(note: this is a quick cut-n-paste the formating sucks see the web version much better) Rapid Climate Change By the 20th century, scientists had rejected old tales of world catastrophe, and were convinced that global climate could change only gradually over many tens of thousands of years. But in the 1950s, a few scientists found evidence that some changes in the past had taken only a few thousand years. During the 1960s and 1970s other data, supported by new theories and new attitudes about human influences, reduced the time a change might require to hundreds of years. Many doubted that such a rapid shift could have befallen the planet as a whole. But the 1980s and 1990s brought proof (chiefly from studies of ancient ice) that the global climate could indeed shift, radically and catastrophically, within a century . perhaps even within a decade. This essay covers large one-way jumps of climate. For short-term cyclical changes, see the essay on The Variable Sun. A shorter and generalized story of "the discovery of rapid climate change" is available as an article in Physics Today. "Many climate changes are well described as relatively small deviations from a reference state, often assumed to be in equilibrium... [But this] linear approach does not hold for abrupt climate change, in which a small forcing can cause a small change or a huge one..." . National Academy of Sciences, 2002.(1) ---- Climate, if it changes at all, evolves so slowly that the difference cannot be seen in a human lifetime. That was the opinion of most people, and nearly all scientists, through the first half of the 20th century. To be sure, there were regional excursions, such as long spells of drought in one place or another. But people expected that after a few years "the weather" would automatically drift back to its "normal" state, the conditions they were used to. The planet's atmosphere was surely so vast and stable that outside forces, ranging from human activity to volcanic eruptions, could have no more than a local and temporary effect. - LINKS - <=Public opinion <=Simple models Looking to times long past, scientists recognized that massive ice sheets had once covered a good part of the Northern Hemisphere. But the Ice Age had evidently ended tens of thousands of years ago, and it was an aberration. During most of the geological record, the Earth seemed to have been bathed in rather uniform warmth. This opinion became so fixed that, as one meteorologist complained, geology textbooks in 1990 were still copying down from their predecessors the venerable tradition that the age of the dinosaurs, and nearly all other past ages, had enjoyed an "equable climate."(2) The glacial epoch itself seemed to have been a relatively stable condition that lasted millions of years. It was a surprise when evidence turned up, around the end of the 19th century, that the recent glacial epoch had been made up of several cycles of advance and retreat of ice sheets . not a uniform Ice Age but a series of ice ages. Some geologists denied the whole idea, arguing that every glaciation had been regional, a mere local variation while "the mean climate of the world has been fairly constant."(3) But most accepted the evidence that the Earth's northern latitudes, at least, had repeatedly cooled and warmed as a whole. Global climate could change rapidly . that is, over the course of only a few tens of thousands of years. Probably the ice could come again. That gave no cause to worry, for it surely lay many thousands of years in the future. <=Climate cycles A very few meteorologists speculated about possibilities for more rapid change, perhaps even the sudden onset of an ice age. The Earth's climate system might be in an unstable equilibrium, W.J. Humphreys warned in 1932. Although another ice age might not happen for millions of years, "we are not wholly safe from such a world catastrophe."(4) The worst scenario was offered by the respected climate expert C.E.P. Brooks. He suggested that a slight change of conditions might set off a self-sustaining shift between climate states. Suppose, he said, some random decrease of snow cover in northern latitudes exposed dark ground. Then the ground would absorb more sunlight, which would warm the air, which would melt still more snow: a vicious feedback cycle. An abrupt and catastrophic rise of tens of degrees was conceivable, "perhaps in the course of a single season."(5) Run the cycle backward, and an ice age might suddenly descend. <=Simple models =>Chaos theory Most scientists dismissed Brooks's speculations as preposterous. Talk of sudden change was liable to remind them of notions popularized by religious fundamentalists, who had confronted the scientific community in open conflict for generations. Believers in the literal truth of the Bible insisted that the Earth was only a few thousand years old, and defended their faith by claiming that ice sheets could form and disintegrate in mere decades. Hadn't mammoths been discovered as intact mummies, evidently frozen in a shockingly abrupt change of climate? Scientists scorned such notions. Among other arguments, they pointed out that ice sheets kilometers thick must require at least several thousand years to build up or melt away. The physics of ice, at least, was simple and undeniable. <=Public opinion The conviction that climate changed only slowly was not affected by the detailed climate records that were recovered, with increasing frequency from the 1920s through the 1950s, from layers of silt and clay pulled up from the ocean floor. Analysis showed no changes in less than several thousand years. The scientists failed to notice that most cores drilled from the seabed could not in fact record a rapid change. For in many places the mud was constantly stirred by burrowing worms, or by sea floor currents and slumping, which blurred any abrupt differences between layers. Lakes and peat bogs retained a more detailed record. Most telling were studies in the 1930s and 1940s of Scandinavian lakes and bogs, using ancient pollen to find what plants had lived in the region when the layers of clay ("varves") were laid down. Major changes in the mix of plants suggested that the last ice age had not ended with a uniformly steady warming, but with some peculiar oscillations of temperature.(6) The most prominent oscillation . already noticed in glacial moraines in Scandinavia around the turn of the century . had begun with a rise in temperature, named the Allerød warm period. This was followed by a spell of bitterly cold weather, first identified in the 1930s using Swedish data. It was dubbed the "Younger Dryas" period after Dryas octopetala, a graceful but hardy Arctic flower whose pollen gave witness to frigid tundra. (The glacial period that preceded the Allerød was the "Older Dryas.") The Younger Dryas cold spell was followed by a more gradual warming, ending at temperatures even higher than the present. In 1955 the timing was pinned down in a study that used a new technique for dating, measuring the radioactive isotope carbon-14. The study revealed that the chief oscillation of temperatures had come around 12,000 years ago. The changes had been rapid . where "rapid," for climate scientists at mid-century, meant a change that progressed over as little as one or two thousand years. Most scientists believed such a shift had to be a local circumstance, not a world-wide phenomenon. Certainly there was no data to drive them to any other conclusion, for it was impossible to correlate sequences of varves (or anything else) between different continents. That would only become possible when radiocarbon dating overcame the many inaccuracies and uncertainties that beset the technique in its early years. (7*) <=>Climate cycles The tundra flower Dryas <=Carbon dates Even swifter changes could show up in the clay varves derived from the layers in the mud of lake beds laid down each year by the spring runoff. But there were countless ways that the spring floods and even the vegetation recorded in the layers could have changed in ways that had nothing to do with climate . a shift of stream drainages, a forest fire, the arrival of a tribe of farmers who cleared the land. Abrupt changes in varves, peatbeds, and other geological records were easily attributed to such circumstances. Scientists could win a reputation by unraveling causes of kinks in the data, but for climatology it all looked like nothing but local "noise."(8) Thus it was easy to dismiss the large climate swings that an Arizona astronomer, Andrew Ellicott Douglass, reported from his studies of tree rings recovered from ancient buildings and Sequoias. Other scientists supposed these were at most regional occurrences, and indeed even regional climate changes scarcely seemed to affect the trees that most scientists looked at (the American Southwest was exceptional in its radically varying climate and precariously surviving trees). It didn't help that Douglass tried to correlate his weather patterns with sunspots, an approach most meteorologists thought hopelessly speculative. <=Solar variation If researchers had found simultaneous changes at widely different locations, they might have detected a broad climate shift. But carbon-14 dating remained fraught with uncertainties, and matching up the chronologies of different places was difficult and controversial. Further, even a massive and global climate change could bring rains in one locale, cold in another, and not much shift of vegetation at all in a third. So each study remained isolated from the others.(9) Some pointed out that the overall rate of advance and retreat of the great ice sheets had been no faster than present-day mountain glaciers were seen to move.(10) That was compatible with "the uniformitarian principle." This geological tenet held that the fundamental forces that molded ice, rock, sea, and air did not vary over time. Some further insisted that nothing could change otherwise than the way things are seen to change in the present. The uniformitarian principle was cherished by geologists as the very foundation of their science, for how could you study anything scientifically unless the rules stayed the same? The idea had become central to their training and theories during a century of disputes. Scientists had painfully given up traditions that explained certain geological features by Noah's Flood or other one-time supernatural interventions. Although many of the theories of catastrophic geological change were argued on fully scientific grounds, by the end of the nineteenth century scientists had come to lump all such theories with religious dogmatism. The passionate debates between "uniformitarian" and "catastrophist" viewpoints had only partly brought science into conflict with religion, however. Many pious scientists and rational preachers could agree that everything happened by gradual natural processes in a world governed by a reliable God-given order.(11) =>Public opinion Nowadays temperatures apparently could not rise or fall radically in less than millennia, so the uniformitarian principle declared that such changes could not have happened in the past. The principle thus went hand-in-glove with a prevailing "gradualist" approach to all things geological. Alongside physical arguments that the great masses of ice, rock and water could not be quickly changed, paleontologists subscribed to a neo-Darwinian model of the evolution of species which argued that here too change must be continuous and gradual. All that seemed to apply to climate. Textbooks pointed out, for example, that there were plausible reasons to believe that tropical rain forests had scarcely changed over millions of years, so the climates that sustained the orchids and parrots must have been equally stable. There was no reason to worry about the fact that old carbon-14 dates were accurate only within about a thousand years plus or minus, so that a faster change could hardly have been detected. If there were unmistakeable fluctuations like the Younger Dryas, presumably those had regional rather than global scope . restricted to the vicinity of the North Atlantic or an even narrower area (few studies had been done anywhere else). <=>Climatologists In 1956, a change at the fastest speed that anyone expected was discovered by the carbon-14 expert Hans Suess, studying the shells of plankton embedded in cores of clay pulled from the deep seabed by Columbia University's Lamont Geological Observatory. Suess reported that the last glacial period had ended with a "relatively rapid" rise of temperature . about 1°C (roughly 2°F) per thousand years.(12) The rise looked even more abrupt when David Ericson and collaborators inspected the way fossil foraminifera shells varied from layer to layer in the Lamont cores. They reported a "rather sudden change from more or less stable glacial conditions" about 11,000 years ago, a change from fully glacial conditions to modern warmth within as little as a thousand years. They acknowledged this was "opposed to the usual view of a gradual change."(13) Indeed Cesare Emiliani, who often disagreed with Lamont scientists, published an argument that the temperature rise of some 8°C had been the expected gradual kind, stretching over some 8,000 years.(14) More was at stake than simple dating. A graduate student in the Lamont group, Wallace Broecker, put a bold idea in his doctoral thesis. Looking at this and other data, he found "a far different picture of glacial oscillations than the usual sinusoidal pattern." Like Brooks, he suggested that "two stable states exist, the glacial state and the interglacial state, and that the system changes quite rapidly from one to the other." (A revision of Brooks's 1926 book on Climate through the Ages was published in 1949, and it was popular enough to be reprinted in 1970.)(15) This was only one passage in a thick doctoral thesis that few people read, and sounded much like Brooks's speculations on cataclysmic changes, long since dismissed by scientists as altogether implausible. <=Uses of shells <=Climate cycles =>Simple models After considerable debate, Emiliani won his point. The rapid shift that Ericson had reported was not really to be found in the data. Like some other sudden changes reported in natural records, it reflected peculiarities in the method of analyzing samples, not the real world itself. Yet mistakes can be valuable, if they set someone like Broecker to thinking about overlooked possibilities. Sometimes the mistake even turns out to reflect a valid understanding, when, as Broecker later remarked, "...you go back around and actually the discovery itself was valid, even though the thing that led to it was wrong."(16) By 1960, three Lamont scientists . Broecker, Maurice Ewing, and Bruce Heezen . were reporting a variety of evidence, from deep-sea and lake deposits, that a radical global climate shift of as much as 5-10°C had indeed taken place in less than a thousand years.(17) While it would necessarily take many thousands of years to melt the great ice sheets, they had realized that meanwhile the atmosphere and the ocean surface waters, which were less massive, could be fluctuating on their own. Broecker speculated that the climate shifts might reflect some kind of rapid turnover of North Atlantic ocean waters . a natural place for an oceanographer to look. =>Chaos theory =>The oceans A few scientists responded with more specific models. Most important was a widely noted paper by Ewing and William Donn, who were "stimulated by the observation that the change in climate which occurred at the close of the [most recent] glacial period was extremely abrupt." Their model proposed ways that feedbacks involving Arctic ice cover could promote change on a surprisingly rapid scale.(18) Following up, J.D. Ives drew on his detailed field studies of Labrador to assert that the topography there could support what he called "instantaneous glacierization of a large area." By "instantaneous" he meant an advance of ice sheets over the course of a mere few thousand years, which was roughly ten times faster than most scientists had imagined.(19) However, the Ewing-Donn theory turned out to have fatal errors, and most scientists continued to doubt that such swift changes were possible. <=>Simple models Further information came from studies of fossil pollen recovered from layers of peat laid down in bogs. Those undertaking such work had not set out to study the speed of climate change . their inquiry was mostly a routine, plodding counting of hundreds of specks under the microscope, assembling data on vegetation shifts to catalog the way ice sheets came and went. But the carbon-14 dates offered surprises for an attentive eye. For example, a 1961 study mentioned in passing that at one location in Wisconsin, the transition from glacial-period pines to oak trees had taken at most 200 years.(20) Earth scientists had to be careful in describing such results, for rapid change remained a touchy question. During the 1950s, Immanuel Velikovsky and others had excited the public with popular books describing abrupt and marvelous upheavals in the Earth's history. Every mammoth frozen in permafrost was offered as proof that the world's climate could change catastrophically overnight. Experts were weary of explaining to students and newspaper reporters that the scenarios were sheer fantasy. The battle against Velikovsky and his ilk only reinforced geologists' insistence on the uniformitarian principle, which they took as a denial of any change radically unlike changes seen in the present. Ideas of catastrophic change were also tainted by the way they were used, persistently and increasingly, by zealots who sought "scientific" proof for their fundamentalist interpretation of passages in the Bible. (Typical was the complaint of a paleontologist who prefaced his 1992 book with a disclaimer: "in view of the misuse that my words have been put to in the past, I wish to say that nothing in this book should be taken out of context and thought in any way to support the views of the 'creationists'...")(21) If pollen types did shift abruptly in some bog, that could easily be accounted for scientifically as an artifact of a purely local change. There seemed to be no good evidence, nor physical cause, of any swift global upset. <=Public opinion =>Sea flooding =>Simple models Hints to the contrary came unexpectedly from entirely different fields. In the late 1950s, a group in Chicago carried out tabletop "dishpan" experiments using a rotating fluid to simulate the circulation of the atmosphere. They found that a circulation pattern could flip between distinct modes. If the actual atmospheric circulation did that, it would change weather patterns in many regions almost instantly. On a still larger scale, in the early 1960s a few scientists created crude but robust mathematical models which demonstrated that global climate really could change to an enormous extent in a relatively short time, thanks to feedbacks in the amount of snow cover and the like.(22) <=Simple models <=Simple models <=Sea flooding Probably it was no coincidence that this new readiness of scientists to consider rapid and disastrous global change spread in the early 1960s. That was exactly when the world public was becoming anxious over the possibility of sudden global catastrophe. Alongside the fantasies of Velikovsky, and increasingly shrill warnings from Bible fundamentalists, there were sober possibilities of disaster brought on by nuclear war, not to mention threats to the entire planet from chemical pollution and other human industrial ills. <=Public opinion Now that theoretical ideas and the general trend of opinion alike made it easier for climate scientists to envision sharp change, they were increasingly able to notice it in their data. Broecker in particular, looking at deep-sea cores, in 1966 pointed to an "abrupt transition between two stable modes of operation of the ocean-atmosphere system," especially a "sharp unidirectional change" around 11,000 years ago.(23) It proved possible to build simple fluid-flow models that showed how such a change could be promoted by a switch in the pattern of ocean currents. Improved deep-sea records, going back hundreds of millennia, brought additional information. By comparing the irregular curves from a number of cores, Broecker noticed that the general pattern of glacial cycles was not a simple symmetric wave. It looked more like a sawtooth where "gradual glacial buildups over periods averaging 90,000 years in length are terminated by deglaciations completed in less than one tenth this time."(24) <=The oceans <=>Climate cycles =>Sea flooding <=Chaos theory <=Climatologists =>Climate cycles The view was supported by data gathered independently at the University of Wisconsin-Madison, where Reid Bryson was already interested in rapid climate changes. In the late 1950s, supported by an Air Force contract to study weather anomalies, he had been struck by the wide variability of climates as recorded in the varying width of tree rings. And he was familiar with the Chicago "dishpan" experiments that showed how a circulation pattern might change almost instantaneously. Bryson brought together a group to take a new, interdisciplinary look at climate, including even an anthropologist who studied the ancient native American cultures of the Midwest. From bones and pollen they deduced that a disastrous drought had struck the region in the 1200s . the very period when the flourishing towns of the Mound Builders had gone into decline. It was already known that around that time a great drought had ravaged the Anasazi culture in the Southwest (the evidence was constricted tree rings in ancient logs from their dwellings). Compared to this drought of the 1200s, the ruinous Dust Bowl of the 1930s had been mild and temporary. A variety of historical evidence hinted that the climate shift had been world-wide. And there seemed to have been distinct starting and ending points. By the mid 1960s, Bryson concluded that "climatic changes do not come about by slow, gradual change, but rather by apparently discrete 'jumps' from one [atmospheric] circulation regime to another." (25*) <=Government <=Climatologists Next the Wisconsin team reviewed carbon-14 dates of pollen from around the end of the last ice age. In 1968, they reported there had been a rapid shift around 10,500 years ago, and by "rapid" they meant a change in the mix of tree species within less than a century (they quoted a "half-life" as short as 55 years). That was about as fast as a forest could adjust, so the climate itself could have changed even faster. Perhaps the Younger Dryas was not just a local Scandinavian anomaly. Bryson and his collaborators were developing a systematic technique for translating their counts of different kinds of pollens into a record of rainfall and temperature. It was a technique "built on a foundation of debatable assumptions," as one reviewer observed, yet still "a major step forward." They produced for the American Midwest the most accurate, detailed, and comprehensive climate record available anywhere.(26) Looking at hundreds of carbon-14 dates spanning the past dozen millennia . dates that improvements had made accurate enough to give a reasonable correlation among widely dispersed sites . they believed they could confirm Bryson's disturbing conclusion. Climate change generally did not come smoothly, but in a steplike pattern; periods of "quasi-stable" climate ended in relatively swift transitions.(27) In a 1974 followup, they spoke more boldly of stable periods interrupted by catastrophic "discontinuities," when "dramatic climate change occurred in a century or two at most."(28) The "at most" was a confession that the power of pollen studies was limited. For even if the climate changed overnight, it could take a century or more for the mix of trees in a forest to evolve until it accurately reflected the new conditions. =>Aerosols <=>Chaos theory Further, it did not take a global climate change to transform any particular forest. Strictly local events could do that. There was no way to correlate climate changes in different parts of the world precisely, since radiocarbon measurements had a wide range of error and other dating techniques were still worse. This limitation of the data did not worry most experts, for they felt it was sheer speculation to propose any physical mechanism that could change the entire world's climate in less than a thousand years or so. Yet confirmation of changes at that rate, at least, was coming from a variety of other work. An example was George Kukla's study of snail shells and pollen in layers of loess (wind-blown dust) in Czechoslovakia . another study that was designed to investigate gradual shifts, but in which a close look at the data revealed unexpectedly abrupt transitions.(29) The emerging picture of severe instability was reinforced by studies of cores drilled from the Greenland and Antarctic ice caps, and by deep-sea cores which covered much longer times. Evidently the hundred-thousand-year glacial cycles did follow a sawtooth pattern: in each cycle the slow descent into a relatively stable cold state had ended with a mysteriously abrupt rise of temperature. As Emiliani put it in 1974, "We used to think intervals as warm as the present lasted 100,000 years or so. Instead, they appear to be short, infrequent episodes."(30) Or as another respected climatologist explained, in place of the old view of "a grand, rhythmic cycle," the new evidence showed a "much more rapid and irregular succession" in which the Earth "can swing between glacial and interglacial conditions in a surprisingly short span of millennia (some would say centuries)."(31) <=Climate cycles <=Climate cycles Within these larger transitions, relatively quick secondary oscillations showed up in various data, such as carbon-14 studies of ancient glacier moraines and lake levels. (32*) Above all there was the Younger Dryas. Evidence from shells in a few excellent deep-sea cores showed a geographically widespread temperature oscillation. Not everyone was swayed by this evidence, however, for any of it could have been confused by various chemical and biological effects. (33*) The most convincing evidence came from a long core of ice drilled at Camp Century, Greenland by a Danish group headed by Willi Dansgaard in cooperation with Americans led by Chester Langway, Jr. The proportions of different oxygen isotopes in the layers of ice gave a fairly straightforward record of temperature. Mixed in with the expected gradual cycles were what the group called "spectacular" shorter-term shifts . including, once again, an oscillation around 12,000 years ago. Some of the shifts seemed to have taken as little as a century or two. Nobody could be sure of that, however, for the odd wiggles in the data might well represent not a world climate shift, but only local accidents in the ice.(34) Camp Century graph of temperatures <=Solar variation =>Public opinion A group of glacial-epoch experts reached something close to consensus in a meeting at Brown University in 1972. Reviewing the Camp Century ice cores, new foraminifera studies by Emiliani, and other field evidence, the scientists agreed that interglacial periods tended to be short and to end more abruptly than had been supposed. In view of the cooling that had been reported in the Arctic since the 1940s, they suspected we might right now be near the end of the present interglacial period. The majority concluded that the current warm period might possibly end in rapid cooling within the next few hundred years . "a first order environmental hazard."(35) <=>Climate cycles =>Government = Milestone Bryson, Stephen Schneider, and a few others took the concern to the public. They insisted that the climate we had experienced in the past century or so, mild and equable, was not the only sort of climate the planet knew. For all anyone could say, the next decade might start a plunge into a cataclysmic freeze, drought, or other change unprecedented in recent memory, but not without precedent in the archeological and geological record. (Link from below) While Bryson warned that the increasing pollution of the atmosphere would shade the Earth and bring rapid cooling, this was not the only possibility. The growing realization that small perturbations could trigger sudden climate change also impressed scientists who were growing concerned about the rising level of the greenhouse gas carbon dioxide ( CO2). Perhaps that might bring serious global warming and other weather changes within as little as a century or two. =>Public opinion As abrupt changes became more credible they were seen in still more kinds of evidence. One example was the shells of beetles, which are abundant in peat bogs, and so remarkably durable that they can be identified even 50,000 years back. Beetles swiftly invade or abandon a region as conditions shift, so the set of species you find gives a sensitive measure of the climate. Russell Coope, studying bog beetles in England, turned up rapid fluctuations from cold to warm and back again, a matter of perhaps 3°C, around 13,000 years ago. It all happened within a thousand years at most, he reported (if the change had been even faster his data could not have showed it).(36) This singular approach got a skeptical response from other scientists as they pursued the well-established study of pollens, for they were accustomed to seeing more gradual transformations of forests and grasslands. The fluctuations in Coope's records were easily dismissed as local peculiarities of English beetles. The Camp Century cores, too, might tell little about change on a global scale. The data might be sensitive to changes of ice cover in the seas near Greenland, or to a local shift of the ice cap's glacial flow. Other evidence, especially oxygen isotopes in shells from deep-sea cores that reflected conditions in the entire North Atlantic, showed changes only over several thousand years. Nevertheless, as pieces of evidence accumulated, a growing number of scientists found it plausible that the climate over large regions, if not the entire world, had sometimes changed markedly in a thousand years or even less. Perhaps one reason was that the early 1970s meanwhile saw further development of global energy-balance models in which a few simple equations produced radical instability. In particular, Mikhail Budyko in Leningrad pursued calculations about feedbacks involving ice cover, and suggested that at the rate we were pumping CO2 into the atmosphere, the ice covering the Arctic Ocean might melt entirely by 2050. Conversely, a buildup of snow and ice might reflect enough sunlight to flip the Earth into a glaciated state.(37) These ideas prompted George Kukla and his wife Helena to inspect satellite photos of Arctic snow cover, and they found surprisingly large variations from year to year. If the large buildup seen in 1971 were repeated for only another seven years, the snow and ice would reflect as much sunlight as during a glacial period. "The potential for fast changes of climate," they warned, "evidently does exist on the Earth."(38) <=Simple models <=Simple models Meanwhile glacier experts developed ingenious models that suggested that global warming might provoke the ice sheets of Antarctica to break up swiftly, shocking the climate system with a huge surge of icewater.(39) (Link from below) Bryson and other scientists worked harder than ever to bring their concerns to the attention of the scientific community and the public. As Broecker put it, any decade now the world could be hit by a severe "climatic surprise."(40) <=>Sea flooding =>Public opinion Most scientists spoke more cautiously. When leading experts had to state a consensus opinion, as in a 1975 National Academy of Sciences report on plans for international cooperation in atmospheric research, they were circumspect. Evaluating past statistics, the panel concluded that predictable influences on climate made for only relatively small changes, which would take centuries or longer to develop. Any big jerks that might matter for current human affairs were likely to be just "noise," the usual irregularities of climate. The panel agreed that there was a significant "likelihood of a major deterioration of global climate in the years ahead," but they could not say how rapidly that might happen. Scientists of the time disagreed on whether the greatest global risk was cooling by atmospheric pollution or greenhouse effect warming. No doubt the present warm interglacial period would end sooner or later, but that might be thousands of years away. About the only thing the scientists fully agreed on was that they were largely ignorant.(41) <=Government As a landscape that looks smooth from a distance may display jagged gullies when seen through binoculars, so sharper and sharper changes appeared as measuring techniques got better. An example was an analysis that Emiliani published in 1975 of some deep-sea cores from the Gulf of Mexico. Thanks to unusually clear and distinct layers of silt, he found evidence of a remarkable event around 11,600 years ago: a rise of sea level at a rate of meters per decade.(42) Another striking example was a 1981 study of a few sediment cores that had accumulated very rapidly, giving excellent time resolution. They showed a startling cooling around 11-12,000 years ago . as much as 7-10°C in less than a thousand years . before the warming resumed. One expert warned that temperatures in the past had sometimes jumped 5°C in as little as 50 years. (43*) =>Sea flooding Was there anything that could have caused such leaps of temperature? An expert noted that most of his colleagues "take the European late-glacial chronology as standard for the whole world, in the belief that climatic changes must have been broadly synchronous because they were cosmically caused."(44) And vast cosmic causes, for example a modulation of sunlight, seemed unlikely to push truly rapid world-wide changes. A close look at the best evidence found only events affecting the North Atlantic region (where most of the experts did their work). A local trigger for the Younger Dryas in particular was suggested by the fossil shorelines of a giant lake of fresh water that had been dammed up behind the North American ice sheet. Evidence suggested that as the ice melted back, the entire lake had suddenly been released to flood down the St. Lawrence River. That could well have set off changes that temporarily altered the region's climate (45*) =>The oceans Other ideas were more global in scope. Had an eruption of icebergs following the sudden disintegration of Arctic Ocean ice sheets cooled the entire North Atlantic Ocean? Perhaps a cluster of volcanic eruptions had affected the whole Northern Hemisphere?(46) Or a catastrophic disintegration of Antarctic ice sheets might have sent forth masses of ice to cool all the Earth's oceans? (See above) Then again, the changes might be purely chaotic, autonomous and unpredictable stutterings between different quasi-stable modes of the planet's climate system? <=>Chaos theory There were all too many feedback forces that might turn a slow local temperature change into an abrupt global one. The more traditional candidates included changes in ice and snow cover, ocean currents, or the pattern of wind circulation and storms. During the 1980s, further speculations lengthened the list. Perhaps a rise in global temperature would cause methane to bubble out of vast expanses of warming peat bogs and tundra? Since methane was a greenhouse gas, which blocked heat radiation even more effectively than CO2, its release would cause more warming still in a vicious feedback circle. Or what about clathrates . peculiar ices that locked up huge volumes of methane in the muck of cold seabeds . perhaps these would disintegrate and release greenhouse gases? <=Other gases It was getting easier for scientists to consider such colossal transformations, for uniformitarian thinking was under attack. By the early 1980s, some geologists were stressing the importance of rare events like the enormous floods that had drained temporary lakes during the melting of the continental ice sheets. In biology, Stephen Jay Gould and a few others were arguing that the evolution of at least some species had proceeded in "punctuated" bursts.(47) Other scientists were offering plausible scenarios of cosmic catastrophes that might happen only once in tens of millions of years. Had a stunning climate change, following the fall of a giant asteroid, exterminated the dinosaurs in a single frozen year? Could something like that befall us? <=World winter Many scientists continued to look on such speculations as little better than science fiction. The evidence of abrupt shifts that turned up in occasional studies may seem strong in retrospect, but at the time it was not particularly convincing. Any single record could be subject to all kinds of accidental errors. The best example was in the best data on climate shifts, the wiggles in measurements from the Camp Century core. These data came from near the bottom of the hole, where the ice layers were squeezed tissue-thin and probably folded and distorted as they flowed over the bedrock. Broecker later remarked that the relatively smooth temperature record of oxygen isotopes in deep-sea sediments "tended to lull scientists into concluding that the Earth's climate responds gradually when pushed." Many continued to believe that the oceans could only vary gradually over thousands of years, with a thermal inertia that must moderate any climate changes. These scientists should have realized that the top few meters of ocean exchange heat only slowly with the rest. And they should have recalled that at most places in the deep sea, sediments accumulate at only a few centimeters per thousand years, with the churning by burrowing worms blurring any record of change. (48*) Ice did not have these problems, so further progress would depend on getting more and better ice cores. Ice drilling was becoming a little world of its own, inhabited by people of many nations (Dansgaard's "Danish" team spoke eight different languages). Their divergent interests made for long and occasionally painful negotiations. But the trouble of cooperation was worth it for bringing in a variety of expertise, plus (no less essential) a variety of agencies that might grant funds.(49) The ancient ice that drilling teams hunted was at places barely possible to reach . eventually they penetrated not only the polar ice caps, but mountain icefields from Peru to Tibet . and the teams had to somehow get there with tons of equipment and supplies. The outcome was a series of engineering triumphs, which could turn into maddening fiascos when a costly drill head got irretrievably stuck a mile down. Engineers went back to their drawing-boards, team leaders contrived to get more funds, and the work slowly pushed on. There is a supplementary site on the History of Greenland Ice Drilling, with some documentation of the US "GISP" projects of the 1980s. <=>International A drilling team <=>Climate cycles <=>Models (GCMs) <=>Modern temp's A breakthrough came after the ice drillers went to a second location, a military radar station named "Dye 3" some 1,400 kilometers distant from Camp Century. By 1981, after a decade of tenacious labor and the invention of an ingenious new drill, they had extracted gleaming cylinders of ice ten centimeters in diameter and in total over two kilometers long. Dansgaard's group cut out 67,000 samples, and in each sample analyzed the ratios of oxygen isotopes. The temperature record showed what they called "violent" changes . which corresponded closely to the jumps at Camp Century. Moreover, the most prominent of the changes in their record corresponded to the Younger Dryas oscillation that had been recorded in pollen shifts all over Europe. It showed up in the ice as a swift warming interrupted by "a dramatic cooling of rather short duration, perhaps only a few hundred years."(50) Dye 3 leaders A particularly good correlation came from a group under Hans Oeschger. An ice drilling pioneer, Oeschger was now measuring oxygen isotopes in glacial-era lake deposits near his home in Bern, Switzerland. That was far indeed from Greenland, but his group found "drastic climatic changes" that neatly matched the ice record. The severe cold spells became known as "Dansgaard-Oeschger events." They seemed to be restricted to the North Atlantic and Europe. (51*) = Milestone As ice drillers improved their techniques, making ever better measurements along their layered cores, they found a variety of large steps not only in temperature but also in the CO2 concentration. (52*) This was a great surprise to everyone. Since the gas circulates through the atmosphere in a matter of months, the steps seemed to reflect world-wide changes. Other scientists promptly pointed out that the observations might be a mere artifact . the amount of gas absorbed might change with the local temperature in Greenland because of the physical chemistry of ice. It remained clear that something had made spectacular jumps. A variety of other evidence for very abrupt climate changes was accumulating, and some began to entertain the notion of such change on a global scale. =>Biosphere =>Models (GCMs) Most of these scientists, after presenting their data, could not resist adding a few suggestive words about possible causes. Dansgaard's group was typical in speculating about "shifts between two different quasi-stationary modes of atmospheric circulation."(53) That was the most common idea about how climate might change rapidly, harking back to the "dishpan" experiments of the 1950s. It implied transient variations of wind patterns within broad limits, and mostly concerned how weather might change in a particular region. The new thinking about grand global shifts urged a broader view. It was hard to see how the atmosphere could settle into an entirely new state unless something drastic happened in the oceans. For it is sea water, not air, that holds most of the heat energy and most of the moisture and CO2 of the climate system. The question of century-scale shifts, now a main topic in climatology, came to rest on the desks of ocean scientists. =>The oceans Their response was prompt. Experts mooted various hypotheses about how changes in the surface waters might affect CO2 levels. There were complex linkages among temperature, sea water chemistry, biological activity, and the chemical nutrients that currents brought to the surface. There were also reasons to believe that the pattern of North Atlantic Ocean circulation could change on a short timescale. Since the circulating waters carry tremendous quantities of heat northward from the tropics, if the circulation ground to a halt, temperatures in many regions of the Northern Hemisphere would immediately plunge. <=The oceans Broecker began to warn that the ocean-atmosphere climate system did not necessarily respond smoothly when it was pushed . it might jerk. In 1987, he wrote that scientists had been "lulled into complacency." People were increasingly taking their cue from elaborate supercomputer simulations of the general circulation of the atmosphere, failing to realize that the models, in the very way they were constructed, allowed only smooth and gradual changes. An "unstable" model would have been reworked by its authors until it produced more consistent results. Broecker strongly suspected that "changes in climate come in leaps rather than gradually," posing a drastic threat to human society and the natural world. And indeed new computer models, laboring to incorporate interactions between air and sea, hinted that he was right. (54*) <=The oceans =>Public opinion <=>Models (GCMs) After 1988 =>after88 Early in the 1990s, further revelations startled climate scientists. The quantity, variety, and accuracy of measurements of ancient climates was increasing at a breakneck pace . compared with the data available in the 1970s there were orders of magnitude more now in hand. The first shock came from the summit of the Greenland ice plateau, a white wasteland so high that altitude sickness was a problem. From this location all ice flowed outward, so it was hoped that even at the bottom, three kilometers deep, the layers would be relatively undisturbed by the movement. Early hopes for a new cooperative program joining Americans and Europeans had broken down, and each team drilled its own hole, some three kilometers (two miles) deep. Competition was transmuted into cooperation by a decision to put the two boreholes just far enough apart (30 kilometers) so that anything that showed up in both cores must represent a real climate effect, not an accident due to bedrock conditions. The match turned out to be remarkably exact for most of the way down. The comparison between cores showed convincingly that climate could change more rapidly than almost any scientist had imagined.(55) For more on ice drilling, see Joel Genuth's Greenland Ice Sheet Project (GISP) site and the official GISP Web site.