Emissions of nitrous oxide, a climate super-pollutant hundreds of times more potent than carbon dioxide, are rising faster than previously thought—at a rate that not only threatens international targets to limit global warming, but is consistent with a worst-case trajectory for climate change, a new study suggests.
The findings, reported Wednesday in the journal Nature, underscore the need for strong climate policies that do not focus solely on carbon dioxide, the dominant greenhouse gas.
The study, arguably the most comprehensive assessment of the global nitrogen cycle ever conducted, found that nitrous oxide (N2O) emissions caused by human activities have increased by 30 percent since 1980. Those emissions, more than two-thirds of which come from agriculture, account for nearly half of all nitrous oxide released over the past decade, with the rest coming from natural ecosystems.
Nitrous oxide is the third most important greenhouse gas after carbon dioxide and methane, and is responsible for roughly 7 percent of global warming since preindustrial times. It is nearly 300 times more potent at warming the planet than carbon dioxide, which means that even small sources of emissions can have an outsized impact on the climate.
Nitrous oxide is also the largest contributor to atmospheric ozone depletion that is not controlled by the Montreal Protocol, an international agreement developed in the 1980s to phase out ozone depleting chemicals through mandatory emissions reductions. Some policy experts say the agreement should now be extended to include nitrous oxide.
Emissions reported in the current study are in line with, or slightly higher than, a "worst case" emissions scenario by the United Nations Intergovernmental Panel on Climate Change.
Such a scenario assumes that growth in fossil fuel power production and the use of nitrogen-based fertilizer, the driving force in human-caused nitrous oxide emissions, continue unabated.
Under the worst case scenario, the world's average temperature would rise by approximately 4.3 degrees Celsius above pre-industrial times, far higher than the limit of 1.5 to 2 degrees Celsius of warming targeted in the Paris climate agreement.
"Current nitrous oxide emissions are not tenable," said Hanqin Tian, director of the International Center for Climate and Global Change Research at Auburn University and the study's lead author. "The numbers are very large, and the increases are very rapid."
The study looked at both natural and human caused, or "anthropogenic," sources of nitrous oxide emissions as well as natural "sinks," biochemical processes that break down nitrous oxide. The study combined "bottom-up" inventories, estimates of known nitrous oxide emissions, with "top-down" assessments based on measurements of nitrous oxide concentrations in the atmosphere, from 1980-2016. Each of the past four decades had higher N2O emissions than the prior decade with some of the highest growth seen in Brazil, China and India where there have been large increases in crop production and livestock.
Curbing nitrous oxide emissions is difficult because the vast majority of anthropogenic emissions are driven by the use of nitrogen-based fertilizer, which emits nitrous oxide as it breaks down in the soil if not taken up by plants. Farmers worldwide depend on nitrogen fertilizer to increase crop production.
However, more efficient fertilizer use can curb nitrous oxide emissions. N2O emissions from agriculture in Europe decreased by 21 percent between 1990 and 2010 in response to agricultural policy that favors optimization and reduction of fertilizer use, according to the study. The policy, known as the "Nitrates Directive," was developed to protect European waters from nitrogen pollution, which causes algae blooms and "dead zones" in freshwater and marine environments and occurs when too much nitrogen fertilizer or manure is used.
Tian says the policy can serve as a model for other regions.
"People think food production and nitrous oxide emissions conflict, but the European experience offers hope that we can increase food production but also reduce N2O emissions," Tian said.
David Kanter, an environmental studies professor at New York University, said nitrogen pollution is still a major issue in European waters and the Union's regulatory policies would be difficult to replicate elsewhere.
"The EU system has been partially successful but not a complete success story," said Kanter, the vice-chair of the International Nitrogen Initiative, an effort aimed at optimizing the benefits of nitrogen use in food production while minimizing its negative environmental impacts. "It will really depend on the regulatory status of other countries. The farm lobby, which is one of the most powerful political forces, particularly in developing countries, makes it [nitrogen fertilizer use] really hard to regulate."
Kanter noted that nitrous oxide is the largest remaining cause of ozone depletion and warned that if emissions continue to increase, they could undermine the decades-long effort to restore the so-called "ozone hole" under the Montreal Protocol.
"There are some success stories, but overall this is not a rosy picture," Kanter said. "This is a wake-up call."
Anthropogenic emissions of nitrous oxide may actually be slightly higher than those reported in the current study. Eleven adipic acid plants in China, chemical plants that make a key ingredient of nylon and polyurethane, likely emit hundreds of thousands of metric tons of nitrous oxide per year despite proven, low cost abatement technology that could reduce 95 percent or more of total emissions, according to a recent InsideClimate News investigation.
The investigation found that nitrous oxide emissions from adipic acid plants in China may equal the greenhouse gas emissions of approximately 25 million automobiles, more than all the cars in California, Beijing and Shanghai combined.
The new study in Nature uses emissions inventories that assume most of the plants' emissions are abated. However, interviews with plant operators, government officials and outside experts suggest abatement technology that was previously installed under an international emissions trading program may have ceased operation when funding for the program dried up.
If the vast majority of emissions from the plants are not abated, global anthropogenic emissions of nitrous oxide could be approximately two percent higher than stated in the study.
Wilfried Winiwarter, a senior research scholar on greenhouse gases with the International Institute for Applied Systems Analysis, based in Laxenburg, Austria and a co-author, said some uncertainty remains related to emissions from the Chinese plants and the higher figure "cannot be ruled out."
Winiwarter said the increase would not significantly change the findings of the current assessment, but may offer an opportunity to jumpstart emission reductions.
"We should start where it is most easily possible, and this is where it is most easily possible," Winiwarter said of any potential emission reductions at adipic acid plants. "You have a dozen or less plants that have to be equipped. This is easy to do."
Climate policy advocates said the current study provides greater clarity on the enormity of global nitrous oxide emissions and underscores the need for decisive action to reduce emissions.
"The study helps build the groundwork for a global plan to reduce N2O emissions to sustainable levels," said David Doniger, a senior strategic director with the Natural Resources Defense Council.
Doniger and others said the best way to reduce emissions of nitrous oxide is through the Montreal Protocol, a binding international agreement that has proven highly successful in reducing other pollutants that deplete atmospheric ozone and warm the planet.
Adding nitrous oxide would continue to significantly expand the scope of the Protocol, which was recently amended to include hydrofluorocarbons (HFCs), chemical refrigerants that are both ozone depleters and potent greenhouse gasses.
Adding nitrous oxide would require broad international support, but, if approved, would provide a stronger regulatory framework for emissions reductions than the Paris climate agreement, which is voluntary.
Durwood Zaelke, president of the Institute for Governance & Sustainable Development, said the high rate of emissions reported in the study, and the "climate emergency" facing the planet, are further justification for the Montreal Protocol to start regulating nitrous oxide.
"It is able to move fast and it's been effective with nearly 100 chemicals that it has controlled in the past," Zaelke said. "You've got to put your best players into the game, and if we are trying to solve climate change, the best player is the Montreal Protocol."