The present-day ice sheets of Greenland, and the glaciers in Alaska and arctic Canada, are residual from that once much more extensive ice covering of the Northern Hemisphere. But it was only in the nineteenth century that the existence of those great ice sheets of the past began to be recognized. Although some of our distant ancestors lived cheek by jowl with the gigantic ice caps, the small glaciers that still survived in high mountain regions by the dawn of modern civilization gave few clues to the earlier extent of ice. The massive ice sheets of Greenland and the Antarctic were far from the consciousness of most of the world’s population and remained largely unexplored until the late nineteenth and early twentieth centuries. Except for a few small mountain glaciers in Switzerland, there were no glaciers close to the centers of learning that could serve as examples. The story of the ice ages had to be worked out from other, much more tenuous, evidence. Like most other scientific advances, the realization that the Earth has periodically been gripped in ice ages didn’t come in a single Eureka! moment. Rather, it developed over a period of time and through the efforts of many naturalists and other close observers of the natural landscape. It came at a time when the science of geology was still young, when the concept that the Earth had an almost inconceivably long history was still controversial, and when the practice of making careful and systematic observations of the natural world was still relatively novel. The ice age had left its marks abundantly on the lands of the Northern Hemisphere. The signs were familiar to farmers and travelers, but for the most part their origins were obscure. It took keen observation, insight, and imagination to recognize in these marks the events that they actually record. And in spite of the fact that by the early part of the nineteenth century many scientists had discarded the notion that nearly all features of the landscape resulted from the biblical Flood, such ideas died hard. Some theologians and others prominent in society thundered “blasphemy” at the idea of an ice age. Even if they didn’t have strictly theological objections, when the idea that northern Europe had once been buried beneath a huge glacier was first proposed, many contemporary scientists summarily dismissed it. There was no analog. They could not conceive of such a drastic transformation of the countryside where they now saw only farmland, forests, and rural villages. From the perspective of a single human lifespan, or even on the timescale of a few generations, the Earth appeared to be quite an unchanging place.
In hindsight, it is easy to say that the geological evidence for ice ages was overwhelming and to wonder why such periods in the Earth’s past were not recognized earlier. And to be fair, even in the eighteenth century, nearly a hundred years before the term “ice age” was coined, there were already a few bold scientists who had begun to recognize the significance of the evidence. They and others who studied the Earth by careful observation were gradually eroding the influence of theologians who tried to shoehorn virtually every observation of the natural world into a literal biblical framework. Still, widespread debate about the reality of ice ages only began in earnest in the 1830s. The very first use of the term, as far as is known, was in a short, humorous poem written by a German botanist named Karl Schimper, who read and distributed copies of his little literary contribution to friends and colleagues at a scientific gathering in Switzerland in February 1837. Schimper was a brilliant but delusional scientist who was eventually committed to an asylum, where he died in 1867. He never became a formal participant in the debate about ice ages, nor did he produce any published works on the subject, but he was a close friend and colleague of the forceful and charismatic Swiss naturalist Louis Agassiz, who today is the person most closely associated with the formulation of ideas about a global ice age. Significantly, Agassiz was brought up literally in the shadows of the Alps, and glaciers—small mountain glaciers to be sure, but glaciers nevertheless—were part of the natural landscape of his childhood. By all accounts, Agassiz, a biologist whose first love was fossil fish, was a vigorous, highly intelligent, and very observant scientist. Like most of his contemporaries, he was initially skeptical about the claim that Alpine glaciers had been much more extensive in the past. But his conversion was rapid when he realized that many of the same landscape features that he observed being produced by contemporaneous mountain glaciers were also present far afield, in the ice-free valleys of his native country and even far beyond. Rural folk who encountered such features in their daily lives had reached a similar conclusion much earlier than Agassiz. The only way they could explain the large and exotic boulders they sometimes found plopped down in their fields was that they had been carried there by ice. That meant that in the past the glaciers must have extended far beyond their current boundaries.
As I hope will become apparent in this book, there is much that can be learned about the Earth, especially its climate, through careful study of the ice ages of the past. The story of how ideas about ice ages have developed, from the work of Agassiz in the 1830s to that of modern laboratories in the twenty-first century, is also a wonderful illustration of how science progresses: not on a smooth trajectory, but in fits and starts and sometimes even with “backward” steps, with long periods of accumulation of evidence and gestation of ideas, a certain amount of serendipity, occasional brilliant flashes of insight, and, especially in more recent times, technological advances. Perhaps because of the scale of the phenomena associated with ice ages, the subject has attracted its share of brilliant, charismatic, and eccentric characters, beginning with Louis Agassiz himself. A few are discussed in some detail later in this book: a self-educated Scot who made the connection between the Earth’s orbit around the sun and ice ages; a Serbian mathematician who worked out—by hand, long before the advent of computers—a mathematical framework for determining temperature changes through time at any latitude on Earth; and an iconoclastic American schoolteacher- turned-academic who proved that parts of the northwestern United States had been ravaged by floods beyond imagining as ice age glaciers melted back into Canada.
Louis Agassiz began discussing his ideas about an ice age at scientific gatherings in 1837, and within a few years, in 1840, he had published his observations and theory in a book. What was truly radical about his treatment was his proposal that ice had covered most of Europe during the ice age, even, perhaps, most of the land on Earth. As is often the case with new concepts, this one did not initially win many adherents. However, the debate about the reality of ice ages quickly became one of the most fiercely argued controversies of nineteenth-century science. It continued, unabated, for decades.
And the eventual acceptance of the ice age theory was far from the end of the story. Since that time, literally hundreds, perhaps even thousands, of scientists have pursued research into the causes and effects of the ice ages, and many thousands of scientific papers have been written on the subject. In the course of that work, Agassiz’s contributions have been remembered in small ways and large. When researchers discovered evidence of a vast ice-dammed lake that had formed along the margins of the melting ice age glaciers in the central part of North America, they named it Lake Agassiz. In Winnipeg, Canada, which lies within the area that had been covered by the waters of glacial Lake Agassiz, there is even an Agassiz microbrewery. Agassiz, who complained when he came to the United States about the American practice of drinking iced tea with lunch instead of wine, undoubtedly would have been pleased.
In principle, the idea of an ice age is a simple one—in the past, it was colder, glaciers were much more extensive than they are today, and huge ice sheets covered large sections of the continents that are now free of ice. However, understanding the phenomenon and determining how an ice age occurs, and what the ramifications are for the Earth and all its inhabitants, is far from simple. Today, it is difficult for anyone to be an expert in every aspect of ice age studies: the intellectual challenge presented by the geological evidence, with its multiple puzzles, has attracted the efforts of geologists, chemists, physicists, mathematicians, biologists, and climatologists. The work has taken on additiona
l urgency in recent years because of mounting concern about the future of the Earth’s climate system. While at first thought this might seem odd—the dominant problem today is global warming, not cooling—it has become clear that our planet has experienced huge climate shifts during the current ice age (as we shall see, the Earth today is still in the grip of an ice age). Understanding how these changes in the global climate occurred in the past, and what their effects were, is a key step toward predicting future changes. But in spite of the great advances that have been made in working out the details of what actually happened during the ice age, there is still much uncertainty about how, and especially why, an ice age actually begins. To be sure, there are hypotheses, but none have yet attained the status of an accepted scientific theory. Much remains to be done.
Louis Agassiz built his ice age theory within the framework of the then-popular catastrophist view of Earth history: the idea that rapid, large-scale events were responsible for many geological observations. He didn’t really concern himself with a mechanism; he just assumed that temperatures had plummeted suddenly and the Earth “froze.” He envisioned glaciers extending as far south as the Mediterranean Sea in Europe, and deep into North America. However, later research has shown that Agassiz’s ice age was neither as rapid in onset as he proposed nor just a single cold period. We now know that the Earth’s most recent ice age comprises a long succession of ice incursions deep into Europe (although not as far as the Mediterranean) and North America, separated by much warmer periods.
It is often not appreciated that today’s climate is just a geologically short warm spell in this continuing ice age. But in addition to the ice sheets of Greenland and Antarctica, mountainous regions today sustain permanent ice fields even in the tropics. The brilliant white cap on Mt. Kilimanjaro described by Hemingway in The Snows of Kilimanjaro is actually a permanent glacier, in spite of the fact that Kilimanjaro is only 300 km (roughly two hundred miles) from the equator. The Andes too host equatorial glaciers. If you were an astronaut circling the Earth at the end of a northern winter, you would observe that nearly half the land area and more than a quarter of the oceans were white with snow and ice. Only a fraction of this is permanent glaciers, but still, about 75 percent of all the fresh water on our “blue” planet is frozen in glaciers. Even so, in comparison with the average of the past few million years, the present-day interglacial climate is benign. The last time the Earth was as warm as it is today was about 120,000 years ago; for most of the time since then it has been much, much colder.
All of the evidence we have about past climates suggests that the Earth has been progressively cooling for the past 50 or 60 million years. Before then, most of the world had experienced warm temperatures—the fossil remains of tropical and subtropical plants and animals from those times are found even north of the Arctic Circle. Sometime near 35 million years ago, there was an especially sharp drop in global temperatures—this is when, most researchers believe, glaciers began to form in Antarctica. However, although temperatures continued to fall as the Antarctic icecap grew, it was not until about 3 million years ago that permanent glaciers appeared in abundance in the Northern Hemisphere, again accompanied by an abrupt temperature decrease. This is generally agreed to be the start of the current ice age, and since that time, most climate changes around the globe have been associated with the waxing and waning of ice sheets in the Northern Hemisphere. Fortunately for us, the glaciers have withdrawn to high altitudes and latitudes during the present warm period. But on average, for the past few million years, the Earth has been considerably colder than over most of its four and a half billion years of existence. During much of Earth history, except for short, rare, intervals, glaciers such as the one on Kilimanjaro have been absent. In contrast, within the current ice age, warm periods with moderate climates similar to the present have been short by geological standards, generally lasting only ten to twenty thousand years. We are already about ten thousand years into the current warm episode. If history is any guide, and if human activities don’t warm the Earth too severely, the ice will return, and quite soon on a geological timescale. The sites of cities such as Montreal and Edinburgh and Stockholm, and perhaps even New York and Chicago, will be buried deep in glacial ice, as they were in the past.
You might reasonably ask: How do we know these things? How do we know that the Earth has been cool for the past few million years, compared to the rest of its history? How do we know that the Earth is still locked in an ice age characterized by a series of advances and retreats of ice over North America and Europe? One of the aims of this book is to answer these questions, and also to delve into some of the history behind the answers. In doing so, I hope also to illustrate the startling ingenuity of some of the scientists who have investigated such questions, and the deep curiosity about how the Earth works that has pushed them toward their goals. And also to show why such work is important for understanding—and perhaps even shaping—man’s impact on our small planet.
Before embarking on this discussion, however, it is worth reviewing a few general aspects of ice ages. First is the meaning of the term itself, because its usage can be confusing. Initially, “ice age” referred to Agassiz’s original concept, a period of cold at some fairly remote time in the past, during which most of Europe was covered with thick glaciers. As already noted, however, it was fairly soon discovered that there had been a whole series of “ice ages” that were really part of the same episode, separated by short warm periods during which forests grew and animals roamed over land that had once been buried deep in ice. Climate zones marched up and down the continents as the temperature changed and glaciers grew and then melted back again. We know this about the ice age of the past few million years because the fossil record of changing animal and plant species is quite well preserved. But there have been ice ages in the much more distant past, too—hundreds of millions and even billions of years ago. About those intervals we know much less, but they too probably had both cold and warm periods. In current usage, the term “ice age” properly refers to an entire cold episode, including its short warm periods. Hence, it’s possible to talk about the “current ice age,” which seems a contradiction in terms on a hot summer’s day. Large-scale advances and retreats of ice during an ice age are usually referred to as glacials and interglacials respectively. Ice ages finally end when the cycles of these glacials and interglacials cease, and permanent ice, if it exists at all, becomes a minor feature of high mountains and polar regions. Our current ice age is often referred to as the Pleistocene Ice Age, taking its name from the subdivision of the geological timescale that more or less coincides with the time during which the Northern Hemisphere has experienced glaciation. Although the current ice age actually began somewhat before the start of the Pleistocene, I shall use that nomenclature in this book as a convenient way to distinguish it from others in the geological record.
In addition to understanding the usage of the term “ice age,” there is also the question of what, exactly, a glacier or icefield is, and how it comes about. Glaciers are actually nothing more than huge accumulations of snow. Pressure from the weight of overlying snow transforms those fluffy flakes that so delight children on a winter’s day into the hard ice of a glacier, brittle enough to crack open in deep crevasses and strong enough to pluck solid rock from a mountainside and carry it down a valley. At the very surface of a glacier, the winter snow is as loosely packed as the drifts you shovel from your driveway after a storm (of course, that snow really doesn’t seem to be so loosely packed when you’re at it, but that’s another story . . . ). But dig down into a glacier, and you’ll find the individual snowflakes—themselves just exquisitely formed crystals of ice—packed together much more tightly, the air between them forced out. Probe even more deeply, and you’ll discover that the snow has recrystallized into a continuous mass of solid ice, as clear as the ice cubes in your refrigerator.
A bona fide glacier must be permanent. Generally, this implies that sufficient
fresh snow must accumulate during the cold months to offset melting during the summer, although on a year-to-year basis, glaciers may expand or contract, depending on local and global climatic conditions. Today, most glaciers around the world are in retreat because of the warming climate, and it appears that the rate of melting is accelerating. This has been documented spectacularly in places such as the Alps, where historical records have been kept and dated sketches and photographs are available to compare with the present extent of ice. Even over periods as short as a few decades, satellite images show that dramatic shrinkage of mountain glaciers has occurred in the Andes, the Himalayas, and elsewhere. It is estimated that many small mountain glaciers will vanish completely within ten to twenty years unless there is an abrupt and unexpected change in the present warming trend.
Frozen Earth: The Once and Future Story of Ice Ages Page 3
