Four Decades of Antarctic Ice Sheet Mass Balance from 1979–2017

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Reviewers: E.R. and and data; J.M. analyzed M.M. research; and M.J.v.W., designed M.v.d.B., E.R. contributions: Author from increasing 1979–2017, and in 1, decade Table accel- per 2, an (Fig. Gt/y reflects loss 6 94 mass factor in of change a eration This by ). S1 Fig. is, Appendix, that SI 2009–2017, in Gt/y 166 1989–2000, in Gt/y 40 Antarctica. future Results the for total results the the SLR. to to of Antarctica regions implications of various contribution the and of budget, contribution and mass processes the SMB between dynamics, partitioning decadal ice the a 2), bal- on (Fig. loss mass mass scale sheet ice regional time in ice acceleration by the the SMB 1979–2017, derive for of ance We reconstruction models. of climate surround- period atmospheric the entire plus the basins, covers 176 and 1 of (Fig. total islands of a ing high- discharge include ice that and continental regions inventories, the 18 assess drainage surface veloc- to topography revised of sheet resolution reconstructions ice (SMB), modeled of balance series thickness, mass time ice observa- of annual updated decades improved ity, four or use 1979, We to tions. back extending and 2017 to for remain. except uncertainties 1992–2017, where and Antarctica, 1992–2011 East periods time the for ments b owo orsodnesol eadesd mi:[email protected] Email: addressed. be should correspondence whom To e rplinLbrtr,Clfri nttt fTcnlg,Pasadena, Technology, of Institute California Laboratory, Propulsion Jet nacdplrwsele uhmr icmoa epwater glaciers. deep the circumpolar toward as more come push to to westerlies decades likely polar in enhanced are Antarctica sectors from same rise The sea-level con- dominate period. major entire a the deep been over circumpolar has tributor which subsurface Antarctica, salty, East warm, including sea- water, to flow on closest glacier enhanced areas impact by dominated in its is loss regional mass document The a rise. to level from comprehensive, model products a Antarctic climate output using the atmospheric and decades of record four balance satellite last mass precise the the over of Sheet state Ice the evaluate We Significance ee epeetrslsfo h opnn ehdupdated method component the from results present we Here, ± tyi h 1ytm eid17–90t 50 to 1979–1990 period time 11-y the in Gt/y 9 h oa asls rmAtrtc nrae from increased Antarctica from loss mass total The (RACMO2.3p1 PNAS d ecirJ a Wessem van J. Melchior , | aur 2 2019 22, January .Tepro fstudy of period The S1). Fig. Appendix, SI ± ANT27 8G/ n19–09 n 252 and 1999–2009, in Gt/y 18 . y . SMB y raieCmosAttribution-NonCommercial- Commons Creative d nttt o aieadAtmospheric and Marine for Institute yearly | o.116 vol. www.pnas.org/lookup/suppl/doi:10. 1979 2014; | d https://doi.pangaea.de/ , o 4 no. | 1095–1103 ± ± https:// 14 26

ENVIRONMENTAL INAUGURAL ARTICLE SCIENCES Table 1. Mass balance of Antarctic glaciers per region (A–K) for the Antarctic Peninsula, West Antarctica, East Antarctica, and total including surrounding islands

Area, SMB79−08,D79−89,D89−00,D99−10,D09−17, M, SLR, SLE, Glacier Region Mkm2 Gt/y Gt/y Gt/y Gt/y Gt/y Gt mm cm

Larsen D-G I00-J 61.7 24.8 ± 3.0 24.8[0] 24.8 24.8 24.8 ± 3.0 0 0.0 5 West Graham I-I00 25.1 89.6 ± 10.4 90.3[4] 91.7 93.7 98.2 ± 5.0 −140 0.4 1 East Graham I-I00 4.4 3.3 ± 1.0 3.3[0] 3.3 3.3 3.3 ± 1.0 0 0.0 0 Larsen C I-I00 18.1 14.5 ± 2.1 15.1[3] 15.9 15.2 15.3 ± 2.0 −32 0.1 1 Larsen B I-I00 9.0 6.4 ± 0.9 7.3 [4] 7.4 12.5 16.2 ± 1.4 −160 0.4 0 Drygalski I-I00 1.0 1.4 ± 0.2 1.4[4] 2.4 5.4 5.5 ± 0.6 −83 0.2 0 Larsen A I-I00 1.3 1.3 ± 0.1 1.3 [4] 2.2 5.0 3.3 ± 0.1 −60 0.2 0 Wordie I-I00 10.6 14.8 ± 1.9 16.0[1] 17.7 18.8 21.5 ± 1.4 −138 0.4 1 Wilkins I-I00 14.7 14.6 ± 0.8 14.6[0] 14.6 14.6 14.6 ± 0.8 0 0.0 1 George VI H0-I 80.1 70.3 ± 4.0 72.6[1] 74.4 78.5 79.1 ± 4.7 −217 0.6 15 Stange H0-I 15.0 16.5 ± 1.0 19.8[2] 17.6 19.2 18.9 ± 1.3 −91 0.3 2 Antarctic Peninsula 280 293 ± 27 302 307 326 336 ± 26 −909 2.5 28 Ronne J-J00 644.7 149.4 ± 8.5 152.3[5] 152.1 151.0 149.8 ± 8.0 −77 0.2 158 Fox H-H0 4.0 5.1 ± 0.3 5.4[2] 7.0 8.2 9.3 ± 0.6 −85 0.2 1 Ferrigno H-H0 9.4 7.3 ± 0.4 11.3[2] 13.6 13.9 13.3 ± 1.0 −218 0.6 2 Venable H-H0 14.9 12.7 ± 0.8 14.7[2] 17.8 17.5 16.7 ± 0.9 −152 0.4 3 Abbot H-H0 26.7 29.2 ± 1.6 30.0[2] 30.9 35.0 30.4 ± 2.5 −96 0.3 4 Cosgrove G-H 8.0 5.4 ± 0.3 5.3[1] 5.1 5.3 5.2 ± 0.6 6 0.0 1 Pine Island G-H 181.4 72.5 ± 4.6 80.2[1] 89.5 106.5 133.2 ± 5.8 −1066 3.0 51 Thwaites G-H 192.8 82.4 ± 4.9 87.0[1] 94.2 101.5 117.3 ± 3.9 −634 1.8 65 Haynes G-H 9.8 8.0 ± 0.5 10.9[1] 11.2 11.9 12.9 ± 1.0 −139 0.4 2 Crosson G-H 12.8 9.7 ± 0.5 13.9[1] 18.5 23.4 30.5 ± 1.4 −435 1.2 3 Dotson G-H 17.4 16.3 ± 0.9 17.1[1] 19.0 22.8 30.0 ± 2.0 −211 0.6 3 Getz F-G 85.9 73.2 ± 4.4 76.6[5] 76.2 90.3 90.0 ± 4.8 −363 1.0 22 Land F-G 13.3 10.6 ± 0.6 12.9[5] 13.4 14.2 14.6 ± 0.6 −121 0.3 3 Hull F-G 16.9 11.8 ± 0.7 10.1[5] 9.9 10.5 11.1 ± 0.4 55 −0.2 4 Nickerson F-G 10.4 7.1 ± 0.4 7.3[5] 7.9 8.6 8.0 ± 0.7 −32 0.1 1 Sulzberger E0-F 39.4 17.5 ± 1.0 16.7[3] 16.6 16.4 16.6 ± 1.7 37 −0.1 6 Withrow E0-F 2.4 1.0 ± 0.1 1.3[1] 1.3 1.3 1.3 ± 0.1 −11 0.0 0 Richter E0-F 0.7 0.5 ± 0.0 0.8[1] 0.8 0.8 0.8 ± 0.1 −11 0.0 0 Ross West E0-F 788.4 107.2 ± 5.6 96.2[1] 85.2 76.6 72.6 ± 4.2 916 −2.5 194 West Antarctica 2,115 653 ± 38 676 696 741 789 ± 42 −2640 7.3 528 Ross East E-E0 1,649.5 63.8 ± 3.8 65.2[4] 65.2 64.1 64.1 ± 5.5 −32 0.1 976 David D0-E 213.5 7.5 ± 0.4 9.5[3] 10.2 9.7 9.2 ±0.4 −84 0.2 118 Lillie D0-E 15.7 3.2 ± 0.2 3.3[1] 3.4 3.5 3.6 ± 0.2 −11 0.0 2 Rennick D0-E 52.3 6.1 ± 0.4 6.0[3] 6.0 5.6 5.7 ± 0.5 8 0.0 11 Slava D0-E 5.0 2.7 ± 0.2 2.9[1] 2.9 2.9 2.9 ± 0.3 −9 0.0 1 Matusevitch D0-E 17.3 3.9 ± 0.2 4.1[5] 4.2 4.3 4.5 ± 0.2 −16 0.0 6 Cook D-D0 308.1 37.7 ± 2.2 37.0[3] 40.2 40.7 40.6 ± 2.0 −66 0.2 158 Ninnis D-D0 178.7 22.4 ± 1.3 23.9[1] 23.1 23.2 23.0 ± 1.0 −34 0.1 95 Mertz D-D0 84.6 19.3 ± 1.1 18.8[3] 18.2 18.4 17.6 ± 0.8 38 −0.1 37 Dibble D-D0 32.2 15.6 ± 0.9 18.5[3] 17.7 19.9 19.9±1.4 −129 0.4 12 Frost C0-D 155.0 43.8 ± 2.6 49.0[4] 46.7 47.3 49.4 ± 1.8 −159 0.4 84 Holmes C0-D 29.7 16.8 ± 1.0 20.1[1] 19.9 21.9 21.7±1.4 −152 0.4 11 Moscow C0-D 221.6 46.1 ± 2.7 49.3[3] 50.0 47.5 47.0 ± 2.1 −93 0.3 128 Totten C0-D 556.0 64.7 ± 3.8 72.6[3] 69.0 69.2 71.4 ± 2.6 −236 0.7 385 Vincennes Bay C0-D 134.6 34.8 ± 2.1 35.7[4] 36.7 35.6 36.2 ± 0.5 −49 0.1 66 Denman C-C0 265.5 52.7 ± 3.1 55.4[3] 59.1 57.7 59.2±4.2 −191 0.5 149 West C-C0 213.7 42.3 ± 2.5 41.9[2] 41.7 41.6 43.1 ± 2.3 6 0.0 115 Publications C-C0 3.5 5.4 ± 0.3 5.5[1] 5.6 5.8 5.9±0.5 −12 0.0 13 Amery B-C 1,338.2 75.3 ± 4.4 76.6[5] 77.3 76.7 77.4 ± 3.6 −65 0.2 777 Rayner Thyer A0-B 118.4 16.6 ± 1.0 17.8[4] 18.6 16.6 16.8 ± 2.1 −32 0.1 54 Shirase A0-B 207.3 17.9 ± 1.1 18.0[3] 16.7 16.1 16.0 ± 0.7 46 −0.1 118 Baudouin A-A0 299.6 26.5 ± 1.6 27.2[4] 28.4 26.9 27.1 ± 2.0 −36 0.1 134 Borchgrevink A-A0 172.3 20.5 ± 1.2 21.3[2] 22.4 20.3 19.9 ± 1.7 −21 0.1 72 Lazarev A-A0 32.6 6.4 ± 0.4 6.0[4] 5.2 4.9 4.8±0.4 45 −0.1 7 Nivl A-A0 27.7 4.3 ± 0.3 4.5[3] 4.5 4.2 4.5 ± 0.6 −6 0.0 8 Vigrid A-A0 35.7 3.7 ± 0.2 3.8[3] 3.7 3.8 3.7± 0.3 −3 0.0 15 Jutulstraumen A-A0 190.9 29.9 ± 1.8 26.7[3] 29.5 30.6 30.0 ± 2.2 33 −0.1 73 Jelbart A-A0 20.0 8.2 ± 0.5 9.0[1] 9.8 9.1 9.1 ± 0.8 −41 0.1 3 Atka K-A 1.4 0.6 ± 0.0 0.9[2] 0.9 0.9 0.9 ± 0.2 −9 0.0 0 Ekstrom K-A 16.0 4.9 ± 0.3 4.8[3] 4.5 4.5 4.5 ± 0.7 11 0.0 2 Quar K-A 2.8 0.9 ± 0.1 1.0[1] 1.0 1.0 1.0 ± 0.2 −3 0.0 0 Riiser Larsen K-A 92.8 18.8 ± 1.1 19.2[4] 19.2 17.0 16.7 ± 2.3 25 −0.1 19 Stancomb Wills K-A 124.0 22.0 ± 1.3 22.4[2] 21.3 21.5 20.5 ± 1.6 17 0.0 46 Filchner J00-K 2,146.9 110.1 ± 6.5 110.7[1] 111.3 107.1 106.3 ± 5.7 43 −0.1 1223 East Antarctica 9,794 1,075 ± 63 1,108 1,114 1,100 1,104 ± 69 −1211 3.4 5169 Islands 163.0 77.0 ± 4.5 77.0 77.0 77.0 77.0 ± 4.5 0 0.0 0 Total 12,353 2,098 ± 133 2,163 2,194 2,244 2,306 ± 142 −4,760 13.2 5,725

Area of drainage basin in 103 km2, average SMB for 1979–2008 with 1-σ uncertainty in billions of tons per year, average ice discharge (D) for 1979–1990, 1989–2000, 1999–2010, and 2009–2017 with 1-σ uncertainty in billions of tons per year , cumulative mass gain (M) in billions of tons for 1979–2017, SLR (millimeters) for 1979–2017, and SLR equivalent for each basin (centimeters). Source of ice thickness: [1] Bedmap-2, [2] Mass Conservation, [3] Griggs and Bamber DEM, [4] Balance ﬂux and speed, and [5] Ice ﬂoatation from TDX DEM. See complete SI Appendix, Table S1 for reference.

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ENVIRONMENTAL INAUGURAL ARTICLE SCIENCES Fig. 2. Ice mass balance of Antarctica using the component method (SMB, on grounded ice minus ice discharge, D, at the grounding line) for (A) 1979–1990, (B) 1989–2000, (C) 1999–2009, and (D) 2009–2017. The size of the circle is proportional to the absolute magnitude of the anomaly in D (dD = SMB1979−2008 − D) or SMB (dSMB = SMB − SMB1979−2008). The color of the circle indicates loss in dD (dark red) or dSMB (light red) versus gain in dD (dark blue) or dSMB (light blue) in billions of tons (1012 kg) per year. Dark color refers to dD; light color refers to dSMB. Plots show totals for Antarctica, Antarctic Peninsula, West Antarctica, and East Antarctica. Background is the total mass balance spread into the drainage basins color-coded from red (loss) to blue (gain).

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ENVIRONMENTAL INAUGURAL ARTICLE SCIENCES Table 2. Mass balance (dM = SMB − D) and anomaly in ice discharge dD = SMB1979−2008 − D for West Antarctica, Antarctica Peninsula, East Antarctica, and Antarctica over decadal time scales from 1979–2017 in billions of tons per year and total contribution to SLR in millimeters (1 mm = 360 Gt/y) dM, dD 1979–1989, Gt/y 1989–1999, Gt/y 1999–2009, Gt/y 2009–2017, Gt/y 1979–2017, mm

West Antarctica −11.9 ± 3 −34.0 ± 4 −55.6 ± 5 −158.7 ± 8 6.9 ± 0.6 −9.9 ± 2 −23.7 ± 3 −70.6 ± 4 −131.9 ± 6 Peninsula −16.0 ± 2 −6.2 ± 3 −29.1 ± 3 −41.8 ± 5 2.5 ± 0.4 −7.6 ± 1 −12.1 ± 2 −31.3 ± 2 −45.1 ± 3 East Antarctica −11.4 ± 4 −9.2 ± 7 −81.8 ± 9 −51.0 ± 13 4.4 ± 0.9 −34.5 ± 3 −43.3 ± 5 −28.2 ± 6 −36.7 ± 9 Antarctica −40.0 ± 9 −49.6 ± 14 −165.8 ± 18 −251.9 ± 27 13.9 ± 2.0 −52.0 ± 7 −79.3 ± 11 −130.2 ± 13 −213.9 ± 20

so the mass loss is likely smaller than that. Farther south, the unnamed glaciers (ANT77452S12220, ANT77453S124427W, northern Larsen A glaciers lost 1 Gt/y after the ice-shelf collapse and ANT77446S12656W), and especially DeVicq, Berry, and in 1995, but only over a short period (17). In contrast, the 4-Gt/y Venzke on the western flank (Fig. 1). Farther west, glacier mass loss of Drygalski Glacier has continued from 1995 to speed-up and enhanced losses are detected on Hull (18% out present. The Larsen B glaciers accelerated in 2002 when the of balance) and Land (14%), but the glaciers draining into Nick- ice shelf collapsed and were still losing 8.4 Gt/y in 2017 (18, erson (1-cm SLE) and Sulzberger (6-cm SLE) ice shelves yield 19). We estimate a small dynamic loss for the Larsen C glaciers no evidence for mass loss or speed-up. (1 Gt/y) and negligible loss for the Larsen D–G glaciers (basin On the west Ross Ice Shelf (basin E0F), we have a unique sit- I00-J) (Dataset S1 and Fig. 1). These areas do not hold much SLE uation along Siple Coast (194-cm SLE), where the glaciers have (Table 1). continued their slow down starting from a state of balance in the In the west, the glaciers feeding Wordie Ice Shelf (1.3-cm SLE) 1970s, with a mass gain reaching 20 Gt/y mass gain in 2017. In lost 1 to 2 Gt/y in 1979–2003, increasing to 8 Gt/y in 2017, with the drainage of Ronne Ice Shelf (basin J-J00, 158-cm SLE), we speed-up detected over the entire drainage. The mass loss for find no change in glacier speed and a negligible mass gain over Wilkins Ice Shelf is small since the glaciers melt completely at the entire period (26). their grounding line and do not speed up (20). In total, the mass loss of West Antarctica is dominated by For the much larger George VI and Stange ice shelves (basin a sector spanning from George VI to Land glaciers, about H0-I), with a combined SLE of 17 cm, we estimate signifi- 2,400 km in length, with 92% of the signal from the ASE. West cant glacier losses of 9 and 2 Gt/y in 2017, respectively, ver- Antarctica contributed 6.9 ± 0.6-mm SLR since 1979. sus balance fluxes of 70 ± 4 and 16 ± 1 Gt/y. Riley, Millet, Ryder, Goodenough, and AN77204S6636W sped up in the East Antarctica. On East Ross Ice Shelf (basin E-E0, 976-cm north in the mid-2000s. Three large glaciers (AN77252S6729W, SLE), we detect no change in speed since the 1960s, as in ref. 27. AN77319S7007W, and AN77326S6324W) with deep basins sped We confirm a speed-up of Byrd in 2009 during a lake drainage up in the south (21, 22), while Nikitin and Hall glaciers sped up event (28). In Victoria Land (basin D0E), David Glacier (118-cm into Stange Ice Shelf. SLE) fluctuated at the 10% level with a small mass loss. With the Overall, the cumulative mass loss from the Antarctic Penin- RACMO2.1 SMB model, the balance flux of David Glacier was sula is dominated by the West coast (George VI, West Graham 57% too high. The improved modeling of wind transport of snow Land, Wordie, and Stange) and Larsen A and B since 1995 and in RACMO2.32p1 confirms a lower-than-average snowfall accu- 2002, for a total 2.5 ± 0.4-mm SLR since 1979. The mass loss mulation (29) and a balance flux closer to the glacier discharge. tripled after the 1990s, averaged 24 Gt/y in 1979–2017, with an We have no evidence for mass loss in Victoria Land based on acceleration of 16 Gt/y per decade. velocity changes or ice front position (30), except for Matusevitch Glacier, which sped up in 2008, losing 1 Gt/y in 2017, or 27% of West Antarctica. Inthe Bellingshausen Sea (BS) (basin H-H0), its balance flux. significant glacier changes on Fox (1-cm SLE) and Ferrigno (2- In basin D-D0, the glaciers draining into Cook Ice Shelf cm SLE) glaciers since 1979 have doubled their mass loss to (158-cm SLE) averaged a 2 to 3 Gt/y dynamic loss over the sur- 11 Gt/y in 2017. The glaciers flowing into Venable Ice Shelf (3- vey period. The western Cook Ice Shelf disintegrated in the 1970s cm SLE) slowed down in recent years, lowering their loss from (31). Glacier flow was high in the 1980s, decreased in the 1990s, 4 to 3 Gt/y. Abbot Ice Shelf doubled its mass loss from 2 to and increased in the 2000s. Farthest west, the glaciers drain- 4 Gt/y with glacier speed-up, consistent with reports of retreating ing into Ninnis (95-cm SLE) and Mertz (37-cm SLE) followed grounding line (23). Farther west (basin G-H), the dominant mass opposite trends: a small loss for Ninnis versus a small gain for loss is from the Amundsen Sea Embayment (ASE), as reported Mertz. Ninnis ice tongue broke up in 2008, followed by speed- extensively elsewhere (24, 25). Pine Island, Thwaites, Haynes, up, whereas Mertz lost a part of its floating tongue in 2013 but Pope, Smith, and Kohler hold a combined 125-cm SLE and experi- did not speed up. We detect no significant signal along Adelie enced a 136 Gt/y loss in 2017, with a disproportionate contribution Coast (36-cm SLE), which hosts few ice shelves. from Dotson and Crosson ice shelves (94% and 228% of the bal- In basin C0-D, Dibble (12-cm SLE), Frost (84-cm SLE), and ance flux versus 78% for Pine Island, 44% for Thwaites, and 49% De Haven and Holmes (11-cm SLE) combine for a dynamic for Haynes). The loss from Pine Island has stabilized since 2012 loss of 12.7 Gt/y in 2017 versus 9 Gt/y in 1979–2003. Frost but remains the largest loss (58 Gt/y) in Antarctica and elsewhere and Holmes develop small ice shelves that regularly disinte- (37 Gt/y for Thwaites and 32 Gt/y for Kohler/Smith). grate in warmer-than-usual summers (32). The glacier draining A new result is the mass loss from Getz Ice Shelf (22-cm into Moscow University Ice Shelf (128-cm SLE) shows little SLE, basin F-G) of 16.5 Gt/y in 2017, or 23% of its balance change, with a 3 Gt/y loss in 1979–2003 versus a 0.3 Gt/y gain in flux versus 5 Gt/y in 1979–2003 (i.e., a tripling in mass loss). 2017. This situation contrasts with Totten Glacier (385-cm SLE), We detect a progressive acceleration of Brennen Inlet, three which slowed down from the 1990s to 2000s, sped up until 2009,

1100 | www.pnas.org/cgi/doi/10.1073/pnas.1812883116 Rignot et al. Downloaded by guest on September 30, 2021 Downloaded by guest on September 30, 2021 otn c oge ehv oeiec o ogtr change A-A long-term (basin for Land evidence Maud its no balance in Queen have changes near in We with is level tongue. 20% SLE) ice 7% the (118-cm floating at or speed Glacier Gt/y in Shirase 0.7 fluctuations flux. despite losing mass Rayner its with of 2008–2016, Rayner in and SLE) SLE) A (19-cm (54-cm Wilma-Robert (basin on measured Land speed-up those detect Enderby in We to 1970s. is similar SLE) the remarkably (777-cm negative in B-C) velocities slightly (basin a with Shelf only balance, Ice has mass. Amery SLE) losing balance. (13-cm and mass Shelf equilibrium above Ice largest flowing its Publication but is balance, near Philippi, Farther is 2016. SLE) glacier, in 1957– (115-cm 18% in Shelf by Ice up 16% West sped by west, and 2000–2008 decelerated in 10% Scott 22% is and neighbor up glacier 1996 The Its sped 1957–2016. shelf balance. in ice C-C of 43% the out (basin and and 1957–1996 SLE 1970s in the 33% 149-cm since by a 16% up hold sped which Denman Shelf, Ice an Shackleton 1996 between km experienced S2). 17 Fig. Vanderfjord of Appendix, (SI 1996. retreat and 2017 line in Bond grounding discharge spectacular (32). ice 2008–2016, a time in lowest accelerating through a most, extent from the in changed In have vary 2003–2017. Underwood that float- in short with sections Gt/y coast, the ing 7.3 near flow Bond, fast to Anza, develop Adams, Underwood 1979–2003 and Vanderfjord, SLE), increas- in (66-cm (33), Gt/y Bay flux Vincennes balance 5.7 the of from 10% ing through about increased at has remains loss but mass speeds time Totten maintaining 2000. while year in recently, those until above again down slowed and inte al. oscil- et Rignot decadal shelves, Pacific ice ASE, the sea the melts the In (43), glaciers. on 1) the (Fig. carved destabilizes streams troughs and ice deep former by through which floor transport, glaciers Ekman more the through force shelf consis- that reaches continental westerlies is the the evolution on of This contraction CDW polar 1970s. the the with since tent increasing been have the small only for and evidence (42) no sectors C-G is losses. Larsen mass there the in Similarly, CDW 1). access of the presence (Fig. block Nickerson must CDW of and warm front east in farther of than floor shallower sea is The Shelf rates. Ice West melt balance on ice-shelf mass near-zero CDW low experiences of and it influence since Ice the shelves Nickerson of ice that limit Antarctic posit western We exhibit the loss. indeed marks mass which Shelf no (40), and Getz Ross melt and and ice-shelf documented (40), Sulzberger low ASE well on (39), not been BS but 38), has (41), (37, to CDW Peninsula Western them of the allows presence on and ice The glaciers, the faster. (CDW) the melts flow vigorously of water which buttressing deep 36), reduces (35, circumpolar shelves, shelf salty, continental to the warm, attributed on of been have intrusion They the synchronous. 3 and and widespread 2 (Figs. are and sectors Antarctica, B and East A and in Larsen and Land, Peninsula Wilkes sec- western BS the Antarctica, and ASE West the by in dominated tors, is Antarctica of loss mass The 4.4 of Discussion is contribution Antarctica total East a 1979. in since with (SI SLR loss Land, 0.9-mm level mass Wilkes 3% the to by total, 2 dominated In the S4). at Fig. fluctuating Appendix, the velocities Finally, glacier J negligible. with (basin is ance, Shelf loss Ice Filchner resulting of the basin but melange ice (34), the between and inter- Shelf time-dependent in Ice to Brunt due slow-moving level the 19% with to actions 5 the at SLE) speed (6- (19-cm in varies K-A) west (basin and the Shelf 1 to Ice (Table Riiser-Larsen neighbors and and SLE) SLE) cm (73-cm Jutulstraumen for infiatcagshv ae lc nDna lce and Glacier Denman on place taken have changes Significant h asls nAE S iks n etr Peninsula Western and Wilkes, BS, ASE, in loss mass The .I h S n S h lce changes glacier the BS, and ASE the In S4). Fig. Appendix, SI .Sacm-il lce bsnK-A) (basin Glacier Stancomb-Wills S1). Dataset 0 00 n nyasalms loss mass small a only and ) K ,2-mSE si bal- in is SLE) 1,223-cm -K, 0 B a e c shelves. ice few has -B) 0 ± ). utsno sesetfrteyas19–07(0 o West for (10) 1992–2017 years (−83.7 the Antarctica for assessment multisensor 2 to lines. shallow grounding to too CDW is warm depth of floor to access sea the exposed the enable stated because potentially as likely Nickerson), areas and most Wilkins of earlier, (e.g., rapidly number melt A not stable do are loss. CDW Shelves) no Ice Filchner exhibit and and far- Ross CDW, areas (e.g., undiluted Conversely, CDW (3). nearly from High rates thest by melt (40). caused ice-shelf high sector are more with sector eastern manifest is this the which in than Getz, losses loss CDW mass of largest incoming sector the limited to find western and we exposed the rates Similarly, on (48). melt acceleration (>400– glaciers ice-shelf and CDW the low of of its depth loss the explains mass above which seafloor m), a 700 on Abbot floats whereas (47), Shelf trough Ice Belgica the through transport CDW We several (46). last the 1970s over unique the is until loss decades. rapid 1940s recent the the that in conclude that developed bal- hypothesis the near instability contradicts was a an which ASE however, by that 1970s, find followed the We in 2002–2009 45). ance (44, in 2010–2016 loss in higher loss lower the glacier subsequent explains and which transfer loss, heat ocean the modulate lations c hl,a o h tnobWlsGair(34). may Glacier Stancomb-Wills or slower-moving the surrounding for melt the as por- shelf, and ice-shelf ice Shelf fast-moving Ice in the Shackleton between increase of tion interactions an complex and from enhanced result shelf CDW reveal may ice of speed-up the ice-shelf intrusion recent mod- but of The presence glacier, CDW. the suggests ified the which rates, near melt high a data experiences on oceanographic grounded There no upstream. is immediately Denman are slope insta- retrograde contrast, marine steep In a a developing with future. ridge of near risk first the low the at for in is slope bility hence prograde and a fast (33) by a km basin from 50 marine protected deep is modi- a Totten found (53). into Totten retreat survey of recent front A in (52). CDW fied Antarctica East a of sector by this compensated not were that between decades. (51) retreats subsequent 1980s in large readvance early experienced the have and shelves regu- 1962 Shackleton, ice disintegrate Conger/Glenzer, of Frost/Holmes West and half (31), (32), and lost years 1970s Cook low-sea-ice the in period: in larly time in shelf that degradation losses ice during the higher its shelves with even ice corroborated with years major is period, few of evolution last entire This the the 1980s. in over the only but not loss 50) mass (49, a detect We Antarctica. Gravity the reconcile balance mass to our with suffice numbers. results correction (GIA), would Experiment Climate GIA Gt/y adjustment and the 10–50 Recovery isostatic of order Revisions the glacial Antarctica. of the East uncertain- for in residual especially correction Similarly, by decade). affected in per is cm ties 5 estimate or gravity mm/y (5 IMBIE-2 level the 10% changes the detect at to required SMB (basin be plateau in will high data the With altimetry on of Antarctica. level decades cm/y B-C), East 5 the in at especially levels volume accumulation changes, translating mass in into uncertainties would estimate, large changes Antarctica by altimetry affected East IMBIE-2 is in The however, data uncertainties. mod- thickness reduce SMB ice Improved further additional trends. and decadal errors estimating els the but when SMB low and thickness are ice in uncertainties by affected (+5 assessment IMBIE-2 (−57.0 tica (168.9 109 Antarctica versus for higher are losses (−28.3 sula u asblnenmesaewti roso h IMBIE- the of errors within are numbers balance mass Our efficient with consistent is Ferrigno and Fox of loss high The ehv nopeeifrainaottepeec D in CDW presence the about information incomplete have We East Land, Wilkes from loss dynamic the is result emerging An ± ± ± 6G/)bcuew eotals o atAntarc- East for loss a report we because Gt/y) 56 ty essagi ihalreucranyi the in uncertainty large a with gain a versus Gt/y) 2 tyversus Gt/y 1 ± PNAS tyversus Gt/y 8 | aur 2 2019 22, January ± 6G/)(al ) u siaeis estimate Our 1). (Table Gt/y) 46 −20 −94 ± 5G/) u u overall our but Gt/y), 15 ± ± | 7G/)adtePenin- the and Gt/y) 27 tyfr1992–2017 for Gt/y 5 o.116 vol. | o 4 no. | 1101

ENVIRONMENTAL INAUGURAL ARTICLE SCIENCES To the East of Denman, the troughs occupied by Ninnis and larger losses in the 1980s. These sectors are all close to CDW Cook hold the most potential for rapid retreat if CDW can and experiencing high ice-shelf melt rates. Enhanced intrusion of access the glaciers (Fig. 1D). To the west of Denman, West Ice CDW being the root cause of the mass loss in the ASE and the Shelf appears to be stable or changing slowly, suggesting that West Peninsula, we posit that a similar situation is taking place in CDW does not have easy access to the cavity, but the bathymetry Wilkes Land, where novel and sustained oceanographic data are beneath and in front of the ice shelf is unknown (Fig. 1D). critically needed. Our mass balance assessment, combined with Table 1 lists the major glacier systems in terms of mass bal- prior surveys, suggests that the sector between Cook/Ninnis and ance, cumulative mass loss, and SLE (more details are given in West ice shelves may be exposed to CDW and could contribute Dataset S1). Sectors with significant SLE include ASE (125 cm), multimeter SLR with unabated climate warming. Getz (22 cm), George VI (15 cm), Land/Hull (7 cm), and Fox/Ferrigno (5 cm) in West Antarctica and the Peninsula, but Materials and Methods only the ASE may be conducive to marine ice sheet instability Ice Velocity, Thickness, and Discharge. The first comprehensive velocity map (i.e., includes bed channels extending well below sea level in of Antarctica combined data from multiple satellite sensors at different the deep interior along mostly retrograde slopes). The north- epochs (54). We use a time series of yearly averaged ice-sheet velocity ern Peninsula does not hold a large SLE: 4 mm for Larsen that spans from 1992 to 2017 with an error of a few meters per year cal- B, 1 cm for Larsen C, and 5 cm for Larsen D-G. Sectors culated from a weighted average by instrument (11). For 1979–1989, we with weak potential include Totten (385 cm) and Vanderfjord use Landsat MSS 1–5 and TM 4–5 data (SI) with errors in speed of 10– 20 m/y. Ice thickness is from (i) BEDMAP-2 (1); (ii) BedMachine Antarctica Glacier in Vincennes Bay (66 cm) because they are protected (13), which combines radar-derived thickness with ice motion vectors, SMB, by prograde bed slopes (33). Sectors at risk and requiring and ice elevation changes from altimetry; and (iii) ice-shelf thickness based more observation include West Ice Shelf (115-cm SLE), Den- on ERS-1 altimetry data from the year 1994 (55) or from (iv) a TanDEM-X man/Shackleton Ice Shelf (149-cm SLE), Cook (158-cm SLE), (TDX) DEM of Antarctica from May–July 2013 and 2014 at 30-m spacing and Ninnis (95-cm SLE). In sum, the northern sector of West (56). If ice thickness is not of sufficient quality, we assume a 1979 ice Antarctica is losing mass rapidly and could entrain the progres- flux in balance with the average SMB for 1979–2008 and scale the results sive collapse of a large share of West Antarctica and its 5.1-m based on changes in ice velocity. We constrain fluxes with ice thickness and SLE. In Wilkes Land, East Antarctica, the ice sheet loss is two time-variable velocity for 78% of Antarctic Peninsula, 96% of West Antarc- to three times slower, but this sector holds an equally large, tica, and 79% of East Antarctica. We use a 10-m uncertainty in thickness for direct radar measurements and 30 m for hydrostatic equilibrium as in multimeter SLE. ref. 7. We use a reference velocity map from ref. 3 to calculate reference Over the last four decades, the cumulative contribution to sea fluxes. We compare the reference velocity with velocity data from differ- level from East Antarctica is not far behind that of West Antarc- ent years to calculate a scaling factor over the fastest parts of the glacier tica, that is, East Antarctica is a major participant in the mass loss and apply the results to the reference flux to obtain time series of yearly from Antarctica despite the recent, rapid mass loss from West ice fluxes. Antarctica (Table 1). Our observations challenge the traditional view that the East Antarctic Ice Sheet is stable and immune Ice Drainage, SMB, and Total Mass Balance. We use a TDX DEM smoothed to change. An immediate consequence is that closer attention over 10 ice thicknesses to derive drainage boundaries based on surface slope (56). In places where flow direction is known with confidence (errors less should be paid to East Antarctica. ◦ In the decades to come, it is likely that SLR from Antarctica than 2 ) we replace the direction of surface slope with the flow direction from a reference velocity of Antarctica (SI Appendix, Fig. S3). We will originate from the same general areas, which are nearest to use the RACMO2.3p1 in our assessment. Total mass balance, SMB − D, of the sources of warm CDW and therefore directly sensitive to grounded ice in each basin is summed up by regions (A–K), East, West, a strengthening and contraction of the polar westerlies toward Peninsula, and for all of Antarctica. We calculate a mean mass loss on a Antarctica that bring more CDW in contact with the glaciers. As decadal time scale by fitting the time series of monthly data and mea- ice-shelf melt increases, the glaciers will feel less resistance to surement errors with a linear regression. The slope of the regression is flow, accelerate, and contribute to SLR. the mean mass loss for the decade and the 1-σ value of the regression is the error of the calculated decadal mass loss. Over the entire period 1979– Conclusions 2017, we perform a quadratic regression to obtain a mean mass loss and Using revised inventories, improved thickness mapping, and time an acceleration in mass loss per decade. Cumulative dSMB and dD values are calculated in reference to SMB for the 1979–2008 reference period. The series of velocity and SMB, we present four decades of mass velocity, grounding line data, and drainage basins are available at National balance in Antarctica that reveal a mass loss during the entire Snow and Ice Data Center (NSIDC), Boulder, CO as MEaSURES-2 products. The period and a rapid increase over the last two decades in parts ice-thickness data are publicly available from NSIDC, BEDMAP-2, and other of Antarctica closest to known or suspected sources of CDW references. from observations of high ice-shelf melt rates, ocean temperature, or based on ocean model output products. This evolution ACKNOWLEDGMENTS. We thank the Polar Space Task Group, European Space Agency, Canadian Space Agency, and Japan Aerospace Exploration of the glaciers and surrounding ice shelves is consistent with a Agency from the Synthetic Aperture Radar data and German Aerospace strengthening of the westerlies caused by a rise in greenhouse Center/Airbus for the TDX DEM used in this study. This work was per- gas levels and ozone depletion that bring more CDW on the formed at the University of California Irvine and at California Institute of continental shelf. While the mass loss from the Peninsula and Technology’s Jet Propulsion Laboratory under a contract with the National Aeronautics and Space Administration Cryosphere Science (NNX13AI84A) West Antarctica have been well documented and reported else- and MEaSUREs (NNX13AI84A) program. M.v.d.B. acknowledges funding where, we note that the Wilkes Land sector of East Antarctica from the Polar Program of The Netherlands Organization for Scientific has been a major participant to SLR over the last 40 y, with Research and The Netherlands Earth System Science Centre.

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