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Runaway greenhouse effect
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A runaway greenhouse effect occurs when, on a planet with substantial reserves of greenhouse gases in liquid or solid form, some forcing occurs to begin to gasify them, leading via positive feedback to complete gasification of these reserves.
The planet Venus is believed to have experienced a runaway greenhouse effect, which led to its oceans boiling away. The term is also used to describe a more constrained runaway effect on earth such as that which may have occurred at the Permian-Triassic extinction event.
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Encyclopedia
A runaway greenhouse effect occurs when, on a planet with substantial reserves of greenhouse gases in liquid or solid form, some forcing occurs to begin to gasify them, leading via positive feedback to complete gasification of these reserves.
The planet Venus is believed to have experienced a runaway greenhouse effect, which led to its oceans boiling away. The term is also used to describe a more constrained runaway effect on earth such as that which may have occurred at the Permian-Triassic extinction event. Terrestrial climatologists often use the term 'abrupt', rather than 'runaway' terms, when describing such scenarios.
Feedbacks
Positive feedbacks do not have to lead to a runaway effect, as the gain is not always sufficient. With radiation from a planet increasing in proportion to the fourth power of temperature, in accordance with the Stefan-Boltzmann law, the positive feedback effect has to be very strong to cause a runaway effect (see gain). An increase in temperature from greenhouse gases leading to increased water vapor which is a greenhouse gas causing further warming is a positive feedback. This is not a runaway effect on earth. Positive feedback effects are common and always exist (e.g ice-albedo feedback) while runaway effects are much rarer and cannot be operating at all times.
Venus
A runaway greenhouse effect involving CO2 and water vapor may have occurred on Venus. In this scenario, early Venus may have had a global ocean. As the brightness of the early sun increased, the amount of water vapor in the atmosphere increased, increasing the temperature and consequently increasing the evaporation of the ocean, leading eventually to the situation in which the oceans boiled, and all of the water vapor entered the atmosphere. On Venus today there is little water vapor in the atmosphere. If water vapor did contribute to the warmth of Venus at one time, this water is thought to have escaped to space. Venus is sufficiently strongly heated by the Sun that water vapor can rise much higher in the atmosphere and be split into hydrogen and oxygen by ultraviolet light. The hydrogen can then escape from the atmosphere and the oxygen recombines. Carbon dioxide, the dominant greenhouse gas in the current Venusian atmosphere, likely owes its larger concentration to the weakness of carbon recycling as compared to Earth, where the carbon dioxide emitted from volcanoes is efficiently subducted into the Earth by plate tectonics on geologic time scales.
Earth
The situation on Earth is very different to that which existed on Venus, as any terrestrial runaway effect is not irreversible on geological timescales, nor will it lead to boiling of the oceans. Earth's climate has swung repeatedly between warm periods and ice ages during its history. In the current climate the gain of the positive feedback effect from evaporating water is well below that which is required to boil away the oceans.
Events which would meet Benton and Twitchet's definition of a runway greenhouse have been suggested as a cause for the Paleocene-Eocene Thermal Maximum and the great dying. Similar conditions may exist on Earth today, with Buffett and Archer suggesting that a similar event may be possible as a result of global warming.
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