Runaway greenhouse effect

A runaway greenhouse effect is a process in which a net positive feedback between surface temperature and atmospheric opacity increases the strength of the greenhouse effect on a planet until its oceans boil away. An example of this is believed to have happened in the early history of Venus. On the Earth, the IPCC states that "a 'runaway greenhouse effect'—analogous to [that of] Venus—appears to have virtually no chance of being induced by anthropogenic activities."

Other large-scale climate changes are sometimes loosely called a "runaway greenhouse effect" although it is not an appropriate description. For example, it has been hypothesized that large releases of greenhouse gases may have occurred concurrently with the Permian–Triassic extinction event or Paleocene–Eocene Thermal Maximum. Other terms, such as "abrupt climate change", or tipping points could be used when describing such scenarios.

This term was coined by Caltech scientist Andrew Ingersoll in a paper that described a model of the atmosphere of Venus. Water vapor initially in the atmosphere of Venus absorbed outgoing radiation which caused the planet to heat and increased water vapor. High abundance of water vapor in the atmosphere allowed photodissociation to occur, with lighter hydrogen gas escaping to space and oxygen reacting with surface rocks. This model is supported by the deuterium/hydrogen ratio on Venus which is 150 times greater than the D/H ratio on Earth.

Positive feedbacks do not have to lead to a runaway effect, as the gain is not always sufficient. A strong negative feedback always exists (radiation from a planet increases in proportion to the fourth power of temperature, in accordance with the Stefan-Boltzmann law) 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 itself a greenhouse gas) causing further warming is a positive feedback, but not a runaway effect, on Earth. Positive feedback effects are common (e.g. ice-albedo feedback) but runaway effects do not necessarily emerge from their presence.

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