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 theamount 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, Eurasia, 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 ofHomo 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.

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. 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 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 a mile 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. For 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. One is that a little ocean water did remain exposed (perhaps because of some kind of localized warming at a 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 to tectonics 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 as tranquil as all that. As Earth warmed, it probably had the wildest weather it has ever experienced, with hurricanes powerful enough to raise 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 ocean vents undoubtedly went on as if nothing were amiss, but all other life on Earth probably came as close as it ever has to checking out entirely. It was all a long time ago and at this stage we just don’t know.

Compared with a Cryogenian outburst, the ice ages of more recent times seem pretty small scale, 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. Whole continents 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 out boulders and long lines of gravelly moraines, but dumped entire landmasses—Long Island and Cape Cod and Nantucket, among others—as they slowly swept along. It’s little wonder that geologists before Agassiz had trouble grasping their monumental capacity to rework landscapes.

If ice sheets advanced again, we have nothing in our armory that could deflect them. In 1964, 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 the Richter scale. Along the fault line, the land rose by as much as twenty feet. The quake was so violent, in fact, that it made water slosh out of pools in Texas. And what effect did this unparalleled 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, over hundreds 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 history 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 or so in an event known to science as the Younger Dryas. (The name comes from the arctic plant 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-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 of Scandinavia for that of the Mediterranean in just two decades. Locally, changes have been even more dramatic. Greenland ice cores show the temperatures there changing by as much as fifteen 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 pretty well unimaginable.

What is most alarming is that we have no idea—none—what natural phenomena could so swiftly rattle Earth’s thermometer. As Elizabeth Kolbert, writing in theNew 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 Gulf Stream to swerve to the south, like a driver trying to avoid a collision. Deprived of the Gulf Stream’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’t veer as before. Instead, we were given the period of unusual tranquility known 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 tendency 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 to do is conduct a vast unsupervised experiment on it.” It has even been suggested, with more plausibility than would at first seem evident, that an ice age might actually be induced by a 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. That simply shouldn’t have been possible.

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 continually 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. How dinosaurs 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 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 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 fifty years 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 collapse is all the more possible. If so, sea levels globally would rise—and pretty quickly—by between fifteen and twenty feet on average.

The extraordinary fact is that we don’t know which is more likely, a future offering us eons of perishing frigidity or one giving us equal expanses of steamy heat. Only one thing is certain: we live on a knife edge.

In the long run, incidentally, ice ages are by no means bad news for the planet. They grind up rocks and leave behind new soils of sumptuous richness, and gouge out fresh water lakes that provide abundant nutritive possibilities for hundreds of species of being. They act as a spur to migration and keep the planet dynamic. As Tim Flannery has remarked: “There is only one question you need ask of a continent in order to determine the fate of its people: ‘Did you have a good ice age?’ ” And with that in mind, it’s time to look at a species of ape that truly did.