A few days after I arrived in Riau, as I marched to the jungle to see one of the fires, I looked back at where my footprints sank some 12 inches into the peat and saw smoke rising from my tracks.

It’s here, in the peat burning below the forests, where the greatest climate impact from palm production can be seen. When forests are cleared to make way for oil palm plantations, the area is usually burned, and most of Riau’s massive fires burn on peat — swampy layers of partially decayed vegetation that spreads up to 60 feet deep beneath most of the province’s forests.

Peatlands hold up to 28 times as much carbon as rainforests growing on mineral soil. The peat is so carbon rich that if it is buried long enough, say for a million years or so, pressure, time and heat will turn it into coal. A single hectare of peatland rainforest can release 6,000 metric tons of planet-warming carbon dioxide when it’s converted into a plantation. Researchers estimated that in 2012 nearly 70 percent of the carbon released during the transformation of Sumatran rainforests into palm plantations came from peatlands, a 75 percent increase over their portion of emissions in the 1990s and an indication that palm is increasingly expanding into peat.

And it’s not just CO2: In 2013 the nation’s then president, Susilo Bambang Yudhoyono, apologized to Singapore and Malaysia for the brown cloud from Sumatran fires that shattered air pollution records in the neighboring nations, filled hospitals with tens of thousands of smoke-sickened patients and forced officials to close schools. Indonesian aircraft seeded clouds above the fires with 100 tons of salt in hopes of bringing rain to drench fires smoldering in the peat.

When the forests ignited again six months later, more than 9,000 tweets bombarded the president’s office. During an emergency trip to Riau he said he was “ashamed” of the fires. Nearly 50,000 Sumatrans sought treatment for the impacts of smoke on their lungs, eyes and skin. Aircraft again seeded the clouds.

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