Climate Change: The Science of Global Warming and Our Energy Future [2 ed.] 9780231172820, 0231172826, 9780231172837, 0231172834, 9780231547871

Table of contents :
Table of Contents
Preface
Prologue
Part I. The Climate System
1. The Atmosphere
2. The World Ocean
3. Ocean–Atmosphere Interactions
4. The Carbon Cycle and How It Influences Climate
Part II. Climate Change and Its Drivers
5. The Concept of Radiation Balance, a Scientific Framework for Thinking About Climate Change
6. Radiative Forcing, Feedbacks, and Some Other Characteristics of the Climate System
7. Learning from the Climate of the Distant Past
Part III. Consequences of Climate Change
8. The Climate of the Recent Past and Impacts on Human History
9. Observing the Change
10. Greenland, Antarctica, and Sea-Level Rise
Part IV. The Future
11. Climate Models and the Future
12. Climate Change Risk in an Unknowable Future
13. Energy and the Future
Epilogue
Notes
Glossary
Bibliography
Index

Citation preview

CLIMATE CHANGE

THE SCIENCE OF GLOBAL WARMING AND OUR ENERGY FUTURE SECOND EDITION

EDMOND A. MATHEZ

AND

JASON E. SMERDON

Climate Change

Climate Change

THE SCIENCE OF GLOBAL WARMING AND OUR ENERGY FUTURE

Second Edition EDMOND A. MATHEZ and JASON E. SMERDON

COLUMBIA UNIVERSITY PRESS

NEW YORK

Columbia U niversity Pr ess Publishers Since 1893 New York Chichester, West Sussex cup.columbia.edu Copyright © 2018 Edmond A. Mathez and Jason E. Smerdon All rights reserved Library of Congress Cataloging-in-Publication Data Names: Mathez, Edmond A., author. | Smerdon, Jason E., author. Title: Climate change : the science of global warming and our energy future /     Edmond A. Mathez and Jason E. Smerdon. Description: Second edition. | New York : Columbia University Press, [2018] |     Includes bibliographical references and index. Identifiers: LCCN 2017054632 | ISBN 9780231172820 (cloth : alk. paper) |  ISBN 9780231172837 (pbk. : alk. paper) | ISBN 9780231547871 (e-book) Subjects: LCSH: Climatic changes. | Global warming. Classification: LCC QC981.8.C5 M378 2018 | DDC 551.6—dc23 LC record available at https://lccn.loc.gov/2017054632

Columbia University Press books are printed on permanent and durable acid-free paper. Printed in the United States of America Cover image: Patrick Eden / © Alamy

To our children, Sophia and Lucas, and Anaïs and Emile, and to all the others of their generation who will bear many of the burdens of climate change and who will ultimately be tasked with finding a path to a more sustainable future.

CONTENTS

Preface

xi

Prologue 1

Weather and Climate 1 The Climate System 2 Climate Change: Separating Facts from Fears 5 Thinking About the Future in the Face of Uncertainty The Story 9

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PART I. THE CLIMATE SYSTEM 1. The Atmosphere 13

The Composition of Air 13 The Compressibility and Pressure of Air 15 Mechanisms of Heat and Mass Transfer 16 The Thermal and Compositional Layering of the Atmosphere 21 The General (Global) Circulation of the Atmosphere 29 Key Points in This Chapter 40 2. The World Ocean 43

Important Properties of Water 44 The Ocean’s Layered Structure 49 The Ocean’s Surface Currents 53 Global Flows of Water Through the Ocean 57 The Hydrological Cycle 63 Key Points in This Chapter 65 3. Ocean–Atmosphere Interactions 69

Exchanges at the Ocean–Atmosphere Interface 69 The El Niño–Southern Oscillation 71 Other Modes of Ocean and Atmosphere Variability 88 Key Points in This Chapter 97

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4. The Carbon Cycle and How It Influences Climate 101

Reservoirs of Carbon 104 The Carbon Cycle 105 The Acidification of the Ocean 118 Uncertainties in the Carbon Cycle 126 Key Points in This Chapter 128

PART II. CLIMATE CHANGE AND ITS DRIVERS 5. The Concept of Radiation Balance, a Scientific Framework for Thinking About Climate Change 133

Electromagnetic Radiation 136 The Greenhouse Effect 142 Earth’s Radiation Balance 143 Geographical and Seasonal Variations in Energy Balance Key Points in This Chapter 160

152

6. Radiative Forcing, Feedbacks, and Some Other Characteristics of the Climate System 163

Radiative Forcing 163 Greenhouse Gases as Forcing Factors 164 Aerosols 171 Land-Use Change 174 Natural Forcing Factors 175 Feedbacks 182 Tipping Points 188 Committed Warming 189 Key Points in This Chapter 190 7. Learning from the Climate of the Distant Past 193

The Ice Age and Paleoclimatology 193 Earth’s Orbital Characteristics and Milankovitch Theory 197 Five Million Years of Climate 202 One Hundred Thousand Years of Climate Change 211 A Lesson from the Distant Past: The Paleocene–Eocene Thermal Maximum 220 Key Points in This Chapter 224

PART III. CONSEQUENCES OF CLIMATE CHANGE 8. The Climate of the Recent Past and Impacts on Human History 229

Climate Proxies 229 Early to Mid-Holocene Climate Change and Human Development 238 The Rise and Fall of Civilizations 246 Key Points in This Chapter 260

CONTENTS

9. Observing the Change

265

A Century of Warming 266 Precipitation and Drought 275 Why Some Water Supplies Are in Jeopardy 283 Severe Storms and Other Extreme Events 284 The Sensitive Arctic 290 Key Points in This Chapter 302 10. Greenland, Antarctica, and Sea-Level Rise

305

Recent Sea-Level Rise and the Factors Contributing to It 306 The Greenland Ice Sheet 313 The Antarctic Ice Sheets 318 Future Sea-Level Rise 321 Key Points in This Chapter 328

PART IV. THE FUTURE 11. Climate Models and the Future 333

What Are Climate Models? 334 Peering into the Future 346 Responding to Our Climate Future 351 Key Points in This Chapter 352 12. Climate Change Risk in an Unknowable Future 355

Climate–Emissions Uncertainties 355 What Is Risk? 357 Some Properties of Climate Change Risk Climate Change and Human Strife 365 Key Points in This Chapter 372

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13. Energy and the Future 375

Electricity Circa 2017 377 Coal: A Vast Enterprise 380 Natural Gas: A Bridge Fuel? 390 The Nuclear Option 395 Wind and Solar Power: Icons for the Future 404 What About Hydropower? 413 Intermittent Power and the Electrical Grid 416 Key Points in This Chapter 418 Epilogue 421

Notes 425 Glossary 453 Bibliography 463 Index 489

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PREFACE

Th e ge n e sis of t h e first edition of Climate Change: The Science of Global Warming and Our Energy Future was a document to support the development of the American Museum of Natural History’s special exhibition Climate Change: Threat to Life and Our Energy Future, which opened in New York City in October 2008 and for which one of us (EAM) served as co-curator. Thus the original book was highly generalized, written, as it was, for a relatively broad audience. Since the time of writing the first edition and through our continued classroom experiences, however, it became apparent that university students need a more substantial introduction to climate science. Thus was born this second edition, which is much expanded from the first. Its target audiences include undergraduate students in beginning to upper-division climate courses, as well as graduate students in nontechnical programs such as education, journalism, and environmental policy. We seek to appeal to this wide university audience with a conceptual focus on the subject matter, avoiding complex mathematical formulations and layering content by consigning detailed or focused arguments to boxes. We hope that this approach will provide instructors with a basis for more in-depth classroom investigation. This edition also takes on a somewhat more didactic character than the first. Each chapter ends with a roundup called “Key Points in This Chapter,” in which the chapter’s salient points are listed. Most chapters also contain a section titled “Back-of-the-Envelope Calculation,” in which a simple computation illustrates one of the relevant chapter principles. Each “Historical Note” presents a biographical sketch of a scientist central to the development of climate science, providing the historical context of our present knowledge. Readers should also understand that this book relies mainly on the primary scientific literature. We nevertheless have limited the citations on each subject to a relatively few recent references, the intent being to provide a bibliography that allows the student practical entry into the broader literature. We must emphasize, however, that the narrative is based on an enormous body of work involving thousands of people, not all of whose work is cited. Finally, similar to the first edition, this second edition is a narrative account, written more as a story than in the style of a traditional textbook.

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The story of this book sets out the scientific basis for our understanding of climate change. It is divided into four parts, beginning with a description of Earth’s present climate system. We explain the workings and interactions of the atmosphere and the ocean; how they move heat around the planet and bring us familiar climate phenomena, such as the El Niño– Southern Oscillation and the monsoons; and the all-important carbon cycle, which determines the carbon dioxide content of the atmosphere. Part II explores the drivers of climate change. We define the scientific framework that enables us to systematically think about climate change—the related concepts of radiation balance and radiation forcing—and investigate the greenhouse effect and other drivers of climate change. Our knowledge of how the climate system works today rests, in part, on our knowledge of how it changed in the past. We therefore also delve into paleoclimate, focusing on the global climate record of the past 2.6 million years. Part III concerns the consequences of climate change. To appreciate how climate change can affect humanity, we first turn our attention to how humans were affected by climate change over roughly the past 10,000 years. We then describe climate change as it has been observed in the twentieth and twenty-first centuries and its consequences to date. All this brings us to the future. Computer models of the climate system help us understand how climate may change in the coming decades and centuries. We therefore begin Part IV with a description of climate models and their projections of what might come to pass. No one knows what the future will bring, of course, so we devote some effort to casting climate as a matter of risk. To deal with an unknown future, humanity invented the concept of risk, in which cost-benefit analysis provides a basis for rational decision making in the face of uncertainty. We thus apply the concept of risk in the context of adaptation to climate change and attempts at mitigation. Among the many aspects of these efforts, it becomes immediately apparent that of paramount concern is our ability to control emissions from the burning of fossil fuels and to apply alternative technologies to satisfy the world’s insatiable appetite for energy. This appetite is dominated by one overriding issue: how we are going to provide for the world’s electricity needs. Because of its centrality to future climate, we have chosen to focus on this matter in the final chapter of this book, at the same time recognizing that the production of energy is now (and has been) a rapidly changing enterprise. This book found the support of many people. One of the joys of its writing was how much we learned from each other and from our many colleagues, many of whom are at the Lamont-Doherty Earth Observatory of Columbia University. We are also indebted to Kent Short, Scott Mandia, Dennis Hartmann, Alessandra Grannini, Mingfang Ting, A. Park Williams, and several anonymous reviewers who suffered through early drafts of the manuscript and provided insightful suggestions for its improvement. The content of the book was also strongly influenced by the innovative legacy of climate courses taught at Columbia University, and we are grateful to the instructors who shared their support, insights, and materials over the years. Several colleagues deserve particular mention: Stephanie Pfirman, Jerry McManus, Yochanan Kushnir, Mark Cane, Ben Cook, Linda Sohl, Steve Cohen, and Louise Rosen. JES is also grateful to Henry and Lana Pollack, who inspired his transition into climate science and who remain aspirational examples of effective public communication and outreach. This book also would not have been possible without the unfailing

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enthusiasm and hard work of our editor, Patrick Fitzgerald, and the technical support of manuscript editor Irene Pavitt and designer Milenda Lee, all of Columbia University Press, and of Saebyul Choe of the American Museum of Natural History. Finally, none of this would have been possible without the inspiration, support, and patience of our families. Edmond A. Mathez Jason E. Smerdon

Climate Change

Satellite view of Iceland and surrounding waters

Climate is a dynamic “system” that ultimately is driven by the energy of the Sun, but results from the dynamic interactions among Earth’s atmosphere, ocean, rock, ice, and life—all of which are observable in this photograph. The lighter ocean colors are due to phytoplankton blooms. The image was taken by NASA’s satellite Terra on June 21, 2004. (J. Descloitres, MODIS Rapid Response Team, NASA/Goddard Space Flight Center, Visible Earth, https://visibleearth.nasa.gov/view.php?id=71461)

PROLOGUE

“R a i n, h e av y at t i m e s , will begin in late morning and continue into the evening hours as a cold front sweeps across the area . . .” Ah, the weather forecast—what would we do without it? There is no shortage of conversation about the weather, which, after all, touches our daily lives. For some, the weather is very important—especially if their harvest depends on it. For others, it is more tangential. We’re thinking of ourselves here; on most days, we just want to know about our treks to work in New York City. Will rain or snow make it impossible or just more miserable than usual? And then there is climate. What might a climate forecast be like?1 “The next decade will bring persistent showers and mild temperatures from January through March and extensive periods of no rainfall at all throughout the summer months.” Hmm . . . that seems a bit remote from our immediate worry of getting to work. Although it may appear harder to connect the implications of climate to our daily lives, it is relevant. Climate dictates the kinds of clothes we keep in our closets and the way the buildings around us are made. It may drive our decisions about where to live, when we decide to visit places around the world, and what kind of car we choose to buy. So how are they different, weather and climate? Weather and Climate Upon reflection, it becomes clear that there are essential differences between weather and climate, even though they are inextricably linked. Weather refers to conditions in the atmosphere at any one time. The familiar radar images on television show that local weather systems develop and dissipate rapidly over the course of hours to a day. On a continent-wide scale, weather systems form and decay over days to a week or so. A persistent weather system, such as a warm spell, may last for a couple of weeks or even more, especially in mid-latitudes, where the tracks of weather systems are commonly determined by the position of the polar jet stream, as chapter 1 explains. Climate, in contrast, can be thought of as the “average weather” for a particular region over some period of time. We place “average weather” in quotes because climate itself is changing, so a weather average must always be defined over a specific time interval that may

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be different if determined over another. In any event, “region” can refer to the entire globe, as in global warming; to a large landmass, such as eastern North America; or to a small land area, as in the “microclimate” of a particular valley in a larger wine-producing region. Although we have become adept at forecasting weather hours to a day or so ahead, predictions beyond that become progressively more uncertain with distance into the future. Weather is inherently chaotic. Strictly speaking, the term chaotic in this context means that small differences in initial conditions result in large differences in how a system will eventually develop. In other words, to predict weather accurately, we would have to know the temperature, humidity, barometric pressure, wind velocity, precipitation, and other characteristics of a weather system everywhere across an affected region, and even then prediction would be accurate for only the next week or two. Being an average condition, climate is not chaotic—at least not in the same way that weather is. Instead, it displays stable and distinctive patterns of change on specific timescales. Examples include annual changes such as monsoons, which are shifts in winds that bring seasonal rains to a number of regions in the tropics and subtropics. They also include fluctuations that occur only every several years, the most notable of which is the El Niño–Southern Oscillation (ENSO) phenomenon, referring to the periodic shifts of winds and ocean currents that bring warm water to the equatorial eastern Pacific Ocean and dry conditions to the western equatorial Pacific, and that influence climate in far-flung parts of the globe. What does all of this mean for those forecasts that we originally imagined? On a day-today basis, climatologists like to boil down the differences between climate and weather to their essence: You dress for the weather and build a house for the climate. Or how about: Climate is what you expect; weather is what you get. If you are a dog owner, you may prefer: Weather is like the dog running back and forth, and climate is like the leash driving the ultimate path. Whatever your preference—and you may have your own—these examples are illustrative of the differences and dependencies of weather and climate. One additional point must be emphasized. Climate change is a long-term phenomenon. This inherently protracted characteristic, at least in terms of human timescales, creates one of the conundrums surrounding attempts to reduce carbon dioxide (CO2) emissions from the use of fossil fuels, the main culprit in global warming. It is simply difficult to marshal either the individual or the collective will to make the changes necessary to avoid the negative impacts of global warming because they generally do not appear to affect our immediate lives. The Climate System Climate is a dynamic system resulting from the combined interactions of various parts of Earth with one another and with the Sun. The components include the atmosphere; the ocean (hydrosphere); glaciers, terrestrial ice sheets, and sea ice (collectively known as the cryosphere); the living biomass (biosphere); and even the solid Earth (lithosphere). Think of it as your body, with all its parts interacting in an interlocking whole. And like your body, the climate system is not just a set of physical interactions, but also a complex chemical system, with matter flowing through its various parts and influencing its characteristics.

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The atmosphere, being the medium in which we live, is the part of the climate system that affects us most directly. The atmosphere contains greenhouse gases—the gases that absorb infrared (IR) radiation—and in this way, it keeps Earth’s surface in a habitable temperature range. Indeed, without such gases, Earth’s surface would be frozen and lifeless. The atmosphere also plays a major role in transporting heat and moisture around the planet. Because Earth is a sphere, the Sun’s heat is more intense near the equator than near the poles. This uneven distribution generates winds that carry heat from the equator toward the poles and from the surface to the upper atmosphere. Additionally, the atmosphere is not isolated from the ocean. The ocean circulates, in part driven by the winds and guided by the positions of continents, and thereby also transports heat toward the poles. Indeed, the ocean holds far more heat than does the atmosphere, but it flows much more slowly. Many of these interactions are also important in the transport of moisture around the planet. For instance, much of the rain that falls on land was originally evaporated from ocean water. The atmosphere therefore takes up enormous amounts of moisture and redistributes it around the globe based on its large-scale patterns of circulation. Finally, the atmosphere also holds ozone (O3), which shades the surface of Earth from much of the lethal ultraviolet (UV) radiation received from the Sun. As for the chemical interactions, the most important are the exchanges of carbon among the atmosphere, ocean, and biosphere (which includes the dead biomass held mainly in soil). In fact, we can think of these spheres as reservoirs where nearly all the carbon on or near Earth’s surface is stored. This description leads to the concept of the carbon cycle, referring to the flow of carbon among the various reservoirs. In months to decades, photosynthesis by plants and decay of organic materials affect the amount of CO2 in the atmosphere, but over longer periods, it is the ocean that exerts the dominant control on atmospheric CO2 content because the amount of carbon in the ocean is nearly 50 times that in the atmosphere. If we think of the climate system as something like our body, the atmosphere and the ocean are its main organs, and the carbon cycle is the circulation system that connects them to each other and to other organs. Most of the carbon (more than 99.9 percent) on Earth exists not in the ocean, atmosphere, or biosphere (the “surface” reservoirs), but in a deep reservoir in the form of rocks—that is, the lithosphere. The lithosphere is part of the climate system mainly because carbon flows between it and the reservoirs on Earth’s surface, but this flow is far slower than the flow of carbon among the surface reservoirs. Over millions of years, a close balance has apparently persisted between two processes: • The flow of carbon from the surface to the rock reservoirs by means of the removal of CO2 from the atmosphere and the ocean through the formation of carbonate and other carbon-bearing rocks • The return of CO2 to the atmosphere by means of the breakdown of those rocks at the high temperatures and pressures of the deep Earth In fact, this long-term balance appears to have acted as a natural, planetary thermostat, maintaining conditions on Earth’s surface that have allowed for liquid water to be stable and

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that have been conducive to the evolution and survival of life since nearly the beginning of Earth’s history. The different parts of the climate system also interact through feedbacks, or phenomena that amplify or diminish the forces that act to change climate. An example helps to envision them. As the Arctic warms due to the buildup of greenhouse gases, sea ice melts. As sea ice melts, there is less bright ice to reflect solar energy back to space, and the ocean absorbs more energy. The greater absorption of energy, in turn, further warms the ocean and overlying

TA B L E P. 1

D I F F E R E N T T I M E S C A L E S O F S O M E W E AT H E R A N D C L I M AT E P H E N O M E N A

Timescale

Example phenomena

Daily

Warm days and cool nights due to solar heating and Earth’s rotation

3–7 days

Weather events, such as the passage of fronts

Months

Eastward-propagating weather disturbances across the tropical Indian and Pacific oceans due to planetary-scale fluctuations in wind patterns

Yearly

Warm summers, cool winters, and shifts in zones of precipitation due to the tilt of Earth’s spin axis and its orbit around the Sun; monsoons, notably on the Indian subcontinent, due to summer heating of landmasses that draws moist winds off oceans

23–36 months

Periodic wind and temperature oscillations in the equatorial stratosphere due to internal atmosphere dynamics

2–7 years

ENSO events, in which changes in equatorial Pacific Ocean currents and winds result in dramatic shifts in rainfall in equatorial regions globally and in lesser shifts in the climate of some temperate regions

1–3 decades

Generally ill-defined oscillations, such as the Atlantic Multidecadal Oscillation, a fluctuation in ocean water temperature over the entire North Atlantic Ocean that affects temperature in adjacent landmasses

Centuries

Irregular fluctuations that have led to multi-century cold or warm periods, such as the Medieval Climate Anomaly (or Medieval Warm Period) and the Little Ice Age, the causes of which are uncertain but may be related to one or more natural phenomena, such as variations in solar irradiance and volcanism

10,000– 100,000 years

Regular variations in orbital parameters (the slow oscillations in Earth’s tilt relative to the orbital plane, precession, and eccentricity of its orbit around the Sun) that affect the amount of energy reaching the Northern Hemisphere and are responsible for the approximately 100,000-year glacial cycles of the past 1 million years

Millions of years

Changes in the positions of continents, in solar luminosity, and in the composition of the atmosphere, all of which affect climate globally

Source: J. R. Christy, D. J. Seidel, and S. C. Sherwood, “What Kinds of Atmospheric Temperature Variations Can the Current Observing Systems Detect and What Are Their Strengths and Limitations, Both Spatially and Temporally?,” in Temperature Trends in the Lower Atmosphere: Steps for Understanding and Reconciling Differences, ed. T. R. Karl et al. (Washington, D.C.: Climate Change Science Program, Subcommittee on Global Change Research, 2006), 29–46.

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atmosphere, causing even more ice to melt. In this way, the melting of ice amplifies the warming due to greenhouse gases alone. This feedback in part accounts for why the Arctic is more sensitive to global warming than is the rest of the planet. Feedbacks can be complex and can operate in unpredictable ways, and they are one reason that projecting future climate is fraught with uncertainty. The climate system is complicated in other ways, one of which is that the various climate phenomena operate on different timescales (table P.1). Some of these phenomena and their associated timescales are familiar—for example, the daily variations of warm days and cool nights, and the annual passage of the seasons. Other phenomena occur on longer or irregular intervals but on timescales that are understandable, and still others occur on timescales beyond the human experience and are consequently difficult to imagine. Our knowledge of the last may also be incomplete because the evidence for them is buried (commonly and literally) in the geological record. Climate Change: Separating Facts from Fears What we do know from the available records, both geological and observational, is that the climate is changing. Hardly a day goes by without some mention of it in the news. Earth’s climate is warming; CO2 and other greenhouse gases have been building up in the atmosphere mainly as a consequence of the burning of fossil fuel; and the scientific evidence is now overwhelming that this buildup is causing the warming. These statements are the facts of climate change. Less certain are how much the climate will warm in response to growing emissions and to what extent the warming will change the world around us. Should the warming be substantial, it may have huge negative impacts on biodiversity, ecosystems, agriculture, ocean life, the global economy, and the well-being of human societies everywhere. These possible results are the fears of climate change. It is important to separate the facts from the fears. The facts give us insight, but the fears reflect the risks. Ultimately, we have to understand the risks if we are to develop intelligent policies to deal with global warming. To assess the risks, we need the knowledge, so let us start with the facts.

Obse rvations of Climate Change: The Facts In addition to CO2, the greenhouse gases include methane (CH4), nitrous oxide (N2O), ozone (O3), and water vapor (H2O). These gases reside mostly in the troposphere, the lower 10 to 15 kilometers (30,000–50,000 feet) of the atmosphere, where the weather occurs. Here, the greenhouse gases absorb heat radiated from Earth’s surface and thus act as a giant insulating blanket. Greenhouse gases have been building up since the beginning of the industrial age, but only since 1958 has the CO2 content of the atmosphere been measured directly, beginning first on the top of Mauna Loa in Hawai‘i, as described in chapter 4.2 The remarkable Mauna Loa record shows that the amount of atmospheric CO2 has been continuously climbing over the years. In 1958, the average CO2 content of the atmosphere was 315 parts per million (ppm)

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by volume (0.0315 percent); by 2015, it had reached about 400 ppm and was rising at a rate of nearly 3 ppm a year. Both the rate of increase and the amount of CO2 presently in the atmosphere are greater than at any time in the past 800,000 years, the time over which the most detailed paleoclimatic record of atmospheric CO2 content exists, and probably much longer.3 Furthermore, a number of observations make quite clear that the CO2 is originating mainly from the burning of fossil fuels. At the same time, global mean surface air temperature has been rising, too. The warming began around 1910 and has proceeded in two distinct intervals, the first from 1910 to 1940, and the second beginning in the late 1970s and continuing. From 1880 to 2012, global mean surface air temperature increased by 0.85°C (1.53°F), and over the past three decades, the rate of increase has accelerated to 0.27°C (0.49°F) per decade.4 What is causing the warming? The evidence is overwhelming that it is a result of the rising levels of greenhouse gases in the atmosphere. First, there is the basic physics: greenhouse gases absorb radiant heat, or infrared energy, which we know from measurement and observation, and such energy is being emitted from Earth’s surface. As was recognized more than a century ago, the climate therefore should warm as the concentration of greenhouse gases in the atmosphere increases. Second, there are no other known natural forces external to the climate system that can account for the warming. For instance, some have suggested that changes in the amount of solar irradiance (that is, the amount of sunlight) reaching Earth may be causing the warming rather than the increase in the greenhouse gas content of the atmosphere. There is certainly indirect evidence that irradiance does change with time and that this may explain some of the cool and warm spells in the past. But except for the 11-year sunspot cycle, which represents only a minuscule fluctuation in irradiance, there have been no detectable changes in solar output since the advent of precise measurements by satellite in 1978, yet climate has rapidly warmed since then.5 Internal variations in the climate system—that is, fluctuations occurring on timescales of years to multiple decades and resulting mainly from the system’s dynamic nature—may conceivably account for warming, at least regionally. These variations include phenomena such as ENSO and the Atlantic Multidecadal Oscillation (AMO), both of which are oscillatory phenomena in the ocean and atmosphere that result in large-scale redistributions of heat and are discussed fully in chapter 3. However, a variety of observations argue against internal variations as being responsible for the warming. First, warming has been occurring more or less everywhere—it is a global, not a regional, phenomenon, as would be expected if the warming were due to internal variability.6 Second, the lower atmosphere below about 10 kilometers (33,000 feet) has also been warming, while parts of the upper atmosphere have been cooling, as expected from the basic theory of greenhouse gas warming. Third, both the annual average maximum (daytime) temperature and the annual average minimum (nighttime) temperature have increased, but the nighttime temperature has increased more than the daytime temperature. This observation is consistent with what would be expected from increased insulation by greenhouse gases, as explained in chapter 5. Fourth, the oceans have been warming by far more than can be accounted for by natural internal variations in the climate system.7

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Thus CO2 and other greenhouse gases are increasing in the atmosphere, and at the same time, Earth’s climate is warming. The scientific evidence overwhelmingly points to the buildup of greenhouse gases in the atmosphere as the cause of the warming because (1) it is an expectation of the basic physics, (2) it is consistent with all the observations of the present-day climate system and the recent record of climate change, and (3) no one has found an alternative hypothesis that can account for those observations.

P otential Consequences of Climate Change: The Fears The fears concern how much the planet will warm and what the repercussions may be, but there is much uncertainty about this future. Climate change permeates the entire environment, so numerous effects, ranging from loss of sensitive ecosystems to increased occurrences of extreme weather events, appear likely. But the more profound and more distant potential consequences are those that are more uncertain. Two potential ramifications that may have a severe impact on society illustrate both the fears and their associated uncertainties. The first and, perhaps, the more frightening is the possibility of harsh and extensive droughts significantly affecting worldwide agriculture and resulting in widespread famine. A number of regions are particularly vulnerable to drought, including western North America, the eastern Mediterranean, Southeast Asia, and the Sahel of Africa (the southern borderland of the Sahara Desert). About 1,000 years ago, for example, western North America experienced a number of “megadroughts,” each of which lasted for several decades over a 300-year interval of relative warmth.8 Such megadroughts have not been seen since. The megadroughts, and the multiyear droughts that have plagued these areas since, appear to be related to conditions in the tropical oceans, but exactly how those conditions influence rainfall patterns is not completely understood. The theory is that warming increases the probability of the occurrence of megadroughts; the fear is that such droughts will occur and have severe economic consequences.9 History is replete with examples of changes in climate that caused localized famine, which, in turn, resulted in massive societal disruptions, including conflict. In today’s world, where trade is global and many economies are intertwined, we might expect localized disruptions to play out differently than in the past. But maybe not. While warming increases the likelihood of drought, it also increases the likelihood that severe and extensive droughts could occur simultaneously in many of the world’s major food-growing regions. Although this may not be likely, especially anytime soon, it is not too difficult to imagine global famine and a cascade of dire and largely unforeseen consequences that follow. History teaches us that this is not an idle concern for the modern world, as demonstrated in chapter 12. Substantial sea-level rise is the second serious concern. Sea level is currently rising at a rate of 3.2 ± 0.4 millimeters (0.13 inch) a year, equivalent to 32 centimeters (13 inches) a century.10 The main contributions are the melting of glaciers, thermal expansion of the ocean (warm water is less dense than cold water and therefore occupies more space), and loss of ice from the Greenland and West Antarctic ice sheets. How much or how quickly Greenland and Antarctic ice will disappear is poorly constrained, so the extent of future sea-level rise

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is uncertain. This is reflected in the numerous estimates of twenty-first century sea-level rise that range from about 30 to 150 centimeters (1–5 feet).11 The stakes, nonetheless, are high. Worldwide, two-thirds of the cities with populations of more than 5 million people are vulnerable to the effects of rising sea level (the most serious of which are flooding during storms and coastal erosion). A sea-level rise of just 0.5 meter (20 inches) could threaten 10 percent of the world’s population, amounting to some 700 million people, 75 percent of whom live in Asia. The rise will be gradual, but even a 1-meter (40-inch) rise in this century will impose enormous economic costs and possibly also disrupt society in ways that are difficult to foresee. Thinking About the Future in the Face of Uncertainty As noted, we know neither exactly how much or how rapidly sea level will rise, nor how drought will affect the global food supply in the distant future. Yes, we are, for the most part, ignorant. But this is exactly the point. We are smart enough to know that we are putting ourselves at risk, but we are not so smart that we can precisely gauge the risk. Speaking of risk, this concept was invented to deal with an uncertain future. Most of us buy insurance to mitigate risk, such as the personal financial risk associated with a house burning down. We can also buy insurance, in a sense, to alleviate the effects of climate change by adopting policies that seek to minimize the change. But there is a big difference in the two cases: while insurance should allow us to buy a new house, if climate change unleashes globally drastic calamities, unlikely as this might be, we are out of luck because we will not be able to buy a new planet. The important points are that efforts to limit climate change and to mitigate its impacts are exercises in risk management, and that understanding the problem in that light should help guide our response. Again, this perspective is developed in chapter 12. It is worth pointing out two characteristics of the climate system that further exacerbate the uncertainty of our future. First, the climate system possesses inertia: it takes time for the system to reach a new balance in response to the forces that have acted to change it. In other words, even if greenhouse gas emissions were to be immediately capped at today’s levels, warming would continue for several decades. By one estimate, there is currently more than 0.6°C (1.1°F) worth of warming already locked in, or “in the pipeline,” since the year 2000.12 Second, as the climate changes, it can reach tipping points, or large abrupt shifts in response to the forces that were gradually causing it to change. The geological record is replete with instances of abrupt and dramatic shifts in climate. On a related note, students often ask us why, considering that climate has changed dramatically in the past in response to only natural forces, we should concern ourselves with human-induced changes. The answers are simple. First, complex societies were not around to experience the huge shifts of the past. The climate of the past 11,600 years, known to geologists as the Holocene, has been stable by the standards of the past 1 million years, and complex societies have been around for only about the past 6,000 of those years. Second, the current human-induced changes are proving to be far more rapid than any natural changes. So the climate system has within it the possibility of bringing about changes that are both

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more dramatic and more rapid than societies have ever experienced, and that could challenge their abilities to adapt. The Story This book is divided into four parts and takes a somewhat unconventional approach to presenting its subject. Part I is not about climate change; rather, it is about the climate system. The concept of the Earth system, of which the climate system is a part, is fundamental in geological thought, and understanding how the climate system works—in other words, how the components of the climate system interact dynamically and chemically with one another—is a necessary prerequisite to understanding how climate responds to the forces that are acting to change it. Thus Part I recounts the fundamental characteristics of the atmosphere and the ocean, and the ways in which they interact dynamically and chemically with each other through the carbon cycle. Part II introduces the equally fundamental concept of radiation balance, which is the scientific framework that has emerged for thinking about climate change. Here we describe the many factors that influence radiation balance. We also explore the fascinating story of past climate changes, or paleoclimate, which gives us essential insight into how climate is changing today and how it will change in the future as more greenhouse gases are injected into the atmosphere. The story focuses on the past 3 million years, but we also visit a more distant time to seek additional insight. Part III concerns the numerous consequences of climate change. It begins by exploring how climate change since the end of the last glaciation has influenced the course of human history. It then documents the rapid increase in global temperature that has occurred over the past century and some of the changes that we are beginning to experience as a consequence of that warming. These include changes in patterns of precipitation and drought and in the occurrence of severe weather events. The Arctic is especially sensitive to warming and, at the same time, has an important influence on global climate, so we also investigate the changes there. As noted, sea-level rise is an important concern, leading us to examine what is happening to the Greenland and West Antarctic ice sheets. Part IV is about the future. Although climate models tell us a great deal about the behavior of today’s climate, they are the main means of portraying future climate, and for that reason they are included here. We argued earlier that mitigating climate change is an exercise in risk management, and recognizing this serves as a basis for developing intelligent policies to alleviate the effects. For these reasons, we devote some attention to climate risk. Finally, obviously central to the future is how the world is going to satisfy its insatiable appetite for energy while keeping carbon emissions in check. It is the vastness of the energyproducing enterprise that astonishes, and we take on this subject as the final chapter. That is the story. It is complex, it suggests that we face a difficult future, but it also implies that we can avoid the most dire consequences of climate change by intelligent action.

Part I THE CLIMATE SYSTEM

The crescent moon as seen through Earth’s thin upper atmosphere

The atmosphere is the medium of climate. It insulates Earth and keeps it warm, transports heat from the tropics to the poles and from the surface to the heights, and transfers water from the oceans to land by precipitation. A crew member of the International Space Station took this photograph from about 360 kilometers (225 miles) above Earth. The cloud deck is about 6 kilometers (3.7 miles) high. (NASA, Johnson Space Center, https://eol.jsc .nasa.gov/SearchPhotos/photo.pl?mission=ISS008&roll=E&frame=8951)

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THE ATMOSPHERE

T h e atmo sph e r e is t h e protective blanket that makes life possible on Earth. It is the air we breathe; it protects us and all other organisms that live on the surface from the Sun’s deadly ultraviolet (UV) radiation; it insulates Earth and regulates its surface temperature so that liquid water exists now and apparently has always existed somewhere on the surface since nearly the beginning of Earth history. Without the atmosphere, Earth’s surface would be frozen, and life would not exist here. Earth’s atmosphere reaches about 500 kilometers (300 miles) above its ocean and land surface. It is layered in terms of composition, temperature, and dynamical behavior, and it has a composition that is unique among planets in our solar system and, if not unique, apparently extremely rare among planets in other solar systems. The atmosphere itself transports the majority of energy from the equator to the poles, and its winds propel ocean-surface currents that comprise the second important part of poleward heat transport. Furthermore, winds transport water evaporated from the ocean to the land, where it may fall as rain or snow, thus driving part of the hydrological cycle. The atmosphere is also one path by which carbon moves between the ocean and the land and other carbon reservoirs (chapter 4). Despite its importance, the atmosphere constitutes only a minuscule part of Earth (Backof-the-Envelope Calculation). The total mass of the atmosphere is 5.14 × 1018 kg, which is a mere four-thousandth of the mass of the ocean (1.39 × 1021 kg) and one-millionth that of the solid Earth (5.98 × 1024 kg). If not from the surface, then at least from space, the true extent of Earth’s thin skin of atmosphere is apparent (see Prologue opening figure). This chapter provides a general overview of the nature of the atmosphere and the way it circulates. The important concept of radiation balance and the role of greenhouse gases in climate and climate change are discussed separately and in detail in chapters 5 and 6. The Composition of Air From a planetary perspective, Earth’s atmosphere is unique because it contains oxygen (O2). No other planets in our solar system, and probably very few in other solar systems, possess oxygenated atmospheres. In fact, early in its history, Earth’s atmosphere was also essentially

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BACK- OF- THE- ENVELOPE CALCULATION The Vastness of the Atmosphere The atmosphere is very important for life on Earth and moves a tremendous amount of energy and mass around the surface of the planet. Despite its great importance, however, the atmosphere represents a very small portion of Earth’s thickness and mass. To help put this in perspective, let’s consider a few scaling exercises to help place the relative quantities in context. The diameter of the solid Earth is about 12,742 kilometers (7,918 miles), while about 80 percent of Earth’s atmospheric mass is in the troposphere, with an approximate height of 16 kilometers (52,000 feet) above Earth’s surface. Those are distances that are hard to grasp, so let’s consider scaling the numbers to something more familiar. The diameter of a men’s professional basketball is about 25 centimeters (9.8 inches). If we represent Earth with the basketball, how thick would its corresponding atmosphere be? The answer is simple: we just have to multiply 25 centimeters by the ratio between the thickness of the troposphere and the diameter of Earth (0.12 percent). The answer is 0.03 centimeter (0.01 inch), or about the thickness of three sheets of standard paper. We can perform a similar calculation for mass. The mass of Earth is about 6.0 × 1024 kg, while the mass of the troposphere is about 75  percent of the mass of the atmosphere, or 3.85  ×  1018 kg. The mass ratio of the troposphere to the solid Earth is therefore about 6.4 × 10−7. Because this ratio is much smaller than the distance ratio, let’s consider a more massive object that is familiar to us all: an automobile. Let’s assume that the average mass of a car is about 1,500 kilograms (3,307 pounds). If the solid Earth were only as massive as a car, the proportional mass of the atmosphere would therefore be about 1 gram (0.035 ounce). That is the approximate mass of a paperclip! Both of these exercises are surprising, given our everyday experience with the vastness of the atmosphere. Despite its apparent scale from our perspective, however, the atmosphere is a very thin veneer that encapsulates our vast planet.

O2-free. The early atmosphere may have been dominated instead by nitrogen-bearing compounds. Nitrogen was part of the material that accreted to form Earth, but because none of the minerals that solidified from that early molten mass take up nitrogen, it was likely left as a major component of the early atmosphere. The addition of oxygen to the atmosphere apparently began later, with the emergence of photosynthesis as an important metabolic process. By 2.4 billion years ago—more than 2 billion years after Earth formed—an atmosphere containing perhaps a few tenths of a percent of O2 began to develop. The reasons, therefore, that Earth’s atmosphere is unique among planets are that Earth harbors life and that early Earth provided an environment conducive not just to the establishment of life, but also to the evolution of complex forms of life.1 For the next 2 billion years, Earth’s atmosphere probably contained only a few tenths to thousandths of a percent of O2, with the modern O2-rich atmosphere developing just several hundred million years ago.2 As O2 built up in the atmosphere, so did ozone (O3). Ozone is notable because it absorbs UV radiation, which is lethal to life in large doses. According to

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the fossil record, life spread from water to land about 400 million years ago. Thus the emergence of terrestrial life may have been made possible by the presence of enough O3 in the upper atmosphere to shade the surface from UV radiation. Today, just three gases—nitrogen (N2 [78.1 percent]), O2 (20.9 percent), and argon (Ar [0.93 percent])—make up 99.96 percent of dry air by volume. The remaining gases are carbon dioxide (CO2), methane (CH4), O3, nitrous oxide (N2O), and a host of other “trace” gases—those that exist in only minuscule quantities. Additionally, water vapor is almost always present in the atmosphere but in varying amounts. Its concentration depends strongly on temperature and on a hot day may reach several percent regionally. In the context of climate, water vapor and CO2 are particularly important because both are greenhouse gases, but their roles differ in important ways. The anthropogenic (due to human activities) rise in the CO2 content of the atmosphere is the root cause of the current warming, but water vapor acts as a feedback; that is, its concentration increases due to the warming from other causes, and by being a greenhouse gas itself, it amplifies that warming. Carbon dioxide also has a long residence time in the atmosphere of thousands of years, while the water vapor content fluctuates rapidly. At Hawai’i’s Mauna Loa Volcano monitoring station, the average annual concentration of CO2 in the atmosphere first exceeded 400 parts per million (ppm) by volume (0.0400 percent) in 2015 (chapter 4). This concentration is up from about 280 ppm at the beginning of the Industrial Revolution around 1750, and is currently rising at a rate of 2.7 ppm a year. Other greenhouse gases—such as CH4, O3, and N2O—are also on the rise but exist in much lower abundances and have limited residence times in the atmosphere. Some of the trace gases are also potent greenhouse gases. The differing roles of CO2 and water vapor, as well as the contributions to warming by the other greenhouse gases, are described in detail in chapter 6. The Compressibility and Pressure of Air A fundamental property of air—for that matter, any gas—is its compressibility, or the amount that its volume changes in response to the pressure that is exerted on it. The higher the pressure on the gas, the smaller its volume becomes (and the greater its density). The volume also depends on temperature, such that the higher the temperature, the higher the volume (and the lower the density). The temperature (T), pressure (P), and volume (V) of gases (in the ranges that they are found in the atmosphere) are related by the ideal gas law,3 PV = nRT, where n is the number of moles of gas (1 mole = 6.02 × 1023 molecules [or atoms for monoatomic gases]) and R is the universal gas constant (8.314 J/mole K, where J is a joule, a unit of energy, and K refers to the unit of temperature known as a Kelvin; 1 Kelvin is equivalent in magnitude to 1 degree Celsius, but the two temperature scales are different [273 K is the same temperature as 0°C]). Because air is compressible, its weight causes air pressure to be much greater at the surface than at altitude. The mass of the atmosphere is such that a column of air 1 centimeter

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FIGURE 1.1 The decrease in air pressure with altitude Air pressure is highest at the surface because of the weight of the overlying air, which amounts to 1,034 g/cm2 (14.7 lb/in2). Half of the mass of the atmosphere is below an altitude of 5.6 kilometers (3.5 miles).

(0.394 inch) on a side at its base and extending from sea level to the outer limit of the atmosphere weighs 1,034 grams (10,340 kg/m2 [14.7 lb/in2]). This is the “air pressure” around us. Air pressure decreases exponentially with altitude (figure 1.1). In other words, there is a lot more air in a given volume at Earth’s surface than in the same volume high in the atmosphere, and the air near the surface is relatively dense, whereas air high in the atmosphere is “thin.” For an atmosphere at its mean temperature, about 50 percent of its mass is below an altitude of 5.6 kilometers (18,370 feet) above sea level. Mechanisms of Heat and Mass Transfer

The Convective Motions of the Atmo sphere The relationships among the pressure, temperature, and volume of air help explain why the atmosphere is in continuous convective motion. Convection refers to the buoyant rising of

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less dense fluid (gas or liquid or, for that matter, even a ductile solid) and the sinking of more dense fluid.4 The lower part of the atmosphere is heated primarily from below by Earth’s surface. The density of a parcel of air that is warmed at the surface decreases (becomes more buoyant), and that parcel rises if the air around it is cooler and therefore denser. The heating of the air parcel occurs through the transfer of sensible heat, or heat associated with a change in temperature, and occurs principally by conduction from Earth’s surface. Conduction refers to the transfer of heat by collisions between molecules of warmer matter, in which the molecules possess relatively high kinetic energies, and those of cooler matter, in which the molecules possess relatively low kinetic energies. When these collisions occur, they increase the kinetic energy of the molecules in the cooler matter and therefore increase its temperature. In the atmosphere, convection occurs on all scales from meters (hot air rising from a chimney), to kilometers (warm air rising and cold air descending below a thunderhead), to 1,000 kilometers (620 miles) or more (convective rise of the atmosphere over the warm equatorial Pacific Ocean). On the large scale of weather patterns, convection spawns winds to replace the rising air. This reflects the fact that rising air masses create zones of relatively low atmospheric pressure, and sinking air masses result in zones of relatively high atmospheric pressure. Again, this occurs on various scales and intensities, from the extreme low pressures that characterize the 100 kilometer–wide (62-mile) cores of hurricanes (also termed cyclones and typhoons), to the gentle highs that characterize the global-scale belts of deserts in the subtropics.

Adiabatic Temperature Change Vertical motions of air result in adiabatic (from the Greek word adiabatos [impassable]) heating or cooling. An adiabatic process is one in which there is no change in the heat content of the system. For the atmosphere, this means that a particular parcel of air that adiabatically rises or sinks neither gains nor loses heat. This requires that the rising or sinking is accompanied by a change in the temperature of the air. For example, as an air parcel rises and expands due to the pressure decrease, its temperature drops. In the idealized case, the temperature reduction is in proportion to the extent of the volume increase. The reason is that the expansion of the parcel of air represents an expenditure of energy on its surroundings, and the temperature drop represents that expenditure. We commonly experience this phenomenon when, after releasing gas from a pressurized can of, say, spray paint or deodorant, we find that the can has cooled. Conversely, if the parcel is adiabatically brought back to its original pressure, its volume decreases and its temperature increases, with both returning to their original values.5 For a thermodynamic explanation of adiabatic changes, see box 1.1.

Wate r Vap or in th e Atmosph ere Water vapor plays an important role in climate because it is a greenhouse gas, serves as an important mechanism of heat transport, and forms clouds and precipitation.6 The amount

BOX 1.1 Thermodynamic Background Adiabatic processes are ones in which no heat is exchanged between a system and its surroundings. To understand adiabatic processes, it is useful to consider the First Law of Thermodynamics, ΔE = E2 − E1 = q + w, where ΔE is the total change in energy of the system, E2 and E1 are its final and initial states, q is the heat change, and w is the work done on the system. (Here we choose the convention whereby positive values of q and w mean E2 > E1.) The First Law states that (1) energy is always conserved (it is neither created nor destroyed in any non-nuclear process), and (2) heat and work are equivalent (both are forms of energy transfer from one to another system). With the First Law in mind, now let us consider what happens to a parcel of air as it rises several kilometers into the atmosphere. To help our thinking, let us imagine the air parcel as a spherical balloon with a mass that remains constant; that is, the walls of the balloon are impermeable (figure B1.1.1). (This is actually not too far from reality. Because the thermal conductivity of air is low and air density depends on temperature, in the real atmosphere air tends to exist and move around as thermally discrete parcels.) As the balloon rises, the atmospheric pressure around it decreases. The volume of the balloon thus increases so that the inside pressure equals the outside pressure. The expansion of the system (the balloon) against its surroundings is the work done by the system (and thus has a negative value) and is given by the expression w = – PΔV. If the rise is adiabatic, the balloon neither gains nor loses heat, so q = 0. Then, according to the First Law, the expenditure of work by the system during the expansion must equal its expenditure of energy: ΔE = q + w = 0 − PΔV. Because the change in internal energy is proportional to the temperature change, we may write, – PΔV = constant × ΔT. In other words, ΔT ≠ 0; more specifically, T (and ΔE) must decrease in proportion to the work done by the system on its surroundings. Conversely, were the balloon to be brought back to the surface, its volume would decrease and the air pressure and temperature inside it would increase, with all three returning to their original values if the expansion and contraction of the balloon were done reversibly. The ΔT for a reversible, adiabatic expansion of an ideal gas may be calculated from the heat capacities (the ratio of heat change to temperature change of a substance, discussed further in chapter 2) of the gas at constant V (Cv) and at constant P (Cp), with the assumption

q = 0, 6T