Melting of Northern Greenland During the Last Interglacial A

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Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | , ; 2006 , ) and temper- Discussions Christensen et al. The Cryosphere 2009 ; , C temperature increase CAPE members ◦ 2009 , Kopp et al. ; 2006 , Clark and Huybers 3518 3517 ). Nevertheless, the reconstructed warming of C higher than today ( ; ◦ ). Evidence from fossil coral reefs suggest a sea 1999 3,4 2007 , , 2007 , Overpeck et al. ), while greenhouse gas concentrations were comparable IPCC ). The warming was caused by a change in seasonal in- Petit et al. 2004 , 2006 , Jansen et al. and K. H. Nisancioglu Loutre et al. 1,2 ). Studies of the geological past provide valuable information on the long term response This discussion paper is/has been under review for the journal The Cryosphere (TC). Please refer to the corresponding final paper in TC if available. Climate and Environmental Physics, Physics Institute,Oeschger University Centre of for Bern, Climate Change Bern,Bjerknes Research, Switzerland Centre Bern, for Switzerland Climate Research,UNI Bergen, Research, Norway Bergen, Norway – Published: 21 December 2011 2007 The last interglacial, thethousand years Eemian, ( is consideredlevel the high warmest stand period of of 4 the to past 8 m 150 ( flow. The analogy with thefeedbacks present could warming suggests increase that the inthe rate coming currently of decades, observed positive melting mass in loss the of south. the northeastern1 GrIS, exceeding Introduction of the ice sheet, masking slowerand processes its that contribution dominate to the sea long term level rise. fate of the GrIS of the GrIS toa warm period periods. 126 000 yrprojections We for before simulate the present end the (126and ka) of GrIS retreats this with during century. significantly, Arctic the despitestrong The temperatures Eemian melting moderate northeastern comparable interglacial, in part melt to the of rates. northeast the is GrIS Unlike not is the compensated unstable by south high and accumulation, west, or fast ice The Greenland ice sheetprimary (GrIS) contributor is to losingthinning global at mass eustatic its margins at sea has raised anthat level concerns these rise. about increasing observations its represent rate, stability. the However, Large it making transient is melting adjustment conceivable it of areas the the and fastest reacting rapid parts for the end of this century ( Abstract atures on and aroundOtto-Bliesner Greenland et 4–5 al. solation ( to preindustrial values ( the Arctic during the Eemian is similar to the projected 2.8–7.8 The Cryosphere Discuss., 5, 3517–3539,www.the-cryosphere-discuss.net/5/3517/2011/ 2011 doi:10.5194/tcd-5-3517-2011 © Author(s) 2011. CC Attribution 3.0 License. 1 2 3 4 Received: 21 November 2011 – Accepted: 10 December 2011 Correspondence to: A. Born ([email protected]) Published by Copernicus Publications on behalf of the European Geosciences Union. Melting of Northern Greenland during the last interglacial A. Born 5 15 10 20 25 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | , , , ) ey ff 3.2 ). In Cu Colville 1992 ). Ice and , discusses and 2005 400 000 yr) and 4 , Braconnot et al. > Dansgaard et al. Amante and Eakins ). Johnsen et al. Marti et al. ). Section ). The low estimate requires ), indicating that the GrIS cov- 2010 3.3 , 2006 , 2004 , Lemark presents results of the analysis, divided ; 3 3520 3519 2001 ), quantification of ice sheet stability (Sect. , 3.1 . Section 2 Otto-Bliesner et al. NGRIP-members ). However, due to its very old age ( ; ). Climate fields are interpolated bilinearly to match the 2007 1997 2011 ). A recent study concluded moderate melting of the southern , Hammer et al. , , 2010 ) or 2–3 m ( , 2010 , 2000 ), forced with monthly temperature and total precipitation from the Insti- , ). Two isolated ice caps in northern and northeastern Greenland (Hans Willerslev et al. 1997 Alley et al. , 2011 Chappellaz et al. Born et al. , ; ). The horizontal resolution of the ice sheet model is 10 km with 90 vertical layers. ; Climate forcing of the upper boundary is provided from simulations of the coupled We simulate the GrIS at 126 ka, using a three-dimensional ice sheet model, forced Eemian sea level estimates provide little constraint on the size of the GrIS, ranging Previous model studies of the last interglacial find substantial melting of the southern Greve model IPSL CM4.(126 ka), Three glacial climate inception modeltions. (115 ka) All experiments employ and constant are preindustrial present used, daytrial (1850 ice AD, greenhouse topography, forced surface 0 gas conditions ka) with and concentrations. boundary preindus- Eemian parameters For condi- are the adjusted Eemian to 126 and kafor the values. the glacial While this Eemian, inception, generally orbital it ispares a is good well probably approximation with not existing realistic2008 proxy for data, the GrIS. also However, in the the simulation vicinity com- of the GrIS ( stresses. This simplification iseral valid extent, for flowing ice slowly masses overcan that a are horizontal valley glaciers. bed. thin compared Ice For shelves totion. the can their GrIS, not lat- however, be this simulated, is nor a valid and widely used approxima- horizontal resolution of thecoarse ice representation sheet of the model. GrIS surface Temperature in fields IPSL CM4 are and corrected the for evolving ice the topography 2 Model description We use a( three-dimensional, thermomechanical ice sheet model,bedrock SICOPOLIS topographies 2.9 are initialized2009 with modern observationsIce ( dynamics are represented in the shallow-ice approximation, neglecting longitudinal and the presentation ofsummarizes the ensemble results. simulations (Sect. tut Pierre Simon Laplace Coupled Model 4 (IPSL CM4) ( models are described ininto the Sect. simulation of the GrIS (Sect. of ice sheet stability. This study is organized as follows. The ice sheet and climate this result is validated with a large ensemble simulation and theoretical considerations melting of the southern dome ofmelting the of GrIS, whereas the the northernmost high estimate partthe requires reconstructed of additional sea the level GrIS. high Bothevidence stand studies for of explain the the a presence last substantial of interglacial, part Eemian but of are ice not at in the accord corewith with locations a discussed climate above. simulationto from previous studies, a strongest comprehensive melting coupled occurs climate in model. the northern In GrIS. The contrast robustness of GrIS, supporting the persistenceet of al. the southern dome and probably Dye-3 ( and the northwestern1985 core at Camp Century contain Eemian ice ( southeastern Greenland, at the Dye-3Eemian core ( site, basal icethe is lack of also direct found evidence tocertain of predate ( Eemian the ice, an Eemian glaciation at this site remains un- Tausen Iskappe and Flade Isblink)the present do interglacial not ( hold ice older than the thermal optimum of from partial melting to aEemian total sea loss level of include 7 a mtic sea potentially ice level large equivalent. sheet. but The unknown Due highest contributionof to estimates from potential the of the archives, expansion Antarc- few of reliableGreenland ice proxy ice at data cores. the exist The last for bottom glacial Eemian sections maximum ice of and extent the the except summit erosion cores for (GISP2, the GRIP, NGRIP) ered these locations.ice The in isolated its ice bottom cap section, on dated the to Renland be peninsula also older contains than 130 ka ( part of the Greenland iceand sheet, Marshall equivalent to a global sea level rise of 4–5.5 m ( 5 5 15 20 10 25 25 15 10 20 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | , , ). 1 Reeh Yokoyama Velicogna ), related to ). 2 . This corrected 2009 1 , − ). These processes ). About half of its cur- 2009 cient proxy data, the ice , 6.5 K km ffi 2006 − N as a normalized scalar in- , ◦ , middle). Therefore, only a fraction 4 van den Broeke et al. Pritchard et al. Oerlemans et al. 3522 3521 , left). However, despite lower melt rates, ice in ). Large regions become ice-free, most notably in 4 3 ), contributing considerably to the rise of eustatic ) probably eclipse smaller but sustained trends that 2009 ). However, there is no justification to extrapolate recent , 2009 , 2011 , ). To estimate this duration, a transient simulation of the GrIS is car- 2011 Rignot et al. , Bhattacharya et al. van den Broeke et al. ) and to estimate the solid fraction of total precipitation, which is used as accumu- ; ect in regions with low accumulation and weak ice transport. In this regard, the dry In order to investigate the impact of Eemian climate on the GrIS mass balance in- A new simulation with climate forcing fixed to 126 ka values is used to investigate the While a transient simulation is generally preferred for the investigation of the GrIS ff warmer air masses strongly enhance the melting. the northeast disappears morethe rapidly very than low in accumulation otherof in the regions. melted this This ice regionback is is dominates (Fig. primarily replaced the due by further to new development: snow as and the the ice strong surface melts positive to ice-elevation feed- lower elevations, dependent of changes in icethe topography, Eemian we (126 compare ka) the andinitialization, simulated preindustrial mass i.e. climate balance before (0 for ka) changesmelting for in increases the ice everywhere first topography (Fig. time take step place. after model With Eemian climate, the northeast and southwest. TheAfter locations 5700 of model all years deeprise NEEM ice core becomes of sites ice-free, 4.2 remain m. defining glaciated. Ice an Note, at upper however, limit that Dye-3 for evidence becomessimulation sea for with thinner, level Eemian constant but ice 126 ka does at forcing. not NEEM disappear is until not the definitive. end of the 10 000 yr whether this is agood good approximation for starting that date period.transient or As experiment if the is interpolated the seasonal only presentEemian climate an day climate forcing shape approximation of evolution, this the and of simulationFor the not is the ice an solely Eemian sheet accurate this used is is to representation estimated constrain a of to the be melt between duration. 5000most likely and response 7000 of yr the (Fig. GrIS.a After 5000 global yr, sea the level loss rise of of Greenland ice 3.5 m corresponds to (Fig. index that is maximum and minimum at 126 and 115during ka, respectively. the Eemian,periods this longer approach than is atsheet limited is most initialized by with a present the day few ice ability and millennia. bedrock to topographies at run Due 130 ka. climate to It is insu models unclear for extrapolation before 126 ka and after 115 ka. This does not occur with the insolation can not sustain their presentvolumes rate of of retreat ice. for veryareas On long, the ( and other concern relatively hand, limited recently observed large and highly variable melt onto the period 130 todex. 110 Linear ka interpolation using has June insolation not at been 65 used because it yields unrealistic results for the e northeastern sector of the GrIS is the most vulnerable region of the ice sheet (Fig. IPSL CM4 for 126 and 115 ka, respectively. The climate model data was interpolated in SICOPOLIS, both with a fixedtemperature atmospheric is lapse used rate to of calculate1991 melt following the positive degree day method ( are capable of causing a strongtionable sea indicator level rise for in ice the sheet long term. stability, because Melting alone moderate is melting a ques- may have a large ried out spanning the period 130 to 110 ka, forced with two time slice simulations of lation. No further modifications are made to the climate forcing fields. 3 Results Observations show that ice loss in Greenland is accelerating in recent yearssea ( level ( 3.1 Simulation of the Greenland iceThe sheet duration of Eemianand melting Esat is not well-constrained by proxy data (e.g. 2009 changes of the GrIS torent long-term mass trends loss ( is dueretreating to calving dynamic fronts adjustments of ( tidewater glaciers ( 5 5 25 15 20 10 25 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | , ) ) t 6 (6) (7) (1) (2) (3) (4) (5) 1997 ( ), an imbal- ) and ABL( 5 t ( H Ritz et al. ). , right). Regions of 5 4 must be matched by equal ects not only marginal ice areas but ff , right). The coupling scheme for CCSM3 6 3524 3523 1 ) (Fig. absolute change in relative surface mass balance − balance to zero: ) only depend on the balance between accumulation t F ( 2006 ABL ACC , H provides a measure of stability (Fig. , ) α 0 ka (absolute surface mass balance) = 0 ), where ACC is approximately constant, and − erent regions is quantified as the relative surface mass bal- ) t t ff ACC = . . = ) yields: ) , middle). The 1 − t F 3 ) ) 4 )/ACC t + t + ( ABL( ACC t αt H ABL) 0 ka t ) ABL( ∂ e t − ABL ( ) and ( − ABL( = = ( α∂ 1 − ABL ) α − Otto-Bliesner et al. t αH − ( ) = = − t (ACC is a constant factor characterizing the elevation dependence of ablation. Com- ACC ). All experiments have been run for 10 000 yr in order to reach quasi- relative changes in absolute surface mass balance ) ) 1 t t ACC − = = α ) ) ABL ACC 126 ka t ACC = = t ABL( ( F H As the GrIS loses mass, thinning in the northeast is faster than in the south (Fig. By how much would the ice flow have to increase to counteract the imbalance in Thus, The positive ice-elevation feedback, the increased melting with lower elevations, can In equilibrium and without lateral ice transport, the ratio of melt to accumulation H ABL( ABL( t ⇒ t t t t ∂ out modification. is identical to the description above, i.e. the interpolated climate fields are applied with- ering a wide range of parameter(Table space, extending the methodology of equilibrium. left), consistent with theing transient the evolution of higher the vulnerabilityclimate original of forcing experiment the and from northeastern corroborat- a region.(CCSM3) 130 ( ka This simulation result of is reproduced the with Community Climate System Model 3 3.3 Ensemble simulations To assess the robustness of our results, a large ensemble of experiments was run cov- surface mass balance? InACC, steady melting state, ABL changes and in ice flow ice elevation due to accumulation ice retreat are identifiedIn by particular, large manifold changes, imbalances thuscumulation. reach an far unsustainable In inland increase contrast in inalso the to melting. ice northeast the that due southwest, is to this dynamically the connected a low to ac- the large interior body of the ice sheet. least stable region with a required fivefold increase in ice flow (Fig. ⇒ changes in ice volume transport. Here again, the northeastern GrIS is identified as the (ABL are negligible (Fig. vary in time: ∂ The vulnerability ofance, di the ratio ofice, ablation changes to in accumulation, iceACC elevation ABL/ACC. and Neglecting ablation lateral ABL( transport of 3.2 Stability criterion be simplified as ABL( ∂ where bining Eqs. ( ance in the ABL/ACC ratioessentially grows reflects exponentially due increased to melting the and ice-elevation thus feedback. ice This loss while changes in accumulation ∂ dividing by accumulation: ∂ ABL/ACC must be one to hold ice elevation invariant. Following Eq. ( 5 5 25 20 15 10 10 20 15 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | , , ). Born ; 2006 , Funder de Vernal 2008 Fyke et al. ). The iden- , ; 123 ka; 2007 ∼ 2011 , , ectively simulated by ff IPCC Otto-Bliesner et al. ). This has been confirmed ; Braconnot et al. ; ). This indicates that seawa- 1998 2005 ), suggesting that the ice margin , Robinson et al. , 2006 2010 ; , , ). However, isotopic analysis of the 1994 , 2003 2008 , , ). An early analysis of basal ice from Dye-3 3526 3525 Alley et al. ). A 20 % increase in precipitation was recon- Souchez et al. Lhomme et al. ; 2011 ; ; , erent studies. The main finding of the present study, 1992 ff Otto-Bliesner et al. 1994 , 1985 2000 the south Greenland coast has been interpreted to imply ; , , , ff Carlson et al. ). Considerable melting of the GrIS in the south is supported 2006 Tarasov and Peltier , ). However, several melt layers have been found in the bottom Houmark-Nielsen et al. ). Colville et al. 2008 , erent regions of the GrIS under the climate of the last interglacial 1989 ff ). The impact of contingent climate biases is e , 2007 Johnsen et al. , Greenland has recently been found to source from all 3 south Green- ff ey and Marshall 2011 Dansgaard et al. ff , Koerner Cu Overpeck et al. Mangerud and Funder 2010 ; ). Biases of the simulated climate, especially at high northern latitudes and at , Eemian ice was also found in the isolated ice cap on the Renland peninsula in east- Note however, that the existence of ice at Dye-3 and in southern Greenland during An estimate on the contribution of ice sheets to sea level rise is one of the most Currently available proxy data is inconclusive on the shape of the Eemian GrIS. Previous model studies of the Eemian identified the southern GrIS as the most vul- Willerslev et al. the last glacial cycle ( ter was present far inlandmodel following results. the penultimate Marine glacial conditions period, in consistent northeastern with our Greenland did not end until late in eastern Greenland. However, therethe are widespread last marine interglacial, deposits particularly1989 in during this area the from period of peak warmth ( the last interglacial does notsectors preclude of the a Greenland considerable ice sea sheet. level Unfortunately contribution there from are other no deep ice cores in north- retreated considerably during the Eemian. ern Greenland ( tification of key regions is a necessary requirement for which the Eemian, despite a structed for the lowest 5with m our of ice, interpretation aswarmer that well climate as ice and numerous increased persisted melting. melt due layers. This to is highpressing consistent accumulation needs for and the despite assessment of the future climate change ( by advanced dating techniques providing( direct evidence of ice predating the Eemian Century ( section of Dye-3, whichinterglacial document ( a warm period with persistent ice during the last concluded that ice at this location disappeared during the Eemian, as well as at Camp and Hillaire-Marcel glacial flour o land Precambrian terranes andthrough thus the significant Eemian ice ( remained on southern Greenland et al. changing the melt parameters ofthe the high ice vulnerability sheet of model,ters which changing the the was northeastern ice found sheet GrIS. tomodel dynamics not Similarly, unlikely makes change as the a well. wide dependence on range the of choice of parame- icePollen in sheet marine sediments o considerable melting of the GrIS to allow for growth of extensive vegetation ( during the early Holocene ( by the finding of a greater discharge of glacial flour during the last interglacial than studies ( the high elevations of thesome Greenland discrepancies ice between sheet, di arehowever, is potentially unlikely large to and be the mightwith result explain of climate model forcing biases. of Thehave results a been have second been validated reproduced to comprehensive simulate climate Eemian model. climate consistent Both with climate proxy data models in previous warm climate such as the lastwith interglacial is low northern accumulation Greenland can because notapplicable its dry to compensate climate modern increased climate melting. change. This general finding is nerable ( 2011 The stability of di has been investigated usingof a sensitivity three-dimensional experiments iceaccumulation sheet and and model, detailed ablation. a analysis large The of ensemble main the finding regional is imbalance that between the most vulnerable region in a 4 Discussion and summary Considerable melting of theeral northeastern recent GrIS, simulations however, has ( also been found in sev- 5 5 15 20 25 10 25 20 15 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | , , , 3520 3518 3525 , 3534 records from 3526 , 2010. 2 3520 , 2006. 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Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 2007 , 1 1 − − K K 1 Abdalati 1 1 − − − 2 − ). As in the south, Pa 1 1 − − erent climate forcings, ff 65 mW m ∼ , in days) (Abdalati, 2007), and ns in the south, however, the northeast t, in m/yr). As in the south, the northeastern ). 6 6 3534 3533 Observed average duration of melt season in 1979-2007 (left parametergeothermal heat flux 30..10..80 value standard value unit degree-day factor for icebasal sliding 10..2..20 5..2..15 14 mm day 11.2 m a degree-day factor for snow 3..8 5 mm day flow enhancement factor 1, 3 3 dimensionless Parameter range for ensemble experiments and standard values used in the simula- Fig. 1. average total precipitation in 1958-2001 from ERA-40 (righ region of the icereceives sheet very shows little precipitation a to long balance melt the melting. season. Unlike regio Observed average duration of melt season in 1979–2007 (left, in days) ( Fig. 1. and average total precipitationthe in northeastern 1958–2001 region from of the ERA-40however, ice the (right, sheet northeast in shows receives a m very yr long little melt precipitation season. to Unlike balance regions the in melting. the south, Table 1. tions. Heat flux ispermutation non-uniform of in the the standard above experiment. parameters One has experiment been for carried every out, possible for two di giving a total of 5184 model simulations (see Fig. Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | st

Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | d 7,000 olely used 00 model years n approximation ote, however, that inner, but does not corresponds to a global −110 tably in the northeast and a (right), after 5,000 years of d 110 ka. The model was initialized −115 outhwest. The southern dome does not ry at the end of the last interglacial. old black lines. Circles show locations of stment is visible in the ﬁrst 1000 years. After de Isblink, Camp Century, NEEM, NGRIP, s are well reproduced. With 126 ka forcing, −120 9 7 time (ka) 3536 3535 −125

−130

300 600 500 400 average ice thickness (m) thickness ice average Modelled ice thickness (in m) for present day (left) and 126 k Modelled ice thickness (in m) for present day (left) and 126 ka (right), after 5000 yr Fig. 3. the Greenland ice sheetdisappear. melts mostly from the northeast and s simulation. Regions defined for this study are separatedGRIP, by Renland, b Dye-3). Preindustrial ice area and thicknes ice core sites (from north to south: Hans Tausen Iskappe, Fla Average ice thickness evolution of the GrIS between 130 and 110 ka. The model was Average ice thickness evolution of the GrIS between 130 ka an A new simulation with climate forcing fixed to 126 ka values is used to investigate the mo Fig. 3. of simulation. Regions definedlocations for of this ice study coreCentury, are NEEM, sites separated NGRIP, GRIP, (from by Renland, Dye-3). bold northreproduced. black Preindustrial With to lines. ice 126 south: ka area Circles forcing,southwest. and Hans the show thickness The Greenland Tausen southern are ice Iskappe, dome well sheet does Flade melts not mostly Isblink, disappear. from Camp the northeast and initialized with preindustrial icefirst and 1000 yr. bedrock After topographies.last that, interglacial. Some ice adjustment melts for is 5000 visible to in 7000 the yr, followed by a recovery at the end of the Fig. 2. Fig. 2. likely response of the GrIS. After 5,000 years, the loss of Greenland ice sea level rise of 3.5 m (Fig.southwest. 3). The Large regions locations become of ice-free,NEEM all most becomes no deep ice-free, defining ice an core upperevidence sites limit for for remain Eemian sea glaciated. ice level rise at After of NEEM 5,7 4.2 is m. not N definitive. Ice at Dye-3 becomes th with preindustrial ice and bedrock topographies.that, Some ice adju melts for 5,000 to 7,000 years, followed by a recove the interpolated seasonal climate forcingand of not the an transient accurate experiment representationto is of constrain only the Eemian a melt climate duration. evolution,years For this (Fig. the simulation 2). Eemian is this s is estimated to be between 5,000 an 5 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | DiscussionDiscussion Paper | Discussion Paper | Discussion Paper Paper | | DisDiscussioncussion Paper Paper | |

Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | , ) and ) and 1 1 − − erent from ccumu- ff . For mass exponen- . For mass flow 0 ka ka 0 ) ABL − /(ACC-ABL) us independent from changes ACC ( 126 ka / ka 126 ) urther enhanced by melting of the lowest he ice divide moves east, thereby drawing so as a result of higher atmospheric tem- ortheast and ice flow can effectively slow et which receives most accumulation. throughout the entire ice sheet, mostly in agnitude less than the increase in melting. southwest is a relatively thin band restricted ice flux over a large region is unrealistic. In d. Accumulation of snow is reduced in the me factor. The northeastern GrIS is identified uantified by relative changes in surface mass ABL , as a measure of instability. Values greater s, accumulation slightly increases in central and − 0 ka 10 12 3538 3537 ). Right: ratio between melt anomaly and prein- ACC 1 ( − )/ACC 0 ka Ratio between melt anomaly and preindustrial accumulation melt anomaly ‘126 ka - 0 ka’ (color, m/yr) and initial ice ele- ABL ectively slow down ice loss. Moreover, where there is melting Left: − , as a measure of instability. Values greater than 1 increase ff Right: accumulation anomaly ‘126 ka - 0 ka’ (color, m/yr) and total a ka 0 126 ka ACC )/ Middle: ka 0 ABL − ka Anomalies and ratios of change at the first model time step, th 126 Anomalies and ratios of change at the first model time step, thus independent from Ratio of absolute surface mass balances, (ACC-ABL) Ratio of absolute surface mass balances, ABL Fig. 4. in ice topography and dynamics: vation (contours, m). lation at 0 ka (contours, m/yr). ( south and west of theperatures ice and sheet vapor content, due especially to at higher higher temperatures. altitude Al tially if not balanced byWith ice 126 flow. ka climatelow-lying boundary regions conditions, and melting in increases large regions in northern Greenlan north-eastern Greenland, but approximatelyThe two northeastern orders region of of m Greenland isbalance. the least stable as q flow to balance surface imbalances,GrIS it is needs identified as to the increase leastregion by stable is region. the unrealistic. same The required In factor. fivefoldis contrast, The increase a the northeastern in ice relatively region flux thin of over band manifold afurther surface restricted large enhanced mass to by balance the melting in of margin.the the the Steep northeast lowest southwest surface and elevation slopes ice. ice are This flowin already dynamical can the present regime e west, and is the di southeast ice of divide the moves ice east, sheet thereby which drawing receives more most ice accumulation. from the stable region to the Fig. 5. total accumulation at 0 ka (contours, m yr than 1 increase exponentiallyditions, if melting not increases balanced throughout bylarge regions ice the in flow. entire northern ice Greenland. Withthe Accumulation sheet, 126 ice ka of sheet mostly snow climate due is in boundary toand reduced low-lying higher in vapor con- temperatures. content, the regions south especially Also and and at asnorth-eastern west higher in a of altitudes, Greenland, result accumulation of but slightly higher increasesin approximately atmospheric in melting. two temperatures central and orders The ofchanges northeastern in magnitude region surface less mass of than balance. Greenland the is increase the least stable as quantified by relative Fig. 4. changes in ice topography and dynamics: left: melt anomaly “126–0 ka” (color, m yr initial ice elevation (contours, m). Middle: accumulation anomaly “126–0 ka”dustrial (color, accumulation, m yr (ABL Fig. 5. to balance surface imbalances, it needs to increase by the sa contrast, the region of manifoldto surface mass the balance margin. in the Steepelevation surface ice. slopes This are dynamical already regime present is and f different from the n as the least stable region. The required fivefold increase in down ice loss. Moreover, where there is melting in the west, t more ice from the stable region to the southeast of the ice she Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper |

Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 800 rglacial ulnerable ensate in- eenland ice 600 06). Considerable is shown in green with al recent simulations 3 400 Biases of the simulated d and southern (red) regions, as a GRL average thickness (m) 200 emble members with a different set of third order are included for visibility. The reen with circles spaced every 1000 years. dent of the simulated climate. o a reduction in total ice thickness, while ice , for 126 ka in IPSL CM4 (left) and 130 ka in 0 0

700 600 500 400 300 200 100 regional average thickness (m) thickness average regional 14 3539 800 600 400 GRL average thickness (m) 200 Simulated average ice thickness in northeastern (black) an 0 erent set of parameters, run for 10 000 yr. Polynomial fits of first and third order are 0 ff

700 600 500 400 300 200 100 Simulated average ice thickness in northeastern (black) and and southern (red) regions, regional average thickness (m) thickness average regional Previous model studies of the Eemian identified the southern GrIS as the most v climate, especially at high northern latitudes and at the high elevations of the Gr (Tarasov and Peltier, 2003; Robinson et al., 2011; Fyke et al., 2011). melting of the northeastern GrIS, however, has also been found in sever Fig. 6. function of the average ice thickness of the entire ice sheet CCSM3 (right). Each data point corresponds to one of 5184 ens parameters, run for 10,000 years. Polynomial fits of first and main finding is that the most vulnerable region in a warm climate such as the last inte transient evolution of the simulationIce of thickness figure in 3 northeastern is Greenland shownin in responds southern g rapidly Greenland is t more stable. This result is indepen is northern Greenland becausecreased its melting. dry This climate general with finding is low applicable accumulation to can modern not climate change. comp (Cuffey and Marshall, 2000; Lhomme et al., 2005; Otto-Bliesner et al., 20 5 included for visibility. The transient evolution of the simulation of Fig. circles spaced every 1000reduction yr. in Ice total thickness iceindependent in thickness, of northeastern while the ice simulated Greenland climate. in responds southern rapidly Greenland to is a more stable. This result is Fig. 6. as a function ofand the 130 average ka ice in thickness CCSM3 ofwith (right). the a Each entire di data ice point sheet, corresponds for to 126 one ka of in 5184 IPSL ensemble CM4 members (left)