Peat Burning Scales Super-linearly with Fire-Season Magnitude in the West Kalimantan–Sarawak Border Region, 2012–2025
Analysis of 2012–2025 fire data in the West Kalimantan–Sarawak border region reveals that peat burning scales super-linearly with overall fire-season magnitude, causing haze burdens to increase disproportionately during El Niño years as a small fraction of recurrently burning peat cells drives the majority of cumulative emissions.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
Every year, as the dry season settles over the islands of Borneo, a thick, choking haze often rolls across the region, blurring the horizon and forcing schools to close. This smog is not just smoke from ordinary forest fires; it is largely the product of peatlands—ancient, waterlogged soils rich in decaying plant matter—that have been drained and set alight. When these peat fires burn, they do not roar with flames like a wood fire; instead, they smolder deep underground, releasing a massive amount of toxic particles that linger in the air for weeks. For decades, scientists and policymakers have known that the severity of these fire seasons is linked to the El Niño–Southern Oscillation, a climate pattern that brings hotter, drier weather to the region. When the ocean warms in the Pacific, rainfall in Borneo drops, the peat d out, and the fires spread. But a lingering question remained: does a stronger El Niño simply mean more fires everywhere, or does it fundamentally change the type of fires, causing a disproportionately larger share of them to ignite in the most dangerous peatlands?
A new study by Takaaki Torii, a researcher at the University of Shizuoka, has answered this question by looking at fourteen years of satellite data from the border region between West Kalimantan in Indonesia and Sarawak in Malaysia. The research reveals a startling pattern: the amount of peat burning does not just rise in step with the total number of fires; it explodes. The study shows that peat fires scale in a super-linear way, meaning that as the overall fire season gets bigger, the portion of fires burning on peat grows much faster than the total count. In simple terms, if the total number of fires in a season doubles, the number of peat fires increases by roughly four times. This finding suggests that the worst haze years are not merely "more of the same" but are qualitatively different, dominated by the most polluting type of combustion.
To uncover this relationship, the researcher analyzed over 1.5 million fire detections recorded by NASA's VIIRS satellites between 2012 and 2025. Each fire was mapped against a global map of peatlands to determine whether it was burning on peat or on mineral soil. The data showed a clear trend: in years with negative climate signals, known as La Niña, which bring wetter conditions, peat fires made up less than 5% of all detections. However, in years with strong El Niño signals, which bring severe drought, the share of peat fires climbed dramatically, reaching as high as 51% in 2019. The study confirms that the intensity of the El Niño event is a powerful predictor of how much of the landscape will burn on peat, but it clarifies that this happens because El Niño drives the overall size of the fire season, which in turn triggers the super-linear surge in peat burning.
The research also challenges the idea that the relationship between climate and fire composition is a simple, direct switch. While the total number of fires and the share of peat fires both rise with El Niño, the data shows that the peat share is actually better predicted by the sheer magnitude of the fire season than by the climate index itself. This means that once a fire season reaches a certain threshold of intensity, the landscape undergoes a shift where an increasing fraction of the available peat becomes dry enough to ignite. It is not that El Niño specifically targets peat; rather, as the drought deepens, the "ignitable zone" expands from dry grasslands and mineral soils to include vast swathes of peat that would otherwise remain wet and safe.
Another critical discovery concerns where these fires happen over time. The study mapped the locations of every peat fire over the fourteen-year period and found that the damage is heavily concentrated in a small number of specific spots. While thousands of grid cells on the map experienced a peat fire at least once, a tiny fraction of those locations burned repeatedly, year after year. Specifically, the cells that burned in seven or more of the fourteen years accounted for nearly 58% of the total energy released by all peat fires in the region. In contrast, the locations that burned only once contributed a mere 2% of the total energy. This indicates that a small set of persistent hotspots, likely driven by permanent drainage infrastructure, is responsible for the majority of the cumulative emissions, while the rest of the peatland burns only during the most extreme droughts.
The study also looked at how these fires affect the neighboring Malaysian state of Sarawak, where the smoke often drifts. Using computer models to track the path of air masses, the researchers found that the super-linear relationship between fire season size and peat burning survives the journey. Even after the smoke travels across the border, the air reaching Sarawak is still disproportionately loaded with peat smoke during intense fire seasons. However, the signal linking the smoke directly to the El Niño climate index weakens as it travels, suggesting that the composition of the haze is governed more by the total scale of the fires in Kalimantan than by the specific climate conditions right at the border.
Looking ahead, the study offers a sobering perspective on the 2026 fire season, which was developing as the research was being finalized. By early September 2026, the number of fire detections had already surpassed the record-breaking 2015 season, a year known for catastrophic haze. The data suggests that if the scaling relationship holds, this new season will not just be bigger, but will be dominated by peat fires to an even greater degree, leading to a haze burden that grows faster than the fire count alone would suggest. The research concludes that while restoring peatlands is essential, the most effective strategy for reducing emissions may be to focus intensely on those few, persistent hotspots that burn every single year, as they are the primary drivers of the region's cumulative smoke problem.
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