Greenhouse effect

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The Greenhouse Effect (or "atmospheric effect") is a general attribute of planets and moons with atmospheres. It is an imbalance between surface radiation and top-of-atmosphere radiation due to the presence of greenhouse gases. For example, in the case of the Earth, the surface emits 390 W/m² (averaged over a year and the whole surface), but the emission at the top of the atmosphere is 235 W/m², giving a global-average greenhouse effect of 155 W/m². The top-of-atmosphere outgoing radiation balances the incoming 235 W/m² of solar radiation (342 W/m² incident minus 107 W/m² reflected). The term "greenhouse effect" is something of a misnomer, since actual greenhouses operate by a different mechanism.

The Physics of the Greenhouse

The essential condition for a greenhouse effect is the presence in a planetary atmosphere of gases that absorb (and emit) in the thermal radiation band of the planetary surface, or lower atmospheric levels. Frequently, the greenhouse gases will be transparent, or nearly so, to incoming solar radiation (ozone is an exception). The surface thermal radiation band will be in the long-wave infrared region (3.5 µm - 100 µm). The greenhouse gases are triatomic (or more, but note the exception for Titan) molecules which absorb energy into a variety of rotational, bending, and stretching modes. Common solar-system gases that meet the requirements are water vapor (H₂O), carbon dioxide (CO₂), ozone (O₃), and methane (CH₄). For Earth, H₂O, CO₂, and O₃ provide most of the greenhouse. Common diatomic gases, such as nitrogen (N₂) and oxygen (O₂), do not have a strong enough dipole to interact well with infrared.

External Links

• [1]Astrobiology article on Titan's very unusual greenhouse.

Additional Bibliography

• Houghton, JT, 19977. The Physics of Atmospheres, 3rd Ed, Cambridge Univ. Press.
• Thomas, GE, and K Stamnes, 1999. Radiative Transfer in the Atmosphere and Ocean, Cambridge Univ. Press.