Rising Heat Means More Emissions from Rice Paddies

From the latest issue of Food, Nutrition & Science, researchers predict GHG emissions to rise in connection to rice paddies.

Rising world temperatures will likely increase greenhouse-gas (GHG) emissions from rice paddies, according to a study from Trinity College Dublin, Northern Arizona University and UC Davis. Researchers estimate that, worse case scenario, the combination of rising CO2 and warming will about double the amount of GHG emissions per kg rice yield by the end of the 21st century.

Agriculture is a major source of GHG emissions, and rice paddies, which account for 11% of the planet’s arable land, are responsible for 20% of agricultural CH4 emissions – that’s four times higher than the total greenhouse gas emissions of either wheat or maize based on a per-hectare basis. Efforts to reduce the impact of such emissions are complicated by the increasing food needs of the world’s population. One report suggests that world cereal production will increase by 1.3% a year until 2025 to meet the demands, yet stabilizing climate change means reducing emissions associated with agricultural production.

Since finding a balance between the right amount of agricultural production to both produce enough food and reduce emissions is key, researchers in this study worked to develop metrics that could be used to solve the problem. The effects of both warming temperatures and elevated CO2 were observed. Increased CO2 stimulated CH4 emissions by 42.2% and rice yields by 24.6% (one interesting side effect of increased CO2 is actually increased yields in rice). However, warming temperatures significantly decreased rice yield by 14.6% per degree Celsius temperature increase. Warming did not seem to affect CH4 emissions.

“CO2 partly negates the effect of warming. However, since temperatures are expected to increase by at least a few degrees over the next century, it seems likely that over time, the positive effects of CO2 enrichment will be overshadowed by the negative effects of warming,” says Dr. Kees Jan van Groenigen, lead author of the study.

Researchers then looked to see if CO2 enrichment and warming showed interactive effects. Based on a 4 degree Celsius increase in temperature (climate change models from the Intergovernmental Panel on Climate Change point to land temperatures rising in rice-growing regions by 4 degrees Celsius), the combination of warming, and increased CO2, CH4 emissions increase by 58%, decreasing rice yields by 33.8%.

Because global food demand will increase in the future, reducing greenhouse gas emissions from agricultural land only addresses part of our future challenges, says van Groenigen. Ideally, we’d reduce emissions and maintain or increase agricultural yields. Similarly, any technological developments that increase agricultural yields, but increase GHG emissions to an even larger extent only address a part of our challenges.

“What we need are high yields and low GHG emissions. To balance these two needs, we need to assess agricultural practices by both yield and CH4 emissions simultaneously, hence the yield-scaled emissions. If this number goes up, that means that GHG emissions go up more than yields, or that yields are decreased more strongly than GHG emissions. Similarly, if management practices that reduce GHG emissions reduce yields to a larger extent, than yield-scaled emissions go up too – also unfavorable,” says van Groenigen.

Though the interaction between climate change and efforts to reduce CH4 emissions is not fully understood, mitigation practices are clearly needed in rice agriculture. Mid-season drainage and replacing urea as a fertilizer with ammonium sulphate can reduce emissions by 52-60% and 40%, respectively. Straw management also affects CH4 emissions. Replacing some of the animal protein in our diet with plant protein would help a lot to reduce total GHG emissions from agriculture too.

While the effects of warming will continue to intensify as temperatures continue to rise, there is a glimmer of hope, adds van Groenigen. Rice production systems, compared with other cereals, show a large potential for reduction in yield-scaled CH4 emissions through better management practices.