665

In this module, we will consider the distinctions among global change, global climate change, and global warming. We will then explore the processes that underlie changes in global climates and, more specifically, global temperatures.

Learning Objectives

After reading this module you should be able to

Throughout this book, we have highlighted a wide variety of ways in which the world has changed as a result of a rapidly growing human population. Human activity has placed increasing demands on natural resources such as water, trees, minerals, and fossil fuels. We have also emitted growing amounts of carbon dioxide, nitrogen compounds, and sulfur compounds into the atmosphere. Our agricultural methods depend on chemicals, including fertilizers and pesticides. Finally, a growing population faces challenges of waste disposal, sanitation, and the spread of human diseases.

Change that occurs in the chemical, biological, and physical properties of the planet is referred to as global change. As you can see in FIGURE 62.1, some types of global change are natural and have been occurring for millions of years. Global temperatures, for example, have fluctuated over millions of years. During periods of cold temperatures, Earth has experienced ice ages. In modern times, however, the rates of change have often been much higher than those that occurred historically. Many of these changes are the result of human activities, and they can have significant, sometimes cascading, effects. For example, as we saw in Chapter 17, emissions from coal-fired power plants and waste incinerators have increased the amount of mercury in the air and water, with concentrations roughly triple those of preindustrial levels. This mercury bioaccumulates in fish caught thousands of kilometers away from the sources of pollution. Because mercury has harmful effects on the nervous system of children, women who might become pregnant and children are advised to avoid eating top predator fish such as swordfish and tuna. Far-reaching effects on this scale were unimaginable just 50 years ago.

One type of global change of particular concern to scientists is global climate change, which refers to changes in the average weather that occurs in an area over a period of years or decades. Changes in climate can be categorized as either natural or anthropogenic. For example, you might recall from Chapter 4 that El Niño events, which occur every 3 to 7 years, alter global patterns of temperature and precipitation (see FIGURE 11.3 on page 120). Anthropogenic activities such as fossil fuel combustion and deforestation also have major effects on global climates. Global warming refers to a specific aspect of climate change: the warming of the oceans, land masses, and atmosphere of Earth.

666

The physical and biogeochemical systems that regulate temperature at the surface of Earth—the concentrations of gases, distribution of clouds, atmospheric currents, and ocean currents—are essential to life on our planet. It is therefore critical that we understand how the planet is warmed by the Sun and how the greenhouse effect contributes to the warming of Earth.

The Sun-Earth Heating System

The ultimate source of almost all energy on Earth is the Sun. In the most basic sense, the Sun emits solar radiation that strikes Earth. As the planet warms, it emits radiation back toward the atmosphere. However, the types of energy radiated from the Sun and Earth are different (see FIGURE 5.1 on page 45). Because the Sun is very hot, most of its radiated energy is in the form of high-energy visible radiation and ultraviolet radiation—also known as visible light and ultraviolet light. When this radiation strikes Earth, the planet warms and radiates energy. Earth is not nearly as hot as the Sun, so it emits most of its energy as infrared radiation—also known as infrared light. We cannot see infrared radiation, but we can feel it being emitted from warm surfaces like the heat that radiates from an asphalt road on a hot day.

Differences in the types of radiation emitted by the Sun and Earth, in combination with processes that occur in the atmosphere, cause the planet to warm. Using FIGURE 62.2, we can walk through each step of this process. As radiation from the Sun travels toward Earth, about one-third of the radiation is reflected back into space. Although some ultraviolet radiation is absorbed by the ozone layer in the stratosphere, the remaining ultraviolet radiation, as well as visible light, easily passes through the atmosphere. Once it has passed through the atmosphere, this solar radiation strikes clouds and the surface of Earth. Some of this radiation is reflected from the surface of the planet back into space. The remaining radiation is absorbed by clouds and the surface of Earth, which become warmer and begin to emit lower-energy infrared radiation back toward the atmosphere. Unlike ultraviolet and visible radiation, infrared radiation does not easily pass through the atmosphere. It is absorbed by gases, which causes theses gases to become warm. The warmed gases emit infrared radiation out into space and back toward the surface of Earth. The infrared radiation that is emitted toward Earth causes Earth’s surface to become even warmer. This absorption of infrared radiation by atmospheric gases and reradiation of the energy back toward Earth is the greenhouse effect.

667

The greenhouse effect gets its name from the idea that solar radiation causes a gardener’s greenhouse to become very warm. However, the process by which actual greenhouses are warmed by the Sun involves glass windows holding in heat whereas the process by which Earth is warmed involves greenhouse gases radiating infrared energy back toward the surface of the planet.

In the Sun-Earth heating system, the net flux of energy is zero; the inputs of energy to Earth equal the outputs from Earth. Over the long term—thousands or millions of years—the system has been in a steady state. However, in the shorter term—over years or decades—inputs can be slightly higher or lower than outputs. Factors that influence short-term fluctuations include changes in incoming solar radiation from increased solar activity and changes in outgoing radiation from an increase in atmospheric gases that absorb infrared radiation. If incoming solar energy is greater than the sum of reflected solar energy and radiated infrared energy from Earth, then the energy accumulates faster than it is dispersed and the planet becomes warmer. If incoming solar energy is less than the sum of the two outputs, the planet becomes cooler. Such natural changes in inputs and outputs cause natural changes in the temperature of Earth over time.

The Gases That Cause the Greenhouse Effect

668

Throughout the book we have seen that certain gases in the atmosphere can absorb infrared radiation emitted by the surface of the planet and radiate much of it back toward the surface. As we have seen, gases in the atmosphere that absorb infrared radiation are known as greenhouse gases.

The two most common gases in the atmosphere, N₂ and O₂, compose 99 percent of the atmosphere. Because these two gases do not absorb infrared radiation, they are not greenhouse gases and do not contribute to the warming of Earth. This means that greenhouse gases make up a very small fraction of the atmosphere. The most common greenhouse gas is water vapor (H₂O). Water vapor absorbs more infrared radiation from Earth than any other compound, although a molecule of water vapor does not persist nearly as long as other greenhouse gases. Other important greenhouse gases include carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), and ozone (O₃). All of these gases have been a part of the atmosphere for millions of years, and have kept Earth warm enough to be habitable. In the case of ozone, we have seen that its effects on Earth are diverse. Ozone in the stratosphere is beneficial because it filters out harmful ultraviolet radiation. In contrast, ozone in the lower troposphere acts as a greenhouse gas and can cause increased warming of Earth. It also is an air pollutant in the lower troposphere because it can cause damage to plants and human respiratory systems. There is one other type of greenhouse gas, chlorofluorocarbons (CFCs), which does not exist naturally. It occurs in the atmosphere exclusively due to production of CFCs by humans and, as we discussed in Chapter 15, these CFCs have contributed to a hole in the ozone layer over Antarctica.

Although we commonly think of the greenhouse effect as detrimental to our environment, without any greenhouse gases the average temperature on Earth would be approximately –18°C (0°F) instead of its current average temperature of 14°C (57°F). Concern about the danger of greenhouse gases is based on our understanding that an increase in the concentration of these gases—as has occurred due to human activities—can cause the planet to warm even more than usual.

The contribution of each gas to global warming depends in part on its greenhouse warming potential. The greenhouse warming potential of a gas estimates how much a molecule of any compound can contribute to global warming over a period of 100 years relative to a molecule of CO₂. In calculating this potential, scientists consider the amount of infrared energy that a given gas can absorb and how long a molecule of the gas can persist in the atmosphere. Because greenhouse gases can differ a great deal in these two factors, greenhouse warming potentials span a wide range of values. For example, water vapor has a lower potential compared with carbon dioxide. The remaining greenhouse gases have much higher values, either because they absorb more infrared radiation than a molecule of CO₂ or because they persist longer in the atmosphere than CO₂. TABLE 62.1 shows the global warming potential for five common greenhouse gases. Compared with CO₂, the greenhouse warming potential is 25 times higher for methane (CH₄), nearly 300 times higher for nitrous oxide (N₂O), and up to 13,000 times higher for CFCs.

The effect of each greenhouse gas depends on both its warming potential and its concentration in the atmosphere. Although carbon dioxide has a relatively low warming potential, it is much more abundant than most other greenhouse gases, except for water vapor, which can have a concentration similar to carbon dioxide. While human activity appears to have little effect on the amount of water vapor in the atmosphere, it has caused substantial increases in the amount of the other greenhouse gases. Among these, carbon dioxide remains the greatest contributor to the greenhouse effect because its concentration is so much higher than any of the others. As a result, scientists and policy makers focus their efforts on ways to reduce carbon dioxide in the atmosphere.

Given what we now know about how greenhouse gases work, the concentrations of each gas, and how much infrared energy each gas absorbs, we can understand how changes in the concentrations of greenhouse gases can contribute to global warming. Increasing the concentration of any historically present greenhouse gas should cause more infrared radiation to be absorbed in the atmosphere, which will then radiate more energy back toward the surface of the planet and cause the planet to warm. Likewise, producing new greenhouse gases that can make their way into the atmosphere, such as CFCs, should also cause increased absorption of infrared radiation in the atmosphere and further cause the planet to warm.

669

As we have seen, greenhouse gases include a variety of compounds such as water vapor, carbon dioxide, methane, nitrous oxide, and CFCs. These gases have natural and anthropogenic sources. After reviewing the different sources of greenhouse gases, we will discuss the relative ranks of the different anthropogenic sources.

Natural Sources of Greenhouse Gases

Natural sources of greenhouse gases include volcanic eruptions, decomposition, digestion, denitrification, evaporation, and evapotranspiration.

Volcanic Eruptions

Over the scale of geologic time, volcanic eruptions can add a significant amount of carbon dioxide to the atmosphere. Other gases and the large quantities of ash released during volcanic eruptions can also have important, short-term climatic effects. A volcanic eruption emits a large quantity of ash into the atmosphere. The ash reflects incoming solar radiation back out into space, which has a cooling effect on Earth. In 1991, for example, Mount Pinatubo in the Philippines erupted and spewed millions of tons of ash into the atmosphere, as far as 20 km (12 miles) high (FIGURE 62.3). The large amount of ash in the atmosphere reduced the amount of radiation striking Earth, which caused a 0.5°C (0.9°F) decline in the temperature on the planet’s surface. Because the ash and small particles eventually settle out of the atmosphere, such effects usually last only a few years.

Decomposition and Digestion

When decomposition occurs under high-oxygen conditions, the dead organic matter is ultimately converted into carbon dioxide. As we saw in our discussion of landfills in Chapter 16, methane is created when there is not enough oxygen available to produce carbon dioxide. This is a common occurrence at the bottom of wetlands where plants and animals decompose and oxygen is in low supply. Wetlands are the largest natural source of methane.

670

A similar situation occurs when certain animals digest plant matter. Animals that consume significant quantities of wood or grass, including termites and grazing antelopes, require gut bacteria to digest the plant material. Because the digestion occurs in the animal’s gut, the bacteria do not have access to oxygen and methane is produced as a by-product. A single termite colony can contain more than a million termites (FIGURE 62.4). Termites are abundant throughout the world—especially in the tropics—and represent the second largest natural source of methane.

Denitrification

As we learned in Chapter 3, nitrous oxide (N₂O) is a natural component of the nitrogen cycle that is produced through the process of denitrification. Denitrification occurs in the low-oxygen environments of wet soils and at the bottoms of wetlands, lakes, and oceans. (FIGURE 7.3 on page 85 shows the nitrogen cycle.) In these environments, nitrate is converted to nitrous oxide gas, which then enters the atmosphere as a powerful greenhouse gas.

Evaporation and Evapotranspiration

As we stated earlier, water vapor is the most abundant greenhouse gas in the atmosphere and the greatest natural contributor to global warming. In Chapter 3 we examined the role of water vapor in the hydrologic cycle. (FIGURE 7.1 on page 80 shows the hydrologic cycle.) Water vapor is produced when liquid water from land and water bodies evaporates and by the evapotranspiration process of plants. Because the amount of evaporation into water vapor varies with climate, the amount of water vapor in the atmosphere can vary regionally.

Anthropogenic Sources of Greenhouse Gases

As shown in FIGURE 62.5, there are many anthropogenic sources of greenhouse gases. The most significant of these are the burning of fossil fuels, agricultural practices, deforestation, landfills, and industrial production of new greenhouse chemicals.

Burning Fossil Fuels

Tens to hundreds of millions of years ago, organisms were sometimes buried without first decomposing into carbon dioxide. In FIGURE 7.2 on page 83 we outlined the process by which the carbon contained in these organisms, called fossil carbon, is slowly converted to fossil fuels deep underground. When humans burn these fossil fuels, we produce CO₂ that goes into the atmosphere. Because of the long time required to convert carbon into fossil fuels, the rate of putting carbon into the atmosphere by burning fossil fuels is much greater than the rate at which producers take CO₂ out of the air and both the producers and consumers contribute to the pool of buried fossil carbon.

Because fossil fuels differ in how they store energy, each type of fossil fuel produces different amounts of carbon dioxide. For a given amount of energy, burning coal produces the most CO₂. In comparison, burning oil produces 85 percent as much CO₂ as coal, and natural gas produces 56 percent as much. As we saw in Chapter 12, from the perspective of CO₂ emissions, natural gas is considered better for the environment than coal. The production of fossil fuels, such as the mining of coal, and the combustion of fossil fuels can also release methane and, in some cases, nitrous oxide.

Particulate matter may also play an important role in global warming. Although particulate matter, also known as black soot, may reflect solar radiation under some conditions, recent findings suggest that it may be responsible for up to one-quarter of observed global warming during the past century. Particulates that fall on ice and snow in the higher latitudes absorb more energy of the Sun by lowering the albedo. As the snow and ice begin to melt, the particulates become more concentrated on the surface. The increased concentration raises the amount of solar radiation absorbed, which increases melting. This positive feedback system might help explain warming that occurred early in the last century, when atmospheric concentrations of greenhouse gases had not yet increased much but soot from the burning of coal was widespread.

671

Agricultural Practices

Agricultural practices can produce a variety of greenhouse gases. Agricultural fields that are overirrigated, or those that are deliberately flooded for cultivating crops such as rice, create low oxygen environments similar to wetlands and therefore can produce methane and nitrous oxide. Synthetic fertilizers, manures, and crops that naturally fix atmospheric nitrogen—for example, alfalfa—can create an excess of nitrates in the soil that are converted to nitrous oxide by the process of denitrification.

Raising livestock can also produce large quantities of methane. Many livestock such as cattle and sheep consume large quantities of plant matter and rely on gut bacteria to digest this cellulose. As we saw in the case of termites, gut bacteria live in a low-oxygen environment and digestion in this environment produces methane as a by-product. Manure from livestock operations will decompose to CO₂ under high-oxygen conditions, but in low-oxygen conditions, for example in manure lagoons that are not aerated, it will decompose to methane.

Deforestation

Each day, living trees remove CO₂ from the atmosphere during photosynthesis, and decomposing trees add CO₂ to the atmosphere. This part of the carbon cycle does not change the net atmospheric carbon because the inputs and outputs are approximately equal. However, when forests are destroyed by burning or decomposition and not replaced, as can happen during deforestation, the destruction of vegetation will contribute to a net increase in atmospheric CO₂. This is because the mass of carbon that made up the trees is added to the atmosphere by combustion or decomposition. The shifting agriculture described in Chapter 11, which involves clearing forests and burning the vegetation to make room for crops, is a major source of both particulates and a number of greenhouse gases, including carbon dioxide, methane, and nitrous oxide.

Landfills

As we saw in Chapter 16, landfills receive a great deal of household waste that slowly decomposes under layers of soil. When the landfills are not aerated properly, they create a low-oxygen environment, like wetlands, in which decomposition causes the production of methane as a by-product.

Industrial Production of New Greenhouse Chemicals

The creation of new industrial chemicals often has unintended effects on the atmosphere. In Chapter 15 we looked at CFCs, the family of chemicals that serves as refrigerants used in air conditioners, freezers, and refrigerators. CFCs were used in the past until scientists discovered that they were damaging the protective ozone layer. As we discussed in Chapter 15, the nations of the world joined together to sign the Montreal Protocol on Substances That Deplete the Ozone Layer, which phased out the production and use of CFCs by 1996. Unfortunately, many of the alternative refrigerants that are less harmful to the ozone layer, including a group of gases known as hydrochlorofluorocarbons (HCFCs), still have very high greenhouse warming potentials. As a result, developed countries will phase out the use of HCFCs by 2030.

672

Ranking the Anthropogenic Sources of Greenhouse Gases

We have seen that there are multiple anthropogenic sources of greenhouse gases. What is the relative contribution of each source? FIGURE 62.6 shows the major anthropogenic sources of greenhouse gases in the United States. FIGURE 62.6a shows that the three major contributors of methane in the atmosphere are the digestive processes of livestock, landfills, and the production of natural gas and petroleum products. The major contributor of nitrous oxide, shown in FIGURE 62.6b, is agricultural soil because they receive nitrogen from synthetic fertilizers, combustion, and industrial production of fertilizers and other products. Finally, looking at the numbers for carbon dioxide in FIGURE 62.6c, we see that approximately 94 percent of all CO₂ emissions come from industrial processes and the burning of fossil fuels.