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" 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.
A runaway greenhouse effect involving carbon dioxide 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. Some evidence for this scenario comes from the extremely high Deuterium to Hydrogen ratio in Venus' atmosphere, roughly ~150x that of Earth, since light hydrogen would escape from the atmosphere more readily than its heavier isotope, Deuterium. 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, 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'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 increased atmospheric water vapor, as well as Earth being too far away from the Sun at its current luminosity for such to occur is well below that which is required to boil away the oceans. Climate scientist John Houghton has written that "[there] is no possibility of [Venus's] runaway greenhouse conditions occurring on the Earth". However, climatologist James Hansen disagrees. In his Storms of My Grandchildren he says that burning coal and mining shale oil will result in runaway greenhouse on Earth. A re-evaluation in 2013 of the effect of water vapour in the climate models showed that James Hansen's outcome might be possible, but requires ten times the amount of CO2 we could release from burning all the oil, coal, and natural gas in Earth's crust.  Further, Benton and Twitchett have a different definition of a runaway greenhouse; events meeting this definition have been suggested as a cause for the Paleocene-Eocene Thermal Maximum and the great dying.
Most scientists believe that a runaway greenhouse effect is actually inevitable in the long term as the Sun gradually gets bigger and hotter as it ages. Such will potentially spell the end of all life on Earth. As the Sun becomes 10% brighter in about one billion years' time, the surface temperature of Earth will reach 47 °C (117 °F), causing the temperature of Earth to rise rapidly until it becomes a greenhouse planet similar to Venus today.
According to astrobiologists Peter Ward and Donald Brownlee in their book The Life and Death of Planet Earth, the current loss rate is approximately one millimeter of ocean per million years, but this rate is gradually accelerating as the sun gets warmer, to perhaps as fast as one millimeter every 1000 years. Ward and Brownlee predict that there will be two variations of this future warming feedback: the "moist greenhouse" where water vapor dominates the troposphere while water vapor starts to accumulate in the stratosphere, and the "runaway greenhouse" where water vapor becomes a dominant component of the atmosphere that the Earth starts to undergo rapid warming that could send its surface temperature to over 900 °C (1,650 °F) as the atmosphere will be totally overwhelmed by water vapor, causing its entire surface to melt and killing all life, perhaps in about three billion years' time. Either way, the loss of oceans will inevitably turn the Earth into a primarily desert world with the only water left being a few evaporating ponds scattered near the poles, and huge salt flats around what was once the ocean floor, much like the Atacama Desert in Chile or Bad Water in Death Valley, where the last life may remain for a few billion more years. Ironically, because of this, in the former case the loss of oceans will actually save the last life instead of destroying it completely. However, complex life like plants and animals will be long extinct before this happens as the loss of oceans will cause plate tectonics to come to halt; water is a lubricant for tectonic activity and the loss of all water will make the crust too hard and dry to be subducted, therefore causing the carbon cycle to cease altogether (there would be no more volcanoes to return CO2 back into the atmosphere).
Physics of the runaway greenhouse
Normally, when a planet's radiation balance is perturbed (e.g., by increasing the amount of sunlight it gets or changing the greenhouse concentration, see Radiative Forcing) it will transition to a new temperature until a stabilizing feedback known as the Stefan-Boltzmann response restores an equilibrium between the amount of energy the body absorbs and that which it emits. For example, if the Earth received more sunlight it would result in a temporary disequilibrium (more energy in than out) and result in warming. However, because the Stefan-Boltzmann response mandates that this hotter planet emit more energy, eventually a new radiation balance can be reached and the temperature will be maintained at its new, higher value.
However, when the planet has an operating water vapor feedback, the efficiency of the greenhouse effect increases as the temperature does, and so the outgoing radiation to space increases less rapidly than for a pure Stefan-Boltzmann radiator behaving like a blackbody. Eventually, the infrared absorption increases so much that the amount of energy escaping to space no longer depends on the temperature at the surface, and asymptotes to a fixed value, the Kombayashi–Ingersoll limit. If the amount of energy that the planet receives from the star (or from internal heat sources) exceeds this value, radiative equilibrium can never be achieved. The result is a runaway that continues until the water vapor feedback ceases, which may be when the entire ocean is evaporated and dispersed to space.
Connection to habitability
The concept of a habitable zone has been used by planetary scientists and astrobiologists to define an orbital region around a star in which a planet (or moon) can sustain liquid water. Under this definition, the inner edge of the habitable zone (i.e., the closest point to a star that a planet can be until it can no longer sustain liquid water) is determined by the point in which the runaway greenhouse process occurs. For sun-like stars, this inner edge is estimated to reside at roughly 84% the distance from the Earth to the sun although feedback such as cloud-induced albedo increase could modify this estimate somewhat.
- Atmosphere of Venus, an example of a runaway greenhouse effect
- Avoiding dangerous climate change
- Runaway climate change
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