Global warming is speeding breakdown of major greenhouse gas, research shows - Earth - Earth Sciences - Environment

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Scientists at the University of California, Irvine have discovered that climate change is causing nitrous oxide, a potent greenhouse gas and ozone-depleting substance, to break down in the atmosphere more quickly than previously thought, introducing significant uncertainty into climate projections for the rest of the 21st century.

Satellite data reveal shifting lifetime

Using extended satellite observations from NASA's Microwave Limb Sounder spanning two decades (2004–2024), researchers from UC Irvine's Department of Earth System Science found that N₂O's atmospheric lifetime is decreasing at a rate of 1.4% per decade. This shift, which is due to climate change-driven alterations in stratospheric circulation and temperature, is comparable in magnitude to differences across the various emissions scenarios currently used by the Intergovernmental Panel on Climate Change for climate assessments.

The UC Irvine scientists shared their findings in a paper published today in Proceedings of the National Academy of Sciences.

A critical but overlooked feedback

"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."

According to climate scientists, nitrous oxide is the third-most-important long-lived greenhouse gas after carbon dioxide and methane, and it's currently the dominant ozone-depleting substance produced by human activities. With atmospheric concentrations reaching approximately 337 parts per billion in 2024 and increasing at about 3% per decade, understanding N₂O's behavior is critical for both climate change mitigation and stratospheric ozone protection efforts, according to Prather.

Changing sink complicates future projections

The research reveals that projecting atmospheric N₂O abundance involves not just understanding emissions from agriculture, industry, and natural sources, but also accounting for how climate change affects the stratospheric sink where N₂O is destroyed. The stratosphere is the atmospheric layer about 10 to 50 kilometers above Earth's surface.

Key findings outlined in the paper include the revelation that the current mean lifetime of N₂O is 117 years, but that is decreasing at approximately a year and a half per decade. The decrease in N₂O lifespan is consistent with observed changes in stratospheric circulation and temperature patterns. When extrapolated to the year 2100, the lifetime change produces shifts in atmospheric nitrous oxide equivalent to significant shifts in Intergovernmental Panel on Climate Change greenhouse gas emissions scenarios.

The study's authors note that while a buildup of atmospheric carbon dioxide results in warmer temperatures near Earth's surface, CO₂ cools the stratosphere, which affects the chemical reactions that destroy N₂O and produce nitrogen oxides that deplete ozone.

"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.

Uncertainty rivals emissions scenarios

The research demonstrates that the uncertainty introduced by the changing N₂O lifetime is comparable to the uncertainty across different Shared Socioeconomic Pathways, the scenarios used by climate scientists to project future greenhouse gas concentrations under different policy and development assumptions.

For example, the scientists found that a continuation of the observed lifetime decrease trend would reduce projected N₂O levels by an amount equivalent to shifting from a high-emissions scenario (SSP3-7.0) to a moderate-emissions scenario (SSP1-2.6 or SSP2-4.5)—without any change in actual emissions.

According to Prather, the study's conclusions have important implications for climate models and projections through 2100, global warming potential calculations for N₂O, ozone depletion assessments, international climate policy under the Paris Agreement, and agricultural and industrial emissions reduction strategies.

How nitrous oxide behaves aloft

Nitrous oxide accumulates in the lower atmosphere from both natural sources such as soils and ocean water and human activities including agriculture, fossil fuel combustion, and industrial processes. It is then transported into the tropical stratosphere by global circulation patterns, where ultraviolet radiation and chemical reactions destroy it.

The primary sink, accounting for 90% of N₂O eradication, is breakdown by sunlight in the middle and upper stratosphere, approximately 25 to 40 kilometers above Earth's surface. The remaining 10% is annihilated through reaction with excited oxygen atoms.

During this process, some N₂O molecules produce nitrogen oxides that catalyze ozone destruction, making N₂O the most important human-emitted ozone-depleting substance in the current era, following the phaseout of chlorofluorocarbons under the Montreal Protocol—the outcome of Nobel Prize-winning research conducted by UC Irvine Professor F. Sherwood Rowland and postdoctoral researcher Mario Molina.

Next steps for climate modelers

The study authors note that while their observational analysis and theoretical understanding point clearly to climate-driven changes in the N₂O lifetime, comprehensive chemistry-climate model experiments are needed to fully quantify all the feedback mechanisms involved, particularly the complete chain of N₂O to nitrogen oxides to ozone to N₂O photolysis (breakdown by sunlight) to the N₂O lifetime.

Also needed are further studies into regional variations in stratospheric circulation, interactions with other atmospheric composition changes, and refinement of projections under different climate scenarios.

"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." 

Provided by University of California, Irvine   

by Brian Bell, University of California, Irvine

edited by Gaby Clark, reviewed by Robert Egan 

Source: Global warming is speeding breakdown of major greenhouse gas, research shows