The major gases in the atmosphere, nitrogen and oxygen, are transparent to both the radiation incoming from the sun and the radiation outgoing from the Earth, so they have little or no effect on the greenhouse warming. Greenhouse gases such as carbon dioxide, methane, and chlorofluorocarbons have been increasing steadily in the atmosphere. These gases block heat reflected from the EarthΓÇÖs surface into the atmosphere, preventing it from escaping into space, thus contributing to global warming and increasing the surface temperature of the Earth.
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The chart at right shows that average atmospheric carbon dioxide concentrations observed at Muana Loa, Hawaii increased approximately 40 parts per million by volume (ppmv) between 1958 and 1995. The small fluctuations in the curve are seasonal variations due primarily to the withdrawal and production of carbon dioxide by terrestrial life. Notice that minimum values occur during the northern hemisphere summers (when global photosynthetic activity is greatest) and maximum values occur six months later.
