Significance Fifty years ago, it was speculated that the marine-based West Antarctic Ice Sheet is vulnerable to warming and may have melted in the past. Testing this hypothesis has proved challenging due to the difficulty of developing in situ records of ice sheet and environmental change spanning warm periods. We present a multiproxy record that implies loss of the West Antarctic Ice Sheet during the Last Interglacial (129,000 to 116,000 y ago), associated with ocean warming and the release of greenhouse gas methane from marine sediments. Our ice sheet modeling predicts that Antarctica may have contributed several meters to global sea level at this time, suggesting that this ice sheet lies close to a “tipping point” under projected warming.

Abstract The future response of the Antarctic ice sheet to rising temperatures remains highly uncertain. A useful period for assessing the sensitivity of Antarctica to warming is the Last Interglacial (LIG) (129 to 116 ky), which experienced warmer polar temperatures and higher global mean sea level (GMSL) (+6 to 9 m) relative to present day. LIG sea level cannot be fully explained by Greenland Ice Sheet melt (∼2 m), ocean thermal expansion, and melting mountain glaciers (∼1 m), suggesting substantial Antarctic mass loss was initiated by warming of Southern Ocean waters, resulting from a weakening Atlantic meridional overturning circulation in response to North Atlantic surface freshening. Here, we report a blue-ice record of ice sheet and environmental change from the Weddell Sea Embayment at the periphery of the marine-based West Antarctic Ice Sheet (WAIS), which is underlain by major methane hydrate reserves. Constrained by a widespread volcanic horizon and supported by ancient microbial DNA analyses, we provide evidence for substantial mass loss across the Weddell Sea Embayment during the LIG, most likely driven by ocean warming and associated with destabilization of subglacial hydrates. Ice sheet modeling supports this interpretation and suggests that millennial-scale warming of the Southern Ocean could have triggered a multimeter rise in global sea levels. Our data indicate that Antarctica is highly vulnerable to projected increases in ocean temperatures and may drive ice–climate feedbacks that further amplify warming.

The projected contribution of the Antarctic ice sheet to 21st-century global mean sea level (GMSL) ranges from negligible (1) to several meters (2, 3). Valuable insights into the response of ice sheets to warming may be gained from the Last Interglacial (LIG) (or Marine Isotope Stage [MIS] 5e in marine sediment records; 129,000 to 116,000 y before present or 129 to 116 ky) (4⇓⇓⇓⇓–9). This period experienced warmer polar temperatures and higher GMSL (+6 to 9 m, possibly up to 11 m) (4, 10⇓⇓–13) relative to present day, and was the most geographically widespread expression of high sea level during a previous warm period (4, 10). LIG sea level cannot be fully explained by Greenland Ice Sheet melt (∼2 m) (8), ocean thermal expansion, and melting mountain glaciers (∼1 m) (4), implying substantial Antarctic mass loss (3, 4, 14, 15). Half a century ago, John Mercer was the first to propose that the marine-based West Antarctic Ice Sheet (WAIS) is vulnerable to a warming atmosphere through loss of buttressing ice shelves and may have made a significant contribution to global sea level during the LIG (5⇓–7). Recent work has further demonstrated that extensive deep, marine-based sectors of the East Antarctic Ice Sheet (EAIS) may have accelerated melting, thus contributing to higher LIG sea levels (14). While an isotopic signature of a relatively cool LIG climate preserved in the Mount Moulton blue ice field (16) may be explained by substantial WAIS mass loss (17), no direct physical evidence has yet been identified (4, 18). Temperature estimates derived from climate model simulations provide an indirect measure of change but typically suggest ∼1 °C less warming than proxy-based reconstructions (4, 8, 19). When used to drive ice sheet models, these climate anomalies are not sufficient to remove the floating ice shelves that buttress ice flow from central Antarctica (20). In an attempt to bypass these problems, ice sheet models have been driven by a wide range of prescribed climate scenarios; however, these suggest widely different sensitivities dependent on model physics and parameterization (21, 22), with >2 °C (and in some instances >4 °C) ocean warming required for the loss of the WAIS, exceeding paleoclimate estimates (3, 9, 20, 23) and different sensitivities of Antarctic ice sheet sectors (18, 24, 25).

Here, we report a high-resolution record of environmental change and ice flow dynamics from the Patriot Hills Blue Ice Area (BIA), exposed in Horseshoe Valley (Ellsworth Mountains; Methods) (Fig. 1A). Horseshoe Valley is a locally sourced compound glacier system (i.e., with negligible inflow) that is buttressed by, but ultimately coalesces with, the Institute Ice Stream via the Horseshoe Valley Trough, making the area sensitive to dynamic ice sheet changes across the broader Weddell Sea Embayment (WSE) (26). Due to strong prevailing katabatic airflow, an extensive BIA (more than 1,150 m across) has formed to the leeward side of the Patriot Hills, where ancient ice is drawn up from depth within Horseshoe Valley (Fig. 1E). Regional airborne and detailed local ground-penetrating radar (GPR) surveys show a remarkably coherent series of dipping (24 to 45°) layers, broken by two discontinuities, which represent isochrons across the Patriot Hills BIA, extending thousands of meters into Horseshoe Valley. A “horizontal ice core” across the BIA spans the time intervals 0 to 80 ky and 130 to 134 ky (Methods and SI Appendix, Fig. S5) constrained by analysis of trace gases and geochemically identified volcanic layers exposed across the transect, which have been Bayesian age modeled against the recently compiled continuous 156-ky global greenhouse gas time series (CO 2 , CH 4 , and N 2 O) (27) on the AICC2012 age scale (28) (Fig. 1B and Methods). The record is located 50 km inland from the modern grounding line of the Filchner–Ronne Ice Shelf in the WSE (29) and close to the Rutford Ice Stream, one of the largest methane hydrate reserves identified in Antarctica [total organic carbon estimated to be 21,000 Gt (30), equivalent to ∼2,000 y of the current carbon emission rate of 10 GtC/year (https://www.co2.earth/global-co2-emissions)]. Today, precipitation at the site is delivered via storms originating from the South Atlantic or Weddell Sea (31). Crucially, the Ellsworth Mountains also lie in a sector of the continent that is highly responsive to isostatic rebound under a scenario of substantial WAIS mass loss, potentially preserving ice from around the time of the LIG in small valley glaciers and higher ground areas (32).

Fig. 1. Location and age profile of the Patriot Hills BIA. (A) Location of Antarctic ice and marine records discussed in this study and austral spring–summer (October to March) SST trends (over the period 1981 to 2010; HadISST data). (B) Trace gas (circles), tephra (triangles), and boundary (square) age solutions for surface ice along transect B–B′ relative to an arbitrary datum along the transect (displayed in D). The dashed lines denote unconformities D0–D2 at their surface expression. (C) Basal topography of the Ellsworth Subglacial Highlands (West Antarctica) with the locations of airborne radio-echo sounding transect A–A′ (displayed in E) and Rutford Ice Stream (IS) (29). The Horseshoe Valley, Independence, and Ellsworth troughs are given by the initials HV, IT, and ET, respectively. (D) The location of Patriot Hills in Horseshoe Valley (LIMA background image) with the BIA climate line (marked by transect B–B′), dominant ice flow direction, and distance to grounding line. (E) Airborne radio-echo sounding cross-section of ice within Horseshoe Valley, Independence, and Ellsworth troughs (modified from ref. 29). Digitization highlights basal topography (brown), lower basal ice unit (gray), and upper basal ice unit (red) as well as internal stratigraphic features (black for observed, dashed for inferred, and purple for best estimate).

Ocean Warming What could be the cause of this ice loss in the South Atlantic sector of the Southern Ocean? Recent work has proposed that the iceberg-rafted Heinrich 11 event between 135 and 130 ky (during Termination II) may have significantly reduced North Atlantic Deep Water (NADW) formation and shut down the Atlantic meridional overturning circulation (AMOC) (42), resulting in net heat accumulation in the Southern Hemisphere (the bipolar seesaw pattern of northern cooling and southern warming) (43, 44) (Fig. 4A). Under this scenario, surface cooling during Heinrich 11 increased the northern latitudinal temperature gradient and caused a southward migration of the Intertropical Convergence Zone and midlatitude Southern Hemisphere westerly airflow (14, 45). Importantly, Heinrich 11 was probably one of the largest of the iceberg-rafting events over the last 140 ky (including H-1 and H-2) and during a time of likely weakened AMOC (42). In the Southern Ocean, the associated northward Ekman transport of cool surface waters (something akin to today; Fig. 1A) was likely compensated by increased delivery of relatively warm and nutrient-rich Circumpolar Deep Water (CDW) toward the Antarctic margin (14, 34, 43, 45, 46), potentially leading to enhanced thermal erosion of ice at exposed grounding lines (43, 47). This interpretation is supported by the enriched benthic foraminifera 13C values into the LIG (46), a proxy for the influence of NADW on CDW in the south, implying northern (warmer) waters were reaching far south for much of this period (and a cause of persistent loss of ice volume) (Fig. 2I). The unambiguous precise correlation between the Patriot Hills ice and West Antarctic marine records (34) afforded by the Termination II tephra demonstrates that the warming recorded in the BIA is coincident with a major, well-documented peak in marine temperatures and productivity around the Antarctic continent and in the Southern Ocean (34, 45, 46) (Fig. 2). The subsequent delivery of large volumes of associated freshwater into the Southern Ocean during the LIG would have reduced Antarctic Bottom Water (AABW) production (46), resulting in increased deepwater formation in the North Atlantic (43, 48, 49) (Fig. 4C). Recent modeling results suggest that increased heat transport beneath the ice shelves can drive extensive grounding-line retreat, triggering substantial drawdown of the Antarctic ice sheet (2, 14, 20) (Fig. 4B). Of concern, warming of the ocean cavity in the WSE is projected to increase during the 21st century (50). Fig. 4. Ocean–atmospheric interactions during Termination II and the LIG. Panels show changing Atlantic meridional overturning circulation (AMOC) in response to iceberg discharge (A and B) in the North Atlantic (Heinrich event 11) during Termination II and (C) from the Antarctic Ice Sheet (AIS) during the LIG, with inferred shifts in atmospheric circulation including midlatitude Southern Hemisphere westerly (crossed circle) airflow and Intertropical Convergence Zone (ITCZ) (14, 43, 45, 46, 48). The vertical arrows denote CH 4 and heat flux associated with Antarctic coastal easterly (dot in circle) and westerly (crossed circle) airflow (30, 47). AABW, AAIW, CDW, NAIW, and NADW define Antarctic Bottom Water, Antarctic Intermediate Water, Circumpolar Deep Water, North Atlantic Intermediate Water, and North Atlantic Deep Water, respectively. With Southern Ocean warming and concurrent ice sheet retreat, the large methane reservoirs in Antarctic sedimentary basins (e.g., Rutford Ice Stream) could have become vulnerable to release (30) and may have contributed to elevated atmospheric levels through the LIG (8, 27) (Fig. 2D). High-latitude open water and sea ice are rich in microbial communities, components of which may be collected by passing storms and delivered onto the ice sheet (e.g., prokaryotes, DNA), offering insights into offshore environmental processes (51, 52). To investigate environmental changes prior to and after the ice sheet reconfiguration recorded in the Patriot Hills BIA, we applied an established ancient DNA methodology and sequencing to provide a description of ancient microbial species preserved within the ice (Methods). Methane-utilizing microorganisms were found in three samples along the Patriot Hills transect and were absent from other samples on the transect and laboratory controls. While such microbes are not obligate methylotrophs and can be present in nonmethane-dominated environments (53), they would be expected to be at very different abundances to what we find. The most striking feature of the Patriot Hills BIA genetic record was detected immediately prior to inferred ice loss, where Methyloversatilis microbes dominated the detectable microbial diversity (∼130 ky) (Fig. 2E and SI Appendix, Fig. S15). Methyloversatilis was only found in high abundance in this sample (with trace amounts identified at ∼22 ky). Crucially, Methyloversatilis are facultative methylotrophs and live on single and multicarbon sources (54), consistent with elevated levels of CH 4 and active methane oxidation by Methyloversatilis or other methanotrophic taxa in marine sediments or in the water column during the end of Termination II (SI Appendix). More work is needed to explore the potential for microbial methane utilization in this unique environment.

Acknowledgments C.S.M.T., C.J.F., M.I.B., A.C., and N.R.G. are supported by their respective Australian Research Council (ARC) and Royal Society of New Zealand fellowships. Fieldwork was undertaken under ARC Linkage Project (LP120200724), supported by Linkage Partner Antarctic Logistics and Expeditions. J.W. and K.W. undertook GPR survey of the Patriot Hills record through the Natural Environment Research Council Project (NE/I027576/1) with logistic field support from the British Antarctic Survey. S.M.D. acknowledges financial support from Coleg Cymraeg Cenedlaethol, the European Research Council, and the Fulbright Commission (259253 and FP7/2007-2013). K.K. was supported by Japan Society for the Promotion of Science and the Ministry of Education, Culture, Sports, Science and Technology’s Grants-in-Aid for Scientific Research (15KK0027 and 17H06320). We thank Dr. Chris Hayward and Dr. Gwydion Jones for electron microprobe assistance; Kathryn Lacey and Gareth James for help with preparing the tephra samples; Drs. Nelia Dunbar, Nels Iverson, and Andrei Kurbatov for discussions on the tephra correlations; CSIRO GASLAB personnel for support of gas analysis; Prof. Bill Sturges and Dr. Sam Allin of the Centre for Ocean and Atmospheric Sciences (University of East Anglia, Norwich, UK) for performing the sulfur hexafluoride analyses; Levke Caesar (Potsdam Institute for Climate Impact Research) for preparing the recent trend in SSTs in Fig. 1; Vicki Taylor (British Ocean Sediment Core Research Facility, Southampton, UK) for assistance with marine core sampling; and Dr. Emilie Capron (British Antarctic Survey) for advice on reconstructing early southern LIG temperatures. We thank CSIRO’s contribution, which was supported in part by the Australian Climate Change Science Program, an Australian Government Initiative. We also acknowledge Johannes Sutter, Torsten Albrecht, and Jonathan Kingslake for advice and data on their model simulations. We also thank the editor and two anonymous reviewers for their insightful comments that helped improve this manuscript.

Footnotes Author contributions: C.S.M.T., C.J.F., and N.R.G. designed research; C.S.M.T., C.J.F., N.R.G., N.P.M., E.v.S., R.T.J., D.E., M.R., D.P.T., S.M.D., C.B.R., Z.A.T., M.I.B., N.C.M., M.K., J.W., K.W., L.S.W., C.M.R., H.M., P.G.A., A.R., A.M., C.-D.H., M.E.W., C.J.M., J.Y., and A.C. performed research; L.S.W. and A.C. contributed new reagents/analytic tools; C.S.M.T., C.J.F., N.R.G., N.P.M., E.v.S., R.T.J., D.E., M.R., S.M.D., C.B.R., Z.A.T., M.I.B., N.C.M., M.K., J.W., K.W., L.S.W., P.G.A., T.v.O., M.C., A.M., S.R., K.K., C.-D.H., M.E.W., and A.C. analyzed data; and C.S.M.T. wrote the paper with input from all authors.

The authors declare no competing interest.

This article is a PNAS Direct Submission. J.S. is a guest editor invited by the Editorial Board.

Data deposition: The isotopic datasets generated in this study are available at the publicly accessible National Oceanic and Atmospheric Administration (NOAA) Paleoclimatology Database (https://www.ncdc.noaa.gov/paleo/study/28610).

This article contains supporting information online at https://www.pnas.org/lookup/suppl/doi:10.1073/pnas.1902469117/-/DCSupplemental.