greenhouse effect: see global warming.

greenhouse, enclosed glass house used for growing plants in regulated temperatures, humidity, and ventilation. A greenhouse can range from a small room carrying a few plants over the winter, to an immense heated glass building called a hothouse or conservatory, covering acres of ground and used for forcing fruits or flowers out of season. Greenhouses have long been used for holding plants over cold seasons and for growing tropical plants and less hardy fruits, but only in this century has the greenhouse been used for forcing vegetables. Now millions of dollars' worth of plant products are raised yearly in greenhouses. See cold frame.

Warming of the Earth's surface and lower atmosphere caused by water vapour, carbon dioxide, and other trace gases in the atmosphere. Visible light from the Sun heats the Earth's surface. Part of this energy is radiated back into the atmosphere in the form of infrared radiation, much of which is absorbed by molecules of carbon dioxide and water vapour in the atmosphere and reradiated toward the surface as more heat. (Despite the name, the greenhouse effect is different from the warming in a greenhouse, where panes of glass allow the passage of visible light but hold heat inside the building by trapping warmed air.) The absorption of infrared radiation causes the Earth's surface and lower atmosphere to warm more than they otherwise would, making the Earth's surface habitable. An increase in atmospheric carbon dioxide caused by widespread combustion of fossil fuels may intensify the greenhouse effect and cause long-term climatic changes. Likewise, an increase in atmospheric concentrations of other trace greenhouse gases such as chlorofluorocarbons, nitrous oxide, and methane resulting from human activities may also intensify the greenhouse effect. From the beginning of the Industrial Revolution through the end of the 20th century, the amount of carbon dioxide in the atmosphere increased 30percnt and the amount of methane more than doubled. It is also estimated that the U.S. is responsible for about one-fifth of all human-produced greenhouse-gas emissions. Seealso global warming.

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Building designed for the protection of tender or out-of-season plants against excessive cold or heat. Usually a glass- or plastic-enclosed structure with a framing of aluminum, galvanized steel, or such woods as redwood, cedar, or cypress, it is used for the production of fruits, vegetables, flowers, and any other plants requiring special temperature conditions. It is heated partly by the sun and partly by artificial means. This controlled environment can be adapted to the needs of particular plants.

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