The storm, described in the journal Frontiers in Astronomy and Space Sciences, was remarkable not because it was the most extreme on record, but because of its rare combination of characteristics. On January 19, the north-south component of the interplanetary magnetic field turned sharply southward, opening an efficient channel for solar wind energy to pour into Earth’s magnetosphere. The auroral electrojet index, or AE, surged to 2,597 nanotesla, while the planetary Kp index climbed to 8-minus, marking severe global disturbance. The storm persisted through January 20, driving the Dst index down to minus 236 nanotesla, a signature of an intense event.

What followed was a deluge of energetic particles. The solar energetic particle event accompanying the January 18 coronal mass ejection persisted through January 19, and the flux of protons with energies above 10 megaelectronvolts, measured by the GOES satellites, peaked near 37,000 particle flux units on January 19. That figure ranks third in the recorded history of 10 MeV proton fluxes, exceeded only by the March 1991 event at 43,000 pfu and the October 1989 event at 40,000 pfu. Meanwhile, magnetospheric electrons rained down into the polar atmosphere throughout the storm’s main and recovery phases.

When these energetic particles slam into the upper atmosphere, they ionize air molecules and set off a cascade of chemistry that ultimately destroys ozone. The ionization produces odd hydrogen compounds, collectively known as HOx, which include the hydrogen atom, the hydroxyl radical, and the hydroperoxyl radical HO2. It also generates odd nitrogen compounds, or NOx, which are more stable and can be carried by winds far from where they were born. Both families act as catalysts: in two-step reaction cycles, each molecule converts ozone and atomic oxygen into two ordinary oxygen molecules without being consumed itself, allowing a single catalyst to destroy thousands of ozone molecules.

The HOx cycle dominates in the mesosphere, at altitudes of roughly 65 to 85 kilometers, where the pressure ranges from about 0.01 to 0.1 hectopascals. There, the hydroxyl radical reacts with ozone to form water and oxygen, and a subsequent reaction regenerates the radical while destroying another ozone molecule. NOx chemistry, by contrast, becomes more important in the stratosphere, where nitric oxide reacts with ozone to produce nitrogen dioxide, which in turn reacts with atomic oxygen to release NO again. Because NOx survives in the darkness of polar night, its influence can linger for weeks and be transported across vast distances by atmospheric circulation.

To track this chemistry in action, the researchers turned to the Microwave Limb Sounder on NASA’s Aura spacecraft, which has been scanning the atmosphere since 2004 from a quasi-polar orbit 705 kilometers up. By measuring thermal emission at millimeter and submillimeter wavelengths along the atmospheric limb, MLS provides daily near-global coverage with vertical resolution of about 3 to 6 kilometers, from the upper troposphere to the mesosphere. Crucially, it measures multiple species simultaneously, including ozone, HO2, and nitric acid, allowing scientists to link the appearance of ozone-destroying catalysts directly to the ozone loss itself.

The observations showed a striking hemispheric asymmetry. In the Northern Hemisphere, HO2 concentrations, which hovered near 0.1 parts per billion before the storm, jumped to between 1.3 and 1.6 ppb at pressures between 0.3 and 0.5 hPa on January 18, and exceeded 1.5 ppb near 0.4 to 0.5 hPa on January 19 and 20, more than an order of magnitude above background. In the Southern Hemisphere, the response was similar in character but weaker in magnitude, with values reaching about 0.8 to 0.9 ppb. After January 20, HO2 in both hemispheres rapidly returned to background levels, confirming that the enhancement was short-lived and tied directly to the particle precipitation.

Nitric acid, a reservoir species for reactive nitrogen, also responded. In the Northern Hemisphere, where wintertime background values are low, HNO3 rose from about 0.3 to 0.5 ppb to 1.1 to 1.2 ppb between January 19 and 20, centered near 0.05 hPa. In the Southern Hemisphere, background levels were already high, between 1.8 and 2.3 ppb, so the storm-driven increase to 2.5 to 2.7 ppb was harder to distinguish from natural variability. Nitric acid matters because it sequesters odd nitrogen and, in the cold polar winter, feeds polar stratospheric clouds whose surfaces activate chlorine compounds that destroy ozone, while its gravitational settling contributes to denitrification that prolongs springtime depletion.

The ozone consequences were dramatic. Following the onset of particle precipitation on January 18, Northern Hemisphere ozone in the 0.01 to 0.1 hPa range fell to roughly 70 percent of its pre-event average, reaching its lowest point on January 20. The Southern Hemisphere saw a milder decline of about 20 percent, peaking on January 19. The altitude of maximum ozone loss coincided precisely with the layer of enhanced HO2, strong evidence that catalytic hydrogen oxide chemistry, ignited by the particle influx, was the primary destruction mechanism. Notably, ozone in the affected layer had not recovered by the end of January, suggesting the storm may also have perturbed the dynamical circulation that transports ozone and its precursors at high latitudes.

Comparing the January 2026 event with earlier storms observed by the same instrument reveals a clear pattern. The January 2005 solar proton event depleted Northern Hemisphere mesospheric ozone by about 60 percent, the January 2012 event by roughly 50 percent, and the January 2026 event by approximately 70 percent, tracking the increasing intensity of the associated particle fluxes. The researchers conclude that the January 2026 storm, with its historically high proton flux and prolonged recovery, ranks among the most powerful space weather events of the satellite era, and that its impact on mesospheric chemistry was correspondingly the most severe of the three events studied.

Subject of Research: Energetic particle precipitation during the January 2026 geomagnetic storm and its impact on high-latitude ozone

Article Title: Ozone depletion in January 2026: impact of energetic particle precipitation

Article References: Mironova, I., Doronin, G., & Mironov, A. (2026). Ozone depletion in January 2026: impact of energetic particle precipitation. Frontiers in Astronomy and Space Sciences, 13, Article 1924724. https://doi.org/10.3389/fspas.2026.1924724

Image Credits: AI Generated

DOI: 10.3389/fspas.2026.1924724

Keywords: ozone depletion, energetic particle precipitation, geomagnetic storm, solar proton event, coronal mass ejection, mesosphere, HO2, nitric acid, Aura MLS, space weather, polar atmosphere, NOx chemistry

The study, led by Isabelle Sangha of the British Antarctic Survey and the University of Cambridge, together with Nathan Abraham, Andrew Orr, Hua Lu, NASA’s Michael Pitts and Lamont Poole, and Michael Weimer of the University of Bremen, targets the UK Earth System Model, UKESM1.1, running within version 13.9 of the Met Office Unified Model. The old PSC scheme in UKESM1.1 was, by the authors’ own assessment, too crude for the task. It assumed that nitric acid trihydrate, or NAT, the solid crystalline component of many PSCs, formed instantaneously in thermodynamic equilibrium whenever conditions allowed, and it fixed every NAT particle at a radius of one micrometre. Liquid supercooled ternary solution droplets, the STS particles that dominate much of the PSC volume, were not formed at all; their effect was approximated with a prescribed surface-area climatology. Ice particles came from the model’s cloud microphysics but were assigned a fixed ten-micrometre radius.

That simplicity mattered because real PSCs are anything but static. NAT particles grow over hours to days, drifting far from equilibrium and reaching sizes large enough to sediment out of the stratosphere, carrying nitric acid with them. This denitrification removes the very nitrogen species that would otherwise lock reactive chlorine back into harmless reservoirs, prolonging ozone destruction, a process especially consequential in the warmer Arctic where denitrification can tip marginal winters into severe ozone loss. Liquid STS droplets, meanwhile, are comparable to or even more effective than solid particles per unit surface area at activating chlorine. A model that cannot form STS, or that freezes NAT into an unrealistic equilibrium snapshot, will misrepresent both the chemistry and the timing of polar ozone depletion, undermining confidence in projections of future ozone recovery as the climate changes.

The fix imported a more sophisticated approach from the ICON-ART modelling framework developed in Germany. In the new scheme, NAT particles nucleate kinetically, beginning at a radius of just 0.1 micrometres when the partial pressure of nitric acid exceeds its saturation value over NAT with a three-kelvin supersaturation threshold, and then grow or shrink through a time-dependent diffusive growth equation that tracks the flux of nitric acid vapour to and from the particle surface. The particles are sorted into nine size bins spanning radii from 0.1 to 22.5 micrometres, with mass conserved as particles outgrow each bin and sedimentation velocities calculated interactively from Stokes’ law. STS droplets, for their part, now form thermodynamically through the uptake of nitric acid and water on aqueous sulphuric acid aerosols supplied by the model’s interactive GLOMAP aerosol microphysics, with the resulting droplet volume converted to surface area for heterogeneous chemistry.

To find out whether the upgrade actually works, the team turned to two of the most capable instruments ever flown for this purpose. The Cloud-Aerosol Lidar with Orthogonal Polarization, CALIOP, aboard the CALIPSO satellite, fired laser pulses into the stratosphere from 2006 to 2023 and used the polarization of the returning light to distinguish spherical liquid droplets from non-spherical solid particles, classifying PSCs into liquid-NAT mixtures, enhanced NAT mixtures, supercooled ternary solutions, ice, and wave ice. The Microwave Limb Sounder on NASA’s Aura satellite, flying in formation with CALIPSO in the A-Train, measured gas-phase nitric acid, water vapour, and ozone. Crucially, the team did not simply compare model output with satellite products; they computed the optical properties that CALIOP would have seen from the modelled particle populations, using Mie theory and T-matrix scattering calculations, and then ran the same classification algorithm on both datasets.

The comparison covered two contrasting winters: the extended 2008 Antarctic winter, when temperatures sat persistently below PSC formation thresholds, and the unusually cold 2009/2010 Arctic winter, which featured temperatures up to nine kelvin below climatology and a major sudden stratospheric warming in late January. The verdict was clear. The old scheme produced a narrow, essentially static distribution of cloud types, dominated by enhanced NAT mixtures, with almost no response to the changing conditions of the Arctic season and no ice-classified points at all. The new scheme broadened the simulated optical properties substantially, capturing the seasonal progression from NAT-dominated clouds in December through increasing STS and ice in January, and then the decline as the vortex warmed, in much closer agreement with CALIOP.

A sensitivity analysis across twelve simulations revealed which knobs mattered most. Increasing the maximum NAT number density shifted more cloud into the NAT mixture and ice categories, an effect most pronounced in the Arctic, where warmer temperatures limit ice formation. Increasing the prescribed ice number density, tested across values from 0.001 to 0.25 particles per cubic centimetre, pushed the optical signatures toward the ice and enhanced NAT regions, with the strongest effect over Antarctica. Yet even the best configurations fell short in one decisive respect: CALIOP observed substantial populations of ice clouds with scattering ratios between 5 and 20, while the simulations peaked around 6, and none of the runs produced the high-number-density wave ice that forms when mountain waves flash-freeze the stratosphere over mountainous terrain.

The gas-phase chemistry told a parallel story. The new scheme’s slower NAT growth and its partitioning of nitric acid between solid and liquid phases brought modelled gas-phase nitric acid closer to MLS observations, particularly within the polar vortex, where the satellite recorded a sharp June drop from eight to ten parts per billion down to roughly two as clouds sequestered the acid. But every simulation, old and new, started the season with a substantial nitric acid deficit, a known low reactive-nitrogen bias in UKESM1.1 that the authors caution may have been partly masked by the old scheme’s tendency to convert all available acid into NAT. Ozone, remarkably, was barely affected except near the edge of the polar vortex, where the new runs agreed better with MLS from August to October. The authors attribute this to saturation: deep inside the vortex, ozone depletion is already near-complete under persistently cold conditions, so changes in cloud microphysics matter most at the margins.

The remaining gaps point the way forward. The prescribed ice and NAT number densities constrain particles to a narrow range of surface areas, and the model lacks a scheme for the adiabatic cooling induced by mountain waves, which an earlier UKESM version had implemented and which can trigger the rapid, localised ice formation that seeds high-number-density NAT mother clouds downstream. Allowing number densities to vary with cooling rate, and reinstating the mountain-wave scheme, could close the gap with the most optically extreme clouds CALIOP observed. For now, the study stands as a careful, technically rigorous demonstration that how a model builds its clouds, particle by particle and bin by bin, determines how well it can be trusted to project the future of the ozone layer, a shield still recovering from a century of human chemistry and one whose fate is written, in part, in the coldest clouds on Earth.

Subject of Research: Updated polar stratospheric cloud parameterisation in the UK Earth System Model evaluated against CALIOP and MLS satellite observations

Article Title: Assessment of an updated polar stratospheric cloud parameterisation for the UK Earth System Model (UKESM1.1) within the UK Met Office Unified Model (v13.9) using CALIOP and MLS observations

Article References: Sangha, I., Abraham, N. L., Orr, A., Lu, H., Pitts, M. C., Poole, L. R., & Weimer, M. (2026). Assessment of an updated polar stratospheric cloud parameterisation for the UK Earth System Model (UKESM1.1) within the UK Met Office Unified Model (v13.9) using CALIOP and MLS observations. Geoscientific Model Development, 19(19), 9857-9877. https://doi.org/10.5194/gmd-19-9857-2026

Image Credits: AI Generated

DOI: 10.5194/gmd-19-9857-2026

Keywords: polar stratospheric clouds, ozone depletion, UKESM1.1, CALIOP, MLS, nitric acid trihydrate, supercooled ternary solution, stratospheric chemistry, Earth system modelling, Antarctic ozone hole, polar vortex, satellite observations

Short-lived halogens, or SLH, are a deceptively simple family of compounds with outsized consequences. The group includes organic very short-lived substances such as bromoform, dibromomethane and methyl iodide, which are emitted largely by biologically active oceans as by-products of algae and phytoplankton metabolism, as well as inorganic halogen families — collectively labelled Cly, Bry and Iy — that cycle rapidly between reactive and reservoir forms. Unlike the infamous chlorofluorocarbons that linger for decades and only break down in the stratosphere, these short-lived species are destroyed quickly, releasing chlorine, bromine and iodine atoms low in the atmosphere where they catalytically attack ozone, alter the abundance of the hydroxyl radical that acts as the atmosphere’s detergent, and reshape the partitioning of nitrogen oxides that govern smog formation.

The new release is technically ambitious. CESM2-SLH embeds offline emission inventories for nine oceanic halocarbons derived from SeaWiFS chlorophyll-a climatology, anthropogenic emissions of dichloromethane and perchloroethylene based on recent inventories, and — crucially — online, interactive sources of inorganic halogens. These online sources include ozone-driven oxidation of iodide at the sea surface, which releases hypoiodous acid and molecular iodine and accounts for roughly three-quarters of all atmospheric iodine, and sea-salt aerosol dehalogenation, in which oxidised halogen reservoirs collide with airborne sea salt and liberate molecular chlorine, bromine and mixed halogens. The team had to retune accommodation coefficients — reducing them by factors of three to eight — because CESM2 represents sea-salt aerosol with a different modal scheme than its predecessor, producing up to ten times larger aerosol surfaces in the boundary layer.

Chemistry itself was expanded dramatically. The mechanism adds twelve chlorine, nine bromine and nineteen iodine species participating in more than two hundred new reactions spanning photolysis, gas-phase kinetics, heterogeneous recycling on aerosols, ice crystals and cloud droplets, and stratospheric activation pathways. Rate constants were updated to the latest Jet Propulsion Laboratory and IUPAC recommendations, and the photolysis of higher-order iodine oxides — species whose inclusion proved essential to reproduce upper-tropospheric iodine observations — is now handled explicitly. The computational price is a twenty to twenty-five percent increase in model runtime, a modest cost for what the chemistry delivers.

The headline result is striking: including short-lived halogens cuts global surface ozone by twenty-one to twenty-eight percent and tropospheric ozone by seventeen to twenty-two percent, with stratospheric columns dropping by up to three percent. Hydroxyl radical abundances fall by two to nine percent and nitrogen dioxide by one to ten percent, depending on configuration and resolution. Because ozone is both a pollutant and a greenhouse gas, and because OH controls the lifetime of methane, these numbers ripple through every projection of future air quality and climate forcing made with the model. The authors emphasise that halogen-driven ozone destruction in the troposphere reaches roughly 660 teragrams per year, dominated — somewhat surprisingly to non-specialists — by iodine chemistry, which alone accounts for up to thirty percent of ozone loss in the tropical free troposphere.

Validation against observations is encouraging. Simulated vertical profiles of organic source gases in the tropical tropopause layer fall within the ranges assessed by the most recent World Meteorological Organization ozone assessment, and the model reproduces the split between source gas injection and product gas injection that determines how efficiently halogens reach the stratosphere. Bromine from short-lived sources adds about five parts per trillion to the stratospheric loading — a twenty-five percent enhancement over long-lived sources — while iodine reaches the stratosphere almost entirely in inorganic form at roughly 0.7 parts per trillion. Comparisons with ship-based campaigns measuring iodine monoxide across the Pacific and Atlantic, and with coastal bromine observations at Cape Verde, show the model capturing magnitudes and day-to-day variability, if not always exact timing.

Perhaps the most consequential finding concerns the Antarctic ozone hole. When short-lived bromine and iodine chemistry is included, springtime ozone destruction over the polar vortex deepens by more than twenty Dobson units — about fourteen percent — in October, confirming earlier CESM1 results with the new model generation. Because the radiative balance of the climate system is exquisitely sensitive to ozone changes in the lowermost stratosphere, this region matters far beyond its thin vertical extent. Previous work by the same groups has shown that these halogens exert an overall cooling influence on climate through coupled effects on ozone, methane, aerosols and stratospheric water vapour, meaning that models lacking this chemistry may systematically misjudge both air quality trajectories and radiative forcing.

The release is deliberately user-friendly. Six new model compsets span nudged and free-running configurations at coarse and fine horizontal resolutions, from 1.9 by 2.5 degrees down to 0.9 by 1.25 degrees, and from low-top configurations reaching about forty kilometres to whole-atmosphere WACCM setups extending into the lower thermosphere. A new namelist section exposes scaling factors that let users adjust sea-salt dehalogenation and washout efficiencies when switching resolutions or meteorological datasets, and a technical appendix walks newcomers through spin-up requirements, emission specifiers, deposition lists and cross-section files. The authors recommend that new users start with the coarse, nudged configuration, where transport is consistent across experiments and chemical signals can be isolated cleanly from dynamical noise.

Not every development from the CESM1 era made the cut. Emission-driven methane simulations, continental inorganic halogen emissions from coal burning, polar sea-ice halogen sources, iodine release from dust, and the expanded sulphur chemistry involving methanethiol and hydroperoxymethyl thioformate are all absent from this first release, flagged for future work. The authors are candid that these omissions mean the simulated halogen influence is, in most respects, a lower limit. Even so, the message to the community is unambiguous: any serious simulation of past, present or future atmospheric composition — from urban smog to the ozone hole — should now include the reactive halogens that the ocean, sea salt and human industry continuously pump into the air. With CESM2-SLH freely available, there is little excuse left to leave them out.

Subject of Research: Implementation of short-lived halogen sources and chemistry in the Community Earth System Model v2

Article Title: Short-lived halogen sources and chemistry in the Community Earth System Model v2 (CESM2-SLH)

Article References: Fernandez, R. P., Cuevas, C. A., Villamayor, J., Feinberg, A., Kinnison, D. E., Vitt, F., Bossolasco, A., Barrera, J. A., Reynoso, A., Tomazzeli, O. G., Li, Q., & Saiz-Lopez, A. (2026). Short-lived halogen sources and chemistry in the Community Earth System Model v2 (CESM2-SLH). Geoscientific Model Development, 19(19), 9325-9376. https://doi.org/10.5194/gmd-19-9325-2026

Image Credits: AI Generated

DOI: 10.5194/gmd-19-9325-2026

Keywords: short-lived halogens, CESM2, atmospheric chemistry, ozone depletion, iodine emissions, bromine, chlorine, sea-salt aerosol, stratosphere, tropospheric ozone, hydroxyl radical, climate modelling

The numbers behind the argument are stark. According to the 2024 Global Nitrous Oxide Assessment, a joint effort by the Climate and Clean Air Coalition, the United Nations Environment Programme, the Food and Agriculture Organization and the International Nitrogen Management System, nitrous oxide has a 100-year global warming potential of 273, making it the third most abundantly emitted greenhouse gas. It accounts for roughly five percent of global greenhouse gas emissions in carbon dioxide equivalents and about ten percent of all warming since the Industrial Revolution. Its ozone-depletion-potential-weighted emissions now exceed those of all other ozone-depleting substances combined. Emissions are rising faster than even the most pessimistic scenarios projected, driven by rising demand for food and animal protein and by industrial chemicals such as nitric and adipic acid.

About three-quarters of anthropogenic nitrous oxide comes from agriculture, specifically the over-application of synthetic fertilizer and manure. Microbes transform excess nitrogen into the gas through nitrification and incomplete denitrification in soils, or indirectly after nitrogen is first lost as ammonia or nitrate. The consequences of continued inaction are severe: if current trends persist while climate policy concentrates on carbon dioxide and methane, stratospheric ozone levels could sink below the lowest recorded values of the 1990s, pushing certain skin cancer rates up by as much as ten percent. Limiting warming to 1.5 degrees Celsius, the authors contend, is likely impossible without ambitious nitrous oxide cuts, which could reduce emissions roughly 40 percent below 2020 levels by 2050 and avoid 235 billion tons of carbon dioxide equivalent.

The co-benefits extend well beyond climate and ozone. Because nitrous oxide sits within the intertwined nitrogen cycle, abatement would also curb ammonia and nitrogen oxides, major air pollutants, and nitrate, a key water contaminant. The assessment estimates that ambitious action could avoid up to twenty million premature deaths by 2050 through improved air quality alone. Policy momentum has begun to build around nitrogen more broadly, with United Nations Environment Assembly resolutions, the 2019 Colombo Declaration and the Kunming-Montreal Global Biodiversity Framework all calling for nitrogen losses to be halved by 2030. Yet the authors observe a troubling disconnect: only eleven Nationally Determined Contributions contain quantitative nitrous oxide targets, covering roughly thirteen percent of global emissions, and the Montreal Protocol has never come close to listing the gas despite its inclusion in the Vienna Convention’s Annex I nearly four decades ago.

Why the oversight? The authors trace it to historical contingencies. International climate governance grew out of concern over fossil carbon dioxide, and it took decades for non-CO2 gases to receive serious attention; methane only got its Global Methane Pledge in 2021, and hydrofluorocarbons were shifted to the Montreal Protocol in 2016 under the Kigali Amendment. On the ozone side, the dominant worry about stratospheric nitrogen oxides in the 1970s and 1980s involved high-altitude aircraft fleets that never materialized, while chlorofluorocarbons were rising faster and proved easier to abate than agriculture, a sector long treated as politically exceptional out of deference to food security and farm lobbies.

The legal case for action under the ozone regime is stronger than most policymakers assume. The 1985 Vienna Convention obliges parties to protect human health and the environment against activities that modify the ozone layer, and its negotiators explicitly flagged nitrogenous fertilizers as a concern. Article 2(10) of the Montreal Protocol allows parties to add substances to its annexes, and Decisions IX/24, XI/20 and XIII/5 establish a working procedure for evaluating new substances. Nitrous oxide was formally added to the Ozone Secretariat’s list of reported new substances in May 2012. The authors conclude that the ozone regime already possesses the legal authority and purview to control the gas, and that a special report from the Protocol’s three assessment panels could provide the scientific foundation for a formal proposal.

What makes the Montreal Protocol uniquely suited to this task is its architecture. Its “start and strengthen” approach has allowed the treaty to evolve as science matured, expanding from its original controlled substances to nearly a hundred chemicals phased out by 99 percent across 198 parties. Independent scientific, environmental effects, and technology assessment panels feed policy-relevant expertise into the process, while the Multilateral Fund has disbursed 4.3 billion dollars across 144 developing countries to finance compliance. Crucially, the Protocol regulates production and consumption rather than diffuse emissions, making enforcement tractable. For agriculture, the authors argue, this translates naturally into targets for nitrogen use efficiency or nitrogen surplus, metrics already tracked at national scale and convertible into nitrous oxide estimates through well-validated emission factors.

The Protocol also has direct experience with a dangerous agricultural input. It eliminated methyl bromide, a soil fumigant, through Multilateral Fund projects that trained tens and even hundreds of thousands of farmers in countries from Argentina to Malawi. The phase-out was painful and drawn out, weakened by industry pressure and generous critical-use exemptions, but it established a template for transitioning away from inputs once considered essential. Nitrous oxide presents harder problems: it is emitted from virtually every agricultural sub-sector, nitrogen inputs cannot simply be banned, and enforcement is more diffuse. Even so, abatement practices such as enhanced-efficiency fertilizers, nitrification inhibitors, precision irrigation and the 4R nutrient stewardship framework can cut agricultural emissions by up to half without sacrificing yields. The authors propose minimum efficiency standards informed by a dedicated task force, alongside a “shared responsibility” model that spreads accountability across fertilizer producers, insurers, financiers and food retailers rather than dumping the regulatory burden on farmers alone.

The fastest wins, however, lie in industry. Nitric and adipic acid production contributes only about five percent of global emissions, but roughly six hundred facilities worldwide can deploy catalytic decomposition or thermal destruction technologies that eliminate over 99 percent of by-product nitrous oxide, often at breakeven prices between zero and twenty dollars per ton of carbon dioxide equivalent. The Kigali Amendment already created a precedent for controlling by-product emissions through its treatment of HFC-23, and the Multilateral Fund has financed destruction obligations in China, Argentina and Mexico. A comparable requirement for industrial nitrous oxide could avoid 2.5 billion tons of carbon dioxide equivalent by 2050 and generate ozone benefits equivalent to some 160,000 tons of CFC-11, building momentum for the harder agricultural phase. Food security concerns can be managed, the authors note, by exempting countries with low nitrogen consumption, an approach modeled on the Protocol’s Article 5 thresholds, which would leave nearly all of sub-Saharan Africa free to increase fertilizer use. Genuine obstacles remain, including pollution swapping within the reactive nitrogen cascade, overlapping mandates across conventions, and a volatile geopolitical landscape that sent fertilizer prices up more than 100 percent after 2021 and over 50 percent in 2026. But the authors insist that instability does not preclude opportunity: roadmap-building, panel reports and demonstration projects now could position a coalition of willing parties to act decisively when the political moment arrives, turning the ozone treaty’s proven machinery against a threat the world can no longer afford to ignore.

The governance gap the authors describe is best understood as a sequencing problem in international environmental law. Each successive wave of atmospheric regulation has targeted the gases whose science was mature and whose abatement was cheapest, leaving the politically awkward remainder for later. Methane followed this pattern, moving from vague coverage under the climate convention to dedicated pledges and reporting frameworks only once satellite-based detection made large emission sources visible and attributable. Nitrous oxide now stands at a comparable inflection point, with growing measurement capacity from atmospheric monitoring networks and emerging satellite instruments offering new possibilities for verifying whether national actions actually reduce concentrations.

The stratospheric stakes deserve particular emphasis. Unlike carbon dioxide, which persists for centuries but does not interact directly with ozone chemistry, nitrous oxide is converted in the stratosphere into nitrogen oxides that catalytically destroy ozone, and this chemistry operates regardless of where the emissions originate. Because the gas has an atmospheric lifetime of roughly a century, every ton emitted today commits the ozone layer to decades of additional depletion. This long memory means that delayed action locks in damage that no future agreement can quickly reverse, in contrast to short-lived pollutants where rapid cuts yield near-term benefits.

The equity dimensions of the proposal also merit attention. Developing countries have historically contributed little to cumulative nitrous oxide emissions, yet many face rising fertilizer demand as they expand food production. Any credible regime would therefore need to mirror the principle of common but differentiated responsibility, combining grace periods, financial support and technology transfer. The authors’ suggestion of exemptions for low-consumption countries reflects this logic, and the Multilateral Fund’s track record suggests that financing mechanisms, once established, can build the technical capacity that ambitious targets presuppose.

Subject of Research: Governance of nitrous oxide emissions under the Montreal Protocol as a pathway to protect both climate and stratospheric ozone

Article Title: Nitrous oxide under the international ozone regime: A new governance pathway for a growing threat

Article References: Kanter, D. R., Ferris, T., Nickson, T., & Reinikainen, T. (2026). Nitrous oxide under the international ozone regime: A new governance pathway for a growing threat. Ambio. https://doi.org/10.1007/s13280-026-02477-w

Image Credits: AI Generated

DOI: 10.1007/s13280-026-02477-w

Keywords: nitrous oxide, Montreal Protocol, ozone depletion, climate change, greenhouse gases, nitrogen pollution, agriculture, fertilizer, environmental governance, Vienna Convention, Kigali Amendment, nitrogen use efficiency