27 ICE TIME
《万物简史英文版》章节: 27 ICE TIME,宠文网网友提供全文无弹窗免费在线阅读。!
The bright sun wasextinguish’d, and the starsDid 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 andassociated tsunamis. It was the biggest volcanic explosion in ten thousand years—150 timesthe 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’seffects 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 J. M.
W. Turner, who could not have been happier, but mostly the world existed under anoppressive, 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 withoutsummer. Crops everywhere failed to grow. In Ireland a famine and associated typhoidepidemic killed sixty-five thousand people. In New England, the year became popularlyknown as Eighteen Hundred and Froze to Death. Morning frosts continued until June andalmost no planted seed would grow. Short of fodder, livestock died or had to be prematurelyslaughtered. In every way it was a dreadful year—almost certainly the worst for farmers inmodern times. Yet globally the temperature fell by only about 1.5 degrees Fahrenheit. Earth’snatural thermostat, as scientists would learn, is an exceedingly delicate instrument.
The nineteenth century was already a chilly time. For two hundred years Europe and NorthAmerica in particular had experienced a Little Ice Age, as it has become known, whichpermitted all kinds of wintry events—frost fairs on the Thames, ice-skating races along Dutchcanals—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 thatwould wreck even a frost fair.
They knew there was something odd about the past. The European landscape was litteredwith inexplicable anomalies—the bones of arctic reindeer in the warm south of France, hugerocks 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 graniteboulders had come to rest high up on the limestone flanks of the Jura Mountains, suggestedthat 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 theexpression 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 ofgeology, had visited Switzerland, he would have seen at once the significance of the carvedvalleys, the polished striations, the telltale strand lines where rocks had been dumped, and theother 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 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 becameone of the first to argue for widespread glaciation. Unfortunately his ideas escaped notice, andfor another half century most naturalists continued to insist that the gouges on rocks could beattributed 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 roadside. Thewoodcutter matter-of-factly told him that the boulders had come from the Grimsel, a zone ofgranite some distance away. “When I asked him how he thought that these stones had reachedtheir location, he answered without hesitation: ‘The Grimsel glacier transported them on bothsides 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 thenotion at scientific gatherings, it was dismissed. One of Charpentier’s closest friends wasanother 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 NaturalHistory at the College of Neuchatel in Switzerland. Another friend of Agassiz’s, a botanistnamed 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 heavilyacross not just the Swiss Alps, but over much of Europe, Asia, and North America. It was aradical notion. He lent Agassiz his notes—then came very much to regret it as Agassizincreasingly 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, yetanother friend, may have had Agassiz at least partly in mind when he observed that there arethree stages in scientific discovery: first, people deny that it is true; then they deny that it isimportant; finally they credit the wrong person.
At all events, Agassiz made the field his own. In his quest to understand the dynamics ofglaciation, he went everywhere—deep into dangerous crevasses and up to the summits of thecraggiest 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 seena 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 ofthe Geological Society, endorsed this view, arguing that the notion that ice could transportboulders presented “such obvious mechanical absurdities” as to make it unworthy of thesociety’s attention.
Undaunted, Agassiz traveled tirelessly to promote his theory. In 1840 he read a paper to ameeting of the British Association for the Advancement of Science in Glasgow at which hewas openly criticized by the great Charles Lyell. The following year the Geological Society ofEdinburgh passed a resolution conceding that there might be some general merit in the theorybut 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 oftimes, 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 theIce Age idea. It was a frustrating time for Agassiz. His marriage was breaking up, Schimperwas hotly accusing him of the theft of his ideas, Charpentier wouldn’t speak to him, and thegreatest 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, theMuseum 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 ofcold. It also helped that six years after his arrival the first scientific expedition to Greenlandreported that nearly the whole of that semicontinent was covered in an ice sheet just like theancient one imagined in Agassiz’s theory. At long last, his ideas began to find a realfollowing. The one central defect of Agassiz’s theory was that his ice ages had no cause. Butassistance 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 ages, was published in the Philosophical Magazine in 1864 and wasrecognized at once as a work of the highest standard. So there was some surprise, and perhapsjust a touch of embarrassment, when it turned out that Croll was not an academic at theuniversity, but a janitor.
Born in 1821, Croll grew up poor, and his formal education lasted only to the age ofthirteen. 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 ofStrathclyde) 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, andgradually began to produce a string of papers, with a particular emphasis on the motions ofEarth and their effect on climate.
Croll was the first to suggest that cyclical changes in the shape of Earth’s orbit, fromelliptical (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 astronomicalexplanation for variations in Earth’s weather. Thanks almost entirely to Croll’s persuasivetheory, 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. When his ingenuity and aptitude wererecognized, 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 ofScience 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, hewas busy taking it into ever more exotic territory in America. He began to find evidence forglaciers practically everywhere he looked, including near the equator. Eventually he becameconvinced that ice had once covered the whole Earth, extinguishing all life, which God hadthen re-created. None of the evidence Agassiz cited supported such a view. Nonetheless, inhis adopted country his stature grew and grew until he was regarded as only slightly below adeity. When he died in 1873 Harvard felt it necessary to appoint three professors to take hisplace.
Yet, as sometimes happens, his theories fell swiftly out of fashion. Less than a decade afterhis 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 withouthesitation.”
Part of the problem was that Croll’s computations suggested that the most recent ice ageoccurred eighty thousand years ago, whereas the geological evidence increasingly indicatedthat 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 theoryfell 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 motionsat all—he was a mechanical engineer by training—developed an unexpected interest in thematter. Milankovitch realized that the problem with Croll’s theory was not that it wasincorrect but that it was too simple.
As Earth moves through space, it is subject not just to variations in the length and shape ofits orbit, but also to rhythmic shifts in its angle of orientation to the Sun—its tilt and pitch andwobble—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 mightbe a relationship between these complex cycles and the comings and goings of ice ages. Thedifficulty 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—whichmeant that determining their points of intersection over long spans of time involved a nearlyendless amount of devoted computation. Essentially Milankovitch had to work out the angleand duration of incoming solar radiation at every latitude on Earth, in every season, for amillion 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 withpencil and slide rule computing the tables of his cycles—work that now could be completed ina 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 wasarrested 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 aweek. The rest of his time was spent working in the library of the Hungarian Academy ofSciences. He was possibly the happiest prisoner of war in history.
The eventual outcome of his diligent scribblings was the 1930 book MathematicalClimatology and the Astronomical Theory of Climatic Changes. Milankovitch was right thatthere was a relationship between ice ages and planetary wobble, though like most people heassumed that it was a gradual increase in harsh winters that led to these long spells ofcoldness. It was a Russian-German meteorologist, Wladimir K?ppen—father-in-law of ourtectonic friend Alfred Wegener—who saw that the process was more subtle, and rather moreunnerving, than that.
The cause of ice ages, K?ppen 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 sunlightis bounced back by the reflective surface, exacerbating the cooling effect and encouraging yetmore snow to fall. The consequence would tend to be self-perpetuating. As snow accumulatedinto an ice sheet, the region would grow cooler, prompting more ice to accumulate. As theglaciologist 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 startfrom a single unseasonal summer. The leftover snow reflects heat and exacerbates the chillingeffect. “The process is self-enlarging, unstoppable, and once the ice is really growing itmoves,” says McPhee. You have advancing glaciers and an ice age.
In the 1950s, because of imperfect dating technology, scientists were unable to correlateMilankovitch’s carefully worked-out cycles with the supposed dates of ice ages as thenperceived, and so Milankovitch and his calculations increasingly fell out of favor. He died in1958, 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 themodel as being anything more than an historical curiosity.” Not until the 1970s and therefinement 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 otherfactors are involved—not least the disposition of the continents, in particular the presence oflandmasses over the poles—but the specifics of these are imperfectly understood. It has beensuggested, however, that if you hauled North America, Eurasia, and Greenland just threehundred miles north we would have permanent and inescapable ice ages. We are very lucky, itappears, to get any good weather at all. Even less well understood are the cycles ofcomparative balminess within ice ages, known as interglacials. It is mildly unnerving toreflect that the whole of meaningful human history—the development of farming, the creationof towns, the rise of mathematics and writing and science and all the rest—has taken placewithin an atypical patch of fair weather. Previous interglacials have lasted as little as eightthousand 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, aroundtwenty 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 thefresh water on Earth is locked up in ice even now, and we have ice caps at both poles—asituation 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 seemquite 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 hotwith no permanent ice anywhere. The current ice age—ice epoch really—started about fortymillion years ago, and has ranged from murderously bad to not bad at all. Ice ages tend towipe 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 Africafollowed by modern humans. Two commonly cited culprits for the present epoch are the riseof 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 theAsian landmass over the last forty-five million years, raising not only the Himalayas, but alsothe vast Tibetan plateau behind them. The hypothesis is that the higher landscape was notonly 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 millionyears 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 changingpatterns of precipitation across at least half the world. One consequence was a drying out ofAfrica, which caused apes to climb down out of trees and go looking for a new way of livingon the emerging savannas.
At all events, with the oceans and continents arranged as they are now, it appears that icewill be a long-term part of our future. According to John McPhee, about fifty more glacialepisodes can be expected, each lasting a hundred thousand years or so, before we can hope fora really long thaw.
Before fifty million years ago, Earth had no regular ice ages, but when we did have themthey tended to be colossal. A massive freezing occurred about 2.2 billion years ago, followedby a billion years or so of warmth. Then there was another ice age even larger than the first—so large that some scientists are now referring to the age in which it occurred as theCryogenian, or super ice age. The condition is more popularly known as Snowball Earth.
“Snowball,” however, barely captures the murderousness of conditions. The theory is thatbecause of a fall in solar radiation of about 6 percent and a dropoff in the production (orretention) of greenhouse gases, Earth essentially lost its ability to hold on to its heat. Itbecame a kind of all-over Antarctica. Temperatures plunged by as much as 80 degreesFahrenheit. 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 iceeverywhere, including around the equator, while the biological evidence suggests just asfirmly that there must have been open water somewhere. For one thing, cyanobacteriasurvived the experience, and they photosynthesize. For that they needed sunlight, but as youwill know if you have ever tried to peer through it, ice quickly becomes opaque and after onlya few yards would pass on no light at all. Two possibilities have been suggested. One is that alittle ocean water did remain 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 warmagain. An icy planet should reflect so much heat that it would stay frozen forever. It appearsthat 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, whichpushed through the buried surface, pumping out lots of heat and gases that melted the snowsand re-formed the atmosphere. Interestingly, the end of this hyper-frigid episode is marked bythe 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 raise waves to the heights of skyscrapersand 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. It was all a long time ago and at this stage wejust don’t know.
Compared with a Cryogenian outburst, the ice ages of more recent times seem pretty smallscale, but of course they were immensely grand by the standards of anything to be found onEarth 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 benearly 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 nothingbut 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 afterthe glaciers’ withdrawal, are still rising back into place. The ice sheets didn’t just dribble outboulders and long lines of gravelly moraines, but dumped entire landmasses—Long Islandand Cape Cod and Nantucket, among others—as they slowly swept along. It’s little wonderthat geologists before Agassiz had trouble grasping their monumental capacity to reworklandscapes.
If ice sheets advanced again, we have nothing in our armory that could deflect them. In1964, at Prince William Sound in Alaska, one of the largest glacial fields in North Americawas 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 thisunparalleled outburst have on the glaciers of Prince William Sound? None at all. They justsoaked 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 toice cores from Greenland we have a detailed record of climate for something over a hundredthousand years, and what is found there is not comforting. It shows that for most of its recenthistory 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 towarm, and quite rapidly, but then abruptly plunged back into bitter cold for a thousand yearsor 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. Therewas also an Older Dryas period, but it wasn’t so sharp.) At the end of this thousand-year onslaught 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 ofScandinavia for that of the Mediterranean in just two decades. Locally, changes have beeneven more dramatic. Greenland ice cores show the temperatures there changing by as much asfifteen degrees in ten years, drastically altering rainfall patterns and growing conditions. Thismust have been unsettling enough on a thinly populated planet. Today the consequenceswould be pretty well unimaginable.
What is most alarming is that we have no idea—none—what natural phenomena could soswiftly rattle Earth’s thermometer. As Elizabeth Kolbert, writing in the New Yorker, hasobserved: “No known external force, or even any that has been hypothesized, seems capableof yanking the temperature back and forth as violently, and as often, as these cores haveshown 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, butall this is a long way from being understood.
One theory is that the heavy inflow of meltwater to the seas at the beginning of theYounger 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 toexplain why a thousand years later when the Earth warmed once again the Gulf Stream didn’tveer as before. Instead, we were given the period of unusual tranquility known as theHolocene, 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 isnatural to suppose that global warming would act as a useful counterweight to the Earth’stendency to plunge back into glacial conditions. However, as Kolbert has pointed out, whenyou 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 arise in temperatures. The idea is that a slight warming would enhance evaporation rates andincrease 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 inNorth America and northern Europe.
Climate is the product of so many variables—rising and falling carbon dioxide levels, theshifts of continents, solar activity, the stately wobbles of the Milankovitch cycles—that it is asdifficult to comprehend the events of the past as it is to predict those of the future. Much issimply beyond us. Take Antarctica. For at least twenty million years after it settled over theSouth Pole Antarctica remained covered in plants and free of ice. That simply shouldn’t havebeen possible.
No less intriguing are the known ranges of some late dinosaurs. The British geologistStephen Drury notes that forests within 10 degrees latitude of the North Pole were home togreat beasts, including Tyrannosaurus rex. “That is bizarre,” he writes, “for such a highlatitude is continually dark for three months of the year.” Moreover, there is now evidencethat these high latitudes suffered severe winters. Oxygen isotope studies suggest that theclimate around Fairbanks, Alaska, was about the same in the late Cretaceous period as it isnow. So what was Tyrannosaurus doing there? Either it migrated seasonally over enormousdistances 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. Howdinosaurs managed to 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 whateverreason, 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 there to fuel the last ice age. They were createdby it.
On the other hand, the next phase of our history could see us melting a lot of ice rather thanmaking it. If all the ice sheets melted, sea levels would rise by two hundred feet—the heightof a twenty-story building—and every coastal city in the world would be inundated. Morelikely, 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 haveincreased 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 youhav