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Periods of extreme heat often lead to increased exposure to ground-level ozone, in addition to other negative effects. This is dangerous for humans, the environment and agriculture. A study led by Forschungszentrum Jülich now provides surprising findings: With strong global warming, ozone pollution could decrease in some regions of the world.

The paper is in the journal npj Clean Air.

The researchers used an innovative "storyline approach": Simulations of plausible climate scenarios that depict various possible developments in global warming—for example, whether extreme weather events such as heat waves or heavy rainfall will become more severe or more frequent.

"Our question was: what impact will global warming have on ozone pollution?" explains the first author, Tamara Emmerichs. "We applied warming levels of +2 °C and +2.75 °C compared to pre-industrial temperature levels—based on the weather conditions of the exceptionally hot summers from 2018 to 2020."

The emissions of pollutants were kept constant in the simulation, allowing the researchers to analyze the pure influence of temperature and humidity.

Unexpected trend reversal with strong warming

The result: with a warming of +2 °C, ozone pollution near the ground initially increases in many regions. One reason for this is the intensified chemical production of ozone, partly driven by rising emissions of precursor substances from plants.

A more significant factor, however, is that higher temperatures make atmospheric chemistry more efficient at producing ozone. Additionally, the ability of vegetation to remove pollutants decreases, further exacerbating air pollution.

Surprisingly, this trend, which was visible in the simulation, reverses with a stronger warming of +2.75 °C: The background concentration of ozone decreases significantly across the Northern Hemisphere. The reason is a more humid atmosphere, in which ozone is broken down more efficiently in the higher atmosphere.

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

Plants as a decisive influencing factor

Plants play a dual role here: they can absorb ozone through their leaves and thus contribute to air purification. At the same time, they transpire water and thus cool their surroundings.

However, plants react to both drought stress and increased exposure to ozone by closing their stomata—this reduces both the cooling effect and ozone uptake.

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

Effects on health and ecosystems

The scientists' simulation therefore shows that, at 2 °C warming, an increase in deaths caused by ozone can be expected worldwide. With even greater warming of 2.75 °C, the number of these ozone-related deaths decreases significantly again—especially in Europe and India.

At the same time, the uptake of harmful ozone by plants increases in certain regions, for example in the boreal forests of the Northern Hemisphere.

The "storyline approach" we used makes it clear that the interactions between climate, weather, chemistry and ecology are complex," says Taraborrelli.

"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."

More knowledge for targeted health protection

The study by the Jülich atmospheric researchers provides valuable insights for the targeted development of regional adaptation strategies for health and environmental protection.

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