by Bill Bryson
It cannot be said too often: all life is one. That is, and I suspect will forever prove to be, the most profound true statement there is.
PART VI THE ROAD TO US
27 ICE TIME
I had a dream, which was not all a dream.
The bright sun was extinguish'd, and the stars
Did wander . . .
--Byron, "Darkness"
IN 1815 on the island of Sumbawa in Indonesia, a handsome and long-quiescent mountain named 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 size of Mount St. 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 had diffused 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 J. M. 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 typhoid epidemic killed sixty-five thousand people. In New England, the year became popularly known as Eighteen Hundred and Froze to 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 in modern times. Yet globally the temperature fell by 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 Dutch canals--that are mostly impossible now. It was a period, in other words, when frigidity was much on people's minds. So we may perhaps excuse nineteenth-century geologists for being slow 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 explanations. One French naturalist named de Luc, trying to explain how granite boulders 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 a popgun. 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 the other abundant 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 of hand the idea that huge boulders had been carried three thousand feet up mountainsides by floods--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. The naturalist Jean de Charpentier told the story of how in 1834 he was walking along a country lane with a Swiss woodcutter when they got to talking about the rocks along the roadside. The woodcutter matter-of-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 himself, but when he raised the notion 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 embrace, and eventually all 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 Neuchâtel 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 ), in 1837, 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 a 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; then they 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 climb them. 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 mad obsession 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 a 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 glaciers could 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 transport boulders 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 a 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 a resolution 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 a moraine, 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. But having 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 l
ast found the esteem 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 just like the ancient one 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 on hydrostatics, electricity, and other scientific subjects from a James Croll of Anderson's University in Glasgow. One of the papers, on how variations in Earth's orbit might have precipitated ice ages, 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 a temperance 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 was able 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 former time parts of the Earth had been in the grip of ice. When his ingenuity and aptitude were recognized, Croll was 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. Eventually he became convinced that ice had 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, in his adopted country his stature grew and grew until he was regarded as only slightly below a deity. When he died in 1873 Harvard felt it necessary to appoint three professors to take his place.
Yet, as sometimes happens, his theories fell swiftly out of fashion. Less than a decade after his death his successor in the chair of geology at Harvard wrote that the "so-called glacial epoch . . . 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 undergone 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 1900s a Serbian academic named Milutin Milankovitch, who had no background in celestial motions at all--he was 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. In particular 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 long spans of time involved 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's temperament. For the next twenty years, even while on vacation, he worked ceaselessly with 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 in 1914 Milankovitch suddenly got a great deal of that when World War I broke out and he was arrested owing to his position as a reservist in the Serbian army. He spent most of the next four 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 most people he assumed that it was a gradual increase in harsh winters that led to these long spells of coldness. It was a Russian-German meteorologist, Wladimir Köppen--father-in-law of our tectonic 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. As snow 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 ice sheets 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. You have advancing glaciers and 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. He died in 1958, unable to prove that 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 his theories 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. It has been suggested, however, that if you hauled North America, Eurasi
a, and Greenland just three hundred miles north we would have permanent and inescapable ice ages. We are very lucky, it appears, to get any good weather at all. Even less well understood are the cycles of comparative balminess 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 an atypical 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--though less 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. Ten percent 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--a 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 picture grows, but it appears that we have had at least seventeen severe glacial episodes in the last 2.5 million 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 landmass 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 not only cooler, but diverted winds in a way that made them flow north and toward North America, 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.