What is the natural vari-ability of our climate?

This simple question is, infact, very hard to answer,from either the theoretical orobservational point of view.However, it is of utmost im-portance if we really want todetect and predict the conse-quences of human activitiesthat have altered several components of the climate system,the most noticeable change being a considerable increase inatmospheric carbon dioxide from the burning of fossil fuels.

The climate system is complex because it is made upof several components (such as the atmosphere, oceans,and ice sheets), each of which has its own response timesand thermodynamic properties. Those components notonly interact nonlinearly with each other, but are con-nected to other complex systems such as the carbon cycle,which regulates greenhouse gas concentrations in the at-mosphere. The climate can be affected by various types ofso-called external forcings or influences (such as changesin insolation) that have different spatial and temporalscales of propagation in the system. A further problem isthat internal rearrangements and resonances make it dif-ficult to determine a true equilibrium state. Indeed, thesteady state is characterized by a significant noise leveland oscillations that are not always easy to distinguishfrom real transient changes of the global climate.

Unstable modelsThe complex climate system has been studied theoreticallyfor the case of the ocean–atmosphere couple. Based on var-ious types of simulations ranging from simple models witha few homogeneous reservoirs to full general circulationmodels,1 it is now recognized that the ocean–atmospheresystem exhibits several stable regimes under equivalentexternal forcings. The transition from one state to anotheroccurs very rapidly when certain climatic parameters at-tain threshold values. Such nonlinear behavior can be il-lustrated by a hysteresis loop, shown in figure 1, that rep-resents the temperature in the North Atlantic Ocean andon adjacent continents as a function of freshwater flux intothe North Atlantic (mainly from rain, rivers, and meltingicebergs). The freshwater flux affects the density of sur-face waters, which in turn controls the large-scale convec-tion occurring in the northernmost part of the AtlanticOcean. At the present time, the surface waters in thenorthern seas become so dense that they sink to the abyss,and that overturning, called the North Atlantic DeepWater (NADW) flux, ventilates the deep layers of theglobal ocean. The convection process is maintained by acontinuous supply of shallow water advected from tropical

regions. The NADW flux be-longs to a wider convectionsystem, known as the globalthermohaline circulationand often represented as aconveyor belt, that connectsthe world’s ocean basins.

According to models, aslight increase in the fresh-water flux above the modern

level F produces a decrease in the NADW convection anda moderate cooling in the North Atlantic (see the upperpart of figure 1). However, the system flips to another stateonce the flux reaches a threshold value F + DF. That state,which has no deep convection, is characterized by surfacetemperatures up to 6°C lower in and around the North At-lantic. Such a drastic transition, known as a bifurcation,can occur very rapidly—over a period of decades. The coldstate is also quite stable: The freshwater flux must be de-creased significantly to another threshold value (F – DF��)before the system can flip back to its original warm state.

That hysteretic mechanism is not the only cause of cli-matic variability, and other types of nonlinear behavior areknown to exist. Another example of feedback and amplifi-cation is associated with drought over land areas at lowlatitudes. In wet climates, rainwater is captured by roots,absorbed by plants, and then returned to the atmosphere,where it may give rise to a significant amount of additionalrainfall. By contrast, in dry climates, much of the rainfallruns off to supply the groundwater, streams, and oceans,and thus reduces any further rainfall.

Evidence for abrupt climate swingsDo we really have any hard evidence for such abruptwarm–cold and cold–warm transitions? Fortunately forhuman societies, such severe and prolonged climate changehas never occurred during the historical period. We there-fore need to make use of paleoclimatology—the study of theclimate before written technical records began being kept—to access much longer time series under a wide range of ex-ternal forcing conditions. During the past decade, many re-searchers have studied ice cores drilled from the Greenlandice sheet and deep-sea sediment cores from the North At-lantic. Both types of archive can be dated and precisely syn-chronized by different techniques, as described in the boxon page 34. Paleotemperatures in Greenland are derivedfrom stable isotope measurements (oxygen-18/oxygen-16and deuterium/hydrogen ratios) on water molecules fromthe ice.2 Sea-surface temperatures (SST) can be evaluated,among other techniques, by analyzing specific molecules(the so-called alkenones) that are synthesized by phyto-plankton and well preserved in sediments.3

Over the past 100 000 years, the temperature fluctu-ations in Greenland and the North Atlantic are clearly par-allel to each other, as seen in figures 2a and 2b. The mostnotable features are millennium-scale warm events, whichare particularly marked in Greenland (increases of more

32 DECEMBER 2002 PHYSICS TODAY © 2002 American Institute of Physics, S-0031-9228-0212-010-2

EDOUARD BARD (bard@cerege.fr) is a professor at Collège de France,where he holds the chaire d’évolution du climat et de l’océan, and worksat CEREGE in Aix-en-Provence, France.

CLIMATE SHOCK:ABRUPT CHANGES OVER

MILLENNIAL TIME SCALESHow will Earth’s climate respond to ongoing changes in greenhouse gases and ocean circulation? Answers aboutthe future might be found in the past.

Edouard Bard

than 10°C) but are relatively subdued in the North At-lantic. And although not so clearly seen in the Greenlandice cores, several drastic cooling events stand out in theoceanic record. They are easily picked out because the cor-responding marine sediments in core samples contain dis-tinctive types of debris carried along by icebergs.3,4 Figure2c shows such a record3 measured in the same North At-lantic sediments as those used to reconstruct the SST timeseries of figure 2b. As the different records show, each pe-riod of increased iceberg rafting is accompanied by drastic

cooling of up to about 5°C.Although the variability illustrated in figure 2 is

based on two particular sites, the climate swings werewidespread in the North Atlantic and over Greenland. Thewarm intervals are called Dansgaard–Oeschger events inhonor of the two pioneering glaciologists, Willie Dansgaardat the University of Copenhagen and Hans Oeschger at theUniversity of Bern, who, in the early 1980s, identified anddescribed the events in Greenland ice cores. The distinctlayers containing ice-rafted debris are found everywhere

between 40 and 60°N in the North Atlantic. Theirimportance was first recognized by Hartmut Hein-rich in the late 1980s. The so-called Heinrich eventsoriginated mainly from the Laurentide ice sheetcovering Canada, as illustrated in figure 3, and ledto the transport of fine-grained sediments eroded

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FIGURE 1. CONCEPTUALIZED CLIMATE SYSTEM representingthe temperature in and around the North Atlantic Ocean as afunction of freshwater input to the northern North Atlantic.The upper branch (red) features strong deep circulation. Alongthe lower branch (blue), the circulation is collapsed. The mod-ern climate, with its freshwater flux F, is on the upper branch,as shown in green. When the fresh water reaches a threshold (F + DF ), the system flips rapidly to the lower, cold regime.Going back to the warm mode would require a greatly reducedfreshwater input (F – DF��). This diagram is highly schematic;the exact position of the modern climate with respect to bifur-cation points is largely unknown. Moreover, the shape (partic-ularly the width DF + DF��) of the hysteresis loop depends onparameters that are external to the ocean–atmosphere system.(For more details, see refs. 1, 5, 11, and 14.)

FIGURE 2. CLIMATIC AND OCEANOGRAPHIC varia-tions in and around the North Atlantic Ocean duringthe past 110 000 years, as revealed in Greenland icecores and North Atlantic sediment cores obtained offthe Iberian Margin. Time progresses from right to left.(a) The Greenland air temperature based on isotopethermometry shows abrupt warm periods called Dans-gaard–Oeschger events. The records were obtainedfrom ice cores by Willie Dansgaard, Sigfus Johnsen,and their collaborators in Copenhagen, Denmark. (b) The sea-surface temperature in the North Atlanticshows episodes of drastic cooling called Heinrichevents. This record compiles our results on biomolecu-lar thermometry with long-chain (37-carbon) organicmolecules called alkenones measured on two sedimentcores at CEREGE (Aix-en-Provence, France). (c) Thepresence of ice-rafted debris, which was revealed in thesediment magnetic property measurements by NicolasThouveny and colleagues at CEREGE, is correlatedwith the drastic cooling in Heinrich events. (d) Theventilation of the deep Atlantic has been reconstructedby Nicholas Shackleton and his colleagues in Cam-bridge, UK, from variations in the carbon-isotope ratiocontained in bottom-dwelling benthic foraminiferafound in the sediment cores. (e) A qualitative measureof the continental climate is the ratio of pollen fromtemperate plants to that from cold-climate plants, asmeasured in marine sediments by Maria-FernandaSanchez-Goni and her collaborators in Bordeaux,France. The methods used to generate these records are described in the box on page 34. (Ice-core data fromref. 2; sediment-core data from ref. 3.)

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34 DECEMBER 2002 PHYSICS TODAY http://www.physicstoday.org

For periods before the past few centuries, there is unfortu-nately no direct way to measure climatic and oceanographic

parameters such as temperature, precipitation, ice cover, orstrength of ocean currents. Paleoclimatologists and paleoceanog-raphers have thus developed a battery of so-called paleoproxiesthat can be empirically calibrated to the climatic parameters ofinterest. Most of these paleoproxies are based on the analysis ofchemical, physical, and biological properties in geologicalarchives such as sediments from oceans and lakes, ice from moun-tain glaciers and large continental ice sheets, groundwater fromdeep aquifers, and stalagmites and other cave deposits.

Dating these records is a prerequisite step that depends onthe age and nature of the archive itself. The most precise datingcan be achieved when seasonal layers are preserved, as in largetrees, massive corals, and recent glaciers. Layers can also befound in relatively rare ocean and lake environments; such lay-ers are called varved sediments. However, dating with annualresolution is limited to the past few millennia. Other datingmethods are based on the natural decay of radioactive isotopes.On the 104–105-year time scale of interest for this article, preciseages are obtained using carbon-14 and thorium-230/uranium-234 measurements. Although both methods were invented inthe 1950s, their precision and accuracy have been boosted dur-ing the past decade by new mass spectrometric techniques.

Past temperatures in Greenland are derived from stable iso-tope measurements on waters melted from the polar ice. At-mospheric temperatures will affect the relative abundances ofvarious isotopes in the ice, liquid, and vapor phases—an effecttermed fractionation. The snow falling on Greenland shows arather large dynamic range of oxygen-18/oxygen-16 and deu-terium/hydrogen ratios, from which a record of past tempera-tures can be extracted. The paleotemperatures obtained thisway have been recently corrected and corroborated using otherphysical and geochemical techniques.17

In the marine realm, two kinds of methods are commonlyapplied for the basinwide mapping of surface paleotempera-tures:18 those based on chemistry, which use a variety of mark-ers (trace metals, stable isotopes, and organic molecules), andthose based on abundances of microfossil species, which rely ona variety of statistical techniques.

Long-chain alkenones are lipids that are present in severaltypes of algae. The number of double bonds identified in thesemolecules is useful for reconstructing the ancient sea-surface tem-perature (SST). Laboratory algae cultures show a clear correla-tion between the growth temperature and the relative abundanceof the di- and tri-unsaturated alkenones (those with two and threedouble bonds). Similarly, using modern (core-top) algae-contain-ing sediments, several workers have found a clear covariation be-tween the unsaturation ratio and the SST. That relationship is ap-plied to paleorecords, but it is largely empirical. It appears thatthe microscopic algae can maintain the viscosity of their internalfluids by adjusting the relative abundance of their lipids. A usefulparallel is provided by the large viscosity difference betweencommon unsaturated triglyceride oils (such as soya, cotton, andsunflower oil) and the corresponding saturated fats (margarine).The alkenone technique works as a proxy because the viscosity isalso related to the ambient temperature.

Oxygen isotope data based on the shells of surface-dwellingplanktonic foraminifera have been used in support of tempera-ture reconstructions, but rarely as a standalone temperatureindex because of the large influences of other environmental pa-rameters on the measured 18O/16O ratio (see PHYSICS TODAY,December 2001, page 16). Other chemical methods includethose based on trace element ratios such as strontium/calcium,uranium/calcium, and magnesium/calcium in the carbonateskeletons of corals and microfossils (shells of planktonic and

bottom-dwelling benthic foraminifera). The general assump-tion is that trace metals are incorporated into the carbonate lat-tice at concentrations that depend on growth temperature.However, observations rarely agree with thermodynamic con-siderations, because living organisms nearly always introduceadditional complexity. (Specialists use the generic term vital ef-fects, which hides their lack of knowledge about still poorlycharacterized mechanisms.)

Water-column properties can be estimated by considering theabundance of various faunal or floral species. The process relieson statistical prediction methods that are entirely empirical.Using modern (core-top) samples, such abundances can be corre-lated with SST or some other property of the ocean. The pri-mary assumption is that, at any given site, the changes with timeare controlled by the same mechanisms that influence the mod-ern spatial patterns of faunal abundance. As one example of suchstatistical techniques, records of the relative abundance of so-called dysoxic benthic foraminifera, which are adapted to oxy-gen-poor conditions in marine bottom waters, can be used to ex-tract information about oxygen depletion in the oceans. Similarstatistical techniques are used with lake and marine sediments tostudy trapped pollen from warm- and cool-climate plants.

The strength of the deep-sea ventilation can be studied withseveral geochemical proxies based on trace elements (such as cad-mium), radioactive isotopes (14C or protactinium-231, for exam-ple) and stable isotopes (such as 13C). Again, the relationships be-tween the paleoproxy and the deep circulation are oftenempirical and depend on biological processes that fractionatechemical and isotopic species during the slow movements ofwater masses. For example, photosynthesis and biological activ-ity preferentially use up the more reactive 12C in surface waters,whereas the dissolution of sinking fecal pellets progressively en-riches deep waters in 12C. Thus, water currently or recently at thesurface is depleted in 12C and enriched in 13C compared to waterthat has been in the deep ocean for a long time. Furthermore, thedeep Pacific waters, due to the longer time since their last contactwith the surface, are depleted in 13C by about one part per thou-sand compared to deep Atlantic waters.

Other biological processes that fractionate isotopes can beexploited in paleoceanography. For example, marine bacteriatypical of oxygen-depleted zones preferentially use nitrogen-14when converting marine nitrates (dissolved NO3

–) into gaseousnitrogen (N2O and N2), leaving the remaining NO3

– enriched in15N. Measurements of the 15N/14N ratio in organic matter pre-served in ocean sediments can thus be used to track changes inthe denitrification processes that are usually associated withoxygen depletion in the ocean.

Another kind of paleoproxy is constructed by considering thesedimentary abundance of minerals and organic compoundslinked to specific oceanographic or climatic processes. For exam-ple, the flux of icebergs can be evaluated qualitatively by count-ing under a binocular microscope the individual grains of conti-nental rocks transported by icebergs (so-called ice-rafted debris)or by measuring directly their total magnetic susceptibility.

To a certain degree, all paleoproxies suffer from uncertain-ties due to perturbations by other environmental variables andto problems related to signal preservation. Such complicationsare inherent to geological archives because, by their nature, allpaleoproxies are linked in some way to a variety of complexnatural processes. Admittedly, the uncertainties represent themain weakness of paleoclimatology and paleoceanography incontrast to modern climatology and oceanography. However,researchers can justify the use of multiproxy reconstructionsbecause, in principle, most of the sources of systematic error (orbias) should cancel out because they are specific to certain bio-logical groups or analyzed chemical species.

The Paleoclimatology Toolbox (Opening a Can of Worms!)

from the underlying rocks. The distribution of these dis-crete melting events can be mapped precisely by studyingthe mineralogical, geochemical, and physical properties ofthe ice-rafted component of sediments.4

The record of deep-ocean ventilation is also correlatedwith the temperature time series, as expected fromocean–atmosphere modeling. The ventilation record can beextracted from the same marine sediments used for study-ing alkenones and ice-rafted debris. It is based on the car-bon isotopes contained in the microscopic shells of unicel-lular organisms that live on the ocean bed (see the box).These so-called benthic foraminifera pick up their 13C/12Cratio from the inorganic carbon dissolved in bottom waters,which itself depends on the oceanic ventilation. High13C/12C ratios in North Atlantic benthic foraminiferal shellsimply vigorous overturning, whereas low 13C/12C ratios in-dicate sluggish deep-sea circulation. The record given infigure 2d shows that the North Atlantic convection over thepast 100 000 years varies essentially among three states:The highest 13C/12C values are observed for the recent past;further back in time, the ocean oscillated between twostates, one characterized by 13C/12C values that were rela-tively high but lower than modern, and the other by stilllower values. As the sedimentary records in figure 2 show,each short period of intense iceberg melting is associatedwith a distinct decrease in North Atlantic ventilation.

Drastic cooling events also affected the nearby Euro-pean continent, as revealed in the plant pollen found in thesame deep-sea sediments that were analyzed for carbon iso-topes and alkenones. The painstaking work of countingpollen was performed for a short time interval between30 000 and 50 000 years before the present. Figure 2e showsthe time evolution of the abundance ratio between thepollen of plants (mainly deciduous trees such as oak) typi-cal of temperate and humid climates and pollen of plants(such as shrubs) typical of cold steppes. The pollen ratio isa useful qualitative measure of climate over the continents,with high values observed during mild periods and low val-ues during the colder events of intense iceberg melting.

Simulating millennium-scale transientsTaken together, those observations seem to support thetheory that rapid climatic changes are due to abrupt tran-sitions of the state of the ocean–atmosphere system, asrepresented by the simplified hysteresis loop of figure 1.The high degree of correlation between temperaturerecords and ocean ventilation records is probably the best“fingerprint,” indicating that both ocean and atmospherewere intimately involved in the abrupt fluctuations. Theexcess freshwater flux into the North Atlantic over thepast 100 000 years is clearly linked to massive influxes ofmeltwater from icebergs.

Several modelers have attempted to simulate the mil-lennium-scale climatic variability of the last glacial periodof 10 000–100 000 years ago, particularly the complex in-terplay between Dansgaard–Oeschger warm phases andHeinrich cold events. At present, models coupling the at-mosphere, ocean, and ice sheets are still unable to correctlysimulate that variability on all scales in both time andspace. To date, all the attempts have been made with sim-plified models using some kind of imposed forcing factor.1

One of the most impressive results so far has been ob-tained by Andrey Ganopolski and Stefan Rahmstorf at thePotsdam Institute for Climate Impact Research in Ger-many. They perturbed a simplified ocean–atmospheremodel with a variable freshwater flux into the North At-lantic.5 As shown in figure 4, this system exhibits a typi-cal hysteresis behavior that is excited by a freshwater flux

cycle of relatively limited amplitude. The Atlantic Oceanresponds in a highly nonlinear way with abrupt and largeamplitude changes in its general overturning rate. Thesefluctuations generate temperature cycles of very large am-plitude both at high latitudes in Greenland and at mid lat-itudes in the North Atlantic.

A highly diagnostic feature is the asymmetric shapeof the warm events, especially for Greenland: The tem-perature warms abruptly to reach a maximum and thenslowly decreases for a few centuries before reaching athreshold, after which it drops back to the cold values thatprevailed before the warm event. Although the Dans-gaard–Oeschger events vary in duration, as seen in figure2a, they have the same asymmetric shape as reproducedby the numerical model. The abrupt warming results froma northward advection of warm Atlantic waters into north-ern seas; the plateau phase corresponds to the “warmmode” of Atlantic Ocean circulation that gradually weak-ens, and the rapid cooling marks the end of deep-water for-mation in the northern seas.

Another key feature is the spatial distribution of themodeled warming event shown in figure 4e. This mapclearly shows that maximal warming is centered on thenorthern Atlantic; farther away, its impact fades, becom-ing much smaller in the Pacific Ocean region and almostnegligible in the Southern Hemisphere.

Such characteristics are fully compatible with existingpaleoclimatic data, which show that temperature variationswere much smaller in the intertropical zone.6 However,these relatively small changes are greatly amplified in thewater cycle: Because the saturated water vapor pressure

http://www.physicstoday.org DECEMBER 2002 PHYSICS TODAY 35

Atlantic Ocean

Icebergs

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FIGURE 3. DURING A HEINRICH EVENT, icebergs surge intothe North Atlantic Ocean. The lower panel illustrates the en-trainment of debris (black) by icebergs and the subsequent sed-imentation of the debris in the deep North Atlantic.

rises exponentially with temperature, rather small temper-ature changes in warm zones translate into relatively largechanges of water availability. Indeed, researchers workingon tropical paleoclimates see evidence that large changes inmoisture availability and tropical rainfall occur duringDansgaard–Oeschger warming events.7

To mimic a Heinrich event, the same model was per-turbed using a major freshwater input into the North At-lantic.5 Such a large flux literally kills off the deep-sea cir-culation, which causes a widespread cold event (figure 4f)that is relatively more pronounced in the mid latitudes ofthe North Atlantic, lower than the latitudes showing max-imal warming in Dansgaard–Oeschger events. Anothercharacteristic feature of the modeled temperature distri-bution is that the intense cooling centered in the North At-lantic is accompanied by a widespread warming in theSouthern Hemisphere. This seesaw effect results from thereduced interhemispheric heat transport and the en-hanced formation of Southern Ocean deep water whenNADW formation is reduced. Evidence for such a seesaweffect is provided by comparisons between ice-core recordsfrom Greenland and Antarctica. In particular, the most re-cent temperature oscillations in Greenland between16 000 and 11 000 years ago are in antiphase with thoseobserved in central Antarctica.

Over the past few years, evidence has been emergingthat the Dansgaard–Oeschger and Heinrich events had astrong and widespread impact on the marine carbon cycle.A global estimation of that effect still requires more work,but several studies have shown that the amount of organicmatter in sediments from several oceanic areas varies inphase with the Dansgaard–Oeschger and Heinrich events.

Convincing records have been obtained not only in theNorth Atlantic but also in the North Indian and West Pa-cific Oceans;8 figure 5 shows a few examples. The changeshave been attributed to large fluctuations of the biologicalproductivity in surface layers of the ocean. Their abrupt-ness could be linked to the injection of nutrients into thesurface layer by wind-induced mixing and upwelling. Anadditional cause could be the direct input of biolimiting el-ements in dust transported by low-latitude winds.

At best, though, the concentrations and mass fluxes ofmarine organic matter in sediments show only a qualita-tive correlation with the biological productivity occurringin the surface layers. The preservation of organic com-pounds depends on ambient conditions in the water col-umn, in particular its oxygen content. Oceanic zones ofhigh biological productivity are characterized by pro-nounced oxygen depletion at intermediate water depths.Ocean convection transporting oxygen-rich water massesmay thus have played an additional role in shaping the or-ganic records. As described in the box, several techniquescan be used to evaluate the severity of oxygen depletion atintermediate depth; figure 5 shows two examples.

The timing and duration of Dansgaard–Oeschger andHeinrich events have been the subject of many hot de-bates. In 1993, Gerard Bond from Columbia Universityidentified some regularity by comparing Greenland andNorth Atlantic time series: The millennium-scale eventsare apparently distributed according to long-term cycles,with warm periods becoming progressively shorter andcooler. Each long-term cycle culminates in a prolonged coldperiod during which a Heinrich event takes place. Otherworkers have applied a variety of frequency-analysis tech-

36 DECEMBER 2002 PHYSICS TODAY http://www.physicstoday.org

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(a) Freshwater flux is the key input to the model. Small perturbations with a 1500-year cycle are sufficient to generate Dans-gaard–Oeschger warming events; a large freshwater input represents a Heinrich event. (b) Resulting variations of the Atlantic ven-tilation, expressed as the North Atlantic Deep Water (NADW) flux. (c) Variations in the air temperature over Greenland. (d) Variations of the sea-surface temperature in the North Atlantic mid latitudes. (e) Maximum air temperature anomaly attainedduring the warm transient of a Dansgaard–Oeschger cycle. The black outlines show the crude continent locations. (f) Maximumair temperature anomaly reached during an iceberg surge associated with a Heinrich event. The greatest cooling during Heinrichevents is found farther south than the greatest warming during a Dansgaard–Oeschger event.

niques. That task has proved to be more complex than ex-pected because of the highly asymmetrical shape of the cy-cles and potential mathematical artifacts linked to sam-pling. Indeed, the predominance of an underlying1500-year cycle has often been proposed but remains con-troversial; a simple inspection of figures 2a and 2b showsthat the records are far from being a simple sine wave.Michael Schulz from the University of Bremen recentlydemonstrated that the prominent 1500-year spectral peakis caused by just three events that represent only 5% ofthe total duration of the glacial period (between 30 000 and35 000 years before the present, visible in figure 2a).9

Other recent studies by Richard Alley at PennsylvaniaState University and by the Potsdam group suggest thatthe Greenland record, by amplifying small fluctuations offreshwater flux into the North Atlantic, exhibits the regu-lar behavior typical of a stochastic resonance system.10

Could it happen again?Although no model has yet reproduced the detailed climaticvariability that occurs during a glacial period, the generalmatch between observational data and modeling stronglysuggests that the envisaged mechanisms are plausible. An-other lesson from the modeling approach is that theocean–atmosphere system is particularly sensitive and un-stable during glacial periods.5,11 Compared to the last gla-cial period, the modern climate is characterized by differ-ences in conditions such as ice-sheet distribution, insolationvariations, and greenhouse gas concentrations in the at-mosphere. Under the present boundary conditions, the hys-teresis loop appears to be significantly wider than duringglaciations. In other words, the modern climate is probablyless prone to frequent jumps between two extreme states.Indeed, the best evidence that the modern climate is rela-tively stable is provided by the temperature records in fig-ure 2. During the Holocene period, which covers the past10 000 years, both the Greenland and North Atlantic tem-peratures define smooth and limited decreasing trends. Pa-leotemperatures reconstructed for low-latitude areas con-firm the relative stability of the Holocene, although recordsobtained for land areas indicate a somewhat larger vari-ability in the water cycle and associated components suchas lake levels and vegetation cover.

The only important event during the Holocene, occur-ring about 8200 years ago, was much smaller and shorterthan the glacial swings.12 Sedimentological and glaciolog-

ical studies suggest that this early Holocene cooling wasdue to freshwater input into the North Atlantic. Lakes lo-cated on the melting front of the Laurentide ice sheet sup-plied a sudden input to the ocean when their drainage wasdiverted toward the Hudson Strait at the very end of thelast deglaciation.

During the glacial period, the 4-km-thick Laurentideice sheet greatly depressed the underlying land. Due to thelong relaxation time scale (a few millennia) for Earth’s vis-coelastic rebound, the ice melted back into its own hole. Be-cause the ice was centered on Hudson Bay, blocking off theBay as well as its outlet, extensive lakes (including thelargest lake on Earth at that time) formed around the iceand could not drain away. Eventually, as the ice shrank, themeltwater managed to escape. It is now understood thatsuch glacier bursts are invariably catastrophic. The heatfrom turbulent drainage flow increases the size of the open-ing, thus enlarging an ice-walled channel. Through such aprocess, it is likely that the largest lake on Earth emptiedinto the North Atlantic during the space of a single summerto a few years. This emptying was immediately followed bya widespread cooling, probably because the fresh water fa-vored sea-ice formation in the North Atlantic. That dramaticevent likely had a transient impact on Atlantic overturning,at least its component in the Labrador Sea.

The early Holocene event is significant not only dueto its magnitude (even though some earlier changes werelarger), but also because it shows that a warm climate canbe perturbed suddenly in a major way. Moreover, thatevent provides an outstanding test bed for numerical mod-els because some of the conditions, such as ice-sheet di-mensions, are more closely comparable to modern condi-tions than earlier events of greater magnitude.

Starting in 1985, Wallace Broecker from ColumbiaUniversity warned that the global thermohaline circula-tion might be the Achilles’ heel of our climate system.13 Ac-cording to him, ocean stability may not hold under futureglobal warming, which will certainly alter the water cycleand hence the Atlantic freshwater budget. Improved mod-els allowed Broecker’s warning to be tested in 1997 byThomas Stocker and Andreas Schmittner in Bern,Switzerland.14 Their transient simulations showed that,when the fossil-fuel burning rate exceeds a particularthreshold, the modern climate could flip to another modecharacterized by cooling over the North Atlantic. That re-gional cooling would be associated with a strong decrease

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Phytoplanktonic biomarker(North Atlantic)

Dentrification intensity(North Indian Ocean)

Benthic fauna adapted to thelack of oxygen (California margin)

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H5FIGURE 5. DANSGAARD–OESCHGER AND HEINRICHevents are reflected in records of the marine carbon cycle dur-ing the last 65 000 years. (a) Concentration of 37-carbonalkenones, a phytoplanktonic biomarker in North AtlanticOcean sediments. We obtained this record at CEREGE on acore sampled off the Iberian Margin. (b) The nitrogen-15/nitrogen-14 ratio in organic matter preserved in North IndianOcean sediments was measured by Mark Altabet from theUniversity of Massachusetts Dartmouth and his colleagues,and is correlated to the oxygen depletion caused by biologicalactivity in the ocean. (c) Oxygen depletion is also reflected inthe relative abundance of benthic foraminifera that haveadapted to the lack of dissolved oxygen in marine bottom wa-ters. The record shown was obtained on sediments from theCalifornia Margin by Jim Kennett and his collaborators atthe University of California, Santa Barbara. The methodsused to generate these records are described in the box onpage 34. (Data from ref. 8.)

in the Atlantic overturning, reminiscent of the scenariothat prevailed during Heinrich events.

Over the past few years, several other climate modelsof increasing complexity have been used to study the am-plitude, duration, and onset conditions of what are now re-ferred to as “climatic surprises.”15 Modeling of the ocean-atmosphere couple is still in its infancy, and the exactgeometry of the hysteresis loop (figure 1) is not well known,even for the modern system. In addition, we still have noconstraint on the position of the modern climate on theupper branch of the loop. For these reasons, an intense de-bate continues in the modeling community about the real-ity of such instabilities under warm conditions.

All the studies so far carried out fail to answer the cru-cial question: How close are we to the next bifurcation? Thisis an urgent problem because modern hydrographic recordsindicate a rapid freshening of North Atlantic waters, whichsuggests that the deep Atlantic ventilation has steadilychanged over the past 40 years.16 A possible answer to thequestion may come after major improvements in climatemodels, which could be achieved by increasing the spatialand temporal resolution of numerical calculations and bytaking better account of the physical processes that link thedifferent components of the climate system. The perform-ance of such a new generation of climate models should firstbe tested on recent oceanographic changes, but also onlonger paleoclimatic time series. In addition to technical ad-vances, any predictions about the climate of the next cen-tury will also depend on scenarios of greenhouse gas emis-sion over the coming years.

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