Inventing the science of polar ice analysis

In the 1950s, in the Wilmette, Illinois, laboratory of the Army's Snow, Ice and Permafrost Research Establishment, young Chester C. Langway Jr. experienced a rare and delicious moment for a scientist. Before him was a material that no other researcher had ever seen—ice from deep in the interior of a high polar glacier. What mysteries did it contain? Just as the roughneck drillers at Site 2 Greenland were out to prove that it was possible to drill deeply into polar ice, Langway was out to prove there were ways to analyze the ice cores that would yield information of scientific value. In the beginning, it was not so much a climate investigation as it was what researchers call a "proof-of-concept" project.

With Henri Bader looking over his shoulder now and then, it was Langway's assignment to tease from the ice information about its progressive age, to prove Bader's hypothesis that the cores contained a unique record of climates past and that the record could be read and understood. Everything depended on a basic idea that at this moment was unproved. It was not enough that the ice contained information about climates past. It had to present that information sequentially. If the past was to be reconstructed, the order of events had to be retained. In one form or another, the annual layering that was so evident and promising near the surface had to survive the transformation of snow into gritty granular material called firn and then into impermeable glacial ice. More than that, at depths where it could prove most interesting, it had to survive the unknown dynamics of thousands of years of ever-increasing pressure from an accumulating overhead load.

Nothing in his studies at Boston University had quite prepared Langway for this challenge. In the vernacular, his mission was to subject deep polar ice to what geologists call stratigraphic analysis, somewhat like studying the sediments in the bottom of lakes. The basic tools of geology and mineralogy were at hand, of course, but this was no pick-and-shovel job. What would the deep ice reveal? Langway could not know and Bader could not tell him what to expect. In the beginning, for all of the hope and promise and scientific salesmanship, and for all of the expense and the slow, slogging labor of drilling in bitter cold, it still was not entirely certain that deep polar ice would reveal much of anything.

The second summer of drilling in the far north of Greenland at Site 2 finally had produced, by August 1957, enough undamaged ice core from as deep as 411 meters to test the concept. Langway brought back 160 meters of core from various depths. Most of it was from near the surface, but he had enough usable deep core to settle the big question about the value—or folly—of drilling into polar ice sheets.

Ernst Sorge in 1930 and Bader himself in a field test in 1954 had seen enough old snow near the surface of the Greenland ice sheet to surmise that the annual layers were intact. Near the surface, at least, the archive was coherent—one year's accumulation could be distinguished from another. Below the surface, however, as the overhead load increases pressure with depth, snow is transformed into firn and then into impermeable, high-density glacier ice. Whether the annual layers survived this metamorphosis was an entirely open question.

It didn't take Langway long to confirm that the ice taken above 100 meters yielded to fairly straightforward methods of investigation. In ice cores taken from temperate glaciers in lower latitudes, where summer temperatures rise above freezing, researchers could observe unmistakable signs of dense summer melt and annual layers of windblown dust and pollen. But these convenient features are absent in glacial ice formed in regions of polar cold, where summer temperatures never rise above freezing. In the "dry-snow zone" of perpetually freezing polar cold, physical characteristics that mark annual layers are more subtle, produced by different processes.

Langway observed that in the dry zone the denser layer represents the winter snowfall, because it is colder, finer-grained, and more densely wind-packed. In the upper 100 meters of core, he was able to identify these regular density variations through the diffuse illumination of a light table, although an interesting reversal took place. In the back-lit effect of the light table, the lighter, whiter-appearing summer layer actually showed up darker than the bluish, more dense winter layer. Langway had the insight to recognize that the light was being scattered by the surfaces of air bubbles embedded in the ice, and the more porous summer layer was allowing less light to be transmitted through the core.

Langway identified the exact density at which the firn transformed into glacial ice and located this "zone of zero permeability" at 71 meters below the surface in the Site 2 core. But could he read the annual layers through this critical zone of transformation? Langway reported that he could, but only because he had continuous core to examine. He wrote: "A major factor in accomplishing the stratigraphic interpretations was the ability to trace continuously the structural features as a function of depth as the features and the surrounding material gradually metamorphosed from the undisturbed surface snow through the firn and into the high density glacier ice." Above 100 meters, the archive was intact and discernible. Langway counted annual layers reaching back 174 years.

For what he called "deep ice" below 100 meters, Langway already knew that "the classical stratigraphic approach" would fail him. He couldn't tell exactly where it would happen because he didn't have continuous ice core samples from below 100 meters, but he felt certain that structural features vanished and density variations were lost somewhere between 100 and 200 meters down. In fact, there was some question whether deep ice retained any features that could be related to original seasonal variations at all. New techniques would be required.

Laboriously, Langway studied the chemical composition of the ice. Here he found himself looking for impurities in one of the purest substances on Earth. "Deep polar ice is purer than triple-distilled lab water," he would recall. Measuring values in parts per million and parts per billion in small wedge-shaped samples of ice core was stretching the limits of mid-twentieth-century chemical technology.

He examined the microscopic structure and orientation of the ice crystals and the air bubbles they enclosed. Although this work did not advance his search, these clues to the age of the ice with depth, the air bubbles—capsules of ancient atmosphere—would prove to be a boon to later researchers.

Most importantly, Langway turned to a technology that was just coming into its own as a peacetime tool in the earth sciences: atomic isotope analysis.

Radiocarbon dating of different segments of the ice core would have been attractive were it not practically impossible. While the method was proving itself in calculating the age of ancient, carbon-rich organic matter such as wood, the state of the art at the time would have required a deep-core ice sample weighing a ton.

What interested Langway most was the ability of instruments known as mass spectrometers to measure accurately the proportion of the different stable isotopes of oxygen. Like a prism that separates the colors of light according to their different wavelengths, a mass spectrometer passes atoms through a magnetic field and spreads out their different isotopes according to their different atomic weights. Separately, two eminent researchers—geophysicist Willi Dansgaard at the University of Copenhagen in Denmark and physicist Samuel Epstein at the California Institute of Technology—were suggesting that the proportion of the rare heavy isotope O to the common form 16O in snow and ice is related to the temperature of its precipitation in high latitudes. A relatively warm temperature produces snow or rain of a relatively higher concentration of the heavier isotope. The differences are small—measured in parts per thousand—but a mass spectrometer can accurately detect them. The method had been tested in firn near the surface of Greenland's ice sheet and had revealed a swing in isotopic composition between summer and winter snowfalls. Whether a record of these seasonal swings survived in deep ice was an open question.

Bader wanted his ice core specialist to accompany the drilling operation to the IGY project in Antarctica, but Langway, having spent two summers in Greenland, begged off. Beginning in the fall of 1957, Langway devoted hundreds of hours at the Wilmette lab to the painstaking preparation of small, pie-shaped wedges of ice core for testing by Epstein's mass spectrometer. Langway prepared 439 samples in all and sent them off to Epstein.

Epstein's work at Caltech was proceeding slowly, and competition for use of the mass spectrometer was great. Langway started to wonder about the future of his project. In 1960, Bader left the Army Corps' lab for a professorship at the University of Miami. Where was Langway going to find the scientific help he needed to pursue his polar ice investigations? Three years passed before the data from Epstein's laboratory arrived.

Europeans always were more interested than Americans in ice. In 1962, at a meeting of the International Glaciological Society in Obergurgl, Austria, Langway was introduced to Hans Oeschger, a Swiss geochemist. Oeschger invited Langway to visit his laboratory at the University of Bern. Buried under 30 meters of earth and enclosed in tons of lead, the facility was designed to detect low-level concentrations of radioactive isotopes in nature. The two men discussed the possibility of using carbon-14, as well as radioactive isotopes of other elements such as beryllium and tritium, to assign ages to different depths of polar ice cores. Mostly, they just got to know each other and realized that they shared an enthusiasm for the study of ancient polar ice. Oeschger was a brilliant scientist and a quietly charming man. The collaboration that was formed by a handshake that day would hasten and enrich the study of climates past. Oeschger would lead the way to the discovery that bubbles of air trapped in the deep-core ice could be measured for their content of such greenhouse gases as carbon dioxide and methane—that they are direct "samples of the ancient atmosphere."

When the stable isotope analyses finally came from Caltech, the results were striking. In a plot of the isotope ratios, a strong seasonal variation was obvious, Langway reported, "with the summer layer richer in 18O and the winter layers depleted in 18O" Measuring the length of the waves during two consecutive seasons, Langway produced estimates of relative snow accumulation from one year to the next.

Langway could report with unqualified success that the new oxygen isotope method confirmed that "original seasonal variations established when the snow was first deposited are still preserved at depth." The archive was intact, deep in the ice, far back in time, an annual calendar legible down to the last bit of core at 411 meters, or 1,336 feet, which Langway estimated fell as snow in the tenth century, AD 934—coincidentally, about the time the Vikings were colonizing Greenland. He could only imagine what analyses of core taken from ice near the bedrock might reveal. Other U.S. researchers were slow to appreciate these findings, but as far as Langway was concerned, "the value of investigating deep ice cores is almost limitless."

When the drillers reached bedrock at Camp Century, Greenland, in 1966, a congratulatory letter arrived on Chet Langway's desk. The Army's facility had changed its name to the Cold Regions Research and Engineering Laboratory and moved from Wilmette, Illinois, to Hanover, New Hampshire, near the campus of Dartmouth University. The well-wisher was Willi Dansgaard. Langway knew Dansgaard only by reputation as one of the leading authorities in the field of stable isotope analysis. Dansgaard congratulated Langway on the technical feat of reaching bedrock at Camp Century and said he would be interested in conducting isotopic analyses of the ice core that had been extracted. He accepted an invitation to visit Langway's lab in Hanover.

When he arrived at the lab, Dansgaard told Langway that he had a contract with the International Atomic Energy Agency to study concentrations of tritium in water samples from around the world and that his lab was equipped like no other laboratory to analyze large numbers of stable isotope samples quickly. This was music to the American's ears. After three days, the two shook hands and agreed to work together on the Camp Century ice.

This arrangement between the American Langway, the Swiss Oeschger, and the Dane Dansgaard would prove to be one of the most fruitful international collaborations in twentieth-century science. The scientific talents, the technical capabilities, and the personal enthusiasm were in place for the systematic investigation of an archive of past climate that is unique in depth and detail. In their hands were the remnants of snow that fell before the last ice age. The antique window looking far out over the deep and varied landscape of Earth's past climates, first jarred loose by Ernst Sorge at Eismitte in the winter of 1930, was about to be flung open. Abrupt climate change was about to be discovered, if not entirely understood.

Dansgaard's analyses of the Camp Century ice core's oxygen isotope values in the late 1960s produced the first continuous record of Earth's climate going back more than 100,000 years. At the University of Copenhagen's mass spectrometry lab, scientists working around the clock had tested nearly 1,600 samples at 218 locations along 4,518 feet of ice. No other record on land or sea could match the Greenland ice sheet for detail or continuous timescale. Not even Antarctica would produce such a high-fidelity record.

The Camp Century core presented climate scientists with the first continuous profile of annual snow accumulation entirely through the last ice age, the Wisconsin, and into the pre-Wisconsin "interglacial" warm period known as the Eemian. When the results were first published in the American journal Science in 1969, even the wording of the title, "One Thousand Centuries of Climate Record from Camp Century on the Greenland Ice Sheet," served notice to researchers who were accustomed to the geological timescale that they could expect a whole new level of detail from deep-core polar ice.

In the long upper section of core representing the 10,000 years or so since the last ice age, variations in the different values of heavy oxygen and light oxygen depicted the Younger Dryas and other lesser episodes of abrupt change, and these benchmarks helped calibrate the data with other climate records such as European pollens and lakebed and seafloor sediments.

In the lower segments, which are composed of snow that fell throughout the Wisconsin ice age, the core revealed a pattern that was nothing like the gradually sloping curves that most geologists would have expected. It was a staggering surprise, difficult to accept, a picture that would confound climate science for years. If the Camp Century core was an accurate archive of climates past, the prevailing view of ice ages as epochs of slumbering stability and slowly evolving change was not even half-right. "It showed all of these perturbations," Langway recalled. "At that time, it wasn't clear. Nobody knew what they meant."

The evidence of abrupt change is almost diabolically misleading. It depicts a climate shifting between different temperature and precipitation regimes at a pace that is totally unexpected. It has the raw and chaotic appearance of unrefined scientific data, which researchers know to be often messy and full of false starts and reversals, apparently meaningless noise. Scientists are trained to look for patterns in such disorder, to carefully select the signals that seem to bear some relation to one another, and so to offer hope of revealing a new clue about the orderly behavior of the world. At the end of the day, order is what science is about, and the ability to find order where others see only chaos is widely regarded as the mark of a great scientist. The idea that the chaos is the signal flies in the face of this tradition. It is just about the last thought that comes to mind—the "last survivor" of a process that has eliminated all of the more reasonable explanations of events. To the eye of an experienced researcher, at first flush the evidence of abrupt climate change naturally looks like data that are in need of more work.

In that first October 1969 article in Science and in a more thorough treatment of the oxygen isotope analyses of the Camp Century core presented at a December symposium the same year at Yale University, Dansgaard and his collaborators presented evidence of order where they could find it, tentatively observing periods of 120, 940, and 13,000 years and suggesting they were caused by variations in sunlight striking Earth. In their search for regularly recurring periods of warming and cooling, for "systematic oscillations," for the causes of climate changes on the large scale of geological time, the scientists were looking over the top of abrupt change. The telltale raggedness in the data was smoothed out by various mathematical filtering techniques. Instances of abrupt change that did not fit a pattern were characterized in the Yale paper as "accidental deviations that do not recur regularly," but Dansgaard was not going to ignore them.

In another brief article in 1972, Dansgaard took a closer look at the most dramatic swings of isotope variations from another angle: "Speculations About the Next Glaciation." Here was evidence that nobody had seen before. The beginning of the last ice age had come with a "rapid" drop in heavy oxygen isotope values, and some 15,000 years earlier an even more dramatic plunge to a cold climate had taken place. "Apparently, within 100 [years] the climate changed from warmer than today into full glacial severity," he wrote. Given the smoothing effect of molecular processes in the ice over time, the change may have come "almost instantaneously," and recovery from "this catastrophic event" may have taken 1,000 years.

The only other deep polar ice core, drilled at Byrd Station in Antarctica, did not reach as far back in time with continuous data. Although the deepest five meters of silty ice representing earlier ages contained several shifts between warm and cold climates, the Antarctic core did not display the kind of "violent oscillations" in oxygen isotope ratios that Dansgaard found in the Camp Century ice. And while the oldest Antarctic ice eventually would prove to be far older than Greenland's, the Greenland team noted "serious disadvantages" in the southern ice sheet. Most importantly, it is discontinuous: Whole years can go by without any new snow accumulating at such key stations as Byrd and Vostok. The fine detail of abrupt climate change shows up in the annually layered Greenland ice sheet like nowhere else in the world.

"The reason for the sudden changes is unknown," Dansgaard wrote. He ruminated about causes and effects. "Could ice surges from the Antarctic continent be responsible for an immediate and extreme cooling of the opposite Pole, in spite of the smoothing effect of the relatively slow coupling via the oceans?" The pattern of changes in the Byrd core suggested one possibility. Could it be that intense volcanic activity for some years "contaminated the stratosphere sufficiently to prevent most of the solar radiation from reaching the surface of the Earth?"

Why did one plunge in temperature usher in an ice age lasting tens of thousands of years whereas another did not? The big dips in isotope values seemed to be consistent with changes in Northern Hemisphere sunlight intensity caused by changes in Earth's orbit, although the dramatic climate changes seemed out of proportion with the subtle astronomical alterations. What mechanism would so drastically amplify such a signal?

Dansgaard asked: Were the sudden decreases in isotope values triggered by low incoming solar radiation, or insolation, in the Northern Hemisphere, where extensive areas of land can be covered by ice? "If so, the conditions for a catastrophic event are present today." He noted that the orbital cycle was moving into another period of low sunlight intensity in the north. "Or, are we faced with more or less accidental events, such as ice surges or intense volcanic activity, that trigger a full glaciation, if the insolation conditions favor such development? Is man's present activity equivalent to such [an] accidental event?"

The scientific rewards from the Camp Century ice core profile established not only the value to climate research of ice coring but more particularly the value of the Greenland ice sheet as a high-resolution climate archive. For sheer detail, no other place in the sea or on the land had produced such a picture of the past. The three researchers could only imagine what a finely resolved set of images would emerge from the profile of an ice core drilled in a Greenland locale chosen specifically for its scientific attributes—for its depth, its accumulation, its temperature, and the stability of its ice.

Greenland's value to climate science came as no surprise to the Scandinavians, of course, especially the Dane Dansgaard, who always chafed at the prevailing American attitude that Greenland was just an empty, featureless landscape, a big white blank on the map, of interest only as a Cold War military lookout base. With no particular affinity for the place, most American scientists saw Greenland the way their National Academy of Sciences had defined its role during the International Geophysical Year (1957-1958), as a logistically convenient proving ground for more scientifically important expeditions to Antarctica.

In the early 1970s, Langway, Dansgaard, and Oeschger embarked on an ambitious, long-term undertaking that was intended to plumb the great ice sheet from top to bottom. The three researchers formalized their collaboration with their national science funding agencies in the United States, Denmark, and Switzerland, respectively, and launched the Greenland Ice Sheet Program. Their plan was to survey the ice sheet as nobody had done before—to gauge its depth, of course, but more than that, to define its flow characteristics and the shape of the underlying bedrock. Like treasure hunters, they would sample the ice at a variety of sites and locate the best places to extract the longest possible continuous profile of climate history. They would drill three cores all the way to bedrock and then, in the field and their laboratories, explore all of the telltale chemical and physical features more thoroughly and carefully than ever, and tease from the ice every bit of data it had to give them.

A comprehensive aerial survey employing new radio-echo soundings, completed in 1974 by a Danish crew, produced the first full three-dimensional profile of the entire ice sheet. In 1975, Langway moved from the Army laboratory in New Hampshire to the State University of New York at Buffalo, accepting an offer of tenure, a department chairmanship, and a new laboratory. From 1971 to 1978, the team spent every summer field season on the ice, experimenting with U.S., Swiss, and Danish drills, and sampled cores at 11 different locations, probing for the sweet spots, the places where the ice was likely to reveal the most about the history of the climates that had deposited its layers. They drilled shallow holes from one end of Greenland to the other, from the northern edge of the glacier high up near the pole at Hans Tausen, at 82°30' North, all the way down to South Dome at 63°33' North, a distance of 1,200 miles.

In his report at the 1969 Yale symposium, Dansgaard argued that the next core, the second core to bedrock, should be drilled in southern Greenland to compare it with Camp Century's far north polar conditions at 77°10' North. Like Europe and eastern North America, southern Greenland is more directly influenced by the Atlantic Ocean than by the Arctic, Dansgaard observed, and such a profile "may contain more direct information about the conditions that led to buildup and extinction of the large Scandinavian and Laurentide ice sheets."

As time passed, however, increasing financial constraints forced on the GISP researchers the unhappy conclusion that they were going to be able to drill not three deep cores, as planned, but only one new core all the way to bedrock. "This limitation made it imperative that the drill site be chosen in the scientifically most favorable area to achieve the program objectives," they reported. Everything pointed to a central Greenland site high up on the ice sheet. In 1975, Dansgaard wrote: "Nowhere else in the world is it possible to find a better combination of reasonably high accumulation rate (which ensures continuity of record), simple ice flow pattern (which facilitates the calculation of the timescale), high ice thickness (which offers a detailed record, even at great depths) and meteorologically significant location (close to the main track of North Atlantic cyclones)." In 1972, the team had used a U.S. thermal drill to extract a core from a depth of 405 meters—1,316 feet—at a central Greenland site called Crête. This remote site was at the crest of the ice cap, 10,405 feet above sea level. Limited to one deep core, the team was hoping to begin drilling to bedrock at Crête in 1977.

Unfortunately, scientific imperatives were overcome by what at the time was seen as a more practical, less risky line of thought. The U.S. Army Cold Regions Research Laboratory had abandoned the ambitious Camp Century experiment, leaving just about everything but the nuclear power generator to be reclaimed by the flowing ice, and had bowed out of its role as logistical and transportation supplier. Now the entire expense of the expedition would have to be borne by civilian science funding agencies. Air transportation that used to be free was now costing $6,000 an hour. The researchers were going into the field with a new Danish ice core drilling rig that had not been tested at great depth, and as the price of operations climbed, the science agency bureaucrats were losing their appetites for risk. Duwayne M. Anderson, director of Polar Programs for the National Science Foundation, acknowledged that budgetary constraints dictated the choice of the Dye 3 site, where a U.S. radar installation already existed; although not an ideal location, it was of "sufficient interest" to justify the project.

Langway, Dansgaard, and Oeschger made the most of it. The new Danish electromechanical drill, battery powered and computer controlled, proved to be both reliable at great depth and fast. It reached 225 meters at the end of the field season in 1979 and 901 meters in 1980, and on August 10, 1981, bedrock was reached at 2,037 meters (6,620 feet), some 647 meters (2,123 feet) deeper than Camp Century. Handling the Dye 3 ice, the team set new standards for efficiency and analysis. While drilling was still under way, in a large "science trench" in the ice, they cut some 67,000 samples of core in a continuous sequence for oxygen isotope analysis. While still on the ice, the scientists tested the core for acidity, dust content, physical and mechanical properties, and chemical composition. Presenting their findings to a symposium in June 1982, less than a year after completing their field operations, the three scientists described Dye 3 as "the most completely recovered, thoroughly recorded, and comprehensively investigated ice core yet to reach bedrock."

Some—but only some—of the same abrupt changes first revealed by the Camp Century core had shown up in ice cores drilled on Devon Island, in Arctic Canada, across Baffin Bay from northern Greenland. Did the oscillations in Camp Century's oxygen isotope values really depict rapid climate changes in the Arctic? The question remained open for the better part of a decade, until the recovery of the core at Dye 3 gave Dansgaard, Osechger, and Langway the corroboration they needed.

"Most of the pronounced oscillations observed in the oxygen isotope profile from the deeper part of the Camp Century ice core were reproduced in the Dye 3 core, confirming their climatic significance and, at least, the regional validity of these data," they reported. Dansgaard took a close look at the evidence of rapid climate change in both cores—the jagged patterns of changing oxygen isotope ratios, designated 5, through the Wisconsin ice age—and found the pattern of correlation especially significant in view of the "completely different ice flow conditions" in two areas nearly 900 miles apart.

Looking for a cause for the sharp changes in isotope ratios, Dansgaard ruled out any local climate conditions in Baffin Bay or ice sheet instability, pointing instead to general climatic changes in the Arctic, "perhaps due to alternations between two different quasi-stationary modes" of the climate system.

Hans Oeschger observed how the temperature-related oxygen isotope variations in the ice and carbon dioxide concentrations in the trapped air bubbles switched back and forth in lock step with one another. He saw how the cores revealed the same patterns of abrupt change known as the Younger Dryas in the lake sediments and peat bogs of Europe. It was all part of this curiously "bistable climatic system." What could explain such abrupt changes?

In a separate paper in 1984, Oeschger and colleagues at the University of Bern, joined by Dansgaard and Langway, described the different timescales in the patterns that emerged along the oxygen isotope profile and proposed different causes for them. Although the idea that "changing seasonal solar irradiation due to variations of the earth's orbital parameters is the primary cause for the glaciation cycles" was gaining ground with new data, the corresponding changes are "relatively slow compared to the climate fluctuations revealed in the Greenland ice and in European lake sediments," they wrote. "These events, however, can be linked to oceanic changes, namely advances and retreats of cold polar water in the North Atlantic Ocean." The southward advance of polar water coincides with colder climate in Europe, they noted, and its retreat with a warmer Europe. "This strongly suggests that the continental climate shifts were an effect of changes of the surface conditions of the North Atlantic Ocean."

From out of the Greenland ice, a new picture was coming into focus. The landscape is not smooth. The grand swayings that appear to define the coming and going of the ice age are riveted with large staccato punctuations of abrupt change. It is not a matter of the pace of change picking up and then slowing down. All along the way, different drummers are pounding out different rhythms. Different mechanisms are at work.

The Camp Century core, the Dye 3 core, the lake sediments in Switzerland, the pollen profiles in Denmark and elsewhere in Scandinavia all told the same story. Suddenly Earth's climate began pulling out of the ice age about 14,700 years ago. Then, just as suddenly, after only about 2,000 years, it plunged back toward glacial conditions for a thousand years or more. And then, abruptly, climate conditions recovered and began a more gradual warming toward the relative stability of the past 10,000 years. The scientists wrote that each of the three climate changes is marked by an abrupt shift in oxygen isotope ratios, signifying drastic changes in central and northwestern Europe at those times.

Duwayne Anderson at the National Science Foundation congratulated the scientists and engineers of the Greenland Ice Sheet Program for "an effort that now can be described as brilliant" and for their papers that "provide a 'shower' of knowledge and information " Thirteen years of work on Camp Century and then GISP had brought new methods of analysis, new technologies, and a whole new realm of data to a fascinating new branch of earth science, christened "paleoclimatology." A provocative new concept of climate change was born.

As a stand-alone science product, the Dye 3 core itself would not be seen as a singular contribution to the archive of Earth history. Its greatest value was in corroborating many of the details revealed first and more effectively at Camp Century, by a core that continues to challenge researchers into the twenty-first century.

In the end, nothing could entirely rescue the Dye 3 profile from the fact that this primary core to bedrock was drilled at a location on the ice sheet that failed the principal scientific criteria laid down by the researchers themselves. Given that they were getting only one core to bedrock, it was simply a bad place to drill. "Complicating the ice core analyses is the extremely complex ice flow trajectory over the hilly bedrock upstream of the recovery site," they wrote. "This required further field work in 1982 and 1983 in order to establish two of the fundamental time-series (i.e., annual snow accumulation and temperature change)."

Being so far south, the Dye 3 location was too warm. In the current climate, at least, summer temperatures often rise above the melting point of water, and atmospheric carbon dioxide readily dissolves in the meltwater, leaving spuriously high concentrations of the gas in the affected ice. The scientists lamented the fact that "this situation invalidates some of the Holocene record (past 10,000 years) of the Dye 3 core as a source of valid information on one of the most urgent problems in modern climatology, i.e., the interaction between climate and atmospheric CO2, including the turnover of CO2 in the terrestrial reservoirs." The scientists thought, but couldn't be sure, that the Dye 3 region probably was too cold for summer melting during the ice age.

Finally, even though the core retrieved was in better condition than Camp Century's, the Dry 3 profile did not reach as far back in time, at least not in continuous sequence. Missing from Dye 3 was ice deposited on southern Greenland before the beginning of the last ice age, during the last interglacial, when climate was more like the current Holocene regime. Climate scientists call this period, about 120,000 years ago, the Sangamon, or Eemian. Dansgaard reported that the mountainous bedrock upstream of Dye 3 had disturbed the layers in the deepest 87 meters of the core. "This makes it impossible to study a time period of great scientific interest: the Eem/Sangamon interglacial and its termination. Detailed studies of this event are important, because they might shed light on the significant problems of why, how, and when the present interglacial period will end."

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