Uncontrolled greenhouse effect

The term was coined by Caltech scientist Andrew Ingersoll in an article describing a model of the atmosphere of Venus ^[7] . Initially, water vapor in the atmosphere of Venus absorbed radiation reflected from the surface, which caused the planet to heat up and increased the evaporation of water, which led to the appearance of positive feedback. The high content of water vapor in the atmosphere allows for photodissociation, while lighter hydrogen gas escapes into space, and oxygen reacts with surface rocks. This model is confirmed by the deuterium / hydrogen ratio on Venus, which is 150 times greater than on Earth .

Positive feedback should not lead to an uncontrolled greenhouse effect, since the gain is not always sufficient for this. There is always a strong negative feedback (the radiation of the planet increases in proportion to the fourth degree of temperature in accordance with the Stefan-Boltzmann law ), therefore the amplitude of the positive feedback must be very strong in order to cause an uncontrolled greenhouse effect (see gain ). An increase in temperature due to greenhouse gases, leading to an increase in the amount of water vapor (which is itself a greenhouse gas), which causes further warming, is undoubtedly a positive feedback effect and exists on Earth, but does not become uncontrolled ^[8] . Systems with positive feedback are very common (for example, the albedo of the ice-water system), but an uncontrolled effect does not always occur in them.

An uncontrolled greenhouse effect with the participation of carbon dioxide and water vapor could occur on Venus ^[9] . In this case, perhaps there was a global ocean on Venus. As the brightness of the young Sun increased, the amount of water vapor in the atmosphere increased, increasing the temperature and, therefore, increasing the rate of evaporation of the ocean, eventually leading to a situation where the oceans boiled and all the water vapor moved into the atmosphere. Today, there is almost no water vapor in the atmosphere of Venus ^{[10] [11]} . If water vapor really once contributed to the heating of Venus, then it is assumed that this water has completely gone into outer space . This scenario is supported by the extremely high ratio of deuterium to hydrogen in the atmosphere of Venus, about 150 times larger than the Earth's, since light hydrogen more actively left the atmosphere than its heavier isotope, deuterium ^{[12] [13]} . Venus is heated quite strongly by the Sun, so water vapor can rise into the upper atmosphere and split into hydrogen and oxygen under the influence of ultraviolet radiation . Then, hydrogen leaves the atmosphere, and oxygen recombines with the rocks. The carbon dioxide dominating in the current atmosphere of Venus owes its presence to a weak mechanism of carbon circulation compared to the Earth, where carbon dioxide discharged from volcanoes is effectively immersed back into magma at geological time scales due to active plate tectonics ^[14] .

Throughout history, the Earth’s climate has repeatedly changed between warm and ice ages. In the current climate, the gain coefficient of positive feedback from an increase in the amount of atmospheric water vapor, as well as the distance from the Earth to the Sun with its current brightness, is significantly lower than that required for the potential evaporation of the oceans ^[15] . Climate scientist John Hewton wrote that "at the moment there is no possibility of a repetition of the greenhouse effect of Venus on Earth" ^[16] . However, climatologist James Hansen does not agree with this view. In the book “ en: Storms of My Grandchildren ”, he says that burning coal and producing shale oil will lead to the uncontrolled greenhouse effect of the Earth ^[17] . Redefining the effect of water vapor in climate models in 2013 showed that the result of James Hansen could in principle be possible, but it requires ten times more CO ₂ than we could get from burning all the oil, coal and natural gas in the earth’s crust ^{[18 ]} . In addition, Benton and Twitchett use a different definition of an uncontrolled greenhouse effect ^[4] , events corresponding to this definition are a possible cause of the Paleocene-Eocene thermal maximum and the great extinction .

Far Future

Most scientists believe that an uncontrolled greenhouse effect is inevitable in the long run, as the sun becomes larger and brighter with time. This could potentially mean the end of all life on Earth. After about a billion years, the Sun will become 10% brighter, the temperature of the Earth’s surface will reach 47 ° C, which will lead to a rapid increase in the temperature of the Earth and its oceans to a boil, until it becomes a greenhouse planet similar to the current Venus.

According to the book of astrobiologists Peter Ward and Donald Brownley “Life and death of the planet Earth” ^[19] , now the ocean loss rate is about one millimeter per million years, but this rate will gradually accelerate as the temperature of the Sun rises, and may reach one millimeter for 1000 years. Ward and Brownley believe that there are two possible scenarios: a “wet greenhouse”, where water vapor prevails in the troposphere and begins to accumulate in the stratosphere, and “uncontrolled greenhouse”, where water vapor will become the main component of the atmosphere, the Earth will begin to experience sharp warming, it the surface will heat up to 900 ° C, as a result of which it will melt and destroy its entire life, possibly in about three billion years. In any case, the loss of the oceans will inevitably turn the Earth into a predominantly deserted world, with the only ponds remaining in the form of several vaporizing ponds near the poles, and with huge wastelands in place of what used to be the ocean floor, like the Atacama Desert in Chile or Badwater in Death Valley , where life can remain for several billion years. Because of this, in the latter case, the loss of the oceans will save the rest of life, and not destroy it completely. However, complex life, such as plants and animals, will die out long before this happens, because the loss of the oceans will stop plate tectonics; water is a lubricant for tectonic activity, and the loss of all water will make the earth's crust too hard and dry to undergo subduction , as a result of which the carbon cycle will stop completely (volcanoes supplying CO2 to the atmosphere will also cease to exist).

Usually, when the equilibrium of the planet’s radiation is disturbed (for example, by increasing the amount of sunlight it receives or by changing the concentration of greenhouse gases), it goes to a new temperature until the stabilizing feedback, known as the Stefan-Boltzmann reaction , regains equilibrium between the amount of energy absorbed and emitted by the planet. For example, if the Earth suddenly received more sunlight, this would lead to a temporary imbalance of radiation (more received than emitted) and, as a result, to warming. However, since the Stefan-Boltzmann law requires that a planet with a higher temperature emits more energy, a new radiation balance will ultimately be achieved, and the temperature will be maintained at a new, higher level.

However, when the planet has a positive feedback mechanism based on water vapor, the effectiveness of the greenhouse effect increases as the temperature rises. Therefore, the amount of radiation going into space increases more slowly than for a pure Stefan-Boltzmann emitter, which behaves like a completely black body . In the end, infrared absorption increases so much that the amount of energy released into space no longer depends on the surface temperature and asymptotically approaches the Combayashi-Ingersoll limit ^{[20] [21]} . If the amount of energy that a planet receives from a star (or from internal heat sources) exceeds this value, an equilibrium of radiation will never be achieved. The result is an uncontrolled process that continues until the feedback involving water vapor disappears, which can happen when the entire ocean evaporates and dissipates into space.

The concept of a habitable zone is used by planetologists and astrobiologists to determine the orbital region around a star in which a planet (or moon) can maintain surface water in a liquid state. In accordance with this definition, the inner edge of the habitable zone (that is, the closest point to the star at which the planet can still hold liquid water) is determined by the point at which an uncontrolled greenhouse effect begins to occur. For solar-type stars, this inner edge is estimated to be approximately 84% of the distance from the Earth to the Sun ^[22] , although other feedback effects, such as increased albedo due to powerful clouds, may slightly change this estimate.

• The atmosphere of Venus (an example of an uncontrolled greenhouse effect)
• Snow earth
• Heat