Human amplification of drought-induced biomass burning in Indonesia since 1960

Much of the interannual variability in global atmospheric carbon dioxide concentrations has been attributed to variability of emissions from biomass burning(1-3). Under drought conditions, burning in Indonesia is a disproportionate contributor to these emissions, as seen in the 1997/98 haze disaster(1,4). Yet our understanding of the frequency, severity and underlying causes of severe biomass burning in Indonesia is limited because of the absence of satellite data that are useful for fire monitoring before the mid-1990s. Here we present a continuous monthly record of severe burning events from 1960 to 2006 using the visibility reported at airports in the region. We find that these fires cause extremely poor air quality conditions and that they occur only during years when precipitation falls below a well defined threshold. Historically, large fire events have occurred in Sumatra at least since the 1960s. By contrast, the first large fires are recorded in Kalimantan ( Indonesian Borneo) in the 1980s, despite earlier severe droughts. We attribute this difference to different patterns of changes in land use and population density. Fires in Indonesia have often been linked with El Nino(1,2,5-12), but we find that the Indian Ocean Dipole pattern is as important a contributing factor.