The greenhouse effect is the trapping of some solar radiation by a planet's atmosphere, increasing the temperature on and near the surface. Without the greenhouse effect, the Earth would be about 14-36K cooler.
The term "greenhouse" is used to describe this phenomenon since these gases act like the glass of a greenhouse to trap heat and maintain higher interior temperatures than would normally occur. The atmospheric gases most responsible for this effect are water vapor (H2O), carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O) and ozone (O3), collectively known as greenhouse gases. This "greenhouse effect" occurs naturally in our atmosphere and is responsible for the earth's surface temperature which allows life on Earth.
Visible light from the Sun is able to pass through the atmosphere and reach the planet's surface where much of it is absorbed, thereby warming the surface. Some of the heat is radiated back at longer infrared wavelengths and the greenhouse gases in the atmosphere absorb some of this radiation, thereby warming up and eventually passing some of the energy back to the surface.
The degree of the greenhouse effect is dependent primarily on the concentration of greenhouse gases in the planetary atmosphere. For example, while the planets Venus, Earth, and Mars have similar amounts of incident solar radiation, the dense, carbon dioxide-rich atmosphere of Venus causes a runaway greenhouse effect with surface temperatures hot enough to melt lead, the atmosphere of Earth causes a greenhouse effect of habitable temperatures, and the thin atmosphere of Mars causes a minimal greenhouse effect.
The term 'greenhouse effect' originally came from the greenhouses used for gardening, but it is a misnomer since greenhouses operate differently. A greenhouse is built of glass; it heats up primarily because the sun warms the ground inside it, which warms the air near the ground, and this air is prevented from rising and flowing away. This can be demonstrated by opening a small window near the roof of a greenhouse: the temperature will drop considerably. It has also been demonstrated experimentally [1]. Greenhouses thus work by preventing convection; the greenhouse effect however reduces radiation loss, not convection.
In recent years many researchers see the Greenhouse effect as a significant contributing factor to the current global warming, due to the increased concentration of some greenhouse gases in the atmosphere as a result of human activity. Climatologists are concerned that increased levels of greenhouse gases in the atmosphere might cause more heat to be trapped.
The hypothesis that a man-made increase in greenhouse gas concentration would lead to a higher global mean temperature was first postulated in the late 19th century by Swedish chemist and 1903 Nobel Laureate Svante Arrhenius (see global warming hypothesis), although, his peers largely rejected that theory.
The theory that human greenhouse gas emissions are connected with the observed heating of the Earth's atmosphere in the 20th century has steadily gained adherents in the scientific community since the 1980s, to the extent that many scientific bodies around the world have strongly endorsed it. Even climate researcher Stephen Schneider, who thought in the 1970s that human industrial emissions might lead to global cooling, has become a strong proponent of the global warming hypothesis.
Automobile exhausts, coal-burning power plants, factory smokestacks, and other waste vents of the industrial age now pump six billion tons of carbon dioxide and other greenhouse gases into the earth's atmosphere each year. Concentrations of greenhouse gases in the atmosphere are currently at approximately 25% above pre-industrial values. This is considerably higher than at any time during the last 420,000 years, the period for which reliable data exists, from ice cores. From less direct geological evidence it is believed that values this high were last attained 40 million years ago.
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[1] R. W. Wood: Note on the Theory of the Greenhouse; Philosophical magazine, 1909, vol 17, p319-320. For the text of this online, see http://www.wmc.care4free.net/sci/wood_rw.1909.html
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