Nitrous oxide is leaking into the atmosphere at a dangerous pace

The colossal amount of nitrogen used as fertilizer in agriculture is leading to an increase in emissions of nitrous oxide, a lesser-known greenhouse gas that contributes to climate change and the depletion of the ozone layer.

Nitrous oxide emissions are increasing at a rate of about 2% per decade, and in 2018 the gas’s concentration in the atmosphere was about 22% above pre-industrial levels, according to a study published today in Nature, the first to do a thorough accounting of atmospheric nitrous oxide. The paper was five years in the making, and involved 48 research institutions from around the world.

“Now that we have started to draw pathways to net zero emissions, we must understand all greenhouse gases,” said Pep Canadell, co-author and executive director of the Global Carbon Project. “It has taken us a very long time to understand how nitrous oxide emissions work, but we have learnt that emissions from natural sources haven’t changed, while human emissions have increased a lot.”

Nitrous oxide, or NO, is the third-most abundant greenhouse gas after carbon dioxide and methane. The gas has about 300 times more warming power than carbon dioxide over the course of a century, and stays in the atmosphere warming the planet for 116 years. Unlike other gases that destroy the ozone layer, which protects Earth from ultraviolet radiation, its emissions haven’t been banned.

Another paper published in Nature in March showed that the Earth’s ozone shield has almost totally recovered due to the1987 Montreal Protocol, which banned a class of harmful chemicals known as chlorofluorocarbons. But the remaining ozone hole over the Antarctic is highly volatile and depends on weather conditions. After it shrunk dramatically in 2019, it’s grown to its largest size in 15 years, Europe’s Copernicus Atmosphere Monitoring Service said on Tuesday.

NO is released into the atmosphere mainly through the fertilizers that make crops and pastures grow faster and greener. Every year, more than 220 million metric tons of nitrogen are spread on croplands as fertilizer and on pasturelands as manure produced by livestock. Agriculture is responsible for around 70% of human emissions of the gas, researchers found.

Developing nations that are highly dependent on agriculture in East and South Asia, Africa, and South America are the main contributors to the increase in nitrous oxide emissions over the last two decades, the researchers found. “The main problem we have in many parts of the world is that food production is subsidized by the state through agriculture subsidies that make fertilizers cheaper,” Canadell said. “If we cut fertilizer subsidies a little, farmers and corporations would start to be more careful about the amount of nitrogen they use.”

Farmers forced to pay market prices for fertilizers in countries that have eliminated or reduced subsidies have almost immediately cut nitrogen use, Canadell said. Other research shows that excess nitrogen in the soil can lead to slightly lower crop output.

Still, developing nations are often reluctant to change agricultural policies. “These are countries that might not have food security,” Canadell said. “The last thing they want to talk about is changing something as essential as the production of food.”

The U.S. has emitted roughly the same amount of NO each year for the last two decades, but an increase in agricultural production during that time suggests a more efficient use of nitrogen fertilizers there. Emissions from Europe have declined as the region has implemented policies to cut emissions, increase land productivity, and reduce water pollution, but there are doubts about how much further they can fall.

Significant curbs from high-emitting countries would go a long way toward slowing the accumulation of nitrous oxide in the atmosphere. But eventually, new technologies will be needed to capture emissions that can’t be eliminated through efficiency measures.

“There’s no doubt we’ll need solutions because there will always be some leaking,” Canadell said. “But before we get there, there are opportunities in Brazil, China and other emerging economies where there is an important acceleration in emissions.”