When it comes to the global climate crisis, carbon dioxide emissions represent a problem that's massive, intractable and running short on time to solve. But it's not the only problem.
Other pollutants are rapidly warming our climate, too, sending scientists on a race to understand their implications before it's too late. For years, experts have warned about the risks from one pollutant in particular—nitrous oxide—and yet there's been little global action on it.
The reason: "It is intimately connected to food," said Ravi Ravishankara, an atmospheric chemist at Colorado State University who co-chaired a United Nations panel on stratospheric ozone from 2007 to 2015.
Nitrous oxide is 300 times more potent than carbon dioxide, and it also depletes the ozone layer. Since it also has a shorter life span, reducing it could have a faster, significant impact on global warming.
But the largest source of nitrous oxide is agriculture, particularly fertilized soil and animal waste, and that makes it harder to rein in. "One could imagine limiting carbon dioxide, less methane, less of lots of things. But nitrous oxide is so much a food production issue," Ravishankara said.
Since the 1960s, fertilizer use has shot up globally, helping usher in the "Green Revolution," which fed millions around the world. In the U.S. alone, the use of fertilizer has risen more than 200 percent over the past 60 years, even though the amount of cropland has stayed relatively constant. At the same time, the number of large industrialized livestock operations has also gone up, creating more manure "lagoons" and excess manure, which is often over-applied on cropland.
A 2013 report by the United Nations found that since the pre-industrial era, nitrous oxide emissions from human activities have increased 20 percent. At the time, the authors wrote that if nothing was done, those emissions were expected to double by 2050.
Despite nitrous oxide's role depleting the ozone layer, it is not included in the Montreal Protocol on Substances that Deplete the Ozone Layer, an international treaty that aims to restore the ozone layer by phasing out certain substances.
Here's what you should know about the potent pollutant:
So, What Is Nitrous Oxide?
Like other greenhouse gases, nitrous oxide absorbs radiation and traps heat in the atmosphere, where it can live for an average of 114 years, according to the EPA. That puts it in a sort-of middle ground of super pollutants.
Compared with carbon dioxide, which can live in the atmosphere for hundreds of years, nitrous oxide is around a relatively short time. But it stays in the atmosphere longer than other short-lived climate pollutants like black carbon (which exists in the atmosphere for days) or methane (which is around for 12 years).
Nitrous oxide also poses a second threat: while in the stratosphere, nitrous oxide is exposed to sunlight and oxygen which converts the gas into nitrogen oxides. Nitrogen oxides can damage the ozone layer, which humans rely on to prevent most of the sun's ultraviolet radiation from reaching earth's surface.
That double-threat effect results in the gas's potency. One pound of N2O warms the atmosphere about 300 times the amount that one pound of carbon dioxide does over a 100 year timescale. Its potency and relatively long life make N2O a dangerous contributor to climate change.
Where Does Nitrous Oxide Come From?
About 40 percent of nitrous oxide emissions come from human activities, and of those, the majority are from the way we use land—particularly agriculture. In the United States, about 75 percent of all N2O emissions from human activity are attributed to agriculture.
Especially in larger farming operations, livestock manure presents a two-fold emissions problem: it emits an enormous amount of methane, but it can create nitrous oxide too. "When the manure doesn't get access to oxygen, toward the bottom of the pit, it starts to convert into nitrous oxide," said Ben Lilliston, Director of Rural Strategies and Climate Change at the Institute for Agriculture and Trade Policy. "This also happens when manure gets overapplied to crop land."
When farmers add nitrogen fertilizer to their soil to help stimulate plant growth, only about half gets taken up by the plant, according to Neville Millar, a senior research coordinator at Michigan State University. The rest can be washed away in groundwater, or off-gassed as nitrous oxide or other gases.
Agriculture isn't the only culprit, though. Nitrous oxide is also emitted when fuels are burned, though how much depends on what type of fuel, and which combustion technology is used. It's also generated as a byproduct of the production of chemicals like nitric acid (used for fertilizer) or adipic acid (used to make nylon and other synthetic products). The treatment of domestic wastewater can also generate nitrous oxide.
What About Natural Sources?
In the summer of 2013, a pilot flew a tiny plane low over thawing Arctic permafrost as part of a study to determine what climate-forcing pollutants were being emitted and how much. The researchers behind the study, a team from Harvard and NOAA, thought they would find methane—and they did.
Permafrost is frozen land that contains ancient soil, sediment and organic material from plants and animals. It covers about a quarter of the Northern Hemisphere. As the Arctic warms at roughly twice the rate of the rest of the world, permafrost is beginning to thaw, and as it does, the ancient materials are exposed to oxygen, which causes them to release gases that are further contributing to warming.
Researchers globally have been trying to understand just how much methane could be contained in the permafrost. But the data collected in 2013—and published in a report in the journal of Atmospheric Chemistry and Physics earlier this year—also showed that nitrous oxide was also being emitted from the permafrost, at roughly 12 times the rate previously assumed.
"When I hit that first result, I went back through them and calculated them again. I thought I had made some little error," said Jordan Wilkerson, a graduate student who first discovered the nitrous oxide. "I looked a couple times and got the same answer."
The study covered about 120 square miles and only during the month of August. As such, it's hard to extrapolate what these findings could mean Arctic-wide.
But what's significant, says NOAA's Ron Dobosy, who was a co-author on the study, is that until Wilkerson's discovery, the Arctic was considered to be very nitrogen poor. Now it's clear that nitrous oxide emissions are present, and need to be studied further. "Methane is being covered more and more heavily. It's time for nitrous oxide to be covered more too," said Dobosy.