Prior to beginning this discussion, read Chapters 7 and 8 in your course textbook. Imagine that you are a resident of Arzaville, a community whose

8 Sustaining Our Atmosphere and Climate

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Learning Outcomes

After reading this chapter, you should be able to

• Differentiate between the troposphere and the stratosphere and between weather and climate. • Describe different sources of air pollution and their environmental and health impacts. • Identify different ways to reduce air pollution. • Describe causes and impacts of stratospheric ozone depletion. • Explain the relationship between the Earth’s energy balance and the greenhouse effect. • List evidence for the warming of the Earth and humanity’s role in global climate change. • Describe the impacts of global climate change on the environment, economy, and human

health. • Identify different ways to address global climate change.

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Section 8.1 Earth’s Atmosphere

Beijing, a city of 17 million people, has some of the worst air pollution of any major city in the world. When the 2008 Summer Olympic Games were held in the Chinese capital, there was widespread concern over the effect air pollution might have on the athletes as well as spectators attending the games. Some athletes with preexisting health conditions refused to participate, while others caused a stir by arriving at the games wearing protective face masks.

Chinese government leaders took action to address concerns and avoid the possible embarrass- ment of hosting a major global event shrouded in smog. Two months before the Summer Games were set to begin, heavily polluting factories were shut down, large-scale construction projects were halted or scaled back, and measures were taken to drastically limit the number of cars and trucks on the road. Overall, close to $20 billion was spent to clean up the city’s air, and the efforts did appear to have some positive impact. Reduced emissions from cars and factories combined with favorable weather conditions to avert terrible smog conditions, even though most major air pollutants continued to exceed safe levels throughout most of the games.

Aware that China was going to implement these pollution restrictions, scientists and pub- lic health experts saw an opportunity to study the effects of air pollution on human health. One such study recruited 125 healthy premedical students in Beijing and began monitoring their heart and lung conditions before the pollution restrictions were put in place, during the restrictions and the period of the Summer Games, and for months afterward. The results of this research were published in the Journal of the American Medical Association in 2012 and showed clearly that during the period of the pollution restrictions, the heart and lung condi- tions of the research subjects improved dramatically (Rich et al., 2012). A different study examined birth weights of babies born in Beijing hospitals before, during, and after the pollu- tion restrictions. It found that women who were 8 months pregnant during the period of the 2008 Summer Olympic Games gave birth to heavier and healthier babies, compared to the other two groups (Rich et al., 2015).

The story of the 2008 Summer Olympic Games is a dramatic example of how pollution can affect human health. And yet it is just one example of how our quality of life worsens when we use the air around us as a dumping ground. This chapter will examine three different envi- ronmental challenges that result from our disregard for the air and atmosphere: air pollu- tion, stratospheric ozone depletion, and global climate change. Each challenge has a different cause and operates on a different timescale, geographic scale, and location.

The bulk of this chapter will focus on what is perhaps the most serious of the three challenges and arguably the greatest environmental challenge facing the world today: global climate change. As its name suggests, global climate change is a global problem with no easy, quick fix. In fact, many environmental scientists are worried that our current inaction in the face of global climate change is locking in catastrophic levels of change for the planet for centuries to come.

8.1 Earth’s Atmosphere

A recurring theme in this book is the degree to which we tend to take for granted and over- look the natural systems that surround us. That’s certainly also the case for our atmosphere. When human populations were small and geographically spread out, pollutant emissions

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Section 8.1 Earth’s Atmosphere

could mix and disperse and not pose much of a problem. However, as our population approaches 8 billion, and as our activi- ties involve ever greater consumption of energy and other material, our impacts on the atmosphere are becoming more severe. To fully understand how human activities lead to air pollution, ozone depletion, and runaway climate change, we need a basic understanding of the atmosphere itself. What is the atmosphere? How does this crit- ical form of natural capital help make life on Earth possible in the first place?

Because our atmosphere cannot really be seen or touched, we often use interesting ways to describe it. Our atmosphere has been called a “thin envelope” or a “thin blanket” of gases sur- rounding the Earth and held in place by gravity. Despite the atmosphere extending miles into the sky, it’s been pointed out that if the Earth were an apple or a peach, the thickness of the atmosphere would be equivalent to the thickness of the skin on those fruits. Our atmosphere provides us with the air we breathe, helps regulate climate in ways that make the planet hab- itable, and contains gases that filter out dangerous forms of solar radiation that could other- wise make life on the surface of the Earth impossible.

The Earth’s atmosphere is divided, from lowest to highest, into four layers—the troposphere, the stratosphere, the mesosphere, and the thermosphere. For the purposes of this chapter, we will focus on the two lowest layers, the troposphere and the stratosphere (see Figure 8.1).

Johnson Space Center/NASA When viewed from the International Space Station, Earth’s atmosphere is a thin blue layer over the surface of the planet.

Figure 8.1: Earth’s atmosphere

The troposphere and the stratosphere are the layers of the atmosphere that most directly affect our daily lives.

Troposphere

Stratosphere

Ozone layer

Earth

Mt. Everest

9 km

10 km

50 km Ozone layer

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Section 8.1 Earth’s Atmosphere

Troposphere The troposphere ranges in height from 8 kilometers (5 miles) in polar regions where air is cold and dense to 18 kilometers (11 miles) over tropical regions where warm air causes ris- ing air currents. The troposphere is the region of the atmosphere that most directly impacts our day-to-day lives. It’s where we live and breathe and is where virtually all the water vapor, clouds, and weather on the planet exist. Air in the troposphere is 78% nitrogen (N2), 21% oxygen (O2), and 0.9% argon (Ar). The remaining one tenth of 1% is made up of “trace” gases like carbon dioxide (CO2), neon (Ne), methane (CH4), and nitrous oxide (N2O), as well as water vapor. As we’ll discuss, some of these gases, though present in very small quantities, exert an important influence on the Earth’s climate system. The top of the troposphere is known as the tropopause, something of a boundary between the troposphere and the stratosphere above.

Stratosphere The stratosphere ranges in height from 18 to 50 kilometers (11 to 31 miles) above the Earth’s surface. The chemical composition of air in the stratosphere is similar to that of air in the tro- posphere, with a couple of key exceptions—air in the stratosphere contains almost no water vapor, and it has about 1,000 times more ozone (O3). While ozone is a dangerous air pollutant near the Earth’s surface, it plays a critical role in the stratosphere. Most of the ozone in the stratosphere is concentrated in an area 18 to 26 kilometers (11 to 16 miles) above the Earth in a region known as the ozone layer. Ozone molecules in the ozone layer effectively absorb certain wavelengths of incoming ultraviolet (UV) solar radiation that can seriously damage living tissues. For this reason, the stratosphere and the stratospheric ozone layer have been referred to as “Earth’s sunscreen.” You’ll learn more about the ozone layer, and threats to it, in Section 8.3.

Weather Versus Climate Energy from the sun is constantly striking the atmosphere. About one fourth of this energy is reflected by clouds and the atmosphere back to space, about one fourth is absorbed by clouds and atmospheric gases, and about half reaches the Earth’s surface. The solar energy that is absorbed by clouds, atmospheric gases, and surfaces on Earth is reemitted as longer wave infrared or heat energy, warming the air and causing evaporation. This warms the air near the surface and causes it to rise, before that air cools again and sinks. This rising and falling of air creates what is known as convective circulation.

Convective circulation patterns help drive the planet’s weather, since heat and moisture get moved around the globe in the process. Weather consists of the day-to-day changes in tem- perature, atmospheric pressure, precipitation, humidity, and wind. In contrast, climate refers to average temperature and precipitation patterns in a given area over a longer period, such as a year. As you’ll see, confusion over the difference between weather and climate is one of the reasons why global climate change is misunderstood.

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Section 8.2 Air Pollution

8.2 Air Pollution

Air pollution is the presence of substances (air pollutants) in the atmosphere in high enough concentrations to harm humans, other organisms, and inanimate structures such as buildings. Air pollutants can come from natural sources like volcanoes, forest fires, and dust storms, as well as from a variety of human activities. Once in the atmosphere, air pollutants can create problems based on their concentration; how they react with sunlight, water vapor, and other chemicals in the atmosphere; how they are transported from one place to another; and how and where they eventually wash out of the atmosphere (atmospheric deposition). The chapter will first examine major sources and types of air pollution and then consider their impacts and possible ways to control and address the air pollution challenge.

Sources and Types of Air Pollution Environmental scientists typically start by breaking air pollution down into two major cat- egories. Primary air pollutants are those that are emitted directly into the atmosphere, such as particulate matter and sulfur dioxide emitted from burning coal in a power plant. Sec- ondary air pollutants are formed through chemical reactions in the atmosphere between primary air pollutants and other substances. Environmental scientists also find it helpful to distinguish between air pollutants that come from a stationary source like a coal-fired power plant versus those that are emitted by a mobile source like a car or truck.

Primary Pollutants Carbon monoxide (CO) is an invisible, odorless, tasteless gas that results from the incom- plete combustion of carbon-based fuels, primarily from motor vehicle exhaust. A second major source of CO is firewood burning and forest fires. Carbon monoxide is dangerous because of its ability to bind to hemoglobin and impair the delivery of oxygen to tissues. Overexposure to carbon monoxide can result in headaches, nausea, fatigue, and ultimately heart damage and death.

Nitrogen oxides (NOX) include nitric oxide (NO) and nitrogen dioxide (NO2). Most NOX emissions come from internal combustion engines in vehicles, as well as from wood burning and forest fires. NO2 is a reddish- brown gas that can cause lung irritation and respiratory disease; it can also react with water vapor to form a secondary pol- lutant known as nitric acid (HNO3). Nitric acid is one of the causes of acid deposi- tion, or acid rain, which is precipitation— including dust, gas, and fog—that contains acid and falls to the ground. Acid deposition can damage plants, harm aquatic life, and even eat away at statues and the exterior of buildings.

Sebastian Kunigkeit/picture-alliance/dpa/AP Images Michel Picaud, president of the charity Friends of Notre-Dame de Paris, points out damage caused by acid deposition at the famous Notre Dame Cathedral in Paris, France.

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Section 8.2 Air Pollution

Sulfur dioxide (SO2) results from the combustion of fuels that contain sulfur, primarily from burning coal in power plants. SO2 can react in the atmosphere to form a secondary pollutant known as sulfuric acid (H2SO4), which also contributes to acid deposition. Sulfur dioxide and sulfuric acid aerosols or droplets reduce visibility, damage crops and trees, corrode metals, damage exterior surfaces, and cause respiratory problems.

Suspended particulate matter (PM) refers to a range of solid and liquid particles that are small enough and light enough to remain suspended in the air. Particulate matter can come from natural sources (pollen, spores) as well as from human activities (soot and smoke from fuel combustion, dust from construction projects). Scientists and environmental regulators classify particulate matter into two categories based on their size. Fine particulate matter, or PM-10, are particles with a diameter of 2.5 to 10 micrometers. (For comparison, a typical human hair is roughly 70 micrometers in diameter.) Ultrafine particulate matter, or PM-2.5, are particles smaller than 2.5 micrometers in diameter. Public health experts are particularly concerned about PM 2.5 pollution because smaller particles are more likely to enter deep into the lungs, where they can damage tissue and impair lung, heart, and brain function.

Volatile organic compounds (VOCs) are a range of chemical compounds that originate from both natural sources (e.g., plants) and human activities. VOCs are labeled as “volatile” because they are compounds that can easily become vapors or gases. Major anthropogenic, or human sources, of VOCs include gases that escape from dry-cleaning solvents, gasoline fumes, paint fumes, and plastics manufacturing. VOCs are a major contributor to the formation of ozone pollution, described in more detail later.

Lead (Pb) is a metal, particulate air pollutant that can enter the atmosphere from combustion of leaded fuels as well as from waste incinerators, lead smelters, coal burning, mining, and battery manufacturing facilities. Lead is toxic to the human nervous system and can impede brain functioning and development. Prior to 1986 the main source of atmospheric lead pol- lution in the United States was leaded gasoline combustion. The phaseout and banning of leaded gasoline in the United States during the 1980s is a regulatory success story, and public health research has confirmed that in the years following the ban, average IQs of children actually increased. Globally, most countries now ban leaded gasoline, although a handful still allow the use of this product.

Ozone We briefly touched on some secondary pol- lutants such as nitric acid and sulfuric acid, but one secondary pollutant that needs greater attention is ozone. Ozone (O3) is formed in the troposphere as a result of reac- tions between VOCs and NOX. These chemi- cal reactions need energy from sunlight to occur, so ozone pollution is also referred to as a photochemical oxidant. Ozone mixes with other pollutants in the troposphere to form photochemical smog.

John Partipilo/The Tennessean/Associated Press Some cities issue warnings about air quality when the smog is bad enough to affect public health.

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Section 8.2 Air Pollution

Recall that ozone in the stratosphere is “good ozone” and necessary for the survival of life on the planet as we know it. However, ozone near the Earth’s surface, in the troposphere where we live and breathe, is a different story. One easy way to remember this distinction is to use the phrase “ozone, good up high, but bad nearby.” Down here ozone is “bad.” It can cause breathing problems, throat and eye irritation, and coughing and can aggravate heart and lung conditions. Ozone is also damaging to plant tissues as well as human-made materials like fabric, paints, and rubber. It’s for these reasons that “ozone alerts” are issued in many met- ropolitan areas during periods when oz one pollution is bad enough to significantly impact public health. In particular, ozone alert days are especially dangerous to vulnerable popula- tion groups like small children, the elderly, and people suffering from respiratory and heart conditions.

Impacts of Air Pollution It’s evident from the review of primary and secondary air pollutants that there are a wide range of known health impacts associated with exposure to air pollution. These include heart and lung damage, respiratory conditions like asthma, brain impairment, and cancer. Some of these conditions are chronic or long term, while others—like an asthma attack or a heart attack—are acute and can occur suddenly.

Because changes to human health are caused by so many interacting factors (environment, diet, genetic traits), it’s sometimes difficult to pinpoint the specific impact of any one factor like air pollution. However, by analyzing large population data sets and controlling for other factors, scientists and public health experts are developing a much better understanding of just how significant a role air pollution plays in human health. For example, a 2019 report in the journal Lancet Planetary Health estimates that 4 million children worldwide develop asthma every year as a result of air pollution (Achakulwisut, Brauer, Hystad, & Anenberg, 2019). While conditions are worse in countries like India and China, air pollution is still a major contributor to new childhood asthma cases in many parts of the United States as well. A 2019 report by the Health Effects Institute estimated that air pollution will shorten the average expected life span of a child born today by almost 2 years. Likewise, research by the University of Chicago’s Energy Policy Institute indicates that the average person alive today would live 2.6 years longer if that person were not exposed to the worst levels of primary and secondary air pollutants described previously (University of Chicago, n.d.). Finally, WHO (2014) estimates that as many as 7 million premature deaths occur worldwide each year as a combined result of outdoor and indoor air pollution.

Air pollution is bad not only for humans but also for other animals and organisms. Plants are particularly sensitive to certain forms of air pollution. It’s estimated that tropospheric ozone pollution leads to crop losses of 5% to 15% for major crops like corn, soybeans, and wheat (United Nations Economic Commission for Europe, n.d.). Ozone pollution and acid deposition can also impact forest health and productivity. Trees that are weakened by exposure to air pollution are also less able to withstand pest and disease outbreaks. Finally, air pollution can even impact life underwater in aquatic ecosystems. For example, acid deposition can change the chemistry and pH of lakes and streams. Because many fish and other aquatic organisms are adapted to living in a narrow range of chemical conditions, these changes can stress and even kill them.

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Section 8.2 Air Pollution

Lastly, air pollution can damage exterior surfaces, statues, and buildings. Some of the most obvious effects are aesthetic, since dust, soot, and other particles darken and discolor exte- rior surfaces. Acid deposition can eat away at limestone and marble statues and building facades and cause them to crumble and erode. Ozone pollution can weaken and deteriorate rubber products like tires and can degrade fabrics and other materials. Acid deposition and other air pollutants can even corrode metal and weaken highways and bridges. A 2014 study in the American Journal of Engineering Research estimated that structural corrosion linked to air pollution was costing the Indian economy $45 billion each year (Rao, Rajasekhar, & Rao, 2014).

Addressing Air Pollution Over the long term, one of the most effective ways to address many air pollution problems is to drastically reduce combustion of fossil fuels and transition to a greater reliance on renew- able energy sources. As we saw in Chapter 7, however, that transition is still underway and could take decades to complete. As a result, we still need to take steps to limit air pollution for the immediate future.

One way to reduce air pollution and limit the negative impacts of this environmental chal- lenge is through regulation. The first real federal regulation of air pollution in the United States came in 1955 with the Air Pollution Control Act. This act was spurred in part by “killer smog” events in Donora, Pennsylvania, in 1948 and London, England, in 1952. Both of these events occurred when weather conditions worked to trap heavy air pollution from industries, coal-burning stoves, cars, and trucks in the lower atmosphere. By the time the air cleared, it had killed dozens of people in the small town of Donora and thousands in London, while sickening many more. However, the 1955 Air Pollution Control Act had a very limited impact,

and growing public disgust with worsening air pollution prompted the passage of the Clean Air Act (CAA) in 1970. The CAA sets air-quality standards for six specific pollut- ants (carbon monoxide, lead, nitrogen diox- ide, ozone, particulate matter, and sulfur dioxide), known as criteria air pollutants, and then imposes fines and penalizes viola- tors of those standards. Additional amend- ments in 1977 and 1990 further strength- ened the CAA.

Another regulatory approach that has proved effective in limiting sulfur dioxide pollution is known as cap and trade. Under cap and trade, the EPA sets maximum allow- able levels (“caps”) for SO2 emissions and

then issues (or sells) permits to large-scale stationary facilities like coal-fired power plants that emit large amounts of SO2. These facilities then have to find a way to lower their SO2 emissions to match the number of permits they have or purchase (“trade”) additional permits from facilities that have extra permits as a result of reducing their own SO2 emissions beyond

Ina Fassbender/picture-alliance/dpa/AP Images The Clean Air Act prompted the development of the catalytic converter, which reduces pollutant emissions from cars and trucks.

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Section 8.3 Stratospheric Ozone Depletion

what they were required to do. The SO2 cap-and-trade program has proved very effective at reducing sulfur pollution and provides an economic incentive for companies to move “beyond compliance” so that they can be in a position to profit from the sale of extra permits.

As with most forms of regulation, many industries furiously resisted the enactment of the CAA and subsequent amendments. These industries lobbied Congress and launched pub- lic relations campaigns arguing that the CAA would harm the economy without necessar- ily delivering any measurable public health or other benefits. However, one research study after another has shown this position to be wrong. Research conducted by Harvard Univer- sity scientists in 1993 and 2006 attempted to quantify the public health benefits achieved through CAA regulations (Dockery et al., 1993; Laden, Schwartz, Speizer, & Dockery, 2006). This research showed a clear and unmistakable connection between cleaner air and lower rates of premature death, although it estimated that there are still as many as 75,000 prema- ture deaths each year in the United States as a result of air pollution. A 2011 study by the EPA estimated that from 1990 to 2020 the CAA would impose direct costs on the economy of $65 billion. However, over that same period the CAA regulations would yield $2 trillion in benefits in the form of reduced illnesses, death, and lost work time. The 30-to-1 benefit–cost ratio in this analysis demonstrates that while environmental regulations might impose some neces- sary costs on specific industries and economic sectors, they can yield significantly greater benefits to the broader society as a whole.

Regulations like the CAA can also spur innovation and the development of new technolo- gies to help control pollution. For example, most particulate matter from stationary sources like power plants can be controlled by using a device known as an electrostatic precipitator. Likewise, wet scrubbers can be used to remove PM and SO2 pollution from stationary sources. In terms of mobile sources like cars and trucks, the CAA prompted the development of a key piece of technology known as the catalytic converter. Catalytic converters reduce pollutant emissions from cars and trucks by 90%. As a result, and even though vehicle miles driven have more than tripled since 1970, air pollution from mobile sources is actually significantly lower today than it was decades ago.

Overall, while dramatic progress has been made in reducing the impacts of air pollution in countries like the United States over the past 50 years, this is not the case everywhere. Rapidly growing emerging economies like China and India are grappling with far more serious air pol- lution levels. And even in the United States and other developed countries, air pollution still takes a toll on public health, the environment, and infrastructure. As we’ll discuss, addressing the air pollution challenge by reducing levels of fossil fuel use will also help address the chal- lenge of global climate change—another of those win–win scenarios.

8.3 Stratospheric Ozone Depletion

The gradual formation of the ozone layer over 1 billion years ago represents one of the single most important preconditions for the evolution of life on land. Prior to that, Earth’s surface was constantly being bombarded with UVB and UVC radiation. This UV radiation is carcino- genic and mutagenic, so the surface of the planet was mostly devoid of life.

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Section 8.3 Stratospheric Ozone Depletion

The only protection from this radioactive bombardment was underwater, and it’s there that single-celled marine bacteria and algae first emerged. Recall that marine bacteria and algae are capable of photosynthesis and that one by-product of photosynthesis is oxygen. As the numbers of these organisms increased, there was a gradual buildup of oxygen in the atmo- sphere. When an oxygen (O2) molecule is struck by UVB or UVC radiation, it splits into two free oxygen atoms (O + O). These free oxygen atoms are then available to combine with an oxygen molecule to form ozone (O + O2 = O3). Over hundreds of millions of years, as oxygen levels increased in the atmosphere, so did levels of ozone, resulting in the formation of the ozone layer.

Ozone in the stratosphere shields us from harmful UV radiation by undergoing a constant process of destruction and re-creation. Incoming UV radiation strikes an ozone molecule and breaks it apart into O and O2, but in the process the energy in that UV radiation is used up and dissipated as heat. After that, the free O can recombine with O2 to once again form O3. It’s believed that this constant cycle of ozone creation and destruction has been going on in the stratosphere for over 1 billion years. Much like aquifer recharge (Chapter 5), this ozone creation–destruction process can be likened to a bathtub filled with water, with the faucet on but the drain also open. As long as the water is flowing into the bathtub (ozone creation) at the same rate it is being drained out (ozone destruction), the level of water in the bathtub (or ozone in the stratosphere) will remain in a dynamic equilibrium.

Causes of Ozone Depletion It’s only in the past 100 years that human activities have thrown off that dynamic equilibrium. Stratospheric ozone depletion is often confused with climate change, with some thinking that the resulting “hole” in the ozone layer was responsible for letting in more heat. In fact, stratospheric ozone depletion is fundamentally a separate issue from climate change: Recall that less ozone means more UV radiation, not more heat. The two problems are related only because they are both the result of human-generated chemicals in the atmosphere.

In the case of stratospheric ozone depletion, the issue stems from the development, mass production, and pervasive use of a class of chemicals known as ozone-depleting substances. Chief among these substances is a product known as chlorofluorocarbons (CFCs). CFCs were first invented in the 1920s as a refrigerant and a propellant for spray cans, but they did not come into widespread production until the 1950s. CFCs offered a number of advantages over other chemicals being used for these applications at the time—namely that they were nontoxic, inexpensive to produce, light, and extremely stable and chemically nonreactive. The latter two characteristics are what led to CFCs becoming a culprit in stratospheric ozone depletion. Because CFCs are so light, they tend to float upward once released to the atmo- sphere. And because CFCs are stable and nonreactive, they do not chemically react or wash out of the atmosphere once put there. As CFCs came into greater and greater use, more of these chemicals were being released into the atmosphere, where they slowly floated toward the stratosphere.

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Section 8.3 Stratospheric Ozone Depletion

In some ways the fact that CFCs seemed to float away made them an ideal chemical com- pound. However, recall that in nature “there is no away” and that “everything must go some- where” (Commoner, 1971, p. 39). It was this basic understanding of nature that led scientists to begin to ask questions about the fate of increasing levels of CFCs being released to the atmosphere. In 1973 atmospheric scientists Mario Molina and F. Sherry Rowland began to speculate that CFC molecules could be reaching the stratosphere, where extremely intense UV radiation would be strong enough to break them apart. A single chlorine atom broken free from a CFC molecule would then be available to react with and “destroy” an ozone molecule in a cycle that could repeat itself thousands of

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