More heat, cleaner air? The surprising ozone connection

Periods of extreme heat usually go hand in hand with spikes in ground-level ozone – a lung-irritating pollutant that harms people, crops, and ecosystems.

A new modeling study led by Forschungszentrum Jülich lands with a twist: if global warming becomes strong enough, surface-level ozone could actually decline across large parts of the Northern Hemisphere.

The work doesn’t overturn what we know about heat and smog days today, but it does reveal how future temperature and humidity shifts might reshape ozone chemistry in unexpected ways.

Modeling future clean air chemistry

The researchers used an innovative “storyline approach,” running climate simulations that play out plausible futures under stronger warming while holding emissions steady. That choice let them isolate the direct effects of temperature and humidity, independent of any changes in pollution.

The experts anchored their scenarios to the weather patterns of Europe’s exceptionally hot summers from 2018 to 2020 and asked a simple, pointed question:

“Our question was: what impact will global warming have on ozone pollution,” said lead author Tamara Emmerichs.

In one set of simulations, global temperatures rose by +2 °C (+3.6 °F) relative to pre-industrial levels; in another, by +2.75 °C (+4.95 °F).

Heat speeds ozone creation

At +2 °C (+3.6 °F), the model behaves much as air quality scientists might expect. Warmer air speeds up the photochemical reactions that create ozone from precursor gases.

Plants, stressed by heat and drought, tend to close their stomata, reducing the natural “scrubbing” of pollutants from the air. Vegetation can also emit more volatile organic compounds in hotter conditions, feeding ozone chemistry. The net effect in many regions: higher near-surface ozone.

Push the warming to +2.75 °C (+4.95 °F), though, and something surprising happens. The background concentration of ozone drops markedly across much of the Northern Hemisphere.

The simulations attribute this to a more humid atmosphere, which accelerates ozone destruction higher up, ultimately lowering the amount that reaches the surface.

“This effect becomes more dominant at higher temperatures,” Emmerichs said. “The result is a net decrease in ozone pollution near the ground.”

Plants as double agents

Vegetation emerges as a pivotal player. Plants can help by absorbing ozone through their leaves and cooling their surroundings via transpiration.

But under drought or ozone stress, they shut down those very pathways – closing stomata to conserve water, which reduces both cooling and pollutant uptake.

“Our study shows that plant responses to stress are a central, previously underestimated factor in the future development of ozone pollution,” said Jülich atmospheric researcher Domenico Taraborrelli.

Ozone shifts reshape ecosystems

Those chemical and biological shifts carry real-world consequences. In the model world at +2 °C (+3.6 °F), ozone-related mortality increases globally.

At +2.75 °C (+4.95 °F), the number of ozone-linked deaths declines again – especially in Europe and India.

Ecosystems feel the changes, too: the simulations project increased ozone uptake by plants in certain regions, such as the boreal forests, altering the balance of plant stress and pollutant removal.

Many forces drive clean air

The message isn’t that hotter is healthier. It’s that climate-chemistry-ecology interactions don’t move in straight lines.

 “The ‘storyline approach’ we used makes it clear that the interactions between climate, weather, chemistry, and ecology are complex,” Taraborrelli said.

“Every single aspect of climate change is the result of a large number of these interrelationships and overlapping factors, and is difficult to examine in isolation.”

By fixing emissions in their experiments, the authors highlight just how much meteorology and biology alone can swing air-quality outcomes.

Preparing for climate’s air surprises

For planners and health agencies, the findings offer a sharper lens for regional adaptation. If humidity patterns and plant stress responses can flip ozone trends under stronger warming, then heat-health planning, crop protection, and forest management will need to account for those thresholds and feedbacks.

“It shows that strong warming can have not only negative but also sometimes positive effects on air quality,” Taraborrelli said. “At least in individual cases, such as here for ozone pollution.”

Nothing here suggests that pushing the planet into higher warming is a viable “strategy” for clean air – extreme heat carries a cascade of harms well beyond ozone, from lethal heatwaves to wildfire smoke and crop losses.

The study demonstrates that air quality futures hinge on more than just emissions cuts. They also depend on the evolving chemistry of a warmer, wetter – or drier – atmosphere and on how plants across landscapes respond to stress.

Understanding those links now can make tomorrow’s protection of public health and ecosystems more precise and more effective.

The study is published in the journal npj Clean Air.

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