The largest monthly release of Fire Radiative Power (FRP) − the rate of electromagnetic energy released by fire14 − detected by TERRA and AQUA satellites in Sumatra (since records began in July 2002) was in June 2013 (FRP = 383 Gigawatts) (Fig. 1a). Singapore's 24-hr Pollutants Standards Index (PSI) reached an all-time record 246 on 22 June 2013 (seven consecutive days > 101 including three consecutive days > 236; rated “very unhealthy”), almost doubling its previous record of 138 from 19 September 1997 (twelve days >101 between 13 Sept. - 25 Oct.) (Fig. 1a). This trans-boundary haze event is remarkable as neither ENSO nor IOD conditions occurred in 2013. By contrast, the last major episode of extreme air pollution over Singapore had occurred in 2006, when both ENSO and IOD conditions preceded major fires in Sumatra (and in Indonesian Borneo)15, resulting in a peak in Sumatra's fire activity in October 2006 (FRP = 366 Gigawatts) and a peak in Singapore's 24-hr PSI on 07 October 2006 (128; rated “unhealthy”) (Fig. 1a).

Figure 1 Singapore's air quality (1997–2013) and Sumatra's fire activity (2002-2013) and rainfall (1997–2013). (a), 24-hour PSI in Singapore (top) and monthly FRP (in Gigawatts) from Sumatra (bottom) measured by the MODIS instruments on-board the TERRA and AQUA satellites. (b), monthly rainfall in Riau province (top) and monthly FRP from the three-million ha study area (bottom). (c), a scatterplot of monthly FRP from the study area fitted using a power function with mean rainfall in the preceding two months. Each cross represents one calendar month (n = 134; July 2002 to August 2013), with June 2013 represented by a filled circle. (d) The same data as in c presented in Log-Log. The solid line shows the linear relation between the logarithm transformed variables: Log (FRP) = 41.1 – 6.13 Log (Rainfall). The hashed lines show the 95% prediction bounds of the fitted curve. Full size image

Our investigations of the June 2013 fires in Sumatra determined that a three-million ha area of Riau Province in Central Sumatra (1.6% of Indonesia's landmass; one LANDSAT scene; see bottom inset in Fig. 2a) was the source of 71% (271 Gigawatts) of Sumatra's FRP in that period (Fig. 1a, b). It also accounted for 72% of the area burned across the entire island as detected by the same satellites in the same month (Supplementary Fig. 1). We investigate this area in greater detail in the following sections.

Figure 2 The three-million ha study area in Riau province, Sumatra (location see inset). (a), Fire hotspots. MODIS daily hotspots distribution for June 2013 (yellow dots) overlaid on a post-fire LANDSAT OLI imagery (12 August 2013) displayed in false colours (RGB: 6-5-4). (b), Burned areas. An estimated 163,336 ha burned in the study area: red (non-forest), green (forest), orange (Acacia plantation) and cyan (cloud). Peatlands are shown in darkest shade of grey; superimposed are the seven locations of the UAV transects. The bottom inset is a UAV snapshot over peatlands deforested 3 years prior to the June 2013 fire, where dead carbonized tree trunks and an excavator preparing land for oil palm are clearly visible. (c), Pre-fire Deforestation. Loss of species-rich Dipterocarp forest from 1990 until May 2013. Light brown: non forest in 1990. Orange: deforested between 1990-2008. Purple: deforested between 2008 and May 2013. The study area lost 1.72 million ha (78%) of forest between 1990 and May 2013 (including 1 million ha on peat). (d), Pre-fire land-ownership map. Industrial oil palm and Acacia plantations developed by companies in concessions are shown in yellow and in khaki, respectively. Concessions (for both oil palm and Acacia) occupied by communities are shown in black. Lands outside concessions are in white. Forest cover (unoccupied land) one month before fire is shown in dark green. Maps created using ArcMap v10.0 geospatial processing program. The data used to generate the maps presented in this figure are made available online at http://www.cifor.org/map/fire/. Full size image

Daily fire hotspots (also from TERRA and AQUA satellites) revealed a peak in fire activity during the week of 18–24 June (Supplementary Fig. 2). A brief dry period preceded this fire. For the twelve months leading up to and including June 2013 (July 2012-June 2013) Riau was wetter than average, receiving 2,530 mm of rain compared to the annual mean of 2,350 mm for 1961–2013 (Supplementary Fig. 3a). However, May and June 2013 registered rainfall deficits compared to the monthly means (Supplementary Fig. 3b). Monthly FRP in the study area was correlated with rainfall over the month of FRP measurements and the month before (Log-Log fit; r2 = 0.55, p<0.01, n = 134). A 1% decrease in rainfall induced a 6% increase in FRP (Fig. 1c, d). Correlations over one, three and four months were lower (r2 = 0.43, r2 = 0.43, r2 = 0.31, respectively). A similar relationship has been observed previously in Central Sumatra15.

LANDSAT satellite imagery acquired shortly before and shortly after the fire indicates that 163,336 ha (including 137,044 ha, or 84% on peat) burned in the three-million ha study area (Fig. 2a, b). We validated this assessment using a Unmanned Aerial Vehicle (UAV or “drone”) one month after fire at seven sites, spanning 1,301 ha (Fig. 2b) and observed an accuracy of 85% for burned areas (Supplementary Fig. 4, Supplementary Table 1, 2), with 96% of MODIS fire hotspots falling within the burned areas extent (Fig. 2a, b).

Only 7% (12,037 ha) of burned lands were classified as ‘forest’ before the fires (accuracy of 97%; Supplementary Table 3, 4). This was mainly small degraded remnants of drained peat-swamp forest. Most burned lands were classified as ‘non-forest’ (81%; 133,216 ha) (Fig. 3). However, over half of burned areas (58%; 94,308 ha) were forested five years previously (Fig. 2b, c). Comparison with the corresponding UAV-based vegetation map reveals that 57% of burned ‘non-forest’ areas were nonetheless ‘forest cemeteries’, i.e. a mosaic of scrub and exposed soil, with stumps, downed trunks and branches (Fig. 3). The burned ‘non-forest’ areas on peat (68%; 111,561 ha) generated the bulk of the FRP (Supplementary Fig. 5). The imagery also detected areas where planted Acacia forests (Acaciacrassicarpa A.Cunn ex Benth. widely called “Acacia” though it was recently renamed Racosperma crassicarpum (A. Cunn. ex Benth.) Pedley.) and oil palm plantations (Elaeis guineensis Jacq.) had been damaged by fire (Fig. 3).

Figure 3 Vegetation cover of the burned areas (163,336 ha) before the June 2013 fires. The ‘Non-forest’ category contains a broad mix of vegetation types, which we identified through a comparison against a more detailed vegetation classification derived from the Unmanned Aerial Vehicle (UAV). This comparison was only performed in the portions of UAV imagery identified as ‘unburned’ (567 ha). The error bar is calculated as +-1 Standard Deviation, (n = 7 UAV transects). Inset, ‘Non-Forest’ is dominated by scrubs and exposed soils, young (<5 years old) and mature (>5 years old) oil palm plantations. Oil palm plantations either belong to small- and medium-scale agriculturalists or to companies. The ‘Forest’ category includes logged and drained natural forests. The ‘Acacia’ category indicates closed-canopy industrial plantations on peatlands. The ‘Cloud’ class indicates areas that were obscured by clouds and cloud shadows on the pre-fire LANDSAT imagery. Full size image

We found that 52% of the total burned area (84,717 ha) was within concessions, i.e. land allocated to companies for plantation development (Fig. 2b, d). However, 60% of burned areas in concessions (50,248 ha, or 31% of total burned area) was also occupied by communities (Methods; Supplementary Fig. 6). This presence makes attribution of fires problematic. The remaining 48% of the total burned land (79,012 ha) was owned by Indonesia's Ministry of Forestry (under central government). These areas were deforested prior to fires and their ownership is often contested by the local government. The detection of two excavators by the UAV preparing land for planting in the burned areas one month after fire suggests fires were associated with agriculture (lower inset in Fig. 2b).

We estimate that the June 2013 fires released 172 ± 59 Tg CO 2 -eq of GHG into the atmosphere during the week of 18–24 June in the study area (Table 1; Methods and Supplementary methods). Carbon emissions were 31 ± 12 Tg C. Uncertainties were around 39% and 35% of total C and total GHG emissions, respectively. These emissions represent 5–10% of Indonesia's reported annual GHG emissions for 2000–200516 and 26% of average annual C emissions from fires in tropical Asia (-10 to 10N, 60–190E) between 2003–2008 modelled using the Global Fire Emission Database (GFED)17. Ninety percent of the emissions originated from peat and CO 2 -eq emissions were mainly in the form of CO 2 (55.3%) and CH 4 (44.5%). Total CH 4 emissions represented 4–6% of average annual emission rate for the whole of Southeast Asia in 2000–200918. N 2 O emissions were negligible (0.3% of total CO 2 -eq emissions).

Table 1 Carbon emissions from fires. Average value ± SE of fuel load (FL), combustion completeness (CC), burned areas (detailed as burned on mineral soils + peat soils for each vegetation cover defined in Fig. 3), emission of carbon dioxide (CO 2 ), carbon monoxide (CO), methane (CH 4 ), nitrous oxide (N 2 O), mono-nitrogen oxides (NOx), total carbon (C) emission (CO 2 +CO+CH 4 ) and total emission of greenhouse gases (GHG) (CO 2 +CH 4 +N 2 O). Total emission of GHG were calculated using 20 (GHG 20YGWP ) and 100 year (GHG 100YGWP ) global warming potentials (GWP) for CH 4 and N 2 O. The lack of appropriate emission factors for other GHG species prevented their inclusion Full size table

Our results demonstrate that the Indonesian fires of 2013 behind the record air pollution episode in Singapore were triggered by a seasonal two month dry spell in an otherwise rainy year. These fires were short-lived and confined to recently deforested peatlands in a localized area in Central Sumatra (in Riau Province), reflecting ongoing conversion to oil palm plantations. The area affected was much smaller than the 9.7–11.7 million ha that burned in 199711,12. However, the emissions of GHG and smoke during this brief localized event (one week and 1.6% of Indonesia's land) were disproportionately large because of the peat. These fires generated unprecedented atmospheric pollution in Singapore because of their proximity and the prevailing south westerly monsoon winds (Supplementary Fig. 7a).

During the last major drought years (1997 and 2006) fires peaked in Southeast Sumatra (Supplementary Fig. 7b) and in southern Borneo from August through October, but their impacts in Central Sumatra were less extreme15. Riau experiences a bi-modal annual rainfall pattern with peaks centred on November and April (Supplementary Fig. 2b)19. It responds less to sea surface temperature anomalies than Indonesia's other fire-prone regions19. The major Riau fires of 2005, 2013 and the recent 2014 event occurred during the regular short seasonal dry spells (< 2 months) centred on February and June. February fires (e.g. in 2005 and early 2014) are associated with prevailing north easterly monsoon winds and thus generally cause little problem of trans-boundary haze (Fig. 1a&b; Supplementary Fig. 7c). In June, as observed in 2013, prevailing winds carry any emissions directly from Riau to Singapore (Supplementary Fig. 7a). The brief droughts, that seldom exceed two months, pose a challenge to forecasting severe fire events in Riau. While the 2013 fire event may initially appear anomalous, we expect such events to be increasingly frequent with ongoing peatland development.

Peat forests in Sumatra have declined by 18,400 km2 (4.6% yr−1) over the last two decades20. This reflected timber cutting, plantation development and fires20. Some of these deforested lands remain undeveloped and persist in a degraded and seasonally fire-prone state21. Deforestation elevates local temperatures, reduces precipitation and limits soil moisture; this heightens climatic variability and likelihood of drought and influences regional climate22,23. The convergence of these trends with the frequent use of fire by humans may, over time, render the emissions of peatland fires in Central Sumatra during ‘wet’ years increasingly similar to that of ‘dry’ ENSO/IOD years. Assessing the state and vulnerability of remaining peatlands, would help identify where vigilance is most required.

The Indonesian government has encouraged investment in oil-palm and pulpwood industries resulting in rapid large-scale plantation expansion and associated developments24. In 2011, Indonesia implemented a moratorium on new plantation concessions in an effort to protect remaining forests and peatlands25. However, such policies did not prevent the June 2013 fires. Our results show that these fires occurred mostly in already-cleared peatlands. Burn locations suggest ignition by both communities and companies. Most fires are lit in order to prepare land for cultivation24 but some are likely accidental, while others may be arson26: we still know too little concerning these specific events and the intentions and safeguards used.

Efforts to avoid major haze events require that all land users control fire use during any dry periods. Given land use practices in the region and the frequent conflicts among land users, this will be challenging5. We advocate active protection of remaining peatland areas and cessation of further drainage. Financial incentives for forest protection are not competitive with commercial land values and future payments for reducing emissions from deforestation and forest degradation (REDD) are unlikely to change this24. Unless strong action is taken Indonesia's peatlands are likely to remain a major source of GHG and aerosol emissions.