L'effetto serra è un processo mediante il quale la radiazione dall'atmosfera di un pianeta riscalda la superficie a una temperatura superiore a quella che sarebbe senza la sua atmosfera. Questo effetto è causato da alcuni gas presenti nell'atmosfera, come il vapore acqueo, che possono intrappolare più energia dal sole, riscaldando la superficie e rendendo l'atmosfera una forza benefica per la vita sul pianeta. Senza questi gas, la temperatura media sul nostro pianeta sarebbe molto più bassa, rendendo praticamente impossibile la vita sulla Terra.
The greenhouse effect is a process that occurs after energy from a planet's host star goes through the planet's atmosphere and heats the planet's surface. When the planet radiates the heat back out as infrared radiation, greenhouse gases in the atmosphere absorb some of the heat and then radiate some of it back down towards the planet, warming it.[1][2][3] Without the greenhouse effect, the Earth's average surface temperature would be about −18 °C (−0.4 °F)[4][5] compared to Earth's actual average surface temperature of approximately 14 °C (57.2 °F).[6] In addition to the naturally present greenhouse gases, human-caused increases in greenhouse gases are trapping greater amounts of heat.[7][8] Burning fossil fuels has increased atmospheric carbon dioxide by about 50% over pre-industrial levels, which is the primary driver for global warming.[9]

Matter radiates energy at a wavelength related to its temperature. The Sun is about 5,500 °C (9,930 °F), so it emits most of its energy in near infrared and visible wavelengths (as sunlight). The Earth's average surface temperature is about 15 °C (59 °F), so it emits longer-wavelength infrared radiation (radiated heat).[8] Different gases absorb different wavelengths of radiation. Earth's atmosphere is made of gases that are transparent to visible wavelengths of light, allowing sunlight through to heat the surface. Most gases in the atmosphere are also transparent to infrared radiation, but a small proportion of the atmosphere is made up of greenhouse gases that absorb it. These greenhouse gas molecules absorb some of the heat the Earth emits and then re-emit the heat in all directions, passing it to the surrounding air and warming other greenhouse gas molecules. Heat radiated downwards further increases the temperature of the surface, which then returns heat to the atmosphere in a positive feedback cycle.

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 radiated heat from escaping into space, thus preventing the planet from cooling.[10] A runaway greenhouse effect involving carbon dioxide and water vapor appears to have occurred on Venus. However, 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. The primary way greenhouses retain energy from the sun is by preventing the movement of air (blocking convection), although their panels also retain heat by limiting heat radiation and conduction.
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