Why does Indonesia burn almost every dry season, and why does El Niño make the haze worse?

Indonesia burns almost every dry season because fire is the cheapest way to clear land, and the land that burns, peat, can smoulder underground for months. The Southeast Asian haze is caused primarily by slash-and-burn land clearing; it flares up every dry season to varying degrees and is generally worst between July and October and during El Niño events. Using fire to clear land costs about US$5 per hectare, against up to US$200 per hectare with machines and chemicals, which is why it remains so tempting.

Undisturbed humid tropical forests are very resistant to fire, with rare fires only during extraordinary dry periods. Drained peat swamp forest is a different matter. Excessive drainage dries out the top layer of peat, and because of its high carbon content dry peat is extremely susceptible to burning in the dry season. Peat fires often smoulder underground for months even despite surface rain.

El Niño makes it worse because it takes the rain away: it is associated with reduced rainfall over Indonesia, India and northern Australia, and a positive Indian Ocean Dipole can add drought and increased fire potential. In 1997 the El Niño made the dry season even drier than normal, and one of the worst haze years followed. As of October 2026, drier conditions due to El Niño had contributed to roughly 230,000 acres burning in Indonesia in July alone, more than the prior six months together, with smoke reaching Singapore, Brunei and Malaysia.