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Sources of Methane
What are the sources of methane (Figure 10)? The 1992
Report of the Intergorvernmental Panel on Climate Change (IPCC) lists the largest
natural source of methane to be wetlands, which produce 115 teragrams (1012
grams) of carbon annually. The uncertainty in these numbers, however, is very
large. Termites are very significant producers of methane in that they eat wood
and release methane in the digestion process. The ocean produces about 10
teragrams per year of methane, and fresh water and methane hydrate contribute
smaller amounts.
Anthropogenic sources include the coal mining, natural gas, and petroleum industries at about 100 teragrams, which is almost as much as natural wetlands. Rice paddies produce on the order of 60 teragrams by means of a process where methane produced in the soil is able to travel up to the hollow stem of the rice plant and be released into the atmosphere without passing through the water, which would tend to suppress the evolution of methane gas (Figure 11).
Enteric fermentation, the digestion process in ruminant animals such as cattle, sheep and goats, produces very large amounts of methane. Animal wastes produce about 25 teragrams; domestic sewage, 25 teragrams; landfills about 30 teragrams; and biomass burning, about 40 teragrams. Some landfills are now being tapped for their methane as a source of power production. This makes good sense on the basis of global warming in addition to getting a "free" source of combustion gas. Burning one methane molecule produces one CO2 molecule, but the global warming potential is reduced by a factor of 20 because the carbon dioxide molecule is only about one-twentieth as effective as the methane molecule in absorbing infrared radiation.
Increases in animal populations are contributing to the increase in atmospheric methane. Figure 12 shows recent increases in several different classes of livestock. If humans continue to have an appetite for meat, the upward trend in animal production and resulting production of methane will likely continue. A particular situation to watch is the development and possible dietary changes in China. If we examine the eating habits of Japan, South Korea, and other Asian nations that have developed very rapidly, one of the significant changes that occurs during economic development is that people's eating habits change from eating primarily grains, mainly rice in these cases, to substantial increases in meat. The big question on the horizon right now is what's going to happen in China? China has an enormous population and it is developing extremely rapidly. If China follows the pattern of other Asian nations, the demand for meat will increase dramatically. I have estimated that if every person in China at a MacDonald's Quarter Pounder every 10 days, raising the beef to meet this demand would consume all the corn grown in Iowa.
