Methane

The primary sources of methane in the atmosphere are agriculture and the use of fossil fuels. The main natural sources of methane are from decomposition of organic matter in wetlands, rice paddies, and bogs. Termites are also a source of methane. Methane was once called "swamp gas" and "marsh gas" because it was commonly observed to evolve from wetland areas filled with decaying organic material such as leaves and other vegetation.

Methane is a by-product of the digestion of farm animals such as cows and pigs (in a process called enteric fermentation). The amount of methane depends on the diet of the animals and how the manure they produce is handled. The tendency toward larger farms favors production methods that release more methane, although the opportunity is also greater to contain it.

Methane is a main component of natural gas (the other component being ethane). Leaky natural gas pipelines inadvertently release methane, which is also released during the process of extracting natural gas or petroleum from the ground.

One molecule of methane absorbs 20 times the amount of sun energy of a molecule of carbon dioxide. However, methane is now at a level of 1774 parts per billion (ppb) in the atmosphere, which is 165 times less prevalent than carbon dioxide. (As Austin Powers might say, this is parts per billions, not millions, which in the case of concentration is much less.) For this reason, methane has less than a third of the overall impact on climate that carbon dioxide has. Studies of the relative proportion of carbon isotopes in the atmosphere indicate that the human-contributed level of methane is well above the natural level. Methane has increased from fossil fuel use, but emissions from agriculture have been stable. In contrast to carbon dioxide, the growth rate of methane emissions overall actually has declined since the 1990s. Figure 6-4 shows the increase of this greenhouse component.

The main methane sink is a photochemical process that removes methane from the atmosphere. The molecules involved are water vapor, ozone, and oxygen

Concentrations of Greenhouse Gases from 0 to 2005

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Carbon dioxide (CO2) Methane (CH4) Nitrous oxide (N2O)

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Carbon dioxide (CO2) Methane (CH4) Nitrous oxide (N2O)

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Figure 6-4 Concentration of three greenhouse gases: carbon dioxide, methane, and nitrogen dioxide. Current levels of all three are unprecedented for the past 2000 years.

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Figure 6-4 Concentration of three greenhouse gases: carbon dioxide, methane, and nitrogen dioxide. Current levels of all three are unprecedented for the past 2000 years.

interacting in the presence of light to destroy the methane molecule. Together they form a piece of a molecule called a hydroxyl radical (OH-) that reacts with the methane and whose presence in the atmosphere serves as an indicator of how long methane will persist. The average lifetime for methane in the atmosphere is about 12 years, which is much less than carbon dioxide.

Methane clathrate is a slushy semifrozen mix of methane gas and ice typically found in the northern hemisphere's tundra permafrost regions and in sediment layers on the ocean floor. Some scientists have speculated that a sudden release of large amounts of methane from deposited methane clathrate called the, clathrate gun theory, might be a cause of abrupt past and possibly future climate changes. However, one study of ice core samples from Greenland performed by Dr. E. Brook of Oregon State University did not find a sudden increase in methane levels in past atmospheres that might have accounted for precipitous historic temperature increases.

More than a third of the nitrous oxide put in the atmosphere by humans comes from the use of nitrogen-based fertilizers in agriculture. Nitrous oxide is the well-known dental anesthetic known as "laughing gas." Some nitrous oxide is formed as a result of biomass burning and certain industrial processes such as nylon production.

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