CIESIN Thematic Guides

Methane Emissions from Flooded Rice Cultivation

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. In "Methane Emission from Rice Fields: Wetland Rice Fields May Make a Major Contribution to Global Warming," Neue explains that in wetland rice soils, flooding a field cuts off the oxygen supply from the atmosphere, resulting in anaerobic fermentation of soil organic matter. Methane, a major end product of anaerobic fermentation, is released from submerged soils to the atmosphere through the roots and stems of rice plants. Estimates of global methane emission rates from rice fields range from 20 to 100 Tg per year (1 Tg=1 million tons), which corresponds to 6 to 29 percent of total annual anthropogenic methane emission.

Chapter 4 of the Environmental Protection Agency report Policy Options for Stabilizing Global Climate (Lashof and Tirpak 1990) provides data on current emissions of methane by source; regional contribution to greenhouse warming; wetland area and associated methane emissions (Figure 4-13); rough rice production (Figure 4-15); and rice area harvested (Figure 4-16).

According to an experiment detailed in "Factors Affecting Methane Production Under Rice," exogenous organic matter appears to be the largest contributor to methane production from flooded rice soils (Delwiche and Cicerone 1993). Figure 2 illustrates the influence of added organic matter on methane emissions from rice soils. In "Methane Production and Emission in Coastal Ricefields of Texas," the authors found that as the rice growing season progressed, methane production at lower depths and farther away from the plants increased in proportion with root density (Fisher et al. 1990).

Experimental studies indicate that reliable data on the importance of rice fields as a source of atmospheric methane can only be obtained by continuous measurement. Schutz et al. (1990) report on a continuous methane sampling and analyzing system in "Methane Emission from Italian Irrigated Ricefields."