20.11.2019 |

Nitrous oxide emissions are increasing faster than predicted

Nitrous oxide emissions from agriculture are on the rise, scientists warn (Photo: CC0)

Emissions of nitrous oxide, a powerful greenhouse gas, are increasing more rapidly than previously thought, according to a new study published in the journal “Nature Climate Change”. A team of scientists has found that global emissions of nitrous oxide (N₂O), commonly known as laughing gas, are higher and growing faster than estimated by the Intergovernmental Panel on Climate Change (IPCC). “Nitrous oxide is a powerful contributor to global warming. It is 265 times more effective at trapping heat in the atmosphere than carbon dioxide and depletes our ozone layer,” the authors warn. In the early 20th century the Haber-Bosch process for the synthesis of ammonia from nitrogen in the atmosphere was developed. “The increased nitrogen availability has made it possible to produce a lot more food,” explains lead author Rona L. Thompson from the Norwegian Institute for Air Research (NILU). “The downside is of course the environmental problems associated with it, such as rising N₂O levels in the atmosphere.” 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 have greatly enhanced emissions of N₂O, the NILU states in a press release.

The researchers looked at N₂O emissions for the years between 1998 and 2016. “Conventional analysis of N₂O emissions from human activities are estimated from various indirect sources. This include country-by-country reporting, global nitrogen fertiliser production, the areal extent of nitrogen-fixing crops and the use of manure fertilisers,” the authors state in a blog article published on “The Conversation”. “Our study instead used actual atmospheric concentrations of N₂O from dozens of monitoring stations all over the world. We then used atmospheric modelling that explains how air masses move across and between continents to infer the expected emissions of specific regions,” they added. Thompson and her team found that N₂O emissions increased globally by 1.6 million tonnes of nitrogen per year between 2000-2005 and 2010-2015. The fastest growth has been since 2009.

“The regions of East Asia and South America made the largest contributions to the global increase,” the authors write in the abstract of the study. “China and Brazil are two countries that stand out. This is associated with a spectacular increase in the use of nitrogen fertilisers and the expansion of nitrogen-fixing crops such as soybean,” they point out in their blog article. The study found that the emissions reported for those two countries, based on the IPCC method, are significantly lower than those which the scientists inferred from actual nitrous oxide levels in the atmosphere. While the IPCC method assumes a constant emission factor (the amount of N₂O emitted relative to the amount of N-fertilizer used), the recent study concludes that N₂O emission may have a non-linear response when levels of nitrogen input are high. “Our results suggest that reducing nitrogen fertilizer use in regions where there is already a large nitrogen surplus, will result in larger than proportional reductions in N₂O emissions”, Thompson says. “This is particularly relevant in regions such as East Asia, where nitrogen fertilizer could be used more efficiently, without reducing crop yields”.

The authors admit that reducing N₂O emissions from agriculture will be very challenging, given the expected global growth in population, food demand and biomass-based products including energy. However, they stress that all future emission scenarios consistent with the goals of the Paris Agreement require a cut in nitrous oxide emissions. In most cases, they would need to decline between 10% and 30% by mid-century. “This will require changes in human diet and agricultural practices and, ultimately, improved nitrogen use efficiency,” they write in the paper. In the United States and Europe, emissions were fairly stable over the past nearly two decades. “In Europe and North America, we have succeeded in decreasing growth in nitrous oxide emissions,” said co-author Eric Davidson of the University of Maryland. “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. (ab)

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