Rice feeds more than half the world. From terraced paddies in Southeast Asia to irrigated fields in China and India, it underpins daily meals for billions of people.But the same flooded soils that help rice thrive also create ideal conditions for microbes that release climate-warming gases.In a new study, our team of environment and agriculture scientists found that greenhouse gas emissions from rice paddies have nearly doubled globally since the 1960s, averaging about 1.1 billion tons of carbon dioxide-equivalent emissions per year in the 2010s. That’s roughly equal to the annual emissions of 239 million cars.AdvertisementAdvertisementThis makes rice-growing the largest emissions source in agriculture outside of livestock, and rice demand is expected to keep rising.Farmers have ways to reduce their rice crops’ emissions without lowering their yields. If every grower used the best currently available “climate-smart” options, we found that global rice emissions could be reduced by about 10% by midcentury. However, greater reductions are needed to slow climate change, which would require developing additional, more effective strategies.Why rice emissions have increasedRice emissions have risen for two reasons: the expansion of rice cultivation area and the intensification of management practices.Just over half of the global increase is from the expansion of rice-growing areas. In Africa, for example, the rice-growing area has roughly doubled since the 1960s, helping drive a twofold rise in methane emissions in the region.AdvertisementAdvertisementAt the same time, rice farmers are using more fertilizers and organic amendments, such as straw and manure, planting more productive rice varieties and growing the plants closer together. The result is more rice but also more greenhouse gas emissions.After rice is harvested, one technique for improving soil fertility is to plow the dried rice stalks back into the soil. But this also increases methane emissions. Jingting ZhangWe found that one practice in particular – leaving rice stalks in the field after harvest and then plowing them into the soil to improve soil fertility – was responsible for about 18% of rice’s increase in overall net emissions since the 1960s. The reason: It increases the organic matter in the soil, which microbes then decompose, creating more methane emissions.Rising global temperatures further accelerate microbial activity in the soils, meaning even more emissions.Fertilizer is another major contributor to emissions. Use of synthetic nitrogen increased by about 76% after 2000, boosting nitrous oxide – another powerful greenhouse gas. It contributed about 9% of the increase in total global net emissions from human activities.Irrigation practices also affect emissions. In the past, irrigated rice paddies were kept flooded throughout the growing season, resulting in constant greenhouse gas emissions produced by microbes that thrive in the wet environment. Over the past two decades, however, more farmers have used intermittent flooding – draining their fields periodically.AdvertisementAdvertisementThis change has lowered methane emissions compared with keeping the paddies continuously flooded. However, we found a slight increase in nitrogen oxide emissions as soils cycled between wet and dry, which induces microbes to transform nitrogen in organic matter into nitrogen oxide gases, particularly nitrous oxide.Climate impact of rice productionPutting a full climate price tag on rice production is harder than measuring one greenhouse gas at a time.Rice paddies emit methane and nitrous oxide from wet or flooded soils. They also remove carbon dioxide from the atmosphere as rice grows, and they lose carbon from their soils between crop seasons.A credible global estimate requires consistently accounting for different gases and soil carbon changes, as well as the uncertainty involved in tracking data across space and time.AdvertisementAdvertisementTo do that, we combined three approaches:An ecosystem computer model allowed us to simulate crop growth, water conditions and soil processes to estimate changes in methane, nitrous oxide and soil carbon together.An artificial intelligence-powered machine learning model improved estimates where measurements were sparse to cover all rice regions in the world.And a meta-analysis of more than 1,200 field experiment sites provided direct evidence of how practices such as irrigation, fertilizer use and management of crop residue affect emissions.Together, they allowed us to quantify …
