Emissions of trace gases as a result of human activities could change the atmosphere's radiative properties enough to alter the earth's climate. Greenhouse gases, including water vapor, carbon dioxide, methane, nitrous oxide, chlorofluorocarbons, and ozone, insulate the earth, letting sunlight through to the earth 's surface while trapping outgoing radiation. Atmospheric concentrations of all of these gases are rising due to human industrial and agricultural processes. Atmospheric models indicate that, at the rate these gases are accumulating, the global mean temperature will increase by between 0.2°C and 0.5°C per decade over the next century (Houghton et al., 1990). This increase could have widespread effects on global sea level, seawater temperatures, rainfall distribution, seasonal weather patterns, plant and animal populations, agricultural production, and human settlement and economic systems.

Carbon dioxide is believed to be responsible for about half of the total global warming potential. If current trends continue, carbon dioxide is expected to account for 55 percent of global warming over the next century, or four times more than methane, the second most important heat-trapping gas (Houghton et al., 1990). According to recent estimates, 75 percent of total carbon dioxide emissions from human activities occur as a result of the combustion of fossil fuels, mostly in nontropical countries (Intergovernmental Panel on Climate Change, 1990a). Land use changes are responsible for most of the remainder.

The most significant of these land use changes are occurring in the humid tropics (Dale et al., Appendix, this volume). As forest conversion occurs, carbon stored in vegetation and soils is released as carbon dioxide through the burning and decomposition of biomass and the oxidation of soil organic matter. Agricultural activities that follow forest conversion—including paddy rice culture, cattle raising, and the use of nitrogen fertilizers—are sources of methane and nitrous oxide.