Science: Nitrous Oxide Emissions Now the Most Significant Threat to Ozone

It's commonly known as "laughing gas," but research published this week in Science finds that nitrous oxide is no joke—it has become the world's top human-caused, ozone-depleting emission. Such ozone-depleting substances damage the Earth's atmosphere and are responsible for causing the large hole in the ozone layer over Antarctica.

The new research explains that, under current practices, nitrous oxide emissions from human agricultural and industrial processes are positioned to remain the largest ozone-depleting substance pumped into the atmosphere throughout the 21st century—unless we find a way to effectively control those emissions soon.

To reach these conclusions, A. R. Ravishankara and colleagues from the Earth System Research Laboratory at the U.S. National Oceanic and Atmospheric Administration (NOAA) in Boulder, Colorado, used a well-known atmospheric model to assess the potential damage that the gas could inflict upon the ozone layer.

"We wanted to take a fresh look at all the potentially ozone-damaging anthropogenic emissions, and find out whether nitrous oxide emissions are becoming more significant in relation to the compounds that have already been addressed by the Montreal Protocol," Ravishankara said in an email interview.

The Montreal Protocol is an international treaty that has successfully helped to stem other ozone-depleting emissions around the world and to reverse the growth rate of the Antarctic ozone hole. The Protocol does not currently regulate the use of nitrous oxide, but the researchers suggest that Earth's ozone layer might benefit if it did.

Unlike other more commonly known ozone depleting substances, such as chlorofluorocarbons, or CFCs, nitrous oxide has both natural and human-made sources. Its emissions are largely associated with current agricultural practices, such as the increased use of nitrogen-based fertilizers and the handling of animal waste matter. Other significant sources of the gas include the combustion of fossil fuels, the burning of biomass and biofuel, and industrial processes such as sewage treatment.

"The role of nitrous oxide in ozone depletion has been known for several decades," Ravishankara said. "But our study, for the first time, explicitly calculated that role using the same measures that have been applied to CFCs, halons, and other chlorine- and bromine-containing ozone-depleting substances."

Specifically, the researchers calculated an intrinsic property of nitrous oxide—its ozone depletion potential, or ODP—and then gauged how much human-produced emissions of the gas were being released around the world. They weighted the amount of those emissions with the gas's ODP in order to put nitrous oxide contribution on the same scale as CFCs, halons, and other common ozone-depleting substances.

"What is new in this study is that we have demonstrated that nitrous oxide can be thought of as an ozone-depleting substance in many of the same ways as other gases that are currently regulated by the Montreal Protocol," said Robert W. Portmann, a coauthor of the Science report, in a press release.

The results of the study indicate that nitrous oxide has become the largest and most significant ozone-depleting substance emitted by human activities—and it is expected to remain the most significant throughout this century.

In light of their findings, Ravishankara and colleagues suggest that limiting nitrous oxide emissions in the future could effectively speed the recovery of Earth's ozone layer, and also slow the human-induced forcing on the climate system—a "win-win" scenario for both ozone and climate.