More carbon dioxide in the atmosphere, coupled with rising temperatures, is making rice agriculture a larger source of the potent greenhouse gas methane, according to a study published today in Nature Climate Change by a research team that includes a University of California, Davis, plant scientist.

But the authors note that relatively simple changes in rice cultivation could help reduce methane emissions. 

“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, published in this week’s edition of Nature Climate Change. “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 paddies are one of the largest man-made sources of methane, and rice is the world’s second-most produced staple crop.

Van Kessel and his colleagues gathered findings from 63 different experiments on rice paddies, mostly in Asia and North America. They used a technique called meta-analysis, a statistical tool for finding general patterns in a large body of experimental published data.

The experiments measured how rising temperatures and extra carbon dioxide in the atmosphere affect both rice yields and the amount of methane released by rice paddies.

“Our results show that rice agriculture becomes less climate-friendly as our atmosphere continues to change,” said Kees Jan van Groenigen, research fellow at Trinity College Dublin, and lead author of the study.

As more carbon dioxide enters the atmosphere, rice plants grow faster, the experimental data showed. This growth, in turn, pumps up the metabolism of methane-producing microscopic organisms that live in the soil beneath rice paddies. The end result: More methane.

Overall, the rice paddy experiments revealed that increased carbon dioxide in the atmosphere boosted rice yields by 24.5 percent and methane emissions by 42.2 percent, increasing the amount of methane emitted per kilo of rice.

Unlike carbon dioxide levels, rising temperatures were found to have only small effects on methane emissions. However, because warming decreases rice yield, it effectively increases methane emissions per kilo of rice.

The authors point out several options available to reduce methane emissions from rice agriculture. For instance, management practices such as mid-season drainage and using alternative fertilizers have been shown to reduce methane emissions from rice paddies. By switching to more heat-tolerant rice cultivars and by adjusting sowing dates, yield declines due to temperature increases can largely be prevented, reducing the effect of warming on methane emissions per yield. 

The researchers, who also include Northern Arizona University Professor Bruce Hungate, said the findings underscore the importance of mitigation efforts to ensure a secure global food supply while keeping greenhouse gas emissions in check.

The research was funded by the U.S. Department of Energy's National Institute for Climatic Change Research, the National Science Foundation, the Irish Research Council, and Marie Curie Actions.