The Greenhouse Effect and Climate Change - Greenhouse Gases

The Energy Information Administration of the U.S. Department of Energy (DOE) reported that CO₂ accounted for 83 percent of greenhouse gas emissions in the United States in 2001. (See Figure 2.6.) Methane (CH₄) was second with 9 percent of the total, followed by nitrous oxide (N₂O) with 6 percent and other greenhouse gases, such as hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6), with 2 percent.

Carbon Dioxide

Carbon dioxide (CO₂) is a gas that is a naturally occurring component of Earth's atmosphere. It is also produced by a variety of human and animal sources. These factors are a part of the carbon cycle. (See Figure 2.7.) As shown in Figure 2.6, CO₂ was the most prevalent of the greenhouse gases emitted from 1990 to 2001.

The burning of fossil fuels by industry and motor vehicles is, by far, the leading source of CO₂ in the atmosphere, accounting for 97 percent of the nation's emission of greenhouse gases in 2001. (See Table 2.2.) As populations and economies expand, they use ever-greater amounts of fossil fuels.

The DOE released its International Energy Annual 2001 in March 2003. The report presents data collected on CO₂ emissions related to fossil fuel use around the world. The United States was responsible for the largest portion (23 percent) of such emissions in 2001, followed by Western Europe (16 percent) and China (13 percent). (See Figure 2.8.) The western European countries responsible for most of that region's CO₂ emissions were Germany, the United Kingdom, Italy, and France.

The DOE predicts that CO₂ emissions from developing countries could actually surpass those from industrialized countries before 2020. (See Figure 2.9.) This trend is attributed to increased use of coal in developing countries, particularly China and India, while developed countries rely increasingly on natural gas.

FIGURE 2.6
Greenhouse gas emissions by gas, 2001

THE ROLE OF THE FORESTS AS CARBON SINKS.

Forests act as sinks, or repositories, absorbing and storing carbon. Trees naturally absorb and neutralize CO₂, although scientists do not agree on the extent to which forests can soak up excess amounts. The increasing levels of CO₂ in the atmosphere might conceivably be tolerated in Earth's normal CO₂ cycle if not for the additional complicating factor of deforestation. The burning of the Amazon rain forests and other forests has had a twofold effect: the immediate release of large amounts of CO₂ into the atmosphere from the fires, and the loss of trees to neutralize the CO₂ in the atmosphere. (See Figure 2.10.)

THE ROLE OF THE OCEANS AS CARBON SINKS.

The oceans are, by far, the largest reservoir of carbon in the carbon cycle. The oceans hold approximately 50 times more carbon than the atmosphere and 20 times more than the terrestrial reservoir (land). Oceanographers and ecologists disagree over the carbon cycle–climate connection and over the ocean's capacity to absorb CO₂. Some scientists believe that the oceans can absorb one to two billion tons of CO₂ a year, about the amount the world emitted in 1950. Until scientists can more accurately determine how much CO₂ can be buffered by ocean processes, the extent and speed of disruption in the carbon supply remains unclear.

Methane

Methane (CH₄) is an important component of greenhouse emissions, second only to CO₂. (See Figure 2.6.) While there is less methane in the atmosphere, scientists FIGURE 2.7
The carbon cycle estimate that it may be 21 times more effective at trapping heat in the atmosphere than CO₂. Over the past two centuries, methane's concentration in the atmosphere has more than doubled and scientists generally attribute those increases to human sources, such as landfills, natural gas systems, agricultural activities, coal mining, and wastewater treatment. However, according to the Energy Information Administration, U.S. emissions of methane from energy and waste management sources actually declined between 1990 and 2002, while agricultural emissions increased over the same time period. (See Figure 2.11.) The increase is blamed on domestic animals and the decomposition of their waste, which releases methane. Growing markets for beef and milk products are driving a booming livestock business.

Nitrous Oxide

Nitrous oxide (N₂O) is formed by natural biological sources and by a number of human activities. Although N₂O makes up a much smaller portion of greenhouse gases than CO₂, it is much more (perhaps 310 times more) powerful than CO₂ at trapping heat. Figure 2.12 shows that agriculture is and has been the major source of N₂O emissions in the United States, followed by energy and industrial sources. Specific agricultural sources of N₂O emissions are shown in Figure 2.13.