Finding abrupt change around the world

Confined to the remoteness of Greenland and the special techniques of polar ice analysis, the meaning of the early record of abrupt climate change might never have been widely recognized. For a decade and more after publication of the Dye 3 results, the Greenland researchers' idea of rapid, radical change on timescales that could be measured in decades or even individual years attracted very little interest. Even scientists willing to accept the validity of the data were inclined to a more conservative interpretation: They must have been regional. In the 1980s, who knew what peculiarities of Arctic weather might have caused those strange wiggles in the climate profile of Dye 3? And if it were really so rapid, it just must be the exception that proved the rule. As Darwin himself had proclaimed: Natura non facit saltum—Nature does not make leaps. The idea was just too far outside science's basic teachings to be taken very seriously. Moreover, no other archive of ancient climate yielded the kind of detail that the Greenland researchers claimed to see in polar ice.

In the mid-1990s, the results of the Summit projects emerged into a different scientific landscape. As signs of climate change were becoming apparent, greater future change was an increasingly potent issue, and so was the need to understand the pattern and pace of climate's natural variations. Suddenly, from the Summit projects came this incredible, high-fidelity continuous profile of climate's behavior through the last 100,000 years. For the quality of analysis, for their exhaustive detail, the projects set new standards for such investigations and just as critically important was that by the mid-1990s, signs of abrupt change were showing up in other archives beyond Greenland.

In tandem with the ice core findings were important discoveries by oceanographers, the one group of researchers that had been both well positioned and temperamentally inclined to take on the challenges posed by the early Greenland findings. The advent of postwar nuclear chemistry and advances in marine engineering had given the old science a young face and a reputation for overturning conventional geological thinking. It was oceanographers whose discovery in the 1950s of spreading midocean ridges that had sparked the theory of plate tectonics and finally confirmed Alfred Wegener's concept of continental drift.

Outside of Greenland, perhaps the most important center of activity was the Lamont-Doherty Earth Observatory in New York, where theorist Wally Broecker and others had been on the trail of abrupt change since the mid-1980s. Moreover, Lamont possessed its own treasure trove of paleoclimate data waiting to be tapped. For years, scientists using the facility's research vessels had labored under a standing order from Maurice Ewing, Lamont's founder and first director, to extract a sediment core every day they were at sea.

Beginning in the late 1980s, even as the big ice core drilling projects were just getting under way at Summit, an unlikely researcher using Lamont's enormous depository of ocean sediment cores was opening a new line of research into abrupt climate change.

Gerard C. Bond was not an oceanographer or a climatologist and had no particular interest in the science of abrupt change. Years later, accepting the prestigious Maurice Ewing Medal of the American Geophysical Union (AGU) for this work, Bond would describe himself as a relative newcomer to paleoceanography who had spent half his career as a "real geologist," more interested in the composition of rocks—petrology—and research projects far removed from the ocean. The sharp turn of events grew out of a research grant proposal that Bond drafted with a colleague, geologist Michelle A. Kominz, who had developed a new optical analysis technique. Bond and Kominz proposed to test the new method on the color record of a North Atlantic deep-sea core from the Lamont library, to see if it identified the large-scale orbital cycles in Cambrian Age rocks, and the proposal went to Broecker for review.

"Wally rushed into my office telling me that the core's color record instead revealed the long-sought marine imprint of Greenland's Dansgaard/Oeschger cycles," Bond recalled at the AGU award presentation in San Francisco in 2003. "I had never heard of Dansgaard/Oeschger cycles, but Wally and other Lamont paleoclimatologists ... managed to convince me that they were much more interesting than Cambrian cycles. By the early 1990s, I had shifted my research from rocks to deep-sea mud. In this new field I was surrounded by a baffling array of machines with flashing red lights, toxic chemicals, and coworkers who spoke the languages of chemists, physical oceanographers, and modelers."

Initially, at least, the change in fields was not as great a leap as he might have expected. The North Atlantic deep-sea cores that Bond examined contained grains of material dropped by melting icebergs that were much the same as the sedimentary rocks that were his first geological specialty. He found himself back in the business of petrology, microscopically studying rocky grains in the seabed to determine their origin. Bond showed how far and wide the iceberg armadas of the Heinrich events spread across the North Atlantic and how their timing matched the cold phases of the Dansgaard-Oeschger cycles in the Greenland ice.

Willi Dansgaard quickly responded to the news from Lamont-Doherty by incorporating the North Atlantic findings into the latest Greenland data. So it happened that Bond was the single American coauthor of the famous 1993 Nature article in which Dansgaard and other members of the European Greenland Ice Core Project team first described the results of their project at Summit. Alongside the new oxygen isotope profile from Greenland was a depiction of the visual color record from the Lamont Deep Sea Drilling Program sediment core 609 that had provoked Broecker's excitement and Bond's conversion to paleoceanography. In Dansgaard's estimation, the seafloor record was comparable to the ice core profile. Except where grains of ice-rafted carbonate "corrupt the grey scale," Dansgaard matched nearly all of the oxygen isotope shifts in the Greenland ice with the color record in Bond's sediment core.

As it happened, the white layers composed of carbonate grains in the core that Dansgaard called corruptions of the color record were features that led Bond to a dramatic advance in the science of abrupt climate change. Using his experience in studying the composition of rocks, he identified the source of the glacially scoured rocks in the debris layers and the icebergs that carried them across the Atlantic. Bond demonstrated that the ice age climate was more complex and even more extreme than the profile drawn from the ice cores. Not only was the last ice age punctuated by abrupt episodes of warming that lasted 1,000 years or more, but the seafloor sediments revealed other instances when temperatures rapidly plunged to levels of exceptional cold.

In 1992, Bond and an international team that included Broecker and the German marine geologist Hartmut Heinrich confirmed Heinrich's identification of a type of abrupt climate change in the North Atlantic that seemed to be very different from the events that dominated the Greenland record. In 1988, Heinrich had described telltale layers of ice-rafted debris in sediment cores taken from an area of the northeastern Atlantic known as the Dreizack Seamount. Bond combined the Dreizack data with Lamont data from cores extracted from across the North Atlantic containing microscopic rock particles that fell to the ocean floor during six different "short-lived, massive discharges of icebergs originating in eastern Canada" between 14,000 and 70,000 years ago. At the same times, temperatures had fallen in the atmosphere over Greenland and on the surface of the ocean over a vast area of the North Atlantic. "The cause of these extreme events is puzzling," they wrote in the journal Nature. "They may reflect repeated rapid advances of the Laurentide ice sheet, perhaps associated with reductions in air temperatures, yet temperature records from Greenland ice cores appear to exhibit only a weak corresponding signal."

Bond went on to identify a large cycle of warm periods and progressively more extreme cold periods that ran through the last ice age at intervals of 10,000 to 15,000 years. In 1993, in a Nature article that linked ice core and ocean sediment records, Bond and colleagues described a sawtooth pattern that was composed of a succession of warm-cold Dansgaard-Oeschger cycles that were "bundled" between Heinrich event cold extremes. The pattern would come to be called "Bond cycles."

"The series of saw-tooth shaped cooling cycles is clearly a fundamental structure of the atmosphere and sea-surface records, and must bear a close relation to the Heinrich events and the repeated, massive collapses of the Laurentide ice sheets," they wrote. For the first time, Bond had documented a link between the ice sheets, the ocean, and the atmosphere. "With the evidence in hand we cannot be certain whether the cooling cycles were caused entirely by internal oscillations of the ice sheet, or whether they reflect a mode of climate forcing that caused ice sheets to grow, culminating each time in a prolonged, cold stadial, ice-sheet instability and massive calving." In other words, they could not identify the mechanisms at work. Was the ice sheet driving the changes in climate? Or was some other climate process causing the ice sheet to become unstable periodically?

As Bond looked more deeply into the layers of debris, the mystery grew more complex. In 1995, Bond and Rusty Lotti, curator of Lamont's Ocean Sediment Core Laboratory, found traces of dark basaltic glass in the layers that could only have been scraped from the volcanoes of Iceland by glaciers. This evidence that more than one ice sheet was involved caused them to question the idea that the dynamics of the Laurentide ice sheet were responsible for Heinrich events. Perhaps other changes were causing all North Atlantic ice sheets to advance and retreat.

Whatever their cause, the "bestiary" of abrupt climate changes during the ice age was becoming larger and more interesting. There were Dansgaard-Oeschger events—episodes lasting 1,000 years or so when temperatures rapidly warmed, slowly staggered back over several centuries and then suddenly plunged to ice age cold. There were Heinrich events—times of exceptional cold when armadas of icebergs drifted far and wide over the North Atlantic. There were Bond cycles, bundles of warm-cold D-O events culminating in a frigid Heinrich event. The mechanisms that caused these episodes would be the subject of years of debate and contemplation and, beginning in the 1990s, the objects of a wide-ranging effort to discover their global reach and the abrupt upheavals in climate they implied.

While Bond and colleagues were able to show that the sediments in the North Atlantic bear the imprint of changes noticed in Greenland's ice, the rapid pace of these events could only be inferred from the sharpness of the Heinrich layer contours. The detail in the ocean sediment could not match the resolution of change in polar ice. In most areas of the open ocean, the sedimentary material on the seafloor is deposited too slowly, and the blurring "bioturbation" effect of bottom-dwelling organisms is too great to capture the real pace of events revealed in Greenland.

The ice and the seafloor are sedimentary archives of very different processes. One is a record of turbulent atmospheric events laid down in annual laminations—snowfalls piled deep on the surface and slowly condensed into layers of nearly pure glacial ice. The other is dense, slurry muck—the very slow buildup of decayed matter and skeletal remains of marine organisms and fallout from melting icebergs. Like instruments performing different parts of a symphony, ice is playing the high staccato notes of the atmosphere while the seafloor sediment is grinding out the slow basal tones of the ocean.

Outside of Greenland, where in the world could climate archives that recorded changes on an annual scale be found? Researchers pursuing this question found themselves rummaging through the dustbin of geology, following a line of investigation that had been discarded by an earlier generation. They would embrace the very notion that earlier climate researchers had found so hard to accept—that far distant pieces of evidence could be related to one another. Oceanographers in particular were on the lookout for varves, the annually deposited "couplets" of sediment first identified and named by the Swedish geologist Gerhard de Geer early in the twentieth century. Stitching together distant events along a new timescale, paleoclimate researchers would rely on other concepts first given form by the Swede. It was de Geer who coined the terms geochronology to describe the pattern of time intervals illuminated by the layers and teleconnection to describe a linkage between far-flung events.

Two studies that first overcame the limitations of most ocean sediments were reported in 1992 by researchers from Norway and the United States. Their data, taken from the bed of the Norwegian Sea, north of the North Atlantic, produced marine climate profiles that were more closely akin to the pace and pattern of events seen in the Greenland ice. The cores were taken from an area of the sea in which biological activity deposited sediments much more rapidly than most regions of the world's oceans. An annual layering recorded the seasonal comings and goings of sea ice that first left the surface at the end of the ice age 13,400 years ago and thereafter opened an ocean corridor that was seasonally free of ice along the west coast of Norway up to 72° North.

Studying traces of algae, Nalan Ko9 Karpuz and Eystein Jansen of the University of Bergen analyzed two cores taken from the continental slope of Norway, a site that was critical to detecting important changes in North Atlantic circulation.

Scott J. Lehman and Lloyd D. Keigwin of Woods Hole Oceanographic Institution in Massachusetts studied a core taken from sediment just south of the Bergen team's core in the Norwegian Trench. Comparing their archive to the Dye 3 ice core, the Greenland standard at the time, the Woods Hole team said its "rates of deposition are comparable to those in the Greenland ice cores." Sediment accumulated in the Norwegian Trench at the rate of five meters every 1,000 years, 20 to 50 times higher than in previously studied North Atlantic cores "and much too high to be significantly affected by bioturbation." Lehman and Keigwin analyzed oxygen isotope variations and changes in the populations of cold-water species of plankton in the sediment to trace the movement and retreat of warmer North Atlantic water into the Norwegian Sea during the past 14,000 years.

The Norwegian Sea sediments revealed dramatic changes during the Younger Dryas and, for the first time, documented shifts in ocean currents that were as abrupt as the changes recorded in Greenland in the core taken at Dye 3. At the end of the ice age, about 13,400 years ago, the warmer Atlantic surface water began flowing into the Norwegian Sea. And 11,700 years ago, after about 1,800 years, the flow suddenly shut down again, just as the Greenland cores began picking up signals of an abrupt return to colder, drier, windier conditions.

In their study of the most rapid change recorded in ocean sediments, the Woods Hole team found evidence that "deglacial shifts in the poleward flow of warm Atlantic surface waters occurred extremely rapidly, typically within a 40-year period." Sea surface temperatures typically jumped 9°F. "These led to equally large and rapid changes in atmospheric temperatures," to shifts in Atlantic circulation and ice sheet melting rates. Directly downwind of these events, the alpine meadows of Scandinavia were intermittently blooming with the white flowers of Dryas octopetala as the Fennoscandian ice sheet waxed and waned.

The data added an important piece to the Younger Dryas puzzle, giving researchers a chain of events extending from the Greenland ice, through the ocean sediments of the far north, to the glacial moraines and terrestrial sediments of northern Europe. Other researchers would incorporate other processes into the scenario of sudden, high-latitude warming such as the shrinking of the Laurentide ice sheet, the meltwater flood into the North Atlantic, the subsequent collapse of the North Atlantic's circulation, and the sudden return of ice age cold to the region.

More generally, the Norwegian cores proved that abrupt change could be detected in certain ocean sediments. Together with the new ice core announcements in 1993, this "proof of concept" encouraged oceanographers to search beyond the continental margins of the North Atlantic for signs of the Younger Dryas cold reversal, the Dansgaard-Oeschger cycles, and the Heinrich events.

The obvious places to look for more annually deposited sediments were the submerged slopes of the continents, where seasonal rains or meltwater runoff or other regular changes over the nearby landscape leave telltale patterns on the seafloor. These continental margins are among the biologically most productive marine environments in the world, where the sea from top to bottom is alive with organisms. Researchers eventually would discover large areas of modern ocean shoreline where the seafloor intersects a biologically unproductive layer of ocean known as the "oxygen-minimum zone" that preserves layers in sediments. Varying in depth, the middle zone is a layer of water between the nutrient-rich depths and the highly productive surface region. Organic matter decays as it sinks, depriving the water column of oxygen necessary for the survival of bottom-dwelling organisms whose burrowing behavior obliterates the telltale layers of the sediment. Also especially valuable are areas in which the sediments are deprived of oxygen because the seafloor is isolated physically from the surrounding ocean by a sill, or submerged ridge.

Since the early 1990s, all of the world's oceans have yielded high-resolution sediments bearing the familiar signs of abrupt climate changes. As the signs of abrupt change became more and more global in scope, in fact, researchers began to doubt the original proposition that processes in the little North Atlantic were really responsible for all of this change. Among the most valuable sites are those in the North Pacific off the California coastline, in the northern Arabian Sea off the coast of Pakistan, in the tropical Atlantic north of Venezuela, and in the subtropics near the island of Bermuda.

In the eastern Pacific, the offshore Santa Barbara Basin already was well known for upper layers of sediment that recorded centuries of modern climate history. These layers had been used to study changes in fish populations and interesting short-term climate variations such as El Niño. At 600 meters below the surface, the Santa Barbara sediments are deprived of oxygen by the presence of a surrounding sill that curbs ocean mixing in the deepest 100 meters or so. In 1992, the basin's potential as an archive of ancient climate was realized when the research vessel JOIDES Resolution of the Ocean Drilling Program extracted a 200-meter core dating back 160,000 years.

"It was a gold mine of scientific information on climate change," said marine geologist James P. Kennett. "Everybody was absolutely amazed at the quality of the site." In 1995, Kennett and B. Lynn Ingram reported that the annual layering from the runoff of winter rains was preserved in some sections of the core but obliterated by burrowing worms in other sections. They ascribed the interruptions to changes in ocean ventilation that altered the depth of the oxygen-minimum zone. As it happened, the pattern was just the signal that researchers were looking for, because it demonstrated the global reach of abrupt climate change.

In 1996, Kennett and Richard J. Behl compared the Santa Barbara Basin sediment pattern with the new GISP2 ice core record and found a close correspondence: The intervals when the laminations were preserved, when the basin bottom was anoxic, matched the record of Dansgaard-Oeschger warm events in the Greenland climate. It was what Kennett called "the first clear demonstration of Dansgaard/Oeschger cycles in another ocean, the biggest ocean, the Pacific Ocean, and at mid-latitudes."

A major turning point had been reached. "All of a sudden, everybody says, ah ha, this is a big process and it covers a large part of the globe," said Kennett. "And everything was so abrupt, it had to be through the atmosphere, the teleconnections. There was a fundamental change in the thinking about the importance of these D/O cycles after that in terms of the magnitude of these processes. Our work immediately showed that the surface oceans not only changed, but were capable of changing very, very abruptly, in the order of decades, just like Greenland. So the oceans were sort of in lock-step with the atmosphere as recorded in Greenland."

Later studies of sediments farther south, from the continental margin off the coast of Baja California, closely matched the Santa Barbara Basin results, although Joseph Ortiz and colleagues suggested that a different mechanism was at work. Kennett explained the changes in terms of changes in the ventilation of the seafloor by large-scale ocean currents. Ortiz pointed to more local changes in marine productivity between warm and cold periods, a process more likely to be the result of differences in wind patterns.

On the other side of the globe, different ocean and atmospheric conditions preserved annually layered tropical Indian Ocean sediments in the northeastern Arabian Sea that revealed a finely detailed profile of abrupt climate change. The continental shelf off Pakistan is a region of intense upwelling of nutrients and such robust biological productivity that the respiration of marine organisms periodically depletes the water of dissolved oxygen.

In 1998, a team of German researchers led by Hartmut Schulz presented sediment records from the Arabian Sea that are remarkably similar to Greenland climate oscillations over the past 110,000 years. When Greenland and North Atlantic temperatures were relatively high during the warm periods of a Dansgaard-Oeschger cycle, strong southwest monsoon activity led to high biological productivity; an oxygen-depleted Arabian seafloor; and dark, carbon-rich, well-preserved annual bands. Pale, carbon-poor disturbed laminations marked times of weaker monsoons and coincided with cold North Atlantic Heinrich events. Schulz wrote in Nature that these links between high-latitude and low-latitude climate events suggested "the importance of common forcing agents such as atmospheric moisture and other greenhouse gases."

Among the most extensive and valuable climate records to come out of an ocean floor core is the one from an area of the tropical Atlantic known as the Cariaco Basin in the southern Caribbean Sea off the coast of Venezuela. The Cariaco Basin is enclosed by especially shallow sills, at 146 meters and 120 meters, which isolate its deep water from ocean circulation. This means that the basin reflects only changes that occur in surface conditions, in the upper 100 meters of ocean. The changes are well preserved as laminations in the sediments because below 300 meters its waters are often depleted of oxygen. Deposits accumulate on the floor of the basin at the rate of 40 centimeters every 1,000 years, about 10 times faster than in the open Atlantic.

The north coast of South America is highly sensitive to climate change, and the basin sediments offer remarkably detailed records of features that are especially valuable in tracing the rapid changes revealed by the Greenland ice cores. The annual climate cycle, which leaves a distinctive "couplet" on the basin floor, is driven by changes in the strength of the northeast trade winds and the seasonal north-south migration of the rainy Intertropical Convergence Zone, where the merging northern and southern trade winds spawn great convective storms. A dry season from October to May is a time of strong trade winds and powerful coastal upwelling of organic nutrients from the deep ocean. The upwelling lays down a light-colored, organic-rich layer. A rainy season between June and October features weaker winds and no upwelling. Local river runoff leaves a dark band of mineral grains.

"The most striking large-scale feature of the Cariaco record is a significant increase in the thickness of the light laminae during the Younger Dryas cold period," oceanographer Alan E. S. Kemp told a meeting of the Royal Society in 2003. Kemp was describing the work of Konrad A. Hughen who compared their Younger Dryas data with results from the European ice core at Summit and found similarities even in small details. It seemed clear that high and low latitudes were responding to the same events, and Hughen's new radiocarbon studies confirmed that the Atlantic's overturning circulation of cold and warm currents was involved. The plunging return to cold conditions in the North Atlantic sharpened the temperature contrast between the high latitudes and the Tropics, increasing the strength of the trade winds, the upwelling along South America's northern shores, and consequently the thickness of the light-colored sediment layer composed of surface-dwelling marine organisms.

Other researchers extended the Cariaco Basin record back 90,000 years, closely measuring subtle changes in the reflectivity of the dark and light layers along 37 meters of sediment core. In 2000, Larry C. Peterson produced a climate profile through the last ice age that closely reproduces the abrupt comings and goings of the warm Dansgaard-Oeschger events recorded in the oxygen isotope profile of the U.S. GISP2 ice core.

Other researchers showed that the impact of Heinrich events extended down through the middle latitudes, well beyond the main belt of ice-rafted debris across the floor of the North Atlantic. Studying a varved sediment core from the continental slope of southern Spain, the French researcher Edouard Bard showed that "all latitudes of the eastern North Atlantic were strongly affected" by the three Heinrich events during the final 15,000

years of the last ice age. In the western Mediterranean, an analysis of varved sediments by Spanish sedimentologist Isabel Cacho found "a strong parallelism" between oscillations of sea surface temperatures and Dansgaard-Oeschger events, as well as five episodes of "drastic changes in the surface hydrography" that corresponded with Heinrich events. On the continents themselves, archives of abrupt climate change are found in varved sediments in the beds of lakes and layered mineral deposits in the stalactites and stalagmites in caves.

In southwestern France, inland of the Atlantic coast in a cave near Villars, Dominque Genty presented stable oxygen and carbon isotope profiles of a stalagmite that recorded D-O events during 50,000 years of the last ice age. In the eastern Mediterranean, studies by Miryam Bar-Mathews of stalagmites and stalactites in Soreq Cave in Israel, in the foothills of the Judean Mountains near Jerusalem, yielded a record of drought in the Middle East that corresponded with Greenland and North Atlantic records.

In China, a team led by Yongjin J. Wang reported in 2001 that oxygen isotope records of five stalagmites from Hulu Cave near Nanjing "bear a remarkable resemblance to oxygen isotope records from Greenland ice cores, suggesting that East Asian Monsoon intensity changed in concert with Greenland temperature between 11,000 and 75,000 years before the present."

By 2002, the global extent of abrupt climate changes was documented by the German marine geologist Antje H. L. Voelker who compiled a database of 183 entries of climate records covering the period between 45,000 and 15,000 years ago. As the evidence accumulated, a more complex picture of climate changes emerged. The popular scenario of switches between warm and cold episodes driven by North Atlantic circulation changes seemed to satisfy most researchers as long as the rapid climate episodes were seen as regional in scope. Collapse or decline of the thermohaline circulation in the North Atlantic implied an opposite reaction in the south as world ocean currents reorganized the transport of tropical heat. What most researchers expected to observe was a "seesaw" pattern of cold-warm switches between the Northern and Southern Hemispheres. But this was not what they found.

Although the Southern Hemisphere is notoriously underrepresented in any compilation of world climate records owing to the predominance of ocean, the results clearly showed most climate changes moving in the same direction at the same time as Northern Hemisphere changes rather than opposite them. As researchers refined their ability to correlate the timing of changes among different records, this question of "synchrony" or "asynchrony" became central to understanding the mechanisms behind abrupt change.

In and around Antarctica, where ice generally accumulates more slowly and so leaves a less detailed climate profile than in Greenland, the evidence would prove especially confusing and contradictory. Ice core records at some locations were in opposite phase with the North Atlantic, supporting the seesaw idea, while records at other locations seemed to move in concert with the Northern Hemisphere. In 1998, European scientists led by the Swiss geophysicist Thomas Blunier synchronized the timing of large and rapid variations in the methane gas content of trapped air bubbles in both Antarctic and Greenland ice. Their comparison of the cores showed that during some large Dansgaard-Oeschger warming episodes, at least, Antarctic temperatures spiked upward more than 1,000 years sooner than temperatures in Greenland. On average, they reported, between 47,000 and 23,000 years ago, "Antarctic climate change leads that of Greenland" by 1,000 to 2,500 years.

Another notable study in 1995 by glacial geologist Thomas V. Lowell established the timing of the advances and retreats of glaciers in the Lake District of the Chilean Andes at 41° South and the Southern Alps of New Zealand at 44° South. Far distant from Greenland and large ice sheets, both adjacent to the Pacific and influenced by the midlatitude southern westerlies, they were what Lowell, writing in Science, called "prime localities for determining whether the North Atlantic climatic pulses were regional events or were part of a global signature." The evidence from both areas was clear: The glaciers in the midlatitudes of the south were moving in the same direction at the same time as the Northern Hemisphere ice sheets. As Lowell wrote, "The implication of global symmetry that arises from our Southern Hemisphere paleoclimate data underscores a fundamental lack of understanding of how rapid climate changes originated and were propagated globally."

Later, in 1999, a group of Swiss researchers led by Susan Ivy-Ochs and George H. Denton confirmed Lowell's conclusions with more precise dating techniques. Using the Swiss accelerator mass spectrometer, they measured the concentration of the cosmogenic isotope beryllium-10 at the exposed surfaces of boulders on the Lake Misery moraines at Arthur's Pass in the Southern Alps of New Zealand and an Egesen moraine complex at Julier Pass in the Swiss Alps. This study found "synchronous glacier advances both in the Southern Alps and in the European Alps during the European Younger Dryas."

In 2002, writing in the Proceedings of the National Academy of Sciences, Lowell noted that a similar pattern had been observed at widely scattered locations. "Such distribution implies an overall cooling of the atmosphere, not simply a regional redistribution of the heat balance," he wrote. From such a perspective, more investigations might answer more detailed questions about the timing of these cooling effects, and so "contribute to an explanation for the causes of abrupt climate changes." By the end of the 1990s, the broad questions about the geographical spread of the major abrupt changes discovered in Greenland's ice had been resolved. With the possible exception of Antarctica, they seemed to be global in scope. At the same time, however, the search for a theory, for an explanation of the causes of abrupt climate change that was consistent with the data, did not prove as congenial to the methods of climate science.

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