Greenhouse Effect

Definition

The warming of a planet by the “greenhouse effect” is caused by atmospheric gases that are very efficient absorbers in the infrared but not in the visible. Symmetric, nonpolar molecules such as H₂, N₂, and O₂have no electric dipole and cannot absorb or emit photons. They are therefore not greenhouse gases. The visible light of the parent star reaches and heats the planetary surface and is reemitted in the infrared, where part of the energy is absorbed by the atmospheric greenhouse gases with some radiated back toward the surface. Water, carbon dioxide, and methane are the greenhouse gases that raise the surface temperature on Earth to an average of +15 °C instead of −17 °C. The “runaway greenhouse” occurs when greenhouse conditions vaporize part of a water reservoir, the resulting water vapor increases the greenhouse effect, and this feedback cycle leads to loss of the entire surface water and, as a secondary effect, to the photodissociation of the atmospheric water vapor...