Indonesia is in the throes of a wildfire catastrophe.

Amid a so-called “Super El Niño” that has supercharged local drought conditions, thousands of active fire hotspots are spreading and multiplying across the country, fuelling a transnational haze crisis that is the region’s worst in more than a decade.

But what makes Borneo’s fires particularly worrying is the fact that they are burning through one of the world’s largest tropical peatland ecosystems.

This 4.5-million-hectare swathe of forest and swamp, under normal conditions, serves as a valuable reservoir for ancient carbon.

When these peatlands catch fire, they risk releasing millions of tonnes of carbon dioxide, carbon monoxide and methane into the atmosphere – a “pulse” of greenhouse gas emissions that has earned them the sobriquet of “carbon bomb”.

“Peatlands are incredibly effective carbon stores because their waterlogged conditions slow decomposition, allowing carbon to accumulate underground for thousands of years,” explains Dr Nisa Novita, a peatland researcher at Indonesian conservation NGO Yayasan Konservasi Alam Nusantara (YKAN).

“But this also means they can become a ‘carbon bomb’ when they are drained or burned, because that carbon can be rapidly released into the atmosphere.

“Tropical peatlands in Indonesia are among the world’s most effective natural carbon stores. The deeper the peat, the greater the carbon stock – and in Kalimantan, we have found peat deposits that can exceed 20 metres in depth.”

A peatland that deep has been known to harbour more than 6,000 tonnes of carbon – roughly equivalent to the annual CO₂ emissions of 4,800 cars – per hectare. And when it burns, says Dr Nisa, that fossilised carbon can be released into the atmosphere “very rapidly”.