Types of Greenhouse Gases

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Greenhouse gases occur naturally in the environment and also result from human activities. By far the most abundant greenhouse gas is water vapor, which reaches the atmosphere through evaporation from oceans, lakes, and rivers.

Carbon dioxide is the next most abundant greenhouse gas. It flows into the atmosphere from many natural processes, such as volcanic eruptions; the respiration of animals, which breathe in oxygen and exhale carbon dioxide; and the burning or decay of organic matter, such as plants. Carbon dioxide leaves the atmosphere when it is absorbed into ocean water and through the photosynthesis of plants, especially trees. Photosynthesis breaks up carbon dioxide, releasing oxygen into the atmosphere and incorporating the carbon into new plant tissue.

Humans escalate the amount of carbon dioxide released to the atmosphere when they burn fossil fuels, solid wastes, and wood and wood products to heat buildings, drive vehicles, and generate electricity. At the same time, the number of trees available to absorb carbon dioxide through photosynthesis has been greatly reduced by deforestation, the long-term destruction of forests by indiscriminate cutting of trees for lumber or to clear land for agricultural activities.

Ultimately, the oceans and other natural processes absorb excess carbon dioxide in the atmosphere. However, human activities have caused carbon dioxide to be released to the atmosphere at rates much faster than that at which Earth’s natural processes can cycle this gas.

Methane is an even more effective insulator, trapping over 20 times more heat than does the same amount of carbon dioxide. Methane is emitted during the production and transport of coal, natural gas, and oil. Methane also comes from rotting organic waste in landfills, and it is released from certain animals, especially cows, as a byproduct of digestion.

Nitrous oxide is a powerful insulating gas released primarily by burning fossil fuels and by plowing farm soils. Nitrous oxide traps about 300 times more heat than does the same amount of carbon dioxide. The concentration of nitrous oxide in the atmosphere has increased 17 percent over preindustrial levels.

In addition, greenhouse gases are produced in many manufacturing processes. Perfluorinated compounds result from the smelting of aluminum. Hydrofluorocarbons form during the manufacture of many products, including the foams used in insulation, furniture, and car seats. Refrigerators built in some developing nations still use chlorofluorocarbons as coolants. In addition to their ability to retain atmospheric heat, some of these synthetic chemicals also destroy Earth’s high-altitude ozone layer, the protective layer of gases that shields Earth from damaging ultraviolet radiation. For most of the 20th century these chemicals have been accumulating in the atmosphere at unprecedented rates.

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