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Tuesday, November 18, 2008

Anti-greenhouse effect

The anti-greenhouse effect is a neologism used to describe two different effects, coming under the header of "the cooling effect an atmosphere has on the ambient temperature of the planet." Unlike the greenhouse effect, which is common, an anti-greenhouse effect is only known to exist in one situation in the Solar System, as well as another which is a misnomer.

The conventional greenhouse effect occurs because the atmosphere is largely transparent to solar radiation, but largely opaque to infrared. In an anti-greenhouse effect, this situation is reversed (i.e. the atmosphere is opaque to solar, but lets out infrared).


The haze containing organic molecules in Titan's upper atmosphere absorb 90% of the solar radiation reaching Titan, but is inefficient at trapping infrared radiation generated by the surface. Although a large greenhouse effect does keep Titan at a much higher temperature than the thermal equilibrium, Titan also exhibits an "anti-greenhouse" effect, which partially compensates for the greenhouse warming, and keeps the surface approximately 9°C (16°F) cooler than would otherwise be expected from the greenhouse effect alone. According to McKay et al., "The anti-greenhouse effect on Titan reduces the surface temperature by 9 K whereas the greenhouse effect increases it by 21 K. The net effect is that the surface temperature (94 K) is 12 K warmer than the effective temperature 82 K. [i.e., the equilibrium that would be reached in the absence of any atmosphere]"

In addition, this effect results in a permanently inverted thermocline on Titan with atmospheric temperatures increasing with increasing altitude above the tropopause. This type of anti-greenhouse effect is only known to occur on Titan, however it is similar to the cooling effects suggested for nuclear winter.


A different mechanism exists on Pluto, which is not a true anti-greenhouse effect. Sunlight striking the nitrogen ice on the surface of dwarf planet Pluto causes it to sublimate; this causes the temperature of Pluto to be about 10°C (20°F) lower than its moon Charon. The sublimation causes cooling, and is analogous to solar radiation evaporating water on Earth; however when this occurs on Earth it is not called an anti-greenhouse effect. This effect was discovered using the Submillimeter Array in Hawaii.

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