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reducing deforestation), removal of radiatively active gases from the atmosphere, and altering the earth's albedo in ways that affect the earth's radiative balance.

Sources of Greenhouse Gas Emissions

This section provides a very brief summary of the magnitudes and sources of greenhouse gas emissions in order to suggest targets for mitigation strategies and some indication of the magnitude of the effort required. It is not intended to be a critical review, but relies on the recent summary compiled by the Intergovernmental Panel on Climate Change (1990, 1991). More information is available in the report of the Effects Panel (Part Two).

The greenhouse gases include carbon dioxide (CO2), chlorofluorocarbons (CFCs), methane (CH4), nitrous oxide (N2O), ozone (O3), and water vapor. Although water vapor continually cycles through the atmosphere, if there is a change in atmospheric temperature, the mean water vapor concentration could change and provide an important positive feedback (i.e., magnify the temperature change). Other gases such as carbon monoxide (CO) and nitrogen oxides (NOx) are involved in chemical reactions in the atmosphere and affect the concentrations of greenhouse gases (in this case, O3). Greenhouse gas emissions come from both anthropogenic (man-made) and natural sources (such as CH4 from wetlands). Table 19.1 lists the primary greenhouse gases, the anthropogenic sources, and the relative contribution of each gas toward greenhouse warming. As shown in this table, CO2 is the single most important greenhouse gas worldwide, but others also make a significant contribution.

Table 19.2 shows the current rates at which greenhouse gases are increasing worldwide. Figure 19.1 breaks down the current worldwide contributions to radiative forcing by source sector emissions during the 1980s. As shown here, energy use that generates emissions of CO2 and other greenhouse gases is the major greenhouse emission source. Table 19.3 shows a recent projection of global emissions from different sources for the years 2000, 2015, and 2050. As CFCs are phased out (presumably), under present international agreements, emissions from energy use are likely to dominate the anthropogenic influence on greenhouse warming.

Even though CO2 contributes about half of the radiative forcing from increased atmospheric concentrations of greenhouse gases, Table 19.4 shows that once in the atmosphere, each molecule of the other greenhouse gases contributes more to global warming than does each molecule of CO2. For example, CFC-11 has, per molecule, 12,400 times the capacity of CO2 to trap heat.

Worldwide, the United States is at present the largest emitter of greenhouse gases (World Resources Institute, 1990). As shown in Figure 19.2, the use of energy in the form of coal, oil, and natural gas is the largest



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