Greenhouse Effect

Photo by Gary Tamin
Photo by Gary Tamin

The Greenhouse Effect, or atmosphere effect, the trapping by the earth's atmosphere of heat energy radiated from the sun. The effect is named after the solar wanning observed in greenhouses, although the latter effect is in fact more directly the result of trapping warmed air below a ceiling of glass or plastic.

All substances absorb energy in the form of radiation, convert it to heat, and then reradiate the energy. The amount absorbed and reradiated varies with the nature of the substance and the wavelength of the radiation. As for the atmosphere, it is nearly transparent for the short, visible wavelengths of the electromagnetic spectrum; the carbon dioxide and water vapor in the air, however, absorb infrared radiation.

An average of about half of the solar radiation intercepted by the earth passes through the atmosphere and is absorbed by the land and water surfaces. These surfaces warm up and emit infrared radiation. The atmosphere absorbs much of this radiation and in turn warms up and re-emits infrared radiation. The part of the atmosphere's reemitted radiation that is directed downward toward the earth, called "counterradiation", is responsible for the greenhouse effect.

There is a continuous exchange of infrared radiation between the earth and the atmosphere. This exchange keeps the air temperature higher than it would be otherwise; one estimate is that the difference at the earth's surface amounts to 104°F (40°C). The burning of coal, oil, and gas (mainly in cities) increases the carbon dioxide content of the air and further raises local air temperatures, but over long periods of time the total heat energy reradiated into space averages out to equal that received from the sun. Climatologists call this the "heat balance".

On a worldwide scale the carbon dioxide content of the air has been increased by about 25% in the past 100 years, and there is some controversy over whether this is causing a general increase in atmospheric temperatures.

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