West ice at a tidewater glacier. Science 365, 369–374 (2019). sheet. Quat. Res. 10, 150–170 (1978). 35. E. Rignot, S. P. Gogineni, B. KW, S. Ekholm, North and northeast Greenland ice 46. E. Rignot, J. Mouginot, Ice flow in Greenland for the international polar year 2008- discharge from satellite radar interferometry. Science 276, 934–937 (1997). 2009. Geophys. Res. Lett. 39, 1–7 (2012). 36. I. Howat, A. Negrete, B. Smith, The Greenland ice mapping project (GIMP) land 47. CReSIS, Data from “Radar Depth Sounder Data.” Center for Remote Sensing of Ice classification and surface elevation data sets. Cryosphere 8, 1509–1518 (2014). Sheets. http://data.cresis.ku.edu/. Accessed 23 January 2020.

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