Study Finds Nitrous Oxide Emissions Grew 40% over 40 Years, Accelerating Climate Change

Emissions of nitrous oxide – a greenhouse gas more potent than carbon dioxide or methane – continued unabated between 1980 and 2020, a year when more than 10 million metric tons were released into the atmosphere primarily through farming practices, according to a new report to which researchers from UC San Diego’s Scripps Institution of Oceanography contributed. 

Agricultural production accounted for 74% of human-driven nitrous oxide emissions in the 2010s – attributed primarily to the use of chemical fertilizers and animal waste on croplands – according to the report “Global Nitrous Oxide Budget 1980-2020,” published today in the journal Earth System Science Data.

At a time when greenhouse gas emissions must decline to reduce global warming, in 2020 and 2021 nitrous oxide flowed into the atmosphere at the fastest rates in history, the international team of researchers led by Boston College reported. On Earth, excess nitrogen contributes to soil, water, and air pollution. In the atmosphere, it depletes the ozone layer, and exacerbates climate change.

Agricultural emissions alone reached eight million metric tons in 2020, a 67% increase from the 4.8 million metric tons released in 1980, according to the study, the most comprehensive study of global nitrous oxide emissions and sinks produced by a team of 58 researchers from 55 organizations in 15 countries.

The unfettered increase in a greenhouse gas with a global warming potential approximately 300 times more powerful than carbon dioxide presents dire consequences for the planet.

 “Reducing nitrous oxide emissions is the only solution since, at this point, no technologies exist that can remove nitrous oxide from the atmosphere,” said the report’s lead author, Hanqin Tian, with the Schiller Institute Professor of Global Sustainability at Boston College.

Scripps geoscientists Jens Mühle and Manfredi Manizza contributed to the study. Mühle provided expertise in measurements of atmospheric N₂O measurement based on the global Advanced Global Atmospheric Gases Experiment (AGAGE) network of stations while Manizza provided global estimates of N₂O emissions from the global ocean, especially for the coastal zones, using an ocean biogeochemical model.

“Unfortunately, AGAGE measurements and those of other measurement networks show that the abundance of N₂O keeps rising in the global atmosphere - as are those of carbon dioxide, methane and many other greenhouse gases – in a time when humanity needs to urgently reduce greenhouse gas emissions to limit the increasing impacts of the climate crisis,” said Mühle, a research scientist at the central calibration and experimental development laboratory of the AGAGE network. 

Mühle noted there is positive news: The previous top emitter, Europe, has reduced its emissions by roughly 30% since the 1980s, but “Other countries and regions clearly need to improve as their N₂O emissions have grown or stayed flat,” he said.  

Manizza noted that globally N₂O emissions from the ocean are about half of the natural emissions and are comparable to those due to human-related activities. The ocean is going through a process of de-oxygenation caused by climate change and increasing ocean stratification.  That could cause an increase in ocean N₂O production especially in currently expanding oxygen minimum zones. 

“We expect the ocean to increase its contribution in the near future if climate change continues to worsen,” said Manizza. “We need to understand and quantify the evolution of oceanic N₂O emissions to continue to have a full global budget and to separate with accuracy  the ocean and land contributions.”

The concentration of atmospheric nitrous oxide reached 336 parts per billion in 2022, a 25% increase over pre-industrial levels that far outpaces predictions previously developed by the Intergovernmental Panel on Climate Change, said Tian, director of the Center for Earth System Science and Global Sustainability at BC’s Schiller Institute for Integrated Science and Society.

The world’s farmers used 60 million metric tons of commercial nitrogen fertilizers in 1980. By 2020, the sector used 107 million metric tons. That same year, animal manure contributed 101 million metric tons for a combined 2020 usage of 208 million metric tons.

The top 10 nitrous oxide emission-producing countries are China, India, the United States, Brazil, Russia, Pakistan, Australia, Indonesia, Turkey, and Canada, the researchers found. Some European countries have seen success implementing policies and practices to reduce nitrous oxide emissions, according to the report. Emissions in China have slowed since the mid 2010s.

In the U.S., agricultural emissions continue to creep up while industrial emissions have declined slightly, leaving overall emissions rather flat. Natural sources of nitrous oxide emissions from soil, fresh- and saltwater have remained stable.

Improved agricultural practices that limit the use of nitrogen fertilizers and animal waste can help reduce greenhouse gas emissions and water pollution. Tian said there is a need for more frequent assessments so mitigation efforts can target high-emission regions and activities. For this, more measurements of N₂O are needed, Mühle said.  

– Adapted from Boston College