The stakes of this audit are higher than they might appear. Ground-based ozone records are the backbone of nearly everything we know about the ozone layer: they underpin the detection of long-term trends, they measure whether the Montreal Protocol is actually healing the stratosphere, and they serve as the yardstick against which every satellite retrieval and reanalysis product is validated. If those yardsticks drift, the entire edifice of ozone recovery science wobbles with them. Yet until now, the most comprehensive performance assessment of the global network dated from 2008, before a wave of new satellite missions, new reanalysis products, and the explosive growth of the Pandonia Global Network reshaped the observing system.

The study covers six instrument types with very different personalities. The Dobson spectrophotometer, invented in the 1920s, measures ozone by comparing ultraviolet light at wavelength pairs that ozone absorbs differently, using a double monochromator to suppress stray light. The Brewer spectrophotometer, designed in the 1970s to automate the process, uses a diffraction grating to measure four wavelengths between 306 and 320 nanometers and can operate unattended in direct sun, moonlight, and zenith-sky modes. Together these are the World Meteorological Organization’s reference instruments, with accuracy better than one percent, traceable through a strict calibration hierarchy from world primary standards down to field instruments. Filter ozonometers, compact devices using fixed interference filters near 306 and 328 nanometers, offer operational simplicity at lower precision. Zenith-sky DOAS instruments, including the SAOZ systems that dominate polar monitoring, retrieve ozone from scattered twilight sunlight and carry a typical accuracy around five percent. Fourier-transform infrared spectrometers retrieve ozone from solar absorption spectra alongside a dozen other gases, and the newest network, Pandora, records direct-sun spectra from 280 to 525 nanometers with central processing through the Pandonia Global Network.

To grade these instruments fairly, the team built a harmonized statistical framework. For every station and every five-year window, they calculated the daily percentage difference between ground-based observations and comparison data from satellite overpasses and four reanalyses: ERA5, MERRA-2, JRA-3Q, and MSR2. Five metrics captured different failure modes: the mean bias, the standard deviation of daily and monthly differences, the amplitude of the seasonal cycle in the differences, and the range of annual means. A well-calibrated instrument should show a stable seasonal signature against fixed benchmarks; drifts in that signature can betray errors in extraterrestrial constants, stray light, or the effective temperature of the ozone layer itself.

The methodological innovation lies in how stations are classified. Rather than imposing one-size-fits-all limits, the team derived percentile-based thresholds from the internal statistics of each network against each benchmark. This matters because the networks differ in precision: the Brewer and Dobson networks show standard deviations of daily differences around two percent, while Filter and UVVIS instruments spread slightly wider, under three percent, especially at high latitudes. Applying Brewer-grade criteria to a UVVIS instrument would artificially condemn data that is perfectly acceptable for its class. The thresholds label each station-period as high quality, medium quality with minor issues, or not assured with major issues, and in most five-year periods fewer than twenty percent of stations worldwide fell into the not-assured category.

The headline numbers are striking. Ground-based annual means generally agree with satellite and reanalysis benchmarks within plus or minus two percent, with typical variability near two percent. The two WMO reference networks agree with each other and with the benchmarks mostly within one percent in recent decades. The world reference Dobson at Boulder and the world reference Brewer in Toronto, along with the European regional reference at Izaña, all show clean long-term records, confirming that the anchor points of the global system are holding steady. The Pandora network, despite being the youngest, shows statistical consistency comparable to the reference networks, with a standard deviation of daily mean differences of 1.83 percent, actually slightly better than Brewer’s 1.97 and Dobson’s 2.04, albeit over a shorter record.

Reanalysis datasets proved both a blessing and a caution. Because most reanalyses assimilate satellite ozone but not ground-based data, they serve as genuinely independent referees, and they allowed the assessment to extend back before the satellite era began in the 1970s. But the pre-satellite portions of ERA5 and JRA-3Q, where ozone is only indirectly constrained by meteorological observations, show biases of up to minus five percent relative to Dobson measurements. The team also caught a sudden two percent shift in MERRA-2 around 2005, when the assimilation system switched its satellite inputs from TOMS and SBUV to OMI and MLS, a reminder that reanalyses carry artifacts of their own. To handle conflicting verdicts from the four reanalyses, the researchers merged their assessments using the Dempster-Shafer evidence theory, which penalizes datasets that repeatedly disagree with their peers.

The audit also exposed the shifting geography of ozone monitoring. The Dobson network peaked at 76 active sites during 1985 to 1994 and has since declined to 51, raising concerns about long-term continuity even as the remaining stations perform superbly. The Brewer network has grown to 87 active sites, making it the largest reference network. Filter ozonometers have contracted from 52 sites to just 20, mostly in Russia. The NDACC FTIR and UVVIS networks report ozone at 22 and 17 sites respectively, while the Pandonia Global Network has surged since 2015 to 138 active sites. Notably, the study found and corrected a data-reporting error in the Filter network archive affecting observations from 2007 to 2024, and documented a confirmed hydrogen lamp failure at the Princess Elisabeth Station in Antarctica that degraded Brewer data in the 2022-23 and 2023-24 seasons before the instrument was recalibrated and returned to service in December 2024.

The practical payoff is a quality-assessed, station-level dataset with flags that data centres can use immediately. The World Ozone and Ultraviolet Radiation Data Centre and its counterparts can now steer users toward the most reliable records for climatologies, trend analyses, and satellite validation, identify sites and periods needing reprocessing, and prioritize calibration campaigns where they matter most. The audit also flags structural vulnerabilities: seventeen percent of Pandora sites are run by university researchers, whose instruments often retire with them, and vast regions including Africa and parts of South America remain thinly covered. A companion paper will turn the tables, using these quality-assured ground records to evaluate the satellites and reanalyses themselves. For a planet betting its ultraviolet protection on a handful of aging spectrophotometers and a rising generation of new instruments, knowing exactly which numbers to trust is the quiet foundation on which the ozone recovery story rests.

Subject of Research: Performance assessment of global ground-based total column ozone monitoring networks from 1940 to 2024

Article Title: Measurement report: Global Total Ozone Records – Part 1: Ground-based monitoring networks performance assessment and status review

Article References: Zhao, X., Fioletov, V., Petropavlovskikh, I., Velazco, V., Redondas, A., Solomatnikova, A., Vigouroux, C., Strong, K., Van Roozendael, M., Pazmino, A., Hanisco, T. F., Cede, A., Tiefengraber, M., Labow, G., Fujiwara, M., van der A, R., Griffin, D., McLinden, C., Kralidis, T., … Lee, S. C. (2026). Measurement report: Global Total Ozone Records – Part 1: Ground-based monitoring networks performance assessment and status review. Atmospheric Chemistry and Physics, 26(19), 13693-13720. https://doi.org/10.5194/acp-26-13693-2026

Image Credits: AI Generated

DOI: 10.5194/acp-26-13693-2026

Keywords: ozone layer, total column ozone, Dobson spectrophotometer, Brewer spectrophotometer, Pandora, satellite validation, reanalysis, Montreal Protocol, WOUDC, NDACC, atmospheric monitoring, data quality

The study, led by Sean M. Davis of the NOAA Chemical Sciences Laboratory in Boulder, Colorado, brought together an international team of more than forty researchers to tackle a deceptively simple question. Over the tropics, the belt between 30 degrees south and 30 degrees north that covers half the planet’s surface and is home to roughly 60 percent of humanity, how much has ozone actually changed since the year 2000, and do the world’s chemistry-climate models agree with what the satellites have seen? The answer, it turns out, depends heavily on which dataset you trust, and that dependence has consequences for how confidently we can forecast ozone’s trajectory through the rest of this century.

To disentangle the picture, the team divided the atmosphere into four vertical layers: the troposphere, which holds about 13 percent of the total ozone column; the lower stratosphere, contributing roughly 28 percent; the middle stratosphere, the largest share at about 41 percent; and the upper stratosphere, which supplies the remaining 18 percent. This layering matters because different physical processes dominate each region. In the lower stratosphere, ozone is governed mainly by transport, as the greenhouse-gas-driven acceleration of the Brewer-Dobson circulation pushes air upward through the tropical tropopause and thins the ozone layer there. In the upper stratosphere, chemistry rules: declining chlorine from the Montreal Protocol and cooling temperatures from rising carbon dioxide both push ozone upward, producing robust recovery. The troposphere adds its own complexity, because ozone there is manufactured from precursor emissions such as nitrogen oxides, which are rising across the developing tropics even as they fall over the midlatitudes of the Northern Hemisphere.

The statistical engine of the study was a dynamical linear model, a flexible regression technique that separates the slow background trend from the noise of natural variability, including the El Niño Southern Oscillation, the quasi-biennial oscillation of equatorial winds, the 11-year solar cycle, and stratospheric aerosol from volcanic eruptions. Applying this method to more than a dozen merged satellite datasets and reanalyses, and to two generations of coordinated chemistry-climate model simulations known as CCMI-1 and CCMI-2022, the researchers computed probability distributions of ozone change over the 2000 to 2021 period and measured how much those distributions overlapped.

The headline numbers look encouraging at first glance. Both model generations simulate an increase in tropical total column ozone since 2000, and the newer CCMI-2022 ensemble, with a mean change of +2.5 Dobson units, agrees slightly better with the observational estimate of +3.3 Dobson units than the older CCMI-1 ensemble does at +1.6 Dobson units. But that apparent agreement conceals systematic differences lurking beneath the surface. In the troposphere, the newer models show a substantial increase of about +1.5 Dobson units since 2000, driven largely by stronger rises in precursor emissions, whereas the older generation shows almost no change at +0.3 Dobson units. That single difference explains most of the gap between the two model generations in total column ozone.

More troubling still is the behavior of the observations themselves. In the upper stratosphere, observational estimates of the 2000 to 2021 change range from +0.6 Dobson units in the GOZCARDS dataset to +2.6 Dobson units in the SAGE-OSIRIS-SAGE composite, with as little as 2 percent overlap between the two distributions. In the middle stratosphere, the spread is equally stark, with one dataset showing essentially no change and another showing an increase of 2.5 Dobson units. Even datasets built largely from the same Aura Microwave Limb Sounder instrument, such as SWOOSH and GOZCARDS, diverge, apparently because of differences in how the pre-2004 data from earlier instruments were processed and merged. In the troposphere, the disagreement is arguably the worst of all: one composite shows zero change while another shows an increase of 2.4 Dobson units, with only 1 percent overlap between them.

Why does this matter so much? Because the models exhibit a tantalizing property that, in principle, could turn observations into a predictive tool. The researchers found that across the older CCMI-1 ensemble, models with faster ozone recovery during the first two decades of the twenty-first century also tend to project stronger ozone growth through the rest of the century, a correlation that holds in every layer of the stratosphere and is especially strong in the middle stratosphere. This kind of relationship, known as an emergent constraint, allows scientists to combine a measured present-day trend with the model-derived correlation to narrow the range of possible futures. Before the satellites can play that role, however, two conditions must hold: the correlation must be robust across model generations, and the observational trend estimates must be precise enough to be useful.

Both conditions fail. The emergent relationship that is so clear in the older model ensemble largely disappears in the newer CCMI-2022 simulations, surviving only as a borderline correlation in the middle stratosphere. The reasons remain uncertain, but the authors point to differences in greenhouse gas forcing between the two experimental designs, improved but changed representations of the quasi-biennial oscillation, and documented differences in stratospheric transport between model generations as plausible culprits. Meanwhile, when the researchers propagated the spread of observational trends through the model-derived relationships, the resulting uncertainty in the observationally informed end-of-century ozone projection was as large as, or larger than, the spread of the raw model projections themselves. Depending on which satellite record is treated as truth, one could conclude that the models either systematically overestimate or systematically underestimate future tropical ozone recovery.

Not everything in the analysis is mired in ambiguity. The lower stratosphere, the region where earlier studies first flagged an unexpected decline, shows the most consistent story. Nearly all observational datasets and both model ensembles agree that tropical lower stratospheric ozone has decreased since 2000, with multi-model means of −0.6 Dobson units for CCMI-1 and −1.1 Dobson units for CCMI-2022, and the observations trending somewhat more negative on average than the models. This decline is broadly consistent with the expected greenhouse-gas-driven speeding up of the shallow branch of the Brewer-Dobson circulation, although questions remain about the roles of tropopause height changes and very short-lived halogen compounds. Looking further ahead, the models robustly project continued lower stratospheric ozone decline through 2100, averaging about −5 Dobson units, even as the upper stratosphere gains roughly +6 Dobson units from chemical recovery.

The ultimate message of the study is a call to action for the observational community. The spread among existing satellite trend estimates, particularly in the troposphere and the middle and upper stratosphere, is wide enough to swamp the signal scientists are trying to measure, and reconciling those differences, possibly by hunting for instrumental drifts in the source records, is now a critical priority. Stable, trend-quality measurements from current and future satellite platforms, combined with a better understanding of why successive model generations behave differently, will be essential before observations can serve as the yardstick against which projections of the ozone layer’s future are judged. Until then, the recovery of the ozone layer over the tropics, the region where billions of people and much of Earth’s biodiversity depend on it, remains a story whose next chapter cannot yet be read with confidence.

Subject of Research: Uncertainties in observed tropical stratospheric and tropospheric ozone trends and their implications for constraining model projections of future ozone change

Article Title: Uncertainties in recent tropical stratospheric and tropospheric ozone changes limit the use of observational constraints for assessing future ozone change

Article References: Uncertainties in recent tropical stratospheric and tropospheric ozone changes limit the use of observational constraints for assessing future ozone change. (n.d.). https://doi.org/10.5194/acp-26-13817-2026

Image Credits: AI Generated

DOI: 10.5194/acp-26-13817-2026

Keywords: ozone layer, tropical ozone, stratosphere, troposphere, satellite observations, chemistry-climate models, Montreal Protocol, Brewer-Dobson circulation, ozone recovery, emergent constraint, CCMI, Dobson units

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