Methane is released into the atmosphere by a number of anthropogenic and natural sources, but anthropogenic emissions used to dominate present-day CH4 budgets, accounting for more than 60% of the total global budget. The anthropogenic sources include rice agriculture, livestock, landfills and waste treatment, some biomass burning, and fossil fuel combustion, while natural sources are wetlands, oceans, forests, fire, termites and geological sources. Estimates of global CH4 sources by various workers have been reflected in Table 16.1.

Cultivated wetland rice soils emit significant quantities of CH4 (Yan et al. 2003). Estimates of rice paddies, as a source of CH4, range from 20 to 100Tgyr-1 (Houghton et al. 1995), which is equivalent to 5-28% of the total CH4 from all anthropogenic sources.

Methane emission levels from a source can vary significantly from one country or region to another, depending on many factors, such as climate, industrial and agricultural production characteristics, energy types and usage, and waste management practices. For example, temperature and moisture have a significant effect on the anaerobic digestion process, which is one of the key biological processes that cause methane emissions in both human-related and natural sources. Also, the implementation of technologies to capture and utilize methane from sources, such as landfills, coal mines, and manure management systems, determine the emission levels from these sources.

Table 16.1 Estimates of the global CH4 sources (Tg CH4 yr ') from different references

Reference Base year

Fung et al. (1991)1980

Lelieveld et al. (1998)1992

Mosier et al. (1998)1994

Houweling et al. (1999)

Olivier et al. (1999)1990

Natural sources Wetlands Termites Oceans Hydrates

Anthro. Sources Energy Landfills Ruminants Waste treatment Rice agricultural

Biomass burning Others

Total sources

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