By the 20th century, scientists understood that in some past geological eras the planet's climate had been radically different. Rejecting old tales of abrupt catastrophe, they believed that global climate changed gradually over many tens of thousands of years. In the 1950s, however, 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 radically within a century — perhaps even within a decade. For one thing, the large-scale circulation of ocean currents might shut down abruptly, disrupting climate around the North Atlantic and beyond. For another, with warming by greenhouse gases underway the stability of the great polar ice sheets came into question, raising a threat of rapid sea-level rise. Worse, there seemed to be feedback loops that could accelerate the warming — perhaps irreversibly. The warming was making it harder for natural systems to remove carbon from the atmosphere, while stimulating the emission of additional gases. Into the 21st century, researchers turned up more and more potential physical and biological feedback mechanisms, interacting with one another in a devilishly complex and potentially unstable system. Some of these processes might take centuries to play out, but some might cause great changes within less than a human lifetime. If humanity's emissions continued to rise, scientists could not rule out passing "tipping points" for an unstoppable catastrophe.
This essay covers large one-way jumps of climate. For short-term cyclical changes, see the essay on Changing Sun, Changing Climate. For a more extensive discussion of rapid breakup of ice sheets, see the essay on Ice Sheets, Rising Seas, Floods. For a more extensive discussion of biological feedbacks see the essay on Biosphere: How Life Alters Climate. For a more extensive discussion of the risk of a North Atlantic circulation shutdown see the essay on Ocean Currents and Climate.
"We are currently provoking the Earth's climate with a steady build-up of greenhouse gases. It would be prudent to find out what it might take to kick the system into one of its alternate modes of operation.." — Wally Broecker, 1994(1) |
| Earth's climate is not stable. Already in the 18th century, scientists discovered ancient fossils of tropical species such as crocodiles in the environs of Paris, and in the mid 19th century they discovered that wooly mammoths had once roamed a France in the grip of a glacial Ice Age. Such climate changes came at a geological pace, too slow to be noticeable in a human lifetime — such was the belief 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. |
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| 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, geologists confidently declared, the Earth had been bathed in uniform warmth. As one meteorologist complained as late as 1991, geology textbooks just copied 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 during 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 rapidly — that is, over the course of only a few tens of thousands of years, shockingly swift in geological terms. Probably the ice could come again. That gave no cause to worry, for it surely lay many thousands of years in the future. |
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| Assuming Stability TOP OF PAGE | |
| 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. |
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| 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 with grass in their stomachs, evidently frozen in a catastrophically abrupt climate change? 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 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 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 hardy but graceful 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 perhaps slightly 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 ("radiocarbon"). 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. |
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| Most scientists believed such a shift had to be a local circumstance around the North Atlantic, 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. | |
| Even swifter changes could show up in the varves in the clay 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 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. He claimed the rings reflected changes in the Sun with global impact. . 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's attempt to correlate his weather patterns with sunspot cycles was an approach most meteorologists had abandoned as hopelessly speculative. | <=Solar variation |
| 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) | |
| In any case it appeared that the rate of advance and retreat of the great ice sheets, at its fastest, 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 there had never been a process of change different from what could be observed, in one place or another, in the present day. 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.(11) |
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| In the historical record, temperatures apparently had not risen or fallen radically in less than millennia, so the uniformitarian principle declared that there must never have been faster changes. 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. 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. As for unmistakable fluctuations like the Younger Dryas, presumably those were restricted to the vicinity of the North Atlantic or an even narrower area (few studies had been done anywhere else). |
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| Changes Over Millennia? (1950s) TOP OF PAGE | |
| 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 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) |
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| 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 Ericson's work and other data, Broecker saw severe changes around the world all dated to about the same time — "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." This was only one passage in a thick doctoral thesis that few people read, and to those few it must have sounded much like Brooks's speculations on cataclysmic changes, long since dismissed by scientists as altogether implausible. They were right to be cautious, for later studies found that the coincidence of changes that Broecker saw at different locations was an artifact of inaccurate dating. (This was not the last time he would glean a valuable idea from foggy data.)(15) |
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| 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 in fact 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. |
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| 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 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. |
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| 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.(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. Frozen mammoths were brought forth again as proof that the world’s climate could change catastrophically overnight (although every arctic hiker knows how swiftly a freeze can come even in summer, or how a misstep in a shifting riverbed could bury the careless in permafrost). 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'...")(21) There seemed to be no good evidence, nor plausible physical cause, for any swift global upset. |
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| Hints of Instability TOP OF PAGE | |
| Hints that the climate system could change abruptly came unexpectedly from fields far from traditional climatology. 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 suggested 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) |
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| 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 warnings from increasingly prominent 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. |
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| 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 the pattern of ocean currents could switch. 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) |
=>sea rise, ice, floods |
| 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 fluid 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."(25*) |
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| 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). 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 for any region.(26) Looking at hundreds of carbon-14 dates spanning the past dozen millennia — dates that improvements had now 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.(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. |
<=>Chaos theory |
| To be sure, 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 down to the exact century, since carbon-14 measurements still had a wide range of error and other dating techniques were 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 that covered much longer timescales. 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."(30) 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)."(31) |
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| 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.(32*) 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. Sea-floor slumping, or various chemical and biological processes in seawater, could easily have confused the data.(33*) 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. | |
| Changes within a Century? (1970s) TOP OF PAGE | |
| 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 anomalies in this particular shaft of ice.(34) |
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| 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 shell studies by Emiliani, and other field evidence, the scientists agreed that interglacial periods tended to be shorter than the textbooks said, not more than ten thousand years, and to end more abruptly than had been supposed. The present interglacial had already lasted ten thousand years. In view of reports of a cooling trend in the Arctic since the 1940s, they suspected our interglacial might be approaching its end. 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) |
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| Stephen Schneider, a scientist at NASA, joined Bryson and a few other scientists to take 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 disastrous freeze, drought, or other change unprecedented in recent memory — but not without precedent in the long record of archeology and paleontology. |
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| When scientists speculated about the future of climate, cooling was not the only option. The growing realization that small perturbations could trigger sudden climate change also impressed a few scientists who were beginning to worry about the rising level of the greenhouse gas carbon dioxide (CO2). Perhaps that might bring serious global warming, not cooling, within as little as a century or two? Many experts dismissed all such speculation, persisting in rejecting any possibility of rapid global change. | |
| In any science, a transformation of fixed opinion usually comes only when independent lines of evidence hammer the point home; any single argument could be faulty, but what if you get the same result in a wholly different way? This was beginning to happen with climate change. To mention only one curious example, consider the shells of beetles. The tiny fragments 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. In the 1970s Russell Coope, painstakingly sorting the remains of English bog beetles, 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 or less, he reported (if the change had been faster his data could not have shown it).(36) 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. Moreover, the wiggles in the data that pointed to abrupt changes at the end of the glacial period came from near the bottom of the hole. The ice layers there were squeezed tissue-thin, and could well have been folded and distorted as they flowed over the bedrock. | |
| Other evidence, notably oxygen isotopes in shells from deep-sea cores that reflected conditions in the entire North Atlantic, changed only over several thousand years. Broecker later remarked that the relatively smooth temperature record in the cores had "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 their massive thermal inertia that must moderate any climate changes. They had failed to realize that the top few meters of ocean exchange heat only slowly with the rest, so the thin surface layer could indeed heat up quickly. And they failed to notice that at most places in the deep sea, sediments accumulate at only a few centimeters per thousand years. Churning by burrowing worms and other creatures within the mud (“bioturbation”) smeared the layers, blurring any record of change.(36a*) | |
| Nevertheless, as more and more kinds of evidence accumulated, a growing number of scientists found it plausible that the climate over large regions, if not the entire globe, had sometimes changed markedly within a thousand years or even less. They were encouraged by theoretical developments in the early 1970s, global energy-balance models in which a few simple equations produced radical instability. In particular, Mikhail Budyko in Leningrad pursued calculations of feedbacks involving ice cover, which suggested that at the rate we were pumping CO2 into the atmosphere, the ice covering the Arctic Ocean in summer 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) |
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| These ideas prompted George Kukla and his wife Helena to inspect satellite photos of Arctic snow cover, where they found surprisingly large variations from year to year. If the large buildup seen in 1971 was 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) | |
| Meanwhile glacier experts — a separate little community with its own special interests and methods — developed ingenious numerical models that suggested that global warming might provoke the ice sheets of Antarctica to break up swiftly, That might shock the oceans with a huge surge of ice water, changing the climate system on a scale of mere decades.(39) This was pure speculation, but it encouraged Bryson and other scientists to work 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" might strike.(40) |
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| 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 abrupt jerks that might matter for current human affairs were likely to be just "noise," the usual irregularities of weather. The panel did agree 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, if at all. |
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| While scientists argued over whether there was a global risk of glacial cooling, or of greenhouse warming, or no risk at all, some journalists wrote sensational stories about the imminent threat of a catastrophic Ice Age. While a few scientists thought it was good for people to be prepared for climate shocks, every expert scoffed at lurid predictions of doom. There was one thing on which all climate scientists agreed — their ignorance. All admitted they would have to do much more work before they could say anything reliable about future climate change.(41) |
<=Aerosols |
| So the work continued, and evidence accumulated. 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 compelling report 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*) |
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| However, a jump like that on a global scale was flatly incredible unless there was some plausible geophysical mechanism that could cause such a change world-wide. An expert noted that most of his colleagues "take the [gradual] 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." The proposed cosmic causes, for example a modulation of sunlight, seemed scarcely strong and swift enough to push rapid world-wide changes. And talk about reorganization of ocean currents, collapse of ice sheets, or whatever was no more than imagination plus a few radically oversimplified equations and hand-waving.(44) |
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| Nevertheless, the idea of big, abrupt transformations was becoming more acceptable in the 1970s, for uniformitarian thinking was coming under attack in a variety of scientific fields. Bold geologists were stressing the importance of spectacular events even if it sounded like Biblical "catastrophism." For example, studies of the peculiar Oregon "scablands" pointed to a gargantuan outburst of glacial water, orders of magnitude greater than any flood recorded in human history. A spacecraft sent to Mars found the planet enveloped in a dust cloud — the entire atmosphere had flipped within months to a different mode, much warmer and windier (it later flipped back). In biology, Stephen Jay Gould and others were arguing that some species had evolved in “punctuated” bursts.(45) Faster still, Luis and Walter Alvarez argued that the entire extinction of the dinosaurs had taken only one frozen year after a giant asteroid struck. |
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| How fast could our own climate change? The answer would require arduous years of effort on research projects far from the public eye. The decisive evidence would come from teams working literally at the ends of the Earth. | |
| Surprises from the Ice (1980s) TOP OF PAGE | |
| 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 in planning a campaign. But the difficulties were worth it for bringing in a variety of expertise, plus (what was also essential) a variety of agencies that could bestow funds. Drilling teams hunted ancient ice in almost impenetrable places, hauling tons of equipment and operating it for months not only in central Greenland but on the barren Antarctic plateau and unexplored mountain ice fields from the Andes to Tibet. The outcome was a series of engineering triumphs, which could fall into maddening fiascos when a precious drill head got irretrievably stuck a mile down in the ice. Engineers went back to their drawing-boards, team leaders contrived to get more funds, and the work pushed ahead.(46) There is a supplementary experimental site on the History of Greenland Ice Drilling, with glimpses into the inner workings of the U.S. "GISP" projects of the 1980s. |
<=>Climate cycles |
| The first breakthrough came after the ice drillers went to a second Greenland 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 within mere centuries — corresponding closely to the changes seen at Camp Century. So the jumps were not accidents in a particular ice core. 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."(47*) |
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| 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 fluctuations (later named "Dansgaard-Oeschger events") seemed to be restricted to the North Atlantic and Europe, widespread but not global.(48*) |
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| 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.(49*) This was a great surprise to everyone. Since the gas circulates through the entire 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, a feature 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. |
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| 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."(50) 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, however, urged a broader view. Above all, 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, most of the moisture, and most of the CO2 of the climate system. The question of century-scale shifts, now a main topic in climatology, came to rest on the desks of oceanographers. |
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| The best evidence for abrupt jumps 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 lake of fresh water that had been dammed up behind the North American ice sheet. Geologists suggested that as the sheet melted back, an ice dam had suddenly broken up and released the entire gigantic lake to flood down the St. Lawrence River (like the outburst that created the Oregon scablands). In 1985 Broecker argued that the flood explained the Younger Dryas. |
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| Drawing on ideas developed by Oeschger and others, Broecker drew attention to the circulation of water in the Atlantic Ocean, of which the Gulf Stream is only one component. The pattern is comprised of huge volumes of warm water drifting slowly north from the tropics to the vicinity of Greenland, where it grows denser from cold and salinity, sinks into the depths, and returns sluggishly southward along the bottom. Broecker argued that the circulation has two modes: the present one, and one in which the water scarcely moves. He proposed that the pulse of glacial fresh water had triggered a switch, temporarily shutting down the circulation. With the ocean no longer carrying tremendous quantities of heat northward from the tropics, temperatures all around the North Atlantic would swiftly plunge — thus the Younger Dryas.(51*) | |
| (The cause of the Younger Dryas would be disputed into the 21st century. A variety of ideas, each with a few supporters, could not be definitively excluded. Had an eruption of icebergs, following the sudden disintegration of Arctic Ocean ice sheets, cooled the entire North Atlantic Ocean? Or did a huge asteroid or comet strike set it off? Or had a catastrophic disintegration of Antarctic ice sheets sent forth masses of ice to cool all of Earth’s oceans? Or perhaps a cluster of volcanic eruptions had affected the whole Northern Hemisphere? The most widely accepted explanation, however, remained Broecker’s proposal of a circulation shutdown.)(52) | |
| Broecker began to warn that the ocean-atmosphere system did not necessarily respond smoothly when it was pushed. It might jerk. In 1987 he wrote that climate scientists had been "lulled into complacency." They were increasingly relying on elaborate supercomputer simulations of the general circulation of the atmosphere and oceans, and failed to realize that these models were built to show only gradual changes. During the early stages of construction, a model would often go wildly astray, and the modelers would revise their parameters and computer code until the output resembled the actual present climate. The authors of an "unstable" model with odd jumps would rework it until it yielded smooth results. Broecker strongly suspected that "changes in climate come in leaps rather than gradually" — posing a drastic threat to human civilization. Through the 1980s computer modelers labored to incorporate interactions between air and sea more fully and accurately, and some of the new simulations hinted that Broecker was right.(54*) |
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| Abrupt Global Change (1990s) TOP OF PAGE | |
| 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; scientists now had in hand orders of magnitude more data than in the 1970s. The first shock came from the very 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 near the bottom of the hole, some 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.(55) |
For more on ice drilling, see Joel Genuth's Greenland Ice Sheet Project (GISP) site and the official GISP Web site. |
| Swings of temperature that in the 1950s scientists had believed must take tens of thousands of years, in the 1970s thousands of years, and in the 1980s hundreds of years, were now found to take only decades. Ice core analysis by Dansgaard's group, confirmed by the Americans' parallel hole, showed rapid oscillations of temperature at irregular intervals throughout the last glacial period. Greenland had sometimes warmed a shocking 7°C within a span of less than 50 years. | |
| For one group of American scientists on the ice in Greenland, a "moment of truth” struck one memorable day in midsummer 1992. As they inspected a cylinder of ice that had been laid down during the last years of the Younger Dryas, they saw a striking change in the ice visible within three snow layers, that is, scarcely three years! The team was first excited, then sobered — their view of how climate could change had shifted irrevocably. The European team reported seeing a similar step within at most five years (later studies found a big temperature jump within a single year). "The general circulation [of the atmosphere] in the Northern Hemisphere must have shifted dramatically," Dansgaard’s group eventually concluded. The finding seemed incredible, and indeed did not get widespread attention. Surely the bulk of the change, some 9°C within a decade, was only a local event restricted to the vicinity of central Greenland?(56*) | |
| It was no longer possible, however, to shrug off severe climate swings over the longer span of a few decades. Oceanographers had been hunting out spots on the seabed where bioturbation by burrowing worms did not smear the record. In some places the sediments accumulated very rapidly, while in others the seawater lacked enough oxygen to sustain life. The first results, from the Norwegian Sea in 1992, confirmed that the decade-scale changes seen in Greenland ice cores were not confined to Greenland alone. Meanwhile changes in the amount of dust carried thousands of miles on the winds were noted in the ice itself, suggesting at least a continental scope for the change. Subsequent work on seabed cores from the California coast to the Arabian Sea, and on chemical changes recorded in cave stalagmites from Switzerland to China, confirmed that the swings found in the Greenland ice had been felt throughout the Northern Hemisphere. The reach of the Younger Dryas temperature changes was further confirmed by a rapid rise in the level of methane (CH4, a potent greenhouse gas) preserved in the ice. The gas must have come from far distant emissions, perhaps out of warmed tundra or tropical wetlands.(57) | |
| Meanwhile, in the late 1980s and early 1990s, improved carbon-14 techniques gave the first accurate dates for sediments containing pollen and other carbon-bearing materials at locations ranging from Japan to Tierra del Fuego. Reliable dates finally allowed correlation of many geological records with the Greenland ice. The results added more evidence that the Younger Dryas events had affected climates, one way or another, all around the globe. The extent and nature of the perturbation was controversial. (It was not until around 2010 that scientists firmly established that the cooling in the Younger Dryas had been restricted to the Northern Hemisphere — the change in ocean circulation had redistributed heat, actually speeding up warming in the Southern Hemisphere.) But scientists were increasingly persuaded that abrupt climate shifts could have global scope, even if they affected different places differently — colder here and warmer there, wetter here and drier there.(57a) |
<=>The oceans |
| Could such drastic variations happen not only during glacial times, but also in warm interglacial periods like the present? That was the most interesting question in 1992, as the European drillers penetrated clear through the last glacial epoch to the preceding "Eemian" period, more than 100,000 years back — a time similar to the current one or perhaps slightly warmer (the world would in fact climb to Eemian temperatures by the early 2020s). Ominously, Dansgaard and his colleagues saw 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 "recent climate stability," Dansgaard warned, "may be the exception rather than the rule," which raised the question whether our climate "will remain stable in spite of the growing atmospheric pollution." Published in Nature in 1993, this would become one of the most widely cited of all climate papers. The ice core data, as Science magazine reported, "shattered" the standard picture of benign, equable interglacials.(58) |
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| Comparison of the two groups' cores, however, gave divergent results. Evidently Dansgaard's measurements, made near the bottom of the core, were distorted by ice flow that stirred together layers from warm and cold periods. Again scientists had benefited from drilling parallel cores. But this time the lesson, valuable if unwelcome, was that they must do more work. (Antarctic cores could not help. Little snow falls there, so the layers of ice were too thin and squashed together to reveal rapid variations.)(59) |
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| Yet for the evolution of how scientists thought about the climate system, one might say that the ice had been broken. Seeing evidence of swift and severe shifts in deep-sea cores, scientists could not dismiss ice core discontinuities as some kind of accidental noise. People recalled that the present climate system was certainly subject to abrupt regional shifts, like the drought revealed by Bryson's group that had devastated native North American cultures. Persuasive new geological evidence blamed extreme prolonged droughts for the downfall of ancient Mayan and Mesopotamian civilizations as well.(60) |
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| At least no climate variation of Younger Dryas magnitude and scope had happened in recorded history. But the European and American researchers warned 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." More broadly, a landmark 2001 study published a temperature record for the entire past 65 million years, and it was far from smooth. "It now appears," the authors concluded, "that extreme aberrations in global climate can arise through a number of mechanisms."(61) | |
| The Fragile Atlantic Circulation TOP
OF PAGE This section gives a short overview of research on the risk of rapid change in the North Atlantic Ocean's circulation. For a more complete history and references, see the essay on Ocean Currents and Climate |
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| Among the most worrisome events in the glacial period preceding the present warm period were the repeated Dansgaard-Oeschger climate fluctuations — for example, the abrupt warming detected in a Greenland core mentioned above, 9° within a few years followed by slower but severe cooling. Scientists will doubt even the best set of data if they have no theory to explain it, but at least one plausible explanation was at hand. The Dansgaard-Oeschger events might have been connected to a transition of the entire North Atlantic Ocean’s circulation pattern, along the lines of Broecker's explanation of the Younger Dryas. People came up with various proposals for things that might have triggered a switch. The most prominent suggestion was a collapse of an Arctic ice sheet an "iceberg armada" into the North Atlantic as far south as Spain. Layers of pebbles on the sea floor far from land, which must have been rafted by icebergs, gave solid evidence of such "Heinrich events," and some of them corresponded to the Dansgaard-Oeschger fluctuations. There had been dozens of these events during the last glacial period, and a few more (called "Bond events") during the millennia as the climate warmed up |
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| Concern that the North Atlantic circulation could shut down was reinforced in the late 1990s by investigation of an "8.2kya event" — an abrupt, severe, century-long North Atlantic cold spell some 8,000 years ago. It resembled the Younger Dryas, but shorter and weaker. Once again, climate changes had spread beyond the North Atlantic region as the ocean's circulation slowed down, probably triggered by a surge of fresh water from the final collapse of the Canadian ice sheet. Apparently the circulation had an inherent weakness. | |
| Did the same type of instability exist today, with no ice sheet covering Canada? There was suggestive evidence that abrupt flips of the North Atlantic circulation had in fact happened long ago during previous periods of warmth between ice ages. "There is surely a possibility," Broecker wrote in 1997, "that the ongoing buildup of greenhouse gases might trigger yet another of these ocean reorganizations."(62) |
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| The media picked up the dramatic image of Europe returning to the frigid conditions of the Younger Dryas. Global warming could bring on a new Ice Age almost overnight! Broecker and others hastened to explain that with such massive volumes of water in ponderous motion, any change would take at least several decades to play out — but a transformation at that rate was still something to worry about. Increasing attention went to what was coming to be named the Atlantic Meridional Overturning Circulation (AMOC) — part of the global "conveyor belt," as Broecker called it, that brought heat to the North Atlantic on the surface and back southward underneath. By now the circulation had been traced onward to the Southern Ocean around Antarctica and into the Pacific, affecting every corner of the globe. |
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| The AMOC might in fact be slowing, according to a troubling 2005 announcement of observations that the amount of heat carried southward by the Atlantic circulation had decreased by as much as 30% since the 1950s. However, the observational record was skimpy. The system was so variable from year to year, and even from decade to decade, that it would take prolonged and dedicated observations to separate long-term changes related to global warming from normal random or cyclic fluctuations. Most climate scientists believed that the circulation was very unlikely to collapse in the 21st century, and not likely to collapse later, "although," some admitted, “the possibility cannot be entirely excluded." |
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| Computer modeling teams had a hard time simulating the AMOC. Even the most elaborate models were unable to reproduce the actual Younger Dryas shutdown. On the other hand, some teams were able to simulate abrupt ocean circulation shifts outside of glacial times, with rapid and severe climate change. In 2020 a group studying seabed sediments were surprised to see big, chaotic changes during past epochs scarcely warmer than the present, confirming the hints from ice cores. "The ocean circulation system," they warned, "may be much less stable than previously thought.(63) |
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| The risk was evaluated periodically by the Intergovernmental Panel on Climate Change (IPCC), tasked by the world's governments with reporting the consensus of experts. In 2007 the panel's experts had agreed that there would probably be a gradual change in the Atlantic Ocean circulation which might affect fisheries, but they believed a dangerous abrupt shift within the century was "very unlikely". By 2021, however, the IPCC experts had only "medium confidence"that there would be no collapse before 2100. That was hardly reassuring, and confidence continued to drop. Just two years later an expert called the IPCC's evaluation "outdated," based as it was on models that during construction had been adjusted to be stable. | |
| Several computer modeling teams attacked the question of an AMOC slowdown with massive runs on the most advanced supercomputers, deploying models that incorporated an ever wider variety of mechanisms. The results remained ambiguous. Different and sometimes idiosyncratic approaches gave results that ranged from confidence that the AMOC was on the verge of a collapse to confidence that nothing of the kind could happen in the present century. (Prospects for the 22nd century were more consistently ominous, but most studies stopped at 2100.) Observations by arrays of sensors in the oceans showed that the circulation had been slowing, apparently more than at any time for centuries. But experts were reluctant to declare that the change was due to global warming, rather than a normal fluctuation which might soon reverse. | |
| There was one significant new observation: a persistent "cold blob" that appeared around 2015 in the waters south of Greenland. That might be evidence that the circulation bringing heat northward was indeed slowing. Or the blob might come from torrents of fresh water now flooding off the melting Greenland ice cap, accompanied by a discharge of icebergs from disintegrating glaciers. The influx of fresh water was bound to inhibit the salty water around Iceland from sinking into the depths, and thus would slow the AMOC — severely altering the climate of Europe and beyond. The risk that global warming would impose an irreversible and catastrophic circulation collapse was uncertain, but undoubtedly rising along with the rise of greenhouse gases.(64) |
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| More Pathways to Catastrophe TOP OF PAGE | |
| In the early 1990s, geologists had found that titanic emissions of greenhouse gases were a likely cause of a spectacular warming that happened 56 million years ago (the "Paleocene-Eocene Thermal Maximum," PETM). At any rate something back then had radically changed climate, with a CO2 level much higher than at present, global heating, and an abrupt change of the deep ocean circulation. The shock to ecosystems brought extinctions and reorganizations so massive that geologists called it the start of a new geological era. Where did the greenhouse gases come from? A leading suspect was clathrates — a kind of ice formed under cold and pressure, found frozen in layers spread through sea floor muds. Clathrates might contain more methane and other carbon compounds than all the world's coal and oil. Studies suggested that warming of the oceans could cause a clathrate deposit to disintegrate in a landslide-like chain reaction, venting a great belch of methane and CO2 into the atmosphere. | = Milestone <=Other gases
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| During the PETM the rise in temperature had proceeded gradually over tens of thousands of years, "rapid" only to a geologist. But the emissions had come stepwise, each step spanning perhaps a few centuries. Similar outbursts showed up at other points in the distant past. Whatever had happened then, it seemed possible that global warming might provoke clathrates to vent gases into the atmosphere at a rate fast enough to bring serious additional warming within a human lifetime, melting still more of the clathrates — a feedback loop that one scientist called "the clathrate gun." The idea sounded like science fiction (indeed science fiction writers used it), spurring widespread controversy and research. |
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| There were other ideas for what caused the PETM (a comet? a reorganization of the global ocean circulation?). In 2004 a group proposed that an eruption of magma into carbon-rich strata under the Atlantic had caused "an explosive release of methane." In 2007 another group offered evidence that the main cause of the disaster was colossal volcanic emissions of CO2 — although there seemed to have been a "massive release of seabed methane" as well. Prolonged arguments settled by 2020 on a burst of volcanism that brought up deeply buried carbon, and perhaps initiated additional carbon emissions from... somewhere. Whatever the exact mechanism, the event indicated (as one group put it in 2021) "the existence of 'tipping points' in the Earth system, which can trigger release of additional carbon reservoirs and drive Earth's climate into a hotter state."(65) | |
| For the sea floor "clathrate gun" in particular, further research inspired by the threat turned out to be reassuring. For one thing, it should take thousands of years for heat to diffuse into the deep sediments where the bulk of clathrates lurk. The idea that greenhouse gas emissions might cause cataclysmic seabed outbursts looked like speculation about a distant future. Even that worry faded after an oil well blew out deep under the Gulf of Mexico in 2010. Along with a devastating oil spill, a great plume of methane bubbled up. All the gas dissolved into the seawater and was oxidized by bacteria before it could reach the atmosphere. Presumably the same would happen to gas released in a clathrate eruption. |
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| Concern persisted, however, for the geology of the clathrate deposits held many mysteries. For example, surveys turned up regions on the sea floor with swarms of giant craters as much as a kilometer across. Had blowouts during past climate crises released methane in amounts massive enough to reach the atmosphere? Worse, in the early 2020s methane was observed bubbling all the way up to the surface in the shallows of frigid polar seas, That suggested, the observers warned, "a potential for rapid transfer of methane to the atmosphere" as the seas warmed. Overall, nobody could be sure whether the reservoirs of methane hidden in the seabed might emerge to accelerate global warming soon, or only in future centuries, or never.(65a) | |
| There was another huge and possibly unstable reservoir of clathrates, along with other forms of carbon: the world's permafrost. The frozen soils, often hundreds of feet deep, store trillions of tons of carbon, comparable to the world's coal deposits. Some experts suspected that the release of some of this carbon had played a role in the PETM, and also in the abrupt warming that ended the Younger Dryas and perhaps other climate catastrophes farther in the past. A closer look was only partly reassuring. An isotope analysis of the methane shifts during the Younger Dryas found that the event mostly involved young carbon, probably from wetlands, rather than fossil carbon from deep in ancient tundra. But a massive release from warming wetlands could be adanger too.(66) | |
| Around 2000 an altogether different mechanism for rapid change showed up in improved observations of Arctic and Antarctic regions. Views from new satellites, plus vigorous programs of precise measurements from airplanes and on the ground, showed that even the largest glaciers could quickly change their speed of travel, while entire floating 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." The accepted wisdom, he explained, "lacking modern observational capabilities, was largely based on 'steady-state' assumptions."(67) Now the possibility that a swift alteration of land or sea ice could shake the global climate (and surge sea level) had to be added to all the other potential feedbacks from global warming. |
For an extended discussion of possible ice collapse see the complete essay on... |
| The new view of climate was reinforced by the last great achievement of the Soviet Union, an ice core drilled with French collaboration at Vostok in Antarctica. By the end of the 1990s their record reached back through nearly four complete glacial-interglacial cycles — and drastic temperature jumps 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 previous warm interglacial periods had certainly been subject to many severe swings of temperature, each 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 (see above). The entire rise of human agriculture and civilization had taken place during a period of exceptionally equable climate — a lucky anomaly. | |
| Paleoclimatologist William Ruddiman argued that the stability was not a coincidence. He amassed convincing evidence that the increase of human population with rice paddies, deforestation, and proliferation of livestock, had added a lot of methane and CO2 to the atmosphere — indeed enough to hold back the gradual cooling that the ice record showed in every other ice-age cycle soon after the temperature peaked. The temperature record of the most recent two centuries in particular happened to show even more unusual stability, compared with what several kinds of new evidence were revealing about severe variations in earlier millennia.(68) | |
| A Paradigm Shift TOP OF PAGE | |
| 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."(69) | |
| The first strong consensus statement had come already 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)."(70) The report’s authors did not emphasize the point, however, and the press seldom mentioned it. |
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| Despite the profound implications of this new viewpoint, hardly anyone rose to dispute it. Not only for climate but for many other complex systems, scientists had come to accept that a small, even random, event could trigger sweeping changes. 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.(71) Policy-makers and the public lagged even farther behind in grasping what the new scientific understanding could mean. |
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| Within a few years that changed. Scientists' attention focused on Antarctic and Arctic ice changes. New evidence and theories suggested that the Greenland and West Antarctic ice sheets might melt much faster than had been suspected, bringing a serious sea-level rise within the next few centuries — or even sooner. "Every time we make a big discovery in Greenland," one NASA researcher lamented, "we find out that the probability of a really fast collapse of the Greenland ice sheet is higher than we previously expected." West Antarctica was starting to look even more unstable.(71a) | |
| Still more alarming, to scientists and the public alike, were startling changes in "before and after" satellite pictures of sea ice in the Arctic. The ice pack was getting thinner and shrinking. "At the present rate," one group of scientists reported, "a summer ice-free Arctic Ocean within a century is a real possibility." That had last been seen millions of years ago, an epoch with a sea level roughly 25 meters above the present. As the Arctic seas opened to the winds there would be profound effects on weather far to southward. The feedback loop foreseen by Budyko a generation earlier was underway (see above). And the sea ice was dwindling faster than modern computer models predicted. Reviewing the familiar feedback where less ice and snow meant more sunlight absorbed, one group commented dryly that approaching some critical point "may produce unexpected system responses."(71b) |
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| Media ranging from science magazines to movies reflected the new science in their distorted mirrors, offering scenarios of a climate that could change to an entirely different state within a few decades or even, according to Hollywood, a few weeks. Around 2005 the term "tipping point" appeared in scientific and news discussions of changes that, once begun, could not be stopped. Some preferred the term "critical threshold" for transformations that might not be rapid, stretching perhaps over centuries, but that would be irreversible once they passed a certain point. On the other hand, some feedback mechanisms might not end with an irreversible tipping over, but would simply amplify whatever change was underway. If we could get the planet to a state where it was not absorbing more energy from the Sun than it was emitting, such feedbacks could reverse and help cooling. Teams set to work to pin down just what thresholds might exist, at what temperature a point of no return might be passed, and how the approach to a transition might be detected..(72*) |
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| Some of the most worrisome thresholds arose from biological and other feedbacks in the carbon cycle. These were expected to develop gradually, not as abrupt "tipping points," but might eventually be irreversible. Although data were spotty (mostly short-term studies of only a few of the countless biological systems), results in the early 2000s were discouraging. More likely than not, as soils got warmer they were releasing additional CO2, methane and other greenhouse gases. A rise in wildfires, accelerated by heat waves, emitted further carbon while degrading the ability of vegetation to take it up. The system of carbon uptake and release by the world's forests, in particular, was so poorly understood that scientists admitted forests held a "potential for major abrupt change."(73*) |
<=>Other gases |
| Arguably the most dangerous carbon storage system was the vast Arctic tundra, where researchers from Canada to Siberia saw methane bubbling out of lakes before their eyes. Permafrost was thawing and emissions climbing much faster than anyone had predicted. A Russian researcher called it an "ecological landslide that
is probably irreversible and is undoubtedly connected to climatic warming." In 2022 a group calculated that a tipping point for a collapse of boreal permafrost might possibly be passed at a global rise of 3°C above the pre-industrial temperature and was likely to be passed around 4°C. The collapse would probably take half a century or more but could conceivably be faster.(73a*) For a full discussion of the history of research on carbon emissions from tundra and other wetlands see the essay Biosphere: How Life Alters Climate. |
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| The good news was, nobody had yet found a mechanism that, outside Ice Age conditions, could plausibly bring a tremendous global climate change faster than over several decades. For example, a thorough review of research reached the not entirely reassuring conclusion that "increased permafrost carbon emissions in a warming climate are more likely to be gradual and sustained rather than abrupt and massive."(73b) The bad news was that if "rapid" meant "within my lifetime," then rapid change was visibly underway. Each ten-year period since the 1970s had been noticeably warmer on average than the preceding ten years, a global change far swifter than anything recorded in human history. And the feedbacks long anticipated were kicking in one by one. As the oceans warmed, seawater was measurably slower at taking up CO2. Uptake by plants and soils was also lagging or even reversing into net emission of CO2 and methane from tundra, wetlands and wildfires, while humanity's production of greenhouse gases continued to climb relentlessly. |
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| In 2007 experts and the public were alarmed to see the summer Arctic ice pack shrink more swiftly than ever before. In the next few years it recovered slightly, but the long-term trend soon resumed, setting new record lows every few years. It seemed that much of the ice pack could give way to open ocean in summer within just a few decades. In a 2013 study, a National Academy of Sciences panel called the dwindling of Arctic ice an already visible "abrupt" climate change, likely to disrupt weather patterns around the Northern Hemisphere. The panel also emphasized that a gradual temperature rise could have abrupt impacts as critical thresholds were crossed, for example in the extinction of species or in agriculture.(74) | |
| Computer models were generally reassuring — but how reliable were they for such matters? The models.adjusted until they were mostly stable, were unable (as Broecker had remarked decades earlier) to reproduce the sudden shifts actually seen in Ice Age records. One sobering indication was a 2008 report on a new and superbly detailed Greenland ice core. The layers displayed radical changes, the temperature stepping as much as 4°C in a single year. It was not only the air temperature in Greenland that had lurched, but also the amount of dust brought thousands of miles by changing winds, indicating that the entire atmospheric circulation had gone through a massive reorganization from one year to the next. That matched what Broecker had hypothesized half a century earlier: the climate system lurching back and forth between two states, interglacial and glacial. "Neither the magnitude of such shifts nor their abruptness," the team warned, “is currently captured by state-of-the-art climate models." As an expert on past climates noted a few years later, even the best models were "largely untested against actual occurrences of abrupt change. It is a huge leap of faith to assume that simulations of the coming century with these models will provide reliable warning of sudden, catastrophic events."(75) |
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| In its 2001 report the IPCC had briefly mentioned "large-scale discontinuities" in the climate system. They estimated a "moderate" risk that triggering something bad would begin if global warming reached 4°C above the 19th-century level, and a "high" risk starting around 5–6°. Over the next decades scientific opinion turned even gloomier. For example, a widely cited 2008 study identified no less than six elements of the climate system that "could surprise us by exhibiting a nearby tipping point," and worried that "society may be lulled into a false sense of security by smooth projections of global change." In its 2015 report, the IPCC lowered the thresholds to 2° for moderate risk and 4° for high risk. Among other things, scientists now understood that it was not enough to calculate each type of change separately. One type of damage could interact with another type, multiplying the threat. |
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| Computer modelers confirmed that there were many routes to a tipping point. And scientists kept turning up more possible mechanisms for feedbacks that could accelerate warming. Concern spread in the research community as evidence accumulated that there might remain unknown critical thresholds, and the known ones might be passed at lower temperatures than had been expected. In 2018 the IPCC issued a major interim report that saw a risk anywhere above 1.5°C. That meant we could conceivably pass some dangerous threshold before mid-century.(76) | |
| In short, if humanity did not quickly rein in its emissions, we could suffer rapid, unpredictable, self-sustaining and irreversible changes in the entire climate system. Scientists planning the IPCC's 2021 report belatedly resolved to give much more attention than in past reports to abrupt climate change. The report mentioned several "tail" possibilities. For example, it recognized that an ice-sheet collapse might bring a 2-meter sea-level rise by 2100. For the first time the IPCC quietly admitted that beyond what it calculated to be likely, "abrupt responses and tipping points" bringing even worse disasters "cannot be ruled out." | |
| Not all of these involved abrupt changes. There was an unquantifiable risk that warming would set in motion slow but irreversible feedbacks, processes that would inexorably work themselves out over centuries of changing ice sheets, forests, soils and so forth. In Earth's interconnected physical and biological systems, passing one type of threshold could push another system, perhaps on the other side of the globe, past its own tipping point in a domino-like cascade. The end could be a radically hotter planet, severely risking "health, economies, political stability... and ultimately, the habitability of the planet for humans." Dozens of potential physical and biological feedback loops had not been fully incorporated into computer models of climate. As one of the leading IPCC authors explained, "We now consider these 'low probability, high impact' scenarios an increasingly critical part of our work."(77) | |
| One of the biggest worries — very high impact, and low probability only for the short term — was the feedbacks in the global carbon cycle. As the oceans warmed they would take up less of our emissions as a matter of simple physical chemistry, and probably also because of the deterioration of marine ecosystems that sequestered carbon. On land, disturbing reports in the early 2020s suggested that Arctic tundra, tropical forests, and wetlands were all turning from net "sinks" absorbing carbon to net sources. Carbon that had been buried for thousands of years was evaporating out in CO2 and methane. Wildfires and plants dying in droughts were also adding to the atmosphere's rising burden of CO2 . Nobody could calculate a specific global threshold when the greenhouse feedback might run away on its own; as one scientist studying permafrost emissions put it, "local tipping points would be crossed in different places at different times and warming levels." Computer studies, which mostly terminated at 2100, saw nothing abrupt or clearly irreversible. But one study that did look to centuries beyond the present — the first to incorporate long-term carbon cycle feedbacks — warned that the world might eventually heat up to an intolerable level.(78) |
|
| As for thresholds in the present century, in 2022 a team published a synthesis of observations, model results, and expert opinion for many of the proposed "Climate Tipping Points." They concluded that somewhere between a warming of 1.5 and 2°C above pre-industrial, the world was likely to pass the critical thresholds for the collapse of the Greenland Ice Sheet, the collapse of the West Antarctic Ice Sheet, and an abrupt thaw of the boreal permafrost. Indeed we might have passed some of these thresholds already. Less likely but possible above 1.5° would be passing the tipping point for a collapse of the Atlantic Ocean circulation (AMOC) and of vulnerable East Antarctic glaciers. Additional thresholds would be passed with greater warming. As for rapidity, they figured the entire disintegration of any of the colossal ice masses would probably take several millennia, but at an uneven pace with sporadic abrupt rises in sea level. The timescale for Atlantic Ocean circulation or boreal permafrost collapse might be as little as a decade or two, although more likely somewhere between half a century and a few centuries. These were only the most obvious of the potentially irreversible processes that might be triggered by the warming expected before the end of the century, unless nations made major policy changes.(79) | |
| The uncertainty was even greater, and the prospects more disturbing, if you considered the thresholds and feedbacks not individually but all together — a tangled system where everything interacted. Climate experts worried that we might be sliding toward the "hothouse" mode that had prevailed for long eras of Earth's history, a world heated to the poles. The change would play out over centuries, but that was rapid enough to collapse the ecosystems that sustain human civilization. Scientists could not say whether a point of no return might be passed in the present century. Some thought the risk was small, but none thought it negligible. As a leading expert pointed out, "you would not get on an airplane if you thought there was a five percent chance that it was going to crash."(80) | |
| See the summary of Impacts
of Climate Change. |
|
| 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? |
<=>Reflections
|
| 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.(81*) If the conventional beliefs had been the same in 1993 as in 1953 — namely, 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 changes within a decade or so, and later still, possibly within a couple of years. 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. |
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| NOTES |
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). BACK
3. Gregory (1908), p. 340. BACK
5. Brooks (1925), pp. 90-91. BACK
6. Work of Knud Jessen, Johannes Iversen (both Danes) and others, reviewed in Manten (1966). BACK
7. [note removed]
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. BACK
9. For discussion on the above points I am grateful to Ken Brown, Daniel A. Livingstone and other respondents from the QUATERNARY and PALEOCLIM listservs. BACK
10. 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). BACK
11. Palmer (1999); also Huggett (1990), pp. 119-21 and passim. BACK
12. Suess (1956), p. 357; other scientists called this an "abrupt increase:" Ewing and Donn (1956), p. 1061. BACK
13. Ericson et al. (1955); Ericson et al. (1956), quotes p. 388. BACK
15. Broecker (1957), p. V-9; see Broecker and Kunzig (2008), pp. 28-29. A revision of Brooks's 1926 book on Climate through the Ages was published in 1949, Brooks (1949) and it was popular enough to be reprinted in 1970. BACK
16. Broecker, interview by Weart, Nov. 1977, AIP. BACK
17. Broecker et al. (1960a). BACK
18. "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 (1956). BACK
19. Ives (1957), quote p. 87; see also Ives (1958); Ives (1962). BACK
20. West (1961); the abruptness of the transition was noted later by Lamb (1977), p. 80. BACK
21. Ager (1993), p. xi. BACK
22. Budyko (1962); Wilson (1964). BACK
23. Broecker (1966), pp. 299, 301. BACK
24. Broecker and Donk (1970). BACK
25. 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. BACK
26. 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). BACK
27. Bryson et al. (1970), p. 72. BACK
28. Discontinuities: Wendland and Bryson (1974); a century or two: Bryson (1974). BACK
29. Kukla and Kocí (1972), p. 383. BACK
30. Quoted in Alexander (1974), p. 94. BACK
31. Mitchell (1972), pp. 437-38. BACK
32. One author, speculating about the coming of a new ice age, pointed to "evidence of (at least) five rapid hemispheric coolings of about 5°C... 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. BACK
33. Broecker and 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. BACK
34. 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 interviews on GISP tape-recorded 1992-1994, records of Study of Multi-Institutional Collaborations, AIP. See for Camp Century Doel et al. (2016) and for the Cold-War context also Martin-Nielsen (2013). BACK
35. Kukla and Matthews (1972). BACK
. Coope (1977); already in 1970 a cooling within a thousand years or so was seen, although not remarked upon, Coope et al. (1971). BACK
36a. Also, the sluggish response of the massive polar icecaps to change smoothed the oxygen-isotope record. Broecker (1987); for these issues in general, see Palmer (1999); Huggett (1990). BACK
38. Kukla and Kukla (1974), quote p. 713; this was brought to the public, e.g. in Time (1974a). BACK
39. E.g., Flohn (1974), with reference to work by Lorenz, Budyko, and Sellers on instability; Dansgaard et al. (1972), p. 396, speculating on cooling. BACK
41. GARP (1975), from App. A (pp. 186-90) by J. Imbrie, W.S. Broecker, J.M. Mitchell, Jr., J.E. Kutzbach. Speculation: see study by Peterson et al. (2008). BACK
42. Emiliani et al. (1975), for criticism, see Science 193 (24 Sept. 1976): 1268. BACK
43. Ruddiman and McIntyre (1981); 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). BACK
44. Mercer (1969), p. 227. BACK
45.The Bretz flood theory of the scablands, proposed in the 1920s, reached full acceptance only in the 1970s. See Wikipedia, "J. Harlen Bretz,” Punctuated equilibrium was proposed in 1972, Gould (2002), pp. 1006-21 gives his version of the subsequent controversy. BACK
46. For Greenland drilling, see interviews on GISP tape-recorded 1992-1994, records of Study of Multi-Institutional Collaborations, AIP; Martin-Nielsen (2013); Mayewski and White (2002); Alley (2000). For this and especially Lonnie Thompson's high-altitude work see Bowen (2005). BACK
47. 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. BACK
48. 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. BACK
49. 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). BACK
50. Dansgaard et al. (1982), p. 1275. BACK
51. Diversion of glacial meltwater from the Mississippi to the St. Lawrence was suggested by Kennett and Shackleton (1975) and Johnson and McClure (1976); Rooth (1982) suggested this disrupted the North Atlantic circulation; Ruddiman and McIntyre (1981), p. 204, dismissed this, Circulation switch: Broecker et al. (1989) (whose "synthesis of palaeoclimatic observations invigorated the community over the next decade," according to Le Treut et al. (2007), p. 106); Broecker et al. (1990). See "Ocean currents" essay. A computer model by Bryan (1986) supported this, but another model found that a cold North Atlantic surface sufficed to bring a Younger Dryas-like climate, Rind et al. (1986). Teller et al. (2002) review the evidence for a Lake Agassiz outburst as the trigger; McManus et al. (2004) showed there was a sharp decline in ocean circulation. For the history see also Broecker (2010). In 2010 geologists found evidence for a Dryas trigger in a massive release of glacial meltwater, as Broecker had predicted, but by way of the Mackenzie River system of northern Canada rather than the St. Lawrence, Murton et al. (2010. BACK
52. 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). Wikipedia.org has a good article on "The Younger Dryas impact hypothesis," see Firestone et al. (2007) and Kennett et al. (2015). Eruptions: Flohn (1974); tropics: see Steffensen et al. (2008); Pacific Ocean: Walczak et al. (2020). BACK
53. REMOVED
54. "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 (1987), pp. 123, 126; new models: Bryan and Spelman (1985); Manabe and Stouffer (1988). BACK
55. GISP interviews, records of Study of Multi-Institutional Collaborations, AIP. Firsthand accounts are Mayewski and White (2002); Alley (2000); Dansgaard (2004) BACK
56. 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 also Mayewski (1993); five-year steps: Taylor et al. (1997); changes of 2-4°C at Greenland within a single year: Steffensen et al. (2008). Good histories are Alley (2000) and Cox, (2005), ch. 8. BACK
57. First ocean results: Karpuz et al. (1992), Lehman and Keigwin (1992). Changes in ice: Severinghaus et al. (1998), Severinghaus and Brook (1999). BACK
57a. For further references 1987-94 (including also Alaska, Ohio, New Zealand, etc.) see Broecker (1995b), pp. 306-08; for later developments, National Academy of Sciences (2002) and Lynch-Stieglitz (2004), also Cox (2005), ch. 8. Northern only: e.g., Kaplan et al. (2010), Bereiter et al. (2018). Some historical details are in Broecker (2010), ch. 4. BACK
58. Dansgaard et al. (1993); Kerr (1993). Among the ten most-cited climate papers during the early 21st century: Robert McSweeney, "Analysis: The Most 'Cited' Climate Change Papers," CarbonBrief.org July 8, 2015, online here. BACK
59. Alley et al. (1995); Chappellaz et al. (1997), comparing with Vostok cores. BACK
60. Deep-sea cores: Heinrich (1988); Bond et al. (1992), see also this note on Heinrich and this note on Bond in the essay on Ocean Currents and Climate. Maya: Hodell et al. (1995); Mesopotamia: Weiss et al. (1993); for global climate shifts throughout the postglacial period, see also deMenocal et al. (2000). BACK
61. Divergence in cores: Taylor et al. (1993), Grootes et al. (1993). 2004 work: NGRIP (2004) (North Greenland Ice Core Project members, K.K. Andersen et al.); see report by Cuffey (2004) and also Cox (2005), ch. 8. Hammer et al. (1997), Preface, "not yet faced," p. 26,315. Cenozoic aberrations: Zachos et al. (2001). BACK
62. 8.2kya event: Alley et al. (1997), Alley and Ágústsdóttir (2005); Wikipedia has a good overview.“Possibility:”Broecker (1997), p. 1588. BACK
63 ."Excluded:" Clark (2008), p. 9; "less stable:".Stocker (2020), commenting on Galaasen et al. (2020) BACK
64. IPCC (2007c), p. 16. IPCC (2021a), Technical Summary Box TS.3. "Outdated:" Stefan Rahmstorf, "New Study Suggests the Atlantic Overturning Circulation AMOC 'Is on Tipping Course'," RealClimate.org (Feb. 9, 2024), online here. Examples of model studies: reassuring, Bonan et al. (2025); high risk of eventual shutdown, Drijfhout et al. (2025); cold blob, Li and Liu (2025). For some other AMOC model studies see this note in essay on “Ocean currents and climate/” BACK
Clathrate image: methane hydrate, a type of clathrate, exists in huge quantities beneath
the sea floor. When the pressure is removed and it warms, this ice-like
substance will emit the trapped gas. Photo: Gary Klinkhammer, Oregon State
University. BACK
BACK to "Other Greenhouse Gases" essay
65.Appenzeller (1991); Kennett and Stott (1991), updated with evidence from other epochs ("mounting geologic evidence for past pervasive, massive CH4 releases from the marine sediment reservoir") by Kennett et al. (2000); high CO2 and hot tropical oceans: Zachos et al. (2003); other theories, Higgins and Schrag (2006); volcanism: Svensen et al. (2004), Storey et al. (2007), confirmed by Foster et al. (2017), Jones et al. (2019); according to Gernon et al. (2022), much of the carbon was brought up from the deep mantle. "Gun:" Kennett et al. (2003); Koch et al. (1992); Dickens et al. (1995); Norris and Röhl (1999); Katz et al. (1999); Nunes and Norris (2006); an overview is Kunzig (2004). Science fiction: e.g., in passing in the award-winning Robinson (1994). Harvey and Huang (1995) estimated clathrates could bring at worst a 10-25% increase in warming. Outbursts: Dickens (2003); "massive release:" Gehler et al. (2015); see Gutjahr et al. (2017) and Frieling et al. (2019) (with evidence for ocean methane release). "Tipping points:" Kender et al. (2021). BACK
65a. Clathrate warming would take thousands of years: Archer and Buffet (2005). Also reassuring, methane from Black Sea:oil seeps did not reach the atmosphere: McGinnis et al. (2006). Oil spill (Deepwater Horizon): Kessler et al. (2011), see review, Ruppel and Kessler (2017). Craters: Andreassen et al. (2017); see Davies et al. (2024) for another speculative release mechanism. Shallow polar seas: Seabrook et al. (2025), see also Skarke et al. (2014). BACK
66.. Younger Dryas: Nisbet (1990b); Nisbet (1992); far past (ending "snowball Earth" episodes): Kennedy et al. (2008). Young carbon: Petrenko et al. (2017). BACK
67. Rignot and Thomas (2002), p. 1505. BACK
68. "longest stable...": Petit et al. (1999), p. 434. On Ruddiman see footnote in the essay on the Biosphere. BACK
69. National Academy of Sciences (2002), p. 16, see also pp. 1, 119, 121. BACK
70. IPCC (1996a), p. 7. BACK
71. National Academy of Sciences (2002), p. 121. BACK
71a. Josh Willis quoted in Mike Damanskis, "NASA Is Flying Over Greenland to Predict the Future of Our Coastlines," Earther.com (Jan. 5, 2017), online here. BACK
71b. Old data from submarines (some of it released on the initiative of Vice President Gore) revealed thinning of ice, Rothrock et al. (1999); for sea ice extent, Overpeck et al. (2005), Lindsay and Zhang (2005), Stroeve et al. (2005). Weather effects: Francis and Hunter (2006). Computer projection of an ice-free Arctic by Holland et al. (2006) were widely publicized. Ice more sensitive to warming than models predicted: Stroeve et al. (2007). BACK
72. The seminal paper was Lenton et al. (2008), On the "tipping point" concept see this note in the essay on The Public and Climate. "Critical threshold:" Kopp et al. (2016b); J. Hansen pointed out how the term "tipping point" is often misused. BACK
73. For example, Best (2006) speculated that increased organic sediments sent down rivers due to deforestation, factory farming, etc., once buried in the seabed, will be converted to methane by bacteria, perhaps seriously adding to greenhouse gas emissions. Another potential threat was a dieback of the Amazon rainforest. It was the IPCC's conclusion that little was understood about the "potential for major abrupt change... in the uptake and storage of carbon by terrestrial systems," Randall et al. (2007), p. 642
For an early general review of possible abrupt changes in the climate system see Alley et al. (2003). Carbon cycle: e.g., Bellamy et al. (2005), Heath et al. (2005), Govindasamy et al. (2005); ocean biological systems in general had instabilities, e.g., Hsieh et al. (2005). BACK
73a. Tundra "tipping points" suggested i.a. by Foley (2005). Russian (Sergei Kirpotin) quoted by Pearce (2005a), along with a report that "the permafrost of western Siberia is turning into a mass of shallow lakes as the ground melts," lakes were expanding on the North Slope of Alaska, Walter et al. (2006) had found methane hotspots in eastern Siberia where the bubbling gas kept the surface from freezing in winter, etc. See also Lawrence and Slater (2005). Also, melting of permafrost allowed dark shrubs to spread, which would increase local heating, Chapin et al. (2005). Also, more wildfires as the Arctic warmed could bring "exponentially increasing" carbon emissions from the fires themselves and from the thawing of exposed permafrost, Descals et al. (2022), see also Schädel et al. (2026). Permafrost "thawing much more quickly than models have predicted:" Turetsky et al. (2019). Evidence from the past suggested that a little warming could cause extensive melting of permafrost: Vaks et al. (2013), and indicated "a sensitive trigger for a threshold-like permafrost climate change feedback," Martens et al. (2020. Threshold calculation: Armstrong McKay et al. (2022)..BACK
73b. Schuur et al. (2015). BACK
74. Latest data are available from the US National Snow and Ice Data Center. National Research Council (2013). BACK
75. Steffensen et al. (2008). Valdes (2011), see commentary in Nature 486: 183-84 (2012). BACK
76. Note the "Burning embers" diagram in IPCC (2001b), p. 11; see Zommers et al. (2020). Modelers: Wunderling et al. (2021). "Lulled...could surprise:" Lenton et al. (2008), p. 1792, see Lenton et al. (2019). IPCC (2018a), chap. 3. BACK
77. IPCC (2021b), C.3.2 and Fig. SPM.8. "Domino-like cascade," "habitability:" Steffen et al. (2018); for an example of "teleconnection" (Amazon rainforest and ice sheets) see Liu et al. (2023). Dozens not incorporated: Ripple et al. (2023).. “Critical part:” Joëlle Gergis, “We Are Seeing the Very Worst of Our Scientific Predictions Come to Pass in These Bushfires,” The Guardian, Jan. 2, 2020, online here. BACK
78. Local tipping points: Nitzbon et al. (2024);, see also Brovkin et al. (2025); intolerable level: Kaufhold et al. (2025). BACK
79. Armstrong McKay et al. (2022). Another major survey, describing 14 Earth System tipping points: Lenton et al. (2023). BACK
80. Hothouse: Ripple et al. (2026). "Crash:" V. Ramanathan, see this note in essay on “Impacts of Climate Change. BACK
81. 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 PALEOCLIM listservs. BACK
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