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Ozone from Amazon wildfires cuts forest carbon uptake by the equivalent of 24% of fire emissions
Researchers from Exeter, ETH Zurich and James Cook University estimate that fire-driven ozone cuts Amazon carbon uptake by about 24% of the fires' emissions. They warn the strong El Niño now forming could push that hidden loss up again.
The Scientist · Science desk
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What happened
- The team combined nearly three decades of observations with a global vegetation model to track ozone damage, fire activity and drought from 1997 to 2024.
- Ozone damage during the 2023/24 Amazon drought nearly doubled compared with the average of the previous decade, according to lead author Flossie Brown.
- The effect is strongest in major droughts, and the three most extreme Amazon droughts of the past three decades, in 1997/98, 2015/16 and 2023/24, all came during El Niño events.
- Almost all Amazon fires are started by people, and the worst ozone damage is concentrated in the Arc of Deforestation, where farmland is expanding into the rainforest.
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Why it matters
- cost Carbon budgets that book only combustion understate what Amazon fires cost the atmosphere, and the shortfall grows in drought years, when the ozone effect is strongest.
- exposure Standing forest along the Arc of Deforestation faces the largest added carbon loss if this year's El Niño brings a drought like 2023/24, since fire and ozone damage concentrate there.
- decision Governments weighing deforestation controls can count a second carbon return, protection of the remaining forest's uptake, on top of avoided fire emissions, according to the authors.
The 24% is measured against one thing: the carbon the fires themselves release [1]. It is a ratio to fire emissions. It should not be read as a share of the Amazon's total uptake. For every tonne of carbon a fire emits, the remaining forest absorbs roughly a quarter of a tonne less [1].
Ground-level ozone forms when pollutants from sources such as wildfires react in sunlight, and it interferes with plants' ability to take up carbon dioxide [3]. "Forest fires don't just release CO2 when they burn," said Flossie Brown of ETH Zurich, the lead author. "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 afterward." [4]
Count only those two terms and a fire's carbon cost comes to about 1.24 times its combustion emissions. A budget that books only the burning captures about 81% of that total and misses about 19% [16]. Brown described the ozone effect as "a significant loss to the Amazon's carbon budget that hasn't previously been considered" [5].
The study, published in Communications Earth & Environment, takes its figure from a global vegetation model run alongside nearly three decades of observations [2][6]. So the 24% depends on how that model represents ozone damage to leaves [6]. I think the ratio should change how Amazon fire emissions are described. Using it to correct a carbon ledger is a separate step. The published summary gives the effect only as a ratio, with no tonnage and no uncertainty range, and does not discuss forest offsets [1].
The El Niño warning rests on a small sample: three droughts, each of which fell in an El Niño year [8]. A strong El Niño is developing this year and could become one of the most intense on record, and the authors expect another major spike in hidden carbon loss [10]. El Niño itself is a natural cycle and is not caused by human activity [9].
The fires are another matter. Almost all are set by people, through land clearing or agricultural burning that spreads into standing forest [11]. "That means the extra ozone damage we're describing is largely preventable," said Stephen Sitch of the University of Exeter [12]. Alexander Cheesman of James Cook University said air pollution, land clearing and climate change cannot be treated as separate problems in the Amazon's future [13]. "These impacts interact and need to be considered together," he said [14].
What to watch
- Whether this year's El Niño brings an Amazon drought on the 2023/24 scale, and how fire activity in the Arc of Deforestation responds.
- A tonnage and uncertainty range for the ozone loss, the inputs a carbon budget would need to apply it as a correction.
- Whether any carbon-accounting body or forest-offset standard adds an ozone term to its treatment of fire emissions.