Methane Emission Sources

About 70% of current methane emissions are anthropogenic in origin. The manner in which total methane emissions rose over the last century is illustrated by the black line in Figure 6-15. As in the case of carbon dioxide, post-World War II rates increased annually much more quickly than had been the case before. In the last 20 years, however, the emission rate for methane has leveled off (Figure 6-15).

Most of the methane produced from plant decay results from the process of anaerobic decomposition, which is decomposition of formerly living matter in the absence of air, i.e., under oxygen-starved conditions. This process converts cellulose (approximate empirical formula chzo) into methane and carbon dioxide:

2 c:h2o —»—* ch4 + co2

Anaerobic decomposition occurs on a huge scale where plant decay occurs under waterlogged conditions, e.g., in natural wetlands such as swamps and bogs and in rice paddies. Indeed, the original names for methane were swamp gas and marsh gas. Wetlands are the largest natural source of methane emissions, though emissions from this source have decreased sharply over the past centuries as wetlands have been drained. The huge increase in rice production over the same period has presumably led to correspondingly large increases in methane emissions from this source.

The expansion of wetlands that occurs by the deliberate flooding of land to produce more hydroelectric power adds to the total natural emissions of the gas. Deep, small reservoirs produce and emit much less methane than do shallow ones that contain large volumes of flooded biomass, such as those in the Brazilian Amazon, especially if the trees are not first removed. Indeed, the combined global warming effect of the methane and carbon dioxide produced by a large, shallow reservoir created to generate hydroelectric power can, for many years, exceed that of the carbon dioxide that would have been emitted if a coal-fired power plant were used to generate the same amount of electrical power! Hydroelectric power is not a zero-emission form of energy production if land is flooded to create it.

The anaerobic decomposition of the organic matter in garbage in landfills is another important source of methane in air. Food waste in the landfill produces the greatest amount of methane. In some communities, methane from landfills is collected and burned to generate heat, rather than being allowed to escape into the air. Although the combustion of methane produces an equal number of molecules of carbon dioxide, because the per molecule effect of COz molecules is so much smaller than that of CH4 molecules, the net greenhouse effect of the emission is thereby greatly reduced in relation to the amount of C02 absorbed from the air when the plant matter was growing.

The burning of biomass, such as forests and grasslands in tropical and semitropical areas, releases methane to the extent of about 1% of the carbon consumed, along with larger amounts of carbon monoxide (both compounds are products of incomplete—poorly ventilated—combustion).

Ruminant animals—including cattle, sheep, and certain wild animals— produce huge amounts of methane as a by-product in their stomachs when they digest the cellulose in their food. The animals subsequently emit the methane into the air by belching or flatulence. The decrease in the population of some methane-emitting wild animals (e.g., buffalo) in recent centuries has been far exceeded by the huge increase in the population of cattle and sheep. The net result has been a large increase in emissions of methane from animal sources.

It was reported by researchers in 2006 that plants, especially those growing in tropical areas, emit methane into the air as part of their aerobic metabolism, not just through the action of bacteria in anaerobic environments. The rate of emission of methane increases sharply with air temperature, approximately doubling for a 10° rise. If aerobic methane release from plants does occur, some of the observed decrease in global methane emission rates in the 1990s may have occurred because extensive tropical deforestation during that period would have greatly reduced the number of methane-emitting plants. Ironically, tropical forests cleared for livestock production may have produced as much methane as the ruminants that are now raised on that land! However, research reported by other scientists in 2007 failed to confirm that methane is produced aerobically and emitted by living plants.

Methane is released into air when natural gas pipelines leak, when coal is mined and the CH4 trapped within it is released into the air, and when the

BOX 6-3

Continue reading here: Determining the Emissions of Old Carbon Sources of Methane

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