Ozone pollution reduces annual growth of tropical forests by 5.1%, study finds

Ozone gas reduces the growth of tropical forests, leaving about 290 million tons of carbon uncaptured each year, a new study finds.

The ozone layer in the stratosphere protects our planet from harmful ultraviolet rays. Its protection is one of the greatest achievements of environmental action.

But ground-level ozone, formed by the combination of pollutants from human activities in the presence of sunlight, interferes with the ability of plants to absorb carbon dioxide. Ozone is also harmful to human health.

The new study, published in the journal Geosciences of naturecalculates that tropospheric ozone reduces new annual growth of tropical forests by an average of 5.1%.

The effect is stronger in some regions: tropical forests in Asia have lost 10.9% of their new growth.

Tropical forests are essential “carbon sinks”: they capture and store carbon dioxide that would otherwise remain in the atmosphere and contribute to global warming.

“Rainforests play a crucial role in reducing our carbon dioxide emissions,” said Dr Alexander Cheesman, co-lead author of the study, from James Cook University and the University of Exeter.

“Our study shows that air pollution can jeopardize this essential ecosystem service. We estimate that ozone has prevented the capture of 290 million tons of carbon per year since 2000. The resulting cumulative loss is equivalent to a 17% reduction in carbon removal by tropical forests since the beginning of this century.”

The researchers conducted experiments to measure the ozone sensitivity of various tropical tree species and then incorporated the results into a computer model of global vegetation.

Urbanization, industrialization, the burning of fossil fuels and wildfires have led to an increase in the “precursor” molecules – such as nitrogen oxides – that form ozone.

“Ozone concentrations in tropical regions are expected to increase further due to increased precursor emissions and changing atmospheric chemistry in a warming world,” said Dr Flossie Brown, co-lead author and recent graduate of the University of Exeter.

“We found that current and future forest restoration areas – areas critical to climate change mitigation – are disproportionately affected by this elevated ozone concentration.

“It is clear that air quality will continue to play an important, but often overlooked, role in how forests absorb and store carbon.”

Professor Stephen Sitch, from the University of Exeter, added: “Moving to a future with greater environmental protection would lead to a reduction in ground-level ozone, improving air quality and providing the added benefit of improved carbon absorption in tropical forests.”

The article is titled “Reduced productivity and carbon in tropical forests due to exposure to tropospheric ozone.”