Biomass burning in Indonesia is a singularly large source of greenhouse gas emissions globally and aerosol emissions regionally, with severe impacts on air quality and human health. Although fire events have been documented on a case-by-case basis since the 1980s, no continuous record of the fire events exists prior to 1996, due to the absence of satellite estimates or ground-truthed records of fire extent. Here, we provide a continuous record of the severe haze from biomass burning in Indonesia from 1960 to 2006 using the visibility reported at airports, which was found to be an excellent predictor of particulate matter emissions in Sumatra and Kalimantan.

Using the visibility proxy, we show that large fire events have occurred in Sumatra at least since the 1960s, but in Kalimantan only since the 1980s, despite the occurrence of several severe droughts during 1960-1980. This increase in sensitivity of fires to drought in Kalimantan coincides with Indonesia's transmigration program and dramatic changes in land use. Our results show a non-linear relationship between rainfall and fire, whereby fire events occur only during years when rainfall falls below a certain threshold in combination with the presence of human land use. In Kalimantan, this drought is controlled primarily by sea-surface temperatures in the Pacific Ocean, whereas in Sumatra, they are controlled primarily by sea-surface temperatures in the Indian Ocean. Better understanding of these controls may help to predict future fire risk in Indonesia with changing climate and land use.