Greenhouse gases: Background Information

Greenhouse gases are any gases that absorb infra-red radiation. By trapping infra-red radiation they contribute to global warming through a process known as the greenhouse effect. To learn more about the effects of global warming visit the Effects of Global Warming unit.

1. Water Vapor (H₂O)

Water vapor is the largest contributor to the greenhouse effect. The amount of water vapor in the atmosphere is determined primarily by the water cycle rather than by human activity. Water evaporates from the surface. Eventually this water condenses and returns to the surface as precipitation. Once global warming begins to occur the amount of water vapor becomes subject to a positive feedback effect. The warmer the air gets the more water vapor it can hold and the more evaporation will occur.

2. Carbon Dioxide (CO₂)

Carbon dioxide is responsible for a majority (60%) of the greenhouse effect due to human activity. A small amount of carbon dioxide exists naturally in the atmosphere. Humans also produce a lot of carbon dioxide. The United States produces the most on a per capita basis (over 2 tons per person annually). Visit the American Forests web site if you want to know how much carbon dioxide you produce. To some extent the amount of carbon dioxide in the atmosphere is determined by the carbon cycle. The oceans and plants absorb some carbon dioxide, but human activity produces it faster than it can be absorbed. Carbon dioxide may last 50-200 years in the atmosphere.

Sources

Fossil Fuel combustion: The burning of fossil fuels (coal, oil, and natural gas) is the largest single source of greenhouse gases from human activity. Coal produces the most carbon dioxide of all the fossil fuels. Image by indy500.com Location: http://www.indy500.com/gallery/start/tn_0524980005_jpg.jpg
Deforestation: Deforestation is the second largest source of greenhouse gases due to human activity. Destruction of the forests can release the carbon stored in trees. If trees are not replanted they can not absorb carbon dioxide in the future.
Cement manufacturing: Carbon dioxide is produced during the manufacture of cement. When lime (CaCO₃), an ingredient in cement, is heated carbon dioxide is released into the air.

3. Methane (CH₄)

Methane, a hydrocarbon also known as natural gas, is used as a fuel in homes and industry.

Sources

Livestock: Livestock produce methane through the process of "enteric fermentation" of food in their digestive tract, and through their manure. Cattle are the greatest source of methane through these processes, followed by swine.
Agriculture: The main source of methane in agriculture is flooded rice paddies where microorganisms and bacteria decompose anaerobically in the soil.
Waste Dumps: The anaerobic decomposition of wastes in land fills and dumps results in methane. Sometimes the methane is collected and used as a fuel.
Coal Mining and Natural Gas production: Methane can leak when coal is mined. Sometimes it leaks or is deliberately vented during natural gas production.
Wetlands: The microorganisms and bacteria in wetlands create methane when they decompose anaerobically in the soil.

4. Other Greenhouse Gases

Nitrous Oxide (N₂O): This greenhouse gas enters the atmosphere from fertilizers used in agriculture.
Chlorofluorocarbons (CFC's): A very potent greenhouse gas. It is used as a propellant in aerosol cans, creating foam plastics, cooling refrigerators and air conditioners, as a solvent in cleaners, and as an ingredient in fire extinguisher materials. Because it destroys ozone, which is also a greenhouse gas, some of its contributions to the greenhouse effect are balanced out. Recently the levels of CFC's in the atmosphere have stabilized thanks to the Montreal Protocol of 1987 which restricts their use.
Ozone (O₃): This much publicized gas is known more for its ability to block harmful ultra-violet radiation than its ability to absorb infra-red rays, but is nonetheless a greenhouse gas. The amount of ozone is declining in the upper atmosphere but is found in increasing amounts near the earth's surface as a pollutant. You can learn more about ozone by visiting the Ozone Notepad .