greenhouse effect

1. an atmospheric heating phenomenon, caused by short-wave solar radiation being readily transmitted inward through the earth's atmosphere but longer-wavelength heat radiation less readily transmitted outward, owing to its absorption by atmospheric carbon dioxide, water vapor, methane, and other gases; thus, the rising level of carbon dioxide is viewed with concern.

2. such a phenomenon on another planet.

[1935-40]

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Warming of the Earth's surface and lower atmosphere that tends to intensify with an increase in atmospheric carbon dioxide and certain other gases.

Visible light from the Sun heats the Earth's surface. Part of this energy is reradiated in the form of long-wave infrared radiation, much of which is absorbed by molecules of carbon dioxide and water vapour in the atmosphere and reradiated back toward the surface as more heat. This process is analogous to the glass panes of a greenhouse that transmit sunlight but hold in heat. The trapping of infrared radiation causes the Earth's surface and lower atmosphere to warm more than they otherwise would, making the surface habitable. The increase in atmospheric carbon dioxide caused by widespread combustion of fossil fuels may intensify the greenhouse effect and cause long-term climatic changes. An increase in atmospheric concentrations of other trace gases such as chlorofluorocarbons, nitrous oxide, and methane may also aggravate greenhouse conditions. It is estimated that since the beginning of the Industrial Revolution the amount of atmospheric carbon dioxide has increased 30%, while the amount of methane has doubled. Today the U.S. is responsible for about one-fifth of all human-produced greenhouse-gas emissions. See also global warming.

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▪ atmospheric science

 a warming of the Earth's surface and troposphere (the lowest layer of the atmosphere), caused by the presence of water vapour, carbon dioxide, methane, and certain other gases in the air. Of these gases, known as greenhouse gases (greenhouse gas), water vapour has the largest effect.

      The atmosphere allows most of the visible light from the Sun (solar radiation) to pass through and reach the Earth's surface. As the Earth's surface is heated by sunlight, it radiates part of this energy back toward space as infrared radiation. This radiation, unlike visible light, tends to be absorbed by the greenhouse gases in the atmosphere, raising its temperature. The heated atmosphere in turn radiates infrared radiation back toward the Earth's surface. (Despite its name, the greenhouse effect is different from the warming in a greenhouse, where panes of glass transmit visible sunlight but hold heat inside the building by trapping warmed air.)

      Without the heating caused by the greenhouse effect, the Earth's average surface temperature would be only about −18 °C (0 °F). On Venus the very high concentration of carbon dioxide in the atmosphere causes an extreme greenhouse effect resulting in surface temperatures as high as 450 °C (840 °F).

      Although the greenhouse effect is a naturally occurring phenomenon, it is possible that the effect could be intensified by the emission of greenhouse gases into the atmosphere as the result of human activity. From the beginning of the Industrial Revolution through the end of the 20th century, the amount of carbon dioxide in the atmosphere increased 30 percent and the amount of methane more than doubled. A number of scientists have predicted that human-related increases in atmospheric carbon dioxide and other greenhouse gases could lead to an increase in the global average temperature of 1.4 to 5.8 °C (2.5 to 10.4 °F) by the end of the 21st century. This global warming could alter the Earth's climates and thereby produce new patterns and extremes of drought and rainfall and possibly disrupt food production in certain regions. Other scientists involved in climatic research maintain that such predictions are overstated, however.

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