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Princeton University Press
The Long Thaw: How Humans Are Changing the Next 100,000 Years of Earth's Climate

The Long Thaw: How Humans Are Changing the Next 100,000 Years of Earth's Climate

by David Archer


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Product Details

ISBN-13: 9780691148113
Publisher: Princeton University Press
Publication date: 08/15/2010
Series: Science Essentials
Edition description: New Edition
Pages: 192
Product dimensions: 5.50(w) x 8.50(h) x (d)

About the Author

David Archer is professor of geophysical sciences at the University of Chicago, the author of "Global Warming: Understanding the Forecast", and a frequent contributor to the Weblog RealClimate.

Read an Excerpt


How Humans Are Changing the Next 100,000 Years of Earth's Climate

Princeton University Press

Copyright © 2009 Princeton University Press
All right reserved.

ISBN: 978-0-691-14811-3

Chapter One

The Greenhouse Effect

The global warming forecast is not new, nor has it changed much over the last century. The basic physics of the greenhouse effect was described in 1827 by Jean Baptiste Joseph Fourier. Fourier was a mathematician in Bonaparte's army in Egypt. His name is best known for the Fourier transform, a mathematical technique for separating some complicated signal (such as the history of temperature through time, to choose an apropos example) into the sum of simple waves of different frequencies (such as the day/night cycle and the annual cycle), what we call calculating a spectrum.

Fourier's contribution to Earth science is the idea that gases in the atmosphere that absorb infrared radiation could eventually warm up the surface of the earth. He made the analogy of a greenhouse, but the actual name "greenhouse effect" came later. The temperature of a planet is set by a natural thermostat, which balances the planet's energy budget. Energy comes in to the Earth as sunlight and leaves as infrared. The greenhouse effect of a gas changes the outgoing part of the budget, the infrared. All objects warmer than absolute zero shine in the infrared. A hot heating element glows red that we can see; the same object at room temperature glows in the infrared.

The rate of energy loss from an object as infrared radiation depends on the temperature of the object. According to the Stefan-Boltzmann relation, the object loses energy at a rate of [sigma] [T.sup.4], where [sigma] is the Stefan-Boltzmann constant (just a number one can look up in a reference book) and [T.sup.4] is the temperature of the Earth in kelvins raised to the fourth power. When the object is hot, it sheds energy much more quickly than when it is cool.

The planet balances its energy budget by warming up or cooling down until the energy loss to space equals the energy gain from the Sun, as in the top panel of Figure 1. The thermostat is a by-product of the need to balance the energy budget. The idea is analogous to water running through a sink, as in Figure 1, bottom panel. The faucet is on, and water is falling into the sink. The drain is open at the bottom of the sink, and the higher the water level is in the sink, the faster it will drain.

When the faucet is initially turned on, water flows in faster than it flows out, and the water level in the sink rises. The sink fills until water is running down the drain as quickly as it is coming out of the faucet. If we start the sink experiment with too much water in the sink, water would drain faster than it filled until it reached that same balancing water level.

If we give our no-atmosphere planet the same energy input from sunlight that the Earth enjoys, it would have an average temperature of about 3?F or -16?C, sub-freezing temperatures around the world. Fourier's greenhouse effect is what's keeping Earth so much warmer than this poor cold naked planet.

Fourier's insight was to add a layer of atmosphere to the planet, which absorbs and emits infrared radiation (Figure 2). The Earth's surface receives energy from the Sun, as before, and it also receives energy from infrared radiation shining down from the atmosphere. The temperature of the Earth's surface rises to about 86?F or 30?C. That's a bit on the high side, but much closer to the real temperature of the Earth.

The greenhouse gas in Earth's energy balance is analogous to a partial obstruction of the drain at the bottom of the sink. A grape or piece of cucumber falls into the strainer, slowing down the drainage. The water level in the sink rises until it gets deep enough to force water through the obstructed drain as quickly as it flows in from the faucet. Let's hope the sink reaches a new balanced water budget before it overflows.

Just over a century ago, in 1896, Svante Arrhenius, a Swedish chemist, took the most astonishing leap I have ever read in climate science. Arrhenius used measurements of the brightness of infrared radiation from the moon to predict the temperature change you would get from raising C[O.sub.2]. Arrhenius estimated a quantity which we now call the climate sensitivity, abbreviated as [DELTA][T.sub.2x]. This is defined as the amount of warming that the Earth would undergo, on average, from a doubling of the atmospheric C[O.sub.2] concentration. The climate sensitivity is probably the first benchmark that two climate scientists in a bar would use to compare two different climate models.

The moonlight infrared data came from Samuel Pierpont Langley, who was trying to determine the temperature of the moon. The hotter the moon is, thought Langley, the brighter it shines in the infrared, the same story as for the Earth. "The dark rays" as they called them, were separated into different bands of wavelengths (different colors, if we could see them) by using a prism made of salt, because salt is one of the few solid substances that doesn't absorb infrared radiation. The intensity of the different invisible beams was measured using something called a bolometer, a device that measures the rate at which the invisible incoming light warms up a thermometer. It all must have seemed rather spooky.

Arrhenius used the data in a way that was not intended by Langley. Arrhenius looked for changes in the intensity of the "dark rays" that vary with humidity, and with the moon angle overhead, which affects the amount of atmosphere the light had to go through. In the moonlight data, more moonlight is absorbed when the light passes through more C[O.sub.2] or more water vapor. Arrhenius used this relationship in the data to predict how much the Earth would warm if you doubled C[O.sub.2]. It was as though, analyzing the water flowing through our sink, Arrhenius calculated precisely how much the flow would slow down if you put a piece of carrot on the drain trap, obstructing the flow of water through the drain, and how much higher the water level in the sink would be.

The surface of the Earth does not all have the same temperature, though, the way that a sink has only one water level. Arrhenius did his calculation on a latitude and longitude grid, just as climate models do today, writing, "I should certainly not have undertaken these tedious calculations if an extraordinary interest had not been connected with them." After two years of pencil-and-paper arithmetic, he concluded that doubling the C[O.sub.2] concentration of the atmosphere would lead to 4 to 6?C of warming. Today, with the benefit of a century of innovation, hard work, and exploding computing power, we now estimate that doubling C[O.sub.2] would lead to about 2.5 to 4?C of warming. There have been revisions, discoveries, missteps, and wrong directions, as in any science, but on the whole not much has changed in the past century.

So what have climate scientists been doing in the meantime? Climate science has really exploded in the past few decades, as global warming grew from a prediction into an observation in the real world. Globally, about 2 billion dollars per year are being spent on climate change research, 50% of this in the United States. This sounds like a lot of money, and it is, but to put it into perspective, it amounts to only about 5% of the profits from the Exxon Mobil Oil Company. Much of the climate research money is used to pay for satellites that monitor various aspects of the climate of the earth from space. Satellites are expensive. Meanwhile, thousands of scientists worldwide, at universities mostly, are hard at work developing climate models and theory, analyzing meteorological data, and reconstructing climates of the past. This form of research has an entrepreneurial feel to it: individuals or small groups, looking for the new angle that will get them funded and published. The scientific literature about global warming has exploded in the last decades, rising from about a hundred papers per year in the 1980s to a thousand per year today (Figure 3).

Climate science is interdisciplinary enough that it is a challenge to synthesize the bits and pieces. For example to understand climate change in the Arctic requires soil science, forestry, atmospheric and ocean physics, polar bear biology, and other scientific specialties. The state of the warming forecast for the entire globe encompasses so much information that no one human mind could hold it all at one time (not mine, anyway).

In response to warnings of the threat of global warming, the World Meteorological Organization created an organization of scientists charged with the task of summarizing the state of the science, called the Intergovernmental Panel on Climate Change or IPCC. The function of IPCC is not to do new research, but rather to summarize and synthesize all the published scientific papers into coherent reports. The scientists who do the actual work for IPCC are mostly employed by universities and national research laboratories around the world like NASA and NOAA. Working Group I of the IPCC writes the Scientific Assessment reports, while Working Group II reports on Impacts of climate change, and III on Mitigation (reducing C[O.sub.2] emissions, mostly). The most recent IPCC reports were released in spring of 2007. The projections and impacts of global warming as presented in the next two chapters are based on information from this report.

Most of the major ingredients in the global warming forecast were there in the results of Arrhenius' tedious calculations. One important example is called the ice albedo feedback. The word albedo describes the reflectivity of a planet to visible sunlight. Clouds reflect sunlight, as does ice and snow. When sunlight is reflected to space, it would be analogous to water from the faucet in the sink analogy that splashes onto the floor. Since that water doesn't have to go down the drain, the water level in the sink decreases. The water level is analogous to Earth's temperature, which falls if more incoming sunlight is reflected back to space instead of being absorbed.

The coupling between ice and light works out to be a loop of cause and effect called a feedback. The air warms for some external reason like rising C[O.sub.2], and as a result ice and snow melt on the land or ocean surface. The ice and snow are very reflective, which helped keep the planet cool, but the ground or ocean underneath have a greater tendency to absorb incoming sunlight, so the planet warms more than it would have. This is an example of a positive, amplifying feedback.

The Arctic warms more intensely than the tropics, because ice melts in the Arctic and the bare ground absorbs more sunlight than the ice did. You can see it in Arrhenius' results, you can see it in the Arctic climate records of the past few decades, and you can see it in the global warming forecast for the future. Full disclosure: where you can't see it is in Antarctica. It's a bit of a mystery how cold it's been in Antarctica; it may have something to do with the ozone hole.

Another amplifying feedback to global warming involves water vapor. Water vapor is a greenhouse gas, responsible for capturing more of the outgoing infrared radiation in the atmosphere than C[O.sub.2] does. The fact that water vapor is a stronger greenhouse gas than C[O.sub.2] does not mean we needn't worry about rising C[O.sub.2] concentrations. The concentration of water vapor in the atmosphere is controlled by the fact that if the humidity gets too high, it rains. Warmer air can carry more water vapor than cool air can, so warming from rising C[O.sub.2] could lead to more water vapor in the atmosphere. Water vapor warms the Earth still further, because it is a greenhouse gas.

Like the ice albedo feedback, the water vapor feedback is an amplifier of global warming. Unlike ice albedo, which is confined to high latitudes, the water vapor feedback has a rather more uniform effect around the globe, and it about doubles the temperature change we expect from rising C[O.sub.2] alone.

There is uncertainty about how strong the water vapor feedback is. The question is whether a warmer world could be drier or wetter than we expect it to be. The average relative humidity of the Earth's surface is about 80%. Arrhenius assumed that the atmosphere would remain 80% relative humidity as it warmed. A relative humidity of 80% represents more actual molecules of water in the warm atmosphere than in the cooler atmosphere, because warm air holds more vapor than cool air. Modern climate models also predict that the relative humidity will not change much with global warming. If the real atmosphere turns out to get wetter with rising C[O.sub.2] than models predict, for example, the real water vapor feedback would be stronger than we expect.

Though the answer hasn't changed much, the quality of the answer has certainly improved in the last century. Many pieces that Arrhenius simply had to guess at can now be predicted based on a mechanistic understanding of how things work. Just as important, the models that make the predictions have been tested against reality. In the 1930s, scientists were excited by a theory that sunspots controlled climate by changing the intensity of the Sun. A prediction was made, based on the weather patterns of the recent past, that it ought to get drier in Africa during the sunspot minimum of the 1930s. It turned out that Africa got wetter during the sunspot minimum, so that was it for sunspot theory. The intensity of the Sun is currently thought to have a large impact on century-timescale climate fluctuations such as the Medieval Optimum and Little Ice Age climates, described in Chapter 3. But variations in solar intensity in the last few decades have been weak compared with the change in climate forcing from greenhouse gases.

One problem that might seem like a show-stopper for climate forecasting is the discovery in the 1960s by Edward Lorenz that the weather is fundamentally unpredictable beyond a time horizon of a week or two. One popular name for this phenomenon is "chaos," and another is the "butterfly effect." The idea is that two nearly identical states of the weather, differing only a little bit, will tend to diverge from each other, so that a small initial difference between the two will grow with time. Small imperfections in a model of the weather today will grow, until eventually all that is left in the model is amplified garbage. The weather forecast for tomorrow is pretty good, and my impression is that the forecasts have been getting better every year. The weather forecast for 10 days from now however has always been and continues to be pretty much useless. How can we expect to forecast the weather in 100 years, let alone in 100 millennia, if we can't do 10 days?


Excerpted from THE LONG THAW by DAVID ARCHER Copyright © 2009 by Princeton University Press. Excerpted by permission.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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Table of Contents

Acknowledgments xi

Prologue. Global Warming in Geologic Time 1

Section I The Present

Chapter 1 The Greenhouse Effect 15

Chapter 2 We've Seen It With Our Own Eyes 30

Chapter 3 Forecast of the Century 45

Section II The Past

Chapter 4 Millennial Climate Cycles 57

Chapter 5 Glacial Climate Cycles 69

Chapter 6 Geologic Climate Cycles 78

Chapter 7 The Present in the Bosom of the Past 91

Section III The Future

Chapter 8 The Fate of Fossil Fuel Co2 101

Chapter 9 Acidifying the Ocean 114

Chapter 10 Carbon Cycle Feedbacks 125

Chapter 11 Sea Level in the Deep Future 137

Chapter 12 Orbits, CO2, and the Next Ice Age 149

Epilogue. Carbon Economics and Ethics 158

Further Reading 175

Index 179

What People are Saying About This


In this short book, David Archer gives us the latest on climate change research, and skillfully tells the climate story that he helped to discover: generations beyond our grandchildren's grandchildren will inherit atmospheric changes and an altered climate as a result of our current decisions about fossil-fuel burning. Not only are massive climate changes coming if we humans continue on our current path, but many of these changes will last for millennia. To make predictions about the future, we rely on research into the deep past, and Archer is at the forefront of this field: paleoclimatology. This is the book for anyone who wishes to really understand what cutting-edge science tells us about the effects we are having, and will have, on our future climate.
Richard B. Alley, Pennsylvania State University


Books on climate change tend to focus on what is expected to happen this century, which will certainly be large, but they often neglect the even larger changes expected to take place over many centuries. The Long Thaw looks at climate effects beyond the twenty-first century, and its focus on the long-term carbon cycle, rather than just climate change, is unique.
Jeffrey T. Kiehl, National Center for Atmospheric Research


A great book. What sets it apart is that it expands the discussion of the impacts of global warming beyond the next century and convincingly describes the effects that are projected for the next few thousand years. What also sets it apart is how deeply it takes general readers into the scientific issues of global warming by using straightforward explanations of often complex ideas.
Peter J. Fawcett, University of New Mexico

James Hansen

This is the best book about carbon dioxide and climate change that I have read. David Archer knows what he is talking about.
James Hansen, director of the NASA Goddard Institute for Space Studies

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