Nitrous oxide levels are on the rise

Most of us know nitrous oxide (N₂O) as "laughing gas," used for its anesthetic effects. It is however actually the third most important long-lived greenhouse gas, after carbon dioxide (CO₂) and methane. According to new research by scientists from IIASA, the Norwegian Institute for Air Research (NILU), and several other institutions across Europe and the US, agricultural practices and the use of fertilizers containing nitrogen have greatly exacerbated N₂O emissions to the atmosphere over the last two decades.

In their paper published in Nature Climate Change, the authors present estimates of N₂O emissions determined from three global atmospheric inversion frameworks spanning 1998-2016, based on atmospheric N₂O observations from global networks. IIASA contributed N₂O emission inventory data from the Greenhouse Gas and Air Pollution Interactions and Synergies (GAINS) model, including those from industrial processes needed for data alignment.

"We see that the N₂O emissions have increased considerably during the past two decades, but especially from 2009 onwards," says study lead author Rona Thompson, a senior scientist at NILU. "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."

The authors state that N₂O 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 the availability of nitrogen substrates in the environment enormously.

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

The results of the study indicate that N₂O emissions increased globally by 1.6 (1.4-1.7) TgN y-1 (approximately 10% of the global total) between 2000-2005 and 2010-2015. This is about twice the increment 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 authors 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 nitrogen 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.

"We will have to adjust our emission inventories in light of these results, including those in the GAINS model," says study coauthor Wilfried Winiwarter, a researcher in the IIASA Air Quality and Greenhouse Gases Program. "Future increments in fertilizer use may trigger much larger additional emissions than previously thought -- emission abatement, as is already reflected in GAINS results, will therefore become even more prominent and also cost efficient for such situations."

From their inversion-based emissions, the researchers estimate a global emission factor of 2.3 or ± 0.6%, which is significantly larger than the IPCC default for combined direct and indirect emissions of 1.375%. The larger emission factor, and the accelerating emission increase found from the inversions suggest that N₂O emission may have a non-linear response at global and regional scales with high levels of nitrogen input. The authors recommend using more complex algorithms and region-specific emission factors to estimate N₂O going forward.

The outcome of the study implies that to ultimately help lower global N₂O emissions, the use of nitrogen fertilizer in regions where there is already a large nitrogen surplus will have to be reduced, as this would result in larger than proportional reductions in N₂O emissions. This is particularly relevant in regions like East Asia, where nitrogen fertilizer could be used more efficiently without reducing crop yields.