Fires in tropical forests affect more than just the forests

A new study reveals that exactly what burns strongly influences the emissions from such fires.

This photo was taken from the International Space Station in orbit over the border between Bolivia and Brazil in August 2020.

(Photo: Chris Cassidy / NASA)

Christine Forsetlund Solbakken Head of Communications

Presented by: NILU

Published 28 February 2025 - 00:01

In 2020, researchers registered the largest burnt areas in the Amazon and Cerrado regions in South America since 2010.

Annual carbon emissions were also much higher than in the previous decade.

During this intense wildfire season, biomass – organic material – with a dry weight equivalent to around 372 million tonnes went up in smoke. This led to around 40 million tonnes of carbon monoxide (CO) emissions.

Satellite observations and modelling of the fires

Satellites are commonly used to monitor vegetation fires. They can observe heat radiation while the fire is burning or the burned areas afterwards.

"From either the heat radiation or the size of the burned area and additional estimates of fuel availability, we know the amount of biomass burned. Then we can also find out how much CO, CO₂, and other chemical substances the fires have emitted,” says Johannes Kaiser.

He is a senior scientist at NILU and one of the authors behind a recent study where a team of researchers combined various satellite observations and fire models.

Johannes Kaiser is a senior scientist at NILU.

The aim was to describe the available fuel and combustion conditions more accurately. By doing so, they have managed to reduce uncertainties in emission estimates from fires.

Fires behave like bonfires

In their analysis of the 2020 fire season, the researchers included information about different types of fuel – meaning what kind of biomass was available to burn. They also examined moisture conditions and burning behaviour.

A common fuel type is woody debris, which is dead wood material like tree trunks and branches. This, along with other surface litter covering the forest floor, accounts for around 75 per cent of the total biomass that burned in these areas.

“Vegetation fires in many ways behave like bonfires. For example, fine and dry fuel like dead twigs and small branches burn with flames. They can ignite coarser fuel, like logs. The logs will afterwards continue smouldering, unless they are piled up. Then they may flame up,” says Kaiser.

This satellite image from August 2020 shows smoke from some of the deforestation fires along major roads in Amazonas, Mato Grosso, and Pará, Brazil.

He explains that similarly, savanna fires usually have flaming fronts with relatively little smouldering afterwards.

Amazon fires emit more CO than fires in Cerrado

The researchers compared fire emissions and atmospheric models with satellite observations of CO in the atmosphere.

By doing so, they demonstrated that burning of woody debris in the tropical Amazon forest often resulted in smouldering fires. These have significantly higher CO emissions than the more combustion-efficient flaming fires that typically occur in the savanna region Cerrado.

The study thus shows that the majority of CO emissions from fires in the entire region come from smoldering fires in the Amazon.

“Our results not only highlight the critical role of woody debris in fire emissions. They also show how important it is to merge satellite observations of both the vegetation state and the fires into emission models. Only then can we understand the role of fires in the Earth system and reduce their impact in these vulnerable regions,” concludes Kaiser.

Reference:

Forkel et al. 'Burning of woody debris dominates fire emissions in the Amazon and Cerrado', Nature Geoscience, vol. 18, 2025. DOI: 10.1038/s41561-024-01637-5 (Abstract)