The lifetime of nitrous oxide is decreasing more quickly than expected, which is changing climate projections.

Scientists at the University of California, Irvine report that climate change is accelerating the breakdown of nitrous oxide in the atmosphere. This gas is both a powerful greenhouse contributor and a key ozone-depleting substance, and its faster removal is adding new uncertainty to climate projections for the remainder of the 21st century.

Drawing on two decades of satellite measurements from NASA’s Microwave Limb Sounder (2004-2024), researchers from UC Irvine’s Department of Earth System Science determined that the atmospheric lifetime of N2O is shrinking by about 1.4 percent per decade.

This trend reflects climate-driven changes in stratospheric temperature and circulation patterns, and its magnitude is similar to the spread between the emissions scenarios used by the Intergovernmental Panel on Climate Change.

The findings were published in Proceedings of the National Academy of Sciences.

“The change in the life cycle of atmospheric nitrous oxide is a critical piece of the puzzle that has been largely overlooked,” said co-author Michael Prather, UC Irvine professor of Earth system science. “While most research has focused on projecting changing N2O emissions from human activities, we’ve shown that climate change itself is altering how quickly this gas is destroyed in the stratosphere – and this effect cannot be ignored in future climate assessments.”

A major greenhouse gas at stake

Nitrous oxide ranks as the third most important long-lived greenhouse gas after carbon dioxide and methane, and it is now the leading ozone-depleting substance linked to human activity. Concentrations reached roughly 337 parts per billion in 2024 and are rising at close to 3 percent per decade. Prather emphasized that accurately tracking its behavior is essential for both climate mitigation strategies and efforts to protect the ozone layer.

The study shows that estimating future N2O levels requires more than accounting for emissions from agriculture, industry, and natural sources. It also depends on how climate change alters the stratosphere, where this gas is broken down. This atmospheric region extends from about 10 to 50 kilometers above Earth’s surface.

Observations reveal accelerating decay

According to the findings, the average atmospheric lifetime of nitrous oxide is currently about 117 years, but it is shortening by roughly one and a half years each decade.

This trend aligns with observed changes in stratospheric circulation and temperature. When extended to 2100, the shortening lifetime could alter atmospheric N2O concentrations to a degree comparable with major shifts in greenhouse gas emissions scenarios used by the Intergovernmental Panel on Climate Change.

The researchers explain that while rising carbon dioxide warms the lower atmosphere, it cools the stratosphere. This cooling changes the chemistry that destroys N2O and produces nitrogen oxides, which contribute to ozone depletion.

“This cooling, combined with changes in atmospheric circulation patterns, is speeding up the transport of N2O to the regions where it’s destroyed. It’s a feedback loop that adds another layer of complexity to climate projections,” explained co-author Calum Wilson, a UC Irvine graduate student researcher in Earth system science.

Lifetime shifts rival emissions scenarios

The team found that the uncertainty caused by changes in N2O lifetime is similar in scale to the differences among Shared Socioeconomic Pathways, which are used to model future greenhouse gas concentrations under varying policy and development conditions.

For instance, continuing the observed trend in lifetime reduction would lower projected N2O levels by an amount comparable to moving from a high-emissions pathway (SSP3-7.0) to more moderate pathways such as SSP1-2.6 or SSP2-4.5, even if emissions themselves remain unchanged.

Prather noted that these results could affect multiple areas, including climate projections through 2100, calculations of N2O’s global warming potential, assessments of ozone depletion, international climate agreements such as the Paris Agreement, and strategies for reducing emissions from agriculture and industry.

How nitrous oxide is removed

Nitrous oxide builds up in the lower atmosphere from natural sources like soils and oceans, as well as human activities such as farming, fossil fuel use, and industrial production. It is then carried upward into the tropical stratosphere by large-scale atmospheric circulation, where it is broken down by ultraviolet radiation and chemical reactions.

About 90 percent of N2O destruction occurs through sunlight-driven breakdown in the middle and upper stratosphere, at altitudes between 25 and 40 kilometers. The remaining 10 percent is removed through reactions with excited oxygen atoms.

During this breakdown process, some N2O molecules generate nitrogen oxides that help destroy ozone. This makes nitrous oxide the most significant human-produced ozone-depleting substance today, following the global phaseout of chlorofluorocarbons under the Montreal Protocol, which was based on Nobel Prize-winning work by UC Irvine Professor F. Sherwood Rowland and postdoctoral researcher Mario Molina.

Models miss key atmospheric feedbacks

The authors emphasize that although observations and theory strongly indicate that climate change is altering N2O lifetime, more advanced chemistry-climate modeling is needed to fully capture the feedbacks involved. This includes understanding the full sequence from N2O to nitrogen oxides to ozone to N2O photolysis (breakdown by sunlight) and back to its atmospheric lifetime. Additional research is also needed on regional differences in stratospheric circulation, interactions with other atmospheric changes, and improved projections under different climate pathways.

“This work highlights a gap in current Earth system models,” Prather added. “Stratospheric chemistry and dynamics present uncertainties in projecting N2O that are as large as uncertainties across different emissions scenarios. We need to incorporate these effects into the models used for international climate assessments.”