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INT RODUCT ION T O MODERN CL IMAT E CHANG E , S ECOND ED IT ION

This is an invaluable textbook for any introductory survey course on the science and policy of climate change, for both non–science majors and introductory science students. The second edition has been thoroughly updated to reflect the most recent science from the latest IPCC reports, and many illustrations include new data. The new edition also reflects advances in the political debate over climate change. Unique among textbooks on climate change, this text combines an introduction to the science with an introduction to economic and policy issues, and it focuses closely on anthropogenic climate change. It contains the necessary quantitative depth for students to properly understand the science of climate change. It supports students in using algebra to understand simple equations and to solve end-of-chapter problems. Supplementary online resources include a complete set of PowerPoint figures for instructors, solutions to exercises, videos of the author's lectures, and additional computer exercises.

Andrew Dessler is a climate scientist who studies both the science and politics of climate change. His scientific research revolves around climate feedbacks, in particular how water vapor and clouds act to amplify warming from the carbon dioxide that human activities emit. During the last year of the Clinton administration, he served as a senior policy analyst in the White House Office of Science and Technology Policy. Based on his research and policy experience, he has authored two books on climate change: this textbook and The Science and Politics of Global Climate Change: A Guide to the Debate (co-written with Edward Parson; second edition published in 2010). This textbook won the 2014 American Meteorological Society Louis J. Battan Author's Award. In recognition of his work on outreach, in 2011 he was named a Google Science Communication Fellow. He is presently a professor of atmospheric sciences at Texas A&M University. His

educational background includes a B.A. in physics from Rice University and a Ph.D. in chemistry from Harvard University. He also undertook postdoctoral work at NASA's Goddard Space Flight Center and spent nine years on the research faculty of the University of Maryland.

INTRODUCTION TO MODERN CLIMATE CHANGE

Second Edition Andrew Dessler

Texas A&M University

32 Avenue of the Americas, New York, NY 10013-2473, USA

Cambridge University Press is part of the University of Cambridge.

It furthers the University's mission by disseminating knowledge in the pursuit of education, learning, and research at the highest international levels of excellence.

www.cambridge.org

Information on this title: www.cambridge.org/9781107480674

© Andrew Dessler 2016

This publication is in copyright. Subject to statutory exception and to the provisions of relevant collective licensing agreements, no reproduction of any part may take place without the written permission of

Cambridge University Press.

First published 2016

Printed in the United States of America

A catalog record for this publication is available from the British Library.

Library of Congress Cataloging in Publication Data

Dessler, Andrew Emory.

Introduction to modern climate change / Andrew Dessler, Texas A&M University. – [Second edition].

pages cm

Includes bibliographical references and index.

ISBN 978-1-107-09682-0

1. Climatic changes. 2. Climatic changes – Government policy. I. Title.

QC903.D46 2016

551.6–dc23 2015014701

ISBN 978-1-107-09682-0 Hardback

ISBN 978-1-107-48067-4 Paperback

Additional resources for this publication at www.andrewdessler.com

Cambridge University Press has no responsibility for the persistence or accuracy of URLs for external or third-party Internet Web sites referred to in this publication and does not guarantee that any content on such

Web sites is, or will remain, accurate or appropriate.

For Michael and Alex

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Contents Preface Acknowledgments

An introduction to the climate problem

Is the climate changing?

Radiation and energy balance

A simple climate model

The carbon cycle

Forcing, feedbacks, and climate sensitivity

Why is the climate changing?

Predictions of future climate change

Impacts of climate change

Exponential growth

Fundamentals of climate change policy

Mitigation policies

A brief history of climate science and politics

Putting it together: A long-term policy to address climate change

References Index

Preface

Future generations may well view climate change as the defining issue of our time. The worst-case scenarios of climate change are truly terrible, but even middle-of- the-road scenarios portend environmental change without precedent for human society. When future generations look back on our time in charge of the planet, they will either cheer our foresight in dealing with this issue or curse our lack of it.

Yet despite the stakes, the world has done basically nothing to address this risk. The reasons are obvious: The threat of climate change is really a threat to future generations, not the present one, so actions taken by our generation will mostly benefit them and not us. Moreover, such actions may be expensive – reducing emissions means rebuilding our energy infrastructure, and we have no idea how much that will cost. In such a situation, it is easiest to do nothing and wait for disaster to strike – which is why dams are frequently built after the flood, not before. Nevertheless, pushing this problem off onto future generations is a poor strategy. The impacts of climate change are global and mainly irreversible; by the time we have unambiguous evidence that the climate is changing and its impacts are serious, it will be too late to avoid these serious impacts. The only hope that future generations have to avoid serious climate change is us.

I fully believe that the cornerstone of good policy is an electorate that is educated on the issues, and this belief provided me the motivation for writing this book. The goal of this book is to cover the human-induced climate change problem from stem to stern, covering not just the physics of climate change but also the economic, policy, and moral dimensions of the problem. This sets it apart from most other climate change books, which typically do not have a tight focus on human-induced climate change or do not cover the nonscience aspects of the problem.

Such complete coverage of the climate change problem is essential. The science clearly underlies all discussion of the problem, and an understanding of the science is essential to an understanding of why so many people are so worried about it. Climate change, however, is no longer just a scientific problem. Virtually every government in the world now accepts the reality of climate change, and the debate has, to a great extent, moved on to policy questions, including the economic and ethical issues. Thus, one must also understand nonscience aspects of the problem to be truly informed on this issue.

The first seven chapters of the book focus on the science of climate change. Chapter 1 defines the problem and provides definitions of weather, climate, and climate change. It also addresses an issue that most textbooks do not have to address: why the reader should believe this book as opposed to Web sites and other sources that give a completely different view of the climate problem. Chapter 2 explains the evidence that the Earth is warming. The evidence is so overwhelming that there is little argument anymore over this point, and my goal is for readers to come away from the chapter understanding this.

Chapter 3 covers the basic physics of electromagnetic radiation necessary to understand the climate. I use familiar examples in this chapter, such as glowing metal in a blacksmith shop and the incandescent light bulb, to help the reader understand these important concepts. In Chapter 4, a simple energy-balance climate model is derived. It is shown how this simple model successfully explains the Earth's climate as well as the climates of Mercury, Venus, and Mars. Chapter 5 covers the carbon cycle, and feedbacks, radiative forcing, and climate sensitivity are all discussed in Chapter 6. Finally, Chapter 7 explains why scientists are so confident that humans are to blame for the recent warming that the Earth has experienced.

Chapter 8 begins an inexorable shift from physics to nonscience issues. It discusses emissions scenarios and the social factors that control them, as well as what these scenarios mean for our climate over the next century. Chapter 9 covers the impacts of these changes on humans and on the world in which we live. Chapter

10 covers exponential math. Exponential growth is a key factor in almost all fields of science, as well as in real life. In this chapter, I cover the math of exponential growth and explain the concept of exponential discounting. I also touch briefly on the social cost of carbon.

Starting with Chapter 11, the discussion is entirely on the policy aspects of the problem. Chapter 11 discusses the three classes of responses to climate change, namely adaptation, mitigation, and geoengineering, and their advantages, disadvantages, and trade-offs. The most contentious arguments over climate change policy are over mitigation, and Chapter 12 discusses in detail the two main policies advanced to reduce emissions: carbon taxes and cap-and-trade systems.

Chapter 13 provides a brief history of climate science and a history of the political debate over this issue, including discussions of the United Nations' Framework Convention on Climate Change and the Kyoto Protocol. Finally, Chapter 14 pulls the last three chapters together by discussing how to decide which of our options we should adopt, particularly given the pervasive uncertainty in the problem.

Overall, it should be possible to cover each chapter in three hours of lecture. This makes it feasible to cover the entire book in one fifteen-week semester. At Texas A&M, the material in this book is being used in a one-semester class for nonscience majors that satisfies the university's science distribution requirement. Thus, it is appropriate for undergraduates with any academic background and at any point in their college career.

Any serious understanding of climate change must be quantitative. Therefore, the book assumes a knowledge of simple algebra. No higher math is required. The book also assumes no prior knowledge of any field of science, just an open mind and a willingness to learn. To aid in the student's development of a numerate understanding of the climate, there are quantitative questions at the end of many of the chapters, and every chapter also has more open-ended, qualitative questions. In addition, there is a chapter summary at the end of each chapter that reviews and

summarizes the most important takeaway messages from the chapter. A list of important terms is also provided at the end of each chapter. I've put additional readings, video recordings of my lectures, and computer exercises on my Web site, www.andrewdessler.com.

This is not an advocacy book. This is not to say that I do not have opinions. I do, and strong ones. I recognize, though, that shrill advocacy is frequently less effective than a dispassionate presentation of the facts. Thus, my strategy in this book is to simply explain the science and then lay out the possible solutions and trade-offs among them. I firmly believe that an unbiased assessment of the facts will bring the majority of people to see things the way I do: that climate change poses a serious risk and that we should therefore be heading off that risk by reducing our emissions of greenhouse gases.

Every year that our society does nothing to address climate change makes solving the problem both harder and more expensive. I am still optimistic, though, because problems often appear intractable at first. In the 1980s, as evidence mounted that industrial chemicals were depleting the ozone, it was not at all clear that we could avoid serious ozone depletion at a reasonable cost. The chemicals causing the ozone loss, namely chlorofluorocarbons, played an important role in our everyday life – in refrigeration, air conditioning, and many industrial processes – just like the main cause of climate change, fossil fuels, also plays an important role in our society. But the cleverness of humans prevailed. A substitute chemical was developed and it seamlessly and cheaply replaced the ozone-destroying halocarbons – at a cost so low that hardly anyone noticed when the substitution took place.

Solving the climate change problem will be harder than solving the ozone depletion problem – how much harder, no one knows. I am confident, though, that the ingenuity and creativeness of humans is such that we can solve this problem without damaging our standard of living. However, there is only one way to find out, and that is to try to do it.

Acknowledgments

This book could not have been written without the incredible work of the climate science community. Ignored by many, demonized by some, I believe that future generations will look back and say, “They nailed it.” I hope this book does justice to all of our hard work. The first edition of the book was written while I was on faculty development leave from Texas A&M University during Fall 2010. I thank the university for this support.

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An introduction to the climate problem

◈

We begin our trip through the climate problem by defining weather, climate, and climate change and by demonstrating how we use latitude and longitude to describe locations on the Earth. We also discuss something that few textbooks address: why you should believe this book.

1.1 What is climate? The American Meteorological Society defines climate as

The slowly varying aspects of the atmosphere–hydrosphere–land surface system. It is typically characterized in terms of suitable averages of the climate system over periods of a month or more, taking into consideration the variability in time of these averaged quantities.

Mark Twain, in contrast, famously summed it up a bit more concisely:

Climate is what you expect; weather is what you get.

Put another way, weather refers to the actual state of the atmosphere at a particular time. Weather is what we mean when we say that, at 10:53 AM on November 15, 2014, the temperature in College Station, Texas, was 8°C, the humidity was 66 percent, winds were out of the southeast at 8 knots, the barometric pressure was 30.23 inches, and there was no precipitation.

Climate, in contrast, is a statistical description of the weather over a period of time, usually a few decades. It would almost certainly include average temperature as well as a measure of how much the temperature varies about this average value, such as the record high and low temperatures. Figure 1.1 demonstrates one way to look at the climate: It shows the distribution of daily average temperatures in August near Fairbanks, Alaska, for two time periods, 1900–1929 and 1970–1999. During the 1900–1929 period, for example, the most likely daily average temperature was 10°C, which occurred on approximately 16 percent of the days. Extremes occur less frequently; for example, the probability of temperatures above 16°C or below 3°C are small. The climate tells us only the range of probable conditions on a particular day; it contains no information about what the temperature was on any particular day.

Figure 1.1 Frequency of occurrence of daily average temperature in August at 64°N, 150°W, near Fairbanks, AK, for two time periods: 1900–1929 and 1970– 1999

(data obtained from the twentieth-century reanalysis, version 2, www.esrl.noaa.gov/psd/data/gridded/data.20thC_ReanV2.html).

In this book, I frequently use the Celsius scale, the standard temperature scale throughout the world (the Fahrenheit scale more familiar to U.S. readers is only used in the United States and a few other countries). For readers who may not be conversant in Celsius, you can convert from Fahrenheit to Celsius using the equation C = (F – 32) × 5∕9; or from Celsius to Fahrenheit, F = C × 9∕5 + 32. It is also useful to remember that the freezing and boiling temperatures for water on the Celsius scale are 0°C and 100°C, respectively. On the Fahrenheit scale, these

temperatures are 32°F and 212°F. Room temperature is about 22°C, which corresponds to 72°F.

Why do we care about weather and climate? Weather is important for making short-term decisions. For example, should you take an umbrella when you leave the house tomorrow? To answer this question, you do not care at all about the average precipitation for the month, but rather whether it is going to rain tomorrow. If you are going skiing this weekend, you care about whether new snow will fall before you arrive at the ski lodge and what the weather will be while you are there. You do not care how much snow the lodge gets on average.

Climate, however, is more important for long-term decisions. If you are looking to build a vacation home, you are interested in finding a place that frequently has pleasant weather – you are not particularly interested in the weather on any specific day. Plots like Figure 1.1 can help make these kinds of climate- related decisions; the plot tells us, for example, that a house in this location rarely needs air conditioning. If you are building a ski resort, you want to place it in a location that, on average, gets enough snow to produce acceptable ski conditions. You do not care if snow is going to fall on a particular weekend, or even what the total snowfall will be for a particular year.

An example of the importance of both the climate and the weather can be found in the planning for D-Day, the invasion of the European mainland by the Allies during World War II. The invasion required Allied troops to be transported onto the beaches of Normandy, along with enough equipment that they could establish and hold a beachhead. As part of this plan, Allied paratroopers were to be dropped into the French countryside the night before the beach landing in order to capture strategic towns and bridges near the landing zone, thus hindering an Axis counterattack.

There were important weather requirements for the invasion. The nighttime paratrooper drop demanded a cloudless night as well as a full moon so that the paratroopers would be able to land safely and on target, and then achieve their

objectives – all before dawn. The sky had to remain clear during the next day so that air support could see targets on the ground. For tanks and other heavy equipment to be brought onshore called for firm, dry ground, so there could be no heavy rains just prior to the invasion. Furthermore, the winds could not be too strong because high winds generate big waves that create problems for both the paratroopers and the small landing craft that would ferry infantry to the beaches.

Given these and other weather requirements, analysts studied the climate of the candidate landing zones to find those beaches where the required weather conditions occurred most frequently. The beaches of Normandy were ultimately selected in part because of its favorable climate (tactical considerations obviously also played a key role).

Once the landing location had been selected, the exact date of the invasion had to be chosen. For this, it would not be the climate that mattered but rather the weather on a particular day. Operational factors such as the phase of the tide and the moon provided a window of three days for a possible invasion: June 5, 6, and 7, 1944. June 5 was initially chosen, but on June 4, as ships began to head out to sea, bad weather set in at Normandy, and General Dwight D. Eisenhower made the decision to delay the invasion. On the morning of June 5, chief meteorologist J. M. Stagg forecasted a break in the weather, and Eisenhower decided to proceed. Within hours, an armada of ships set sail for Normandy. That night, hundreds of aircraft carrying tens of thousands of paratroopers roared overhead to the Normandy landing zones.

The invasion began just after midnight on June 6, 1944, when British paratroopers seized a bridge over the Caen Canal. At dawn, 3,500 landing craft hit the beaches. Stagg's forecast was accurate and the weather was good, and despite ferocious casualties, the invasion succeeded in placing an Allied army on the European mainland. This was a pivotal battle of World War II, marking a turning point in the war. Viewed in this light, Stagg's forecast may have been one of the most important in history.

Temperature is the parameter most often associated with climate, and it is something that directly affects the well-being of the Earth's inhabitants. The statistic that most frequently gets discussed is average temperature, but temperature extremes also matter. For example, it is heat waves – prolonged periods of excessively hot weather – rather than normal high temperatures that kill people. In fact, heat-related mortality is the leading cause of weather-related death in the United States, killing many more people than cold temperatures do. And the numbers can be staggering: In August 2003, a severe heat wave in Europe lasting several weeks killed tens of thousands of people.

Precipitation rivals temperature in its importance to humans, because human life without fresh water is impossible. As a result, precipitation is almost always included in any definition of climate. Total annual precipitation is obviously an important part of the climate of a region. However, the distribution of this rainfall throughout the year also matters. Imagine, for example, two regions that get the same total amount of rainfall each year. One region gets the rain evenly distributed throughout the year, whereas the other region gets all of the rain in one month, followed by eleven rain-free months. The environment of these two regions would be completely different. Where the rain falls continuously throughout the year, we would expect a green, lush environment. Where there are long rain-free periods, in contrast, we expect something that looks more like a desert.

Other aspects of precipitation, such as its form (rain versus snow), are also important. In the U.S. Pacific Northwest, for example, snow that accumulates in the mountains during the winter melts during the following summer, thereby providing fresh water to the environment during the otherwise dry summers. If warming causes wintertime precipitation to fall as rain rather than snow, then it will run off immediately and not be available during the following summer. This can lead to water shortages during the summer.

As these examples show, climate includes many environmental parameters. What part of the climate matters will vary from person to person, depending on how

each relies on the climate. The farmer, the ski resort owner, the resident of Seattle, and Dwight D. Eisenhower are all interested in different meteorological variables, and thus may care about different aspects of the climate. But make no mistake: We all rely on the stability of our climate. In particular, food production and freshwater availability, two of the most important things we rely on to survive, are greatly affected by the climate. I discuss this in greater depth when I explore climate impacts in Chapter 9.

A final difference between weather and climate is how easy they are to determine. Measuring the weather is pretty easy – just walk outside and look around.1 If you need a higher level of accuracy, you can buy reasonably cheap instruments to measure the temperature, precipitation, or any other variable of interest. Climate, in contrast, is much harder to measure; it requires the gathering of decades of data so that we have sufficiently good, robust statistics, such as I plotted in Figure 1.1.

1.2 What is climate change? The climate change that is most familiar is the seasonal cycle: the progression of seasons from summer to fall to winter to spring and back to summer, during which most non-tropical locations experience significant temperature variations. Precipitation can also vary by season. In fact, almost any climate variable can vary over the course of the year.

The concern in the climate change debate – and in this book – is with long-term climate change. The American Meteorological Society defines the term climate change as “any systematic change in the long-term statistics of climate elements (such as temperature, pressure, or winds) sustained over several decades or longer.” In other words, we can compare the statistics of the weather for one period against those for another period, and if the statistics have changed, then we can say that the climate has changed.

Thus, we are interested in whether today's climate (defined over the past few decades) is different from the climate of a century ago, and we are worried that the climate at the end of the twenty-first century will be quite different from that of today. To illustrate this, Figure 1.1 shows the August temperature near Fairbanks, Alaska, for two periods, 1900–1929 and 1970–1999. Clearly, the temperature distributions in these two periods are different – the temperature distribution at the end of the twentieth century is about 2°C warmer than at the beginning of the century. In other words, the climate of this region has changed. It should also be noted that there is no information on what caused the change – it may be due to global warming or any number of other physical processes. All we have identified here is a shift in the climate.

The shift in the temperature distribution is only ∼2°C, and it might be tempting to dismiss this as unimportant. However, as I discuss in Chapter 9, seemingly small changes in climate are associated with significant impacts on the environment. So you should not dismiss such a change lightly.

In Chapter 2, we will look more closely at data to determine if the climate is indeed changing. Before we get to that, however, there are two things I need to cover. First, in the next section, I discuss the coordinate system I will be using in this book.

1.3 A coordinate system for the Earth I will be talking a lot in this book about the Earth, so it makes sense to define the terminology used to identify particular locations and regions on the Earth.

To begin, the equator is the line on the Earth's surface that is halfway between the North and South Poles, and it divides the Earth into a northern hemisphere and a southern hemisphere. The latitude of a particular location is the distance in the north-south direction between the location and the equator, measured in degrees (Figure 1.2). Latitudes for points in the northern hemisphere have the letter N appended to them, with S appended to points in the southern hemisphere. Thus, 30°N means a point on the Earth that is 30° north of the equator, whereas 30°S means the same distance south of the equator.

Figure 1.2 A schematic plot of latitude.

The tropics are conventionally defined as the region from 30°N to 30°S, and this region covers half the surface area of the planet. The mid-latitudes are usually defined as the region from 30° to 60° in both hemispheres, and these regions occupy roughly one-third of the surface area of the planet. The polar regions are typically defined to be 60° to the pole, and these regions occupy the remaining one- sixth of the surface area of the planet. The North and South Poles are located at 90°N and 90°S, respectively.

Latitude gives the north-south location of an object, but to uniquely identify a spot on the Earth, you need to know the east-west location as well. That is where longitude comes in (Figure 1.3). Longitude is the angle in the east or west direction, from the prime meridian, a line that runs from the North Pole to the South Pole through Greenwich, England, and is arbitrarily defined to be 0° longitude. Locations to the east of the prime meridian are in the eastern hemisphere and have the angle appended with the letter E, whereas locations to the west are in the western hemisphere and have the letter W appended. In both directions, longitude increases to 180°, where east meets west at the international date line.

Figure 1.3 A schematic plot of longitude.

Together, latitude and longitude identify the location of every point on the planet Earth. For example, my office in the Department of Atmospheric Sciences of Texas A&M University is located at 30.6178°N, 96.3364°W. Knowing your location

can literally be a matter of life and death – shipwrecks, wars, and other miscellaneous forms of death and disaster have occurred because people did not know where they were. Luckily for us, GPS (global positioning system) technology, which is probably built into your cell phone, can determine your latitude and longitude to within a few feet.

1.4 Why you should believe this textbook I now have to address an issue that generally does not come up in college textbooks: why you should believe it. Students in most classes accept without question that the textbook is correct. After all, the author is probably an authority on the subject, the publisher has almost certainly reviewed the material for accuracy, and the instructor of the class, someone with knowledge of the field, selected that textbook. Given those facts, it seems reasonable to simply assume that the information in the textbook is basically correct.

But climate change is not like every other subject. If you do a quick Internet search, you will be able to find a Web page that disputes almost every claim made in this textbook. Your friends and family may not believe that climate change is a serious problem, or they may even believe it is a hoax. You may agree with them. This book will challenge many of these so-called skeptical viewpoints, and you may face the dilemma of whom to believe.

This situation brings up an important and interesting question: How do you determine whether or not to believe a scientific claim? If you happen to know a lot about an issue, you can reach your own conclusions on the issue. However, no one can be an expert on every subject; for the majority of issues on which you are not an expert, you need a shortcut.

One type of shortcut is to rely on your firsthand experience about how the world works. Claims that fit with your own experience are easier to accept than those that run counter to it. People do this sort of evaluation all the time, usually unconsciously. Consider, for example, a claim that the Earth's climate is not changing. In your lifetime, climate has changed very little, so this seems like a plausible claim. However, a geologist who knows that dramatic climate shifts are responsible for the wide variety of rock and fossil deposits found on Earth might regard the idea of a stable climate as ludicrous, but in turn might be less likely to accept a human origin for climate change. The problem with relying on firsthand

experience about the climate is that our present situation is unique – people have

never changed the composition of the global atmosphere as much or as fast as is currently occurring. Thus, whatever the response will be, it will likely be outside the realm of our and the Earth's experiences.

Another type of shortcut is to rely on your values: You can accept the claims that fit with your overall worldview while rejecting the claims that do not. For example, consider the scientific claim that secondhand smoke has negative health consequences. If you are a believer in unfettered freedom, you might choose to simply reject this claim out of hand because it implies that governments should regulate smoking in public places to protect public health. Those who are more suspicious of the integrity of big business are going to be more skeptical of the efficacy of vaccines because they believe that corporations are willing to put profits ahead of safety.

Yet another shortcut is to rely on an opinion leader. Opinion leaders are people who you trust because they appear to be authoritative or because you agree with them on other issues. They might include a family member or influential friend, a media figure such as talk show hosts Rush Limbaugh and Jon Stewart, or an influential politician such as Barack Obama or George W. Bush. In the absence of a strong opinion of your own, you can simply adopt the view of your opinion leaders. The problem with this approach is that there is no guarantee that the opinion leaders have a firm grasp of the science.

The most widely accepted approach is to rely on the opinions of experts. When the relevant experts on some subject have high confidence that a scientific claim is true, that is the best indication we have that the claim actually is true. This is not just my view; I am willing to bet it is something you believe in, too. If a friend tells you that he thinks he may be sick, what would you recommend? Your recommendation is likely to be that he should go see a doctor – and not just any doctor, but one who is an expert in that particular ailment.

This is also the view of the U.S. legal system. Many court cases involve questions of science (e.g., what was the cause of death, does a particular chemical cause cancer, does a DNA sample match the defendant). To settle those cases, the court will frequently turn to expert witnesses. These expert witnesses are, as their name suggests, experts on the matter that they are testifying about, and they provide relevant expertise to the court to help evaluate the important scientific questions that a case may revolve around.

To be an expert witness, one must demonstrate expertise in a particular subject. I have served as an expert witness on climate change in lawsuits over the permitting of coal-fired power plants, and the court qualifies me as an “expert” by using my research in climate change as well as the textbooks I have authored as evidence.

It should be emphasized that one must demonstrate specific, recent expertise in the exact area under consideration to be an expert witness. Showing expertise in general technical matters or in a related field is not sufficient. For example, one might consider a scientist with a Ph.D. in another field (e.g., solid-state physics) to have a credible opinion about the science of climate change. This is not so, and a person with a Ph.D. but without specialized knowledge of the climate would not qualify as an expert on matters of climate. That also goes for weather forecasters – climate and weather are different, and being an expert in weather does not qualify someone to be an expert witness on climate. And the requirement for the expertise to be recent rules out those who were experts, say, a decade ago but who have not kept up with the latest discoveries in the field.

There are many more examples that demonstrate that, as individuals and as a society, we rely on experts when evaluating complex technical issues. That is probably a good thing, too, because on a planet with 7 billion people, you can always find someone who will contest any claim, no matter how well established it is. For example, it would be relatively easy to find someone somewhere who would dispute the claim that cigarettes cause health problems. So if everyone's opinion

counted equally, then it would be impossible to ever settle any dispute over a scientific claim – even one as simple as whether the Earth goes around the Sun.

But we also know that even the most trusted expert can be wrong, so the opinion of a single expert should be taken with caution. One way to gain confidence in a particular expert opinion is to ask several experts instead of just one. We frequently do this for important medical decisions by getting a second opinion. For high-stakes medical decisions, you would ideally solicit the opinions of many experts. If all of these experts were to agree, then you would have justifiably high confidence that the recommendations are the best advice that modern medicine can provide.

Climate change is really no different. It is obvious that the relevant experts are the community of climate scientists. And rather than listen to any single individual, we would do best by asking a large, representative sample of the world's climate scientists what they think – and if the vast majority agree on a particular point, then we can have high confidence that this is best estimate science can provide.

This is, in fact, what has already been done. In 1988, as nations began to acknowledge the seriousness of the climate problem, the Intergovernmental Panel on Climate Change (IPCC) was formed. The IPCC assembles large writing teams of scientific experts and has them write, as a group, reports detailing what they know about climate change and how confidently they know it. The reliance on large writing groups reduces the possibility that the erroneous opinions of an individual or a small group make it into the report, much like getting multiple opinions in medicine reduces the chance of a bad diagnosis.

To further minimize the possibility that the group of scientists writing the report are biased in some direction, the scientists making up the writing teams are not assembled by a single person or organization; they are nominated by the world's governments. Thus, the only way the IPCC's writing groups would be biased in some direction is if all of the world's governments nominated appropriately biased individuals. This seems unlikely, particularly since addressing climate change

brings a raft of short-term problems to most governments. Many governments would therefore be happy if climate change disappeared completely as a political issue and therefore have no incentive to nominate scientists biased to the view climate change as a serious problem.

Drafts of the IPCC's reports are reviewed prior to release by other expert scientists, and they undergo a public review and a separate review by the world's governments. In the end, the IPCC's reports are widely regarded as the most authoritative statements of scientific knowledge about climate change, and as such they carry enormous weight in both the scientific and the policy communities. In 2007, the IPCC shared the Nobel Peace Prize in recognition of its work on the climate.

An aside: The Summary for Policymakers

If you have ever tried to read an IPCC report, you know that the 1,000 plus page reports can be baffling for non-experts. That is why every report also has a Summary for Policymakers, a more readable summary of the full report that runs a few dozen pages. Referred to as the “SPMs,” they summarize in more general language the most important conclusions in the main report.

The SPMs also serve another unique function. During a final meeting after the main report is written, representatives from each of the world's governments review a draft SPM written by scientists and vote on every sentence. Only if there is unanimous agreement from all of the world's governments is a sentence included in the SPM. During this process, sentences are frequently rewritten to make them acceptable to the world's governments. If there is nearly unanimous agreement on a sentence, with just one or two countries dissenting, than the sentence can be included in the SPM with a footnote recording the dissent.

The purpose of this exercise is to produce a common set of scientific facts to serve as the basis of future negotiations on policy. By having unanimous agreement on every sentence, no country can later say during policy negotiations that they don't agree with a particular scientific fact – they have already agreed to everything in the SPM.

This means, though, that every country is also trying to mold the SPM to best suit their negotiating position. During the meeting for the SPM for the IPCC's 1995 report, for example, Saudi Arabia and Kuwait argued strenuously to weaken the statements about humans causing climate change. When the rest of the world disagreed, it was then proposed that a footnote would be added to the report noting the disagreement – but the footnote was removed at Saudi Arabia and Kuwait's request because it would have been embarrassing for those two major oil producers to be the only countries in the world to not accept the scientific evidence of human impacts on climate.

In the end, the SPM represents a good summary of our scientific understanding of the climate but one that has an unavoidable hint of political influence in it. To the extent that political wrangling affects the SPM, it is almost always to water down the conclusions – reduce our confidence in scientific statements, lessen the impacts, etc. But despite these flaws, the SPMs should be given considerable deference in policy debates over climate.

Despite the careful process that produces the IPCC reports, it remains controversial in the public debate. To understand why, consider the curious case of cigarettes. Scientists have known since the 1950s that smoking cigarettes is terrible for your health. And in 1964, a landmark report by the U.S. surgeon general laid out the evidence in great detail for the general public. The tobacco companies at that point had two choices: they could accept that their product killed their customers, an

admission that would certainly reduce their profits, or they could fight back by attacking the science.

Perhaps unsurprisingly, they chose to fight the science. In a memo released just after the surgeon general's report, a tobacco executive plotted out the response: “We must in the near future provide some answers which will give smokers a psychological crutch and a self rationale to continue smoking. These answers must also point out the weaknesses in the [Surgeon General's] Report.”2

Good to their word, the tobacco companies proceeded to wage a successful multi-decade campaign to cast doubt on the science. It was only in the 1990s, four decades after the science was actually settled, that the phony public debate over the health impacts of smoking finally faded away.

Today, the tobacco debates are the archetype of the dishonest manipulation of science in pursuit of a particular policy goal.3 And the dishonesty paid handsomely: it effectively delayed by decades public awareness of the strength of the scientific consensus of the dangers of cigarettes, thereby keeping people smoking and delaying government action to restrict its sales. Given how successful the fight waged against tobacco science was, it should come as no surprise that those opposed to political action to address climate change have adopted a similar approach.

Echoing the tobacco debate, one argument frequently made during the climate debate is that there is, in fact, wide disagreement on the science of climate change among climate scientists. As evidence, they will point to Internet petitions and various lists of scientists that dispute the mainstream view. However, a close evaluation of the dissident scientists on these lists and petitions reveals that in almost all cases they should not be considered experts on climate. Although many of the individuals on the lists have technical degrees, and some even have doctorates, their specific training does not include climate change. They would never qualify as an expert witness in a lawsuit on climate change; they do not have the background to

teach a college-level course on the material; and we would never trust the diagnosis provided by a doctor with equivalent expertise.

The only reason that advocates put such transparently unqualified people forward as experts is that legitimate experts with the desired opinions are not available. Thus, the lack of credentials of those on the petitions and lists actually underscores the strong agreement among the relevant experts on the science of climate change.

A second claim we may hear is that climate scientists are manufacturing a crisis to benefit themselves. If climate is a crisis, so the argument goes, then more research funding will flow into the field, the prestige of climate scientists will increase, and scientists will be able to implement their preferred social policies.

There is, in fact, no evidence to support this argument. Rather, this argument relies on the listener simply accepting the obviousness of the claim that an entire scientific field would be willing to engage in scientific misconduct for research funding. What is often lost in this discussion is that all scientific fields have this same incentive. Biomedical fields could invent a new disease or a cure for an existing one to increase funding; physicists studying solid-state physics could invent a discovery that could lead to much faster computers; and space physicists could invent evidence of life elsewhere in the solar system. All of these “discoveries” would increase funding and interest in the particular field of interest.

It turns out that there are strong barriers to such widespread fraud by an entire scientific community. First and foremost, there is a coordination problem: How do you get everyone to go along with the fraud? The answer is that you can not. A scientific field such as climate science is a large, diverse, and intensely competitive endeavor, and the desire to outthink one's peers and show that one is smarter than they are is much greater than the incentive to cooperate in this type of fraud. The reason for this is that success in science is achieved by impressing one's colleagues. One of the best ways to do this is to overturn conventional wisdom, either by showing that previous scientific results are wrong or by suggesting a new theory

that fits the data better. Because this is so beneficial to the reputation of the individual scientist, it provides a strong incentive against participating in any conspiracy.

And the incentives in science do not support such a conspiracy. Money from grants does not generally go into the pockets of the researchers. Most scientists are employees of federal or state governments or private universities, and the amount of money they can pay themselves off grants is extremely limited. Instead, the majority of grant money goes to buy equipment or pay for graduate students. Thus, research funding provides a very weak incentive to cheat.

Finally, the entire underlying premise of the “climate science is corrupt” narrative is questionable. The premise is that, by suggesting that human effects on the climate are well understood, the field gets more research funding than it would otherwise. However, history suggests that whenever a field reaches a conclusion on a problem, funding for further research on that problem goes down. For example, after ozone depletion was confidently attributed to ozone-destroying chemicals known as chlorofluorocarbons in the mid-1990s, the funding for subsequent research rapidly dropped. By saying that they understand the climate system reasonably well, members of the climate science community are not helping their funding. They would do better if they claimed that there was no consensus on why the climate is warming. In that case, it would almost certainly be a high priority for most policymakers to fund research into determining the cause of the warming.

But while there has never been widespread fraud by an entire scientific community, there have been cases in which advocates opposed to political action have falsely tried to cast doubt on the science. One such example was discussed earlier: leaked documents from tobacco companies clearly show the intent of these companies was to generate doubt in the public's mind about the health effects of cigarette smoking.

Because of this, we should be leery of the argument that “the experts can't be trusted.” It goes against both common sense and our experience in the real world,

and it should only be accepted if extraordinary evidence is provided. In the climate change debate, such evidence is clearly lacking.

An aside: But I still don't trust the IPCC

If you talk to people about climate change, you will find some people who, no matter what arguments you make, will simply not accept the IPCC reports. To those people, you should point out the many other reports written by authoritative organizations, such as the U.S. National Academy of Sciences and the U.K. Royal Society.4 Or you can look at the statements put out by the scientific societies that climate experts belong to. In October 2009, for example, a collection of U.S. scientific organizations sent a letter to the U.S. Senate stating that climate change is a serious problem facing the entire human race and that emissions of greenhouse gases have to be dramatically reduced for us to avoid the most severe impacts. Signatories of this letter include the American Association for the Advancement of Science, the American Chemical Society, the American Geophysical Union, the American Institute of Biological Sciences, the American Meteorological Society, the American Society of Agronomy, the American Society of Plant Biologists, the American Statistical Association, the Association of Ecosystem Research Centers, the Botanical Society of America, the Crop Science Society of America, the Ecological Society of America, the Natural Science Collections Alliance, the Organization of Biological Field Stations, the Society for Industrial and Applied Mathematics, the Society of Systematic Biologists, the Soil Science Society of America, and the University Corporation for Atmospheric Research. Comparable non-U.S. scientific organizations in other countries have also endorsed the mainstream view of the science of climate change.

The science in this book follows the reports of the IPCC and the other relevant national and international scientific organizations. That, in a nutshell, is why you should believe this book. The alternative views on climate change you might see or hear, such as those from skeptical friends or the Internet, do not come from a process as credible as the IPCC's and therefore should not have the same standing. It should be emphasized that this does not mean the IPCC's reports are correct – any scientific claim is at risk of being overturned by future research. Nevertheless, the IPCC's reports do accurately represent what the relevant experts think about the science, which is the best guide there is for non-experts.

1.5 Chapter summary

1.5 Chapter summary Weather refers to the exact state of the atmosphere at a point in time; climate refers to the statistics of the atmosphere over a period of time, usually several decades in length or longer.

Climate change refers to a change in the statistics of the atmosphere over decades. Such statistics include not just the averages but also the measures of the extremes – how much the atmosphere can depart from the average.

Temperatures expressed in this book are in degrees Celsius; conversion from Fahrenheit can be done with this equation: C = (F − 32) × 5/9.

Any position on the surface of the Earth can be described by a latitude and longitude. Latitude is a measure of the position in the North-South direction, while longitude is a measure of the position in the East-West direction. The tropics cover the region from 30°N to 30°S; mid-latitudes cover the region from 30° to 60° latitude; and the polar regions cover from 60° to 90° latitude.

In our society, we frequently rely on experts for advice on highly specialized or technical fields. For climate change, the IPCC reports represent the opinion of the world's experts, and the science described in this book reflects the IPCC's scientific views.

Those opposed to policy action on climate change frequently make doubt of the science of climate change a central part of their argument. In so doing, they are following the strategy employed by the tobacco companies to keep public debate about the health effects of smoking alive for decades after the issue was settled by scientists.

Additional reading

The IPCC's reports are available online from www.ipcc.ch.

A. E. Dessler and E. A. Parson, The Science and Politics of Global Climate Change: A Guide to the Debate, 2nd ed. (Cambridge: Cambridge University Press, 2010). Chapter 2 discusses how the scientific and policy processes work and how assessments (like the IPCC) help bridge the gap between them.

N. Oreskes and E. M. Conway, Merchants of Doubt: How a Handful of Scientists Obscured the Truth on Issues from Tobacco Smoke to Global Warming (London: Bloomsbury Press, 2010). This is an important book about how deception is used to mislead the public on matters ranging from the risks of smoking to ozone depletion to the reality of global warming.

Union of Concerned Scientists, Smoke, Mirrors, and Hot Air: How ExxonMobil Uses Big Tobacco's Tactics to Manufacture Uncertainty on Climate Science (Cambridge, MA: Union of Concerned Scientists, January 2007). This is a description of how the tactics employed by the tobacco companies to cast doubt on the science of the health impacts of smoking are now being used by oil companies to cast doubt on the science of climate change (available online at www.ucsusa.org/assets/documents/global_warming/exxon_report.pdf).

Most scientific societies have statements affirming the science of climate change presented in this book. This includes statements from the American Geophysical Union and the American Meteorological Society.

See www.andrewdessler.com/chapter1 for links to the above material and additional resources for this chapter.

Terms Climate

Climate change

Equator

Latitude

Longitude

Mid-latitudes

Opinion leader

Polar region

Prime meridian

Summary for Policymakers

Tropics

Weather

Problems 1. Determine the latitude and longitude of the White House, the Kremlin, the Pyramids of Giza, and the point on the opposite side of the Earth to where you were born. Use an online tool (e.g., Google Earth) or an atlas (which you can find in any library).

2.

a) Convert the following temperatures from degrees Fahrenheit to degrees Celsius: 300, 212, 70, 50, 32, and 0°F

b) Convert the following temperatures from degrees Celsius to degrees Fahrenheit: 150, 100, 70, 50, 0, and –10°C

3.

a) The temperature increases by 1°C. How much does it increase in degrees Fahrenheit?

b) The temperature increases by 1°F. How much does it increase in degrees Celsius?

c) This is true: I told a reporter that the Earth has warmed by 0.8°C over the last century. When it appeared in print, the sentence said: Dessler said that the Earth has warmed by 33°F over the last century. Where did the reporter go wrong?

4. What temperature has a numerical value that is the same in degrees Celsius as it is in degrees Fahrenheit?

5. Find a two-digit temperature in degrees Fahrenheit for which, if you reverse the digits, you get that same temperature in degrees Celsius (e.g., find a temperature, such as 32°F, for which the Celsius equivalent would be 23°C; this example, of course, does not work).

6. Why do you believe that smoking causes cancer? (If you do not believe this, then why do you believe that smoking does not cause cancer?) What would be required to get you to adopt the opposing view?

7. Find two friends who have strong but opposing views of climate change.

a) Ask both of them why they believe what they do and what would be required for them to adopt the opposing view. It is important to understand where their views come from; if they argue, say, that glaciers are retreating or not, find out where they get their facts.

b) Which of these positions appears more credible? Why?

c) Can you use their views on climate change to predict their views on other issues (abortion, gun control) and their political affiliation?

8. Practice reading a graph. These questions all refer to Figure 1.1.

a) What fraction of days have an average temperature of 15°C during the 1900–1929 and the 1970–1999 periods?

b) For the 1900–1929 period, what is the warmest temperature that occurred? What about the 1979–1999 period?

c) What temperature(s) have an equal probability of occurring in the two periods?

d) For the period 1970–1999, estimate the fraction of days that have a temperature 15°C or greater.

e) Same as d, but for the period 1900–1929.

f) Compare the answers to d and e. What does this tell you about the changes in extreme heat under even modest warming?

9. Give examples of situations when weather affected your or your family's life. Then do the same for climate.

10. Those opposed to the IPCC's scientific conclusions have set up their own summary of the science of climate change, which they call the NIPCC. Do some online research and then compare and contrast the credibility of the two reports.

11. In the climate debate, few institutions are attacked as frequently as the IPCC. Using web searches, identify some arguments made by those arguing that the IPCC cannot be trusted.

1 There are, of course, siting issues in measuring the weather. Depending on your location, the weather you measure when you walk outside may not be terribly representative of the weather of the larger areas.

2 From the Legacy Tobacco Documents Library, legacy.library.ucsf.edu/tid/ctv74e00/pdf;jsessionid=F68A45A37FAF5E3AD23A5E84A7EEE463.tobacco03

3 The movie “Thank You for Smoking” (www.imdb.com/title/tt0427944/) is a great parody of the debate.

4 See the additional reading for this chapter at www.andrewdessler.com/chapter1 for links to these reports and other examples.

2

Is the climate changing? ◈

In this chapter, I address the question of whether the Earth's climate is currently changing and how it has changed in the past. You will see overwhelming evidence that the climate is indeed changing and that it has changed significantly over the Earth's entire history. I will not discuss the causes of climate change here, though – we will do that in Chapter 7.

In Chapter 1, climate change was defined as a change in the statistics of the weather over a period of several decades. In this chapter, the statistic I will primarily focus on is global average temperature for two reasons. First, the most direct impact from the addition of greenhouse gases to the atmosphere is an increase in temperature. Changes in other variables, such as precipitation or sea level, are a response to the temperature change. Second, we have the best data for temperature. The technology for measuring it is centuries old, and people have been measuring and recording the temperature with reasonable global coverage since the middle of the nineteenth century. In addition to direct temperature measurements, there are other techniques, such as studying the chemical composition of ice and rocks, that allow us to indirectly infer the temperature of the Earth over nearly its entire 4.5- billion-year history.

Rather than analyze temperature directly, however, we will instead analyze temperature anomalies, defined as the difference between the temperature and a reference temperature; the reference temperature is usually the average temperature over a previous multi-decadal period. For example, the monthly average temperature for Australia in August 2009 was 19.4°C, while the August 1979–2009 average for that location was 15.4°C. Thus, that region's temperature anomaly in August 2009 was +4.0°C, relative to the 1979–2009 baseline. Had we picked a

different reference period, the anomaly would change. As an example, Figure 2.1 shows the global pattern of anomalies for June 2009.

Figure 2.1 The monthly surface temperature anomaly in June 2009 in degrees Celsius. The reference temperature for the anomaly is the average of the June temperatures from 1979 to 2009

(data obtained from the MERRA reanalysis).

Why use anomalies rather than absolute temperature? The main reason is that absolute temperature can vary sharply over short distances, such as between a city and a nearby rural area, or between two nearby sites at different altitudes. You may have noticed this, for example, if your car displays outside temperature on its dashboard. As you drive a few miles, you might see the temperature change by a few degrees, particularly if you are driving into and out of a city.

Anomalies, however, are constant over much longer distances: If it is a degree warmer than average in a city, then it will be a degree warmer than average a few kilometers away from the city, even if the absolute temperatures differ by a few degrees. Figure 2.1 shows this – regions of warm and cold anomalies tend to be hundreds, sometimes even thousands, of kilometers across. This means that

calculations of global average temperature anomalies require only about a hundred or so temperature stations spread across the globe. Measuring the absolute temperature of the planet would require many more stations – more stations, in fact, than exist.

Another advantage of using anomalies is that you can measure changes in a quantity even if you cannot measure the absolute value of the quantity. Imagine, for example, that you want to determine if a child is growing. The most obvious way to do this is to measure the child's height every few months. But, if you could not do that, an alternative would be to measure his height relative to, say, a mark on the wall. If the top of his head is 1 inch below the mark one year, even with it the next year, and 1 inch above the following year, then you can be confident that the child is growing 1 inch per year – even if you never know the child's absolute height. This is the situation for the Earth's temperature. We cannot measure the Earth's absolute temperature with high accuracy (because measuring it would require an extremely dense network of temperature measurements, as discussed earlier), but we can measure the temperature anomaly with high accuracy – high enough to see clear warming.

I will also generally focus in our discussion on global average quantities. The reason is that the climate of a region can vary significantly just due to weather variability – i.e., particular regions can experience climate extremes (e.g., a heat wave) that are completely independent of climate change. However, these local variations are usually balanced by an opposite extreme elsewhere: if one region is undergoing a heat wave, there is likely another region that is undergoing a cold wave (as seen in Figure 2.1). By averaging over the globe, we rid ourselves of most of this weather variability and more clearly isolate the smaller climate change signal.

2.1 Recent climate change

2.1.1 Surface thermometer record

People have been measuring the local air temperature at locations all over the globe for centuries. They were originally made manually using liquid-in-glass thermometers, but in recent decades these have been replaced by automated electronic thermometers. By combining these measurements, scientists can estimate the global average surface temperature anomaly of the Earth over the past 150 years, and that time series is plotted in Figure 2.2a.

Figure 2.2 (a) Global annual average temperature anomaly; the gray line is the annual average, and the black line is a smoothed time series. (b) Smoothed temperature anomaly time series for three different regions of the planet: the northern hemisphere (24°N-90°N), the tropics (24°N-24°S), and the southern hemisphere (24°S-90°S). In both plots, the reference temperature used in calculating the anomalies is the 1951–1980 average.

Data are from the NASA GISS Surface Temperature Analysis [Hansen et al., 2010], downloaded from data.giss.nasa.gov/gistemp/.

The data clearly show that the Earth is warming. From 1880 to 2012, the average surface temperature of the Earth rose by 0.85°C. The warming has not been uniform but occurred primarily in two distinct periods, from 1910 to 1945 and from

1976 to 2002. Superimposed on the slow warming trend are many bumps and wiggles that are unrelated to climate change. Despite this short-term variability, the recent warming is basically continuous, with every decade since the mid-twentieth century warmer than previous decades. The three warmest years in the record were 2005, 2010, and 2014.

It is also worth noting that the year-to-year variations in the global-average temperature are quite small – just a few tenths of a degree. This is much smaller than the local temperature variations where you live. Thus, warming of a few degrees Celsius, which are predicted for the twenty-first century, would run off the scale in Figure 2.2. While this does not tell us the effect of such warming, it should certainly compel our attention.

Figure 2.3 shows how the warming of the twentieth century was distributed across the planet. Clearly, the warming is occurring just about everywhere – thus justifying the “global” part of “global warming.” However, the warming has not been entirely uniform. Probably the most obvious difference is that land areas warmed more than the ocean. Figure 2.2b shows temperatures for the northern hemisphere, tropics, and southern hemisphere, and it shows that the northern hemisphere warmed more than the tropics or the southern hemisphere and that the tropics and southern hemisphere warmed about equally.

Figure 2.3 The distribution of warming (in °C) between 1901 and 2012. Regions where data are too sparse to produce an estimate are white.

Adapted from Figure SPM.1 of IPCC [2013].

In science, no single data set is ever considered definitive, and that is particularly true of the surface thermometer record. This network of thermometers was not designed for climate monitoring, and, over the years, the network has undergone many changes. Changes in the types of thermometer used, station location and environment, observing practices, and other sundry alterations all have the capacity to introduce spurious trends in the data.

For example, imagine you have a thermometer that is in a rural location in the late nineteenth century. Over time, a nearby city expands so that by the 1980s, the thermometer is completely surrounded by the city. Because cities tend to be warmer

than nearby rural locations, this would introduce a warming trend in the data not caused by a warming climate.

Scientists know about these problems, and, to the extent possible, they adjust the data to take them into account. For example, the impact of a city growing up around a thermometer can be assessed by comparing the measurements from that thermometer to nearby thermometers that have remained rural for the entire period. The temperature record in Figure 2.2 includes adjustments to account for as many of these issues as possible.

Nevertheless, uncertainty in the data remains, as a result of both uncertainties in the adjustments and uncertainties that cannot be adjusted for. To account for this, scientists put error bars on the trend. An error bar is the scientists' estimate of the potential error in their estimate. The name “error bar” comes from the fact that, on a plot, error bars are frequently indicated as bars extending from the data. For the warming from 1880 to 2012 of 0.85°C, the error bar is 0.20°C, meaning that the warming is very likely between 0.65°C and 1.05°C.

Given all of the possible problems in these data, it would be foolish to rely entirely on this one source to determine if and how much the Earth was warming. Scientists therefore turn to other data sets to verify this result. In the rest of Section 2.1, I describe the other data sets used to build confidence in the surface thermometer data set.

2.1.2 Satellite measurements of temperature

It is possible to measure global average temperature from orbit, and the United States has been flying instruments on satellites to make that measurement since 1978. Figure 2.4 shows the time series of satellite measurements of the global monthly average temperature anomaly. These data show a general warming trend over this period of approximately 0.14°C per decade (1.4°C per century).

Figure 2.4 Satellite measurements of the global monthly average temperature anomaly (black line). The gray line is temperature from the surface thermometer record. Anomalies in this plot are relative to the 1981–2010 period

(data obtained from the University of Alabama, Huntsville; downloaded from www.nsstc.uah.edu/data/msu/t2lt/uahncdc_lt_5.6.txt).

As with all data sets, though, this one has its own set of problems and uncertainties. First, satellites actually measure the average temperature of the lowest 8 km of the atmosphere, from the surface to about the altitude where airliners fly.

Thus, it is not actually a measurement of the surface temperature, although the temperature of this layer of the atmosphere should track the surface temperature.

Another issue with these data is orbital drift of the satellites carrying the instruments. Imagine that a satellite flies over a location at 2 PM each day and makes a measurement of that location's temperature. Over time, the satellite's orbit drifts so that it flies over that location later and later each day. After a few years, the satellite is flying over that location at 3 PM. Because temperatures rise throughout the day, it is generally warmer at that location at 3 PM than it is at 2 PM. Thus, the drift in the satellite's orbit would by itself introduce a warming trend, even if the climate were not actually changing. This artifact must also be identified and adjusted for.

Other issues include calibration of the satellite instruments, which were never designed to make long-term measurements, and the shortness of the satellite record (just a few decades long), both of which also introduce uncertainty into the observed warming. As with the surface thermometer record, these issues are known and adjusted for, to the extent possible.

One way to gain confidence in the satellite and surface thermometer records is to compare them; this is done in Figure 2.4 (the same surface thermometer data, but annually averaged, was plotted in Figure 2.2). The excellent agreement between these two independent temperature measurements provides strong confirmation of the reality of the warming of the climate seen in both data sets.

It is also worth noting that superposed on the long-term warming trend in Figure 2.4 are lots of shorter-term ups and downs. These are not random but can be assigned to various physical causes, mainly El Niño-La Niña cycles and volcanic eruptions. During El Niño events, the Earth warms several tenths of a degree Celsius. El Niño's opposite is La Niña, and during those events the Earth cools several tenths of a degree. These El Niño-La Niña events cause temporary fluctuations in temperature lasting a few years but no long-term changes in the climate. Volcanic gases emitted during eruptions cool the climate by blocking sunlight – after a few years, the effluents are removed from the atmosphere and the

climate returns to normal. These processes will be discussed in more detail later in the book.

An aside: Has global warming stopped?

“The planet has largely stopped warming over the past 15 years, data shows.”

– FoxNews.com, September 27, 20131

One claim that has arisen recently in the public debate over climate change is that the Earth was warming, but it stopped warming five, ten, or fifteen years ago. The implication of this argument is that climate change is therefore nothing to worry about.

So has climate change stopped? Figure 2.4 showed that superimposed on the slow warming trend are lots of bumps and wiggles associated with random variability from things like weather (a particularly cold winter or a particularly warm summer) or volcanic eruptions (which cool the planet) or natural cycles like (El Niño-La Niña cycles).

These sources of short-term variability do not have anything directly to do with climate change and do not cause any long-term changes in the climate. The bumps and wiggles do, however, make determining trends over short time periods (e.g., a decade) problematic. To illustrate this, Figure 2.5 shows monthly average global surface temperature anomalies between 1970 and 2013. Over this period, the planet warmed rapidly, at a rate of 1.7°C/century.

Figure 2.5 A plot of monthly and global average surface temperature from the surface thermometer record (gray line) along with short-term trend lines (black lines). This figure is an adaptation of SkepticalScience's escalator plot (www.skepticalscience.com/graphics.php?g=47 and www.skepticalscience.com/still-going-down-the-up-escalator.html)

Also shown on Figure 2.5 are short-term trends based on endpoints that were carefully selected to produce cooling trends. As you can see, it is possible to generate a continuous set of short-term cooling trends, even as the climate is experiencing a long-term warming. All you have to do is start the trend calculation during a particularly hot year (e.g., an El Niño year) and then end it in a cool year (e.g., a La Niña or volcanic year).

The existence of these short-term negative trends allows someone to disingenuously claim during almost any year covered by Figure 2.5 that global warming had stopped or even that the Earth had entered a cooling

period. There is even a term for this deceptive argument: “Going down the up escalator.”

There are two aspects of this worth emphasizing. First, claiming that global warming has stopped requires careful selection of the endpoints. This process of intentionally selecting data to yield a result counter to the full data set is known as cherry picking. Many of the skeptical claims you will hear in the public debate over climate are based on cherry picking a large data set in order to find the small number of exceptions that support the claim. Second, it is only possible to find cooling over short time periods. Over periods lasting several decades, the long-term warming dominates and even the most egregious endpoint selection cannot generate a cooling trend. We will return to this point when we talk about climate predictability in Section 8.7.

So two independent, direct measurements of temperature show the planet is warming, but this question is so important that even more confirmation is required. To do this, we turn to other measurements that, while not direct measurements of temperature, nevertheless tell us something about the temperature of the planet: the amount of ice on the planet, the heat content of the ocean, and sea level.

2.1.3 Ice

Because ice melts reliably at 0°C, it is a dependable indicator of temperature. In particular, if the warming trend identified in the surface thermometer and satellite records is correct, then we should expect to observe the Earth's ice disappearing. In this section, I show that ice is indeed disappearing, thus confirming the warming seen in the other data sets.

2.1.3.1 Glaciers

Glaciers form in cold regions when snow that falls during the winter does not completely melt during the subsequent summer. As snow accumulates over millennia, the snow at the bottom is compacted by the weight of the overlying snow and turns into ice. This process eventually produces glaciers hundreds, or even thousands, of feet thick.

The length, areal extent, and total volume of glaciers have been monitored for decades and, in some cases, centuries. For example, Figure 2.6 shows changes in average glacier length (relative to the length in 1950) for five world regions over the past few centuries. It shows that glaciers began retreating around 1800, with the recession accelerating later in the nineteenth century. The pattern of glacier retreat is consistent worldwide, confirming that the warming we are now experiencing is truly global.

Figure 2.6 Change in mean glacier length over time, measured relative to length in 1950, for five world regions

(the source is Figure 3.2 of Dessler and Parson, 2010, which was based on Figure 4.13 of Lemke et al., 2007).

Decreases in precipitation or decreases in cloudiness could also cause glaciers to recede. However, the fact that glaciers are receding all over the planet means that, whatever is causing the changes, it must be global. And there is no evidence of global trends in either cloudiness or precipitation that could cause the reduction in glacier lengths. We do, however, have evidence of global trends in temperature (e.g., Figure 2.3). Thus, the recession of glaciers is consistent with the global warming of the climate seen in the surface therm