Nitrous oxide (N2O) is a long-lived greenhouse gas that began to accumulate in the atmosphere prior to the industrial era. Nitrous oxide belongs to a class of so-called “super pollutants” that simultaneously contribute to climate change and degrade the health of people and ecosystems. It has about 270 times the impact on global warming per unit mass compared to carbon dioxide, and accounts for about 8% of total anthropogenic global warming.1 Nitrous oxide is a significant ozone-depleting substance projected to remain so throughout the 21st century.2
Anthropogenic sources of N20 originate from a massive disturbance of nitrogen cycles at local to global scales. Most nitrogen on the planet is in the atmosphere, where it exists primarily as diatomic nitrogen gas (N2), accounting for about 78% of the air volume. However human activity has dramatically increased the abundance of other forms of nitrogen that form a “cocktail” of nitrogen pollutants with wide-ranging health and environmental impacts.3
Global emissions of N20 increased by 62% from 1970 to 2022.4 That rate of increase is the fastest among all major greenhouse gases. Soil and nutrient management in agriculture are the dominant sources of N2O emissions. Nitrogen-based fertilizers (urea, ammonium nitrate) are used to boost crop yields. Excess nitrogen not taken up by plants is transformed by soil microbes through processes like nitrification and denitrification, releasing N₂O into the atmosphere. Global application of nitrogen-based fertilizers increased by a factor of four since 1970.
Manure from livestock contains high levels of nitrogen. When manure is applied to fields as fertilizer, microbial activity in the soil converts that nitrogen into N₂O. Manure stored in lagoons, piles, or tanks releases N₂O via multiple chemical transformation pathways.
In the industrial sector, the chemical industry releases N20 in the manufacture of nitric and adipic acid. Nitric acid is used in the manufacture of fertilizers, explosives, and other chemicals, while adipic acid is used in the production of nylon, adhesives, and food additives.
The combustion of biomass releases N20. This includes burning of crop residues such as rice or wheat straw to prepare a field for next year’s planting; burning forest to clear land for agriculture or other forms of development; natural and human-induced wildfires; and the combustion of wood, pellets, or other forms of biomass for heat and electricity.
The combustion of fossil fuels releases N2O in two ways. First, during combustion nitrogen in the air reacts with oxygen at high temperatures, forming nitrogen oxides (NOx), including N₂O. Second, coal and oil contain nitrogen compounds that N₂O during combustion.
In China, N20 emissions from agricultural soils (33%) and manure management (16%) accounted for nearly half of total emissions in 2022. The manufacture of nitric and adipic acid accounted for another 25% of N20 emissions. In the United States agricultural soils alone accounted for three-quarters of N2O emissions in 2022, followed by manure management, the processing of wastewater, and the generation of electricity with fossil fuels.
In Brazil, manure accounts for about 60% of total N20 emissions. Brazil has one of the largest cattle population in the world, resulting in large quantities of manure production. Much of the cattle grazing occurs on open pastures where the manure is often left to decompose directly on the land, increasing N2O emissions. The country’s warm, humid climate accelerates the decomposition of manure, and with it, the production of N2O.
The increase in nitrous oxide emissions over the past 50 years is the result of several interrelated factors related to how we produce food and consume energy. Fertilizer use has increased due to population growth and food demand, the introduction of high-yield variety plants in the Green Revolution that rely on fertilizers, the intensification of agriculture where arable land is limited, and technological advances that have made fertilizers cheaper and more accessible.
The increase in the demand for food has included an increased demand for meat, dairy and other animal products. Higher incomes and changes in social norms in developing nations have increased the demand for meat. Technological advances and livestock production have made animal products cheaper, and globalization has made them more accessible.
Nitrous oxide is the most significant ozone-depleting substance currently emitted into the atmosphere. Its destructive impact equals the combined effect of all other ozone-depleting substances emitted today. By 2050, ambitious efforts to reduce nitrous oxide emissions could deliver ozone protection benefits comparable to those achieved by the 2007 Montreal Protocol, which accelerated the phase-out of hydrochlorofluorocarbons (HCFCs).5
Reductions in N2O emissions requires focused actions across agriculture, industry, and waste management. Key changes needed in agriculture include more precise applications of fertilizer, the integration of nitrogen-fixing crops (e.g., legumes) to reduce fertilizer use, the use of slow-release fertilizers, and better tillage and irrigation practices. 6 Together these practices form the “4 R’s of nutrient management” (right source, right rate, right time, and right place.7
N2O emissions from manure can be reduced through the optimization of the N content of the livestock food, storing manure in a covered facility, the used of anaerobic digesters that capture methane reducing conditions that favor N2O emissions, and adding so-called “chemical inhibitors” to manure to slow the conversion of ammonium to nitrate, reducing N₂O production.
1 Smith, C., et al., 2021, “The Earth’s Energy Budget, Climate Feedbacks, and Climate Sensitivity Supplementary Material. In Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernment Panel on Climate Change (V. Masson-Delmotte, V. et al. (eds.), available from https://www.ipcc.ch/.
2 United Nations Environment Programme and Food and Agriculture Organization. 2024. “Global Nitrous Oxide Assessment,” Nairobi. https://doi.org/10.59117/20.500.11822/46562
3 Sutton, Mark A., Clare M. Howard, David R. Kanter, Luis Lassaletta, Andrea Móring, Nandula Raghuram, and Nicole Read. “The Nitrogen Decade: Mobilizing Global Action on Nitrogen to 2030 and Beyond.” One Earth 4, no. 1 (January 22, 2021): 10–14. https://doi.org/10.1016/j.oneear.2020.12.016.
4 Hoesly, Rachel, Steven J Smith, Noah Prime, Hamza Ahsan, Harrison Suchyta, Patrick O’Rourke, Monica Crippa, et al. “CEDS V_2024_07_08 Release Emission Data.” Zenodo, July 24, 2024, https://doi.org/10.5281/zenodo.12803197
5 United Nations Environment Programme and Food and Agriculture Organization, op. citLink
6 Hassan, Muhammad Umair, Muhammad Aamer, Athar Mahmood, Masood Iqbal Awan, Lorenzo Barbanti, Mahmoud F. Seleiman, Ghous Bakhsh, et al. “Management Strategies to Mitigate N2O Emissions in Agriculture.” Life 12, no. 3 (March 17, 2022): 439. https://doi.org/10.3390/life12030439.
7 U.S. Department of Agriculture, “Following Nutrient Management Guidelines can Help to Reduce Nitrous Oxide Emissions,” accessed December 6, 2024, https://tinyurl.com/w62bwdkc