Brown added that this hidden effect amounts 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."

Why do humans bear most of the blame?

Almost all fires in the Amazon are started by people, mainly to clear land for agriculture and cattle ranching. These fires sometimes escape control and spread into surrounding rainforest, destroying large stretches of woodland.

The damage does not end when the flames die out. The pollution produced during burning lingers in the atmosphere, feeding ozone formation that continues to stress trees well beyond the fire's edge. Even areas that never burned directly can lose part of their capacity to pull carbon out of the air.

How do droughts and El Niño make it worse?

The impact is not constant; it worsens under harsh climatic conditions. Severe droughts put vegetation under greater stress, compounding the ozone damage.

One key factor is El Niño, the warm phase of a natural climate cycle that shifts weather patterns worldwide. El Niño is not caused by human activity, but global warming raises baseline temperatures and can intensify some of its effects.

Three of the most severe Amazon droughts in recent decades — 1997–1998, 2015–2016 and 2023–2024 — all coincided with El Niño events. During the 2023–2024 drought, ozone-related damage nearly doubled compared with the average of the previous decade.

The researchers caution that a strong El Niño could again create damaging conditions for the rainforest. But they stress that El Niño should not be confused with the root cause of forest loss: logging and burning tied to human land use remain the fundamental drivers, and those can be directly addressed.

What would protect the rainforest?

The findings show that wildfire impacts stretch far beyond the CO2 released as forests burn. Ozone pollution quietly weakens surviving trees, and when this effect overlaps with extreme climate conditions, the consequences compound.

Stephen Sitch of the University of Exeter said: "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."

Alexander Cheesman of James Cook University put it succinctly: "These impacts interact and need to be considered together."

The takeaway for policymakers is that saving the Amazon means more than preventing trees from burning. It also means preserving the surviving forest's ability to keep absorbing CO2 — which requires action on deforestation, air pollution and climate change together. Ignore any one of these connections, and you overlook part of the problem.

As with any modeling study, the estimates depend on the assumptions built into the vegetation model and the observational record, and the researchers note that impacts vary with climatic conditions. Still, the study identifies a carbon loss that most accounting has simply missed — one invisible in the smoke, but written on the leaves.