Anti-greenhouse effect
The anti-greenhouse effect is a process that occurs when energy from a celestial object's sun is absorbed or scattered by the object's upper atmosphere, preventing that energy from reaching the surface, which results in surface cooling – the opposite of the greenhouse effect. In an ideal case where the upper atmosphere absorbs all sunlight and is nearly transparent to infrared energy from the surface, the surface temperature would be reduced by 16%, which is a significant amount of cooling.
This effect has been discovered to exist on Saturn's moon Titan. In Titan's stratosphere, a haze composed of organic aerosol particles simultaneously absorbs solar radiation and is nearly transparent to infrared energy from Titan's surface. This acts to reduce solar energy reaching the surface and lets infrared energy escape, cooling Titan's surface. Titan has both a greenhouse and an anti-greenhouse effect which compete with one another. The greenhouse effect warms Titan by 21 K while the anti-greenhouse effect cools Titan by 9 K, so the net warming is 12 K.
It has been suggested that Earth potentially had a similar haze in the Archean eon, causing an anti-greenhouse effect. It is theorized that this haze helped to regulate and stabilize early Earth's climate. Other atmospheric phenomena besides organic hazes act similarly to the anti-greenhouse effect, such as Earth's stratospheric ozone layer and thermosphere, particles formed and emitted from volcanoes, nuclear fallout, and dust in Mars's upper atmosphere.
Outside of the Solar System, calculations of the impact of these hazes on the thermal structure of exoplanets have been conducted.
Energy balance theory
Energy balance
To understand how the anti-greenhouse effect impacts a planet or large moon with its host star as an external source of energy, an energy budget can be calculated, similar to how it is done for Earth. For each component in the system, incoming energy needs to equal outgoing energy to uphold the conservation of energy and remain at a constant temperature. If one energy contributor is larger than the other, there is an energy imbalance and the temperature of an object will change to reestablish a balance. Energy sources across the whole electromagnetic spectrum need to be accounted for when calculating the energy balance. In the case of Earth, for example, a balance is struck between incoming shortwave radiation from the Sun and outgoing longwave radiation from the surface and the atmosphere. After establishing a component's energy balance, a temperature can be derived.Ideal anti-greenhouse effect
In the most extreme case, suppose that a planet's upper atmosphere contained a haze that absorbed all sunlight which was not reflected back to space, but at the same time was nearly transparent to infrared longwave radiation. By Kirchhoff's law, since the haze is not a good absorber of infrared radiation, the haze will also not be a good emitter of infrared radiation and will emit a small amount in this part of the spectrum both out to space and towards the planet's surface. By the Stefan–Boltzmann law, the planet emits energy directly proportional to the fourth power of surface temperature. At the surface, the energy balance is as follows,where is the Stefan–Boltzmann constant, is the surface temperature, and is the outgoing longwave radiation from the haze in the upper atmosphere. Since the haze is not a good absorber of this longwave radiation, it can be assumed to all pass throughout to space. The incoming solar energy must be scaled down to account for the amount of energy that is lost by being reflected to space since it is not within the planet-atmosphere system. In the upper atmosphere, the energy balance is as follows,
where is the incoming solar energy flux, is planetary albedo, and is the effective mean radiating temperature. The incoming solar flux is divided by four to account for time and spatial averaging over the entire planet and the factor is the fraction of the solar energy that is absorbed by the haze. Replacing with in the second equation, we have,
and the ratio equals or 0.84. This means that the surface temperature is reduced from the effective mean radiating temperature by 16%, which is a potentially significant cooling effect. This is an ideal case and represents the maximum impact the anti-greenhouse effect can have and will not be the impact for a real planet or large moon.
Outdated concept of anti-greenhouse effect
Earlier discussions in the scientific community pre-dating the current definition established by Dr. Christopher McKay in 1991 referred to the anti-greenhouse effect as a precursor to the Late Precambrian glaciation, describing it more as a carbon sequestration process. This is no longer the current usage of the term, which emphasizes surface cooling due to high-altitude absorption of solar radiation.Comparison to negative greenhouse effect
The negative greenhouse effect is a phenomenon that can produce localized, rather than planetary, cooling. Whereas the anti-greenhouse effect involves an overall temperature inversion in the stratosphere, the negative greenhouse effect involves a localized temperature inversion in the troposphere. Both effects increase outgoing thermal emissions—locally in the case of a negative greenhouse effect and globally in the case of the anti-greenhouse effect.On Titan
The organic haze in Titan's stratosphere absorbs 90% of the solar radiation reaching Titan, but is inefficient at trapping infrared radiation generated by the surface. This is due to Titan's atmospheric window occurring from roughly 16.5 to 25 micrometers. Although a large greenhouse effect does keep Titan at a much higher temperature than the thermal equilibrium, the anti-greenhouse effect due to the haze reduces the surface temperature by 9 K. Because the greenhouse effect due to other atmospheric components increases it by 21 K, the net effect is that the real surface temperature of Titan is 12 K warmer than the effective temperature 82 K. In the ideal anti-greenhouse case described above, the maximum impact of the organic haze on Titan is 82 K = 13 K. This is higher than the 9 K found on Titan.The organic haze is formed through the polymerization of methane photolysis products and nitriles, meaning the products combine into longer chains and bigger molecules. These methane-derived polymers can be made of polycyclic aromatic hydrocarbons and polyacetylene. The distribution of these polymers is not vertically uniform in Titan's atmosphere, however. The nitrile and polyacetylene polymers are formed in the upper atmosphere while the PAH polymers are created in the stratosphere. These polymers then aggregate to form haze particles. The opacity to sunlight of this organic haze on Titan is determined primarily by the haze production rate. If haze production increases, opacity of the haze increases, resulting in more cooling of the surface temperature. Additionally, the presence of this organic haze is the cause of the temperature inversion in Titan's stratosphere.