Firefighters at work Samarinda, Borneo. Forest fires in this region were unknown before human activity escalated in recent years (Image: Action Press/Rex Features)

Human activities have turned the world’s third largest rainforest region into a tinderbox that climate change will ignite. So concludes a new study of fire in the forests of Sumatra and Borneo in Indonesia.

Investigators examined the fire history of Indonesian forests by analysing half a century of visibility records at local airports.

“During the late 1970s, Borneo changed from being highly fire-resistant to highly fire-prone during drought years,” says Robert Field, an atmospheric physicist from the University of Toronto. “The abrupt transition can be attributed to rapid increases in deforestation and population growth.”


Field says that human invaders clearing patches of trees for farming destabilised the forest ecosystem, making it drier and more vulnerable to future drought.

Droughts, usually during El Niños in the Pacific, have triggered huge fires in Indonesia seven times since 1960. But Field found that until about 1980, the fires were restricted to Sumatra, where human activity and deforestation rates were highest. The forests of Borneo did not burn.

Then, after humans began invading Borneo’s interior on a larger scale, with rates of deforestation rising above 2%, these forests become vulnerable too. The worst fires were in 1997 to 1998, causing smogs in cities hundreds of kilometres away and at least one plane crash.

The biggest source of smoke and carbon-dioxide emissions during forest fires in southeast Asia is not the trees themselves, says Field, but the burning of peat in the deep swamps on which many forests grow. Once alight, the peat can burn for months (see Bog barons: Indonesia’s carbon catastrophe).

Field’s findings add to growing concern about the Indonesian government’s announcement earlier this month that it has ended a two-year moratorium on turning peatlands into oil palm and tree plantations. The move is bound to cause more fires, says Field.

Journal reference: Geoscience (DOI: 10.1038/NGEO443)