Boreal and subarctic peatlands comprise a carbon pool of 455 Pg that has accumulated during the postglacial period at an average net rate of 0.096 Pg/yr (1 Pg = 1015g). Using Clymo's (1984) model, the current rate is estimated at 0.076 Pg/yr. Longterm drainage of these peatlands is estimated to be causing the oxidation to CO2 of a little more than 0.0085 Pg/yr, with conbustion of fuel peat adding °0.026 Pg/yr. Emissions of CH4 are estimated to release ° 0.046 Pg of carbon annually. Uncertainties beset estimates of both stocks and fluxes, particularly with regard to Soviet peatlands. The influence of water table alterations upon fluxes of both CO2 and CH4 is in great need of investigation over a wide range of peatland environments, especially in regions where permafrost melting, thermokarst erosion, and the development of thaw lakes are likely results of climatic warming. The role of fire in the carbon cycle of peatlands also deserves increased attention. Finally, satellite—monitoring of the abundance of open water in the peatlands of the West Siberian Plain and the Hudson/James Bay Lowland is suggested as a likely method of detecting early effects of climatic warming upon boreal and subarctic peatlands.

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Northern Peatlands: Role in the Carbon Cycle and Probable Responses to Climatic Warming.

This review shows that thermodynamic and kinetic constraints largely prevent large-scale methanogenesis in the open ocean water column. One example of open-ocean methanogenesis involves anoxic digestive tracts and fecal pellet microenvironments; methane released during fecal pellet disaggregation results in the mixed-layer methane maximum. However, the bulk of the methane in the ocean is added by coastal runoff, seeps, hydrothermal vents, decomposing hydrates, and mud volcanoes. Since methane is present in the open ocean at nanomolar concentrations, and since the flux to the atmosphere is small, the ultimate fate of ocean methane additions must be oxidation within the ocean. As indicated in the Introduction and highlighted in Table 3, sources of methane to the ocean water column are poorly quantified. There are only a small number of direct water column methane oxidation rates, so sinks are also poorly quantified. We know that methane oxidation rates are sensitive to ambient methane concentrations, but we have no information on reaction kinetics and only one report of the effect of pressure on methane oxidation. Our perspective on methane sources and the extent of methane oxidation has been changed dramatically by new techniques involving gene probes, determination of isotopically depleted biomarkers, and recent 14C-CH4 measurements showing that methane geochemistry in anoxic basins is dominated by seeps providing fossil methane. The role of anaerobic oxidation of methane has changed from a controversial curiosity to a major sink in anoxic basins and sediments. © 2007 American Chemical Society.

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Oceanic methane biogeochemistry.

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

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Atmospheric methane and global change

Previous studies on ice core analyses and recent in situ measurements have shown that CH 4 has increased from about 075–173 μmol/mol during the past 150 years Here, we review sources and sink estimates and we present global 3D model calculations, showing that the main features of the global CH 4 distribution are well represented The model has been used to derive the total CH 4 emission source, being about 600 Tg yr -1  Based on published results of isotope measurements the total contribution of fossil fuel related CH 4 emissions has been estimated to be about 110 Tg yr -1  However, the individual coal, natural gas and oil associated CH 4 emissions can not be accurately quantified In particular natural gas and oil associated emissions remain speculative Since the total anthropogenic CH 4 source is about 410 Tg yr -1 (∼70% of the total source) and the mean recent atmospheric CH 4 increase is ∼20 Tg yr -1 an anthropogenic source reduction of 5% could stabilize the atmospheric CH 4 level We have calculated the indirect chemical effects of increasing CH 4 on climate forcing on the basis of global 3D chemistry-transport and radiative transfer calculations These indicate an enhancement of the direct radiative effect by about 30%, in agreement with previous work The contribution of CH 4 (direct and indirect effects) to climate forcing during the past 150 years is 057W m −2 (direct 044W m −2 , indirect 013 W m −2 ) This is about 35% of the climate forcing by CO 2 (16W m −2 ) and about 22% of the forcing by all long-lived greenhouse gases (26 W m −2 ) Scenario calculations (IPCC-IS92a) indicate that the CH 4 lifetime in the atmosphere increased by about 25–30%during the past 150 years to a current value of 79 years Future lifetime changes are expected to be much smaller, about 6%, mostly due to the expected increase of tropospheric O 3 (→OH) in the tropics The global mean concentration of CH 4 may increase to about 255 μmol/mol, its lifetime is expected to increase to 84 years in the year 2050 Further, we have calculated a CH 4 global warming potential (GWP) of 21 (kgCH 4 /kgCO 2 ) over a time horizon of 100 years, in agreement with IPCC (1996) Scenario calculations indicate that the importance of the climate forcing by CH 4 (including indirect effects) relative to that of CO 2 will decrease in future; currently this is about 35%, while this is expected to decrease to about 15% in the year 2050 DOI: 101034/j1600-08891998t01-1-00002x

Changing concentration, lifetime and climate forcing of atmospheric methane

Measurements of carbon-14 in small samples of methane from major biogenic sources, from biomass burning, and in "clean air" samples from both the Northern and Southern hemispheres reveal that methane from ruminants contains contemporary carbon, whereas that from wetlands, pat bogs, rice fields, and tundra is somewhat, depleted in carbon-14. Atmospheric (14)GH(4) seems to have increased from 1986 to 1987, and levels at the end of 1987 were 123.3 +/- 0.8 percent modern carbon (pMC) in the Northern Hemisphere and 120.0 +/- 0.7 pMC in the Southern Hemisphere. Model calculations of source partitioning based on the carbon-14 data, CH(4) concentrations, and delta(13)C in CH(4) indicate that 21 +/- 3% of atmospheric CH(4) was derived from fossil carbon at the end of 1987. The data also indicate that pressurized water reactors are an increasingly important source of (14)CH(4).

https://science.sciencemag.org/content/sci/245/4915/286.full.pdf

B. Deck

Papers

Carbon-14 in methane sources and in atmospheric methane: the contribution from fossil carbon.