Nitrous oxide emissions from rice farms are a cause for concern for global climate

Intermittently flooded rice farms can emit 45 times more nitrous oxide as compared to the maximum from continuously flooded farms that predominantly emit methane, according to a new study published in Proceedings of the National Academy of Sciences. This raises the prospect that rice farming across the world could be responsible for up to twice the level of climate impact relative to what was previously estimated.

According to an accompanying global analysis released by Environmental Defense Fund (EDF), methane and nitrous oxide emissions from rice farms could have the same long-term warming impact as about 600 coal plants (1,900 MMT per year CO₂e₁₀₀). In the short-term, this warming impact could be as much as 1,200 average-sized coal power plants (3,600 MMT per year CO₂e₂₀) because nitrous oxide lasts many more decades in the atmosphere than methane.

The short-term vs long-term climate tradeoff
The authors also found an inverse correlation between methane and nitrous oxide emissions from rice farming: water and organic matter management techniques that reduce methane emissions can increase nitrous oxide emissions. This is crucial because nitrous oxide is a long-lived greenhouse gas that traps several times more heat in the atmosphere than methane over both 20 and 100-year time frames.
 
“The full climate impact of rice farming has been significantly underestimated because up to this point, nitrous dioxide emissions from intermittently flooded farms have not been included,” said Kritee, Ph.D., senior scientist at EDF and the lead author of the paper. “Increasing pressure on limited water resources under a changing climate could make additional rice farming regions look to intermittent flooding to address water limitations and concerns about methane emissions. Water management on rice farms needs to be calibrated to balance water use concerns with the climate impacts of both methane and nitrous oxide emissions.”

To monitor and mitigate rice farming’s nitrous oxide impact, the authors call on 1) scientists to map flooding regimes and measure nitrous oxide emissions at a diversity of rice farms across the world; 2) countries to report these emissions; and 3) rice producers to optimize water, nitrogen and organic matter use to reduce emissions of these two important greenhouse gases.

“It is essential for scientists to measure both nitrous oxide and methane emissions from rice fields in order to develop policies effective at meeting food demand while mitigating rice farming’s climate impacts,” said Kritee.

Rice is a critical source of nutrition for the world’s rapidly growing population, providing more calories to humans than any other food. But growing rice is also resource-intensive: rice cultivation covers 11% of the earth’s arable land, consumes one-third of irrigation water.

Focus on methane leaves nitrous oxide underestimated 
Most studies on the climate impacts of rice have measured methane emissions from continuously flooded rice farms. In addition, current climate mitigation strategies for rice production focus on reducing methane emissions by alternate wetting and drying, or intermittent flooding. Most rice producing countries – including the United States and the world’s biggest producers of rice (China, India, Indonesia, Vietnam and Bangladesh) – assume continuous flooding and/or do not clearly report emission factors associated nitrous oxide emissions from rice production as part of their national greenhouse gas inventories submitted to the United Nations.

The findings of today’s study suggest that under intermittent flooding nitrous oxide emissions from rice farms across the world might be 30 to 45 times the maximum of current estimates, equivalent to annual climate pollution from 200 coal power plants to our atmosphere without accounting for methane emissions.

With the help of local partners, the authors investigated greenhouse gas emissions from rice farms across southern India and found that nitrous oxide emissions from rice can contribute up to 99% of the total climate impact of rice cultivation at a variety of intermittently flooded farms.   These emissions contributed substantially to global warming pollution – far more than the estimate of 10% previously suggested by multiple global rice research organizations.

Solutions to reduce methane and nitrous oxide
The authors found that carefully chosen farming techniques at individual farms reduced net greenhouse gas emissions from rice cultivation by as much as 90 percent by integrating shallow (mild-intermittent) flooding with co-management of nitrogen and organic matter. If all irrigated rice farmers only used the proposed shallow flooding instead of continuous or intense forms of intermittent flooding, estimates in the accompanying analysis shows that the rice farms with irrigation have the potential to reduce their global climate impact by 60 percent (450-550 MMT CO₂e₁₀₀ years).

“We now know nitrous oxide emissions from rice farming can be large and impactful,” said Richie Ahuja, a co-author of this study. “We now also know how to manage the problem. Major rice producing nations in Asia are investing to improve the agriculture sector and could benefit from the suggested dual mitigation strategies that lead to water savings, better yields, and less climate pollution.”

For additional resources, including a global risk map of rice nitrous oxide emissions, visit edf.org/riceN2O.