Nitrous Oxide is a non-toxic gas consisting of one oxygen and two nitrogen atoms (N2O). The main sources of nitrous oxide are natural processes in soils and the ocean, chemical fertilizers, and deforestation. Ice core data suggests that the atmospheric concentration of N2O was approximately constant before the Industrial Revolution started. The annual increase in the atmospheric concentration of N2O is estimated to be about 0.3 percent, according to the IPCC.

Chlorofluorocarbons (CFCs) are non-toxic gases composed primarily of carbon, hydrogen, chlorine, and fluorine. They are used mainly as cleaning agents, refrigerants, fire retardants, and in aerosol sprays. Since they are chemically inert in the troposphere, they have a long residence time, and reach the stratosphere where they are photodissociated by ultraviolet radiation. Then, free chlorine atoms destroy ozone catalytically. Thus, besides being important greenhouse gases, CFCs are responsible for destroying the ozone layer that protects humans from harmful ultraviolet radiation.

Methane is an odorless, non-toxic, flammable gas consisting of one carbon and four hydrogen atoms (CH4). Mainly anaerobic or oxygen-deprived processes produce methane. It forms when the digestion of organic material by bacteria releases single carbon atoms. The main sources of methane are wetlands, the combustion of fossil fuels, animals, rice plantations, biomass burning, landfills, termites, and the oceans. Methane is removed from the atmosphere primarily by reaction with hydroxyl radicals (OH). The atmospheric concentration of methane has also been steadily increasing since the beginning of the Industrial Revolution. The atmospheric concentration of CH4 has been increasing at the rate of about 1 percent per year.

Ozone (O3) is formed via the dissociation of molecular oxygen by ultraviolet radiation. It is the most photochemically active greenhouse gas. Ozone is an important greenhouse gas because it strongly absorbs ultraviolet radiation. About 90 percent of the atmospheric ozone is found on the stratospheric ozone layer. However, the ozone concentration in very polluted areas can exceed the stratospheric levels.

The concentration of water vapor increases exponentially with temperature. This fact and the presence of water reservoirs such as lakes and oceans make the opacity of the atmosphere a strong function of surface temperature. Thus, global increases in temperature (global warming) produce a positive feedback in the climate system. Increases in the surface temperature causes increases in the amount of atmospheric water vapor that produce further increases in the surface temperature. This positive feedback makes the greenhouse effect increase dramatically with temperature. Above a critical value of the solar forcing, the atmospheric water vapor feedback becomes so large that equilibrium is not possible and a runaway greenhouse occurs. In fact, there is evidence that the dependence of the concentration of water vapor on temperature might lead to multiple climate equilibria.

See ALSo: Carbon Dioxide; Climate Feedbacks; Greenhouse Effect; Methane Cycle; Nitrous Oxide.

BIBLIogRAPHY. A.P. Ingersoll, "The Runaway Greenhouse: A History of Water on Venus," Journal of Atmospheric Science (v.26, 1969); Intergovernmental Panel on Climate Change, Climate Change 2007: The Physical Science Basis (Cambridge University Press, 2007); M. Komabayashi, "Discrete Equilibrium Temperatures of a Hypothetical Planet with the Atmosphere and Hydrosphere of one Component Two-Phase System under Constant Solar Radiation," Journal of the Meteorological Society of Japan (v.45, 1969); N.O. Renno, "Multiple-Equilibria in Radiative-Convective Atmospheres," Tellus (v.49, 1997); G.C. Simpson, "Some Studies in Terrestrial Radiation," Royal Meteorological Society (v.16, 1927).

Nilton O. Renno University of Michigan

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