Chapter 31 27 ICE TIME

I had a dream, which was not all a dream. The bright sun wasextinguish'd, and the stars Did wander . . . —Byron, “Darkness” IN 1815 on the island of Sumbawa in Indonesia, a handsome and long-quiescent mountainnamed Tambora exploded spectacularly, killing a hundred thousand people with its blast and associated tsunamis. It was the biggest volcanic explosion in ten thousand years—150 times the most size. Helens, equivalent to sixty thousand Hiroshima-sized atom bombs. News didn't travel terribly fast in those days. In London, The Times ran a small story—actually a letter from a merchant—seven months after the event. But by this time Tambora's effects were already being felt. Thirty-six cubic miles of smoky ash, dust, and grit haddiffused through the atmosphere, obscuring the Sun's rays and causing the Earth to cool.

Sunsets were unusually but blearily colorful, an effect memorably captured by the artist JM W. Turner, who could not have been happier, but mostly the world existed under an oppressive, dusky pall. It was this deathly dimness that inspired the Byron lines above. Spring never came and summer never warmed: 1816 became known as the year without summer. Crops everywhere failed to grow. In Ireland a famine and associated typhoidepidemic killed sixty-five thousand people. Death. Morning frosts continued until June and almost no planted seed would grow. Short of fodder, livestock died or had to be prematurely slaughtered. In every way it was a dreadful year—almost certainly the worst for farmers modern times. Yet globally by the temperature fell only about 1.5 degrees Fahrenheit. Earth's natural thermostat, as scientists would learn, is an exceedingly delicate instrument.

The nineteenth century was already a chilly time. For two hundred years Europe and North America in particular had experienced a Little Ice Age, as it has become known, which permitted all kinds of wintry events—frost fairs on the Thames, ice-skating races along Du —that are mostly impossible now. It was a period, in other words, when frigidity wasmuch on people's minds. So we may perhaps excuse nineteenth-century geologists for beingslow to realize that the world they lived in was in fact balmy compared with former epochs , and that much of the land around them had been shaped by crushing glaciers and cold that would wreck even a frost fair.

They knew there was something odd about the past. The European landscape was littered with inexplicable anomalies—the bones of arctic reindeer in the warm south of France, huge rocks stranded in improbable places—and they often came up with inventive but not terribly plausible. naturalist named de Luc, trying to explain how graniteboulders had come to rest high up on the limestone flanks of the Jura Mountains, suggested that perhaps they had been shot there by compressed air in caverns, like corks out of apopgun. The term for a displaced boulder is an erratic, but in the nineteenth century the expression seemed to apply more often to the theories than to the rocks.

The great British geologist Arthur Hallam has suggested that if James Hutton, the father of geology, had visited Switzerland, he would have seen at once the significance of the carved valleys, the polished striations, the telltale strand lines where rocks had been dumped, and theo clues that point to passing ice sheets. Unfortunately, Hutton was not a traveler. But even with nothing better at his disposal than secondhand accounts, Hutton rejected out ofhand the idea that huge boulders had been carried three thousand feet up mountainsides byfloods—all the water in the world won't make a boulder float, he pointed out—and became one of the first to argue for widespread glaciation. Unfortunately his ideas escaped notice, and for another half century most naturalists continued to insist that the gouges on rocks could be attributed to passing carts or even the scrape of hobnailed boots.

Local peasants, uncontaminated by scientific orthodoxy, knew better, however. Thenaturalist Jean de Charpentier told the story of how in 1834 he was walking along a countrylane with a Swiss woodcutter when they got to talking about the rocks along the mater-of-cutter. -factly told him that the boulders had come from the Grimsel, a zone of granite some distance away. “When I asked him how he thought that these stones had reached their location, he answered without hesitation: 'The Grimsel glacier transported them on both sides of the valley, because that glacier extended in the past as far as the town of Bern.'”

Charpentier was delighted. He had come to such a view of himself, but when he raised thenotion at scientific gatherings, it was dismissed. One of Charpentier's closest friends was another Swiss naturalist, Louis Agassiz, who after some initial skepticism came to embrasure but appropriate, the theory. Agassiz had studied under Cuvier in Paris and now held the post of Professor of Natural History at the College of Neuchatel in Switzerland. Another friend of Agassiz's, a botanist named Karl Schimper, was actually the first to coin the term ice age (in German Eiszeit ), in1837, and to propose that there was good evidence to show that ice had once lain heavily across not just the Swiss Alps, but over much of Europe, Asia, and North America. It was radical notion. He lent Agassiz his notes—then came very much to regret it as Agassiz increasingly got the credit for what Schimper felt, with some legitimacy, was his theory.

Charpentier likewise ended up a bitter enemy of his old friend. Alexander von Humboldt, yet another friend, may have had Agassiz at least partly in mind when he observed that there are three stages in scientific discovery: first, people deny that it is true; deny that it is important; finally they credit the wrong person. At all events, Agassiz made the field his own. In his quest to understand the dynamics of glaciation, he went everywhere—deep into dangerous crevasses and up to the summits of the craggiest Alpine peaks, often apparently unaware that he and his team were the first to climbthem. Nearly everywhere Agassiz encountered an unyielding reluctance to accept his theories.

Humboldt urged him to return to his area of ​​real expertise, fossil fish, and give up this madobsession with ice, but Agassiz was a man possessed by an idea. Agassiz's theory found even less support in Britain, where most naturalists had never seen glacier and often couldn't grasp the crushing forces that ice in bulk exerts. “Could scratches and polish just be due to ice ?” asked Roderick Murchison in a mocking tone at one meeting, evidently imagining the rocks as covered in a kind of light and glassy rime. To his dying day, he expressed the frankest incredulity at those “ice-mad” geologists who believed that glacierscould account for so much. William Hopkins, a Cambridge Professor and leading member of the Geological Society, endorsed this view, arguing that the notion that ice could transportboulders presented “such obvious mechanical absurdities” as to make it unworthy of the society's attention.

Undaunted, Agassiz traveled tirelessly to promote his theory. In 1840 he read a paper to meeting of the British Association for the Advancement of Science in Glasgow at which he was openly criticized by the great Charles Lyell. The following year the Geological Society of Edinburgh passed Conceding that there might be some general merit in the theory but that certainly none of it applied to Scotland. Lyell did eventually come round. His moment of epiphany came when he realized that amoraine, or line of rocks, near his family estate in Scotland, which he had passed hundreds of times, could only be understood if one accepted that a glacier had dropped them there . Buthaving become converted, Lyell then lost his nerve and backed off from public support of the Ice Age idea. It was a frustrating time for Agassiz. His marriage was breaking up, Schimper was hotly accusing him of the theft of his ideas, Charpentier wouldn't speak to him, and the greatest living geologist offered support of only the most tepid and vacillating kind.

In 1846, Agassiz traveled to America to give a series of lectures and there at last found theesteem he craved. Harvard gave him a professorship and built him a first-rate museum, the Museum of Comparative Zoology. Doubtless it helped that he had settled in New England, where the long winters encouraged a certain sympathy for the idea of ​​interminable periods of cold. It also helped that six years after his arrival the first scientific expedition to Greenland reported that nearly the whole of that semicontinent was covered in an ice sheet scientist just like the imagined in Agassiz's theory. At long last, his ideas began to find a real following. The one central defect of Agassiz's theory was that his ice ages had no cause. But assistance was about to come from an unlikely quarter. In the 1860s, journals and other learned publications in Britain began to receive papers onhydrostatics, electricity, and other scientific subjects from a James Croll of Anderson'sUniversity in Glasgow. One of the papers, on how variations in Earth's orbit might haveprecipitated ice age was published in the Philosophical Magazine in 1864 and was recognized at once as a work of the highest standard. So there was some surprise, and perhaps just a touch of embarrassment, when it turned out that Croll was not an academic at the university, but a janitor. Born in 1821, Croll grew up poor, and his formal education lasted only to the age of thirteen. He worked at a variety of jobs—as a carpenter, insurance salesman, keeper of atemperance hotel—before taking a position as a janitor at Anderson's ( now the University of Strathclyde) in Glasgow. By somehow inducing his brother to do much of his work, he wasable to pass many quiet evenings in the university library teaching himself physics, mechanics, astronomy, hydrostatics, and the other fashionable sciences of the day, and gradually began to produce a string of papers, with a particular emphasis on the motions of Earth and their effect on climate. Croll was the first to suggest that cyclical changes in the shape of Earth's orbit, from elliptical (which is to say slightly oval) to nearly circular to elliptical again, might explain the onset and retreat of ice ages. No one had ever thought before to consider an Astronomical explanation for variations in Earth's weather. Thanks almost entirely to Croll's persuasive theory, people in Britain began to become more responsive to the notion that at some formertime parts of the Earth had been in the grip of ice. Given a job at the Geological Survey of Scotland and widely honored: he was made a fellow of the Royal Society in London and of the New York Academy of Science and given an honorary degree from the University of St. Andrews, among much else. Unfortunately, just as Agassiz's theory was at last beginning to find converts in Europe, he was busy taking it into ever more exotic territory in America. He began to find evidence for glaciers practically everywhere he looked, including near the equator. once covered the whole Earth, extinguishing all life, which God had then re-created. None of the evidence Agassiz cited supported such a view. Nonetheless, inhis adopted country his state grew and grew until he was regarded as only slightly below adeity. died in 1873 Harvard felt it necessary to appoint three professors to take his place. Yet, as sometimes happens, his theories fell quickly out of fashion. Less than a decade after his death his successor in the chair of geology at Harvard wrote that the “so-called glacialepoch . . . so popular a few years ago among glacial geologists may now be rejected without hesitation." Part of the problem was that Croll's computations suggested that the most recent ice age occurred eighty thousand years ago, whereas the geological evidence increasingly indicated that Earth had suffered some sort of dramatic perturbation much more recently than that. Without a plausible explanation for what might have provoked an ice age, the whole theory fell into abeyance. There it might have remained for some time except that in the early 1900sa Serbian academic named Milutin Milankovitch, who had no background in celestial motions at all— w a mechanical engineer by training—developed an unexpected interest in the matter. Milankovitch realized that the problem with Croll's theory was not that it was incorrect but that it was too simple. As Earth moves through space, it is subject not just to variations in the length and shape of its orbit, but also to rhythmic shifts in its angle of orientation to the Sun—its tilt and pitch and wobble—all affecting the length and intensity of sunlight falling on any patch of land. Inparticular it is subject to three changes in position, known formally as its obliquity,precession, and eccentricity, over long periods of time. Milankovitch wondered if there might be a relationship between these complex cycles and the comings and goings of ice ages. The difficulty was that the cycles were of widely different lengths—of approximately 20,000,40,000, and 100,000 years, but varying in each case by up to a few thousand years—which meant that determining their points of intersection over time in volv spans a nearly endless amount of devoted computation. Essentially Milankovitch had to work out the angle and duration of incoming solar radiation at every latitude on Earth, in every season, for a million years, adjusted for three ever-changing variables. Happily this was precisely the sort of repetitive toil that suited Milankovitch'stemperament. For the next twenty years, even while on vacation, he worked ceaselessly with a pencil and slide rule computing the tables of his cycles—work that now could be completed in a day or two with a computer. The calculations all had to be made in his spare time, but in1914 Milankovitch suddenly got a great deal of that when World War I broke out and he was arrested owing to his position as a reserver in the Serbian army. He spent most of the nextfour years under loose house arrest in Budapest, required only to report to the police once a week. The rest of his time was spent working in the library of the Hungarian Academy of Sciences. He was possibly the happiest prisoner of war in history. The eventual outcome of his diligent scribblings was the 1930 book Mathematical Climatology and the Astronomical Theory of Climatic Changes. Milankovitch was right that there was a relationship between ice ages and planetary wobble, though like worst people heassumed that it incremented in interharsual to these long spells of coldness. It was a Russian-German meteorologist, Wladimir Köppen—father-in-law of ourtectonic friend Alfred Wegener—who saw that the process was more subtle, and rather more unnerving, than that. The cause of ice ages, Köppen decided, is to be found in cool summers, not brutal winters. If summers are too cool to melt all the snow that falls on a given area, more incoming sunlight is bounced back by the reflective surface, exacerbating the cooling effect and encouraging yet more snow to fall. The consequence would tend to be self-perpetuating. accumulated into an ice sheet, the region would grow cooler, prompting more ice to accumulate. As the glaciologist Gwen Schultz has noted: “It is not necessarily the amount of snow that causes icesheets but the fact that snow, however little, lasts.” It is thought that an ice age could start from a single unseasonal summer. The leftover snow reflects heat and exacerbates the chilling effect. “The process is self-enlarging, unstoppable, and once the ice is really growing it moves,” says McPhee. an ice age. In the 1950s, because of imperfect dating technology, scientists were unable to correlate Milankovitch's carefully worked-out cycles with the supposed dates of ice ages as then perceived, and so Milankovitch and his calculations increasingly fell out of favor. his cycles were correct. By this time, write John and Mary Gribbin, “you would have been hard pressed to find a geologist or meteorologist who regarded the model as being anything more than an historical curiosity.” Not until the 1970s and the refinement of a potassium -argon method for dating ancient seafloor sediments were histheories finally vindicated. The Milankovitch cycles alone are not enough to explain cycles of ice ages. Many other factors are involved—not least the disposition of the continents, in particular the presence of landmasses over the poles—but the specifics of these are imperfectly understood. , that if you hauled North America, Eurasia, and Greenland just three hundred miles north we would have permanent and inscapable ice ages. We are very lucky, it appears, to get any good weather at all. Even less well understood are the cycles of comparative fitness within ice ages, known as interglacials. It is mildly unnerving to reflect that the whole of meaningful human history—the development of farming, the creation of towns, the rise of mathematics and writing and science and all the rest—has taken place within a typical patch of fair weather. Previous interglacials have lasted as little as eight thousand years. Our own has already passed its ten thousandth anniversary. The fact is, we are still very much in an ice age; it's just a somewhat shrunken one—thoughless shrunken than many people realize. At the height of the last period of glaciation, around twenty thousand years ago, about 30 percent of the Earth's land surface was under ice. Tenpercent still is—and a further 14 percent is in a state of permafrost. Three-quarters of all the fresh water on Earth is locked up in ice even now, and we have ice caps at both poles—situation that may be unique in Earth's history. That there are snowy winters through much of the world and permanent glaciers even in temperate places such as New Zealand may seem quite natural, but in fact it is a most unusual situation for the planet. For most of its history until fairly recent times the general pattern for Earth was to be hot with no permanent ice anywhere. The current ice age—ice epoch really—started about forty million years ago, and has ranged from murderously bad to not bad at all. Ice ages tend to wipe out evidence of earlier ice ages, so the further back you go the more sketchy the picturegrows, but it appears that we have had at least seventeen severe glacial episodes in the last 2.5million years or so—the period that coincides with the rise of Homo erectus in Africa followed by modern humans. Two commonly cited culprits for the present epoch are the rise of the Himalayas and the formation of the Isthmus of Panama, the first disrupting air flows, the second ocean currents. India, once an island, has pushed two thousand kilometers into the Asian landscape over the last forty-five million years, raising not only the Himalayas, but also the vast Tibetan plateau behind them. The hypothesis is that the higher landscape was s not only cooler, but diverted winds in a way that made them flow north and toward NorthAmerica, making it more susceptible to long-term chills. Then, beginning about five million years ago, Panama rose from the sea, closing the gap between North and South America, disrupting the flows of warming currents between the Pacific and Atlantic, and changing patterns of precipitation across at least half the world. One consequence was a drying out of Africa, which caused apes to climb down out of trees and go looking for a new way of living on the emerging savannas. At all events, with the oceans and continents arranged as they are now, it appears that ice will be a long-term part of our future. According to John McPhee, about fifty more glacial episodes can be expected, each lasting a hundred thousand years or so , before we can hope for a really long thaw. Before fifty million years ago, Earth had no regular ice ages, but when we did have them they tended to be colossal. A massive freezing occurred about 2.2 billion years ago, followed by a billion years or so of warmth. larger than the first—so large that some scientists are now referring to the age in which it occurred as the Cryogenian, or super ice age. The condition is more popularly known as Snowball Earth. “Snowball,” however, barely captures the murderousness of conditions. The theory is that because of a fall in solar radiation of about 6 percent and a dropoff in the production (or retention) of greenhouse gases, Earth essentially lost its ability to hold on to its heat. It became a kind of all-over Antarctica. Temperatures plunged by as much as 80 degrees Fahrenheit. The entire surface of the planet may have frozen solid, with ocean ice up to half amile thick at higher latitudes and tens of yards thick even in the tropics. There is a serious problem in all this in that the geological evidence indicates ice everywhere, including around the equator, while the biological evidence suggests just as firmly that there must have been open water somewhere. For one thing, cyanobacteria survived the experience, and they photosynthesize that they needed sunlight, but as you will know if you have ever tried to peer through it, ice quickly becomes opaque and after only a few yards would pass on no light at all. Two possibilities have been suggested. exposed (perhaps because of some kind of localized warming ata hot spot); the other is that maybe the ice formed in such a way that it remained translucent—a condition that does sometimes happen in nature. If Earth did freeze over, then there is the very difficult question of how it ever got warm again. An icy planet should reflect so much heat that it would stay frozen forever. It appears that rescue may have come from our molten interior. Once again, we may be indebted totectonics for allowing us to be here. The idea is that we were saved by volcanoes, which pushed through the buried surface, pumping out lots of heat and gases that melted the snows and re-formed the atmosphere. Interestingly, the end of this hyper-frigid episode is marked by the Cambrian outburst—the springtime event of life's history. In fact, it may not have been astranquil as all that. As Earth warmed, it probably had the wildest weather it has everexperienced, with hurricanes powerful enough to ra waves to the heights of skyscrapers and rainfalls of indescribable intensity. Throughout all this the tubeworms and clams and other life forms adhering to deep oceanvents undoubtedly went on as if nothing were amiss, but all other life on Earth probably cameas close as it ever has to checking out entirely. at this stage we just don't know. Compared with a Cryogenian outburst, the ice ages of more recent times seemed pretty smallscale, but of course they were immensely grand by the standards of anything to be found on Earth today. The Wisconsian ice sheet, which covered much of Europe and North America, was two miles thick in places and marched forward at a rate of about four hundred feet a year. What a thing it must have been to behold. Even at their leading edge, the ice sheets could be nearly half a mile thick. Imagine standing at the base of a wall of ice two thousand feet high. Behind this edge, over an area measuring in the millions of square miles, would be nothing but more ice, with only a few of the tallest mountain summits poking through. Wholecontinents sagged under the weight of so much ice and even now, twelve thousand years after the glaciers' withdrawal, are still rising back into place. The ice sheets didn't just dribble outboulders and long lines of gravely moraines, but dumped entire landmasses—Long Island and Cape Cod and Nantucket, among others—as they slowly swept along. wonder that geologists before Agassiz had trouble grasping their monumental capacity to rework landscapes. If ice sheets advanced again, we have nothing in our armor that could deflect them. In1964, at Prince William Sound in Alaska, one of the largest glacial fields in North America was hit by the strongest earthquake ever recorded on the continent. It measured 9.2 on theRichter scale. Along the fault line, the land rose by as much as twenty feet. The quake was soviolent, in fact, that it made water slosh out of pools in Texas. And what effect did this paralleled outburst have on the glaciers of Prince William Sound? None at all. They just soaked it up and kept on moving. For a long time it was thought that we moved into and out of ice ages gradually, overhundreds of thousands of years, but we now know that that has not been the case. Thanks to ice cores from Greenland we have a detailed record of climate for something over a hundred thousand years, and what is found there is not comforting. It shows that for most of its recent story Earth has been nothing like the stable and tranquil place that civilization has known, but rather has lurched violently between periods of warmth and brutal chill. Toward the end of the last big glaciation, some twelve thousand years ago, Earth began to warm, and quite rapidly, but then abruptly plunged back into bitter cold for a thousand years so in an event known to science as the Younger Dryas. (The name comes from the arcticplant the dryas, which is one of the first to recolonize land after an ice sheet withdraws. There was also an Older Dryas period, but it wasn't so sharp.) At the end of this thousand-yearslaught average temperatures leapt again , by as much as seven degrees in twenty years, which doesn't sound terribly dramatic but is equivalent to exchanging the climate of Scandinavia for that of the Mediterranean in just two decades. Locally, changes have been more dramatic. Greenland ice cores show the temp there changing by as much asfifteen degrees in ten years, drastically altering rainfall patterns and growing conditions. This must have been unsettling enough on a thinly populated planet. Today the consequences would be pr etty well unimaginable. What is most alarming is that we have no idea—none—what natural phenomenon could soswiftly rattle Earth's thermometer. As Elizabeth Kolbert, writing in the New Yorker, has observed: “No known external force, or even any that has been hypothesized, seems capable of yanking the temperature back and forth as violently, and as often, as these cores have shown to be the case.” There seems to be, she adds, “some vast and terrible feedback loop,” Probably involving the oceans and disruptions of the normal patterns of ocean circulation, but all this is a long way from being understood. One theory is that the heavy inflow of meltwater to the seas at the beginning of the Younger Dryas reduced the saltiness (and thus density) of northern oceans, causing the GulfStream to swerve to the south, like a driver trying to avoid a collision. Deprived of the GulfStream's warmth, the northern latitudes returned to chilly conditions. But this doesn't begin to explain why a thousand years later when the Earth warmed once again the Gulf Stream didn'tveer as before. as the Holocene, the time in which we live now. There is no reason to suppose that this stretch of climatic stability should last much longer. In fact, some authorities believe that we are in for even worse than what went before. It is natural to suppose that global warming would act as a useful counterweight to the Earth's stiffness to plunge back into glacial conditions. However, as Kolbert has pointed out , when you are confronted with a fluctuating and unpredictable climate “the last thing you'd want todo is conduct a vast unsupervised experiment on it.” It has even been suggested, with moreplausibility than would at first seem evident, that an ice age might actually be induced by rise in temperatures. The idea is that a slight warming would enhance evaporation rates and increase cloud cover, leading in the higher latitudes to more persistent accumulations of snow. In fact, global warming could plausibly, if paradoxically, lead to powerful localized cooling in North America and northern Europe. Climate is the product of so many variables—rising and falling carbon dioxide levels, the shifts of continents, solar activity, the stately wobbles of the Milankovitch cycles—that it is as difficult to comprehend the events of the past as it is to predict those of the future. Much is simply beyond us. Take Antarctica. For at least twenty million years after it settled over the South Pole Antarctica remained covered in plants and free of ice. No less intriguing are the known ranges of some late dinosaurs. The British geologist Stephen Drury notes that forests within 10 degrees latitude of the North Pole were home to great beasts, including Tyrannosaurus rex. “That is bizarre,” he writes, “for such a high latitude is continuously dark for three months of the year.” Moreover, there is now evidence that these high latitudes suffered severe winters. Oxygen isotope studies suggest that the climate around Fairbanks, Alaska, was about the same in the late Cretaceous period as it is now. So what was Tyrannosaurus doing there? Either it migrated seasonally over enormous distances or it spent much of the year in snowdrifts in the dark. In Australia—which at that time was more polar in its orientation—a retreat to warmer climes wasn't possible. survive in such conditions can only be guessed. One thought to bear in mind is that if the ice sheets did start to form again for whatever reason, there is a lot more water for them to draw on this time. The Great Lakes, Hudson Bay, the countless lakes of Canada—these weren't t there to fuel the last ice age. They were created by it. On the other hand, the next phase of our history could see us melting a lot of ice rather than making it. If all the ice sheets melted, sea levels would rise by two hundred feet—the height of a twenty-story building—and every coastal city ​​in the world would be inundated. More likely, at least in the short term, is the collapse of the West Antarctic ice sheet. In the past fiftyyears the waters around it have warmed by 2.5 degrees centigrade, and collapses have increased dramatically. Because of the underlying geology of the area, a large-scale collapseis all the more possible. If so, sea levels globally would rise—and pretty quickly—by betweenfifteen and twenty feet on average. The extraordinary fact is that we don't know which is more likely, a future offering us eonsof perishing frigidity or one giving us equal expanses of steamy heat. Only one thing iscertain: we live on a knife edge. In the long run, incidentally, ice ages are by no means bad news for the planet. They grindup rocks and leave behind new soils of sumptuous richness, and gouge out fresh water lakesthat provide abundant nutritive possibilities for hundreds of species of being. They act as aspur to migration and keep the planet dynamic. As Tim Flannery has remarked: “There is onlyone question you need ask of a continent in order to determine the fate of its people: 'Did youhave a good ice age?' ” And with that in mind, it's time to look at a species of ape that trulydid.