One of the grandchallenges formodern science is

to predict climate. Research-ers especially wish to learnabout large surprises—chang-es that could help one soci-ety flourish or lead anotherto devastation. Will Europereturn to the warmer tem-peratures of 1,000 years ago,when the Vikings settledGreenland and Britons nur-tured vineyards? Or could Californiasuffer extended droughts, lasting cen-turies, just as the region endured rough-ly a millennium ago? Recent concernsabout global warming and the effects ofman-made greenhouse gases have onlyheightened the need to understand thebasic natural processes that cause theclimate to change.

To gain this fundamental knowledge,climatologists have turned to the past.Drilling deep below the surface of icesheets and glaciers in Greenland, Ant-arctica and elsewhere, scientists haveobtained water frozen for tens of thou-sands of years. Trapped in the ice aretrace chemical impurities containing pre-cious information about ancient climate.

Recent work by European and U.S.teams, including us, studying cores ofice from deep drillings in Greenland hasshown that large, rapid changes in cli-mate, typically lasting a few hundred toseveral thousand years, punctuated thelonger cycles between glacial (cold) andinterglacial (warm) periods. Moderncultures have not experienced such dra-matic swings. What caused them? Didthey occur simultaneously in the high

latitudes of the Northern and Southernhemispheres? How were the tropics af-fected? The answers to these questionscould provide a window to the future.Although current concerns about cli-mate change focus on the influence ofhuman activities, ancient shifts may havebeen fated by the heavens.

Celestial Effects

During the 1920s and 1930s, Mi-lutin Milankovitch, a Serbian as-

tronomer, studied how the gravitationalpull from other planets causes subtlechanges in the orbit of the earth. Thesealterations result in different distribu-tions and intensities of sunlight, whichthen lead to dramatic variations in cli-mate over tens of thousands to hundredsof thousands of years. Milankovitch in-vestigated three orbital variables: the

tilt of the spin axis; the pre-cession of the tilt (a motionsimilar to the wobbling of aspinning top); and the eccen-tricity in the orbit around thesun (that is, whether the orbitis almost circular or stretchedout into a more elongated el-lipse). Changes in these threeparameters cause slow butdistinct oscillations in climatewith periods of about 40,000years (governed by tilt),

20,000 years (precession) and 100,000or more years (eccentricity) [see “WhatDrives Glacial Cycles?” by Wallace S.Broecker and George H. Denton; Sci-entific American, January 1990].

Many contemporaries of Milanko-vitch denounced his astronomical theo-ry of climate change, which remainedlargely unproved up to his death in1958. Subsequently, however, scientistsstudying sediments deposited on the bot-tom of the ocean made some landmarkdiscoveries. They found that the pastfew million years have been character-ized by a repeated series of temperaturechanges during which great glaciers ad-vanced and retreated over vast areas,all the time marching to the beat pre-dicted by Milankovitch. For at least thepast half a million years, the basic cli-mate cycle—the period from one glacialor interglacial extreme to the next—hasbeen about 100,000 years, with short-er-term oscillations of roughly 20,000and 40,000 years.

More recently, researchers have soughtto study ancient climate in even finerdetail. To that end, investigators havedirected some of their attention away

Greenland Ice Cores: Frozen in Time80 Scientific American February 1998

Greenland Ice Cores: Frozen in Time

Ice, frozen in place for tens of thousands of years, provides scientists with clues to past—and future—climate

by Richard B. Alley and Michael L. Bender

CENTRAL GREENLAND, where snowhas accumulated for more than 100,000years, offers researchers the opportunityto probe ancient climate change by drill-ing into the glacier below.

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from ocean sediments and have exam-ined cores extracted from depths downto three kilometers (about two miles)below the surface of the great ice capsof Greenland and Antarctica, amongother areas [see “The Antarctic Ice,” byUwe Radok; Scientific American,August 1985]. But scientists could nothave drilled just anywhere in these fro-zen wastelands; they had to find “good”glacial deposits, where snow has accu-mulated over tens of thousands of years.As the snow piles up in such places, itcompresses under its own weight andeventually forms ice, preserving in theprocess a wealth of information aboutthe climate.

Age-Old Question

The crucial prerequisite in the studyof ice cores is the accurate determi-

nation of the age of the specimens. With-out these data, scientists could not buildan overall chronology in which to placetheir other measurements. Fortunately,investigators can often determine theage of the ice by simply counting off thenumber of years.

In places such as central Greenland,where it snows frequently, the ice formsin annual layers that can be analyzed inmuch the same way that yearly growthrings can be used to determine the ageof a tree. The layering in glacial ice isoften noticeable with the naked eye be-cause crystals from summer snow arelarger than those of winter snow. Clima-tologists can also detect the annual lay-ers by measuring the acidity of the ice,which is generally higher for summersnow for reasons that remain somewhatobscure. And researchers can use a la-ser to determine the concentration ofdust particles in an ice specimen. Thenumber of dust particles typically risesin the spring because of the greaterstrength of the wind in that season.

Using these and other indicators, oneof the authors (Alley) worked with hiscolleagues on the U.S. ice-coring projectin Greenland, headed by Paul A. May-ewski of the University of New Hamp-shire, to obtain a chronology that com-pares well with several independentmeasures. For example, an analysis ofthe composition of ash found in certainlayers of ice enabled the team to identi-fy the volcanoes involved and the corre-sponding historical dates of the erup-tions. Such corroborative testing indi-cates that the counting of annual strataintroduces virtually no errors for Green-

Greenland Ice Cores: Frozen in Time

ANNUAL LAYERS, often visiblewith the naked eye, are usuallyhorizontal, but slow movementof the glacier sometimes distortsthem into Z-shaped folds.

AIR BUBBLES become trapped inthe snow as it compresses intoice. These samples of the ancientatmosphere reveal how concen-trations of greenhouse gases suchas carbon dioxide and methanehave changed.

OXYGEN ATOMS in the ice canvary in composition. The relativeabundance of the different typesreflects the temperature at thetime of precipitation.

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HYDROGEN

Slicing through Time

Scientists make many differentkinds of observations in their ef-

forts to decipher the record of cli-mate held in ice cores (such as thecut example at the left). Some keymethods of analysis appear below.

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land ice that is several centuries old. Forthe most recent 11,500 years, the so-called Holocene warm period, layercounting is correct to within 1 percent.Although the accuracy is somewhatpoorer in older ice from colder times, itappears to be as good as that of otherdating techniques to at least 50,000years ago. Arguably annual layers re-main visible past 100,000 years, butthey often appear distorted.

Why are these deep layers so disrupt-ed? Although nominally solid, glaciersand ice sheets spread and thin under theinfluence of gravity, similar to the mo-tion of pancake batter poured on a grid-dle. As the movement continues overtens of thousands of years, the bottomlayers of ice become extremely drawnout (sometimes disappearing altogether)and crumple easily as the glacier oozesover the ground below.

The resulting deformations make itimpossible to count annual layers con-tinuously below a certain depth. For ex-ample, the cores the U.S. team obtainedin Greenland contained layers that,from top to bottom, went from beinghorizontal, to having small wiggles, toshowing Z-shaped folds, to becomingslanted at angles up to 20 degrees in iceolder than about 110,000 years. Corestaken nearby in a parallel effort by Eu-ropean researchers revealed a similarlycomplex (yet different) pattern in iceolder than 110,000 years.

This dissimilarity helped to solve apuzzle. Ice initially believed to be fromthe previous interglacial period—theEemian, which ended about 120,000years ago—indicated severe climatechanges with rapid and repeated swings.The result was both surprising andalarming: climatologists had viewedboth the present and past warm inter-glacial times as stable and free of suchwild shifts.

Scientists thus began a series of care-

ful examinations that have since shownthat flow of the ice disrupted the olderlayers in both the European and U.S.cores. A clue was that the two sets ofclimatic records were virtually identicalduring the most recent 110,000 yearsbut that they could not be positivelymatched for older times. Apparently,these disturbances occurred fartherabove the bed than originally had beenexpected. Paleoclimatologists are nowbeginning to understand such effectsand are developing additional tech-niques, such as computer simulations,to recognize them.

To obtain a reliable Eemian record,the European team has commenced an-other deep-drilling project in Greenland,about 340 kilometers northwest of thetwo earlier drill sites. Layers older than110,000 years at the new location arethought to be higher above the bedrock

than those at the previous bore posi-tions. Thus, distortions from the flowof ice are less likely to be acute.

Ice-Core Secrets

If the Europeans succeed, scientistswill gain important insight into how

the climate operated, in large part fromtheir estimates of ancient temperatures.The primary thermometer used for thispurpose takes advantage of the fact thatwater comes in “light” and “heavy”flavors, or isotopes. Light isotopes haveonly ordinary hydrogen and oxygen;heavy ones contain either hydrogen withan added neutron (deuterium) or oxygenwith one or two extra neutrons (oxygen17 or oxygen 18). The cooling of an airmass causes precipitation, which re-moves more of the heavy water (becauseof its lower vapor pressure) from the

Greenland Ice Cores: Frozen in Time82 Scientific American February 1998

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MEASUREMENTS of ancient tempera-ture (a) and atmospheric methane (b)from the Greenland ice cores have cor-roborated earlier findings about climateat Vostok, Antarctica (c), and for the totalvolume of ice around the world, as deter-mined from the analyses of deep-sea sedi-ments (d). In particular, the different re-search shows a warm period approximate-ly every 20,000 years. A more gradualtrend of cooling until 20,000 years agoand the rapid warming between 20,000and 10,000 years ago follow a longer, ap-proximately 100,000-year cycle.

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moisture-laden atmosphere. Conse-quently, snowfall at an inland site duringcolder times tends to contain “lighter”water, the heavy isotopes having alreadybeen squeezed out of the air as it trav-eled over the ocean and the flanks ofthe ice sheet before reaching the site.

A second thermometer for ancientclimate comes from the current temper-ature of the ice sheet. Just as a frozenroast placed in a hot oven will tend tomaintain the temperature of its formerenvironment by remaining frozen in thecenter even as its outside begins to warm,the ice sheet is actually colder a kilome-ter or two (or about a mile) down thanat the surface. In essence, the older ice“remembers” the extreme temperaturesof the last ice age. Taken together, thetwo thermometers show that the sever-est periods of the last ice age were in-deed quite frigid—on average Green-land was colder by more than 20 de-grees Celsius (36 degrees Fahrenheit)than it is currently.

In addition to records of temperature,ice cores contain a history of precipita-tion. Scientists have, for example, usedthe thickness of an annual layer (after

correction for any distortions caused bythe flow of ice) to gauge the amount ofsnow that year. This analysis revealedthat the coldest periods in central Green-land had four to five times less precipi-tation than today.

Further clues to ancient climate aresupplied by windblown materialstrapped in the ice. Coarser dust parti-cles suggest winds of greater strength.Researchers can, in fact, track past pat-terns of atmospheric circulation by us-ing the composition of the dust to de-termine its source, just as the analysis ofash enables the identification of the vol-canic eruption involved. Other tracematerials found in the ice cores includechemicals from marine algae and ra-dioactive isotopes produced in the airby cosmic rays.

A decreased concentration of thesesubstances indicates either a smaller sup-ply or an increased amount of snowfall,which dilutes these materials. Becausethe annual layering in cores from centralGreenland allows scientists to determinethe rate at which snow accumulated,they have been able to separate the twoeffects. After doing so, they uncoveredup to 100-fold changes over time forsome windblown imports such as calci-um, indicating that much stronger windsand perhaps larger deserts existed dur-ing cold intervals.

The ice is also an excellent storehousefor samples of ancient air. In freshly fall-en snow, gas molecules circulate easilythrough pores between the ice crystals.The massive weight of snow that pilesup on a polar ice sheet, however, pressesdown on the deeper layers, causing thepores to become smaller and smaller un-til, between a depth of about 40 and120 meters, the compression is greatenough that air becomes imprisoned asindividual bubbles in the ice.

Analyses of these tiny air samples byone of us (Bender) and others studyingice from Greenland and Antarctica re-veal how concentrations of atmospher-ic gases have changed over time. In par-ticular, scientists have determined howvarious heat-trapping greenhouse gaseshave varied naturally. From glacial tointerglacial periods, for example, theconcentrations of carbon dioxide andmethane surged by about 50 and 75percent, respectively. This informationhas helped put the recent additional in-creases caused by human activities—30percent for carbon dioxide and 160 per-cent for methane—into perspective.

Studies of trapped gas have an ancil-

lary benefit. Because the global atmo-sphere mixes rapidly, its makeup every-where is almost the same. Thus, research-ers can safely assume that changes in theatmospheric composition occurred si-multaneously in Greenland and Antarc-tica, allowing them to use gas measure-ments to correlate ice cores taken fromopposite sides of the world.

Flickering Climates

Examination of the Greenland icecores has bolstered the results of

previous investigations of Antarctic icecores and deep-sea sediments. Com-bined, the different studies provide solidsupport for Milankovitch’s astronomi-cal theory of climate change. For exam-ple, the different records show warmtemperatures at about 103,000, 82,000,60,000, 35,000 and 10,000 years ago,which roughly reflect Milankovitch’s20,000-year precession cycle.

But perhaps the most striking featuresof the Greenland ice-core results are“interstadial events”: intervals lasting afew hundred to a few thousand yearsduring which Greenland warmed rap-idly, then cooled at first slowly, thenquickly. The Greenland ice cores clearlyshow that between 100,000 and 20,000years ago, approximately two dozen in-terstadial events occurred—all unpre-dicted by Milankovitch theory.

Interestingly, the Greenland ice coresdemonstrate that the methane concen-tration of the atmosphere increased dur-ing each interstadial event. Methane isproduced by bacteria in environmentswhere oxygen is scarce—for example,in tropical swamps and bogs. The high-er methane levels in the ice indicate thatthe tropical wetlands must have ex-panded during interstadial times, evi-dently because of increased rainfall.

An intriguing characteristic of inter-stadial events is their abruptness. Chang-es of perhaps five to 10 degrees C (nineto 18 degrees F) or more, twofold insnow accumulation and up to 10-fold industiness occurred over mere decades,sometimes even during as little as a fewyears. This dramatic behavior was mostprominent during times of intermediatetemperatures over the past 100,000years; the coldest part of the ice age andthe modern period of warmth appearstabler in comparison. Right before andafter the large interstadial jumps, theclimate at times took smaller hops backand forth between warm and cold—abehavior that scientists have dubbed

Greenland Ice Cores: Frozen in Time Scientific American February 1998 83

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flickering. So, in addition to the inter-stadial shifts, the climate apparentlybounces repeatedly between a warmand a cold mode—all when Milanko-vitch would have predicted gradualtransitions.

Beyond Milankovitch

To explain such seemingly erraticbehavior, scientists have begun to

investigate other factors in addition tothe earth’s orbit. Using sophisticatedcomputer models, researchers havesought to understand “teleconnections,”by which changes in the climate of onegeographic region trigger variations else-where. For example, recent investiga-tions indicate that warming in high lati-tudes could alter the circulation of theocean or atmosphere in ways that wouldalso heat the tropics. Such warming atlow latitudes would enable water toevaporate more quickly there. That in-crease in the amount of water vapor (agreenhouse gas) would, in turn, trapmore heat near the surface of the earth.

Similarly, during glacial times the vastcontinental ice sheets and the enormousexpanses of frozen ocean around themreflected much sunlight back to space.In warm times, melting of the ice sheetsallowed more of the sunlight to be ab-sorbed. And the higher concentration of

Greenland Ice Cores: Frozen in Time

Detail c shows the sharpened, pivoting metal “core dogs,”which are designed to ride lightly down the outside ofthe core during drilling. When technicians raise the bar-rel, the dogs, which are slanted slightly upward, dig intothe bottom of the core, freeing it from the surroundingice. The dogs also hold the core inside the barrel whilethe entire assembly is winched up the hole.

Detail b shows the electric motorattached to the tube. This motorspins the cutters against the ice,making chips that are pumpedup the outside of the barrel. Suchdrilling leaves a cylinder, or core,of solid ice inside.

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Detail a shows leaf springs thatfirmly grip the adjacent ice tokeep the upper part of the as-sembly from twisting or spinningwhile the lower part is drilling.

COREDOG

Tools of the Trade

ICECUTTER

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the tip, is lowered into the boreholeon a cable from an indoor platform(right). To prevent the hole from col-lapsing on itself, technicians inject afluid such as butyl acetate (a pineap-ple-smelling food additive) to equal-ize pressure with respect to the sur-rounding ice.

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carbon dioxide, as well as other green-house gases such as methane and watervapor, led to more heat retention.

Because of such interactions, climatol-ogists reasoned that different regions ofthe planet would warm or cool together.It came as a surprise, then, when Thom-as F. Stocker of the University of Bernand Thomas J. Crowley of Texas A&MUniversity independently predicted in1992 that for rapid climatic events,Greenland and Antarctica would changeoppositely.

Studying the effects of ocean currents,Stocker and Crowley noted that today’stepid, salty waters flow in the GulfStream from the equator toward theArctic, where they release heat to theatmosphere, giving northern Europe itsrelatively equable climate. After cool-ing, the salty waters become dense andsink to the deep ocean, where they thenflow southward as part of a great “con-veyor belt.” During the harsh, stadialtimes of the ice ages, the conveyor wasoff or weakened, and the resulting slow-er flow of equatorial waters to the Arc-tic left Greenland and northern Europeparticularly frigid. During interstadialperiods, the conveyor strengthened.

Stocker and Crowley examined howthis oceanic circulation affects the cli-mate of the Southern Hemisphere. Theirspecial insight was to show that the con-veyor cools the south at the same time itwarms the north, and vice versa, becauseof two effects. First, the flow of cold,deep waters to the Southern Hemispherecauses a return current of shallower,warmer waters to the north, whichbrings additional heat there while rob-bing it from the south. Second, slowingof the conveyor causes more warm wa-ters to reach the surface of the seaaround Antarctica, where they release

heat that warms the air inthe far south.

Support for this theoryof out-of-synch climatechange came in the mid-1990s, when scientistsstudying fossil foramini-fera (microscopic seashells)found that the AntarcticOcean was warm whenthe conveyor was not op-erating. Furthermore, inlight of the predictions ofStocker and Crowley,Wallace S. Broecker ofColumbia University re-examined the climate rec-ords during the last degla-ciation, between 20,000and 10,000 years ago. Heshowed that warmingstalled in Antarctica dur-ing times of rapid temper-ature increases in Green-land, and vice versa.

Bender and several oth-er colleagues have shownthat all the major stadial and interstadi-al events recorded in the Greenland icecores are also present in Antarctica, al-though climate changes in the southwere not as large or abrupt. Uncertain-ties in dating the cores precisely havemade it difficult to determine whethercooling in Greenland caused warmingin Antarctica for all these events. Labo-ratories in Grenoble, Bern and else-where are now in the process of extend-ing exact correlations back to the mid-dle of the last ice age.

This interest in the relative timing ofclimate change in polar regions is oneof the factors spurring new efforts toextract deep ice cores from Antarctica.Teams from the U.S., Japan, Europe and

Australia all have deep-drilling projectsunder way. One goal is to obtain corescontaining climate records for the past110,000 years (or earlier, if possible)that can be precisely correlated with themeasurements from Greenland.

These research programs, in additionto the ongoing European effort inGreenland, will help answer fundamen-tal questions about the climate duringoscillations between glacial and inter-glacial periods. Only by obtaining thisfuller understanding of the past can sci-entists begin to predict future climate,including the severity of greenhousewarming in the near term and, furtheroff, the timing of any possible return toan ice age.

Greenland Ice Cores: Frozen in Time Scientific American February 1998 85

The Authors

RICHARD B. ALLEY and MICHAEL L. BENDER were mem-bers of the U.S. team that extracted and analyzed ice cores from cen-tral Greenland in the early 1990s. Both men are currently involvedin the U.S. project drilling for ice cores in West Antarctica. Alley isprofessor of geosciences at Pennsylvania State University and an as-sociate at the Earth System Science Center there. He received hisPh.D. in geology from the University of Wisconsin. His recent re-search includes studying how glaciers and ice sheets record climatechange, how their flow affects sea level, and how they erode and de-posit sediments. Bender is professor in the department of geosciencesat Princeton University. He did his graduate studies at the Lamont-Doherty Earth Observatory of Columbia University. He was withthe Graduate School of Oceanography at the University of Rhode Is-land for 25 years. His current research includes analyzing oxygenand its isotopes as a means to learn about ancient climates.

Further Reading

Abrupt Increase in Greenland Snow Accumulation at theEnd of the Younger Dryas Event. R. B. Alley et al. in Nature,Vol. 362, pages 527–529; April 8, 1993.

Evidence for General Instability of Past Climate from a250-kyr Ice-Core Record. W. Dansgaard et al. in Nature, Vol.364, pages 218–220; July 15, 1993.

Climate Correlations between Greenland and Antarcticaduring the Past 100,000 Years. Michael Bender, Todd Sowers,Mary-Lynn Dickson, Joseph Orchardo, Pieter Grootes, Paul A.Mayewski and Debra A. Meese in Nature, Vol. 372, pages663–666; December 15, 1994.

Large Arctic Temperature Change at the Wisconsin-Holo-cene Glacial Transition. Kurt M. Cuffey, Gary D. Clow, Rich-ard B. Alley, Minze Stuiver, Edwin D. Waddington and Richard W.Saltus in Science, Vol. 270, pages 455–458; October 20, 1995.

EXAMINATION OF ICE CORES by various teamsfrom the U.S., Japan, Europe and Australia is currentlytaking place in Antarctica and Greenland. One goal ofthese studies is to determine how the climate in thesouth and north are related.

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