Global N2O Budget Synthesis

Important new synthesis papers have come forth in 2019 and 2020 on the human impacts of the global nitrous oxide (N2O) budget. 

Most of us know nitrous oxide as “laughing gas,” used for its anesthetic effects. But nitrous oxide (N2O) is actually the third most important long-lived greenhouse gas, after carbon dioxide (CO2) and methane. Nitrous oxide is also one of the main stratospheric ozone depleting substances— and we are releasing more of it into the atmosphere than previously thought.

Aerial photograph of cropland in AsiaIn a paper published in Global Change Biology (DOI: 10.1111/gcb.14514), Tian et al. assessed the effects of multiple anthropogenic and natural factors, including nitrogen (N) fertilizer application, atmospheric N deposition, manure N application, land cover change, climate change, and rising atmospheric CO2 concentration, on global soil N2O emissions for the period 1861–2016 using a standard simulation protocol with seven processbased terrestrial biosphere models. Results suggest global soil N2O emissions have increased from 6.3 ± 1.1 Tg N2ON/year in the preindustrial period (the 1860s) to 10.0 ± 2.0 Tg N2ON/year in the recent decade (2007–2016). Cropland soil emissions increased from 0.3 Tg N2ON/year to 3.3 Tg N2ON/year over the same period, accounting for 82% of the total increase. Regionally, China, South Asia, and Southeast Asia underwent rapid increases in cropland N2O emissions since the 1970s. However, US cropland N2O emissions had been relatively flat in magnitude since the 1980s, and EU cropland N2O emissions appear to have decreased by 14%. In the recent decade, N fertilizer application, N deposition, manure N application, and climate change contributed 54%, 26%, 15%, and 24%, respectively, to the simulated total increase. 

The Tian et al. (2019) paper was followed by another in Nature Climate Change (https://doi.org/10.1038/s41558-019-0613-7) by Thompson et al, (2019) that uses inversions of observations of atmospheric concentrations of N2O to deduce spatial and temporal variation of sources from oceans and the continents.

“We see that the N2O emissions have increased considerably during the past two decades, but especially from 2009 onwards,” said lead scientist Rona L. Thompson from NILU–Norwegian Institute for Air Research. “Our estimates show that the emission of N2O has increased faster over the last decade than estimated by the Intergovernmental Panel on Climate Change (IPCC) emission factor approach.”

Increasing use of nitrogen fertilizers is leading to higher N2O levels in the atmosphere

In the study, Thompson and scientists including Eric Davidson of the University of the Maryland Center for Environmental Science found that nitrous oxide in the atmosphere has risen steadily since the mid-20th century. This rise is strongly linked to an increase in nitrogen substrates released to the environment. Since the mid-20th century, the production of nitrogen fertilizers, widespread cultivation of nitrogen-fixing crops (such as clover, soybeans, alfalfa, lupins, and peanuts), and the combustion of fossil and biofuels has increased enormously the availability of nitrogen substrates in the environment.

“The increased nitrogen availability has made it possible to produce a lot more food,” Thompson said. “The downside is of course the environmental problems associated with it, such as rising N2O levels in the atmosphere.”

Rate of increase has been underestimated
The study authors found that N2O emissions increased globally to approximately 10% of the global total between 2000-2005 and 2010-2015. This is about twice the amount reported to the United Nations Framework Convention on Climate Change based on the amount of nitrogen fertilizer and manure used and the default emission factor specified by the IPCC. The researchers argue that this discrepancy is due to an increase in the emission factor (that is, the amount of N2O emitted relative to the amount of N-fertilizer used) associated with a growing nitrogen surplus. This suggests that the IPCC method, which assumes a constant emission factor, may underestimate emissions when the rate of nitrogen input and the nitrogen surplus are high.

From scientific methods to practical measures
“This new publication demonstrates both how we can solve a problem of growing greenhouse gas emissions and how current efforts are falling short in some regions of the world,” said co-author Eric Davidson of the University of Maryland Center for Environmental Science. “These emissions come primarily from using fertilizers to grow food and increasing livestock herds, but we’ve learned how to produce more food with less nitrous oxide emission.”

“In Europe and North America, we have succeeded in decreasing growth in nitrous oxide emissions, an important contributor to climate change and stratospheric ozone depletion,” he added. “Unfortunately, the same can’t be said for Asia and South America, where fertilizer use, intensification of livestock production, and the resulting nitrous oxide emissions are growing rapidly.

“The good news is that this problem can be solved, but the less good news is that it will take a global effort, and we are far from there yet,” he said.

Acceleration of global N2O emissions seen from two decades of atmospheric inversion” was published in Nature Climate Change.

Stay tuned, because a third important paper, with an author group overlapping with the Tian et al. (2019) and Thompson et al. (2019) papers is now in review in a major journal. It synthesizes the process-based modeling (Tian et al. 2019), atmospheric inversion modeling (Thompson et al. 2019), and additional inventory data to generate a comprehensive global nitrous oxide budget. Unfortunaely, while it shows some progress in some regions and economic sectors, it reveals overall lack of mitigation of this important greenhouse gas at the global scale. It highlights the need for redoubled efforts to reduce emissions and subsequent increases in atmospheric N2O.

Photo by Ibadah Mimpi from Pexels