last several decades, various human activities have released increasing amounts of such gases. At present, there is consensus among the scientific community that these greenhouse gas accumulations will eventually cause warming of both the Earth's surface and lower atmosphere. This phenomenon could eventually alter the planet's climate. One effect of such changes could be an increase in weather extremes: For example, there might be an increase in the frequency or severity of droughts or storms. More specifically, global warming may enhance the frequency and severity of hurricanes. These effects will ultimately impact both human societies and natural ecosystems.
The main anthropogenic greenhouse gases include CO2, CH4, N2O, as well as various members of the halocarbon and related families (5:1027). For roughly the past 10,000 years, the concentration of these various gases in the Earth's atmosphere has remained fairly constant. With increasing population over the last few centuries, however, agricultural and industrial activities have released greenhouse gases in considerable quantities. Since the mid-1700s, for example, the concentration of CO2 alone has increased by approximately 25 percent. Although this release has occurred primarily through the combustion of fossil fuels, land-use changes (e.g., deforestation) have also had a significant impact. Additionally, since about 1750, the concentration of atmospheric CH4 has more than doubled. Sources of atmospheric methane are thought to include rice-fields, animal and domestic waste, coal mining, and the venting of natural gas. Over the la
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