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C.B., and E.J.S., data; G.D.C., analyzed K.M.C., E.D.W. research; and performed J.P.S. and R.B.A., E.D.W., M.K., ice of composition isotopic (δ deuterium the informa- core: (δ thermometric the therefore of from analysis sources tion Our additional two Strategy ). tem- incorporates Reconstruction climatic of (SI averages long-term peratures only about information tain subse- and transition 1). (Fig. deglacial changes the reveals temperature bore- Holocene of profile quent remnant WDC temperature thermal The 3.4-km-deep direct ). Methods a the and in (Materials hole temperatures measured We Reconstruction record. core central 68-ky in only the (10). prevail Greenland by conditions provided favorable with similarly together information Elsewhere, rate, of accumulation combina- Sheet suitable snow wealth the high uniquely Ice the and given is ice Antarctic temperatures, thick site of West borehole This tion using the 16). (15, analyses of (WDC) for site core the ice Divide from history ature domi- greatly imprecise. accumulation transport method heat the small diffusive rendering the that advection, nated meant but site (14), the Antarctic at One information rate climate. such past used of tem- legacy study borehole direct most from ther- the information peratures, isotopic independent the using of mometers quantifications convincing sites core precluded Antarctic East have at rates accumulation low the Moreover, uhrcnrbtos ...adCB eindrsac;KMC,GDC,EJS,CB,T.J.F., C.B., E.J.S., G.D.C., K.M.C., research; designed C.B. and K.M.C. contributions: Author owo orsodnesol eadesd mi:[email protected]. Email: addressed. be should correspondence whom To .J Fudge J. T. , nsuhr onanrne,amnfsaino changing models. CO test atmospheric of further and can manifestation insolation, a transport, recession heat ranges, glacier oceanic mountain with southern coincident in the climate but in of than Hemisphere prediction Antarctica Northern key in earlier a progressed regions, Warming amplify polar models. processes in feedback changes which global to extent the change, times temperature quantifying three global to contemporaneous two the than was bore- larger Antarctica mea- 3.4-km-deep in a with warming along Deglacial data temperatures hole. isotopic situ in combined over- of we To surements limitation, calibration. this absolute relied without come history indicators this reconstruct isotopic cli- to on Earth’s attempts of Prior aspects system. change key mate reveal temperature deglaciation Antarctic last of the through timing and magnitude The Significance )adtentoe stpccmoiino rpe gas trapped of composition isotopic nitrogen the and D) eas fdfuiesotig oeoetmeaue con- temperatures borehole smoothing, diffusive of Because temper- Antarctic West of reconstruction a present we Here 15 IIeIooi Data Ice-Isotopic (SI N) c ihleKoutnik Michelle , and . d INtoe-stpcData Nitrogen-Isotopic SI c olg fErh ca,and Ocean, Earth, of College di .Waddington D. 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EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES 3500 3500 mid- to late Holocene (SI Basis Functions and Fig. S2). Magni- tudes are within ±0.5 ◦C in the final reconstruction. 3000 2011 15 3000 2014 To incorporate δ N data, the temperature history can be 2014 measured model adjusted further by an amount ∆TN determined by densification 2500 2500 physics to reconcile the accumulation rate histories derived two independent ways (Materials and Methods and SI Firn Thickness 2000 2000 Model and Use of Nitrogen Isotope Data): one from Ts (t) and 15 1500 δ N using a model for the evolving firn density and thickness 1500 and a second one from the ice core’s observed annual layer thick- 1000 nesses, corrected for strain using glaciological parameters deter- 242.6 243.0 243.4 mined in the optimization of Eq. 2. The multiplier ω(t) ranges Elevation above bed (m) 500 between 0 and 1, allowing us to control the relative importance of ice and gas isotopic records. It safeguards against possible prob- 0 245 255 265 -0.02 0 0.02 lems with the nitrogen isotope method; the accuracy of ∆TN as Temperature (K) Mismatch (K) a thermometer is not well established, especially given imperfec- tions of firn densification models and irregularities of the gas- Fig. 1. Observed and modeled temperatures in the WAIS Divide borehole. trapping process at the firn base. We examined reconstructions (Left) Ice temperature profile observed in 2014. At this scale the model and with various ω values and used borehole temperatures to iden- measurements are indistinguishable, as are the profiles measured in 2011 tify the range of admissible scenarios (SI Calculation of Limits and 2014. Left Inset expands the upper portion and compares the model to and Tolerance). the 2014 observation. The cold region centered around 1,700 m is a rem- ω > 0.5 nant of the last glacial climate. (Right) Difference between observed tem- Our final reconstruction (Fig. 2) uses for all but peratures and optimal models (model minus observed), for both the 2011 the late Holocene (Materials and Methods and SI Firn Thick- and 2014 measurements.

Fig. S1). Although not perfect thermometers, isotopic data A no isotopes (basis fns.) provide a constraint based on temperature-dependent physi- -30 no isotopes cal processes, and their use avoids a priori assumptions about ice isotopes only climate variability that are otherwise required for interpreting Eq. 2 + perturbations borehole temperatures. The temperature (T ) dependence of δD reconstruction (Eq. 2) arises from atmospheric distillation processes (17) and, despite potential complexities, is usually treated as a linear relationship: -35 δ = γ T + β. Due to gravitational settling of heavy gases, δ 15N measures the thickness of firn (18, 19), the 50- to 120-m-deep porous layer of consolidated snow that blankets the ice-sheet surface. Firn densification proceeds at a rate dependent on both temperature and loading, so firn thickness depends on both tem- Temperature (C) -40 perature and accumulation rate (20, 21). Higher temperatures and slower accumulation both produce thinner firn, reducing δ 15N. To determine the surface temperature history Ts (t), we opti- -45 mized the match between temperatures measured in the bore- 40 30 20 10 0 hole and those calculated with a numerical model of heat transfer B driven by various Ts (t) scenarios as a boundary condition (Mate- reconstruction (Eq. 2) rials and Methods and SI Calculation of Temperatures vs. Depth). -30 limits To start, we filtered the deuterium ice-isotopic record (δD) to linear, sensitivity = 0.80 remove high-frequency variability. Using the time derivative of linear, sensitivity = 0.55 ˙ this filtered history (δ), we then optimized the temperature vari- -35 ation (relative to modern) given by Z −1 ∆Ts (δ) = γ (t) δ˙(t) dt, [1] t -40 where the coefficient γ (t) takes three values (Table S1), one for Temperature (C) each of the three major periods of isotopic change (deglacial, early to mid-Holocene, and late Holocene). Fig. 2A shows the -45 result. This calibrated ∆Ts (δ) is then used without further adjustments in subsequent optimizations of 40 30 20 10 0

3 Thousand Years Before Present X T (t) = T + ∆T (δ) + c · g (t) + ω(t) · ∆T (t) [2] s o s i i N Fig. 2. Optimal West Antarctic Divide surface temperature reconstructions. i=1 The blue curve, our optimal reconstruction using Eq. 2 without random per- turbations, is identical in A and B.(A) Eq. 1 (green), Eq. 2 (blue), and Eq. in which the free parameters are three coefficients ci and a 2 plus random perturbations (red). Also shown are calibrated basis func- constant To , the modern temperature. This relation introduces tions without isotopes (dashed black) and the same with prescribed cooling three basis functions gi (t), with ranges ∈ (0, 1) that reflect the before 21 ka (solid black). (B) Our final reconstruction [Eq. 2 (blue)] and broadest pattern of variations seen in the observed temperature– model-dependent ±2σ limits as defined in SI Calculation of Limits and Tol- depth profile. They allow for adjustments to reconstructed aver- erance (dashed blue), together with linearly scaled ice-isotope curves with ◦ −1 age temperatures of the late glacial period, early Holocene, and sensitivities γ18 = 0.55‰ and 0.80‰ C (gray).

2 of 6 | www.pnas.org/cgi/doi/10.1073/pnas.1609132113 Cuffey et al. Downloaded by guest on September 27, 2021 Downloaded by guest on September 27, 2021 sn c stpsaoecnb rl eetd scnoptimized can 2 as (Fig. rejected, recon- isotopes firmly without be the functions basis Eq. can in optimized alone the isotopes changes contrast, ice significant using In 2A). any (Fig. large regard- temperature without and structed not achieved perturbations without is is model less the improvement negate observed to This and enough model to between profiles. error match its rms temperature–depth illustrates the 1 reduced Fig. one cK. best 1.11 The scenarios. various tested Eq. to cK 1.47 of error root-mean-square a with ( profiles perature ufye al. et Cuffey at 13.7 ka as large (3–6 as maximum perhaps thermal was Holocene site) middle this the to LGM the of γ covariation spatial and that such whether (δ on data function linear ice-isotopic single a of as temperatures treating tradi- of the method from tional derived histories temperature with comparison by Tolerance. and Limits of culation in methodologi- summarized and as parameters choices, cal model from arising uncertainties implied patterns Limits broad the mismatch of altering remaining by without the Calculation eliminated of (SI much be measure- how could variables assess the than To glaciological larger in Tolerance). and still in uncertainties but by and 1), defined ment (Fig. tolerance profile 0.8-cK temperature the the of Data, range Isotope Nitrogen of Use and cient Model ness G aeaefo 0k o2 a hni h aeHolocene late the in the than during ka) colder 23 to was ka Antarctica by 20 West from (average central LGM in that indicates surface 2B) (Fig. the reconstruction temperature surface Our models reconstruc- Discussion nonisotopic optimal simple the our of of shape range the 2A). (Fig. and the of 2B) data both (Fig. half temperature tion from second borehole this apparent the the of warmth from is in comparative strongly cold the emerges That too interval ka. are reconstruc- 15–9 deglaciation, however, our the matches Both, The cold, one well. too reconstruction. spatial tion is our the sensitivity temporal surprisingly, with the whereas, comparison by given in temperature LGM cases two these gives covariation tial aver- reconstruction, climate for age relevant most of the uncertainty presumably of the times Estimates three reconstruction. Divide, our Sheet Ice Antarctic West imately fucranyi nu aibe n rirr oe choices model arbitrary and variables input ( sources in for accounting uncertainty by limits of define can reconstruction we our however, of strategy, context higher- the providing Within proxies information. isotopic frequency possible the the given on and only influences forcing nonthermal preserve climatic temperatures the of borehole averages that long-term given define, to ficult gas and despite structure thermometer firn on a controls transport. as the serves in complexities proxy potential evidence firn-thickness providing the performance, that model the of improves using stages significantly that all note for We performance analysis. model of metrics summarizes r h udauesmo 2σ of sum quadrature the are 2B) (Fig. limits The S4). Table total the considering match excellent an is This S2). Table 18 esetv nteuiiyo u eosrcincnb gained be can reconstruction our of utility the on Perspective h rettlucranyo eosrce eprtrsi dif- is temperatures reconstructed of uncertainty total true The 11. δ ≈ and D γ ω 18 3 0.4‰ n ie ac ewe oe n bevdtem- observed and model between match a gives and ) 2 ± ≈ ±5.5 and Strategy) Reconstruction (SI optimization before 1.8 γ 0. D δ 55‰ δ o1.0‰ to 8 = ◦ 15 ◦ (2σ C fucranyi h G eprtr at temperature LGM the in uncertainty of C eest etru D roye-8content, oxygen-18 or (D) deuterium to refers ,w nrdcdarno etrainterm perturbation random a introduced we N, γ ◦ 18 C .Etmtsof Estimates ). ,weestentwrigfrom warming net the whereas S3), (Table ) −1 γ ◦ 18 C = 2) hra h otnn-iespa- continent-wide the whereas (24), −1 ≈ γ δ 0.8 T 2,2) orsodn oapprox- to corresponding 23), (22, ω aeil n Methods and Materials and + ‰ 1 = β γ ihconstant with , T ◦ rmtmoa covariations, temporal from C γ nta of instead ◦ −1 nAtrtc ag from range Antarctica in A .Ctdetmtsfrthe for estimates Cited C. rmsuiso temporal of studies from and 2,2) i.2B Fig. 25). (22, al S2). Table ω 0 = γ h Coeffi- The and dependent al S2 Table nEq. in ICal- SI plots 1 2 rbblt) aussmlrt rsihl ihrta nsderived ones than atmospheric higher slightly of or to doubling similar values a probability), that concluded CO (35) data oclimate might temperature of this underestimate latitudes. imply an high likely results at most change Our bias, of cores. source ice a be Antarctic East temperature four global 55 of the estimated poleward latitudes the from (6) to history contributing proxies 3.6 only 3A 5.8 as Fig. the as small in of large as curves as estimates are are Some LGM others 34). at whereas (6, depression context temperature model global records a proxy in on interpreted based temperature global-averaged estimated 7 about by ka, warmed 4 com- about 18–15 Antarctica in with interval West expected the that is In find contrast sites. we smaller mid-latitude A on 2–3. focused of parisons factor implies a transi- and climate was range deglacial tion broad the rather through models’ amplification the Antarctic that of middle the in lies isostasy) for (adjusted of thinning Holocene LGM a to from change such temperature constant-elevation a from imply in would effect shown temperatures rate LGM lapse our of 2B limits Fig. the increase, reason, this accumulation by by For temperature deglacial surface preceded LGM known (29–31), reconstructed ky our the warms 15 to last due the in thickening Divide WAIS Tem- at Reconstructed thinning of Interpretation Elevation of on peratures Influence Changes (SI reconstructed Thickness values shift and warmer would to which temperatures velocity, LGM recon- ice lapse with our vertical associated atmospheric influences is increased thinning thickness the directly; ice temperature via of surface structed history elevation the arise surface Further, likely rate. of of variations aspects changes temperature Some Holocene from surface. ice-sheet and Changes. the deglacial at the Thickness temperature of and is Elevation tion of 28). Influence (10, Greenland in inaccurate be temperature, to climatic known to method isotopes is a assump- ice from of It derive sensitivity all the 26). values about the West tions these by 12, but from influenced 27), strongly (6, difference (24, more ocean regional proximal C is climate real Dome the a where and exists Antarctica, F, there that Dome possible smaller, Dome, are Talos Antarctica Land, East in 7 locations from at warming deglacial in fteLM(1k)gvsagoa vrg oln relative cooling average 80 global of the of a in Holocene gives late ka) them to (21 of LGM simula- the model some of climate much tions eliminates various the from but or results Arctic, Averaging the reduces in Antarctic. area operate these land of vegeta- All smaller and 32). snow, 9, sheets, 8, factors ice (4, by tion ice, and sea temperature, albedo: surface surface longwave on controlling of rate dependence lapse the and and emissions moisture, ice, sea soil through transport, exchange heat moisture pro- feedback atmospheric by regions involving polar cesses the in amplified are changes ature Implications. Climatic appropriate. more is temperature constant- and elevation temperature even possibly surface simula- or between smaller correction a negligible model that indicating likely, ice-sheet elevation more smaller are that changes reasoning, suggest all constraints Glaciological geological shown and temperatures tions, 2B. surface Fig. of Tempera- range in the Reconstructed overlaps of which Interpretation tures), on Changes Thickness eetsnhsso lmt estvte siae rmpale- from estimated sensitivities climate of synthesis recent A 3A Fig. 2 ◦ ol nraegoa eprtr y2.2–4.8 by temperature global increase would )ma of mean S) ◦ o9.3 to C aebe xaddb hsmgiue nldn the Including magnitude. this by expanded been have and lutae o u etAtrtchsoycmae to compares history Antarctic West our how illustrates ◦ niae ygairrteti e eln (33). Zealand New in retreat glacier by indicated C .Apasbeuprlmtcs f30m 300 of case upper-limit plausible A S3). 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EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES A proxies both confirm a significant inference from isotopic records -30 12 (6, 26, 41) that the high latitudes of the Southern Hemisphere experienced much of their warming millennia before the Arctic. At 15 ka Greenland had warmed by perhaps 5 ◦C (Fig. 3A), or -35 7 less than 30% of the deglacial total (10), and did not maintain interglacial temperatures until around 11 ka, four millennia later than in West Antarctica. -40 2 Global climate evolution of the last deglaciation is regarded as the superposition of two modes (41): a global warming fol- lowing radiative forcing and an interhemispheric redistribution -45

Temperature (C) of heat following variations in cross-equatorial heat transport global by the Atlantic Meridional Overturning Circulation. Reduced -50 global (cold) Atlantic heat transport between 19 ka and 14.7 ka (Heinrich Sta- Greenland dial 1) cooled Greenland and contributed to the early Antarctic West Antarctica warming. Climate model experiments suggest that this contribu- B -55 tion amounted to no more than a few degrees (42), however, and other factors such as increased insolation and associated 0 sea ice feedbacks were probably important (15, 43). How these processes combined to produce the comparatively large inferred -0.4 warming remains uncertain and offers a potentially illuminating target for further model studies. -0.8 In the period of early warming, 18.5–15 ka, increasing annually albedo forcing integrated insolation and reduced northward oceanic heat trans- greenhouse gas forcing 2 East Antarctica port account for a climate forcing of 1 to a few watts per meter

Normalized Change -1.2 West Antarctica specific to the Southern Hemisphere (SI Climate Forcings Dur- 25 20 15 10 5 0 ing Deglaciation). This can be compared with the global forcings Thousand Years Before Present governing the difference between planetary temperatures of the LGM and Holocene. Taken together, decreasing ice-sheet extent Fig. 3. Comparisons of temperature histories and forcings. (A) This study’s (hence reduced albedo) and increasing greenhouse gas concen- optimal West Antarctic Divide surface temperature history (blue and left trations (CO2 and CH4) account for about 90% of the direct axis) from Eq. 2, central Greenland temperature history (red and left axis) global-averaged climate forcing, with respective magnitudes of from ref. 10, and global-average temperature from studies of the deglacial ∼2.9 W·m2 and 2.2 W·m−2 (2). Fig. 3B plots histories of these period (6) and the Holocene (34) with an LGM temperature 3.6 ◦C below modern (solid green and right axis) and scaled at ages greater than 11 ka two forcings. Antarctic warming leads the greenhouse gas forcing to reach the colder LGM (−5.8 ◦C) implied by ref. 7 (dashed green and right through the deglaciation, and the fractional increases (Fig. 3B) axis). The best-constrained studies yield an LGM temperature between these put an upper limit on the contribution of the gases to Antarctic (5). (B) Various histories of forcings and temperatures (to facilitate compar- warming between 18.5 ka and 15 ka of ∼65%. The magnitude of isons of relative timing, the histories are all scaled to range from zero to the gas forcing by 15 ka was ∼1 W·m−2, equal to or somewhat unity between 18.5 ka and 1 ka, times of nearly identical annually inte- smaller than the forcing from combined insolation and reduced grated insolation at 70 ◦S): optimal West Antarctic Divide surface tempera- ocean transport. Given this similarity, the observation that CO2 ture (solid blue), average of East Antarctic ice cores (26) (dashed purple), could account for roughly half of the warming is consistent with methane and carbon dioxide greenhouse gas climate forcing (46) (solid normative estimates of its climatic impact. black), and ice-sheet area albedo forcing calculated as sea-level change raised to the power 0.8 according to the average relationship between ice- Fig. 3B also shows strong similarities between our West sheet area and volume (section 8.10.3 in ref. 13) (dotted black). Antarctic temperature history and a composite of the ice-isotopic records from interior East Antarctica (26), although the latter covaries yet more closely with the greenhouse forcing. In the by other techniques. Those paleoclimate studies using Antarc- Holocene, a maximum at around 4 ka appears in both parts of tic data have likely underestimated the temperature change in the continent, with a West Antarctic magnitude of 0.6–1.2 ◦C response to the well-constrained change in CO2 and thus under- warmer than the last millennium (0–1 ka). Differences at other estimated climate sensitivity. In particular, the analysis produc- times might record changes of elevation, sea ice, or other fac- ing the lowest estimated sensitivity in the compilation (36, 37) tors. The West Antarctic warming just before 18 ka, attributed (a bit larger than 2.2 ◦C) predicted an Antarctic LGM cooling previously to insolation driving Southern Ocean warming and smaller than even estimates from the East Antarctic isotopic sea ice retreat to which East Antarctic isotopes are less sensitive proxies. Our reconstruction affirms that this is an underpredic- (15), appears in Chile as glacier advance and in New Zealand tion. Antarctica accounts for a small fraction of global area and as glacier retreat punctuated by readvances that left moraines hence does not contribute much directly to climate sensitivity (33, 44). These glaciers are thought to respond to Southern estimates, but the discrepancy may indicate too-weak feedbacks Ocean temperatures as well (45). in some of the models used to assimilate proxy data. A noteworthy aspect of our reconstruction is that by 15 ka, Conclusion the millennial-averaged temperature had risen to within ∼2.5 ◦C The temperature reconstruction reported here provides infor- of its late Holocene value (Fig. 2). The fraction of deglacial mation about the thermal state of firn and ice through time warming accomplished by this time was most likely 0.77±0.05 at the West Antarctic Divide site, an essential input to studies (Table S3). Such early warming of West Antarctica is consistent of the depth–age relationship, interhemispheric climate phas- with inferences from retreat of glaciers in Southern Hemisphere ing, CO2 history, and covariation of temperature with accumu- mountain ranges. Extensive retreat occurred between 18 ka and lation (16, 47, 48). Of greatest immediate interest, however, is 14.5 ka in Patagonia (38), with the Patagonian ice cap attain- our demonstration that the global deglacial temperature change ing near-present dimensions by 15.5 ka (39). Glaciers in the New was amplified by a factor of 2–3 in the Antarctic, that Antarctic Zealand Alps largely completed their major retreat by 15.7 ka warming was largely achieved by 15 ka in coherence with records (40). Thus, our temperature reconstruction and glacial geologic from Southern Hemisphere mountain ranges, and that climate

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Isotope of Data Nitrogen Isotope Calculation of Nitrogen of Use Use Layer and and Model and Model Relation Thickness Age–Depth Firn (SI 53) 47, and (16, identifying time this by Thicknesses). to determined (SI to prior were back 47) records layers thicknesses (15, annual layer counting respectively annual spectrometry, and mass tion and Data spectroscopy Ice-Isotopic laser by Data. Ice-Core the by determined measurements. values PTLS the accurate match to more neighboring uniformly a in shifted measured (52), previously are hole profile air-filled temperature ice (depths the borehole used surrounding the we of m the portion 96 fluid-filled in the temperatures for during mK assessing ∼5.3 again in uncertainties and ( bined 51) Logging 2011 Temperature (50, Polar December (PTLS) Survey Geological System during US the using logged 2014, December were borehole Temperatures. WDC Borehole of Measurement (Eqs. The optimizations parameter. (six). all independent constant in an remains parameters accounts not free latter is simultaneous The which of melt. flux, number basal heat of rate geothermal be the the to and for (known m), thickness 3,450–3,470 the ice range the present parameters: in the free temperature, additional surface measured three mean to modern and plus related variations modeled parameters temperature free of involved surface mismatch optimization such squared Every the temperatures. minimized that values Optimization. Depth). Cal- vs. (SI Temperatures sources and of energy advection, culation time-dependent conduction, one-dimensional for accounting the Depth. equation solve balance of to Function (49) a method volume as Temperatures of Calculation Methods and Materials studies. model for challenge illuminating an ettasot nrae noain n nraigatmospheric increasing oceanic CO and northward insolation, reduced increased of quan- transport, to effects heat helps combined study from warm- our arose early which tify, The Hemisphere, Southern discounted. the be the of can that ing sensitivity but average, lowest on with well ones perform deglaciation the of models .GaesnR,Lne L arte 21)Plrapicto nCS4 Contribu- CCSM4: in amplification Polar (2014) feed- T Mauritsen temperature PL, by Langen last RG, dominated Graversen the amplification was 9. Arctic cold (2014) How T (2006) Mauritsen S F, Rahmstorf Pithan S, 8. Held A, Ganopolski con- TS, dioxide Deimling carbon increasing von by 7. preceded warming Global (2012) al. et JD, changes Shakun temperature of 6. reconstruction global new A (2013) JC Hargreaves JD, Annan 5. archives. paleoclimate from Information (2013) al. et dust. V, atmospheric ice-age Masson-Delmotte by climate 4. of forcing Radiative (2003) al. et T, Claquin 3. .Hwt D icelJB(97 aitv ocn n epneo C oieage ice to GCM a of response and forcing Radiative (1997) JFB Mitchell CD, Hewitt 2. .Hne ,e l 18)Ciaesniiiy nlsso edakmechanisms. feedback of Analysis sensitivity: Climate (1984) al. et J, Hansen 1. b(t ˙ lca transition. glacial feedbacks. albedo surface and rate lapse the from tions models. climate contemporary in backs maximum? glacial deglaciation. last the during centrations maximum. glacial last the 383–464. at pp 5, Stocker Chap York), eds New Change, Press, Climate University on (Cambridge al. Panel et Intergovernmental TF, the of Report Assessment Basis. Science Physical The 2013: Change Dynam eid nsxAtrtciecores. ice Antarctic six in periods Science sensitivity. climate and feedback Cloud conditions: boundary Monogr )and )) 2 uniaiesmlto fti hnmnncudprovide could phenomenon this of simulation Quantitative . 20:193–202. 282:268–271. T 29:130–163. s (t ,w acltdfindniypolsuiga miia model empirical an using profiles density firn calculated we ), iglrvledcmoiinwsue ofidparameter find to used was decomposition value Singular δ δ δ esrmnsof Measurements 15 N(t). Science and epy e Lett Res Geophys ie iutnoshsoiso cuuainrate accumulation of histories simultaneous Given IMaueeto oeoeTemperatures Borehole of Measurement SI .Teaedphrela- age–depth The Data). Nitrogen-Isotopic SI 270:455–458. h hsc fGlaciers of Physics The lmPast Clim lmPast Clim 1k n ycoscreain fgas of cross-correlations by and ka ∼31 δ 33:L14709. fieand ice of D 9:367–376. a Geosci Nat otiuino okn ru oteFifth the to I Group Working of Contribution Nature 7:397–423. rvdsmr information. more provides eprtrsi h fluid-filled the in Temperatures 484:49–55. δ 7:181–184. Esve,Brigo,M) t Ed. 4th MA), Burlington, (Elsevier, 15 δ N(t 15 Clim J ftapdN trapped of N fgsa h et of depth the at gas of ) eue h control- the used We lmDynam Clim 27:4433–4450. IFr Thickness Firn SI 6m.Above m). >96 13:821–834. 1 .Com- ). Geophys 2 Climate and were Clim 2) 4 ti J ta.(03 eetciaeadiesetcagsi etAtrtc com- Antarctica West in changes ice-sheet and climate Recent (2013) al. et EJ, Steig 24. 3 ozlJ ta.(03 antd fiooetmeauesaigfritrrtto of interpretation for scaling isotope/temperature of Magnitude (2003) al. et J, Jouzel 23. stable-isotopic ice-core High-resolution (2005) T firn Ommen Van polar EJ, of Steig DP, Schneider densification 22. the Modeling (2003) C Ritz J-M, model. Barnola empirical C, An densification: Goujon Firn 21. (1980) C Langway firn polar M, in Herron air of 20. fractionation Thermal (2001) M Battle A, Grachev JP, Severinghaus 19. (1992) Y Korotkevich D, Raynaud M, Bender T, Sowers of 18. variation time climate and abrupt Space (2003) of KM Cuffey phasing JL, Kavanaugh interpolar Precise 17. (2015) Members Project Antarctica West Divide in WAIS warming deglacial 16. of Onset (2013) Members Project calibration Divide paleothermometer WAIS and 15. dating age core Ice (1998) al. et AN, Salamatin 14. lmt n adUeCag rga GDC) n ainlOencand (C.B.). Oceanic Fellowships National Change and Global USGS and (G.D.C.), the Climate Program Space (K.M.C.), Administration Change Atmospheric Foundation and Use Family Land Aeronautics acknowledge Martin and National Climate the gratefully We from and M.K.). support J.P.S.) (to additional (to NNX12AB74G 0538657 Grant Administration and Foundation 1338832 1043518 1043528, R.B.A.), Science 0539578, E.J.S.), E.D.W.), (to National (to (to 0440666 1043092 US 0944197, 0537930, 0944199, the C.B.), K.M.C.), (to through (to White. acknowl- 0537661 funded C. gratefully 0539232, W. is are Grants J. referees work and anonymous This Brook, two J. edged. of E. Taylor, comments K. helpful thank The especially and project WDC ACKNOWLEDGMENTS. (http://www.usap-data.org/). Center Data Program Availability. Data 2B Fig. in effects. shown these limits all those of The with model. model 2D one-dimensional tem- a our and from from flow results of comparing variations by upstream of perature coefficient from of timing arising the impact uncertainty and of the the shape assessed values the assess and including to functions strategy, recalculations Tol- basis model and such about Limits used choices of we Calculation altering (SI addition, ice depth– In of history, conductivity thickness erance). ice thermal the and be histories. scale, to age temperature proved variables optimal important recalculated most The and variables input uncertain correc- temperature-history The 47. and 28 refs. tion in described method the modeled to and adjustment sured an defined optimized asso- model then initial strain an the and with thicknesses ciated layer ice-core observed from derived of value. modern observed Calculation calculated the The matches atmosphere. climate current the and for with fractionation, mixing thermal convective settling, near-surface gravitational for accounting trapping, aclto of Calculation history. temperature Data, the of Isotope variations Nitrogen Alternative of explanation). Use and Model Thickness b(t ω coefficient 54). The (21, models densification other firn the alternative and two strain via ice temperatures from reconstructed estimated from one several rate, iterated accumulation of was histories process modern- entire against The calibrated sites. times. been various has and and observations equation day barometric the and els δ ˙ 15 = ae ihteps ,0 years. 2,000 past the with pared eta naci c cores. ice Antarctic central reconstruction. climate interannual Towards iso- 41:63–70. Antarctica: gas from records and sites. characteristics Greenland close-off and Antarctica to 4792. for Application fractionation diffusion: topic heat including 373–385. gradients. temperature seasonal age-gas by ice on constraint possible a and firn the in differences. transport age gas of tracer A ice: polar revisited. precipitation tic age. ice last the during change forcing. orbital local by Station driven Vostok at boreholes deep from profiles Antarctica). temperature (East and isotope on based oteinitial the to ) ntmeaueadacmlto aeacrigt r est mod- density firn to according rate accumulation and temperature on N for 0 ∆T i.S1 Fig. N (t ka, <3 a hncluae safnto ftertoo h adjusted the of ratio the of function a as calculated then was ) lutae h ls ac ewe h piie oe’ two model’s optimized the between match close the illustrates ± ± ±2σ ω ω ω ∆ ∆ ∆T epy Res Geophys J ω epy Res Geophys J l aaaeaalbeoln hog h ..Antarctic U.S. the through online available are data All b(t nE.2. Eq. in σ σ ˙ N = δ (t). .Ti ucindsrbsteculddpnec of dependence coupled the describes function This ). iiso eosrce Temperature. Reconstructed on Limits 15 o .–2k,and ka, 3.5–12 for 1 eaedel netdt aypriiat nthe in participants many to indebted deeply are We efis calculated first We ,following S1), (Fig. possible as well as agreed N(t) lblBoece Cy Biogeochem Global epy Res Geophys J safnto faeorotmzdmdluses model optimized our age of function a As Nature 97(D14):15683–15697. Nature 103(D8):8963–8978. a Geosci Nat b(t ˙ T ece epy Geosyst Geophys Geochem s (t 500:440–444. .Ti rcs a trtdutlmea- until iterated was process This ). 520:661–665. .Cmaio ihmeasured with Comparison ). δ 108(D12):4361. 15 ω 6:372–375. .W lorpae nlsswith analyses repeated also We N. (t r sdi enn iison limits defining in used are ) ω 17(1017):1–14. δ ≈ eiefo udauesums quadrature from derive ω 15 ohi Eq. in both , for 0.5 NSEryEdition Early PNAS N(t δ 15 sn ninitial an using ) epy Res Geophys J h Coefficient The fN of N δ 5k (see ka >15 18 2(7):2000GC000146. O 2 and eperturbed We nartapdin trapped air in ial,we Finally, 2. δ Glaciol J n Glaciol Ann nAntarc- in D 108(D24): | δ IFirn SI 15 ω f6 of 5 δ N(t b(t 15 ˙ for 25: N ) )

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