With all the attention focused on CO2 and methane in concerns about human-made emissions, there is a greenhouse gas whose impact has been largely overlooked. That gas is nitrous oxide, which also happens to be an ozone-depleting substance.

Worse: emissions of nitrous oxide, popularly known as “laughing gas” and widely used in anesthetics, have been on the rise in this new century, according to a team of scientists from the International Institute for Applied Systems Analysis in Austria (IIASA), the Norwegian Institute for Air Research (NILU), as well as other institutions from Europe and the United States.

After carbon dioxide (CO2) and methane, nitrous oxide (N2O) is the third most important greenhouse gas. It is commonly used in agriculture, especially in cheap nitrogen-based fertilizers. The use of these fertilizers has shot up in the past two decades, leading to ever larger N2O emissions worldwide.

Atmospheric levels of N2O has been rising since the mid-20th century in tandem with significant increases in nitrogen-based fertilizers, the spread of such nitrogen-fixing crops as clover, soybeans, alfalfa and peanuts, as well as the burning of biofuels.

“We see that the N2O emissions have increased considerably during the past two decades, but especially from 2009 onwards,” says Rona Thompson, a senior scientist at NILU who was the lead author of a study published in Nature Climate Change. “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,” Thompson adds.

On the one hand, the wider spread of nitrogen-based products has boosted food production, which has been a boon to people globally. On the other hand, however, growing nitrogen use has led to serious environmental problems, including rising levels of N2O in the atmosphere, Thompson notes.

According to the new study, between 2000-2005 and 2010-2015 N2O emission levels increased globally by 1.6 (1.4-1.7) TgN y-1 (approximately 10% of the global total). This rate is twice that 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 discrepancy has been “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,” the study’s authors explain. “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.”

And worse is likely yet to come. “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,” says Wilfried Winiwarter, a researcher at IIASA who was a coauthor of the study.

In order to lower global N2O emission rates, the researchers recommend reducing the use of nitrogen fertilizers in regions with a large existing nitrogen surplus. This applies especially to East Asia, where nitrogen fertilizer could be used more efficiently without reducing crop yields, they argue.

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