By: Leslie Guerra & Justin Freiberg

Background image at the top of this article belongs to Nguyen Huy Kham/REUTERS

The life of a typical rice farmer involves long hours of labor-intensive work under a blazing sun. In the Takéo province of Cambodia, multigenerational families plant, tend, and harvest rice by hand.^[1] This is hard work—it involves constant stooping and twisting on the field, and disease-carrying ticks and flies. Many rice farmers do not earn enough to meet their daily needs, complicating financial access to healthcare.^[2],[3] Another major threat from the rice paddies is invisible to the naked eye and relevant far beyond the paddies of monsoon Asia—methane. This super pollutant plays a major role in raising global temperatures, which are projected to reduce rice yields and worsen farming conditions.^[4]

The large amount of methane emitted from rice paddies is a serious problem, but there is an opportunity for large-scale change.

The Intergovernmental Panel on Climate Change estimates that rice paddies emit approximately 60 million metric tons of methane every year, representing 10 to 12 percent of global methane emissions.^[5] These emissions are spread across more than 165 million hectares of land, with an overwhelming majority in Monsoon Asia.^[6] Over 80 percent of the 144 million rice farmers work on rice fields of less than two hectares in size.^[7],[8] These emissions stand to grow. Due to the growth in rice demand and production expected by 2040, one study estimates that methane emissions from rice paddies in South and Southeast Asia will increase by almost 40 percent.^[9]

Methane is produced from rice paddies through the anaerobic decomposition of organic matter, which primarily consists of rice plant residues and manure, if applied.^[10] Anaerobic conditions, where there is little to no oxygen in the soil, are especially common in rice cultivation as over 75 percent of global rice cultivation occurs in flooded fields.^[11] These conditions lead to higher methanogenic bacteria populations, which grow as the soil submersion period increases. The methane produced by methanogens is emitted into the atmosphere through three mechanisms: plant-mediated transport, ebullition, and diffusion. Plant-mediated transport—the release of methane through plant aerenchyma, the porous tissue that allows for gas exchange—releases 90 percent of the methane. Ebullition, the release of methane from the soil via bubbles, is responsible for almost 10 percent of emissions, and diffusion through the soil and water is responsible for less than one percent. While the aerenchyma is responsible for significant methane release, it also leads to partial oxidation. The aerenchyma delivers oxygen to the rice root and rhizosphere, allowing methanotrophic bacteria to break down some of the methane in these areas. Ultimately, rice paddy methane emissions depend on the factors that influence methanogenic and methanotrophic bacteria.^[12]

There is hope; changing the way rice is farmed can reduce these global emissions by over a third without risking rice yields and farmer livelihoods.^[13]

These agronomic practices can be grouped into four categories: water management, soil amendments, fertilizer management, and biological control—the addition of living organisms.^[14] Alternate wetting and drying, a water management technique that involves periodically draining and re-flooding fields, is particularly promising. This practice can reduce methane emissions by 30 to 70 percent without impacting rice grain yield. It can also cut water usage by 30 percent, lowering costs and conserving resources in an era of increasing water scarcity.^[15] Another potentially impactful strategy is biochar application, which can be applied as a soil amendment to rice paddies. A two-year field experiment in eastern China found that adding rice straw biochar to paddy soil reduced methane emissions by up to 86 percent while increasing yield by over 13 percent.^[16] Using rice straw in this way can also lower methane and carbon dioxide emissions that would have otherwise been produced from straw burning, a common practice across many rice-growing countries. Moreover, changing fertilizer application regimes has the potential to lower methane emissions by upwards of 50 percent. Co-culturing rice with fish or crabs, which has been traditionally practiced in certain regions for centuries, has potential as well.^[17] Alternative solutions underway include breeding low-methane rice varieties and planting rice seeds directly into dry soil.^[14] Certain combinations of these practices can heighten methane reduction in specific environments. For instance, studies have shown that combining alternate wetting and drying with dry seeding can reduce emissions by up to 90 percent.^[18]

Despite the impressive suite of existing methods to reduce methane emissions from rice paddies, we require new structures that both communicate the viability of and lead to the adoption of these practices.

These structures will need to overcome farmers’ hesitation by gaining their trust and providing an incentive to encourage their adoption of low-methane practices. The perception of potential risk by farmers is understandable. While multiple studies have simultaneously assessed methane reduction and yield impacts, various practices and regions have received less attention.^[14] For example, until 2022, there had been no existing research on the impacts of combining the application of organic matter with alternate wetting and drying in central Vietnam.^[19] Rice growing can be highly variable depending on location and climate; therefore, more studies across the diversity of locales where these practices stand to be implemented are crucial.^[20] Creating the economic incentive for this practice change has its own clear challenges. Firstly, the measurement of methane emissions from rice paddies can be difficult and requires expensive equipment and monitoring.^[21] Secondly, the shortage of clear, comprehensive standards has historically led to the generation of questionable carbon credits, which have eroded credit buyers’ trust.^[22]

But in both of these realms, there is reason for optimism.

A number of groups are working toward completing further research, training more farmers, and, recently, creating new revenue streams to incentivize rice farmers to shift their practices. Some are large nongovernmental organizations, while others are smaller startups. Many of these startups have begun working with groups to train farmers in practices that reduce methane emissions, while also connecting them to the voluntary carbon market as an economic incentive. Three impactful groups are highlighted below.

Back to the farmers in the Takéo province of Cambodia—the implementation of methane-reducing agricultural practices has the potential to increase their income through alternative revenue streams.

Higher farmer incomes have their own domino effect, ranging from improved infrastructure to better working conditions.^[23] Keep an eye out for our upcoming blog post, which will delve deeper into mitigation challenges and focus on potential paths forward. Potential solutions are not only as varied as the land utilized by these farmers globally, but also as vast as the emission reductions that can be achieved through meaningful mitigation.