A runaway greenhouse effect is not a clearly defined term, but is understood to mean an event analogous to that which is believed to have happened in the early history of Venus, where positive feedback increased the strength of its greenhouse effect until its oceans boiled away.[1][2] The term is not generally used by the IPCC, which in one of its few mentions says a “runaway greenhouse effect” — analogous to Venus - appears to have virtually no chance of being induced by anthropogenic activities.[3]
Other, less catastrophic events, may loosely be called a "runaway greenhouse". It has been hypothesised that such may have occurred at the Permian-Triassic extinction event.[4][5] Terrestrial climatologists often use the term 'abrupt', rather than 'runaway', when describing such scenarios.[6]
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Positive feedbacks do not have to lead to a runaway effect, as the gain is not always sufficient. Radiation from a planet increasing in proportion to the fourth power of temperature, in accordance with the Stefan-Boltzmann law, provides a negative feedback; so 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.[7] 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.
A runaway greenhouse effect involving CO2 and water vapor may have occurred on Venus.[8] 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.[9]
The situation on Earth is very different from that which existed on Venus, as any terrestrial runaway effect is not irreversible on geological timescales. Potential runaway greenhouse effects on Earth may involve the carbon cycle, but unlike Venus will not involve 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.[10] Climate scientist John Houghton has written that "[there] is no possibility of [Venus's] runaway greenhouse conditions occurring on the Earth".[11]
Benton and Twitchett's have a different definition of a runaway greenhouse;[4] events meeting this definition have been suggested as a cause for the Paleocene-Eocene Thermal Maximum and the great dying.