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When you think of forest fires, you might think of a single fire season,
or time of year when it’s more likely for fires to start and spread.
But some fires can actually overwinter, meaning that they never go out.
Instead, the fire seems to go out but is hidden underground, smoldering in the soil
for the whole winter and flaring up again during the next fire season.
This usually happens in boreal forests, which are forests in cold areas like Alaska,
or Canada’s Northwestern Territories.
And because these fires might appear dead, only to rise from the grave
a few months later, they’ve earned the nickname zombie fires.
And these undead fires might start appearing more often.
Zombie fires can keep burning for up to eight months,
and while they’re dormant, they’re difficult to detect.
When they emerge again, people tend to record them as new fires,
rather than the same fire from last year that never went out.
But although it’s tough to record them,
we’ve known about zombie fires’ existence for a while.
We know that these fires go through four stages.
First, the fire stops burning and its spread decreases,
making people think the fire has gone out.
Second, the fire continues to smolder, unnoticed, underneath snow cover.
Third, the fire springs back up as soon as the fire season starts again.
And fourth, the fire spreads, burning down more of the forest.
Because zombie fires can pop up before the fire season officially starts, in places
with no clear sources of ignition, they can be difficult for fire managers to deal with.
And recently, fire managers in northern regions have started
reporting a significant increase in zombie fires.
Now, because these fires haven’t been recorded and studied very much,
it’s difficult to say what exactly is causing this increase.
But experts think that climate change might be the culprit.
Which is not that surprising,
given that climate change is already affecting regular fire seasons.
But to figure out why more zombie fires are forming,
we need to understand what causes any zombie fires to happen at all.
A recent study looked into how exactly zombie fires form
and identified some conditions that make overwintering more possible.
What they found is that zombie fires are more likely to happen during years when
a large area of the forest is burned.
Larger fires are harder to extinguish, and for a zombie fire to form,
the fire can’t completely go out.
The larger the burned area, the more chances a fire has to overwinter.
But zombie fires also happen more frequently in years with hotter summers.
Looking at the Northwest Territories between 2005 and 2017,
the researchers found that the six hottest summers in that record
all created zombie fires, which came back with a vengeance the year after.
But the seven coolest summers didn’t create any zombie fires.
Now, it isn’t just a matter of average temperature.
The number of exceptionally hot days in a summer
also affects the chances that a zombie fire might be created.
In fact, the more days over a summer that registered a temperature above the 90th percentile,
the greater the amount of forest that burned that season,
and the greater the chances that a zombie fire survived to burn again the next year.
The severity of the fire also makes a difference.
For a fire to overwinter, it needs to burn deep into the ground,
beneath the soil or underneath tree roots.
And burn depth is also correlated with temperature.
The more days over that 90th percentile mark, the deeper fires tend to burn.
So, overall, that means that three things are very important for zombie fires to form:
the temperature of the summer, the amount of burned area, and the burn depth.
And that brings us back to climate change.
Climate change is raising temperatures all over the world,
but nowhere on Earth are temperatures rising faster than in the Arctic.
The Arctic and boreal regions are warming twice as fast as the rest of the world.
In those northern regions, summer temperatures are already much warmer
than they used to be.
This is perfect for fires and could be why officials are seeing
more zombie fires than in previous years.
And with the Arctic continuing to warm,
there are fears that fire seasons in this region will last longer and longer,
and fires will become more severe, leading to even more zombie fires.
This could create a positive feedback loop where stronger fire seasons lead to
more zombie fires, which lead to stronger fire seasons, and so on.
And it isn’t just about fire seasons.
Unfortunately, this could also lead to more climate change.
See, trees are made of carbon and when they’re burned, they release carbon dioxide.
While zombie fires are currently rare, if their frequency increases, the amount of
carbon they burn also increases, putting more carbon dioxide in the atmosphere.
Current research estimates that zombie fires in Alaska and the Northwest
Territories emitted 3.5 teragrams of CO2 between 2002 and 2018.
And 64% of that was emitted during the especially bad fire seasons of 2010 and 2015.
And this is a conservative estimate because this only counts
the carbon from the trees themselves.
We still don’t have a good way to measure the amount of carbon released
during the winter, while the fire survives quietly beneath the earth.
But we do know that these slow-burning fires emit more methane than normal fires,
and methane is a much more potent greenhouse gas than CO2.
Thankfully, since zombie fires are rare,
the amount of emissions they cause is relatively small.
Right now, zombie fires cause less than a percent of all emissions
during fire season, but that number could go up as they happen more often.
One way to stop this is by using air and satellite-based systems
to detect zombie fires early, while they’re still small.
Putting them out before they have a chance to grow
can reduce the amount of damage they do.
And of course, any work we put into stopping climate change can also help us
stop or slow down these positive feedback effects before they get out of hand.
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