The greenhouse effect warms the lower portion of a planet's atmosphere when heat is trapped there by gases--such as water vapor, carbon dioxide (CO2), methane, and nitrous oxides--that prevent it from escaping into space. As a result of their molecular structure, these gases are dominant absorbers and emitters of infrared radiation. They absorb infrared energy from a planet's surface and reemit it in all directions. A significant fraction of this reradiated energy is directed back to the planet's surface, resulting in an increase in average temperatures. This effect is somewhat analogous to the trapping of heat by a greenhouse, but the retention of heat in a greenhouse is due mostly to reduced cooling caused by the prevention of convection: Only a small amount is due to trapped infrared radiation.

The greenhouse effect is a natural phenomenon that has been occurring on Earth and other planets for millions of years. It allows Earth to support life. If heat were not trapped in Earth's atmosphere, the planet would be approximately 33œ Celsius cooler than it is now. A large percentage of Earth's natural greenhouse warming is caused by water vapor. If the greenhouse effect were enhanced, the Earth would become warmer, which could cause problems for humans, plants, and animals.

In the mid-1950's, an enhanced greenhouse effect was recognized as a concern. As a result of anthropogenic (human-induced) activities, atmospheric concentrations of the greenhouse gases (GHGs) were on the rise. This trend was associated with an increasing global atmospheric temperature. Industrialization resulted in an increase in the use of fossil fuels, which increased GHG emissions. The global mean annual temperature rose by approximately 0.5œ Celsius between 1890 and 2000.

Most of that increase occurred after 1970. In addition to rising global temperature, observations of glaciers indicate that more of them are retreating than are growing. For example, eight glaciers that were advancing on Mount Baker in the northern Cascades in 1976 were all melting back at their termini by 1990. Observed climate changes associated with rising global temperatures, retreating glaciers, and increased polar ice melting are consistent with the effects that might be produced by an enhanced greenhouse effect accelerated by burning more fossil fuels. Verification of this connection depends primarily on computer-generated climate models. Existing models must be improved by addressing the coupled, nonlinear, dynamic nature of climate and by expanding the climatological database to include additional factors that influence the greenhouse effect, such as volcanic production of GHGs and the emission of methane from landfills and melting tundra.

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