What are “zombie fires”, what other types of fires are there (and which one has been burning for at least 6,000 years)

From Spain to France and Portugal, from California to Siberia.

The wave of devastating forest fires and flames does not stop.

But there are other lesser known fires that can be just as destructive: smoldering fires.

These fires without flames are so difficult to fight that one of them, in Australia, has been burning for at least six thousand years, Spanish engineer Guillermo Rein, professor of Fire Sciences at the Department of Mechanical Engineering at Imperial College in Australia, told BBC Mundo. London.

And among the smoldering combustion fires, there is an especially complex and surprising type: the so-called “zombie fires”.

Guillermo Rein explained to BBC Mundo what kinds of fires there are, why when escaping we should never run up a slope and how fires that “seem to return from death” come to be produced.

What types of fires there?

There are different kinds of fires depending on where the combustion is seen, the Imperial College expert noted.

“If the combustion occurs in the treetop the flames are very high. Those fires are tremendously dangerous and ferocious.”

“Something more normal and intermediate in ferocity is what happens when what is burning is the surface fuelthe bushes, the grasses, the small trees, the fuel that we see at our height”.

The third type is latent combustionthe fire that has no flame and in which what burns is the ground.

“Not all soils are combustible, but when they are flammable, as in the case of peat, the soil burns without flame.”

William Rein at Imperial College. “Being an engineer means that everything I discover as a scientist, I discover to find solutions, not for the beauty of the process itself, but simply because as an engineer I want to protect and help society.” (Photo: IMPERIAL COLLEGE)

What is peat and how does it ignite?

“If the humidity is very, very high, that is, if it is basically flooded, or it is very cold, like near the poles, the forest vegetation that falls to the ground does not degrade. There are no germs, there are no bacteria that eat it, so it accumulates and compacts,” Rein explained.

What results from that process is peat, a carbon-rich material that can be thousands of years old.

“Basically, when we pick up peat with our hands the deeper we go, we’re talking about forests that may be older than humans coming out of Africa.”

ground from which smoke comes out
In smoldering combustion fires, which do not have a flame, what is burned is the ground. (Photo: COURTESY GUILLERMO REIN)

The peat has to be either very wet or very cold. But if it is dry and hot it becomes a flammable materialexplained engineer Rein.

“Typically what can happen is a forest fire with a flame, for example by lightning or by human action. The flame that lights the peat is extinguished but the peat continues to burn for weeks, months or years”.

Although all smoke is toxic, in the case of smoldering combustion fires “smoke is the most toxic known”.

“It is the smoke that has been recorded, for example, in peat fires in Indonesia. As there is no flame, the smoke is not very hot and it does not rise by buoyancy, it stays on the ground where the people are”, said Rein.

Although they can be devastating, the absence of flames makes it difficult to convey “the vastness or horror of smoldering fires” in the media.

Women in Indonesia wearing face masks from peat fire smoke
Smoke produced by smoldering peat fires in Indonesia. (Photo: GETTY IMAGES)

What are zombie fires?

Zombie fire is a type of smoldering peat combustion.

“The zombie name has been a fantastic success. It is an informal name, but it works very well to communicate”, Rein pointed out.

The term was first used by scientists in Alaska to describe fires that occur in the summer.

“There are flame fires that occur in the summer. Then the flame is typically extinguished for natural reasons, because it hits a river or it rains and everyone is very happy.”

“But they haven’t realized that the peat has caught fire and is starting to burn down.”

When the snow comes it covers the ground completely.

“But the fire continues below. The layer of snow that is insulating protects the latent combustion in the subsoil and when spring comes and the snow melts, the smoldering combustion rises to the surface and is even capable of igniting flames“.

“The fire that was extinguished last summer then comes back in the same place, but people can’t find out what the possible ignition process is.”

“They are zombie fires in the sense that they thought they were dead but they were alive like zombies and then they scared us again.”

Guillermo Rein in a mountain from which smoke comes out, a case of latent combustion
The absence of flames makes it difficult to convey in the media “the immensity or horror of smoldering fires”. (Photo: COURTESY GUILLERMO REIN)

Can there be zombie fires in places where there is no snow?

The term is usually reserved for fires in areas where it is very cold, such as Alaska or Siberia.

Rein pointed out that there are also well-known smoldering fires in places without snow. But it is not known if they could properly be called “zombie fires” because it is unknown if there were flames at first that were extinguished or if they started directly as smoldering combustion, in what is called self-heating or self heating.

Sign marking the Burning Mountain or Montaña Ardiente reserve
The Burning Mountain fire in Australia has been burning for at least 6,000 years below the mountain and is advancing one meter a year. (Photo: COURTESY GUILLERMO REIN)

The oldest fire known to mankind is in Australia and is the Burning Mountain or Burning Mountainas Mount Wigan is popularly called, about 220 km north of Sydney.

“It is a case of latent combustion, but not of peat, but the peat after several million years turned into coal, it is the fossilized forest of antiquity”, explained the Imperial College expert.

It has been burning for at least six thousand years under the mountain following the coal seam“.

Another famous case of latent combustion is that of centraliaa town in Pennsylvania, United States.

“In the 1960s, Centralia was a very small mining town.”

“There was a mine where people worked and extracted the coal,” Rein said.

“And one day on a holiday they made a fire and the fire ended at the entrance to the mine and set the mine on fire. The flames are over but since 1962 they have not been able to put out the smoldering combustion“.

“The fire has moved into the mine galleries below the town of Centralia, creating sinkholes. And smoke comes out everywhere with sulfur, with toxic smoke. That is why the United States government had to close the town.”

Abandoned road that led to Centralia and is covered in graffiti.
The abandoned road leading to Centralia, where a smoldering coal mine fire has been burning since 1962. (Photo: GETTY IMAGES)

How do you fight zombie fires?

“It is really very difficult. In fact, the vast majority of the time they cannot be turned off, only the smallest ones,” Rein pointed out.

“But the ones we are most concerned about, in Siberia for example, start to go very deep and continue to consume fuel. The density of the peat is very high and they produce a lot of smoke. In fact, smoldering fires produce more smoke than smoldering fires.”

The most effective way to put out a zombie fire is to flood the groundsaid the Imperial College expert.

“Peat occurs on sites that are naturally flooded. So, basically, if the water has disappeared, you have to bring it back.”

“In the United States, smoldering combustion fires have been extinguished, flooding the land, but with a gigantic human effort. We are talking about hundreds of water pumps, or 300, 400 firefighters working for a month and moving a lot of water. Or to temporarily divert the course of a river.”

“Another answer is not to turn it off but to reduce its impact, for example, by compacting the soil with tractors. That prevents oxygen from reaching the smoldering combustion. It doesn’t turn it off, but it makes it weaker.”

Firefighters facing the flames in a forest fire
The audiovisual emphasis is always on images of airplanes and helicopters, “but really the action, the credit, is in the people on the ground,” Rein said. (Photo: GETTY IMAGES)

What is the most effective way to combat flame fires?

“The most effective way to put them out is with firefighters on the ground who create ditches in which they remove the fuel, the vegetation. They create a firewall in real time.”

Plane in California releasing water with retardants
The role of helicopters or planes is to drop water with retardant chemicals to slow down the fire and give firefighters more time. (Photo: GETTY IMAGES)

“They cut down trees, remove bushes, remove weeds. Imagine that the physical effort is beastly”, Rein pointed out.

The audiovisual emphasis is always on images of airplanes and helicopters, “but really the action, the merit, is in the people on the ground.”

The role of helicopters or planes is to drop water with retardant chemicals to slow down the fire.

“The water wets the vegetation, slowing down the flame, giving firefighters on the ground more time to build firebreaks.”

What determines how fast a fire spreads? And why when escaping you should never run up a slope?

There are three elements that define the speed of the flame, Rein explained.

“Number one is gas. There are pines, for example, which are less combustible”.

“Another factor is the meteorology, that is, wind and humidity. The fire spreads much faster in the direction of the wind than against it, and a wet fuel is not the same as a dry fuel”.

The third is the topography.

Forest fire
“Fire always goes much faster uphill than downhill. And so if we see a fire and there is a slope, we should not go up.” (Photo: GETTY IMAGES)

“Because of gravity, the flame always goes up. It’s because if a gas is hot it expands, and as the weight per unit volume expands, the density decreases.”

“And the air that is around, which is cold, weighs more, goes down and pushes the lighter air up.”

“So if there is a sloped terrain, the heat has to travel less space up, than down.”

The flame always heats the fuel above it more, so it spreads faster in that direction, Rein explained.

Fire always goes much faster uphill than downhill. And so if we see a fire and there is a slope, we should not go up. We have to go down.”

What are the fires so powerful that they create their own weather?

They are the fires that in English are called mega fires or megafires.

They can’t be turned off, it is absolutely impossible. Firefighters can only wait for it to fade somewhat and turn into a normal fire,” Rein said.

“These fires produce giant flames and drafts. The flames consume oxygen and create winds.”

“And the smoke that goes up into the atmosphere carries a lot of water vapour. Because the combustion process generates water vapour, it is one of the combustion products in engines”.

“These fires bring a lot of water into the atmosphere, which creates clouds, and these clouds are charged with particles and then they create electrical charge, lightning, rain.”

“So what you have is an absolutely gigantic fire that creates a mini weather system with its clouds, with its rains, with its lightning and its winds”.

Is the number of fires increasing globally?

Satellite studies have found that globally the number of fires is neither increasing nor decreasing, Rein said.

“But the number of fires is not the problem. The problem is the damage they cause.”

Forest fire
The number of fires globally is not increasing, but their size and danger are. (Photo: GETTY IMAGES)

“The damage can be measured by the size of the fire, how much area has been burned, or by the danger it has created for humans.”

“And when we measure that, the size or the danger that they have generated, what we see is that year after year the impact of forest fires is increasing“.

What is the most worrying currently in relation to fires?

“I am concerned that unfortunately it is still not understood at a global level what to do with forest fires.”

Rein pointed out that for millions of years there have been fires with beneficial effects on ecosystems, since they create clearings in the forests for new vegetation, or favor the cones of some tree species to open and release their seeds.

Not all fires are bad and if all fires are put out the problem can be even more seriousRein explained.

“This is what has happened, for example, in the United States, with what is known as the ‘fire exclusion‘”.

“The United States has been putting out forest fires for a hundred years. They have excluded fire from many forests and many ecosystems, so the forests have been accumulating fuel, dead matter. And this means that now when there is a fire it will be much bigger.

“It has to be shown that burning every twenty years with a low-intensity fire, in spring, when the vegetation has more moisture and is not very hot and the subsoil is moist, basically cleans the undergrowth.”

It also worries Rein theuncertainty about the future impact of climate changepenthouse.

“Global climate change is warming the planet.”

When the vegetation heats up, it loses water and that increases the flammability of the vegetation, the expert said.

“So what climate change is doing is that globally, not in all places, but in most places, is increasing the flammability of vegetationwhich means that it can catch fire more easily and that when it catches the fire it will be faster and with higher flames”.

Firefighters in Chile throwing water with a hose to fight a forest fire
“Preparing is not just having firefighters with big trucks full of water or having planes and helicopters… There are many things that can be done beforehand.” (Photo: GETTY IMAGES)

What can Latin American countries do to better prepare for forest fires?

“The idea is to prepare, but preparing is not just having firefighters with big trucks full of water or having planes and helicopters.”

“That is very important, but it is the last thing. Basically after that we have nothing.”

“There are many things that can be done beforehand. For example, create awareness at the community level to clean up the forest“.

“The communities that live inside the forest can also be prepared so that when the fire comes it doesn’t destroy everything.”

“That is done a lot in the United States. The communities have to take a look at your immediate surroundings and consider how flammable it is“.

“They can, for example, cut the continuity of fuels, the line of bushes to the house, or change the trees for trees that protect themselves more from the fire, or put roofs that are not made of wood but of ceramic tiles.”

“Obviously they are going to have a scare, they are going to have to evacuate. But that town is much more resistant to fire.”

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