Study warns of 'alarming rise' in long-lived greenhouse gas

Emissions of a greenhouse gas more potent than carbon dioxide or methane increased unabated up to 2020, according to a new report which a University of Leeds academic helped to produce.

Nitrous oxide (N2O) lasts for around 120 years in the atmosphere. It is emitted from a range of sources but is largely driven by unsustainable practices in global food production and growing food demand. Agricultural-related emissions – primarily due to wide spread use of fertiliser – lead the human contribution.

In an era when greenhouse gas emissions must decline to reduce global warming, in 2020 and 2021 N2O was emitted into the atmosphere faster than at any other time in history, according to the new report by the Global Carbon Project. The study was produced by a team of 58 academics, from 55 organisations, in 15 countries, including Dr Chris Wilson from the School of Earth and Environment.

On Earth, excess nitrogen is particularly damaging to water systems and contributes to soil and air pollution. In the atmosphere, it depletes the ozone layer and exacerbates climate change.

Dire consequences

The paper, “Global Nitrous Oxide Budget 1980-2020”, was published in Earth System Science Data. Led by Hanqin Tian at Boston College Schiller Institute for Integrated Science and Society, it shows that agricultural production accounted for 74 per cent of human-driven nitrous oxide emissions in the 2010s. Agricultural emissions reached 8 million metric tons in 2020, a 67 per cent increase since 1980.

This study highlights the need for more considered use of fertilisers within global agricultural systems, otherwise atmospheric N2O concentrations will continue to rise at alarming rates.

Dr Chris Wilson, School of Earth and Environment

The unfettered increase in N2O, a greenhouse gas with a global warming potential approximately 300 times larger than carbon dioxide, presents dire consequences for the planet. The concentration of atmospheric N2O reached 336 parts per billion in 2022, a level beyond even the range of predictions previously developed by the Intergovernmental Panel on Climate Change.

Dr Chris Wilson, scientist at the National Centre for Earth Observation (NCEO) in Leeds’ School of Earth and Environment, said: “This study highlights the need for more considered use of fertilisers within global agricultural systems, otherwise atmospheric N2O concentrations will continue to rise at alarming rates.

“There is plenty of evidence that we can maintain or increase crop yields whilst limiting the amount of fertiliser applied if it is done in a targeted way.”

Drawing on millions of nitrous oxide measurements taken during the past four decades, on land and in the atmosphere, freshwater systems, and the ocean, Tian said the researchers had generated the most comprehensive assessment of global nitrous oxide, to date.

They examined data collected around the world for all major economic activities that lead to nitrous oxide emissions and reported on 18 natural and anthropogenic – meaning caused by humans or their activities – sources; plus three absorbent “sinks” of global nitrous oxide, according to the report.

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.

They also found that some countries have successfully implemented policies and practices to reduce nitrous oxide emissions. Emissions in China have slowed since the mid-2010s, as have emissions in Europe during the past few decades.

Dr Wilson contributed to the ‘inverse modelling’ section of the study, using measurements of atmospheric N2O and a model of atmospheric transport to estimate emissions of the gas over the past 25 years.

In the study, multiple groups did the same type of inverse modelling, so the results were not dependent on a single model. These ‘top-down’ results were then compared to ‘bottom-up’ estimates from other groups who simulated N2O emissions directly. The two methods produced consistent results.

Human impact

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 gasses – carbon dioxide, methane, and nitrous oxide – that assess emissions and sinks to inform further research, policy, and international action.

Global Carbon Project Executive Director Josep Canadell, who is also a research scientist at Australia’s CSIRO Marine and Atmospheric Research, said: “We must reconsider many of our current practices in agriculture with a more rational use of nitrogen fertilisers and animal manure to reduce greenhouse gas emissions and water pollution.”

While there have been some successful nitrogen reduction initiatives in different regions, the researchers 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 percent higher than the average rate of increase in the previous decade.

Tian said there was a need for more frequent assessments so mitigation efforts could be targeted to high-emission regions and economic activities. An improved inventory of sources and sinks will be required if progress is going to be made toward the objectives of the Paris Agreement.