Soil moisture is key to slowing zombie fires

When peat fires smolder underground during the freezing winter months, they sometimes persist for so long that they reignite after the ground warms and thaws in the spring. These “zombie fires” are known to release up to 100 times more carbon into the atmosphere than flaming fires.

In a new study, researchers at Imperial College London have demonstrated how soil moisture content affects the ignition and spread of smoldering peat fires. The team also used computer models to discover how several small peat fires can merge into one large blaze.

“Peat fires are a devastating yet chronically under-researched phenomenon that spurt millions of tons of carbon into the atmosphere every year,” said study first author Dwi Purnomo. “If we can use scientific evidence to help people manage them more effectively, we can perhaps dampen their impact on people and the environment.”

Peat fires are most common in regions like Southeast Asia, North America, and Siberia.  Peat is a natural reservoir of carbon, and zombie fires are fueled by soils that are rich in this and other organic content. 

Unlike the smoke from non-peat fires, which is ejected high into the atmosphere, smoldering peat fires burn close to the ground where they produce a haze that is very threatening to human health. 

Zombie fires are notoriously difficult to control. Even when the flames are extinguished, the fire can smolder up to several feet underground for weeks or months at a time, just waiting to flare back up.

These hazardous peat fires can be sparked naturally by lightning strikes or by human activities, such as controlled burns. 

“Although people have been using controlled burns in agriculture for centuries, starting them on peat soils can be particularly dangerous,” explained study senior author Professor Guillermo Rein. 

“Peat draws the fire underground, which then hides there before coming back like zombies, making detection and extinction very challenging. The effects are felt in plantations, forests, homes, residents’ health and the environment.” 

The new study reveals that burning vegetation on peat soils with a higher moisture content is less likely to sustain smoldering, which means that fires will be less likely to burn uncontrollably. 

The research is the first of its kind to investigate the interaction between smoldering peat and flaming vegetation. The findings may ultimately inform land managers and scientists on the safest ways to clear the vegetation in peatlands. 

“It might seem trivial that drier soils sustain faster and larger smouldering fires, but this work can predict the critical moisture values for ignition,” said Purnomo. “I’ve seen the devastation they can cause and want to help my country and others like it which are affected by peat fires.”

The study is published in the journal Proceedings of the Combustion Institute.

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By Chrissy Sexton, Earth.com Staff Writer