"Methane is produced by anoxic degradation of organic matter, primarily in aquatic soil and sediment systems. Flooded ricefields occupy about 1.5 million km2 of the world's total arable land, and are projected to be a dominant source of atmospheric methane." H. Schutz, H. Rennenberg and W. Seiler, Fraunhofer-Institute for Atmospheric Environmental Research (IAER); A. Holzapfel-Pschorn, Max-Planck-Institute for Chemistry
Methane emission from Italian irrigated ricefields. International Rice Research Newsletter
ciesin.org/docs/004-017/004-017.html

"Atmospheric methane (CH4) is an integral component of the greenhouse effect, second only to CO2 as a contributor to anthropogenic greenhouse gas emissions. Methane’s overall contribution to global warming is significant because it is estimated to be 21 times more effective at trapping heat in the atmosphere than CO2 (i.e., the GWP value of methane is 21). Over the last two centuries, methane’s concentration in the atmosphere has more than doubled (IPCC 1996). Experts believe these atmospheric increases were due largely to increasing emissions from anthropogenic sources, such as landfills, natural gas and petroleum systems, agricultural activities, coal mining, stationary and mobile combustion, wastewater treatment, and certain industrial processes "
epa.gov/globalwarming/emissions/national/methane.html

epa.gov/globalwarming/emissions/national/methane.html
Distribution of methane sources in the US from epa.gov. America has a substantially different distribution of methane sources than other countries.

"By 2020, the world will need to produce 350 million tons more rice per year to feed an anticipated 3 billion more people than in 1992. Yet Neue (1993) recently identified rice field methane emissions as a major source of atmospheric methane." Columbia University.
ciesin.org/TG/AG/ricecult.html

"Public concern about global warming mostly focuses on carbon dioxide, the most prevalent greenhouse gas. Methane (CH4), the major component of natural gas, is second in importance as a greenhouse gas. Methane concentration in the atmosphere has more than doubled during the last 200 years. Its current atmospheric concentration of 1.7 ppm by volume, up from 0.7 ppm in preindustrial times, is much lower than the 345 ppm of carbon dioxide, up from 275 ppm. But one molecule of methane traps approximately 30 times as much heat as does carbon dioxide. The heating effect of the atmospheric methane increase is approximately half that of the carbon dioxide increase (Dickinson and Cicerone 1986, Ramanathan et al. 1985). Continued increase in atmospheric methane concentrations at the current rate of approximately 1% per year is likely to contribute more to future climatic change than any other gas except carbon dioxide (Cicerone and Oremland 1988)and may significantly contribute to a negative feedback system with unpredictable consequences for the whole chemistry of the atmosphere."
ciesin.org/docs/004-032/004-032.html

"The overall budget of atmospheric methane is fairly well established, but the strength of individual sources remains uncertain. Best estimates of sources and sinks from various reports are summarized in Table 1. Anthropogenic sources (340 Tg/yr) predominate over natural sources (160 Tg/yr), and 80% of the total methane emission is of modern biogenic origin. Only 20% is due to fossil carbon sources (Wahlen et al. 1989)."
ciesin.org/docs/004-032/004-032.html

"Wetland rice fields have recently been identified as a major source of atmospheric methane. Although the potential for methane release from rice fields has long been noted (Harrison and Aiyer 1913), the first comprehensive measurements of methane fluxes in rice fields were reported only in the early 1980s (Cicerone and Shetter 1981, Cicerone et al. 1983, Holzapfel-Pschorn et al. 1985, Seiler et al. 1984). As in a natural wetland, flooding a rice field cuts off the oxygen supply from the atmosphere to the soil, which results in anaerobic fermentation of soil organic matter. Methane is a major end product of anaerobic fermentation. It is released from submerged soils to the atmosphere by diffusion and ebullition and through roots and stems of rice plants. Recent global estimates of emission rates from wetland rice fields range from 20 to 100 Tg/yr (IPCC 1992), which corresponds to 6-29% of the total annual anthropogenic methane emission."
ciesin.org/docs/004-032/004-032.html

"Methane Emission from Rice Fields" Heinz-Ulrich Neue
ciesin.org/docs/004-032/004-032.html

"(8/21/2001) Survey Finds Less Than 130 Chinese Alligators Left in the Wild Due to Almost Total Loss of Wetlands Habitat to Rice Paddies."
eces.org/ec/extinction/reptiles.shtml#102201

Adapted from Bachelet and Neue 2002, Bouwman 1989, IPCC 1992, Khalil and Shearer 2002.

"Aside from being an important greenhouse gas, methane also affects the chemistry and oxidation capacity of the atmosphere, for example, by influencing concentrations of tropospheric ozone, hydroxyl radicals, and carbon monoxide. In the stratosphere, it is a sink for chlorine but a source for hydrogen and water vapor (ice crystals). The current burden of methane in the atmosphere is approximately 4700 Tg (1 Tg = 1 million tons; Wahlen et al. 1989), and the global annual emission is estimated to be 500 Tg with an apparent net flux of 40 Tg/yr (Cicerone and Oremland 1988)."
ciesin.org/docs/004-032/004-032.html

The chart for methane sources and sinks lists 420 Tg per year of methane reacting with atmospheric hydroxyl radicals which form water and methyl radicals. These may go on to enter negative feedback loops in catalytic ozone breakdown. The question then arises as to which effects would be worse, those of methane or those of methyl radicals in the atmosphere. Pick your poison.

Click for more Green House Gas Interaction information.