In the Solar System

In the solar system

VenusA 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 if the outgoing thermal radiation was below the Simpson-Nakajima limit but above the moist greenhouse limit. 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. This scenario helps to explain why there is little water vapor in the atmosphere of Venus today. If Venus initially formed with water, the runaway greenhouse effect would have hydrated Venus' stratosphere, and the water would have escaped to space. Some evidence for this scenario comes from the extremely high deuterium to hydrogen ratio in Venus' atmosphere, roughly 150 times 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 while the oxygen recombines or bonds to iron on the planet's surface. The deficit of water on Venus due to the runaway greenhouse effect is thought to explain why Venus does not exhibit surface features consistent with plate tectonics, meaning it would be a stagnant lid planet. 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 through the carbonate-silicate cycle, which requires precipitation to function. EarthEarly investigations on the effect of atmospheric carbon dioxide levels on the runaway greenhouse limit found that it would take orders of magnitude higher amounts of carbon dioxide to take the Earth to a runaway greenhouse state. This is because carbon dioxide is not anywhere near as effective at blocking outgoing longwave radiation as water is. Within current models of the runaway greenhouse effect, carbon dioxide (especially anthropogenic carbon dioxide) does not seem capable of providing the necessary insulation for Earth to reach the Simpson-Nakajima limit. There remains debate, however, on whether carbon dioxide can push surface temperatures towards the moist greenhouse limit. Climate scientist John Houghton has written that "[there] is no possibility of [Venus's] runaway greenhouse conditions occurring on the Earth". The IPCC (Intergovernmental Panel on Climate Change) has also stated that "a 'runaway greenhouse effect'—analogous to [that of] Venus—appears to have virtually no chance of being induced by anthropogenic activities." However, climatologist James Hansen disagrees. In his Storms of My Grandchildren he says that burning coal and mining oil sands will result in runaway greenhouse on Earth. A re-evaluation in 2013 of the effect of water vapor in the climate models showed that James Hansen's outcome would require ten times the amount of CO2 we could release from burning all the oil, coal, and natural gas in Earth's crust. As with the uncertainties in calculating the inner edge of the habitable zone, the uncertainty in whether CO2 can drive a moist greenhouse effect is due to differences in modeling choices and the uncertainties therein. The switch from using HITRAN to the more current HITEMP absorption line lists in radiative transfer calculations has shown that previous runaway greenhouse limits were too high, but the necessary amount of carbon dioxide would make an anthropogenic moist greenhouse state unlikely. Full three-dimensional models have shown that the moist greenhouse limit on surface temperature is higher than that found in one-dimensional models and thus would require a higher amount of carbon dioxide to initiate a moist greenhouse than in one-dimensional models. Other complications include whether the atmosphere is saturated or sub-saturated at some humidity, higher CO2 levels in the atmosphere resulting in a less hot Earth than expected due to Rayleigh scattering, and whether cloud feedbacks stabilize or destabilize the climate system. Complicating the matter, research on Earth's climate history has often used the term runaway greenhouse effect to describe large-scale climate changes when it is not an appropriate description as it does not depend on Earth's outgoing longwave radiation. Though the Earth has experienced a diversity of climate extremes, these are not end-states of climate evolution and have instead represented climate equilibria different than that seen on Earth today. 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. Additionally, during 80% of the latest 500 million years, the Earth is believed to have been in a greenhouse state due to the greenhouse effect, when there were no continental glaciers on the planet, the levels of carbon dioxide and other greenhouse gases (such as water vapor and methane) were high, and sea surface temperatures (SSTs) ranged from 28 °C (82.4 °F) in the tropics to 0 °C (32 °F) in the polar regions. Distant futureMost 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 and its oceans to boil away 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 and starts to accumulate in the stratosphere, and the "runaway greenhouse" where water vapor becomes a dominant component of the atmosphere such that the Earth starts to undergo rapid warming that could send its surface temperature to over 900 °C (1,650 °F), causing its entire surface to melt and killing all life, perhaps in about three billion years' time. In both the moist and runaway greenhouse states the loss of oceans will turn the Earth into a primarily desert world. The only water left on the planet would be in a few evaporating ponds scattered near the poles as well as huge salt flats around what was once the ocean floor, much like the Atacama Desert in Chile or Badwater Basin in Death Valley. These small reservoirs of water may allow life to remain for a few billion more years. As the Sun brightens, CO2 levels should decrease due to an increase of activity in the carbon-silicate cycle corresponding to the increase of temperature. This would mitigate some of the heating Earth would experience due to the Sun's increase in brightness. Eventually though, as the water escapes, the carbon cycle will cease as plate tectonics come to a halt due to the need for water as a lubricant for tectonic activity.

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