Ozone is best known as a component of urban smog, but it is also a potent phytotoxin that costs global agriculture billions of dollars annually and suppresses the land carbon sink worldwide. Tropical forests are particularly exposed. The very traits that make them so productive, including dense canopies and high stomatal conductance, allow a large flux of ozone to enter leaves year-round. Recent experimental work has shown that tropical tree species are as susceptible to ozone damage as, or more susceptible than, many temperate species, meaning earlier studies that assumed lower sensitivity may have underestimated the problem. With the Amazon holding around 120 petagrams of carbon, even modest percentage losses in productivity carry global consequences for climate projections.

The research team employed the Joint UK Land Environment Simulator, JULES, a process-based land surface model that represents photosynthesis, stomatal behaviour, soil moisture dynamics and vegetation competition. Ozone concentrations were supplied by the UK Earth System Model, UKESM1, whose atmospheric chemistry scheme tracks the full oxidation cycles of ozone precursors including nitrogen oxides, carbon monoxide, methane and biogenic volatile organic compounds. The simulations spanned 1997 to 2024, a period punctuated by six major Amazon droughts, and were driven by observed meteorology from the CRU-JRA reanalysis, prescribed atmospheric carbon dioxide and annually varying crop and pasture fractions. By running paired simulations with and without ozone damage, the team isolated the carbon lost to the pollutant in every year of the record.

Validation of the model was a central concern. Simulated gross primary productivity reproduced the general interannual variability captured by two independent satellite products, Fluxsat and GOSIF, and the model’s tropospheric nitrogen dioxide columns matched observations from the OMI instrument on NASA’s Aura satellite, both in magnitude and spatial pattern during drought years. Because long-term surface ozone measurements in the Amazon remain scarce, the team also compared simulated variability against the TES satellite retrieval of lower-tropospheric ozone for 2005 to 2011, finding that the model reproduced interannual patterns with a coefficient of determination of 0.62, rising to 0.73 during August, the peak biomass burning season. The model had previously been evaluated against six Amazonian measurement sites and shown to capture spatial, seasonal and daily ozone variability.

The headline finding is a robust statistical relationship between fire activity and surface ozone. Across the 1997 to 2024 period, fire carbon emissions explained 72 percent of the year-to-year variance in simulated ozone concentrations, and the same linear relationship emerged independently in four additional Earth system models from the CMIP6 archive, despite their differing meteorology and chemistry schemes. Every major drought year except 2015 and 2023 showed above-average fire activity and elevated ozone, with 2024, the year of the most extreme drought and highest fire emissions in recent record, reaching unprecedented levels of both. This confirms that human-ignited fires, concentrated along the Arc of Deforestation where agricultural frontiers press into flammable degraded forest, are the dominant driver of ozone variability across the basin.

Quantifying the carbon consequences, the researchers found that the reduction in net annual carbon uptake caused by fire-related ozone damage equals 24 percent of direct fire carbon emissions, with an uncertainty range of 18 to 35 percent derived from observed variability in tropical tree ozone susceptibility. Put differently, for every tonne of carbon a fire sends directly into the atmosphere, an additional quarter of a tonne is effectively lost because ozone entering leaf stomata suppresses photosynthesis in surviving vegetation. The simulated interannual variability of this indirect loss was substantial, with a standard deviation of 38 teragrams of carbon per year and a total range of 129 teragrams across the study period. The authors caution that this should be read as an order-of-magnitude estimate, since uncertainties in stomatal representation and the ozone damage scheme itself remain unexplored.

A subtler and scientifically intriguing finding concerns the role of stomata, the microscopic pores through which plants both absorb carbon dioxide and take in ozone. Drought triggers stomatal closure, which can shield leaves from rising ozone concentrations, and rising atmospheric carbon dioxide plus warming-driven increases in vapour pressure deficit have produced a long-term declining trend in stomatal conductance across the Amazon. Sensitivity tests attributed this trend roughly equally to carbon dioxide effects and meteorological change. The result has been a gradual weakening of ozone damage over the past decade, with carbon dioxide increases alone avoiding approximately 2 teragrams of carbon loss per year. Yet this apparent protection is fragile. In 2024, record fire emissions overwhelmed the stomatal defence, producing significantly elevated ozone fluxes across the southern and south-western Amazon despite extensive stomatal limitation.

The regional anatomy of drought matters enormously. In the northern Amazon, drought produces only marginal ozone increases during the January to April dry season, and stomatal closure reduces ozone flux, yielding no statistically significant change in ozone damage. In the southern Amazon, by contrast, drought drives much stronger ozone increases during the July to October fire season, accompanied by a significant increase in ozone flux through stomata. During August, ozone damage in drought years reduced net primary productivity by an amount equal to half of the direct drought effect, and in individual years such as 2010 the ozone contribution exceeded the direct drought stress itself. The damage concentrates in the Arc of Deforestation, where almost all fires are human-initiated through clearing or agricultural escapes. The contrast between 2016, when low fire activity spared the region severe ozone damage, and 2024, when record burning erased any protection, illustrates how ignition, not drought alone, determines the outcome.

The study also raises an alarm for remote, previously untouched forest. Simulations of the 2005 and 2024 droughts show elevated ozone fluxes penetrating regions rarely exposed to pollution stress, and the authors hypothesise that vegetation lacking historical ozone exposure may prove especially sensitive. In 2024, forest disturbance from fire reached unprecedented scale even as deforestation rates declined, suggesting that degradation, not just clearance, now drives the fire-ozone-carbon cascade. Because the 2015/16 and 2023/24 droughts have both been partly attributed to anthropogenic climate forcing, and Earth system models project intensifying Amazonian drying through the twenty-first century, the authors argue that reducing deforestation must be paired with reducing forest degradation to cut drought-related fires and the ozone they generate.

The findings carry a clear message for carbon accounting and climate policy. Ozone damage is almost never included in national or global carbon budgets, yet it represents a substantial indirect emission pathway that amplifies the climate impact of every fire. The authors call for sustained ozone monitoring across the Amazon, particularly in remote regions, and for urgent development of more process-based representations of ozone effects on vegetation, including delayed damage, changes to respiration and carbon partitioning, and the sluggish stomatal responses observed in polluted air. As the climate continues to warm and droughts intensify, the invisible chemistry downwind of the flames may prove as consequential for the fate of the Amazon as the flames themselves.

Subject of Research: Fire-derived ground-level ozone damage to Amazon forest carbon uptake under extreme drought

Article Title: Fire-derived ozone intensifies carbon loss in Amazon forests under extreme droughts

Article References: Brown, F., Sitch, S., Folberth, G. A., Mercado, L. M., Cheesman, A. W., Barningham, S., Weber, J., Johnson, B., O’Sullivan, M., & Artaxo, P. (2026). Fire-derived ozone intensifies carbon loss in Amazon forests under extreme droughts. Communications Earth & Environment, 7(1), Article 790. https://doi.org/10.1038/s43247-026-04047-0

Image Credits: AI Generated

DOI: 10.1038/s43247-026-04047-0

Keywords: Amazon rainforest, ground-level ozone, wildfire emissions, drought, carbon cycle, stomatal conductance, land surface modelling, JULES, UKESM1, gross primary productivity, Arc of Deforestation, climate change

Ground-level ozone is not emitted directly by fires. Instead, it is a secondary pollutant, formed when reactive gases produced by combustion react chemically in the presence of sunlight. In the lower atmosphere, ozone is harmful to human health and, crucially, interferes with the physiology of plants. When ozone enters leaf tissue through the same microscopic pores that trees use to absorb carbon dioxide, it damages cells and impairs photosynthesis. For a forest already stressed by drought and fire, this additional burden means that the trees that survive a blaze are less able to resume their role as carbon absorbers in the months and years that follow. The result is a double blow to the climate: fires release carbon dioxide directly when biomass burns, and the ozone they generate suppresses the ability of the remaining forest to capture and store that gas.

The research team quantified this effect using nearly three decades of observations and a global vegetation model, tracking how ozone damage evolved between 1997 and 2024 alongside patterns of fire activity and drought. Their analysis shows that the impact of fire-driven ozone is strongest during major droughts, when hot, dry conditions simultaneously fuel more fires and leave vegetation more vulnerable to pollution damage. Alarmingly, three of the most extreme Amazon droughts of the past three decades — in 1997/98, 2015/16 and 2023/24 — all occurred during El Niño events, the warm phase of a natural, recurring global climate cycle. While El Niño is not caused by human activity, climate change means it now sits atop rising baseline temperatures, making it more likely to push global temperatures to record highs and intensify extreme weather.

Lead author Dr Flossie Brown, of ETH Zurich, emphasized that the problem extends well beyond the flames themselves. Forest fires do not just release carbon dioxide when they burn, she explained; they also contribute to the rise in ground-level ozone, a pollutant that damages the leaves of surviving trees and reduces their ability to keep absorbing carbon afterwards. According to the study, this hidden effect is equivalent to around a quarter of the carbon released by the fires themselves — a significant loss to the Amazon’s carbon budget that had not previously been considered. In other words, every hectare of burned forest inflicts a further, largely unmeasured penalty on the intact forest around it, eroding the region’s capacity to recover the carbon that has been lost.

The drought years examined in the study each left a distinct signature in the data, but the most recent event stands out starkly. Ozone damage during the 2023/24 drought nearly doubled compared with the average of the previous decade, a jump that coincided with record fire activity across the basin. Dr Brown warned that, given a comparably strong El Niño appears to be developing again this year, the researchers are concerned that history could repeat itself. Forecasters have indicated that the emerging El Niño has the potential to become one of the most intense on record, raising the prospect of another major spike in this hidden carbon loss. If drought conditions and fire activity mirror those of 2024, the ozone-driven suppression of photosynthesis could once again strip a substantial fraction of the Amazon’s remaining carbon uptake capacity at precisely the moment it is most needed.

The mechanics of this damage are rooted in the chemistry of the tropical atmosphere. Wildfires release a cocktail of precursor gases, including nitrogen oxides and volatile organic compounds, which react under the intense sunlight of the tropics to produce ozone near the ground. Unlike the stratospheric ozone layer that shields the Earth from ultraviolet radiation, ground-level ozone is a pollutant. In plants, it enters through stomata — the tiny pores on leaf surfaces that regulate gas exchange — and triggers oxidative stress that damages cellular machinery. Stressed trees may close their stomata to limit ozone uptake, but doing so also restricts the influx of carbon dioxide, directly reducing photosynthesis. Over large areas and extended periods, this physiological impairment translates into measurably lower carbon uptake across the forest, compounding the losses from drought stress and fire mortality.

What makes the new findings particularly consequential is that the ozone effect operates on surviving forest rather than burned land. Conventional carbon accounting for wildfires focuses on the immediate emissions from combustion and the slower losses from tree death and decomposition. The ozone penalty, by contrast, is invisible in satellite images of smoke and scorched earth, yet it accumulates across vast swathes of intact forest downwind of burning areas. By combining long-term observational records with a global vegetation model, the researchers were able to isolate this signal and express it in carbon budget terms, showing that the atmosphere suffers significantly more damage from each Amazon fire season than direct emissions alone would suggest.

There is, however, a clear and actionable message embedded in the research. Almost all fires in the Amazon are started by people, whether through deliberate land clearance or through agricultural burning that escapes into standing forest. The worst ozone damage is concentrated in the so-called Arc of Deforestation, the frontier zone where agricultural land is expanding into the rainforest. Professor Stephen Sitch, of the University of Exeter, stressed that this means the extra ozone damage described in the study is largely preventable. Reducing deforestation and forest degradation would cut direct fire emissions, but the study shows it would also protect the Amazon from this secondary, invisible source of carbon loss. The same policy lever, in other words, addresses both problems at once.

Co-author Dr Alexander Cheesman, of James Cook University, argued that the findings demonstrate why air pollution, land-clearing and climate change cannot be treated as separate problems when determining the future of the Amazon. These impacts interact and need to be considered together, he said, but that interaction also means solutions may deliver multiple benefits. Policies that tackle deforestation and degradation address both the carbon dioxide released directly by fires and the ozone pollution quietly eroding what is left of the forest’s ability to soak that carbon back up. In a region where the margin between sink and source grows thinner with each drought, such co-benefits could prove decisive.

As the developing El Niño raises the specter of another record fire season, the study serves as a timely reminder that the true climate cost of Amazon fires is greater than the smoke plumes suggest. The research was funded by the Natural Environment Research Council, partly via the GW4+ Doctoral Training Partnership, and the underlying paper is entitled Fire-derived ozone and intensifying droughts undermine the Amazon carbon sink. Its central conclusion is stark: every fire season in the Amazon now carries a hidden surcharge, paid not in flames but in the diminished photosynthesis of the trees that survive. Protecting the world’s largest rainforest, the authors make clear, requires confronting not only the fires themselves but the polluted air they leave behind.

Subject of Research: Fire-derived ground-level ozone pollution and drought reducing carbon uptake in the Amazon rainforest

Article Title: Wildfire ozone pollution weakening Amazon's ability to absorb carbon

Article References: Wildfire ozone pollution weakening Amazon's ability to absorb carbon. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: Amazon rainforest, ozone pollution, wildfires, carbon sink, El Niño, drought, deforestation, climate change, photosynthesis, Communications Earth & Environment, Arc of Deforestation, ground-level ozone

The core insight of the research is deceptively simple but methodologically powerful. An annual count of compound heatwave-ozone events can rise for two very different reasons: either heatwaves as a whole become more frequent, or a larger share of the heatwaves that do occur coincide with elevated ozone. These two scenarios expose vegetation to fundamentally different conditions, yet both inflate the same summary statistic. To separate them, the team constructed a statistical framework in which the total number of heatwave events was held fixed, so that the coefficient associated with compound events isolated the effect of ozone co-occurrence per se rather than general heatwave activity. This fixed-total parameterization, compared against a conventional separate-count model, revealed how much hidden information the raw compound-event count had been carrying.

The data underpinning the analysis were assembled from some of the most advanced environmental datasets available. Daily maximum temperatures came from the ERA5 atmospheric reanalysis produced by the European Centre for Medium-Range Weather Forecasts, with heatwave thresholds calibrated against the 1983 to 2012 baseline at each pixel using the 90th percentile of warm-season temperatures. A heatwave was declared when daily maxima exceeded this local threshold for at least three consecutive days. Ozone exposure was mapped using the CHAP dataset, a machine-learning reconstruction of daily ground-level ozone at one-kilometer resolution that blends ground stations, satellite retrievals, and reanalysis fields, with a cross-validation coefficient of determination of 0.87. Days were classified as ozone-stressed when the eight-hour daily maximum concentration exceeded 100 micrograms per cubic meter, a widely used operational cutoff in Chinese compound-exposure studies.

For the biological response, the researchers turned to the MODIS MCD12Q2 land-surface phenology product, which tracks the timing of seasonal canopy transitions from vegetation-index time series. Their focal variable was the end of season, or EOS, the date at which the canopy moves toward dormancy in autumn. Croplands were deliberately excluded because planting, irrigation, and harvest schedules dominate their seasonal rhythms and would obscure environmental signals. That left forests and grasslands, the two dominant natural vegetation types covering roughly a quarter and nearly thirty percent of China’s landscape respectively. Using a fixed-effects panel regression over more than 48,000 sampled pixels, the team quantified how year-to-year variation in exposure was associated with year-to-year variation in EOS within each pixel, while absorbing persistent local differences and shared annual fluctuations. Uncertainty was estimated with a spatial block bootstrap of 2,000 replicates to guard against the statistical distortion caused by neighboring pixels sharing conditions.

The mapping of compound stress itself produced striking geography. Compound heatwave-ozone events clustered most densely over the North China Plain and the Fenwei Plain, regions where dense industrial activity and vehicle fleets supply abundant nitrogen oxides and volatile organic compounds, the raw ingredients of ozone, and where persistent anticyclonic systems trap both heat and pollutants over the boundary layer. Roughly one in five land pixels fell into the upper compound-frequency classes. Heatwave-only events, by contrast, were concentrated in Southern China and the Tarim Basin, where monsoon cloudiness and stronger atmospheric ventilation suppress photochemical ozone accumulation despite frequent heat. Ozone-only stress days blanketed the mid-latitudes, with more than thirty percent of the land area experiencing over 114 such days per year. Temporally, the study identified what the authors call a misalignment-overlap pattern: ozone pollution peaks in June, heatwaves intensify in July and August, and so compound exposure compresses into a narrow midsummer window that coincides exactly with the peak growing season.

When the phenological associations were computed, forests and grasslands parted company in an unexpected way. In forests, holding the total number of heatwave events constant, each additional ozone-coincident heatwave was associated with an earlier end of season, by about 0.293 days per event, with a confidence interval comfortably excluding zero. Grasslands showed the opposite tendency, a positive coefficient of about 0.114 days per event, though it was statistically uncertain. The formal grassland-minus-forest contrast was 0.407 days per event, a clear and significant ecosystem difference. Perhaps most revealing was what happened when the conventional parameterization was compared with the fixed-total one: in grasslands, the positive compound-event association largely evaporated once overall heatwave frequency was accounted for, indicating that most of the apparent grassland signal had been borrowed from the general heatwave regime. In forests, by contrast, the negative association actually strengthened, suggesting that ozone co-occurrence carries genuine information about autumn canopy decline beyond the sheer number of heatwaves.

The physiological logic behind this forest response is plausible, though not yet fully demonstrated. Ozone enters leaves almost exclusively through stomata, the microscopic pores that plants open to absorb carbon dioxide, and once inside it triggers oxidative damage that accelerates senescence. Stomatal behavior, however, is not a passive conduit. It responds strongly to temperature, vapor pressure deficit, and the plant’s water status, and hot, dry conditions can partially close stomata, limiting ozone uptake even as ambient concentrations climb. Forests and grasslands differ markedly in rooting depth, canopy structure, and water-use regulation, which is one reason the same atmospheric exposure might translate into different canopy outcomes. The authors caution that resolving the mechanism will require direct measurements of stomatal conductance, plant water status, and stomatal ozone flux, since event-based counts of threshold exceedances compress away the intensity and duration of exposure that determine the physiological dose plants actually receive.

One of the study’s most vivid findings concerns elevation. In forests, the negative association between ozone co-occurrence and end-of-season timing was strongest at low elevations and progressively weakened upward, at a rate of roughly 0.242 days per event per thousand meters. Across the forest elevation range sampled, from about 155 to 3,258 meters, the conditional coefficient swung from minus 0.518 to plus 0.234 days per event, although the high-elevation estimate carried wide uncertainty. Topography reshapes surface energy balance, water redistribution, and evaporative demand, and previous satellite analyses have shown that elevational patterns of autumn phenology cannot be explained by temperature alone. The pattern suggests that forests are most phenologically sensitive to ozone-coincident heatwaves when those events occur under warmer, more water-demanding lowland conditions, a finding consistent with evidence from the 2022 European heat and drought extremes, where forest carbon uptake and autumn canopy activity depended heavily on the seasonal hydrothermal background.

The study’s authors are candid about its limits. The 2014 to 2023 window is short for characterizing rare compound extremes, and annual event counts inevitably compress differences in event timing, duration, intensity, and antecedent conditions, all of which are known to modulate vegetation responses. Satellite-derived end-of-season dates, moreover, capture canopy greenness rather than the exact cessation of photosynthesis, and different remote-sensing proxies can disagree about the timetable of senescence. Integrating solar-induced chlorophyll fluorescence, eddy-covariance carbon flux measurements, and flux-based ozone metrics such as phytotoxic ozone dose would provide a more physiologically grounded picture of how compound exposure propagates from leaf chemistry to landscape-scale phenology.

Even with those caveats, the implications are substantial. Climate projections indicate that heatwaves will grow more frequent and severe, and that ozone episodes will intensify in many rapidly developing regions, making compound exposure an increasingly routine feature of the growing season rather than a statistical curiosity. The finding that the ecological meaning of a compound-event count depends on whether it reflects more heatwaves or more ozone within those heatwaves offers modelers a sharper instrument: vegetation projections that treat compound events as a single undifferentiated category may misattribute the drivers of phenological change. For China’s forests, particularly the densely exposed lowland forests of the north, the message is that air-quality management is climate-adaptation policy. curbing ozone precursors during heatwave episodes could ease an invisible but measurable pressure that pulls the growing season to an early close, with consequences for carbon storage, autumn land-surface energy exchange, and the ecosystems that depend on the length of the green season.

Subject of Research: Associations between compound heatwave-ozone exposure and autumn vegetation phenology across Chinese forests and grasslands

Article Title: Distinguishing heatwave occurrence from ozone co-occurrence reveals contrasting autumn phenological associations across Chinese ecosystems

Article References: Hou, W., Hu, Z., Wang, L., Wang, M., & Liu, X. (2026). Distinguishing heatwave occurrence from ozone co-occurrence reveals contrasting autumn phenological associations across Chinese ecosystems. Environmental and Sustainability Indicators, 32, Article 101528. https://doi.org/10.1016/j.indic.2026.101528

Image Credits: AI Generated

DOI: 10.1016/j.indic.2026.101528

Keywords: heatwaves, ground-level ozone, autumn phenology, end of season, forests, grasslands, China, compound events, satellite remote sensing, fixed-effects regression, elevation gradient, stomatal ozone uptake