Methane anthropogenic

With the addition of CO caused by photochemical oxidation of methane, a significant flux enters the atmosphere annually, but the principal global contributions are terrestrial, anthropogenic and from atmospheric photolysis of methane. 

Concerns over atmospheric methane as a greenhouse gas and the large contribution of biomethanogenesis as a source of this gas make it important to determine the relative significance of various components of this activity. A recent paper (8) summarized estimates (28-30) of source fluxes of atmospheric methane based on several carbon isotopic studies and presented new data on natural sources and biomass burning. These data (Table III) show that of a total flux of 594 million tons (Tg) per year, 83% is produced via biomethanogenesis from a combination of natural (42%) and anthropogenic (41%) sources. 

Over the last three decades, GHG emissions increased by an average of 1.6% per year, with C02 emissions from fossil-fuel use growing at 1.9% per year. Total GHG emissions in 2004 (Kyoto gases)8 amounted to 49.0 Gt C02-equivalent (C02-eq.), a 70% increase since 1970 and a 24% increase since 1990. Total C02 emissions in 2004 amounted to 26.1 Gt (see Fig. 2.7). Emissions of C02 have grown by approximately 80% since 1970 (28% since 1990) and represented 77% of total anthropogenic emissions in 2004. Total methane (CH4) emissions rose by about 40% from 1970 sectorally there was an 84% increase from combustion and the use of fossil fuels,... 

This is a very broad conclusion, and additional measurements must be made. Some of this effort (which is current) should address the problem of other pollutants and condensation nuclei that accompany the nonurban oxidant. Interpretation of these measurements will increase the specificity of separating anthropogenic sources from natural background sources. Theoretical assessments of the existing observations will shed light on the relative roles played by stratospheric injection, plant emission, background methane, and diy deposition on surfaces in the natural portion of the tropospheric ozone cycle. 

Methane is produced by bacteria under anaerobic conditions in wet environments such as wetlands, swamps and rice fields. It is also produced in the stomachs of cattle and by termites. Typical anthropogenic sources are from fossil fuels such as coal mining and as a byproduct in the burning of biomass. The latter sources are considerably heavier in C than the former. Recently, Keppler et al. (2006) demonstrated that methane is formed in terrestrial plants under oxic conditions by an unknown mechanism. The size of this methane source is stiU unknown but it might play an important role for the methane cycle. 

Methane extracted from air bubbles in polar ice up to 350 years in age has a 8 C-value which is 2%c lower than at present (Craig et al. 1988). This may indicate that anthropogenic burning of the Earth s biomass may be the principal cause of the recent C enrichment in methane. 

Values in Table 56.1 are expressed in teragrams per year. A teragram is equivalent to 1012 grams. Table 56.1 indicates that approximately two-thirds of methane comes from human sources, principally related to agriculture. Animal digestion (flatulence) is chiefly from livestock such as cattle. An adult cow emits 600 liters of methane each day. Fossil fuel sources include both combustion and processing of oil, natural gas, and coal. Landfills are the primary anthropogenic source of methane in the United States. 

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