Runaway greenhouse effect

a theory that coal combustion could eventually lead to a degree of global warming causing human extinction.While the term was first coined by Caltech scientist Andrew Ingersoll in a paper that described a model of the atmosphere of Venus, the initial idea of a limit on terrestrial outgoing infrared radiation was published by George Simpson in 1927. The physics relevant to what would later be named the runaway greenhouse effect, was explored by Makoto Komabayashi at Nagoya University. Assuming a water vapor-saturated stratosphere, Komabayashi and Ingersoll independently calculated the limit on outgoing infrared radiation that defines the runaway greenhouse state. That limit is now known as the Komabayashi–Ingersoll limit, to recognize their contributions. ==Physics==

A runaway greenhouse effect occurs when greenhouse gases accumulate in the atmosphere through a positive feedback cycle to such an extent that they substantially block radiated heat from escaping into space, thus greatly increasing the temperature of the planet. The runaway greenhouse effect is often formulated in terms of how the surface temperature of a planet changes with differing amounts of received starlight. If the planet is assumed to be in radiative equilibrium, then the runaway greenhouse state is calculated as the equilibrium state at which water cannot exist in liquid form. Assuming radiative equilibrium, runaway greenhouse limits on outgoing longwave radiation correspond to limits on the increase in stellar flux received by a planet to trigger the runaway greenhouse effect. Two limits on a planet's outgoing longwave radiation have been calculated that correspond with the onset of the runaway greenhouse effect: the Komabayashi–Ingersoll limit At these values the runaway greenhouse effect overcomes the Stefan–Boltzmann feedback so an increase in a planet's surface temperature will not increase the outgoing longwave radiation. The Komabayashi–Ingersoll limit was the first to be analytically derived and only considers a grey stratosphere in radiative equilibrium. A grey stratosphere (or atmosphere) is an approach to modeling radiative transfer that does not take into account the frequency-dependence of absorption by a gas. In the case of a grey stratosphere or atmosphere, the Eddington approximation can be used to calculate radiative fluxes. This approach focuses on the balance between the outgoing longwave radiation at the tropopause,F^\uparrow_\text{IRtop}, and the optical depth of water vapor, \tau_\text{tp}, in the tropopause, which is determined by the temperature and pressure at the tropopause according to the saturation vapor pressure. This balance is represented by the following equations\begin{align} \frac{1}{2}F^\uparrow_\text{IRtop}\left(\frac{3}{2}\tau_\text{tp}+1\right) &= \sigma T_\text{tp}^4 \\ \tau_\text{tp} &= \kappa_{v}p^*(T_\text{tp})\frac{1}{g}\frac{m_v}{\bar{m}} \end{align}Where the first equation represents the requirement for radiative equilibrium at the tropopause and the second equation represents how much water vapor is present at the tropopause. Taking the outgoing longwave radiation as a free parameter, these equations will intersect only once for a single value of the outgoing longwave radiation, this value is taken as the Komabayashi–Ingersoll limit. At that value the Stefan–Boltzmann feedback breaks down because the tropospheric temperature required to maintain the Komabayashi–Ingersoll OLR value results in a water vapor optical depth that blocks the OLR needed to cool the tropopause. The Simpson–Nakajima limit is lower than the Komabayashi–Ingersoll limit, and is thus typically more realistic for the value at which a planet enters a runaway greenhouse state. For example, given the parameters used to determine a Komabayashi–Ingersoll limit of 385 W/m2, the corresponding Simpson–Nakajima limit is only about 293 W/m2. The Simpson–Nakajima limit builds off of the derivation of the Komabayashi–Ingersoll limit by assuming a convective troposphere with a surface temperature and surface pressure that determines the optical depth and outgoing longwave radiation at the tropopause. The moist greenhouse limit Because the model used to derive the Simpson–Nakajima limit (a grey stratosphere in radiative equilibrium and a convecting troposphere) can determine the water concentration as a function of altitude, the model can also be used to determine the surface temperature (or conversely, amount of stellar flux) that results in a high water mixing ratio in the stratosphere. While this critical value of outgoing longwave radiation is less than the Simpson–Nakajima limit, it still has dramatic effects on a planet's climate. A high water mixing ratio in the stratosphere would overcome the effects of a cold trap and result in a "moist" stratosphere, which would result in the photolysis of water in the stratosphere that in turn would destroy the ozone layer and eventually lead to a dramatic loss of water through hydrodynamic escape. This climate state has been dubbed the moist greenhouse effect, as the end-state is a planet without water, though liquid water may exist on the planet's surface during this process. ==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 outgoing longwave radiation limit beyond which the runaway greenhouse process occurs (e.g., the Simpson–Nakajima limit). This is because a planet's distance from its host star determines the amount of stellar flux the planet receives, which in turn determines the amount of outgoing longwave radiation the planet radiates back to space. Calculating the inner edge of the habitable zone is strongly dependent on the model used to calculate the Simpson–Nakajima or moist greenhouse limit. use the more current and accurate HITEMP database, which has led to different calculated values of thermal radiation limits. More accurate calculations have been done using three-dimensional climate models that take into account effects such as planetary rotation and local water mixing ratios as well as cloud feedbacks. The effect of clouds on calculating thermal radiation limits is still in debate (specifically, whether or not water clouds present a positive or negative feedback effect). == Runaway greenhouse effect in the Solar System ==

Venus A runaway greenhouse effect involving carbon dioxide and water vapor likely 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. 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. 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. Earth Early 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. Climate scientist John Houghton wrote in 2005 that "[there] is no possibility of [Venus's] runaway greenhouse conditions occurring on the Earth". However, climatologist James Hansen stated in Storms of My Grandchildren (2009) 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. 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. 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 40 °C (104 °F) in the tropics to 16 °C (65 °F) in the polar regions. Distant future Most scientists believe that a runaway greenhouse effect is inevitable in the long term, as the Sun gradually becomes more luminous as it ages, and will spell the end of all life on Earth. As the Sun becomes 10% brighter about one billion years from now, the surface temperature of Earth will reach (unless albedo is increased sufficiently), causing the temperature of Earth to rise rapidly and its oceans to boil away until it becomes a greenhouse planet, similar to Venus today. The current loss rate is approximately one millimeter of ocean per million years. This is due to the colder upper layer of the troposphere acting as a cold trap currently preventing Earth from permanently losing its water to space at present, even with manmade global warming (this is also the reason why climate change is only going to make extreme weather events worse in the near term, as a warmer atmosphere can hold more moisture, as even with global warming, the cold trap ensures that the current atmosphere will still be too cold to allow water vapor to be rapidly lost to space). This is being overshadowed by shorter-term changes in sea level, such as the currently rising sea level due to the melting of glaciers and polar ice. However, the rate is gradually accelerating, as the sun gets warmer, to perhaps as fast as one millimeter every 1000 years, by ultimately making the atmosphere so hot that the cold trap is pushed even higher up until it eventually fails to prevent the water from being lost to space. Ward and Brownlee predict that there will be two variations of the future warming feedback: the "moist greenhouse" in which water vapor dominates the troposphere and starts to accumulate in the stratosphere and the "runaway greenhouse" in which water vapor becomes a dominant component of the atmosphere such that the Earth starts to undergo rapid warming, which could send its surface temperature to over , causing its entire surface to melt and killing all life, perhaps about three billion years from now. In both cases, 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. The 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. That would mitigate some of the heating Earth would experience because of the Sun's increase in brightness. Eventually, however, as the water escapes, the carbon cycle will cease as plate tectonics come to a halt because of the need for water as a lubricant for tectonic activity. ==Runaway refrigerator effect==

Mars and Earth during the Cryogenian period may have experienced the opposite of a runaway greenhouse effect: a runaway refrigerator effect. Through this effect, a runaway feedback process may have removed much carbon dioxide and water vapor from the atmosphere and cooled the planet. Water condenses on the surface, leading to carbon dioxide dissolving and chemically binding to minerals. This reduced the greenhouse effect, lowering the temperature and causing more water to condense. The result was lower temperatures, with water being frozen as subsurface permafrost, leaving only a thin atmosphere. In addition, ice and snow are far more reflective than open water, with an albedo of 50-70% and 85% respectively. This means that as a planet's temperature decreases and more of its water freezes, its ability to absorb light is reduced, which in turn makes it even colder, creating a positive feedback loop. This effect, combined with the decrease in heat-retaining clouds and vapor, becomes runaway once snow and ice coverage reach a certain threshold (within 30 degrees of the equator), plunging the planet into a stable snowball state. ==See also==