Introduction to modern climate change andrew dessler

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

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© Andrew Dessler 2016

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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

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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.

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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

http://www.esrl.noaa.gov/psd/data/gridded/data.20thC_ReanV2.html
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.