Here's how wildfires can burn underground for months or even years

For months, remnants of last summer's destructive Alkali Lake wildfire have been smouldering underground in northwestern B.C.

Now that spring has come, they're coming out of hibernation, flaming up to the surface. A handful of small wildfires have popped up in recent weeks, growing to cover up to 5,000 square metres.

These subterranean blazes, known as ground fires, are tricky to manage, but they're not uncommon and they're especially prevalent in areas with lots of organic material in the soil, according to fire ecologist Robert Gray.

"You need a combination of very deep ground fuel — that means duff and litter and roots and buried logs — and then you need drought conditions," Gray told CBC.

"If you have that combination, then you have fires that can get into that deep, deep, dried organic material, and with just a little bit of oxygen they can hang on for years."

When this happens, it's because there are just enough tiny spaces in the soil and between pieces of wood material to hold oxygen and keep the combustion going. These fires can smoulder metres below the surface, like the recent peat fires in the U.K.

It can happen even if there's a layer of snow on the ground above — the snow acts as an insulator for the fire, Gray said — but it'll melt if there's enough heat being generated below.

This fire behaviour is why the devastating Fort McMurray wildfire that started in the spring of 2016 wasn't fully extinguished until the summer of 2017.

Firefighters watching for flare-ups

The B.C. Wildfire Service has now stationed a small crew of firefighters in Dease Lake to monitor for new flare-ups from last summer's fire near Telegraph Creek, and keep them under control.

The wildfire destroyed 27 buildings in the tiny community of Telegraph Creek last year, but thankfully the flare-ups this year have been at least four kilometres from human habitation, according to fire information Carolyn Bartos.

Bartos said the wildfire service monitors previous fire sites through the winter by using infrared scanners and overhead flights to search for smoke or flames — and that activity will continue until the fire is fully suppressed.

"I've heard some reports that there has been smoke detected throughout the winter," she said of the Alkali Lake fire.

When wildfires burrow underground, there's not much crews can do to manage them apart from watching and waiting for them to burn themselves out, according to Gray.

"There's a safety element to these things, because they are burning underground. The surface can cave in if it's deep," he said.

"Even if it's only a metre deep underground, if you step anywhere close to it, you can get third-degree burns if you wind up in what we call an ash pit."

The good news is that underground fires smoulder over relatively small areas compared to wildfires at the surface.

But as the climate continues to change, there's a growing likelihood that vestiges of one summer's wildfires will wait out the winter underground and appear again the next year.

"Longer fire seasons, drier conditions, less precipitation — those are going to exacerbate drought conditions, so there's definitely a climate change signal for that," Gray said.