an enhanced greenhouse effect on Earth?
             The Earth is a livable planet because of the greenhouse effect, but the levels of greenhouse gases must fall within a narrow range. Global warming is defined as the human-induced increase in the average global temperature of the atmosphere near the Earth's surface (Botkin et al., 1995). Complex positive and negative feedback cycles affect the atmosphere, therefore the theory of global warming is controversial. The debate centers exclusively on the anthropogenic greenhouse gases, which stem from human activity. Anthropogenic emissions include the release of carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), chlorofluorocarbons (CFCs), and ozone (O3) into the atmosphere.
             Dry air in the Earth's atmosphere is made up of three main gases, nitrogen (79%), oxygen (20%), and argon (1%) (Gebelein, 1997). Water vapour is also usually present in concentrations of up to several percentages. It is the key gas in the natural greenhouse effect, accounting for about 80 percent of the contributing gases (Botkin et al., 1995). Other gases in small amounts make up the remaining 20 percent. Despite their low concentrations, measurable in parts per million (p.p.m.) or parts per billion (p.p.b.) by volume of air, these trace gases contribute significantly to the greenhouse effect. CO2 is the most prominent, but other significant gases include CH4, N2O, and O3 (Rodhe, 1990). Each of these gases is a basic part of natural biogeochemical cycles and is efficient in absorbing infrared radiation from the Earth. An additional important group of greenhouse gases is the commercially produced CFCs (Rodhe, 1990).
             The majority of natural greenhouse warming is due to water in the atmosphere. Water vapour is not yet a direct concern for the future because it is viewed as a natural factor, not an anthropogenic emission. Yet, scientists report that the amount of moisture in the air h...