Greenhouse Gases Carbon Dioxide

The Earth naturally absorbs and reflects incoming solar radiation and emits longer wavelength terrestrial (thermal) radiation back into space [50]. On average, the absorbed solar radiation is balanced by the outgoing terrestrial radiation emitted to space (see Figure 3-2). A portion of this terrestrial radiation, though, is absorbed by gases in the atmosphere. These gases, known as greenhouse gases, have molecules that have the right size and shape to absorb and retain heat. These gases include water vapor (H2O), carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and, to a lesser extent, halocarbons consisting of hydrochlorofluorocarbons, perfluorocarbons, and sulfur hex-afluoride (SF6). The energy from this absorbed terrestrial radiation warms the Earth's surface and atmosphere, creating what is known as the natural greenhouse effect, which makes the Earth inhabitable. Without the natural heat-trapping properties of these atmospheric gases, the average surface temperature of the Earth would be about 60°F lower.

Although the Earth's atmosphere consists of mainly oxygen and nitrogen (i.e., over 99%), neither plays a significant role in enhancing the greenhouse effect because both are essentially transparent to terrestrial radiation [50]. The greenhouse gases comprise the remaining ~1% of the atmosphere, of which over 97% is water vapor. Methane, carbon dioxide,

Solar radiation

Planet's atmosphere contains greenhouse gases Solar radiation passes through gases Solar radiation warms the surface of the earth and heat rises from the surface Some heat is able to pass through the gases Some heat is absorbed and remains in the atmosphere, adding to the overall temperature

FIGURE 3-2. The greenhouse effect. (Adapted from DOE [51] and Peabody Holding Co. [52].)

nitrous oxide, and other gases comprise less than 3% of the greenhouse gases [52]. The composition of the greenhouse gases is [51]:

Carbon dioxide, methane, and nitrous oxide are continuously emitted to and removed from the atmosphere by natural processes on Earth. Anthropogenic activities, however, can cause additional quantities of these and other greenhouse gases to be emitted or sequestered, thereby changing their global average atmospheric concentrations. Natural activities such as respiration by plants or animals and seasonal cycles of plant growth and decay generally do not alter average atmospheric greenhouse gas concentrations over decadal time frames [50]. Climatic changes, which are long-term fluctuations in temperature, precipitation, wind, and other elements of the Earth's climate system, that result from anthropogenic activities can have positive or negative feedback effects on these natural systems.

Overall, the most abundant and dominant greenhouse gas in the atmosphere is water vapor. Human activities, however, are not believed to directly affect the average global concentration of water vapor, although this issue is currently being debated [50,53]. In nature, carbon dioxide is cycled between various atmospheric, oceanic, land biotic, marine biotic, and mineral reservoirs. Of all the greenhouse gases, human activity has the largest influence on carbon dioxide, which is a product of the combustion of fossil fuels. Carbon dioxide concentrations in the atmosphere increased from approximately 280 ppmv in pre-industrial times to 367 ppmv in 1991 [50].

Methane is primarily produced through anaerobic decomposition of organic matter in biological systems. Agricultural processes, such as wetland rice cultivation, enteric fermentation in animals, and the decomposition of animal wastes, emit methane, as does the decomposition of municipal solid wastes [50]. Methane is also emitted during the production and distribution of natural gas and petroleum. Methane is released as a by-product of coal mining and, to a lesser extent, incomplete fossil fuel combustion.

Anthropogenic sources of nitrous oxide include agricultural soils, due, particularly, to the use of fertilizers; fossil fuel combustion, especially from mobile sources; nylon and nitric acid production; wastewater treatment; waste combustion; and biomass burning [50]. Halocarbons that contain chlorine (e.g., chlorofluorocarbons, hydrofluorocarbons, methyl chloroform, and carbon tetrachloride) and bromine (e.g., halons, methyl bromide, and hydrobromofluorocarbons), perfluorocarbons, and sulfur hexafluoride (SF6) are manmade chemicals and not products of combustion. They are, however, powerful greenhouse gases.

The concept of global warming potentials (GWPs) has been developed to evaluate the relative effects of emissions over a given time period in the future [54]. GWPs take into account the differing times that gases remain in the atmosphere, their greenhouse effect while in the atmosphere, and the time period over which climatic changes are of concern. GWPs are intended as a quantified measure of the globally averaged relative radiative forcing impacts of a particular greenhouse gas [50]. It is defined as the cumulative radiative forcing—both direct and indirect effects—integrated over a period of time from the emission of a unit mass of gas relative to some reference gas. Table 3-3 summarizes the greenhouse gases, their major anthropogenic sources, and their GWPs [50]. Carbon dioxide has been chosen as the reference gas; as an example, methane's GWP is 21, which means that methane is 21 times better at trapping heat in the atmosphere than carbon dioxide. GWPs are typically reported on a 100-year time horizon. The EPA uses a time period of 100 years for policy making and reporting purposes.

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