One of the biggest modern day threats to our ozone layer is something we've largely overlooked for decades: nitrous oxide. It's also something we can't feed ourselves without producing.
David Kanter is a postdoctoral research fellow at Columbia University's Earth Institute who has spent his academic career researching nitrous oxide's impact on the ozone layer and what humans can do to reduce our N2O emissions. Most recently, he's looked at the potential economic benefits of stricter environmental regulations to industries that many would assume would suffer under new rules, including companies that produce synthetic nitrogen fertilizers used in food production.
"There have already been lots of people who have argued that controlling nitrogen is good from an environmental perspective and from the farmers' perspectives because they can save money," Kanter told me last week, after presenting some of his most recent research at a Columbia symposium on sustainable development. "No one has yet really looked at the fertilizer industry case. Can you imagine such a case where you can get the environmental benefits, the farmers' savings, and the industry profits?"
Nitrous oxide only accounts for about 5 percent of greenhouse gas emissions in the US, according to the Environmental Protection Agency. But the trouble is it's a particularly nasty emission, one that stays in the atmosphere for about 114 years, the EPA says.
When you compare its ozone depletion potential (a metric for how much damage a substance does to the ozone layer) to other gases, it's actually "the most important ozone-depleting emission," according to a report from the United Nations Environment Programme, co-authored by Kanter.
To put it another way: the EPA says that "the impact of one pound of N2O on warming the atmosphere is almost 300 times that of one pound of carbon dioxide."
Okay, so it's pretty clear that too much N2O is bad news, but where is it all coming from? Most of us only know nitrous oxide as the laughing gas we get if we're having a particularly lucky visit to the dentist, but rest assured David After Dentist isn't to blame for the ozone depletion. In fact, agriculture accounts for 60 percent of nitrous oxide emissions worldwide, according to another report from UNEP.
See, N2O is a natural by-product of the nitrogen cycle: when bacteria in the soil and oceans break down nitrogen compounds, N2O is released. This is true whether the nitrogen is naturally-occurring or put there by human hands.
Humans have been disrupting that natural cycle ever since we figured out how to synthetically produce nitrogen fertilizers. These fertilizers are important: a 2008 article in Nature Geoscience estimates half of the world's population wouldn't have enough food to eat today if we didn't have nitrogen fertilizers. But it also means we're producing a bunch of extra nitrous oxide on top of the natural cycle.
Here's the kicker: the biggest contributor to N2O pollution from agriculture comes from accidentally using more nitrogen than the crops even need, according to an article by Kanter and Woods Hole Research Center's Eric Davidson published last year in Environmental Research Letters. The duo wrote that 50 percent of nitrogen applied to soil isn't even used by crops.
It seems like the solution is simple: if farmers use only the exact amount of nitrogen fertilizers their crops need, they'll save money and reduce N2O emissions while still getting all the benefits of nitrogen-enhanced soil. But that's not actually such an easy feat, requiring careful monitoring of nitrogen levels in the soil and more targeted methods of applying fertilizer.
Nitrogen oxide has also not been a focus of any major climate strategies in the US, so coming up with these new methods hasn't been a priority. In fact, N2O emissions weren't even addressed in the Montreal Protocol, the 1987 treaty designed to protect the ozone layer from depletion.
"There were other sources of nitrogen in the stratosphere that were considered more important at that time," Kanter explained, including the Concorde jet, which produced significant quantities of nitrogen oxides in its exhaust. "Once it was clear the US wasn't developing a Concorde program, the perceived threat of nitrous oxide was lessened."
"Agriculture is the third rail in politics."
The killing of the Concorde program, combined with the fact that CFCs, a class of chemicals that were found in many household items from hairspray to refrigerator coolant, posed a bigger immediate threat to the atmosphere, meant that N2O emissions were largely overlooked in early environmental restrictions.
Now, Kanter argues it's time to include nitrous oxide in our plans to fight climate change, and he thinks there's a win-win way to do it. Even the massive fertilizer manufacturers could stand to gain from new restrictions, Kanter said, because the companies are already investing in new technologies that could solve the problem.
"The same guys that are producing the bad stuff are also coming up with the alternatives," Kanter told me.
Companies from small startups to some of the biggest agriculture suppliers in the world are developing new technologies to make nitrogen fertilizer applications more efficient. Companies have developed sensors to help farmers carefully monitor the nitrogen levels in their soil, and new versions of fertilizer that use chemicals or polymer coatings to prevent unused nitrogen from leaking into the soil and water. Yara, one of the world's biggest fertilizer manufacturers, released a GPS-equipped system that measures nitrogen levels second-to-second and seamlessly communicates with fertilizer application equipment.
The fact that these companies are investing in this technology is a good sign, because that industry is an important piece of the puzzle, Kanter said.
"They have a significant impact on a political level in the sense that they're a very well-organized lobby," he said. "They also have influence on the farmer level. There have been studies surveying farmers about who they trust and take advice from. It's usually other farmers or the guy at the market where they buy their seed and fertilizers. Academics and policy makers are way at the bottom of the list."
In his presentation last week, Kanter suggested that the EPA could independently impose new federal restrictions on nitrogen fertilizer use under Section 615 of the Clean Air Act, which gives the EPA administrator authority to regulate "any substance, practice, process, or activity" that "may reasonably be anticipated to affect … ozone in the stratosphere." But he admits regulating agriculture is difficult and usually done on a state-by-state basis, so we could still be a long way from bringing nitrous oxide emissions into our climate change strategy.
"Agriculture in this country and most countries is the third rail in politics," Kanter said. "It's very difficult to get traction."
Correction: An earlier version of this story stated the Montreal Protocol was signed in 1989. It was signed in 1987.