(First Edition) Chapter 1 – The Basic Science — Easy as 1-2-3
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“What we are now doing to the world . . . by adding greenhouse gases to the air at an unprecedented rate . . . is new in the experience of the Earth. It is mankind and his activities which are changing the environment of our planet in damaging and dangerous ways.”
— British Prime Minister Margaret Thatcher, Nov. 8, 1989 (speech to the United Nations)
“We all know that human activities are changing the atmosphere in unexpected and in unprecedented ways.”
— President George H. W. Bush, Feb. 5, 1990 (remarks to the Intergovernmental Panel on Climate Change)
The two quotes above show that, more than a quarter century ago, the conservative leaders of both the United Kingdom and the United States were already convinced of the reality and the threat of global warming. What made them so sure? In the case of Thatcher, it probably helped that she was a scientist herself (trained in chemistry), which made it easier for her to recognize the underlying scientific ideas. But Bush was not a scientist, and he and many other people of all political persuasions were still able to understand the same ideas. Why? Because they are not very difficult.
In this first chapter, I’ll show you the simple underlying science of global warming. Note that nothing in this chapter is subject to any scientific debate at all, and you’ll find that this basic science is accepted even by the scientists who count themselves as ardent skeptics (as evidence, look ahead to the quote that opens chapter 2). To show you just how simple and solid it is, let’s begin with an example from astronomy.
A Tale of Two Planets
Figure 1.1 Earth and Venus are shown to scale. Why do two planets that are so similar in size and composition have such different surface temperatures?
Figure 1.1 shows the planets Earth and Venus to scale, along with their global average surface temperatures. You can see that both planets are about the same size; they also both have about the same overall composition of rock and metal. But look at the enormous difference in their surface temperatures.1 Earth has temperatures ideally suited to life and our civilization, while Venus is hot enough to melt lead. If you think about it, you might wonder why two planets that are so similar in size and composition would have such drastically different surface temperatures.
It might be tempting to chalk it up solely to the fact that Venus is closer to the Sun than Earth, but that is not the answer. Figure 1.2 shows part of the Voyage Scale Model Solar System, which shows the sizes and distances of the Sun and planets on a scale of 1 to 10 billion. Notice that while it’s true that Venus is closer to the Sun, the difference isn’t really all that great, and it’s not nearly enough to account for such a large temperature difference by itself. Moreover, Venus’s bright clouds reflect so much sunlight that its surface actually absorbs less sunlight than Earth’s, which by itself would lead us to expect Venus to be colder than Earth. So why is Venus so hot?
Figure 1.2 This photo shows the inner portion of the Voyage Scale Model Solar System, located outside the National Air and Space Museum (Washington, D.C.). The locations of the Sun, Venus, and Earth are indicated. The model Sun is the visible gold sphere; on this scale, Venus and Earth are each about the size of the ball point in a pen (about one millimeter in diameter) and can be seen within the glass disks that face outward from their pedestals.
The primary answer is carbon dioxide, a gas that can trap heat and make a planet warmer than it would be otherwise. In fact, as we’ll discuss in more detail shortly, both planets would actually be frozen over if they had no carbon dioxide in their atmospheres at all. Earth has just enough carbon dioxide (plus water vapor; see page 13) to make our planet livable, so in that sense, carbon dioxide is a very good thing for life. But Venus has almost 200,000 times as much carbon dioxide in its atmosphere as Earth, and all this carbon dioxide traps so much heat that the entire surface is baked hotter than a pizza oven — providing clear proof that it is possible to have too much of a good thing (figure 1.3).
This story of Venus and Earth contains almost everything you need to understand the basic science of global warming. It shows that gases like carbon dioxide, which we call greenhouse gases, really do make planets warmer than they would be otherwise, and that the more of these gases a planet has, the hotter it will be.
Global Warming 1-2-3
The lesson from our tale of two planets leads directly to the subtitle of this chapter, in which I say that global warming is as easy as 1-2-3. By this, I mean that for all the arguments you may hear in the media, the basic science of global warming can be summarized in three simple statements, which embody two indisputable scientific facts and the inevitable conclusion that follows from them:
Fact: Carbon dioxide is a greenhouse gas, by which we mean a gas that traps heat and makes a planet (like Earth or Venus) warmer than it would be otherwise.
Fact: Human activity, especially the use of fossil fuels2 — by which we mean coal, oil, and gas, all of which release carbon dioxide when burned — is adding significantly more of this heat-trapping gas to Earth’s atmosphere.
Inevitable Conclusion: We should expect the rising carbon dioxide concentration to warm our planet, with the warming becoming more severe as we add more carbon dioxide.
Figure 1.3 The explanation for Venus’s high temperature is the vast amount of carbon dioxide in its atmosphere, which traps an enormous amount of heat through what we call the greenhouse effect.
Notice the inevitability of the conclusion: As long as both of the facts are true — and I’ll show you why there is no scientific doubt about either of them — then there’s really no way around the conclusion that global warming should be expected.
Of course, knowing that global warming is expected doesn’t tell us how badly or imminently we’ll be affected, and by itself it leaves open the possibility that other factors (such as climate feedbacks) might mitigate or even counteract the expected warming, at least on some time scales. We’ll discuss the debate over these issues in chapter 2. First, however, we’ll turn our attention to the evidence that supports our two facts.
Evidence for Fact 1 (Carbon Dioxide Makes Planets Warmer)
Fact 1 is that carbon dioxide is a greenhouse gas that makes a planet warmer than it would be otherwise. Now, in Q&A format, we’re ready to examine the evidence that makes this a fact rather than a matter of opinion.
Figure 1.4 This diagram explains the greenhouse effect, which makes a planet’s surface and lower atmosphere warmer than they would be otherwise. The yellow arrows represent visible light, the red arrows represent infrared light, and the blue dots represent greenhouse gas molecules.
There is no doubt that higher concentrations of carbon dioxide and other greenhouse gases make planets warmer, because this fact is based on the simple, well-understood, and well-tested physics of what we call the greenhouse effect. Figure 1.4 shows how the greenhouse effect works. Notice the following key ideas:
The energy that warms Earth comes from sunlight, and in particular from visible light (the kind of light that our eyes can see). Some sunlight is reflected back to space, and the rest is absorbed by the surface (land and oceans).
Earth must ultimately return the energy it absorbs back to space. The returned energy takes the form of infrared light, which our eyes cannot see.
Greenhouse gases — which include water vapor (H2O), carbon dioxide (CO2), and methane (CH4, also commonly called natural gas) — are made up of molecules3 that are particularly good at absorbing infrared light. Each time a greenhouse gas molecule absorbs a photon (the technical name for a “piece” of light) of infrared light, it quickly reemits it as another infrared photon, which may head off in any random direction. This photon can then be absorbed by another greenhouse gas molecule, which does the same thing.
The net result is that greenhouse gases tend to slow the escape of infrared light from the lower atmosphere, while their molecular motions heat the surrounding air. In this way, the greenhouse effect makes the surface and the lower atmosphere warmer than they would be from sunlight alone. The more greenhouse gases present, the greater the degree of surface warming. A blanket offers a good analogy. You stay warmer under a blanket not because the blanket itself provides any heat, but because it slows the escape of your body heat into the cold outside air.
It’s basic physics, verified by observations. All objects — including the Sun, the planets, and even you — always emit some form of light,4 but the form depends on the temperature. Hot objects, like the Sun, emit visible light. Cooler objects, like planets and you, emit only infrared light. While we cannot see infrared light with our eyes, we can detect it with infrared cameras and other instruments, and orbiting satellites have directly measured the amount of infrared light being emitted by Earth.
The atmosphere is indeed made mostly of nitrogen and oxygen; together, these two gases make up about 98 for nitrogen and 21% for oxygen). However, molecules of nitrogen and oxygen do not absorb infrared light, and therefore do not contribute to the heating of the surface. In other words, without the relatively small amounts of infrared-absorbing greenhouse gases (such as water vapor, carbon dioxide, and methane) that are present in our atmosphere, all the infrared light emitted from Earth’s surface would escape directly into space, and our planet would be frozen over.
In case you are wondering why some molecules can absorb infrared light and others cannot, it is a result of their structures. In our atmosphere, nitrogen and oxygen both take the form of molecules in which two atoms are bound together; that is, nitrogen is in the form N2 and oxygen in the form O2. In order to absorb photons of infrared light, molecules must be able to vibrate and rotate. This turns out to be fairly difficult for molecules with only two atoms, particularly when both atoms are the same, as in N2 and O2; that is why these molecules do not contribute to planetary heating. In contrast, vibration and rotation are relatively easy for many molecules with more than two atoms, which is why water vapor (H2O), carbon dioxide (CO2), and methane (CH4) all absorb infrared light effectively, making them greenhouse gases.
Although water vapor, carbon dioxide, and methane are the three most important greenhouse gases in Earth’s atmosphere, other trace gases can also act as greenhouse gases, which means they can also contribute to warming. Those that you are likely to hear about and that we’ll discuss a bit more in this book include nitrous oxide (N2O) and industrial chemicals known as halocarbons, which include chlorofluorocarbons (CFCs).
It depends on how picky you want to be. The term comes from botanical greenhouses, but those greenhouses actually trap heat through a different mechanism than planetary atmospheres: Rather than absorbing infrared radiation, greenhouses stay warm primarily by preventing warm air from rising. Nevertheless, atmospheric greenhouse gases and botanical greenhouses have the same net effect of keeping things warmer than they would such as cement production,be otherwise, so I’m personally fine with the term “greenhouse effect.”
Figure 1.5 This diagram shows the experimental setup used by John Tyndall in 1859, when he first measured how gases like carbon dioxide create what we now call the greenhouse effect. The measurements have been repeated and refined ever since. Source: The original illustration is from Tyndall’s 1872 book Contributions to Molecular Physics in the Domain of Radiant Heat; this annotated version is from Wikipedia.
Two major lines of evidence show conclusively that greenhouse gases trap heat. First, scientists can measure the heat-trapping effects of these gases in the laboratory. Although the actual setups are somewhat more complex, the basic idea is simply to put a gas (such as carbon dioxide) in a tube, shine infrared light at it, and measure how much of that light passes through and how much is absorbed. Such measurements were first made more than 150 years ago by British scientist John Tyndall (figure 1.5) and have been repeated and refined ever since.
Second, we can easily confirm that the greenhouse effect raises actual planetary temperatures in the way we discussed earlier for Earth and Venus. If there were no greenhouse effect, a planet’s average temperature would depend only on its distance from the Sun and the relative proportions of sunlight that it absorbs and reflects. I won’t bother you with the mathematical details, but they lead to the simple formula that you can see being applied to Earth in figure 1.6. The formula shows that Earth’s global average temperature would be well below freezing (–16°C, or +3°F) without greenhouse gases. In other words, we need the greenhouse effect to explain Earth’s actual average temperature, which is about 15°C (59°F). The same is true for all other planets: We get correct answers for planetary temperatures only when we use mathematical formulas that include the greenhouse effect.
Figure 1.6 This painting shows the calculation of Earth’s expected average temperature if there were no greenhouse effect. The fact that this temperature (−16°C) is so much lower than the actual average temperature (+15°C) shows that the natural greenhouse effect is what makes Earth warm enough for life. Painting by Roberta Collier-Morales from The Wizard Who Saved the World.
This brings us back to our tale of two planets. For Earth, we find that without the naturally occurring greenhouse effect, our planet would be too cold for liquid oceans and life as we know it. That is why, as we saw in figure 1.3, the natural greenhouse effect is a very good thing for life on Earth. But Venus, with almost 200,000 times as much carbon dioxide in its atmosphere as Earth, clearly has much too much of this good thing.
Figure 1.7 This diagram shows what would happen if Earth magically moved to Venus’s orbit.
Earth actually has about the same total amount of carbon dioxide as Venus, but while Venus’s carbon dioxide is virtually all in its atmosphere, nearly all of Earth’s is “locked up” in what we call carbonate rocks, the most familiar of which is limestone. The reason for this difference is that Earth has oceans and Venus does not.
On both planets, the original source of carbon dioxide was gas released by volcanoes. On Earth, carbon dioxide dissolves in the oceans (which contain about 60 times as much carbon dioxide as the atmosphere), where it then combines with dissolved minerals to form carbonate rocks (which contain almost 200,000 times as much carbon dioxide as the atmosphere). Venus lacks oceans and therefore cannot dissolve carbon dioxide gas, so it all remains in the atmosphere.
A deeper question is why Earth has oceans and Venus does not, and scientists attribute this to the fact that Venus is closer to the Sun (Venus is about two-thirds as far from the Sun as Earth). You can understand the role of distance from the Sun by thinking about what would happen if Earth were magically moved to Venus’s orbit (figure 1.7). The greater intensity of sunlight would immediately raise Earth’s average temperature from its current 15°C to about 45°C (113°F). The higher temperature would increase the evaporation of water from the oceans, putting much more water vapor into the atmosphere — and because water vapor is a greenhouse gas, the added water vapor would strengthen the greenhouse effect and drive temperatures even higher. The higher temperatures, in turn, would lead to even more ocean evaporation and more water vapor in the atmosphere, strengthening the greenhouse effect even further. In other words, we’d have a reinforcing feedback5 process in which each little bit of additional water vapor in the atmosphere would lead to a higher temperature and even more water vapor. The process would rapidly spin out of control, resulting in what scientists call a runaway greenhouse effect. It would not stop until the “moved Earth” became as hot as (or even hotter than) Venus is today.
In fact, something like this probably occurred on Venus long ago. Based on scientific understanding of how the Sun generates energy through nuclear fusion, the Sun should very gradually brighten with time. The rate is so slow that we cannot measure it, but the calculations indicate that the Sun was about 30% dimmer when the planets were born (about 4½ billion years ago) than it is today. This means that the young Venus probably had sunlight of not much greater intensity than Earth does today, and some scientists suspect that Venus may have had oceans at that time. As the Sun gradually brightened, Venus grew hotter until a runaway greenhouse effect set in.
Yes, but it is very weak. The atmosphere of Mars is made mostly of carbon dioxide (about 95 of that on Earth) that the total amount of carbon dioxide is actually quite small. As a result, Mars is warmed only a little by its greenhouse effect, and its greater distance from the Sun makes it quite cold, with an average surface temperature of –50°C (–58°F). Scientifically, the surprise is that Mars shows clear evidence of having had liquid water on its surface in the past. This means that long ago, Mars must have been much warmer than it is today, which in turn means that it must once have had a much stronger greenhouse effect. Scientists have a pretty good idea of why Mars once had a strong greenhouse effect and why the effect ultimately weakened so much, but the full discussion isn’t directly relevant to our topic in this book.The Cosmic Perspective and Life in the Universe).">6
It’s true that there is more water vapor than carbon dioxide in the atmosphere. In fact, there’s about 10 times as much water vapor as carbon dioxide, and water vapor does indeed contribute more than carbon dioxide to Earth’s overall greenhouse warming. However, carbon dioxide is the more critical gas in setting Earth’s temperature.
The reason is that once we increase the carbon dioxide concentration of the atmosphere, the concentration tends to remain higher for many decades, centuries, and even millennia. In contrast, water vapor cycles easily into the atmosphere through evaporation and out of the atmosphere through rain and snow. As a result, the amount of water vapor in the atmosphere at any given time is determined by the temperatures of the ocean and atmosphere. That is, the amount of water vapor in the atmosphere can change in response to temperature changes, but it does not initially cause those changes. Instead — and very importantly — water vapor amplifies climate changes initiated by other factors, because it acts as a reinforcing feedback. For example, if more carbon dioxide raises the global temperature a little bit, the atmosphere can hold more water vapor, which then traps more heat, making the temperature rise even more. Conversely, if the carbon dioxide level drops, the global temperature decreases so that there is less water vapor in the atmosphere, which then traps less heat so that the temperature drops further. This amplification by water vapor is well understood and is necessary to explaining Earth’s natural cycles of ice ages and warm periods, which we’ll discuss in chapter 2.
Yes, but we also have to phrase the question more precisely. Let’s pose it this way: If we suddenly stopped adding carbon dioxide to the atmosphere, how long would it take for the carbon dioxide concentration to drop back down to something closer to its “natural” (preindustrial) value? To answer this question, scientists do calculations based on all the various ways in which carbon dioxide can be removed from the atmosphere, which include uptake by plants, dissolving in the oceans, and the gradual production of seashells and carbonate rocks. The details are fairly complex and subject to some uncertainties, and the answer also depends on how much carbon dioxide we’ve added (in total) by the time we stop adding it, but here’s a brief summary of current understanding:
For the first few decades, uptake by the land and oceans would remove carbon dioxide relatively rapidly, so that about a third of the carbon dioxide we’d added would be removed in 20 to 50 years. But then the rate would slow dramatically: Between 15 of our added carbon dioxide would still remain in the atmosphere after 2,000 years, and it would take tens of thousands of years for the concentration to come all the way back down to its preindustrial valuehere.">7. (This discussion assumes natural processes only, as opposed to “geoengineering” schemes in which we develop technologies that can remove carbon dioxide from the atmosphere.)
After carbon dioxide and water vapor, methane (CH4) is the next most abundant greenhouse gas in Earth’s atmosphere, and though it is much less abundant than carbon dioxide and makes a much smaller overall impact on the climate, its effect is still important. The same is true for other greenhouse gases, and scientists can and do take all these greenhouse gases into account when gauging the strength of the greenhouse effect. However, aside from a brief discussion that you’ll find on pages 22–23, I will generally ignore these other gases in this book, primarily because carbon dioxide has the greatest impact on Earth’s climate and we can therefore keep our science discussions simpler by keeping the focus on it. Nevertheless, the contribution of other greenhouse gases is very important to consider both scientifically and in policy decisions, so while they may be secondary in importance to carbon dioxide, they should not be ignored.
There is no doubt at all about the fact that atmospheric carbon dioxide and other greenhouse gases create a greenhouse effect that makes a planet warmer than it would be otherwise. The heat-trapping effects of greenhouse gases have been measured in the laboratory, and our overall understanding of the greenhouse effect has been further verified by the fact that actual planetary temperatures match calculated temperatures only when we take it into account. Indeed, while a small minority of scientists dispute the threat of global warming, you will not find any legitimate scientists who dispute the basic physics of the greenhouse effect.
Evidence for Fact 2 (Human Activity Is Adding Carbon Dioxide to the Atmosphere)
We now turn to the evidence that establishes Fact 2, which is that human activity, especially the use of fossil fuels, is adding heat-trapping carbon dioxide to Earth’s atmosphere.
Figure 1.8 This graph shows direct measurements of the amount of carbon dioxide in the atmosphere, which have been made on a regular basis since the late 1950s. Source: National Oceanographic and Atmospheric Administration (NOAA). The data are updated monthly, and you can always see the latest at www.esrl.noaa.gov/gmd/ccgg/trends.
The most direct way to measure the amount of carbon dioxide in the atmosphere is to collect and study air samples. Scientists have been making such direct measurements continuously since the late 1950s. Figure 1.8 shows the measurements made using samples collected at the Mauna Loa Observatory in Hawaii. As you can see, the measurements clearly show a rapidly rising concentration of carbon dioxide in Earth’s atmosphere. Measurements made at numerous other sites around the world show similar increases over time.
Notice that the units on the vertical axis are parts per million (ppm), which means the number of carbon dioxide molecules among each 1 million total molecules of air. You can see that the carbon dioxide concentration has recently surpassed 400 parts per million, which is the same as 0.04.">8 In other words, carbon dioxide represents only a tiny fraction of the molecules in Earth’s atmosphere. Nevertheless, as we’ve already discussed, this small amount of carbon dioxide is very important because of its role in the greenhouse effect.
These small wiggles represent seasonal variations in the carbon dioxide concentration. They occur because plants and trees absorb carbon dioxide as they grow in spring and summer, then release it as they decay in fall and winter. The global pattern follows the seasons of the Northern Hemisphere because, as you’ll see if you look at a globe, that is where most of Earth’s land mass — which also means most of the plants and trees — is located. (The Southern Hemisphere is mostly ocean.) The seasonal wiggles peak each year in May because that is when most of the prior year’s vegetation has decayed (releasing its carbon dioxide), while the Northern Hemisphere’s summer growing season is not yet far enough along for new vegetation to have absorbed very much carbon dioxide.
The carbon dioxide concentration varies somewhat from place to place on Earth, so it’s important to choose measurement locations that are relatively unaffected by local conditions and therefore representative of changes occurring in the atmosphere as a whole. The Mauna Loa site was selected by Scripps Institution scientist Charles David Keeling because its high altitude and its location on a relatively isolated island make the air above it representative of a large portion of the global atmosphere. Today, scientists also measure the carbon dioxide concentration at many other locations around Earth, and these measurements confirm that the concentration is rising at a similar rate around the world. Note also that measurements are made by several independent scientific groups (for example, scientists from Scripps and from the National Oceanic and Atmospheric Administration [NOAA]), and these different measurement sets agree very well, further confirming that the measured changes are real. We usually show the Mauna Loa data because they constitute the longest continuous record from any site and because other data confirm that they are representative of the global carbon dioxide concentration. Incidentally, Keeling’s work has proven to be so important that the graph shown in figure 1.8 is often called the Keeling curve in his honor.
Figure 1.9 These photos show the drilling of an ice core, which consists of the compressed snows (with trapped air bubbles) laid down year after year. Scientists can read the past climate history in the layers in much the same way that we can learn about the past from tree rings. Source: NASA, Goddard Space Flight Center; photos copyright by photographer Reto Stöckli; reprinted with permission.
Although we only have direct measurements since the 1950s, scientists have discovered a variety of ways to measure the carbon dioxide concentration from earlier times. The most reliable records come from air bubbles trapped in ancient ice — that is, ice that has remained frozen for long periods of time in glaciers or in Greenland or Antarctica. Although the work is difficult and requires great care, the basic idea is simple: Scientists drill down into the ice and bring up an ice core (figure 1.9). The ice core is made from the accumulated snow of many years, which over time is compressed into solid ice. The deeper layers represent ice laid down at earlier times. The longest ice core ever drilled extended to a depth of about 3.2 kilometers (2 miles) in the Antarctic ice, and represents snows that fell and accumulated over a period of 800,000 years.
By studying the air bubbles in this long ice core, scientists have been able to measure how the carbon dioxide concentration has varied in Earth’s atmosphere over the past 800,000 years. Figure 1.10 shows the results, along with a zoom-out to show the direct measurements made since the 1950s. Notice that the carbon dioxide concentration has risen and fallen substantially many times over the 800,000-year period. These variations must be natural, because they predate the human burning of fossil fuels, which became important only in the past couple hundred years.
Several key facts should jump out at you as you study figure 1.10:
Figure 1.10 This graph shows how the atmospheric carbon dioxide concentration has changed over the past 800,000 years. Source: Data from the European Project for Ice Coring in Antarctica. Also see the animated version of this graphic at www.esrl.noaa.gov/gmd/ccgg/trends/history.html.
The carbon dioxide concentration has varied naturally over the past 800,000 years, but only within a range between about 180 and 290 parts per million. These natural variations are dwarfed by the huge increase that has occurred since the industrial revolution began in about 1750.
Today’s carbon dioxide concentration of about 400 parts per million is already about 40% higher than the pre-industrial concentration (280 parts per million in 1750), which was itself near the highest found at any time in the past 800,000 years.
If we extrapolate into the future, we find that the carbon dioxide concentration is on track to reach double its preindustrial value (560 parts per million) in just 50 to 60 more years, and triple that value (840 parts per million) by the middle of the next century.
Figure 1.11 (Left) This graph shows the amounts and sources of carbon dioxide released by human activity since 1850. (Right) This graph shows that the rate of increase in the total amount of carbon dioxide released by human activity (red curve) tracks almost perfectly with the measured rise in the atmospheric carbon dioxide concentration (black curve). Sources: (left) Reproduced directly from J. M. Melillo, T. C. Richmond, and G. W. Yohe, eds., 2014, Highlights of Climate Change Impacts in the United States: The Third National Climate Assessment (U.S. Global Change Research Program); (right) Scripps CO2 Program (scrippsco2.ucsd.edu/graphics_gallery/mauna_loa_record/mauna_loa_fossil_fuel_trend).
It’s true that there are natural sources that can add carbon dioxide to the atmosphere (such as volcanoes), but we can be very sure that the recent, dramatic rise in carbon dioxide that you see in figure 1.10 is due almost entirely to human activity. Most of this carbon dioxide comes from the burning of fossil fuels, with lesser amounts from deforestation and industrial processes, such as cement production,9 that release carbon dioxide. There are four major reasons why we can be so sure that humans are responsible for the current rise in the carbon dioxide concentration.
First, the rise in atmospheric carbon dioxide coincides almost perfectly with the increased release of carbon dioxide by human activity. The ice core data tell us that for the 1,000 years prior to 1750, the atmospheric carbon dioxide concentration stayed very close to 280 parts per million; the dramatic rise that has taken it past 400 parts per million today began right when the industrial revolution began, which is also when humans first began to use large quantities of fossil fuels. This correlation becomes even clearer when you look at how the release of carbon dioxide due to industry tracks with the increase in the carbon dioxide concentration. The graph on the left side of figure 1.11 shows how the amount of carbon dioxide released by human activity each year has been rising with time, and identifies the amounts from different sources. Now look at the graph on the right side of figure 1.11. At first, you may think you are just looking at a repeat of figure 1.8 with different colors. However, in figure 1.8, the black curve running up the middle was the average trend running through the seasonal wiggles. This time, the seasonal wiggles are shown in black, and the red curve running up the center represents the rising amount of carbon dioxide that has been released into the atmosphere by humanswebsite of the Scripps CO2 Program.">10. The virtually perfect tracking shows that the atmospheric carbon dioxide concentration is rising precisely as we’d expect if its source is human activity.
Second, there really isn’t any other possibility. Scientists have a variety of ways to measure the amount of carbon dioxide that is added by natural sources, such as volcanoes, and it just doesn’t compare to the amounts being released by the burning of fossil fuels and other human activity. In fact, the natural contributions are smaller than about 1% of the human contributions.
Third, the burning of fossil fuels consumes oxygen at the same time that it releases carbon dioxide, which means that if the rising carbon dioxide concentration comes from fossil fuels, there should be a corresponding decrease in the concentration of oxygen in the atmosphere and oceans — and this has indeed been measured. The oxygen decrease probably won’t have direct effects on us, because oxygen makes up about 21. However, it may be contributing to an increase in low-oxygen areas of the ocean, which can be very damaging to ocean life.
Figure 1.12 This graph shows the atmospheric carbon dioxide concentration (black) and the relative abundance of carbon-13 (brown) over the past 1,000 years, as measured from ice cores. The declining abundance of carbon-13 is a “smoking gun” that leaves no doubt that the rising carbon dioxide concentration is coming mostly from the burning of fossil fuels. (The carbon-13 abundance is shown as “δ13,” which is a standard scientific unit used for this purpose.) Source: M. Rubino et al., J. Geophys. Res. Atmos. 118 (2013): 8482–8499, doi:10.1002/jgrd.50668
Fourth, and perhaps most convincing of all, careful chemical analysis of atmospheric carbon dioxide shows changes that only make sense with fossil fuels as the source. The key to understanding this evidence is to recognize that carbon atoms come in three different forms, or isotopes, known as carbon-12, carbon-13, and carbon-14, and the relative abundances of these three isotopes are different in carbon that comes from different sources (such as volcanoes, deforestation, and the burning of fossil fuels). Therefore, we can determine the source of atmospheric carbon dioxide by measuring these isotope abundances. Let’s first consider carbon-14, which is radioactive and exists today only as a result of ongoing production as cosmic rays hit atoms in Earth’s upper atmosphere11. Carbon-14 becomes incorporated into living organisms through respiration, but it decays after the organisms die, and the organisms that made fossil fuels died so long ago that there is no carbon-14 in fossil fuels at all. As a result, if fossil fuels are the source of the rising carbon dioxide concentration, then the relative abundance of atmospheric carbon-14 (compared to ordinary carbon-12) should be falling as the total carbon dioxide rises — and this is just what has been observed.
Even more impressive isotope evidence comes from changes in the relative abundance of carbon-13. Overall, carbon-13 represents about 1.07% of all natural carbon on Earth, but its percentage is slightly lower in living organisms (because life incorporates carbon-12 more readily into living tissues than carbon-13), which means it is also slightly lower in fossil fuels (since they are the remains of living organisms). Figure 1.12 shows ice core data for the past 1,000 years with the total carbon dioxide concentration in black and the relative abundance of carbon-13 in brown. Notice that the carbon-13 abundance has been dropping in tandem with the rise in carbon dioxide, just as we should expect if the rising carbon dioxide comes from the burning of fossil fuels (with their lower abundance of carbon-13). In effect, these isotopic data are a “smoking gun” that leaves no doubt that most of the added carbon dioxide is coming from the burning of fossil fuels.
No. Careful measurements show that only about half of the carbon dioxide released by humans each year is staying in the atmosphere. Much of the rest is being dissolved into the oceans (and some is taken up by plants and soil on land). Measurements confirm that the carbon dioxide concentration is increasing in the oceans in tandem with the increase in the atmosphere. This increase is making ocean water slightly more acidic, which creates the problem of ocean acidification that we will discuss in Chapter 3 as one of the major consequences of global warming.
Worth noting: Although we do not know exactly how much carbon dioxide the oceans are capable of absorbing, scientists expect the rate of uptake to slow as the oceans warm. If it does, then as the oceans absorb less, more of the carbon dioxide released by human activity will stay in the atmosphere, which will increase the rate at which the carbon dioxide concentration is growing even beyond that shown in figure 1.8.
It’s because the natural sources are in a natural balance. That is, while it’s true that the amount of carbon dioxide released from the oceans and by life is far larger than the amount released by human activity, those natural releases are almost perfectly balanced by natural processes that absorb carbon dioxide. For example, not counting the extra added by human activity, the oceans always absorb essentially the exact same amount of carbon dioxide that they release, and plants naturally absorb all the carbon dioxide exhaled by animals (and people). You can be sure this is the case because if it weren’t, the carbon dioxide concentration would always vary wildly over a huge range, rather than staying within the fairly narrow range that you see in figure 1.8. The only way that natural processes can change the carbon dioxide concentration is by being out of balance. Volcanoes can disrupt the balance, since their eruptions add carbon dioxide without removing it, but as noted above, these amounts are small compared to the human release. Deforestation also adds carbon dioxide, because it releases carbon dioxide stored in trees and plants — but this isn’t exactly a natural process, since humans are the cause of most of the deforestation that has occurred in the past few centuries.
Figure 1.13 These graphs show the concentrations measured since the late 1970s of methane, nitrous oxide, and halocarbons, including CFCs and gases used as substitutes for CFCs. Source: NOAA Earth System Research Laboratory, www.esrl.noaa.gov/gmd/aggi/
Yes. The left graph in figure 1.13 shows changes in the atmospheric methane concentration since the late 1970s (the time period for which direct measurements of all the gases shown in the figure are available). Data from ice cores and other sources show that the methane concentration has more than doubled since 1750. Human activity adds methane to the atmosphere in several ways, but the three largest are (1) agriculture, in which methane is released from rice paddies and from the raising of livestock; (2) oil and gas extraction and transport, during which methane can leak directly into the atmosphere; and (3) landfills, in which decomposing wastes release methane.
The middle graph in figure 1.13 shows the rapidly rising nitrous oxide concentration. Nitrous oxide is released primarily through the production and use of fertilizers, which means these emissions are tied to food production. It’s worth noting that because the release of nitrous oxide is unrelated to the use of fossil fuels, it’s essentially a separate problem that we will need to deal with along with carbon dioxide and methane emissions.
The rightmost graph in figure 1.13 shows halocarbons, which come entirely from human manufacturing and do not exist naturally. Notice that while CFC concentrations were rising rapidly in the 1970s and 1980s, their concentrations have since declined. The reason for the decline is that, starting in the 1970s, scientists began to recognize that CFCs could cause destruction of Earth’s atmospheric ozone layer, which protects us from dangerous ultraviolet light from the Sun. As a result, the nations of the world came together to sign the global treaty known as the Montreal Protocol (and subsequent revisions to strengthen it), which has successfully led to a dramatic decline in the production and use of CFCs.
To gauge the combined effects of carbon dioxide and other greenhouse gases, scientists use something called the “annual greenhouse gas index” (AGGI), which you can learn more about from the AGGI section of the NOAA Web site. These measurements indicate that the combined greenhouse warming from all gases emitted through human activity is roughly 50% larger than that from carbon dioxide alone.
There is no doubt that human activity is adding carbon dioxide and other greenhouse gases to the atmosphere. We know the carbon dioxide is from human activity because the rate of increase is what we expect based on the rate at which humans are releasing carbon dioxide; because natural sources are too small to account for the observed increase; because we see a corresponding decrease in atmospheric oxygen, just as expected; and because chemical analysis shows that the added carbon dioxide is coming from the burning of fossil fuels. Moreover, as Margaret Thatcher said in the quote that opens the chapter, we are adding this carbon dioxide at an “unprecedented rate” that is “new in the experience of the Earth,” at least for the past 800,000 years (and probably since our planet was born).
Global Warming 1-2-3: The Inevitable Conclusion
Let’s repeat the “global warming 1-2-3” that we started out with:
Fact: Carbon dioxide is a greenhouse gas, by which we mean a gas that traps heat and makes a planet (like Earth or Venus) warmer than it would be otherwise.
Fact: Human activity, especially the use of fossil fuels — by which we mean coal, oil, and gas, all of which release carbon dioxide when burned — is adding significantly more of this heat-trapping gas to Earth’s atmosphere.
Inevitable Conclusion: We should expect the rising carbon dioxide concentration to warm our planet, with the warming becoming more severe as we add more carbon dioxide.
In this chapter, I’ve shown you the evidence that explains why Facts 1 and 2 are scientifically supported beyond any reasonable doubt. Because statement 3 — that we should expect global warming to be occurring — follows inevitably, you can probably see where concern for global warming comes from.
Of course, knowing we should expect global warming does not tell us how fast it will occur or how detrimental its consequences will be. To understand those issues, we need to investigate the climate in more detail, and we’ll do that in the next chapter. But if we keep adding carbon dioxide to the atmosphere, our planet will become warmer. You might therefore begin asking yourself, how much warming are you willing to risk?
Note: These web pages offer the FIRST edition of the global warming primer for you to read freely. The NEW second edition (published 2024) is currently available only in book form (print $18; e-book $8). More details here.
