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Overview
Macdougall describes the awesome power of cataclysmic floods that marked the melting of the glaciers of the Pleistocene Ice Age. He probes the chilling evidence for "Snowball Earth," an episode far back in the earth's past that may have seen our planet encased in ice from pole to pole. He discusses the accumulating evidence from deep-sea sediment cores, as well as ice cores from Greenland and the Antarctic, that suggests fast-changing ice age climates may have directly impacted the evolution of our species and the course of human migration and civilization. Frozen Earth also chronicles how the concept of the ice age has gripped the imagination of scientists for almost two centuries. It offers an absorbing consideration of how current studies of Pleistocene climate may help us understand earth's future climate changes, including the question of when the next glacial interval will occur.
Product Details
| ISBN-13: | 9780520954946 |
|---|---|
| Publisher: | University of California Press |
| Publication date: | 02/15/2013 |
| Sold by: | Barnes & Noble |
| Format: | eBook |
| Pages: | 278 |
| File size: | 6 MB |
About the Author
Read an Excerpt
Frozen Earth
The Once and Future Story of Ice Ages
By Doug Macdougall
UNIVERSITY OF CALIFORNIA PRESS
Copyright © 2013 The Regents of the University of CaliforniaAll rights reserved.
ISBN: 978-0-520-95494-6
CHAPTER 1
Ice, Ice Ages, and Our Planet's Climate History
The American author and historical popularizer Will Durant once wrote, "Civilization exists by geological consent, subject to change without notice." That is not a new idea, even if Durant phrased it especially well, but nowadays many historians scoff at the notion of environmental determinism, the possibility that climate or geology may have seriously affected the course of human history. And yet there are still many places on this planet where Durant's observation rings true, especially places with extremes of climate. One such is the arctic regions, particularly Greenland. Ninety-five percent of that island country is covered by ice. Towns and villages cling to the coastline; at their backs loom glaciers a thousand meters thick: gleaming, white, blue, clear, transparent ice. The icecap weighs on the land like a lead brick on a floating plank, pressing it down below the level of the surrounding sea. If the ice were suddenly removed, the waters of the ocean would rush in to take its place. The glaciers seem fixed and static, but in reality they are dynamic, in constant slow movement outward from their thick centers. New snowfall adds to their mass every year, but at the margins they calve off apartment-block-sized chunks of themselves and send flotillas of weirdly shaped icebergs sizzling and crackling and sometimes eerily and silently floating down the fjords to the sea. The icebergs carry pieces of Greenland with them too, sand, pebbles, and boulders gouged and scraped from the land, later to be dropped far out at sea as the ice melts. The Inuit of Greenland have lived with the ice of glaciers for thousands of years. They are truly people of the ice age. Most of the rest of us have been affected by the ice age too, but in less obvious ways.
Permanent icefields—that is, large glaciers—are not common in mainland North America. In the mountainous west, in Alaska and in the Yukon, there are small high-altitude glaciers, but in the overall scheme of things, they are fairly minor features of the landscape. However, as a boy, like many others both in North America and northern Europe, I grew up surrounded by the work of ice. Like most others, I was, at the time, completely unaware of that fact. I am not referring to the ice of a skating rink or of a January puddle. Rather, this was ice just like that of Greenland today, or of Antarctica, ice of vast extent and kilometers thick that blanketed huge swathes of the Northern Hemisphere thousands of years ago. It reached down from centers in Canada and Scandinavia and covered the sites of cities such as Boston, Detroit, and Hamburg. Its legacy is everywhere even today, from the geography of our waterways to the distribution of native peoples in the New World. It ground up solid rock to make the sand of countless beaches and the soil of midwestern farms in the United States. It sculpted rolling hills and long valleys across the landscape. It scraped up soil and rocks as it flowed, and dumped the debris as terminal moraines in places like Cape Cod and Long Island, New York, far from its original home. It even picked up diamonds from still-undiscovered deposits in Canada and transported them to the United States, twenty thousand years before NAFTA was conceived.
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 additional 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.
(Continues...)
Excerpted from Frozen Earth by Doug Macdougall. Copyright © 2013 The Regents of the University of California. Excerpted by permission of UNIVERSITY OF CALIFORNIA PRESS.
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