The natural process by which certain gases in the atmosphere absorb heat radiated from the Earth's surface and re-emit it, keeping the planet far warmer than it would otherwise be; human emissions are strengthening it and causing global warming.
Sunlight reaches the Earth mostly as short-wave visible radiation, which passes through the air easily. The warmed surface then gives off energy as long-wave infrared radiation. Gases made of three or more atoms, or with uneven charges such as carbon dioxide and water vapour, absorb this infrared energy and send part of it back downwards. The lower atmosphere and surface therefore stay warmer than they would be if heat escaped straight to space.
The physics was worked out in the nineteenth century. Joseph Fourier suggested in the 1820s that the atmosphere traps heat; John Tyndall measured the absorption of infrared by gases around 1859; and Svante Arrhenius calculated in 1896 that doubling atmospheric CO2 would raise temperatures by several degrees. Charles David Keeling's measurements at Mauna Loa, begun in 1958, showed a steady rise in CO2, now known as the Keeling Curve.
The name comes from an analogy with glass greenhouses, although real greenhouses stay warm mainly by blocking the escape of warm air rather than by trapping radiation.
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