The greenhouse effect is a natural phenomenon that traps radiation within the earth's atmosphere. Natural greenhouse gases include water vapor, carbon dioxide, nitrous oxide, methane, and ozone, all essential to support life. The enhanced greenhouse effect, the direct result of human activities, increases concentrations of these gases in the atmosphere, and leads to pollution of the lower atmosphere and contributes to global warming. These gases let in sunlight but tend to insulate Earth against the loss of heat, as do the glass walls of a greenhouse. A higher concentration of the greenhouse gases means a warmer climate. For example, the twentieth century was been 1° warmer on worldwide average than the nineteenth century—warming at a rate 20 times faster than average.

Carbon dioxide (CO₂) is considered the predominant greenhouse gas and has the greatest impact on global heat. From April 1958, when monthly measurements of CO₂ from atop...