Methane is the most abundant organic chemical in Earth's atmosphere, and its concentration is increasing with time, as a variety of independent measurements have shown. Photochemical reactions oxidize methane in the atmosphere; through these reactions, methane exerts strong influence over the chemistry of the troposphere and the stratosphere and many species including ozone, hydroxyl radicals, and carbon monoxide. Also, through its infrared absorption spectrum, methane is an important greenhouse gas in the climate system. We describe and enumerate key roles and reactions. Then we focus on two kinds of methane production: microbial and thermogenic. Microbial methanogenesis is described, and key organisms and substrates are identified along with their properties and habitats. Microbial methane oxidation limits the release of methane from certain methanogenic areas. Both aerobic and anaerobic oxidation are described here along with methods to measure rates of methane production and oxidation experimentally. Indicators of the origin of methane, including C and H isotopes, are reviewed. We identify and evaluate several constraints on the budget of atmospheric methane, its sources, sinks and residence time. From these constraints and other data on sources and sinks we construct a list of sources and sinks, identities, and sizes. The quasi-steady state (defined in the text) annual source (or sink) totals about 310(±60) × 1012 mol (500(±95) × 1012 g), but there are many remaining uncertainties in source and sink sizes and several types of data that could lead to stronger constraints and revised estimates in the future. It is particularly difficult to identify enough sources of radiocarbon-free methane.

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Biogeochemical aspects of atmospheric methane

The geographic and seasonal emission distributions of the major sources and sinks of atmospheric methane were compiled using methane flux measurements and energy and agricultural statistics in conjunction with global digital data bases of land surface characteristics and anthropogenic activities. Chemical destruction of methane in the atmosphere was calculated using three-dimensional OH fields every 5 days taken from Spivakovsky et al. (1990a, b). The signatures of each of the sources and sinks in the atmosphere were simulated using a global three-dimensional tracer transport model. Candidate methane budget scenarios were constructed according to mass balance of methane and its carbon isotopes. The verisimilitude of the scenarios was tested by their ability to reproduce the meridional gradient and seasonal variations of methane observed in the atmosphere. Constraints imposed by all the atmospheric observations are satisfied simultaneously by several budget scenarios. A preferred budget comprises annual destruction rates of 450 Tg by OH oxidation and 10 Tg by soil absorption and annual emissions of 80 Tg from fossil sources, 80 Tg from domestic animals, and 35 Tg from wetlands and tundra poleward of 50°N. Emissions from landfills, tropical swamps, rice fields, biomass burning, and termites total 295 Tg; however, the individual contributions of these terms cannot be determined uniquely because of the lack of measurements of direct fluxes and of atmospheric methane variations in regions where these sources are concentrated.

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Three‐dimensional model synthesis of the global methane cycle

Environmental Impacts of Solid Waste Landfilling

Emission rates of CH4 from four Japanese paddy fields were measured throughout the cultivation period in 1988 by using the closed chamber method. Large seasonal variations of the CH4 flux were observed. The emission was closely related to the decrease of the redox potential (Eh) in paddy soils. Drainage and supplementary application of mineral fertilizer substantially reduced the CH4 emission. Emission rates of CH4 differed markedly with the soil types. The highest rate was observed in a paddy field consisting of Peat soil (44.8 g-CH4/m2 during a cultivation period), followed by Gley soil (8.0-27.0). The emission rates in the Andosols were significantly lower (0.6-12.6). Application of rice straw at a rate of 6-9 t/ha to the paddy fields increased the CH4 emission rates 1.8- to 3.5-fold. Application of compost slightly increased the CH4 emission. Annual emission rates of CH4 from individual plots were positively correlated with the contents of readily mineralizable carbon (RMC) in paddy soils col...

https://www.tandfonline.com/doi/pdf/10.1080/00380768.1990.10416797

Effect of Organic Matter Application on Methane Emission from Some Japanese Paddy Fields

This paper addresses three related questions: (1) What factors control the efficiency of carbon burial in sediments? (2) Are rates of anaerobic organic matter degradation intrinsically lower than aerobic rates? (3) How important are anaerobic processes in the global marine sediment carbon economy? Carbon burial efficiency (the ratio of the carbon burial rate and the carbon flux to the sediment surface) was estimated from literature data for a range of environments and was shown to be a function of sedimentation rate. No difference independent of sedimentation rate was found between aerobic and anaerobic sediments. A review of recent microcosm and laboratory studies shows that anaerobic rates are not intrinsically lower than aerobic rates; fresh organic matter degrades at similar rates under oxic and anoxic conditions. Aerobic decomposition rates near the sediment surface are typically greater than anaerobic rates at depth because the most labile carbon is consumed before it can be buried in the a...

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Anaerobic mineralization of marine sediment organic matter: Rates and the role of anaerobic processes in the oceanic carbon economy

On the basis of 17 ecosystems, it is estimated that 9.1×1014 g CH4/year are emitted into the atmosphere from the biosphere. Enteric fermentation in animals and humans, decomposition of organic wastes, and biomass burning contribute an additional 2.0×1014 g CH4/yr. Various fossil sources emit another 1×1014 g CH4/yr. When all sources are considered, they emit 12.1×1014 g CH4 each year. As with earlier inventories, this study indicates that the fossil methane contribution is less than 10% of the total annual global production rate. Chemical kinetic relationships are established between the bacteriamediated anaerobic decomposition of humic matter, the mean residence time (MRT) of humus, and methane fluxes. These equations and the 14C specific activity are used to obtain an average MRT of 1365 years for the earth's 1.8×1018 grams of humic carbon. Use of the global methane production rate and the concentration of atmospheric methane results in an average 3.3 year residence time and an average global hydroxyl radical concentration of 2.7×106 per cm3.

J. C. Sheppard

Papers

Inventory of global methane sources and their production rates