Nitrous oxide emissions grew 40% from 1980–2020

A new report by the Global Carbon Project has revealed that emissions of nitrous oxide — a greenhouse gas more potent than carbon dioxide or methane — have significantly increased over the past 40 years, primarily as a result of farming practices. Notably, agricultural production accounted for 74% of human-driven nitrous oxide emissions in the 2010s, due to the use of commercial fertilisers and animal manure on croplands.

Established in 2001, the Global Carbon Project analyses the impact of human activity on greenhouse gas emissions and Earth systems, producing global budgets for the three dominant greenhouse gases — carbon dioxide, methane and nitrous oxide — that assess emissions and sinks to inform further research, policy and international action. The Global Nitrous Oxide Budget 2024 was produced by team of 58 researchers from 55 organisations in 15 countries and published in the journal Earth System Science Data.

Study co-author Professor Eric Davidson, from the University of Maryland Center for Environmental Science, described nitrous oxide as “the forgotten greenhouse gas”, but it is far from harmless. Excess nitrogen contributes to soil, water and air pollution here on Earth, while in the atmosphere, it depletes the ozone layer and exacerbates climate change.

“In addition to being an important, potent, long-lived gas that contributes significantly to climate change, it also destroys the protective ozone layer in the stratosphere,” Davidson said. “This paper, from a large, international and interdisciplinary team, demonstrates that N₂O emissions are increasing unabated, mostly from agriculture, but also from industries that could be cutting their emissions with existing and affordable technology.”

Drawing on millions of nitrous oxide measurements taken during the past four decades on land and in the atmosphere, freshwater systems and the ocean, the researchers have generated the most comprehensive assessment of global nitrous oxide to date. They examined data collected from around the world for all major economic activities that lead to nitrous oxide emissions and reported on 18 anthropogenic and natural sources and three absorbent ‘sinks’ of global nitrous oxide.

They found that human-induced N₂O emissions increased by 40% in the past four decades, and in 2020 and 2021 nitrous oxide flowed into the atmosphere at a faster rate than at any other time in history. Agricultural emissions reached 8 million tonnes in 2020 — a 67% increase from the 4.8 million tonnes released in 1980, according to the study. In 2022, the concentration of atmospheric nitrous oxide reached 336 parts per billion — a 25% increase over pre-industrial levels that outpaces predictions previously developed by the Intergovernmental Panel on Climate Change.

Graph of 2000 years of atmospheric nitrous oxide concentrations. Observations taken from ice cores and the atmosphere. Image ©Bureau of Meteorology/CSIRO/Australian Antarctic Division.

“This emission increase is taking place when the global greenhouse gases should be rapidly declining towards net zero emissions if we have any chances to avoid the worst effects of climate change,” said Boston College Professor Hanqin Tian, lead author of the study.

The world’s farmers used 60 million tonnes of commercial nitrogen fertilisers in 1980. By 2020, the sector used 107 million tonnes. That same year, animal manure contributed 101 million tonnes for a combined 2020 usage of 208 million tonnes. This increase in a greenhouse gas with a global warming potential approximately 300 times larger than carbon dioxide presents dire consequences for the planet, according to the report.

According to Tian, the top five emitters by volume of anthropogenic N₂O emissions in 2020 were China (16.7%), India (10.9%), USA (5.7%), Brazil (5.3%) and Russia (4.6%). Australia comes in at number seven, having kept its anthropogenic N₂O emissions stable over the past two decades. The report also acknowledged that emissions in China have slowed since the mid-2010s, while rising agricultural emissions and declining industrial emissions in the US have left overall emissions rather flat.

“The once top emitter, Europe, has reduced its emissions since the 1980s by 31%, through industrial emission reductions,” Tian said. “However, emerging economies have grown in response to growing population and food demand.”

Global Carbon Project Executive Director Josep Canadell, a research scientist at CSIRO, added, “While there have been some successful nitrogen reduction initiatives in different regions, we found an acceleration in the rate of nitrous oxide accumulation in the atmosphere in this decade. The growth rates of atmospheric nitrous oxide in 2020 and 2021 were higher than any previous observed year and more than 30% higher than the average rate of increase in the previous decade.”

While improved practices in agriculture around the use of nitrogen fertilisers and animal manure will help reduce greenhouse gas emissions, Tian believes there is a need for more frequent assessments so mitigation efforts can be targeted to high-emission regions and economic activities. An improved inventory of sources and sinks will also be required if progress is going to be made towards the objectives of the Paris Agreement.

“Nitrous oxide emissions from human activities must decline in order to limit global temperature rise to 2°C as established by the Paris Agreement,” Tian said. “Reducing nitrous oxide emissions is the only solution since at this point no technologies exist that can remove nitrous oxide from the atmosphere.

“For net zero emission pathways consistent with the Paris Agreement to stabilise global temperatures below 2°C, anthropogenic N₂O emissions need to decline on average by around 20% by 2050 from 2019 levels,” Canadell added.

Top image credit: iStock.com/zetter