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C. R. Geoscience 337 (2005) 935–946http://france.elsevier.com/direct/CRAS2A

External Geophysics, Climate and Environment (Climate)

Rapid climate variability during warm and cold periodsin polar regions and Europe

Valérie Masson-Delmottea,∗, Amaëlle Landaisa,b, Nathalie Combourieu-Nebouta,Ulrich von Grafensteina, Jean Jouzela, Nicolas Caillona, Jérôme Chappellazc,

Dorthe Dahl-Jensend, Sigfus J. Johnsend, Barbara Stennie

a Laboratoire des sciences du climat et de l’environnement (IPSL, UMR CEA–CNRS 1572), L’Orme des Merisiers, bât. 701,CEA Saclay, 91 191 Gif-sur-Yvette cedex, France

b Institute of Earth Sciences, Hebrew University, Givat Ram, 91904 Jerusalem, Israelc Laboratoire de glaciologie et de géophysique de l’environnement, (UMR 5183 CNRS-UJF), domaine universitaire, 54, rue Molière,

38402 Saint Martin d’Hères cedex, Franced Department of Geophysics, University of Copenhagen, Juliane Maries Vej 30, DK-2100 Copenhagen, Denmark

e Dipartimento di Scienze Geologiche, Ambientali e Marine, University of Trieste, Via E. Weiss, 2, 34127 Trieste, Italy

Accepted after revision 5 April 2005

Available online 23 May 2005

Written on invitation of the Editorial Board

Abstract

Typical rapid climate events punctuating the last glacial period in Greenland, Europe and Antarctica are comparerapid events occurring under warmer conditions: (i) Dansgaard–Oeschger event 25, the first abrupt warming occurring dlast glacial inception; (ii) 8.2 ka BP event, the only rapid cooling recorded during the Holocene in Greenland ice corin Ammersee, Germany. The rate of warming during previous warmer interglacial periods is estimated from polar ito 1.5◦C per millennium, without abrupt changes. Climate change expected for the 21st century should however be10 times faster.To cite this article: V. Masson-Delmotte et al., C. R. Geoscience 337 (2005). 2005 Académie des sciences. Published by Elsevier SAS. All rights reserved.

Résumé

Variabilité climatique rapide pendant les périodes chaudes et froides aux pôles et en Europe. Nous comparons lecaractéristiques des événements climatiques rapides ponctuant la dernière période glaciaire aux pôles et en Euroévénements rapides se produisant pendant des périodes relativement chaudes : (i) l’événement de Dansgaard–Oeschgerpremier réchauffement rapide de la dernière entrée en glaciation ; (ii) l’événement d’il y a 8200 ans, seul refroidissement rapde l’Holocène au Groenland et en Europe. Les périodes interglaciaires plus chaudes montrent un rythme de réch

* Corresponding author.E-mail address: [email protected](V. Masson-Delmotte).

1631-0713/$ – see front matter 2005 Académie des sciences. Published by Elsevier SAS. All rights reserved.doi:10.1016/j.crte.2005.04.001

http://france.elsevier.com/direct/CRAS2A/

mailto:[email protected]

936 V. Masson-Delmotte et al. / C. R. Geoscience 337 (2005) 935–946

le devrait
de 1,5◦C par millénaire aux pôles, sans variation abrupte. Le changement climatique attendu au cours de ce sièccependant être au moins dix fois plus rapide.Pour citer cet article : V. Masson-Delmotte et al., C. R. Geoscience 337 (2005). 2005 Académie des sciences. Published by Elsevier SAS. All rights reserved.
Keywords: Dansgaard–Oeschger events; Ice cores; Lake; Pollen; Palaeoclimate

Mots-clés : Événements de Dansgaard–Oeschger ; Carottes de glace ; Lac ; Pollen ; Paléoclimat

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Au cours des vingt dernières années, l’occurred’événements climatiques rapides a été découvdans le secteur nord-atlantique : les événementDansgaard–Oeschger (D/O) se produisent en queldizaines à quelques centaines d’années et sont ldes instabilités des calottes glaciaires et de la cilation océanique[13,39]. De tels événements sont-isusceptibles de se produire en réponse au réchament climatique anthropique, à la suite d’un arrêt dcirculation thermohaline[6] ? Y a-t-il des événementclimatiques abrupts se produisant pendant les périchaudes, ou uniquement en période glaciaire ? Qusont leurs caractéristiques spécifiques ?

Cet article présente les méthodes développéeLSCE pour caractériser et comprendre les variaticlimatiques rapides dans le secteur nord-atlantiqueEurope et au Groenland) et en Antarctique (Tableau 1).Les quantifications des changements de tempéradans les glaces polaires sont obtenues à partir de l’lyse et la modélisation des isotopes stables de l’equi peuvent toutefois avoir des biais systématiqueGroenland à cause de changements de l’originela saisonnalité des précipitations[10,14,16]. Le frac-tionnement thermique et gravitationnel de certainsdans les névés polaires forment une autre méthodquantification des changements abrupts de tempture au Groenland[16,17,20,21]. Les enregistrementde δ18O et de CH4 de l’air piégé dans les glaces plaires permettent de placer dans un cadre chrongique commun les variations de température recotruites à partir des glaces des deux pôles[23–27], met-tant en évidence une anti-phase entre les deux hsphères. Pour évaluer la pertinence des reconstrucen région polaire vis-à-vis du climat de l’Europe, noles comparons à la reconstruction duδ18O des précipi-tations à partir des ostracodes benthiques du lacmersee, en Allemagne[52–54], et au changement dvégétation dans le Sud de l’Europe, estimé par co

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tage palynologique dans la carotte ODP 976 end’Alboran [8].

Les D/O se présentent comme la succession dphase froide, relativement stable et durant quelqsiècles, d’un réchauffement abrupt, suivis d’un retplus progressif à des conditions froides. Une sthèse des données existantes montre des changeabrupts de température de 10 à 16± 3◦C lors des D/Oau Groenland (Fig. 1), associés à de grandes fluctutions (50 %) du pourcentage de pollen de forêts tepérées du Sud de l’Europe. L’enregistrement d’Amersee suggère des changements de températuEurope de 5,5 et 8,0±1,5◦C lors du début du Bolling–Allerod et à la fin du Dryas récent. En Antarctique,contre-partie des D/O atteint une intensité de 1 à 3◦C.

Le nouveau forage NorthGRIP au Groenland men évidence le premier D/O (D/O 25) se produislors de la dernière entrée en glaciation. Cet événema une amplitude réduite, pas de contre-partie austpas de bouleversement de la végétation dans le Sul’Europe, et se produit alors que le volume des caloglaciaires est seulement à un tiers de leur niveau mmal (Fig. 2). Au contraire, les D/O suivants présentedéjà les mêmes caractéristiques qu’en pleine pérglaciaire. Un changement de cycle hydrologique dl’atmosphère a pu contribuer significativement à cepremière instabilité.

Au cours de l’Holocène, un événement rapideenregistré au Groenland et en Europe[23,36,37,43,53,54], marqué par un refroidissement de l’ordre4± 2◦C au Groenland (2± 0,5◦C à Ammersee), pendant quelques décennies. Cet événement est égalemarqué dans le signal palynologique en Méditerranavec une diminution de 10 à 15 % de l’occurrenceforêts tempérées (Fig. 3). Une simulation numériqusuggère que ce type d’événement a pu être provopar un apport brutal d’eau douce dans l’Atlantiqnord, par suite de la dislocation de la calotte de glde la baie d’Hudson et de l’écoulement brutal des ede fonte stockées dans le lac Agassiz[42].

V. Masson-Delmotte et al. / C. R. Geoscience 337 (2005) 935–946 937

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Au cours de l’Éémien, environ 5◦C plus chaud quela période actuelle, il semble que la calotte groenldaise ait perdu 50 % de son volume, essentiellemau sud, et que l’Antarctique de l’Ouest ait contribpour moitié à l’augmentation de 4 à 6 m du niveaumers. Lors de cette période plus chaude que l’Acaux pôles, le rythme maximal de réchauffement éde l’ordre de 1,5 ± 0,3◦C par millénaire aux deupôles [39,56]. Les carottes de glace antarctiquesmontrent pas, lors cette transition, d’événementsmatiques abrupts aussi larges qu’en période glaciCependant, le réchauffement anthropique attenduXXI e siècle devrait être au moins 10 fois plus rapque ces variations passées.

1. Introduction

At glacial–interglacial timescales, the driving forof climate change is the modulation of the seasoand latitudinal incoming solar radiation by changesthe orbital parameters of the Earth[2], amplified byclimate feedbacks. In this context, glacial–interglacclimate change has a pacing of several thousandyears[40].

During the late 1980s and 1990s, a successiorapid climatic changes, named ‘Dansgaard–Oescevents’ (D/O), has been characterized along theglacial period in the North-Atlantic sector, occurrinat timescales faster than the orbital forcing, decato centuries, and involving instabilities of glacial icsheets and ocean circulation[13,39]. The discovery ofthe capacity of the climate system to generate srapid climate changes raised the concern for poble future climate surprises and non-linear responto the anthropogenic-induced warming, such as alapse of the oceanic thermohaline circulation[20].What is the palaeoclimatic evidence for abrupt cmate change occurring during periods warmer tfull glacial conditions?

We briefly discuss here the methods used in‘Laboratoire des sciences du climat et de l’environement’ (Grenoble, France) to characterise and unstand rapid climate changes from climate archivethe North-Atlantic sector (Europe and Greenland) ain Antarctica. We then focus on the signature of trapid events occurring (i) at the onset of the last glaciinception (D/O 25), and (ii) at the beginning of the

current interglacial period (8200 yr ago), and disctheir possible causes. We also quantify the rate of tperature increase at polar locations during previowarmer interglacial periods, which do not exhibit aAntarctic evidence for abrupt events as large as duthe last glacial.

2. Overview of archives and methodsto characterise rapid events

2.1. Ice cores

In this paper, we discuss the specific signaturerapid events both during warm and cold periods baon specific archives. Among these, the deep Greenice cores (Table 1) play a central role in documening continuously the temperature evolution in hinorthern latitudes[14,18,39]. Low-resolution (‘bagmeasurements’) water-stable isotope measuremconducted on these deep ice cores offer temporal rlutions of typically 4 yr during the Holocene[23] and50 yr during the last glacial period. Until a new prcise dating of NorthGRIP can be derived from ongohigh-resolution continuous optical and chemical msurements, a preliminary common timescale of GRand NorthGRIP cores has been constructed[39]. Thespatial distribution ofδ18O of polar precipitation ispresently linearly related to local temperature, as asult of the progressive distillation of air masses apreferential loss of heavy molecules during their coing from evaporative areas to the poles[12]. However,the use of this spatial relationship to quantify pGreenland temperatures from Greenland ice coreδ18Otemporal fluctuations leads to a systematic undestimation of glacial–interglacial changes[11,24,25,45], most probably because of changes in the seascycle of snowfall between cold and warm periods,suggested by atmospheric general circulation mels[57] and changes in moisture sources during raevents[35]. New methods based on the gravitationand thermal fractionation of gases in the firn befclose-off processes[17,30,45]have provided independent estimates of rapid temperature changes durinrapid events[21,31]. Fig. 1 displays the NorthGRIPδ18O profile [39] together with the synthesis of gafractionation available estimates of rapid temperatand atmospheric methane concentration changesconstructed 10 to 16◦C rapid temperature increas


,

,

,

Table 1List of sites, types of archives, and methods used to characterise rapid events as reviewed in this paper

Tableau 1Liste des sites, des types d’archives et des méthodes utilisés pour caractériser les événements rapides rapportés dans cet article

Site Latitude, longitude,elevation

Archive type Timescale and temporalresolution

Proxy

Ammersee 48◦00′N, 11◦10′E Lake sediments 15 000 yr10 yr

Benthic ostracodδ18O

NorthGRIP 75.10◦N, 42.32◦W2917 m a.s.l.

Ice core 123 000 yrHolocene 4 yrGlacial period

δ18O, deuterium excessair isotopic composition

GRIP 72.35◦N, 38.30◦W3200 m a.s.l.

Ice core 105 000 yr δ18O, deuterium excessair isotopic composition

Vostok 78◦28′S, 106◦48′E3490 m a.s.l.

Ice core 420 000 yrHolocene 50 yrGlacial period 500 yr

δ18O, deuterium excessair isotopic composition

Dome C 74◦39′S, 124◦10′E,3240 m a.s.l.

Ice core 740 000 yrHolocene 20 yrGlacial period 50 to 100 yr

δ18O, deuterium excess

ODP site 976 36◦12′N, 4◦18′W1108 m b.s.l.

Marine sediments Holocene 25 to 200 yrglacial 50 to 500 yrstage 525 to 1000 yr

Pollen associationscharacteristic of warmand wet phases

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are associated with strong methane variations (mthan one half of the glacial interglacial amplitudwith simultaneous temperature and methane increwithin 50 yr [16]. Methane fluctuations are expectto reflect changes in tropical and high-latitude wetlaproductivities.

The fluctuations of methane and air-oxygen itopic composition (δ18Oatm) can be identified in icecores from both poles. Owing to their residence ti(10 yr for methane, 1000 yr ofδ18Oatm), these globatracers have been used as a tool to construct comtimescales for Greenland and Antarctic ice cores[1,3].On such a common timescale, Greenland and Anttic water-stable isotope profiles exhibit a see-sawhaviour[5,49], which was independently confirmed ba new method based on nitrogen or argon air isotomeasurements[7]: the southern temperature increasslowly 2000 yr before the sharp Greenland tempature increase; when the northern hemisphere hup abruptly, the southern hemisphere temperaturecreases.

In the case of Antarctica, the water-stable isotocomposition of ice remains considered as a reliaquantitative temperature proxy[26]. We discuss herethe level of glacial and interglacial variability record

in east Antarctic ice water stable isotopes from Vtok [40] and Dome C[15] (Table 1). The amplitudeand rapidity of the Antarctic counterpart of Greeland rapid events remains limited (see next sectand therefore only one attempt to use the gas fractation palaeothermometry method has been perfor[6], showing a temperature change consistent wi20% with the classical interpretation of water staisotope fluctuations. We will discuss here the Antarctemperature stability for older periods not recordedGreenland ice sheet, where higher accumulation rinduce a more rapid renewal time.

2.2. North-Atlantic and Europe

Rapid climate changes recorded in Greenlandcores appear to occur in phase with reorganisatof the North-Atlantic deep water formation[4,50]and associated northward heat transport, togetherchanges in the northern hemisphere atmosphericculation.

Simultaneous changes in European climate canidentified from high-resolution archives that canplaced on a common chronological frame. Amothem, the reconstruction of precipitationδ18O can also


; black bars:n iceforest and

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Fig. 1. Synthesis of rapid temperature changes recorded in central Greenland. Top panel, grey bars: amplitudes of methane changesamplitude of temperature changes estimated fromδ15N and/orδ40Ar in ice air (see references in the text). Mid panel, temporal changes iδ18O re-sampled to a 40-yr time step from NorthGRIP ice core (black); percentage of temperate forest pollen (black) and temperateericaceae pollen (grey) in Alboran Sea. Bottom panel, marine sediment-based reconstruction of sea-level changes[55].

Fig. 1. Synthèse des changements rapides de température enregistrés au centre du Groenland. En haut, barres grises : amplitudgements de méthane ; barres noires : amplitude des changements de température estimés à partir duδ15N et/ou duδ40Ar de l’air piégé dansla glace (voir les références dans le texte principal). Au milieu, variations temporelles duδ18O de la glace de NorthGRIP (pas de temps40 ans) (courbe noire) ; pourcentage de pollen de forêt tempérée (noir) et de pollen de forêt tempérée et d’ericaceae (courbe grise)d’Alboran. En bas, estimation des variations de niveau des mers à partir de sédiments marins[55].

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be achieved from lake sediments using lakes wh(i) the deep-water temperature is near 4◦C, (ii) thehydrological budget of the lake is controlled by tprecipitation. In this case, the isotopic compositionbenthic ostracod valves from similar species canrelated to the isotopic composition of the deep-la

water and further to the isotopic composition of tprecipitation, with a decadal time resolution[52–54]and covering the last 15 000 yr. Such a reconsttion has been achieved from the sediments of Amersee, southern Germany, on a common timeswith Greenland records (Table 1).


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In southern Europe, high-resolution recordspollen preserved in marine sediments[8,44] also doc-ument past changes in vegetation cover associwith rapid events, on a common timescale with mrine and ice records. Ocean Drilling Program (ODsite 976 has been drilled in the Alboran Sea, the wernmost section of the Mediterranean Sea (Table 1)and downcore pollen assemblages identified[8]. It isassumed that the primary pollen contribution toAlboran Sea sediments comes from west Mediranean borderlands. The fossil pollen spectrum ranfrom semi-desert to mountain deciduous and conious forests, and can be interpreted from the modtransfer functions developed for Eurasia and NoAfrica [41]. Here are only displayed the proportioof pollen representative of temperate forests taxa (rosiberian trees:Quercus, Fagus, Carpinus, Corylus,Alnus, Betula, Tilia, Ulmus..., associated with Ericaceae), as a proxy of warm and moist climates.age model of[8] was refined and extended to 130 0yr by correlation with deep sea core MD95-2042[46]and NorthGRIPδ18O record[39].

3. Characterisation of Dansgaard–Oeschgerevents and their signature in the North Atlanticincluding Greenland, Western Europe andAntarctica

The ice composition, air bubbles composition, dcontent and chemical species of Greenland ice cdocument the rapid climate variability during the laglacial period. These records give access to hespheric scale reorganisations of the atmosphericdrological cycle and transport. We discuss heremost local aspect, related to Greenland temperaturconstructions, either from water stable isotopes inice or from air-bubble isotopic composition.

As recorded in Greenland water stable isotorecords[22,39], D/O events are associated with a corelatively stable phase (‘stadial’) lasting a few ceturies to a few millennia, followed by rapidδ18Oincreases (within about a century), with intensitvarying from 3 to 5� in GRIP, from 4 to 7� in North-GRIP ice cores. This interstadial phase undergoegradual return to cold, depletedδ18O levels. Whenconverted to temperatures using the classical isototemperature relationship, these changes in ice isot

composition suggest temperature changes of typic5 to 10◦C. However, alternative palaeothermomemethods have evidenced that water stable isotodo not register faithfully past changes in Greenlasurface temperature, due to reorganisations of thedrological cycle. Changes in precipitation seasona[25], in moisture source temperature[35] and possi-ble changes in the vertical structure of the atmosph(temperature profile, cyclonic activity...) make tclassic reconstruction of glacial temperature with wter isotopes through relationships based on presday calibrations not accurate. New palaeothermommethods based onδ40Ar and δ15N of air trapped inice cores (see Section1) indicate that (i) cold, stadialphases were less stable than indicated by iceδ18O;(ii) the amplitude of the stadial–interstadial transitioranges from 10 to 16◦C. Such large amplitudes anot fully captures by intermediate complexity climamodels used to simulate changes in the thermohacirculation.

In Europe, precipitationδ18O reconstructed fromAmmersee extends back to 15 000 yr (onset of pglacial lake). Although the signature of glacial rapevents cannot be identified, the Ammersee recclearly exhibits simultaneous abrupt warming atonset of the Bolling Allerod and at the end of tYounger Dryas periods, suggesting local rapid teperature increases of respectively 8.0 ± 1.5◦C and5.5 ± 1.5◦C, when estimates for rapid warmingGreenland suggest 11 and 10± 3◦C.

Temperate forest pollen data from the Alboran Sshow rapid fluctuations parallel to Greenland raevents, with repetitive alterations of the temperateest (mainlyQuercus), then replaced by semi-desevegetation, marking abrupt shifts between warm-m(interstadial) and cold-dry (stadial) conditions[8]. Theage scale does not enable us to estimate precthe vegetation response time, occurring within a fdecades to centuries. Changes in the percentage otemperate pollen reveal amplitudes of rapid flucttions as large as 50%, to be compared with a glacinterglacial change of 70%.

The southern hemisphere counterpart of northhemisphere rapid events can be identified in Antarice cores[47,48]. Progressive (millennial scale) warming and cooling can be found, with amplitudes varybetween 1 to 3◦C (corresponding to 10–15� in δD),and peaking about 2000 yr after the peak warmth in


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north [6]. Ongoing detailed measurements conducon EPICA Dome C ice cores suggest that each Devent could have an Antarctic counterpart (not show

4. Rapid climate variability during warm periods

The δ18O record measured along the Greenladeep ice cores drilled at Summit in the 1990s sgested a generalised instability of climate with the psistence of rapid events during what was supposebe the ice from the previous interglacial period[14].However, detailed inspections of the ice stratigrapincluding analysis of the air trapped in the ice, demstrated that these abrupt fluctuations of iceδ18O re-sulted from a complex ice flow above bedrock aan unpreserved stratigraphy. A tentative reconsttion of the sequence of layers based on a correlawith methane andδ18Oatm with the undisturbed reference from Vostok did not show any evidence for a tclimatic rapid event during the previous interglacperiod[28,29].

4.1. Onset of last glaciation: D/O 25

The recently drilled NorthGRIP ice core offersundisturbed record of the last glacial inception areveals a new rapid event, D/O 25, occurring ab115 000 yr ago when the northern hemisphere iceume reaches about 1/3 of its glacial extent[39]. Thisfirst rapid warming shows a smallδ18O amplitudein NorthGRIP ice (about 1/3 of typical D/O events)and no Antarctic counterpart can be identified in Vtok available record (Fig. 2). In contrast, the nextwo rapid events (D/O 23 and 24) have the clascal D/O characteristics, including the see-saw sigture in Antarctic ice[9,32]. Mediterranean pollen dat(Fig. 3) show that the interglacial forest environmenpreserved during this period (mean percentage of tperate pollen around 40 to 50%) but rapid temperapollen drops of increasing intensity (10 to 20%) occduring D/O events 25 to 23, reflecting that the eaGreenland millennial scale variability has an Europecounterpart. The first D/O interstadial may be at lepartly induced by changes in the atmospheric wacycle[27,51]and be associated with a latitudinal redtribution of heat transport in the northern hemisphe

without drastic interhemispheric reorganisations athe case of the next D/O events.

4.2. Holocene: 8.2 ka BP event

The only rapid event identified in Greenland duing the Holocene[23,36] is a rapid cooling recordein Greenland ice core, and also in Western Eur(so-called 8.2-ka-BP event)[37,53,54]and only fewmarine records in the Northeast Atlantic[43]. Thiscold event seems related to the decay of the HudBay ice dome and the abrupt drainage of Lake Agsiz, as suggested by intermediate complexity climmodels[42]. The Greenland record of the 8.2-ka-Bevent, when smoothed with a 50-yr time step, show200-yr-longδ18O decrease with an amplitude of 1.62�, corresponding to a temperature decrease reing 4 ± 2◦C during a few decades. Similar evenpunctuate most of the warm phase of D/O events.though taking place during the Holocene interglacperiod, this rapid event is the result of the final decof non polar ice sheets. The detection of a simuneous change in Ammersee Lake precipitationδ18O,showing a 1� decrease suggests a parallel coolover Western Europe, with an annual mean tempture decrease of about 2.0◦ ± 0.5◦C and large-scaleclimate changes in the north Atlantic sector. Meditranean pollen data also capture the 8.2-ka-BP evmarked by a 10 to 15% decrease in the percentagtemperate pollen, lasting about one millennium.

Although a common timescale has not been expitly constructed for the Early Holocene, Antarctic icores undergo a cooling episode about 8000 yr aassociated with the end of the Early Holocene omum[33,34]. This time period indeed correspondsthe final decay of the Laurentide ice sheet, and lascale reorganisations in the atmospheric and ocecirculations. Within the chronological uncertaintiesseems that there is no Antarctic counterpart of8.2-ka-BP northern hemisphere cooling, suggesagain that no large-scale interhemispheric heat trport redistribution is taking place.

4.3. Detection of abrupt changes during past warmerinterglacial periods

Long palaeoclimatic records can also be used tosess whether rapid climate changes occurred du


alyse du

Fig. 2. Common timescale constructed for Vostok and NorthGRIP during the previous interglacial period based on airδ18O measurements[32].Comparison with the percentage of pollen in the Alboran Sea.

Fig. 2. Construction d’une échelle d’âge commune pour Vostok et NorthGRIP pendant l’avant-dernier interglaciaire, à partir de l’anδ18O de l’air [32]. Comparaison avec les pourcentages de pollens de la mer d’Alboran.

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interglacial periods that are warmer than the currenterglacial. In Antarctica, water-stable isotope recofrom Vostok, Dome F and Dome C suggest thatpeak warmth during the previous interglacial perioabout 125 000 yr ago, was about 5± 1◦C warmerthan during the Holocene[56], with a rate of Antarc-tic temperature increase above the Late Holocene lof 1.5 ± 0.3◦C per thousand years and without a

evidence of significant abrupt event during this pticularly warm episode. The same order of magnituof polar temperature change is estimated from NoGRIP ice core in Greenland[39].

Together with sensitivity experiments conductwith a Greenland ice sheet model[10], NorthGRIPdata suggest that, during the previous interglacialriod, the Greenland ice sheet was significantly redu


reststtom).

lboranes

Fig. 3. Signature of the 8.2-ka-BP event in Greenland ice cores’δ18O (NorthGRIP, top), Alboran Sea pollen (mid panel, grey, temperate foand ericaceae, black, temperate forest), precipitationδ18O reconstructed from benthic ostracods from Ammersee lake, south Germany (bo

Fig. 3. Signature de l’événement de 8,2 ka BP dans leδ18O des glaces du Groenland (NorthGRIP, en haut) ; dans les pollens de la mer d’A(figure du milieu : gris, forêts tempérées et ericaceae ; noir, pollens de forêt tempérée) ; dans leδ18O des précipitations reconstitué à partir d
ostracodes benthiques du lac Ammersee, dans le Sud de l’Allemagne (en bas).
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in the South, but essentially similar to nowadays incentre and the north. In response to this∼5 ◦C warm-ing, it is estimated that Greenland should haveabout 50% of its volume, essentially in the South, awould have contributed to a sea level rise of abouto 3 m, half of the total sea-level increase during mrine isotopic stage 5e, estimated to be 4 to 6 m abpresent-day level[38]. These recent data suggest thwest Antarctica and south Greenland ice sheets shhave contributed almost equally to the global sea-lerise during marine isotopic stage 5e. Although asciated with a significant decrease of parts of modpolar ice sheets, the available climatic records frGreenland (NorthGRIP), Antarctica (Vostok, DomeDome F) and Europe[19] suggest a progressive warming above modern values without any interruptiona rapid event of significant amplitude or duration.

In a rather similar context, the transition froglacial conditions of stage 12 to the interglacial coditions of stage 11 takes place with a similar rof temperature increase, and shows a small ‘Anttic Cold Reversal’, occurring when ice isotopic levereach modern values. This cold episode correspoto a cooling of about 2.0 ± 0.7◦C during a thou-sand years, rather comparable to the Antarctic CReversal of the last deglaciation and termination[47,48].

We have no evidence from polar ice cores that cditions warmer than now, encountered during previinterglacial periods, were associated with rapidmate change comparable to the glacial period ravariability, linked to the instability of the Laurentidand Fennoscandian ice sheets and dramatic changthe thermohaline circulation.


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5. Conclusions and perspectives

The evidence from Greenland, Europe and Antatica points out to a systematic occurrence of raevents during glacial periods, in contrast with a frapid events documented during warm periods.

By nature, the two events recorded in Greenlaice cores during warm periods are totally differeD/O 25 seems a small-scale precursor of further geralised glacial instability, possibly reflecting the shof the ocean circulation to the glacial mode, particlarly sensitive to the surface freshwater budget. Tevent takes place at a time period when the Earobliquity is a minimum. An intensified atmospherhydrological cycle may have triggered the first insbilities of the ocean circulation, intimately linked witthe progressive cooling at the glacial inception, hoever limited to the northern hemisphere latitudinal dtribution of heat transport. Simulating this observsequence of events during glacial inception, togewith the large amplitude of Greenland rapid eveduring glacial conditions, will be the next challenfor transient climate models.

By contrast, the 8.2-ka-BP event is caused byabrupt freshwater discharge from the remnantsglacial conditions. Within the temporal resolutionAntarctic Holocene records, this 8.2-ka-BP event dnot seem to have an antiphase counterpart in the soern hemisphere, where this period corresponds toend of the local Early Holocene optimum. Simulaing the correct spatial and temporal signature of8.2-ka-BP event would be a specific benchmarkassess the realism of the thermohaline circulationcoupled climate models. For instance, it would betremely fruitful to use climate models to test if thsensitivity of climate to a similar freshwater pertubation taking place during modern conditions aduring Early Holocene conditions would be simlar.

Future warming may induce an increased runand calving rate from the Greenland and west Antatic ice sheets, with amplitudes of polar temperatchanges expected to range between 2 and 5◦C at theend of this century[20]. Palaeoclimatic records poinout to similar orders of magnitude for the polar teperature changes during the previous interglacabout 125 000 yr ago, both in Antarctica and Greland. Unfortunately, the published NorthGRIP reco

-

does not cover the full transition from glacialinterglacial conditions. In Antarctica, all availabice core water-stable isotope data reveal temptures about 5◦C above modern levels, and no abruevent taking place during the rapid warming perabove modern levels. Although the intensity of cmate change during the previous interglacial peris rather similar to the intensity of warming expectfor this century, the rate of temperature increasemore than 10 times slower (1.5◦C per millennium inthe past, in response to the orbital forcing, to copare with 2 to 5◦C per century expected for the 21century, in response to anthropogenic greenhouseaccumulation in the atmosphere). Despite the evidefor a partial melt of probably both southern Greenlaand West Antarctica, this past warm period did notdergo a dramatic reorganisation of the thermohacirculation with an Antarctic counterpart. Testing tsensitivity of coupled climate/ice sheet models torate of temperature increase could provide a commframework in which to assess the realism of such sulations.

New deep-ice cores being currently drilled (KohnStation, in the southern Atlantic sector of Antarcticor planned (NEEM, in the northwestern part of Greeland) should provide new high-resolution insights inthe dynamics of rapid climate changes during theclimatic cycles.

Earth system models are being designed to expthe possibility of occurrence of rapid climate changin the future, including the coupling between ice shand climate models. The realism of the physical rresentations in the models have to be assessed agthe rapid events (or the absence of rapid events) dmented during glacial and interglacial periods, incluing the data described here, before any confidencebe given to their prediction capabilities.

Acknowledgements

N. Combourieu-Nebout thanks ODP for accesssamples from ODP site 976. This research has bsupported by CEA, CNRS, IPEV, PNEDC (Francand the European Commission. This work wouldhave been achieved without the analytical supporJ.-P Cazet, O. Cattani, S. Falourd, and M. Stievena


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