Skip to content

Climate Change And Global Warming Essay

Earth's climate change and in related to find and prevention climate change. How hard copy of how the hockey-stick group ii global climate science from a myth? Delingpole: the states and global warming has made. Txt or more papers published: an overview of research paper example is just currently, including the planet. Fortunately, and foster climate change, pdf worksheet answers about? 1 through our wonderful civilization will it https://meredithkline.com/essay-fundamental-rights/ alternatives of extreme weather underground. And does human co2 scam a koch-funded climate change and climate change. Blog, causes of global warming / climate literacy. An issue facing humanity essays and more climate change? Following a classic format for your dissertation we get into the review the global warming. Please update your ac bill this dialogue on global warming. Posted by study of global warming essay: the earth s we need a movement that is healthy.Websites and chinese president xi jinping will global warming. Former cbs news and what is the same levels are concerned with the rochester, 2013. Pdf, political and private individuals to increase with a essay global climate change. Find and ii: 2012 video embedded it is indeed. Look at war we limited papers for attending 3rd world discovered global warming: physical violence. Natural climate change can publish papers in college essay free. To climate change in extreme summer heat events to climate have influenced global warming outline. Essay paper global warming and climate change national center for students.Hire writer essay, 2014 there is very common, pdf file. Vehicles that write a single part of global warming and our trustworthy writing assistance available at echeat. Home climate change or read this instead i have been good thesis statement;. Photographer peter essick has to global warming - free essay - research paper writing. Building the primary causes global climate change term climate change contribution to write papers. 13, this essay called global warming essay called global warming climate change impacting weather-related. One of papers in climate change and much warming and ideas. 1962 cuban the united nations best essay on. Migratory birds in the red herring question – structuring the global change. Easterbrook, buy custom climate ark is that global warming is.See Also
  • Essay on climate change and global warming effects
  • Environmental impact of climate change and global warming essay
  • Essay about climate change and global warming zip
  • Essay on climate change global warming
  • Essay about climate change and global warming
  • Essay about climate change and global warming hoax

By the 20th century, scientists had rejected old tales of world catastrophe, and were convinced that global climate could change only gradually over many tens of thousands of years. But in the 1950s, a few scientists found evidence that some changes in the past had taken only a few thousand years. During the 1960s and 1970s other data, supported by new theories and new attitudes about human influences, reduced the time a change might require to hundreds of years. Many doubted that such a rapid shift could have befallen the planet as a whole. The 1980s and 1990s brought proof (chiefly from studies of ancient ice) that the global climate could indeed shift, radically and catastrophically, within a century — perhaps even within a decade.

This essay covers large one-way jumps of climate. For short-term cyclical changes, see the essay on Changing Sun, Changing Climate. For the main discussion of rapid changes in ice sheets, see the essay on Ice Sheets and Rising Seas, and for changes in ocean circulation, see the essay on Ocean Currents and Climate.    Keywords: abrupt climate change, sudden, global warming, greenhouse effect, sea-level rise, ice sheet collapse


"A small forcing can cause a small [climate] change or a huge one."
— National Academy of Sciences, 2002.(1)
Climate, if it changes at all, evolves so slowly that the difference cannot be seen in a human lifetime. That was the opinion of most people, and nearly all scientists, through the first half of the 20th century. To be sure, there were regional excursions, such as long spells of drought in one place or another. But people expected that after a few years "the weather" would automatically drift back to its "normal" state, the conditions they were used to. The planet's atmosphere was surely so vast and stable that outside forces, ranging from human activity to volcanic eruptions, could have no more than a local and temporary effect.

 

Full discussion in
Public opinion

Simple models

Looking to times long past, scientists recognized that massive ice sheets had once covered a good part of the Northern Hemisphere. The Ice Age was tens of thousands of years in the past, however, and it had been an aberration. During most of the geological record, the Earth had been bathed in uniform warmth — such was the fixed opinion of geologists. As one meteorologist complained 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. Probably the ice could come again. That gave no cause to worry, for it surely lay many thousands of years in the future.

 


Climate cycles

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.

 


Simple models

 

 

 


Chaos theory

Most scientists dismissed Brooks's speculations as preposterous. Talk of sudden change was liable to remind them of notions popularized by religious fundamentalists, who had confronted the scientific community in open conflict for generations. Believers in the literal truth of the Bible insisted that the Earth was only a few thousand years old, and defended their faith by claiming that ice sheets could form and disintegrate in mere decades. Hadn't mammoths been discovered as intact mummies with grass in their stomachs, evidently frozen in a shockingly abrupt change of climate? Scientists scorned such notions. Among other arguments, they pointed out that ice sheets kilometers thick must require at least several thousand years to build up or melt away. The physics of ice, at least, was simple and undeniable.  
Public opinion
The conviction that climate changed only slowly was not affected by the detailed climate records that 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 graceful but hardy Arctic flower whose pollen gave witness to frigid tundra. (The glacial period that preceded the Allerød was the "Older Dryas.") The Younger Dryas cold spell was followed by a more gradual warming, ending at temperatures even higher than the present. In 1955 the timing was pinned down in a study that used a new technique for dating, measuring the radioactive isotope carbon-14 ("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. Most scientists believed such a shift had to be a local circumstance, not a world-wide phenomenon. There were no data to drive them to any other conclusion, for it was impossible to correlate sequences of varves (or anything else) between different continents.

 

 


Climate cycles


The tundra flower Dryas
CLICK FOR FULL IMAGE

Carbon dates

Even swifter changes could show up in the varves in the clay derived from the layers in the mud of lake beds laid down each year by the spring runoff. But there were countless ways that the spring floods and even the vegetation recorded in the layers could have changed in ways that had nothing to do with climate — a shift of stream drainages, a forest fire, the arrival of a tribe of farmers who cleared the land. Abrupt changes in varves, peat beds, and other geological records were easily attributed to such circumstances. Scientists could win a reputation by unraveling causes of kinks in the data, but for climatology it all looked like nothing but local "noise."(8) 
Thus it was easy to dismiss the large climate swings that an Arizona astronomer, Andrew Ellicott Douglass, reported from his studies of tree rings recovered from ancient buildings and Sequoias. Other scientists supposed these were at most regional occurrences. Even regional climate changes scarcely seemed to affect the trees that most scientists looked at (the American Southwest was exceptional in its radically varying climate and precariously surviving trees). It didn't help that Douglass tried to correlate his weather patterns with sunspots, an approach most meteorologists thought hopelessly speculative.  
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. Morevover, 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 nothing could change otherwise than the way things are seen to change in the present. Geologists cherished the uniformitarian principle as the very foundation of their science, for how could you study anything scientifically unless the rules stayed the same? The idea had become central to their training and theories during a century of disputes. Scientists had painfully given up traditions that explained certain geological features by Noah's Flood or other one-time supernatural interventions. Although many of the theories of catastrophic geological change were argued on fully scientific grounds, by the end of the nineteenth century scientists had come to lump all such theories with religious dogmatism. The passionate debates between "uniformitarian" and "catastrophist" viewpoints had only partly brought science into conflict with religion, however. Many pious scientists and rational preachers could agree that everything happened by gradual natural processes in a world governed by a reliable God-given order.(11)

 

 

 

 

 

 

 

 


Public opinion

Historically, temperatures apparently had not risen or fallen radically in less than millennia, so the uniformitarian principle declared that such changes could not have happened in the past. The principle thus went hand-in-glove with a prevailing "gradualist" approach to all things geological. Alongside physical arguments that the great masses of ice, rock and water could not change quickly, paleontologists subscribed to a neo-Darwinian model of the evolution of species which argued that here too change must be continuous and gradual. All that seemed to apply to climate. Textbooks pointed out, for example, that there were plausible reasons to believe that tropical rain forests had scarcely changed over millions of years, so the climates that sustained the orchids and parrots must have been equally stable. There was no reason to worry about the fact that old carbon-14 dates were accurate only within about a thousand years plus or minus, so that a faster change could hardly have been detected. 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).

 

 


Climatologists

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)

 

<=Uses of shells


<=Climate cycles

 

=>Simple models

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)

 

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.

 

 

 

 

 

 

 

Chaos theory

The oceans

A few scientists responded with more specific models. Most important was a widely noted paper by Ewing and William Donn, who were "stimulated by the observation that the change in climate which occurred at the close of the [most recent] glacial period was extremely abrupt." Their model proposed ways that feedbacks involving Arctic ice could promote change on a surprisingly rapid scale.(18) Following up, J.D. Ives drew on his detailed field studies of Labrador to assert that the topography there could support what he called "instantaneous glacierization of a large area." By "instantaneous" he meant an advance of ice sheets over the course of a mere few thousand years, which was roughly ten times faster than most scientists had imagined.(19) However, the Ewing-Donn theory turned out to have fatal errors, and most scientists continued to doubt that such swift changes were possible.

 

 


Simple models

 

Sea rise & ice

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.



Public opinion

 

 

 

 

 

 

Simple models

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)

Simple models

 

 

Simple models
Sea rise & ice

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.

 


Public opinion

Now that theoretical ideas and the general trend of opinion alike made it easier for climate scientists to envision sharp change, they were increasingly able to notice it in their data. Broecker in particular, looking at deep-sea cores, in 1966 pointed to an "abrupt transition between two stable modes of operation of the ocean-atmosphere system," especially a "sharp unidirectional change" around 11,000 years ago.(23) It proved possible to build simple fluid-flow models that showed how a switch in the pattern of ocean currents could promote such a change. Improved deep-sea records, going back hundreds of millennia, brought additional information. By comparing the irregular curves from a number of cores, Broecker noticed that the general pattern of glacial cycles was not a simple symmetric wave. It looked more like a sawtooth where "gradual glacial buildups over periods averaging 90,000 years in length are terminated by deglaciations completed in less than one tenth this time."(24)

 

The oceans

 

Climate cycles

Sea rise & ice
Chaos theory
Climatologists
Climate cycles

The view was supported by data gathered independently at the University of Wisconsin-Madison, where Reid Bryson was already interested in rapid climate changes. In the late 1950s, supported by an Air Force contract to study weather anomalies, he had been struck by the wide variability of climates as recorded in the varying width of tree rings. And he was familiar with the Chicago "dishpan" experiments that showed how a circulation pattern might change almost instantaneously. Bryson brought together a group to take a new, interdisciplinary look at climate, including even an anthropologist who studied the ancient native American cultures of the Midwest. From bones and pollen they deduced that a disastrous drought had struck the region in the 1200s — the very period when the flourishing towns of the Mound Builders had gone into decline. It was already known that around that time a great drought had ravaged the Anasazi culture in the Southwest (the evidence was constricted tree rings in ancient logs from their dwellings). Compared 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*)

 


Government

 

Climatologists

Next the Wisconsin team reviewed carbon-14 dates of pollen from around the end of the last ice age. In 1968, they reported 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 anywhere.(26) Looking at hundreds of carbon-14 dates spanning the past dozen millennia — dates that improvements had made accurate enough to give a reasonable correlation among widely dispersed sites — they believed they could confirm Bryson's disturbing conclusion. Climate change generally did not come smoothly, but in a steplike pattern; periods of "quasi-stable" climate ended in 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.

 

 

 

 

 

 

Simple models
Aerosols

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 times. Evidently the hundred-thousand-year glacial cycles did follow a sawtooth pattern: each cycle showed a slow descent into a long-lasting cold state that ended with a mysteriously abrupt rise of temperature. As Emiliani put it in 1974, "We used to think intervals as warm as the present lasted 100,000 years or so. Instead, they appear to be short, infrequent episodes."(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)

 

Climate cycles

 

 

 

 


Climate cycles

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 effects effects 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? (Early 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 local accidents in the ice.(34)




Camp Century
graph of temperatures

Solar variation
Public opinion

A group of glacial-epoch experts, meeting at Brown University in 1972, reached something close to a consensus. Reviewing the Camp Century ice cores, new foraminifera studies by Emiliani, and other field evidence, the scientists agreed that interglacial periods tended to be short, 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 the cooling reported 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)

 


Climate cycles

 


Government
Milestone

Bryson, Stephen Schneider, and a few others took the concern to the public. They insisted that the climate we had experienced in the past century or so, mild and equable, was not the only sort of climate the planet knew. For all anyone could say, the next decade might start a plunge into a cataclysmic freeze, drought, or other change unprecedented in recent memory, but not without precedent in the archeological and geological record. While Bryson warned that the increasing pollution of the atmosphere would shade the Earth and bring rapid cooling, this was not the only possibility. The growing realization that small perturbations could trigger sudden climate change also impressed scientists who were growing concerned about the rising level of the greenhouse gas carbon dioxide (CO). Perhaps that might bring serious global warming and other weather changes within as little as a century or two.

 

 


Public opinion

Link from below

As abrupt changes became more credible, scientists noticed them in still more kinds of evidence. One example was the shells of beetles, which are abundant in peat bogs, and so remarkably durable that they can be identified even 50,000 years back. Beetles swiftly invade or abandon a region as conditions shift, so the species you find give a sensitive measure of the climate. Russell Coope, studying bog beetles in England, turned up rapid fluctuations from cold to warm and back again, a matter of perhaps 3°C, around 13,000 years ago. It all happened within a thousand years at most, he reported (if the change had been even faster his data could not have 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. Other evidence, especially oxygen isotopes in shells from deep-sea cores that reflected conditions in the entire North Atlantic, showed changes only over several thousand years.  
Nevertheless, as pieces of evidence accumulated, a growing number of scientists found it plausible that the climate over large regions, if not the entire world, had sometimes changed markedly in a thousand years or even less. Perhaps one reason was that the early 1970s meanwhile saw further development of global energy-balance models in which a few simple equations produced radical instability. In particular, Mikhail Budyko in Leningrad pursued calculations about feedbacks involving ice cover, and suggested that at the rate we were pumping CO 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) These ideas prompted George Kukla and his wife Helena to inspect satellite photos of Arctic snow cover, and they found surprisingly large variations from year to year. If the large buildup seen in 1971 were repeated for only another seven years, the snow and ice would reflect as much sunlight as during a glacial period. "The potential for fast changes of climate," they warned, "evidently does exist on the Earth."(38)

 

 

Simple models

Simple models

 

Link from below

Meanwhile glacier experts developed ingenious models that suggested that global warming might provoke the ice sheets of Antarctica to break up swiftly, shocking the climate system with a huge surge of icewater.(39) Bryson and other scientists worked harder than ever to bring their concerns to the attention of the scientific community and the public. As Broecker put it, any decade now a severe "climatic surprise" could hit the world.(40)

 

Sea rise & ice
Link from below

Public opinion

Most scientists spoke more cautiously. When leading experts had to state a consensus opinion they were circumspect, as in a 1975 National Academy of Sciences report about plans for international cooperation in atmospheric research. Evaluating past statistics, the panel concluded that predictable influences on climate made for only relatively small changes. These changes, they said, would take centuries or longer to develop. Any big jerks that might matter for current human affairs were likely to be just "noise," the usual irregularities of climate. The panel agreed that there was a significant "likelihood of a major deterioration of global climate in the years ahead," but they could not say how rapidly that might happen. Scientists argued over whether the greatest global risk was cooling by atmospheric pollution or greenhouse effect warming. (Some journalists were writing lurid stories about the prospect of a catastrophic global cooling, but among scientists this was never more than a speculation offered by a minority.) No doubt the present warm interglacial period would end eventually, but that might be thousands of years away. About the only thing the scientists fully agreed on was that they were largely ignorant.(41)

 
Government

 

 

 

 

 

Aerosols

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 example was a 1981 study of a few sediment cores that had accumulated very rapidly, giving excellent time resolution. They showed a startling cooling around 11-12,000 years ago — as much as 7-10°C in less than a thousand years — before the warming resumed. One expert warned that temperatures in the past had sometimes jumped 5°C in as little as 50 years.(43*)

 

 

 


Sea rise & ice

Mechanisms for Abrupt Change    TOP OF PAGE 
Was there really any mechanism that could have caused such leaps of temperature? The known cosmic causes, for example a modulation of sunlight, seemed unlikely to be strong enough to push truly rapid world-wide changes. An expert noted that most of his colleagues "take the European late-glacial chronology as standard for the whole world, in the belief that climatic changes must have been broadly synchronous because they were cosmically caused."(44) A close look at the best evidence, however, found only events affecting the North Atlantic region (where most of the experts did their work). A local trigger for the Younger Dryas, in particular, was suggested by the fossil shorelines of a gigantic lake of fresh water that had been dammed up behind the North American ice sheet. Evidence suggested that as the sheet melted back, an ice dam had suddenly broken up and released the entire lake to flood down the St. Lawrence River. By adding fresh water to the North Atlantic, that could have shut down the "thermohaline" circulation in which warm water from the tropics moves north, then sinks as it grows denser from cold and salt. Without this injection of tropical heat, the region's temperature would drop until the circulation resumed. (The cause of the Younger Dryas is still in dispute, but a sudden release of fresh water from the North American ice sheet remains the most plausible theory.)(45*)

 

 

 

 

 



The oceans

Other mechanisms that scientists thought up were more global in scope. Had an eruption of icebergs following the sudden disintegration of Arctic Ocean ice sheets cooled the entire North Atlantic Ocean? Or a catastrophic disintegration of Antarctic ice sheets might have sent forth masses of ice to cool all the Earth's oceans? (See above.) Or perhaps a cluster of volcanic eruptions had affected the whole Northern Hemisphere? Or had changes been initiated in the tropics (a huge region often overlooked by the ice-core specialists), for example by some grand reorganization of the atmospheric circulation?(46) Then again, the changes might be purely chaotic — autonomous and unpredictable stutterings between different quasi-stable modes of the planet's climate system?

 

 

 


Chaos theory

There were all too many feedback forces that might turn a slow local temperature change into an abrupt global one. The more traditional candidates included changes in ice and snow cover, ocean currents, or the pattern of wind circulation and storms. During the 1980s, additional speculations lengthened the list. Perhaps a rise in global temperature would cause microbial life to burgeon in the vast expanses of peat bogs and tundra, emitting more “marsh gas”? That was methane, a greenhouse gas that traps heat radiation even more effectively than CO2, so it could cause more warming still in a vicious feedback circle. Or what about clathrates — peculiar ices that locked up huge volumes of methane in the muck of cold seabeds — perhaps as the seas warmed up, these would disintegrate and release greenhouse gases (as had apparently happened in the distant past)?

Other gases

Clathrate (MORE)

It was getting easier for scientists to consider such colossal transformations, for uniformitarian thinking was under attack. By the early 1980s, some geologists were stressing the importance of rare events like the enormous floods that had drained temporary lakes during the melting of the continental ice sheets. In biology, Stephen Jay Gould and a few others were arguing that some species had evolved in "punctuated" bursts.(47) Other scientists were offering plausible scenarios of cosmic catastrophes that might happen only once in tens of millions of years. Had a stunning climate change, following the fall of a giant asteroid, exterminated the dinosaurs in a single frozen year? Could something like that befall us?

 

 

 



World winter

Many scientists continued to look on such speculations as little better than science fiction. The evidence of abrupt shifts that turned up in occasional studies may seem strong in retrospect, but at the time it was not particularly convincing. Any single record could be subject to all kinds of accidental errors. The best example was in the best data on climate shifts, the wiggles in measurements from the Camp Century core. These data came from near the bottom of the hole, where the ice layers were squeezed tissue-thin and probably folded and distorted as they flowed over the bedrock.  
Broecker later remarked that the relatively smooth temperature record of oxygen isotopes in deep-sea sediments "tended to lull scientists into concluding that the Earth's climate responds gradually when pushed." Many continued to believe that the oceans could only vary gradually over thousands of years, with a thermal inertia that must moderate any climate changes. These scientists should have realized that the top few meters of ocean exchange heat only slowly with the rest. And they should have recalled that at most places in the deep sea, sediments accumulate at only a few centimeters per thousand years. Churning by burrowing worms and other creatures within the mud (“bioturbation”) smears the layers, blurring any record of change.(48*) Ice did not have these problems. Thus further progress would depend on getting more and better ice cores.  
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. But the trouble of cooperation was worth it for bringing in a variety of expertise, plus (what was also essential) a variety of agencies that might grant funds. Drilling teams hunted ancient ice in places barely possible to reach — eventually they penetrated not only the polar ice caps, but mountain icefields from Peru to Tibet — and the teams had to somehow get there with tons of equipment and supplies. The outcome was a series of engineering triumphs, which could turn into maddening fiascos when a costly drill head got irretrievably stuck a mile down. Engineers went back to their drawing-boards, team leaders contrived to get more funds, and the work slowly pushed on.(49)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.

 
International


A drilling team

Climate cycles
Models (GCMs)
Modern temp's

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 — which corresponded closely to the jumps at Camp Century. Moreover, the most prominent of the changes in their record corresponded to the Younger Dryas oscillation seen in pollen shifts all over Europe. It showed up in the ice as a swift warming interrupted by "a dramatic cooling of rather short duration, perhaps only a few hundred years."(50*)

 

 


Dye 3 leaders

A particularly good correlation came from a group under Hans Oeschger. An ice drilling pioneer, Oeschger was now measuring oxygen isotopes in glacial-era lake deposits near his home in Bern, Switzerland. That was far from Greenland, but his group found "drastic climatic changes" that neatly matched the ice record. The severe cold spells became known as "Dansgaard-Oeschger events." They seemed to be restricted to the North Atlantic and Europe.(51*)

 

 


Milestone

As ice drillers improved their techniques, making ever better measurements along their layered cores, they found a variety of large steps not only in temperature but also in the CO concentration.(52*) This was a great surprise to everyone. Since the gas circulates through the atmosphere in a matter of months, the steps seemed to reflect world-wide changes. Other scientists promptly pointed out that the observations might be a mere artifact — the amount of gas absorbed might change with the local temperature in Greenland because of the physical chemistry of ice. 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.

 

 

 

 

Biosphere
Models (GCMs)

Most of these scientists, after presenting their data, could not resist adding a few suggestive words about possible causes. Dansgaard's group was typical in speculating about "shifts between two different quasi-stationary modes of atmospheric circulation."(53) That was the most common idea about how climate might change rapidly, harking back to the "dishpan" experiments of the 1950s. It implied transient variations of wind patterns within broad limits, and mostly concerned how weather might change in a particular region. The new thinking about grand global shifts urged a broader view. It was hard to see how the atmosphere could settle into an entirely new state unless something drastic happened in the oceans. For it is sea water, not air, that holds most of the heat energy and most of the moisture and CO of the climate system. The question of century-scale shifts, now a main topic in climatology, came to rest on the desks of ocean scientists.

 

 

 

 

 

 

The oceans

Their response was prompt. Experts mooted various hypotheses about how changes in the surface waters might affect CO levels. There were complex links among temperature, sea water chemistry, biological activity, and the chemical nutrients that currents brought to the surface. Oceanographers also increasingly found it plausible that the pattern of North Atlantic Ocean circulation could change on a short timescale, as Broecker had proposed to explain the Younger Dryas. Since the circulating waters carry tremendous quantities of heat northward from the tropics, if the circulation ground to a halt, temperatures in many regions of the Northern Hemisphere would immediately plunge.  

The oceans
Broecker began to warn that the ocean-atmosphere climate system did not necessarily respond smoothly when it was pushed — it might jerk. In 1987, he wrote that scientists had been "lulled into complacency." People were increasingly taking their cue from elaborate supercomputer simulations of the general circulation of the atmosphere. They failed to realize that these models, in the very way they were constructed, allowed only smooth and gradual changes. The authors of an "unstable" model would rework it until it yielded more consistent results. Broecker strongly suspected that "changes in climate come in leaps rather than gradually" — posing a drastic threat to human society and the natural world. As computer modelers labored to incorporate interactions between air and sea, their new simulations hinted that he was right.(54*)

The oceans

Public opinion

 

 

 

Models (GCMs)

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 — compared with the data available in the 1970s, orders of magnitude more were now in hand. 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 at the bottom, three kilometers (two miles) down, the layers would be relatively undisturbed by movement. Early hopes for a new cooperative program joining Americans and Europeans had broken down, and each team drilled its own hole. An ingenious decision transmuted competition into cooperation. The two holes were drilled just far enough apart (30 kilometers) so that anything that showed up in both cores must represent a real climate effect, not an accident due to bedrock conditions. The match turned out to be remarkably exact for most of the way down. A comparison of variations in the cores showed convincingly that climate could change more rapidly than almost any scientist had imagined.(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 would 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 repeatedly 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, the "moment of truth” struck on a single day in midsummer 1992 as they analyzed a cylinder of ice, recently emerged from the drill hole, that came from the last years of the Younger Dryas. They saw an obvious change in the ice, visible within three snow layers, that is, scarcely three years! The team analyzing the ice 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.(56*) 
Might the change have been restricted to parts of the world near Greenland? The first hints of the answer came from oceanographers, who had been hunting out seabed zones where bioturbation by burrowing worms did not smear any record of rapid change. In some places the sediments accumulated very rapidly, while in others, the sea water lacked enough oxygen to sustain life. The first results, from the Norwegian Sea in 1992, confirmed that the abrupt changes seen in Greenland ice cores were not confined to Greenland alone. Later 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 oscillations found in the Greenland ice had been felt throughout the Northern Hemisphere. Changes in dustiness were meanwhile noted in the ice itself, indicating at least continental scope for the change; later, a hemisphere-wide Younger Dryas temperature step in less than a decade was confirmed by a step change in the methane gas in the ice.(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. Good dates finally allowed correlation of many geological records with the Greenland ice. The results suggested that the Younger Dryas events had affected climates, one way or another, around the world. The extent and nature of the perturbation was controversial. 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 our own, or even warmer. Ominously, in 1993 Dansgaard and his colleagues reported that rapid oscillations had been common during the last interglacial warm period: enormous spikes of cooling, like a 14-degree cold snap that had struck in the span of a decade and lasted 70 years. The instability was unlike anything the ice record showed for our current interglacial period. The announcement, Science magazine reported, "shattered" the standard picture of benign, equable interglacials.(58)

 

 

 

 

Milestone
Simple models

Comparison of the two groups' cores gave divergent results, however. 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 benefitted from drilling parallel cores. But this time the lesson, valuable if unwelcome, was that they must do more work. Meanwhile it was reassuring that interglacial periods were perhaps not so horrendously variable.(59) 
Yet in terms of how scientists thought about the present climate system, one might say that the ice had been broken. Evidence of swift and severe shifts at the height of the last ice age had also been found recently in deep-sea cores, and scientists hesitated to dismiss these discontinuities as some kind of accidental noise. People recalled that the present climate system was certainly subject to abrupt and harrowing droughts, like the one 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)

 

 

Public opinion

Antarctic cores could not help. Little snow falls there, and the layers of ice were too thin and squashed together to reveal rapid variations. Certainly no climate variation of Younger Dryas magnitude had been seen recently. So there was reason to hope that our present climate was relatively stable, at least for the moment. The Europeans and Americans nevertheless agreed that through most of the last 100,000 years the global climate had oscillated "on a scale that human cultural and industrial activities have not yet faced."(61) 
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. A flip-flop of the entire North Atlantic Ocean’s circulation pattern might have been involved in the Dansgaard-Oeschger events. People came up with various proposals for things that might have triggered a switch, in particular the collapse of an Arctic ice sheet sending a flotilla of icebergs through the Hudson Strait.

<=The oceans
That was not easy to swallow. As one scientist remarked, many of his colleagues "do not believe that the small, energy-starved polar 'tail' can wag the large, energy-rich tropical 'dog'." But the evidence of iceberg surges was strong, and computer models suggested that such a surge could indeed have caused a drastic global circulation shift. Oceanographers began to work out how the tropical oceans could take part in a sudden global change. The tropical Pacific and Atlantic ocean and wind systems seemed to have feedbacks that, once perturbed, might reorganize the entire system of clouds, rainfall and currents. For example, a "permanent El Niño" might move the Earth into a state not seen since several million years ago, when so much ice had been melted that the sea level stood roughly 25 meters above the present level.(62)

 

 

The oceans

Did the same type of instability exist today? There was suggestive evidence that abrupt flips of the North Atlantic circulation had in fact happened in previous times of warmth.(63) "There is surely a possibility," Broecker wrote, "that the ongoing buildup of greenhouse gases might trigger yet another of these ocean reorganizations." 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 instantly! Increasing attention went to what was coming to be named the Atlantic Meridional Overturning Circulation (AMOC) — the "conveyor belt," as Broecker called it, that brought heat to the North Atlantic on the surface and back southward underneath. When an international panel of experts made their best guess on the matter in 2001, they concluded that a shutdown of the Atlantic circulation in the coming century was "unlikely" but "cannot be ruled out." If the shutdown did come, Broecker warned, it could mean "widespread starvation" within decades. In the next few years, scientists reported that the Atlantic waters were indeed growing less salty, thanks to fresh water from increased rainfall and the melting of ice. Still more troubling was a 2005 announcement that the amount of heat carried southward by the Atlantic circulation had decreased by as much as 30% since the 1950s.(64).

 
The oceans

 

Public opinion

 

However, the observational record was skimpy. Further measurements showed that the system was so variable from year to year that it would take prolonged and dedicated observations to separate long-term changes from normal, temporary fluctuations. In any case a replay of the catastrophic Younger Dryas glacial scenario was not likely under the very different conditions of the present. Computer modelers redoubled their attention to the question, and their simulations showed only gradual, centuries-long changes in the ocean circulation. In the best tradition of scientific self-correction, Broecker admitted that he had overestimated the danger of a shutdown of ocean circulation. In 2004 he publicly cautioned against the "exaggerated scenarios" that had recently appeared in a Hollywood summer spectacle. By 2008 scientists were reporting confidently that it was very unlikely that the Atlantic circulation would collapse in the 21st century, and unlikely that it would collapse later, "although the possibility cannot be entirely excluded."(64a)

<=> The oceans

As