In Indonesia, fire beneath the soil prolongs the smoke crisis

The smoke covering parts of Indonesia has a source that extends beyond flames above the trees. In peatlands, the soil itself, rich in accumulated organic matter, can keep burning below the surface. An image acquired by the Terra satellite on September 14 and released by NASA the following day shows plumes over Borneo. The September 14 bulletin from Indonesia's Meteorology, Climatology and Geophysics Agency, known as BMKG, confirms persistent smoke across several regions of the archipelago.

In the Terra image, MODIS—an instrument that records radiation in different spectral bands—combines the surface view with hotspot markers. The Indonesian bulletin adds another observation: imagery from the Himawari-9 satellite at 8 a.m. Western Indonesian Time on September 14 showed smoke in parts of Sumatra, Kalimantan, Maluku and Papua. These records come from different satellites, with their own timing and coverage, and help track the problem's distribution.

Weather conditions favor the persistence of fires. According to BMKG, by the first ten-day period of September, about 81% of monitored seasonal climate zones were in the dry season. That proportion refers to the agency's zones, not 81% of the country's area. The bulletin also recorded 1,637 observation sites with more than 60 consecutive days without rain. The agency links worsening dryness to a very strong El Niño: warming in a band of the tropical Pacific changes atmospheric circulation and often reduces rainfall in parts of Indonesia.

Peat makes this combination especially difficult to control. It consists of plant remains that accumulate in normally waterlogged environments, where decomposition is slow. When that material dries, it becomes fuel for slow combustion that can advance below ground. Water that kept the deposit wet no longer offers the same protection. In an analysis published by NASA on September 3, University of Maryland ecologist Mark Cochrane also linked today's vulnerability to the historical drainage of wetlands for agriculture. Drought aggravates a landscape that has already been altered.

Monitoring requires care in interpreting the numbers. A hotspot is a detection of radiation consistent with fire within an image pixel; it does not necessarily correspond to a whole fire. A single fire can produce several detections. Conversely, thick smoke, clouds, vegetation cover and underground combustion can conceal fire from sensors. A decline in detected hotspots alone therefore does not prove improvement. NASA's analysis explains that the smokiest episodes can be the hardest to observe using this method.

The records support a persistent crisis, with consequences for air quality, visibility and transport, but do not provide a complete estimate of the area still burning underground. BMKG also emphasizes that local rainfall can occur during the dry season; it does not guarantee uniform relief across the archipelago. Tracking this crisis requires combining heat detections, smoke imagery, weather conditions and observations on the ground. The challenge is to extinguish fuel that can remain active even when visible flames disappear.