Amazon wildfire ozone cuts surviving trees' carbon uptake, study finds

Ozone pollution caused by Amazon wildfires is reducing the rainforest's ability to absorb carbon dioxide from the atmosphere, according to a new study published in the journal Communications Earth & Environment. Ground-level ozone is formed when pollutants from sources such as wildfires react in the presence of sunlight, and it interferes with plants' ability to absorb carbon dioxide. The researchers say wildfires therefore have a double impact on the climate: directly releasing CO2 from biomass burning and reducing the ability of remaining forests to capture and store CO2.

The study 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. Lead author Dr Flossie Brown, of ETH Zurich, said forest fires "don't just release CO2 when they burn" and 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 afterward. She said the hidden effect is equivalent to around a quarter of the carbon released by the fires themselves, describing it as a loss to the Amazon's carbon budget that had not previously been considered.

Researchers from the University of Exeter, ETH Zurich and James Cook University used nearly three decades of observations and a global vegetation model to track how ozone damage changed from 1997 to 2024, alongside fire activity and drought. Brown said ozone damage during the 2023/24 drought nearly doubled compared with the average of the previous decade. The study states the effect is strongest during major droughts, and that three of the most extreme Amazon droughts of the past three decades — 1997/98, 2015/16 and 2023/24 — occurred during El Niño events.

The researchers said a strengthening El Niño this year could bring a repeat of the record damage seen in 2024. The article reports that forecasters say a strong El Niño is already developing this year, with the potential to become one of the most intense on record. Brown said: "Given that a comparably strong El Niño appears to be developing again this year, we're concerned we could see history repeat itself."

Almost all fires in the Amazon are started by people, either through land clearing or agricultural burning that spreads into standing forest, according to the study. It states the worst ozone damage is concentrated in the "Arc of Deforestation," where agricultural land is expanding into the rainforest. Professor Stephen Sitch, of the University of Exeter, said the extra ozone damage described is largely preventable, and that reducing deforestation and forest degradation would cut direct fire emissions while also protecting the Amazon from this secondary, invisible source of carbon loss.