Frozen Earth: The Once and Future Story of Ice Ages Read online

  Frozen Earth

  Frozen Earth

  The Once and Future Story of Ice Ages

  WITH A NEW PREFACE

  Doug Macdougall

  UNIVERSITY OF CALIFORNIA PRESS

  BerkeleyLos AngelesLondon

  University of California Press, one of the most distinguished university presses in the United States, enriches lives around the world by advancing scholarship in the humanities, social sciences, and natural sciences. Its activities are supported by the UC Press Foundation and by philanthropic contributions from individuals and institutions. For more information, visit www.ucpress.edu.

  University of California Press

  Berkeley and Los Angeles, California

  University of California Press, Ltd.

  London, England

  First paperback printing 2006

  © 2004, 2013 by The Regents of the University of California

  ISBN: 978-0-520-27592-8

  eISBN: 9780520954946

  An earlier edition was catalogued by the Library of Congress as follows:

  Cataloging-in-Publication Data

  Macdougall, J. D., 1944–.

  Frozen earth : the once and future story of ice ages / Doug Macdougall.

  p. cm.

  Includes bibliographical references and index.

  ISBN 978-0-520-24824-3 (pbk : alk. paper)

  1. Glacial epoch. 2. Paleoclimatology. 3. Global environmental change. I. Title.

  QE698.M125 2004

  551.7'92—dc222004008502

  Manufactured in the United States of America

  22 21 20 19 18 17 16 15 14 13

  10 9 8 7 6 5 4 3 2 1

  In keeping with a commitment to support environmentally responsible and sustainable printing practices, UC Press has printed this book on Rolland Enviro100, a 100% post-consumer fiber paper that is FSC certified, deinked, processed chlorine-free, and manufactured with renewable biogas energy. It is acid-free and EcoLogo certified.

  For Grace and Lorn Macdougall,

  who always encouraged exploration

  CONTENTS

  List of Illustrations

  Acknowledgments

  Preface to the 2013 Edition

  Chapter 1. Ice, Ice Ages, and Our Planet’s Climate History

  Chapter 2. Fire, Water, and God

  Chapter 3. Glaciers and Fossil Fish

  Chapter 4. The Evidence

  Chapter 5. Searching for the Cause of Ice Ages

  Chapter 6. Defrosting Earth

  Chapter 7. The Ice Age Cycles

  Chapter 8. Our Planet’s Icy Past

  Chapter 9. Coring for the Details

  Chapter 10. Ice Ages, Climate, and Evolution

  Chapter 11. The Last Millennium

  Chapter 12. Ice Ages and the Future

  Suggestions for Further Reading

  Index

  ILLUSTRATIONS

  1.Bedrock grooved by Pleistocene Ice Age glaciers in southern Ontario

  2.Marks of Pleistocene glaciation in Canada’s Northwest Territories

  3.Louis Agassiz late in his career, as a professor at Harvard University

  4.A large erratic boulder in a field near Örebro, Sweden

  5.Glacier in Greenland

  6.Cross section through a lateral moraine in Switzerland

  7.Cartoon sketch of William Buckland by Thomas Sopwith

  8.James Croll, who proposed an astronomical theory of ice ages in the 1860s

  9.Plan (above) and side (below) views of the Earth’s orbit around the sun

  10.The wobble in the Earth’s axis relative to the plane of its orbit around the sun

  11.Part of James Croll’s graph of the changes in eccentricity of the Earth’s orbit around the sun

  12.Map of the catastrophic glacial flooding that created the Channeled Scablands in Washington State

  13.J Harlan Bretz, aged 95, at his home near Chicago

  14.Graph showing the “equivalent latitude” for 65° N in summer for the past 600,000 years

  15.Milutin Milankovitch working at his desk in 1954

  16.Graph of climate changes over the past 550,000 years, based on oxygen isotope analyses of deep-sea sediments

  17.The Earth’s major ice ages

  18.The spread of Gondwanaland ice sheets during the Permo-Carboniferous glaciation

  19.A glacially scratched and faceted boulder from the Earth’s oldest known ice age, some 3 billion years ago

  20.Graph of variations in ocean water temperatures over the past 60 million years

  21.Graph of ice-core data from the Antarctic ice cores at Vostok Station

  22.Dryas flowers, which suddenly reappeared in Europe indicating a sudden drop in temperatures

  23.Graph showing the drop in temperature in central Greenland about 12,800 years ago, deduced from isotopes in ice cores

  24.Temperature fluctuations between fifty and twenty thousand years ago as recorded in a central Greenland ice core

  25.Temperature variations through the past millennium as recorded in a Greenland ice core

  26.Painting by Sir Henry Raeburn of the Reverend Robert Walker skating on Duddingston Loch, near Edinburgh, toward the end of the Little Ice Age

  ACKNOWLEDGMENTS

  Many thanks go to those who graciously allowed me to use their photographs in this book: Professor Kenneth Hamblin, Brigham Young University; Dr. John Shelton, La Jolla, California; Professor Michael Hambrey, Liverpool John Moores University; Mr. Vasko Milankovitch, North Balwyn, Australia; and Professor John Crowell, University of California Santa Barbara.

  My agent, Rick Balkin, worked hard to ensure that this project got off the ground in the first place, and Blake Edgar at U.C. Press provided much input along the way, helping to make the final manuscript more readable, and, I hope, a more interesting book. Guy Tapper produced all of the line drawings in his usual professional manner. Heartfelt thanks to all of you.

  PREFACE TO THE 2013 EDITION

  Ice ages are global episodes of extreme climate change. For that reason, understanding them—why they occur, how they impact our planet, what brings them to an end—provides crucial information for anticipating how climate will change in the future and what the effects may be. Since this book was first published, scientists have made great strides toward untangling the complex interplay of factors that affect the Earth’s climate. Much of this progress has come through careful study of ice ages, especially the most recent—the Pleistocene Ice Age.

  What are some of the advances climate scientists have made? They include a clearer understanding of various forcing factors (parameters with the potential to change climate, for example the greenhouse gas content of the atmosphere), improvements in computer modeling of future climate change, and the identification of previously unrecognized processes that may have a profound impact on climate. A key ingredient has been the availability of better physical records of climate—for example, ice cores from the Antarctic that reach further back into the past than those previously available, sediment cores from a Siberian lake that provide a high-resolution record of northern climates over nearly the entire duration of the Pleistocene Ice Age, and cave speleothems (such as stalactites) that accumulate slowly, drip by drip, over long periods. These materials give us a window into the changing ice age environment through the climate proxies they contain: chemical and isotopic properties that reflect past temperatures or other environmental characteristics, biological tracers such as pollen grains that reveal the local climate, and other properties that can give clues to precipitation, windiness, seasonality, and other environmental parameters.

  This short pre
face cannot do justice to the immense amount of research on ice age– and climate-related issues that has been carried out over the past few years. But I’d like to focus on a few specific studies that give a taste of the kind of work being done. The first of these deals with the how and why of our planet’s warming up from the frigid peak of its most recent cold period a little more than twenty thousand years ago, a time that scientists refer to as the Last Glacial Maximum, usually abbreviated as LGM, when thick glacial ice blanketed parts of the United Kingdom and Russia, much of Scandinavia, and large portions of North America, pushing down far south of the Great Lakes.

  The Pleistocene Ice Age, which has gripped the Earth over approximately the past two and a half million years, has not been monotonously cold. Instead, climate has cycled between long icy intervals and relatively short warm periods, like now, which geologists call interglacials. Analyses of gas bubbles trapped in ice cores from Greenland and the Antarctic—tiny samples of air from the past—show unequivocally that the atmosphere had high concentrations of greenhouse gases during the interglacial warm periods and low concentrations during the cold intervals. The ice cores provide information from times long before humans began to influence the atmosphere, so the greenhouse gas variations they record were entirely natural. The question is, were natural increases in carbon dioxide and other greenhouse gases the cause of warm interglacial periods, or were they somehow a result of the higher temperatures? Clearly this is an important question for understanding how the Earth’s climate will respond to greenhouse gas increases caused by humans.

  Until recently, the answer to this question seemed to be that the high concentrations during the interglacials were an effect of the increasing temperatures, not the cause. This conclusion was drawn from detailed studies of Antarctic ice cores, which showed that as the Earth warmed into the interglacials, rising temperatures (measured via isotopic proxies in the ice) slightly preceded the greenhouse gas increases (measured directly in gas bubbles from the same ice cores). The time difference was small, and the results were crucially dependent on accurate dating of the ice cores. But the data seemed robust and the conclusion inescapable. Although higher carbon dioxide levels would have enhanced the warming, something else must have been the primary forcing factor.

  However, recent work by an international group of climatologists, published in the journal Nature, has challenged that conclusion (“Global Warming Preceded by Increasing Carbon Dioxide Concentrations during the Last Deglaciation,” by Jeremy D. Shakun and colleagues, Nature 484, 5 April 2012). These scientists realized that temperature data from the Antarctic ice cores reflect only local temperatures, whereas greenhouse gas results from the same cores provide information for the Earth as a whole because gases in the atmosphere are well mixed globally. The researchers wanted to find out what the story would be if they compared the greenhouse gas results with temperature data that were also averaged globally.

  In their study Shakun and his colleagues examined temperature information from a globally distributed set of eighty different natural records, mostly ice and sediment cores, through the period from the onset of the most recent deglaciation to the point when the Earth’s climate reached approximately its present state (roughly the interval between twenty thousand and eleven thousand years ago). The data show that throughout most of that interval, increasing global average temperatures lagged increasing carbon dioxide concentrations by several hundred years. Surface temperatures averaged for the Earth as a whole evidently changed on a different timescale from those at the sites of the Antarctic ice cores. The authors concluded that greenhouse gases, particularly carbon dioxide, were the primary cause of the global warming and melting of the glaciers.

  However, there is a twist in the tale. Or rather, two twists. The first is that the pattern (although not the total degree) of warming differs between the Northern and Southern Hemispheres. The second is that for a short interval at the very beginning of the deglaciation—in contrast to the rest of the period—the Earth’s average temperature rose by a small amount, about 0.3°C (less than 1°F), before carbon dioxide began to rise. What do these findings imply? Taking the second observation first, it appears that the initiation of deglaciation—the very beginnings of ice sheet melting—occurred not because of increased greenhouse gases in the atmosphere after all but through minor solar heating of the Northern Hemisphere. Because of regular variations in the Earth’s orbit (described in chapter 5), insolation (the amount of solar energy that the Earth’s surface receives) was increasing rapidly at high northern latitudes at this time. The temperature rise this caused was small but still sufficient to initiate the melting of Northern Hemisphere glaciers. In a kind of domino effect, this slight warming set off other processes, including the rise of greenhouse gases, that amplified the initial temperature increase many times over and drove the large-scale interglacial warming.

  One of these feedback processes was a reduction in reflectivity, or albedo, as the area covered by ice decreased. With less snow and ice, the Earth retained more of the sun’s energy instead of reflecting it back into space, raising temperatures further. This is happening today in the Arctic, which is warming more rapidly than other parts of the Earth as the extent of sea ice diminishes. But even more important during the last deglaciation were changes in ocean circulation.

  How does ocean circulation affect climate? To understand this it’s necessary to remember that because of their huge volume, the oceans hold a tremendous amount of heat. Circulating ocean waters carry this heat from one place on the globe to another. Also—and especially important in terms of the warming and cooling cycles of ice ages—the oceans contain a very large amount of carbon. Some of this is present as dissolved carbon dioxide, and much of the rest can easily be transformed into carbon dioxide. In total, the oceans contain about fifty times as much carbon as the atmosphere. The rapid increase in carbon dioxide during the most recent deglaciation apparently happened when changes in ocean circulation released some of that carbon into the atmosphere.

  Ocean circulation is strongly influenced by the geographical distribution of the continents. In the present-day configuration it is largely driven by warm tropical water flowing northward in the Atlantic Ocean, cooling and becoming saltier due to evaporation as it goes. Both these processes make the surface water denser, and in the North Atlantic it sinks, drawing even more tropical water northward to replace it, thus maintaining the circulation pattern (the dense, cold water descends to the deep ocean and flows south toward the Antarctic and eventually into the Indian and Pacific Oceans). Shakun and his colleagues suggest that at the beginning of the most recent deglaciation, the slight warming of northern polar regions caused by increasing insolation slowed or even stopped this pattern of circulation. How did this happen? Fresh water (which is considerably less dense than salty seawater) from the melting glaciers flowed into the North Atlantic, decreasing the density of the surface water to the point where it could no longer sink. This shut down the northward transfer of warm water from the tropics, leading to warming of the Southern Hemisphere and modest cooling, or at a minimum slower warming of, northern polar regions. Climatologists refer to such ocean-driven temperature alternations between hemispheres as the bipolar seesaw. As the southern oceans warmed, Antarctic sea ice cover decreased, and changes in southern ocean circulation released carbon dioxide into the atmosphere, enhancing warming globally.

  If you’re not already familiar with some of these processes, following the scenario just described may set your head spinning. It involves a complex series of interrelated events driven by multiple climate-forcing mechanisms, amplified by feedbacks such as changes in albedo or ocean circulation patterns. But then, all natural systems are complex, and the bottom line from the work of Shakun and his colleagues is that the greenhouse gas carbon dioxide was the primary forcing mechanism for global warming during the most recent deglaciation. Currently, this research is the most extensive and thorough examination of what caused temperatur
es to rise globally from the LGM to the present. It is always possible that future work will change some of the details, but for the moment this is one of our best guides for understanding how climate may react to future changes.

  An interesting aspect of this work is its conclusion that the ultimate trigger for deglaciation was increasing insolation at high northern latitudes, even though—once the ice age glaciers had begun to melt—carbon dioxide was the primary forcing mechanism for the bulk of the warming. One of the earliest workers who attempted to explain glacial cycles, James Croll, recognized the importance of changes in northern insolation more than 150 years ago; later (early in the twentieth century) Mulutin Milankovitch expanded on this idea (see chapters 5 and 7). What these perceptive scientists didn’t understand, though, was that the key role of northern summer insolation in Pleistocene Ice Age cycles was at least partly due to the present-day configuration of the continents.

  Why is this? Think about the current situation: the South Pole lies within the Antarctic continent, the bulk of which is south of 70° latitude. When global temperatures are low, snow and ice can build up quickly to form a continent-scale ice sheet. But exceptional cold is required to maintain year-round sea ice beyond the continent, so such ice does not extend significantly farther north today. In contrast, the North Pole falls in the Arctic Ocean, a small ocean surrounded by continents on which glaciers build up and retreat in response to relatively small temperature changes caused by variations in Northern Hemisphere insolation. Feedback mechanisms then amplify these changes and affect temperatures globally. About twenty-two thousand years ago, during the LGM, such processes allowed glaciers to reach as far south as 40° north latitude in North America. Ice ages in the Earth’s distant past (see chapter 8) occurred at times when the arrangement of continents was radically different from today’s. Undoubtedly insolation changes were important for these too, but likely in quite different ways.