Kilometers above the Earth’s surface, the ozone layer protects humanity and all life from the sun’s harmful ultraviolet rays. But in the troposphere, at ground level, this gas can wreak havoc on planetary health in myriad ways. As temperatures increase due to climate change, the ozone problem is forecast to worsen in many parts of the world — including heavily populated urban and rural areas in the tropics.

Ground-level zone isn’t a direct emission; it’s created when precursors such as methane, nitrogen oxides, carbon monoxide and volatile organic compounds react together with sunlight. These precursors include many anthropogenic sources — especially the burning of fossil fuels (in vehicles, by industry and at power plants) and via agriculture and wildfires, all of which crank up tropospheric ozone levels.

“The problem with ozone is it’s an incredibly active molecule and somewhat unstable in the environment. It interacts with everything it touches,” Nathan Borgford-Parnell, coordinator of the Scientific Advisory Panel and Science Affairs at the Climate & Clean Air Coalition, told Mongabay in an interview.

That makes tropospheric ozone a planetary health problem that threatens human health, food security and biodiversity. “The concerns are quite legion,” says Borgford-Parnell.

Ground-level ozone pollution is getting worse due to what experts have dubbed the “ozone-climate penalty.” Though ozone’s formation is complex, it is closely tied to higher temperatures. That’s why the problem generally peaks in warmer months; which was true in 2024 — officially the hottest year on record, with ozone pollution spiking during European summer heat waves.

The increasingly common formation of heat domes, also due to climate change, add to the problem, as stalled weather systems allow precursor pollutants and ground-level ozone to build up in stagnant air.

In coming decades, as temperatures rise, persistent heat domes form, and wildfires increase, the burden of ozone pollution will very likely intensify in cities and tropical areas in India, Latin America, Africa, Indonesia and elsewhere as these regions face their own ozone-climate penalty.

“Changes in weather patterns, and increasing temperature, biomass burning and ozone precursor emissions [will] accelerate this rise in tropical tropospheric ozone,” says Jayanarayanan Kuttippurath, a climate scientist at the Indian Institute of Technology Kharagpur.

As ozone increases, so do health concerns

In recent years, rising temperatures and grueling heat waves have collided with air pollution to cause dangerous spikes in ground-level ozone, doing increased harm to human health.

Ozone’s high reactivity means it aggressively attacks the airways, causing irritation or exacerbating underlying health issues (including asthma, emphysema and chronic bronchitis), say experts. But health concerns don’t stop there; ozone can stress both the cardiovascular and reproductive systems. Research has also associated ozone exposure with low infant birth weight, particularly in South Asia, the Middle East and North Africa. Other studies have found a correlation with gestational hypertension and high blood pressure.

It’s estimated that ozone contributed to around 365,000 deaths globally in 2019, with more than 70% of mortality occurring in India and China alone. But a recently published paper found those figures may be vastly underestimated; that study determined that around 1.4 million deaths annually can be attributed to ozone exposure.

Ozone’s health impacts ripple out far wider, causing “spiraling impacts in our economies and societies,” says Borgford-Parnell. “It’s truly a powerful air pollutant that harms human health and reduces our resiliency as societies to climatic changes.”

Experts underline that ozone’s harmful effects are not limited by national boundaries, as precursors and the gas itself freely move across borders. One study assessed that more than half the ozone-attributed deaths in Europe are due to pollution originating elsewhere in the world.

Likewise, ozone pollution arising from climate change-intensified wildfires can impact people far from the flames. A 2024 paper found that “when smoke plumes travel long distances southward from Canada, higher air temperatures [act on nitrate precursors] to create higher levels of ozone [over U.S. cities].”

Without swift action to curb climate change and ozone precursor emissions, the “legion” of harm it causes is set to worsen, say researchers.

Ozone harms plants and forests, worsens global warming

Ozone is a greenhouse gas and it’s estimated that around 0.2° Celsius (0.36° Fahrenheit) of current warming globally is due to ozone. Kuttippurath says that its climate impact is of great concern, especially in some heavily polluted regions (including urban India), where it can boost local temperature by up to 0.4°C (0.72°F).

But that’s only one aspect of a complex problem. Ozone also seriously damages plants, limiting their growth, which in turn reduces vegetation’s ability to soak up and store atmospheric CO2. This results in an “effectively doubled” climate impact for this highly reactive gas, says Borgford-Parnell.

Researchers have studied ozone’s harmful effect on plants for decades. Though the sensitivity of plants to ozone varies, it can reduce photosynthesis, disrupt nutrient and water uptake, leave plants more vulnerable to diseases, and more, says Evgenios Agathokleous, a professor at the School of Ecology and Applied Meteorology at Nanjing University of Information Science and Technology, China.

“Overall, we have decreased photosynthesis, growth, biomass, and yield of the plants,” he says. That can cause a “cascade of ecological implications,” from the microbial level in soils, to pollinators and other insects, all the way up to biodiversity dwelling in and among the affected trees, including birds.

There’s also a complicated interplay between ozone, climate change and the biogenic volatile organic compounds (BVOCs) emitted by plants themselves. In areas subject to higher temperatures and high pollution, isoprene, a BVOC emitted by plants, can trigger heightened ozone formation. This raises concerns for some tropical regions that a combination of rapid urbanization, climate change, and plants’ natural BVOC emissions could increase ground-level ozone in coming years.

Ozone’s impacts on forest health may escalate climate change even further. In a 2024 paper, a team led by Alexander Cheesman, a senior research fellow at James Cook University in Australia and the University of Exeter, U.K., found that environmental levels of ozone can effectively diminish tropical forest growth by around 5.1% on average, which would in turn impact carbon stores.

Cheesman says adding all this ozone harm up makes it a “worrying issue.” His team estimates that ozone may have stopped tropical forests absorbing around 290 million metric tons of carbon per year since 2000. Asia is particularly hard hit, as forests there could be losing up to 10.9% in growth per year due to ozone exposure.

This research challenges commonly held assumptions that humanity can bank on forests to store vast amounts of carbon as a climate solution. “If we’re not accounting for this [ozone] damage that might be [worsening] into the near future, then we’re going to have a misunderstanding of the potential carbon sequestration,” Cheesman says.

However, Agathokleous notes that while numerous lines of research show that ozone can lead to decreased growth and lessened biomass accumulation, that doesn’t necessarily mean decreased carbon sequestration, as that occurs over far longer time scales.

But he also underlines the need for far greater monitoring of ozone and its precursors in the tropics, as there are vast information gaps, particularly in Africa and the Southern Hemisphere that could potentially alter understanding of the climate change-ozone link.

Hammering crop yields

The harm to trees and wild vegetation extends to food crops. Each year, ozone pollution is estimated to reduce yields of major food crops such as wheat, soy, rice and maize; that’s expected to worsen as climate change progresses, particularly if precursor emissions rise unabated.

Analysis by Kuttippurath and his team, for example, estimated that ground-level ozone in India led to losses of around 7.39 million metric tons of rice in 2005. That figure increased to 11.46 million metric tons in 2020, at a cost of $2.92 billion dollars, while other recent research found that wheat yields in India can be reduced by up to 40% due to surface ozone.

“The rise of tropospheric ozone is also a food security issue,” Kuttippurath says, especially since about half the world’s population lives in developing or underdeveloped tropical countries, causing additional malnourishment and hunger concerns.

“Increasing ozone in this region aggravates the issues already existing there, and the problems will be many folds than that of the developed regions,” he adds. “This further iterates the need for international cooperation and support in reducing pollution in the tropics and global south.”

Research by Cheesman also suggests some negative impacts on banana growth, but his team was unable to assess any effect on yields. Other research shows that some wheat varieties and common legumes grown in sub-Saharan Africa are sensitive to ozone, likewise sweet potato.

Fortunately for humanity, experts note that some crops and varieties are resilient. Tests on sorghum, a widely grown crop in the tropics, suggested no impact, for example. Work is ongoing to develop and breed ozone-tolerant crop varieties, though greater efforts are needed on this front, says Agathokleous.

Disrupting pollinators

But still another problem has been recognized: ozone’s wide-reaching impact on global insect declines. It, along with other air pollutants, harms pollinators in multiple ways.

James Ryalls, a researcher at the University of Reading, U.K., explains that ozone can degrade floral scents, reducing pollinator foraging ability and making it harder to find flowers, endangering insect health. Other researchers have found that ozone can destroy sex pheromones produced by fruit flies, triggering them to mate with other species, increasing the likelihood they produce infertile hybrids.

“Ozone can also decrease pollination success, ultimately reducing crop yields,” Ryalls wrote in an email, adding that research shows ozone has harmful effects at levels currently deemed “safe,” suggesting that current air quality regulations are not adequately protecting ecosystems.

“Globally, ozone levels are projected to rise until around mid-century before they start decreasing,” Ryalls says. “Ozone pollution is an often overlooked but significant threat to pollinators and global food security. Addressing it requires policy action and interdisciplinary research to develop mitigation strategies that balance human activities with ecosystem health.”

Tackling ozone

Ground-level ozone is a difficult gas to control, but taking steps to reduce it will ultimately benefit air quality and help address climate change, say experts.

Ozone pollution is what experts call “non-linear,” due to the complexity of its formation. That, for example, means that big cuts to precursor emissions won’t necessarily result in major cuts in ozone pollution. In fact, in some instances, reducing precursors can cause a spike in ozone, says Alessandra De Marco, an expert on air pollution and a senior researcher at the Italian National Agency for New Technologies, Energy and Sustainable Economic Development.

“That’s crazy, but it’s dependent on the mechanism of ozone formation,” she says.

Experts also urge caution regarding the “greening” of cities. For example, the common tactic of planting trees in urban areas to reduce pollution can backfire without careful planning. “If you plant trees that are emitting high levels of [natural] volatiles you can increase the ozone concentration,” De Marco says. “You try to solve the problem, and you can actually make it worse … so it’s very important which kind of tree you plant.”

Agathokleous agrees, noting that whatever is green doesn’t immediately mean better air quality, “because trees and plants can be a sink, but also a source of air pollution.” This fact has implications for reforestation efforts too, with fast-growing species such as willow or poplar ultimately increasing ozone formation.

“When we are planting billions of trees, we may drastically affect how ozone behaves in the atmosphere,” Agathokleous says.

Other experts emphasize a multipronged approach to the problem. Borgford-Parnell says his organization’s focus is on methane emission reductions as “the fastest, most impactful way” of reducing ozone pollution. Methane is responsible for creating large amounts of ozone globally, according to research. Cutting it would be a win-win, as methane is also a powerful greenhouse gas.

“Addressing methane to get at ozone is a little bit like using a sledgehammer,” he says, adding that beyond methane, other major targets are reducing NOx emissions from sectors such as transportation. “When you get down to these other precursor gasses, you’re going to the level of a scalpel.”

Others, including Kuttippurath, emphasize the need to transition to cleaner fuels, renewable energy, advanced industrial technologies and more public transportation — all of which can reduce ozone pollution.

“However, transboundary ozone pollution, through atmospheric transport, requires a global strategy and policy implementation. Addressing the issue globally requires international cooperation [via measures] like the Convention on Long-Range Transboundary Air Pollution,” he says.

Also known as the Gothenburg Protocol, it’s designed to slash ozone and other pollutants. The protocol entered into force in 2019 and its 51 signatories are comprised of European and North American countries. It’s recognized as a success for cutting emissions of many major air pollutants.

Yet, even if the Gothenburg Protocol is fully implemented in Europe, ozone pollution will likely still be a persistent threat there, reducing wheat yields by 16.8 million metric tons in 2050, according to an analysis, emphasizing the complexity of this challenge.

Though ozone pollution has long been a mostly ignored “also ran” greenhouse gas and air pollution source, mitigating it is worthwhile, achievable, and a win-win for planetary health, says Borgford-Parnell: “Seeing growing recognition that there’s an overlap between ozone, climate and air quality — and the benefits of addressing it — is really heartening.”

Banner image: Banana crop in Uganda. Research suggests ozone pollution may inhibit banana plant growth. There are research gaps across the tropics concerning ground-level ozone’s effects on forests and important food crops. Image by IFPRI/Milo Mitchell via Flickr (CC BY-NC-ND 2.0).

With climate change, cryosphere melt scales up as a threat to planetary health

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