Rice Agriculture: Factors Controlling Emissions

Recent atmospheric measurements indicate that concentrations of greenhouse gases are increasing. Atmospheric methane concentration has increased at about 1% annually to 1.7 ppmV during the last decades (Khalil and Rasmussen, 1987). The resulting effect on global temperature is highly significant because the warming efficiency of methane is up to 30 times that of carbon dioxide (Dickinson and Cicerone, 1986). Data from polar ice cores indicate that tropospheric methane concentrations have increased by a factor of 2–3 over the past 200–300 years (Khalil and Rasmussen, 1989). The increase of methane concentrations in the troposphere correlate closely with global population growth and increased rice production (Figure 1), suggesting a strong link to anthropogenic activities. The total annual global emission of methane is estimated to be 420–620 Tg/yr (Khalil and Rasmussen, 1990), 70–80% of which is of biogenic origin (Bouwman, 1990). Methane emissions from wetland rice agriculture have been estimated up to 170 Tg/yr, which account for approximately 26% of the global anthropogenic methane budget. Flooded ricefields are probably the largest agricultural source of methane, followed by ruminant enteric digestion, biomass burning, and animal wastes (summarized by Bouwman, 1990).