| Abstract from DBPedia | The greenhouse effect is a process that occurs when energy from a planet's host star goes through its atmosphere and heats the planet's surface, but greenhouse gases in the atmosphere prevent some of the heat from returning directly to space, resulting in a warmer planet. Earth's natural greenhouse effect keeps the planet from having the below freezing temperature that it would have if there were no greenhouse gases. Additionally, human-caused increases in greenhouse gases trap greater amounts of heat, causing the Earth to grow warmer over time. Anything radiates energy related to its temperature: the Sun—at about 5,500 °C (9,930 °F)—sends most of its energy as visible and near infrared light, while Earth's average surface temperature—at about 15 °C (59 °F)—emits longer-wavelength infrared, radiant heat. The atmosphere is transparent to most incoming sunlight, and allows its energy through to heat the surface. Most gases in the atmosphere are transparent to infrared, but the small proportion of the atmosphere that constitutes greenhouse gases absorbs some of the heat emitted by the surface rather than letting it escape into space. These greenhouse gas molecules then emit radiant heat in all directions, passing heat to the surrounding air and warming other greenhouse gas molecules. Radiant heat going downwards further increases the temperature of the surface, which then returns heat to the atmosphere in a positive feedback cycle. Without Earth's natural greenhouse effect the Earth would be more than 30 °C (54 °F) colder. A runaway greenhouse effect occurs when greenhouse gases accumulate in the atmosphere through a positive feedback cycle to such an extent that they substantially block thermal radiation (heat) from escaping into space, thus preventing the planet from cooling. The runaway greenhouse effect occurred with carbon dioxide and water vapor on Venus. It is unlikely that human-caused greenhouse gas emissions alone could trigger a runaway effect on Earth. The term greenhouse effect comes from a flawed analogy to greenhouses, which have transparent glass that passes sunlight but retains heat by physically restricting air movement; radiative effects are not involved.温室効果（おんしつこうか、英: greenhouse effect）とは、大気圏を有する惑星の表面から発せられる赤外線放射が、大気圏外に放出される前に、その一部が大気中の物質に吸収され、再び惑星へ放出されてくることで、そのエネルギーが太陽光のエネルギーに加わって地表や地表付近の大気をさらに暖める効果のこと。。地球が生命の住める温暖な気候なのは温室効果のためであり、温室効果が無い場合は地球の表面温度は氷点下19℃の極寒となると見積もられている。 地球においては大気圏形成（地球の大気を参照）によって同効果が始まるが、大気の構成が長期的に変化しているため、気温への影響も変化がみられる（過去の気温変化を参照）。なお、近代以降の人類活動の結果もたらされたとみられる「地球温暖化」（英: global warming）においては、同効果が主な原因とされている。 (Source: http://dbpedia.org/resource/Greenhouse_effect) |
