Rice Paddies have a Methane Problem

E has written before—at length—about the problems of industrial meat production, one of which involves the large quantities of methane emitted by livestock. But cows aren’t alone in contributing to the planet’s methane problem—rice paddies, too, play a significant role, and thanks to global warming, their impact is on the rise. As a global warming gas, methane is 20 times more potent than carbon dioxide; and studies indicate that rice fields may contribute up to 20% of global methane emissions. Methane is just a natural byproduct of flooded rice fields—underwater microbes produce the global warming gas as they break down organic matter in the water—and a recent study published in Nature Climate Change found that as carbon dioxide levels and temperatures rise, rice fields emit more methane.“Together, higher carbon dioxide concentrations and warmer temperatures predicted for the end of this century will about double the amount of methane emitted per kilo of rice produced,” said Chris van Kessel, professor of plant sciences at UC Davis and co-author of the study. “Because global demand for rice will increase further with a growing world population, our results suggest that without additional measures, the total methane emissions from rice agriculture will strongly increase.”

Rice serves as the primary food source for about 50% of the world population. For the study, researchers from UC Davis and other schools looked at findings from 63 experiments on rice paddies throughout Asia and North America. They found that increased carbon dioxide was spurring greater rice production which in turn increased the metabolism of methane-producing microscopic organisms beneath the soil, driving up methane emissions. The researchers found that rice yields increased by 24.5% due to increased CO2 emissions in the atmosphere, driving up methane emissions by 42.2%.

There are steps farmers could take to reduce these methane levels, including draining fields, switching fertilizers, planting more heat-tolerant rice and adjusting when rice is sown.

Farmers in China began taking at least one of these steps in the 1980s—draining fields mid-season to save water and increase yields—and reducing methane emissions as a result. Ending the practice of using rice straws from previous seasons as fertilizer could also help reduce methane emissions. An article in Nature from 2009 notes that “India is responsible for nearly a third of the estimated global emissions” from rice, adding that “this may be due to the high temperatures in that country’s rice-growing regions, the large cultivation areas and the practice of continuously flooding paddies.”