By the 20th century scientists, rejecting old tales of world catastrophe, 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. The 1980s and 1990s brought proof (chiefly from studies of ancient ice) that the global climate could indeed shift, abruptly 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 small forcing can cause a small [climate] change or a huge one..." --National Academy of Sciences, 2002 ⁽¹⁾

Looking to times long past, scientists recognized that massive ice sheets had once covered a good part of the Northern Hemisphere. The Ice Age was tens of thousands of years in the past, however, and it had been an aberration. During most of the geological record, the Earth had been bathed in uniform warmth--such was the fixed opinion of geologists. 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. The global climate could change abruptly--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.

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 respected climate expert C.E.P. Brooks offered the worst scenario. 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.

The conviction that climate changed only slowly was not affected by the detailed climate records that oceanographers 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 abrupt 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. There were 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⁾

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, peat beds, 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. 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.+

If researchers had found simultaneous changes at widely different locations, they might have detected a broad climate shift. Carbon-14 dating remained fraught with uncertainties, however, and matching up the chronologies of different places was difficult and controversial. Moreover, even a massive and global climate change could bring rains in one locale, cold in another, and little shift at all of vegetation in a third. So each study remained isolated from the others. ⁽⁹⁾

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. Geologists cherished the uniformitarian principle 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.+ ⁽¹⁰⁾

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 change quickly, 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 rainforests 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 unmistakable 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).

In 1956 the carbon-14 expert Hans Suess, studying the shells of plankton embedded in cores of clay pulled from the seabed by Columbia University's Lamont Geological Observatory, discovered a change at the fastest speed that anyone expected. Suess reported that the last glacial period had ended with a "relatively rapid" rise of temperature--about 1C (roughly 2F) per thousand years. ⁽¹¹⁾+ 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." ⁽¹²⁾ Indeed Cesare Emiliani, who often disagreed with Lamont scientists, published an argument that the temperature rise of some 8C had been the expected gradual kind, stretching over some 8,000 years. ⁽¹³⁾+ 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.) ⁽¹⁴⁾ 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.

After considerable debate, Emiliani won his point. The abrupt 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." ⁽¹⁵⁾ 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 5-10C had in fact taken place in less than a thousand years. ⁽¹⁶⁾ 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.+

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. ⁽¹⁷⁾ 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. ⁽¹⁸⁾+ However, the Ewing-Donn theory turned out to have fatal errors, and most scientists continued to doubt that such swift changes were possible.

Further information came from studies of fossil pollen recovered from layers of peat laid down in bogs. The scientists who undertook 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. ⁽¹⁹⁾

Earth scientists had to be careful in describing such results, for abrupt 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 grew 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 zealots used the ideas, persistently and increasingly, as they 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'...") ⁽²⁰⁾+ If pollen types did shift abruptly in some bog, scientists could account for that as an artifact of a purely local change. There seemed to be no good evidence, nor plausible physical cause, for any swift global upset.+

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, weather patterns in many regions would change almost instantly.+ On a still larger scale, in the early 1960s a few scientists created crude but robust mathematical models that 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. ⁽²¹⁾+

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.+

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. ⁽²²⁾ It proved possible to build simple fluid-flow models that showed how a switch in the pattern of ocean currents could promote such a change.+ 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." ⁽²³⁾+

The view was supported by data gathered independently at the University of Wisconsin-Madison, where Reid Bryson was already interested in abrupt 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 with 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." ⁽²⁴⁾

Next the Wisconsin team reviewed carbon-14 dates of pollen from around the end of the last ice age. In 1968, they reported evidence for 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. ⁽²⁵⁾ 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 swift transitions. ⁽²⁶⁾ 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." ⁽²⁷⁾+ 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.

Moreover, 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. ⁽²⁸⁾+ 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 that covered much longer times. Evidently the hundred-thousand-year glacial cycles did follow a sawtooth pattern: each cycle showed a slow descent into a long-lasting cold state that 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." ⁽²⁹⁾ Another respected climatologist explained that the old view of "a grand, rhythmic cycle" must be replaced by 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)." ⁽³⁰⁾+

Within these larger transitions, even quicker secondary oscillations showed up in various data, such as carbon-14 studies of ancient glacier moraines and lake levels. ⁽³¹⁾ Above all there was the Younger Dryas. Evidence from shells in a few excellent deep-sea cores showed a geographically widespread temperature oscillation. Many scientists found this evidence of little interest, however. Various chemical and biological effects could easily have confused the data. ⁽³²⁾

Up through the early 1970s, few of the scientists who studied ancient climates paid much attention to putative short-term changes. Their energies continued to focus on pinning down the grand multi-millennial rhythm of the ice ages and the famous puzzle of its causes.

It was the pursuit of these long cycles, more than any expectation of finding abrupt changes, that attracted scientists to a high-altitude frozen plateau. A Danish group headed by Willi Dansgaard drilled a long core of ice at Camp Century, Greenland in cooperation with Americans led by Chester Langway, Jr. The proportions of different oxygen isotopes in the layers of ice gave a fairly direct record of temperature. But mixed in with the expected gradual cycles, the group was surprised to notice what they 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 represent not a world climate shift, but only local accidents in the ice. ⁽³³⁾+

A group of glacial-epoch experts, meeting at Brown University in 1972, reached something close to a consensus. 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 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 abrupt cooling within the next few hundred years--"a first order environmental hazard." ⁽³⁴⁾+

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.+ While Bryson warned that the increasing pollution of the atmosphere would shade the Earth and bring abrupt 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.

As abrupt changes became more credible, scientists noticed them 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 species you find give 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 3C, 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 shown it). ⁽³⁵⁾ This singular approach got a skeptical response from other scientists who pursued the well-established study of pollens, for they were accustomed to seeing more gradual transformations of forests and grasslands. They easily dismissed the fluctuations in Coope's records 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. ⁽³⁶⁾+ 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." ⁽³⁷⁾

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. ⁽³⁸⁾+ 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 a severe "climatic surprise" could hit the world. ⁽³⁹⁾+

Most scientists spoke more cautiously. When leading experts had to state a consensus opinion they were circumspect, as in a 1975 National Academy of Sciences report about plans for international cooperation in atmospheric research. Evaluating past statistics, the panel concluded that predictable influences on climate made for only relatively small changes. These changes, they said, 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 abruptly 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 eventually, but that might be thousands of years away. About the only thing the scientists fully agreed on was that they were largely ignorant. ⁽⁴⁰⁾+

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. ⁽⁴¹⁾+ Another compelling 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 7-10C in less than a thousand years--before the warming resumed. One expert warned that temperatures in the past had sometimes jumped 5C in as little as 50 years. ⁽⁴²⁾

Was there really any mechanism that could have caused such leaps of temperature? The known cosmic causes, for example a modulation of sunlight, seemed unlikely to be strong enough to push truly abrupt world-wide changes. 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." ⁽⁴³⁾ A close look at the best evidence, however, 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, it had suddenly released the entire lake to flood down the St. Lawrence River. That could well have set off changes that temporarily altered the region's climate ⁽⁴⁴⁾+

Other mechanisms that scientists thought up 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? ⁽⁴⁵⁾ Or a catastrophic disintegration of Antarctic ice sheets might have sent forth masses of ice to cool all the Earth's oceans? Then again, the changes might be purely chaotic, autonomous and unpredictable stutterings between different quasi-stable modes of the planet's climate system?+

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, additional 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?+

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 some species had evolved in "punctuated" bursts. ⁽⁴⁶⁾ 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?+

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. ⁽⁴⁷⁾ 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 (what was also essential) a variety of agencies that might grant funds. ⁽⁴⁸⁾+ Drilling teams hunted ancient ice in 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.

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 more than 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 seen 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." ⁽⁴⁹⁾

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 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. ⁽⁵⁰⁾

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. ⁽⁵¹⁾ 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. Yet clearly 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.+

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." ⁽⁵²⁾ 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 seawater, 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.+

Their response was prompt. Experts mooted various hypotheses about how changes in the surface waters might affect CO₂ levels. There were complex links among temperature, seawater chemistry, biological activity, and the chemical nutrients that currents brought to the surface. Oceanographers also had 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.+

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. They failed to realize that these models, in the very way they were constructed, allowed only smooth and gradual changes. The authors of an "unstable" model would rework it until it yielded 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. As computer modelers labored to incorporate interactions between air and sea, their new simulations hinted that he was right. ⁽⁵³⁾+

After 1988

+

Early in the 1990s, further revelations startled climate scientists. The quantity, variety, and accuracy of measurements of ancient climates were increasing at a breakneck pace--compared with the data available in the 1970s, orders of magnitude more were 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 glacier experts hoped that even at the bottom, three kilometers (two miles) down, the layers would be relatively undisturbed by movement. Early hopes for a new cooperative program joining Americans and Europeans had broken down, and each team drilled its own hole. An ingenious decision transmuted competition into cooperation. The two holes were drilled 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. A comparison of variations in the cores showed convincingly that climate could change more rapidly than almost any scientist had imagined. ⁽⁵⁴⁾ For more on ice drilling, see Joel Genuth's Greenland Ice Sheet Project (GISP) http://www.ngdc.noaa.gov/paleo/icecore/greenland/gisp/gisp.html.

Swings of temperature that scientists in the 1950s believed to take tens of thousands of years, in the 1970s to take thousands of years, and in the 1980s to take hundreds of years, were now found to take only decades. Ice core analysis by Dansgaard's group, confirmed by the Americans, showed rapid oscillations of temperature repeatedly at irregular intervals throughout the last glacial period. Greenland had sometimes warmed a shocking 7C within a span of less than 50 years. During the Younger Dryas transition, drastic shifts in the entire North Atlantic climate were visible within five snow layers, that is, as little as five years! "The general circulation [of the atmosphere] in the Northern Hemisphere must have shifted dramatically," Dansgaard's group concluded. ⁽⁵⁵⁾

There was more. In the late 1980s and early 1990s, improved carbon-14 techniques gave dates for pollen and the like at locations ranging from Ohio to Japan to Tierra del Fuego. The results suggested that the Younger Dryas events had affected climates around the world. The extent of this perturbation, and just how weather had changed in different regions, was controversial. ⁽⁵⁶⁾ But scientists were increasingly persuaded that abrupt climate shifts could have global scope.

Could such variations occur not only in glacial times, but also in warm periods like the present? The layers from the previous warm period were down near bedrock, distorted by ice flow, and here the two groups' cores gave divergent results. (Antarctic cores could not help. Little snow falls there, and the layers of ice were too thin and squashed together to reveal any rapid variations.) Certainly no climate variation of Younger Dryas magnitude had been seen recently. So there was reason to hope that our present climate was relatively stable, at least for the moment. The Europeans and Americans nevertheless agreed that through most of the last 100,000 years the global climate had oscillated "on a scale that human cultural and industrial activities have not yet faced." ⁽⁵⁷⁾

Scientists will doubt even the best set of data if they cannot explain it, but at least one plausible explanation was at hand. A flip-flop of the entire Atlantic Ocean's circulation pattern might have caused the Dansgaard-Oeschger events. People came up with various proposals for things that might have triggered a switch, such as the surge of an ice sheet that released a flotilla of icebergs.+ That was not easy to swallow. As one scientist remarked, many of his colleagues "do not believe that the small, energy-starved polar 'tail' can wag the large, energy-rich tropical 'dog'." ⁽⁵⁸⁾ But the evidence of iceberg surges was strong, and computer models suggested that such events--or even just by the natural instability of the circulation during a glacial period--could indeed have caused a drastic circulation shift with global impact.

Could the same instability hold today? There was suggestive evidence that abrupt flips of circulation had in fact happened in previous times of warmth. In particular, it appeared that during the warm period some 8000 years ago, fresh water bursting out of huge lakes left over from the last ice age had sufficed to halt the North Atlantic circulation, bringing a deep cold spell. ⁽⁵⁹⁾ "There is surely a possibility," Broecker wrote, "that the ongoing buildup of greenhouse gases might trigger yet another of these ocean reorganizations." He was troubled by contemporary measurements of the North Atlantic that suggested that its circulation had slowed down, compared with measurements in earlier decades. (Further observations in 2001-2002 confirmed that the Atlantic's freshwater balance was changing.) Still, the record was so skimpy that nobody could say this was not just a normal, temporary fluctuation. When an international panel of experts made a best guess on the issue in 2001, they reported that a shutdown of the Atlantic circulation in the coming century was "unlikely," but "cannot be ruled out." If so, it would probably change climates all around the North Atlantic, a serious regional cooling brought on by global warming. Broecker warned that the consequences, perhaps only a few decades hence, in a world that would already be pressed to feed its soaring population, could be "widespread starvation." [In 2004, however, he cautioned against "exaggerated scenarios." As others too pointed out, there were no prospects in our era for a deluge of melted glacier water. Any reorganization of the ocean circulation would be gradual-- troublesome but not catastrophic.] ⁽⁶⁰⁾ +

Other mechanisms for catastrophically rapid shifts remained on the table. An example was the clathrate ices, frozen in layers spread through sea floor muds. Clathrates might hold more carbon compounds than all the world's coal and oil. New studies made it plausible that warming of the oceans could cause some of the deposits to disintegrate in a landslide-like chain reaction, which would vent enough methane and CO₂ into the atmosphere to redouble global warming. The idea sounded like science fiction (indeed some science fiction writers used it), and it seemed highly unlikely to happen anytime soon. ⁽⁶¹⁾

In the 1990s, geologists found that such titanic gas outbursts had in fact caused a spurt of warming 55 million years ago. At any rate something back then had radically changed climate, bringing mass extinctions and a new geological era, and clathrates were the leading suspect. The total carbon release that caused this havoc was roughly comparable to the amount of carbon that humanity would emit if we burned all available coal and oil. Back then, the rise in temperature had apparently stretched over tens of thousands of years, "rapid" only to a geologist. But it seemed to have come in abrupt steps, which in some centuries might have pumped greenhouse gases into the atmosphere at a rate fast enough to bring serious change within a human lifetime ⁽⁶²⁾+

Ominously, data showed that sudden climate shifts did not happen only during a glacial period. In 1993, Dansgaard and his colleagues reported that rapid oscillations had been common during the last interglacial warm period--enormous spikes of cooling, like a 14-degree cold snap that had struck in the span of a decade and lasted 70 years. The instability was unlike anything the ice record showed for our current interglacial period. The announcement, Science magazine reported, "shattered" the standard picture of benign, equable interglacials. ⁽⁶³⁾+

Others soon showed that these measurements, made near the bottom of the core, were distorted by ice flow that stirred together layers from warm and cold periods. Interglacials were perhaps not so horrendously variable. ⁽⁶⁴⁾ Yet in terms of how scientists thought about the present climate system, one might say that the ice had been broken. People recalled that the present system was certainly subject to abrupt but harrowing droughts, like the one revealed by Bryson that had devastated native North American cultures. Persuasive new geological evidence blamed extreme prolonged droughts for the downfall of ancient Mayan and Mesopotamian civilizations too. ⁽⁶⁵⁾

An altogether different type of evidence for rapid change came from improved observations of Arctic and Antarctic regions. New views from satellites, plus vigorous programs of precise measurements from airplanes and on the ground, showed that enormous glaciers could quickly change their speed of travel, while entire ice sheets could break up within a matter of months. As one expert remarked, this "ran counter to much of the accepted wisdom regarding ice sheets." That accepted wisdom, he explained, "lacking modern observational capabilities, was largely based on 'steady-state' assumptions." ⁽⁶⁶⁾ Now the plausible possibility that a swift alteration of land or sea ice could transform climate had to be added to all the other potential feedbacks from global warming.+

The new view of climate was reinforced by one of the last great achievements of the Soviet Union, an ice core drilled with French collaboration at Vostok in Antarctica. The record reached back through nearly four complete glacial-interglacial cycles--and drastic temperature changes peppered almost every stretch of data. This Antarctic record was too fuzzy to say whether any of these changes had come and gone on the decade-size timescale of the Younger Dryas. But warm interglacial periods had certainly been subject to big swings of temperature lasting for centuries. Especially striking to the researchers, by contrast, was our own era, the ten thousand years since the last glaciation. It was, "by far, the longest stable warm period recorded in Antarctica during the past 420 [thousand years]." When Bryson, Schneider, and others had warned that the century or so of stability in recent memory did not reflect "normal" long-term variations, they had touched on an instability grander than they guessed. The entire rise of human civilization since the end of the Younger Dryas had taken place during a period of warm, stable climate that was unique in the long record. The climate known to history seemed to be a lucky anomaly. (Paleoclimatologist William Ruddiman suggested that this was no coincidence. Perhaps the rise of agriculture, with its deforestation and rice paddies, had added enough methane and CO₂ to the atmosphere to dampen the normal ice-age cycle?) The well-recorded history of the most recent century or so happened to show even more unusual stability, compared with what new evidence was revealing about severe variations in earlier millennia. ⁽⁶⁷⁾

The accumulation of evidence, reinforced by at least one reasonable explanation (the reorganization of ocean circulation) destroyed long-held assumptions. Most experts now accepted that abrupt climate change, huge change, global change, was possible at any time. A report written by a National Academy of Sciences committee in 2001 said that the recognition, during the 1990s, of the possibility of abrupt global climate change constituted a fundamental reorientation of thinking, a "paradigm shift for the research community." ⁽⁶⁸⁾

The first strong consensus statement had come in 1995 from the Intergovernmental Panel on Climate Change, representing the considered views of nearly all the world's climate scientists.+ The report included a notice that climate "surprises" were possible--"Future unexpected, large, and rapid climate system changes (as have occurred in the past)." ⁽⁶⁹⁾+ The report's authors did not emphasize the point, however, and the press seldom mentioned it.

Despite the profound implications of this new viewpoint, hardly anyone rose to dispute it. Yet while they did not deny the facts head-on, many denied them more subtly, by failing to revise their accustomed ways of thinking about climate. For example, few of the scientists studying pollen in bogs went back to their data and took on the difficult task of looking for catastrophically rapid shifts in the past. "Geoscientists are just beginning to accept and adapt to the new paradigm of highly variable climate systems," said the Academy committee in 2001. Beyond geoscientists, "this new paradigm has not yet penetrated the impacts community," that is, the economists and other specialists who tried to calculate the consequences of climate change. ⁽⁷⁰⁾ Policy-makers and the public lagged even farther behind in grasping what the new scientific view could mean.+

A lesson about how science proceeds can be learned from this history.+ Asked about the discovery of abrupt climate change, many climate experts today would put their finger on one moment: the day they read the 1993 report of the analysis of Greenland ice cores. Before that, nobody confidently believed that the climate could change massively within a decade or two; after the report, nobody felt sure that it could not. So wasn't the preceding half-century of research a waste of effort? If only scientists had enough foresight, couldn't we have waited until we were able to get good ice cores, and settle the matter once and for all with a single unimpeachable study?

The actual history shows that even the best scientific data are never that definitive. People can see only what they find believable. Over the decades, many scientists who looked at tree rings, varves, ice layers, and so forth had held evidence of rapid climate shifts before their eyes. They easily dismissed it. There were plausible reasons to believe that global cataclysm was a fantasy of crackpots and Bible fundamentalists. Records of the past were mostly too fuzzy to show rapid changes, and where such a change did plainly appear, scientists readily attributed it (usually correctly) to something other than climate. Sometimes the scientists' assumptions were actually built into their procedures. When pollen specialists routinely analyzed their clay cores in 10-centimeter slices, they could not possibly see changes that took place within a centimeter's worth of layers. ⁽⁷¹⁾ If the conventional beliefs had been the same in 1993 as in 1953--that significant climate change always takes many thousands of years--scientists would have passed over the decade-scale fluctuations in ice cores as meaningless noise.

First scientists had to convince themselves, by shuttling back and forth between historical data and studies of possible mechanisms, that it made sense to propose shifts as "rapid" as a thousand years. Only then could they come around to seeing that shifts as "rapid" as a hundred years could be plausible. And only after that could they credit still swifter changes. Without this gradual shift of understanding, the Greenland cores would never have been drilled. The funds required for these heroic projects came to hand only after scientists reported that climate could change in damaging ways on a timescale meaningful to governments. In an area as difficult as climate science, where all is complex and befogged, it is hard to see what one is not prepared to look for.

Related:

Ocean Currents and Climate+

Aerosol Hazes+

Simple Models of Climate+

Supplements:

Radiocarbon Dating+

Ice Sheets & Rising Seas+

1. National Academy of Sciences (2002), p. 7.

2. Crowley and North (1991), pp. 234-35; steady climate was still a "paradigm" in various geosciences into the 1980s, according to Schimel and Sulzman (1995).

3. Gregory (1908), p. 340.

4. Humphreys (1932).

5. Brooks (1925), pp. 90-91.

6. Work of Knud Jessen, Johannes Iversen (both Danes) and others, reviewed in Manten (1966).

7. Outside Scandinavia, carbon-14 dating of trees overridden by the North American ice sheet showed the front had advanced and retreated by up to a kilometer a year between 13,600 and 12,200 years ago. Flint (1955).

8. Classic work included that of Johannes Iversen on the arrival of agriculture in Denmark and Leonard Wilson on forest fire and other rapid glacial-era changes in Wisconsin.

9. For discussion on the above points I am grateful to Ken Brown, Daniel A. Livingstone and other respondents from the QUATERNARY and PALEOLIM listservs.

10. Palmer (1999); also Huggett (1990), pp. 119-21 and passim.

11. Suess (1956), p. 357; other scientists called this an "abrupt increase:" Ewing and Donn (1956a), p. 1061.

12. Ericson et al. (1955); Ericson et al. (1956), quotes p. 388.

13. Emiliani (1957).

14. Broecker (1957), p. V-9; Brooks (1949).

15. Broecker, interview by Weart, Nov. 1977, AIP.

16. Broecker et al. (1960a).

17. "Stimulated:" Broecker et al. (1960a), p. 442; Ewing with Heezen had collected some of the crucial cores and noticed the rapid change, Ewing and Donn (1956a).

18. Ives (1957), quote p. 87; see also Ives (1958); Ives (1962).

19. West (1961); the abruptness of the transition was noted later by Lamb (1977), p. 80.

20. Ager (1993), p. xi.

21. Budyko (1962); Wilson (1964).

22. Broecker (1966), pp. 299, 301.

23. Broecker and van Donk (1970).

24. Bryson, personal communications, 2002. Anthropologist: David Barreis. Barreis and Bryson (1965), p. 204; see Bryson and Barreis (1968), chs. 2, 3; Bryson (1968). The causes of the collapse of the great urban center Cahokia and other elements of the Mississippian culture remain controversial today, with climate change a strong contender.

25. Webb and Bryson (1972); reviewer: Bradley (1985), pp. 322-329, quote p. 327; for a general review of "transfer functions" for deducing temperature, see Sachs et al. (1977).

26. Bryson et al. (1970), p. 72.

27. Discontinuities: Wendland and Bryson (1974); a century or two: Bryson (1974).

28. Kukla and Kocí (1972), p. 383.

29. Quoted in Alexander (1974), p. 94.

30. Mitchell (1972), pp. 437-38.

31. One author, speculating about the coming of a new ice age, pointed to "evidence of (at least) five rapid hemispheric coolings of about 5C... each event spread over not more than about a century," Flohn (1974), quote p. 385; one line of evidence was carbon-14 studies of tree stumps in glacial deposits: Denton and Karlén (1973). But their fluctuations lasted several centuries, and the authors predicted not a new ice age but a shift to a mild climate.

32. Broecker and van Donk (1970); Ruddiman & McIntyre too found evidence in deep-sea cores of faunal change (including one core where the warming was interrupted by a cold spell). They called the change "abrupt" although they thought it was spread over a few thousand years: Ruddiman and McIntyre (1973), p. 129; a few years later they realized the spread was due to bioturbation, and the changes were actually "very abrupt." See their review of relevant studies from 1941 to 1977, Ruddiman and McIntyre (1981a), pp. 146-50.

33. Dansgaard et al. (1971); "spectacular": Dansgaard et al. (1972), p. 396; Dansgaard et al. (1973). The Camp Century and later work is discussed in Dansgaard (2004) and in interviews on GISP tape-recorded 1992-1994, records of Study of Multi-Institutional Collaborations, AIP.

34. Kukla and Matthews (1972).

35. Coope (1977); already in 1970 a cooling within a thousand years or so was seen, although not remarked upon, Coope et al. (1971).

36. Budyko (1972).

37. Kukla and Kukla (1974), quote p. 713; this was brought to the public, e.g. in Time (1974a).

38. E.g., Flohn (1974), with reference to work by Lorenz, Budyko, and Sellers on instability; Dansgaard et al. (1972), p. 396, speculating on cooling.

39. Broecker (1975).

40. GARP (1975), from App. A (pp. 186-90) by J. Imbrie, W.S. Broecker, J.M. Mitchell, Jr., J.E. Kutzbach.

41. Emiliani et al. (1975), for criticism, see Science 193 (24 Sept. 1976): 1268.

42. Ruddiman and McIntyre (1981a); another example: century-scale changes in carbon-dated peat bog pollen, including a clear oscillation 11,000-9,000 years ago, Woillard and Mook (1982); in 50 years: Flohn (1979).

43. Mercer (1969), p. 227.

44. Diversion of glacial meltwater from the Mississippi to the St. Lawrence was suggested by Kennett and Shackleton (1975); Johnson and McClure (1976); Ruddiman and McIntyre (1981a), p. 204 dismissed this since they saw no decrease in North Atlantic biological productivity; but later data, as explained by Broecker, supported the idea, Broecker et al. (1989).

45. Mercer (1969) considered breakup of an Arctic Ocean ice sheet; this is cited as a likely explanation by Ruddiman and McIntyre (1981a), pp. 204ff.; see Ruddiman and McIntyre (1981b); eruptions: Flohn (1974).

46. Gould (2002), pp. 1006-21 gives one version of the history, with his characteristically polemical approach.

47. Also, the sluggish response of the massive polar icecaps to change smoothed the oxygen-isotope record. Broecker (1987b); for these issues in general, see Palmer (1999); Huggett (1990).

48. For Greenland drilling, see interviews on GISP tape-recorded 1992-1994, records of Study of Multi-Institutional Collaborations, AIP; Mayewski and White (2002); Alley (2000); Dansgaard (2004).

49. Dye 3: Dansgaard et al. (1982), "violent," "dramatic" (also "drastic"), p. 1273; see also Oeschger et al. (1984); Camp Century CO₂: Neftel et al. (1982); note that they do not discuss a jump that is evident in their data.

50. Siegenthaler et al. (1984), "drastic" p. 149. They found nothing similar in North American records; Barnola et al. (1987) found nothing like it in their Antarctic ice core, but admitted their methods would not detect very rapid changes.

51. A century-scale shift closely correlated with temperature change was found by Oeschger et al. (1984); see also Dansgaard et al. (1984); decade-scale shifts are visible in the data, although not specially remarked upon, in Hammer et al. (1986).

52. Dansgaard et al. (1982), p. 1275.

53. "The basic architecture of the models denies the possibility of key interactions that occur in the real system. The reason is that we do not yet know how to incorporate such interactions into the models." Broecker (1987a), pp. 123, 126; new models: Bryan and Spelman (1985); Manabe and Stouffer (1988).

54. GISP interviews, records of Study of Multi-Institutional Collaborations, AIP. Firsthand accounts are Mayewski and White (2002); Alley (2000).

55. Dansgaard et al. (1989); increasingly abrupt changes were seen on further study, Johnsen et al. (1992); Grootes et al. (1993); jumps of Greenland snow accumulation "possibly in one to three years" were reported by Alley et al. (1993); see Alley (2000); five-year Younger Dryas steps: Taylor et al. (1997); a Younger Dryas temperature step in less than a decade was found to be hemisphere-wide since methane gas changed as well, Severinghaus et al. (1998).

56. For references 1987-94 (including also Alaska, New Zealand, Gulf of California, etc.) see Broecker (1995), pp. 306-08; for later developments, National Academy of Sciences (2002) and Lynch-Stieglitz (2004).

57. Hammer et al. (1997), Preface, "not yet faced," p. 26,315.

58. Alley (1998).

59. Barber et al. (1999).

60. Broecker et al. (1992); quotes: Broecker (1997), p. 1588; IPCC (2001), p. 420; Atlantic freshening: Hansen et al. (2001); Dickson et al. (2002); Curry et al.(2003); "exaggerated:" Broecker (2004); see Weaver and Hillaire-Marcel (2004).

61. Science fiction: notably the award-winning Robinson (1994).

62. Appenzeller (1991); for the late Paleocene event, Kennett and Stott (1991); Koch et al. (1992); Dickens et al. (1995); Norris and Röhl (1999); Katz et al. (1999); a good recent overview is Kunzig (2004).

63. Dansgaard et al. (1993); Kerr (1993).

64. Alley et al. (1995); Chappellaz et al. (1997), comparing with Vostok cores.

65. Maya: Hodell et al. (1995); Mesopotamia: Weiss et al. (1993); for global climate shifts throughout the postglacial period, see also deMenocal et al. (2000).

66. Rignot and Thomas (2002), p. 1505.

67. "longest stable:" Petit et al. (1999), p. 434. Ruddiman and Thomson (2001) #1679; see Kerr (2004).

68. National Academy of Sciences (2002), p. 16, see also pp. 1, 119, 121.

69. IPCC (1996), p. 7.

70. National Academy of Sciences (2002), p. 121.

71. There is a famous comparable case in another field of science. In the 1930s, physicists used thin screens to block extraneous large particles from their instruments as they measured the tiny particles resulting from nuclear reactions. Since they never imagined that an atom could split into two large chunks, they automatically prevented themselves from discovering uranium fission. For discussion on the difficulties of detecting rapid change, I am grateful to Ken Brown, Daniel A. Livingstone and other respondents from the QUATERNARY and PALEOLIM listservs.