Biogeosciences Fire , drought and El Ni ño relationships on Borneo ( Southeast Asia ) in the pre-MODIS era ( 1980 – 2000 )

Borneo (Indonesia) is Earth’s third largest island, and the location of both extensive areas of rainforest and tropical peatlands. It is the site of both regular (seasonal) biomass burning associated with deforestation, land cover change and agricultural production preparations, and occasional, but much more severe, extreme fire episodes releasing enormous volumes of carbon from burning vegetation and peat. These extreme fire episodes are believed to result from anthropogenic practices related to (the still ongoing) forest degradation and clearance activities, whose impact with regard to fire is magnified by the effects of El Ni ño related drought. Since 2000, data from the MODIS Earth Observation satellite instruments have been used to study fire on Borneo, but earlier large fire events remain less well documented. Here we focus on a series of large fire episodes prior to the MODIS era, and specifically a 20 yr period covering both the two strongest El Ni ño events on record (1997–1998 and 1982–1983), along with an unprecedented series of more frequent, but weaker, El Ni ños. For the five El Nĩ nos occurring between 1980 and 2000, we develop quantitative measures of the fire activity across Borneo based on active fire counts derived from NOAA AVHRR Global Area Coverage (GAC) Earth Observation satellite data. We use these metrics to investigate relationships between the strength and timing of the El Niño event, the associated drought, and the fire activity. During each El Nĩ no, we find areas of major fire activity confined within two or three fire sub-seasons (separated by monsoons) and focused in parts of South and Central Kalimantan, and sometimes also in East and/or West Kalimantan. For each El Nĩ no we investigate various lag correlations, and find relationships of similar strength between monthly rainfall deficit and fire, but of more variable strength between indices of El Nĩ no strength (ENSO indices) and rainfall deficit. The two strongest El Ni ño episodes (1982–1983 and 1997– 1998) are accompanied by the most abundant fires (two and three times the active fire count seen in the next largest fire year), and the strongest correlations between measures of El Niño strength, rainfall and fire. We find the most significant positive statistical association between an ENSO index and fire activity to be that between the 16-month (first and second fire sub-seasons) cumulative NINO3 anomaly and the simultaneously recorded active fire count ( r = 0.98, based on the five El Niño episodes between 1980 and 2000), although we find a negative association of equal strength between the cumulative NINO4 index and active fire count when considered over the entire two year duration of each El Ni ño episode (first, second and third fire sub-seasons). Our results confirm that the El Nĩ no phenomenon, via its effect on precipitation, is a primary large-scale, short-term climatic factor that has a strong control on the magnitude of the fire activity resulting from the numerous land cover changes, agricultural preparation practices and human-caused ignitions occurring annually across Borneo. The results also suggest that ENSO forecasting maybe a realistic means of estimating the extent and magnitude of this fire activity some months in advance, thus offering some potential for forecasting effects on the remaining forest and peatland resource and the regional atmosphere. Published by Copernicus Publications on behalf of the European Geosciences Union. 318 M. J. Wooster et al.: Fire, drought and El Niño relationships on Borneo