Rhett Butler – Indonesia's government has been at pains to stress that the recent catastrophe in Sumatra was triggered by a rare meteorological event. Cyclone Senyar formed in the Malacca Strait, an area where the national weather agency notes such storms are "an extremely rare phenomenon," before unloading torrents of rain on Aceh, North Sumatra and West Sumatra.
Meteorologists say the interaction of two cyclones may have produced record rainfall over parts of the island. Scientists are cautious about pinning any single storm on climate change. Yet few now argue that the tragedy was simply an act of nature.
Officials themselves have begun to say the quiet part out loud. Forestry minister Raja Juli Antoni told parliament that "poor forest management" had worsened the disaster, and promised to "review forest governance, consider a moratorium on new permits, and revoke the licenses of violators," according to Reuters. The environment minister, Hanif Faisol Nurofiq, has suspended permits for several companies in the Batang Toru watershed and warned that criminal proceedings are possible if violations are found.
The broader pattern is well documented: Sumatra has lost an estimated 4.4 million hectares of forest since 2001. When the floodwaters receded, viral images showed how extensive clearing shapes disaster impacts on the ground: houses smashed by logs, riverbanks lined with cut timber, and whole slopes stripped to bare soil.
Environmental groups argue that the cyclone merely collided with a landscape that had been made fragile by decades of land conversion. The Indonesian Forum for the Environment (WALHI) estimates that 1.4 million hectares of forest in Aceh, North Sumatra and West Sumatra were cleared between 2016 and 2025 alone, much of it for mining, palm oil and pulpwood.
"The disaster was not just nature's fury, it was amplified by decades of deforestation," WALHI's Rianda Purba told the Associated Press. The Batang Toru catchment, one of the worst-hit areas, now hosts hydropower schemes, a major gold mine, and plantations in what was once a continuous upland rainforest.
The physical mechanisms are well understood. Intact forests slow rainfall, hold soil in place and allow water to percolate into the ground rather than roar off the surface. When forest is replaced by roads, pits, and monoculture estates, that sponge function fails. Analysts writing in The Conversation describe how, in degraded catchments, "the ecosystem loses its natural ability to act as a 'sponge,'" so intense rain turns quickly into destructive runoff and unstable slopes. That pattern was stark in the western block of Batang Toru, where satellite images show thousands of hectares of forest converted, then gouged away by landslides during the storm.
On Sumatra's peatlands the story is subtly different, and just as grim. Draining peat for plantations requires dense canal networks that lower the water table and dry out the soil. Pantau Gambut, an Indonesian NGO, has mapped more than 280,000 kilometers of canals cut through peat ecosystems across the archipelago, much of it inside oil palm and pulpwood concessions. The effect is uniform: lowered water tables across wide areas. As peat dries, it oxidizes and compacts, causing the land surface to sink. The World Resources Institute calls peat subsidence a "sleeping disaster": over time, low-lying peatlands end up below river or sea level, vulnerable to both inland floods and coastal inundation. What once stored water safely in the ground now sheds it rapidly into swollen rivers.
The result is a patchwork of risks. In deforested lowlands and plantation belts, lost canopy cover contributes to higher surface temperatures and heat stress. On steep terrain where natural forest no longer anchors soil, intense rain can produce more frequent landslides. And in peat districts where canals and subsidence have lowered the land, even ordinary high tides or seasonal rains can now trigger serious and persistent flooding. Cyclone Senyar exposed all three patterns, and its consequences unfolded over the following fortnight. It also ripped through some of the island's last biodiversity strongholds: scientists fear that dozens of Tapanuli orangutans, already the world's rarest great ape, may have been wiped out when whole forested slopes in Batang Toru collapsed.
The human geography has shifted too. More people now live in floodplains, on riverbanks and along steep hillsides than a generation ago, as towns expand and new roads push into once-remote valleys. Analysts at the Lowy Institute note the persistence of "spatial planning violations": buildings along rivers, development on floodplains, weak enforcement of existing rules. That pattern means that even if storms do not become more frequent, the number of people and assets in harm's way will keep rising.
Officials and activists disagree on how much to emphasize climate change. Some stress warming seas and extreme rainfall; others, like disaster scholar Eko Teguh Paripurno, warn against treating the cyclone as an unavoidable act of the atmosphere. "We can't just leave it as it's the rain or the climate to blame," he told Mongabay Indonesia. That distinction matters. Countries cannot control where cyclones form. They can control whether watersheds are stripped, peatlands are drained, and housing sprawls into known hazard zones.
What the Sumatra floods show, in stark fashion, is that much of the island now faces systemic disaster risk, the product of 30 years of aggressive land conversion and weak land-use management. Restoring forests and peatlands, enforcing spatial plans and curbing destructive concessions will not make rare cyclones vanish. But without those changes, the next unusual storm could again translate into thousands of avoidable deaths, submerged towns and what Pantau Gambut calls a "cycle of disaster" baked into the landscape itself.