Ozone pollution released by Amazon wildfires is significantly reducing the rainforest's ability to absorb carbon dioxide from the atmosphere, new research reveals.
Ground-level ozone is formed when pollutants from sources such as wildfires react in the presence of sunlight.
This harms human health and interferes with plants' ability to absorb carbon dioxide (CO2) from the atmosphere.
This means wildfires have a double impact on our climate: both directly releasing CO2 from biomass burning and reducing the ability of remaining forests to capture and store CO2.
And the researchers – from the University of Exeter, ETH Zurich and James Cook University – warn that this year's strengthening El Niño could bring a repeat of the record damage seen in 2024.
Hidden carbon loss
The new study, published in the journal Communications Earth & Environment, calculates that fire-driven ozone damage cuts Amazon carbon uptake by an amount equivalent to around 24% of the carbon emitted by the fires themselves.
This effect is strongest during major droughts, and three of the most extreme Amazon droughts of the past three decades (1997/98, 2015/16 and 2023/24) occurred during El Niño events.
El Niño is the warm phase of a natural, recurring global climate cycle. It's not caused by human activity, but climate change means it sits on rising baseline temperatures, making it more likely to push global temperatures to record highs and intensify extreme weather.
With forecasters warning that a strong El Niño is already developing this year – with the potential to become one of the most intense on record – the authors of the news study say we are likely to see another major spike in hidden carbon loss.
Distinct signature
"Forest fires don't just release CO2 when they burn," said lead author Dr Flossie Brown, of ETH Zurich.
"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.
"We found 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 hasn't previously been considered."
The researchers used nearly three decades of observations and a global vegetation model to track how ozone damage has changed from 1997 to 2024, alongside fire activity and drought.
"The Amazon droughts we studied have each left a distinct signature," Dr Brown said.
"Ozone damage during the 2023/24 drought nearly doubled compared to the average of the previous decade. Given that a comparably strong El Niño appears to be developing again this year, we're concerned we could see history repeat itself."
Solutions overlap
Almost all fires in the Amazon are started by people, either through land clearance or agriculture that escapes into standing forest. The worst ozone damage is concentrated in the "Arc of Deforestation", where agricultural land is expanding into the rainforest.
"That means the extra ozone damage we're describing is largely preventable," said Professor Stephen Sitch, of the University of Exeter.
"Reducing deforestation and forest degradation would cut direct fire emissions – but our study shows it would also protect the Amazon from this secondary, invisible source of carbon loss."
Co-author Dr Alexander Cheesman, of James Cook University, said the findings show that air pollution, land-clearing and climate change cannot be treated as separate problems in determining the future of the Amazon.
"These impacts interact and need to be considered together," he said. "But this also means that solutions may have multiple benefits. Policies that tackle deforestation and degradation address both the CO2 released directly by fires and the ozone pollution quietly eroding what's left of the forest's ability to soak it back up."
This research was funded by the Natural Environment Research Council, partly via the GW4+ Doctoral Training Partnership.
