'Zombie fires' smolder under the snow during the winter then rise from the dead come spring. They may get far more common.

• Most wildfires die out by winter. But "zombie fires" can smolder underground then reignite in spring.
• Research suggests more zombie fires survive after hotter summers, so they may become more common.
• Zombie fires happen in northern boreal forests, where the flames feed on peat deep in the soil.

Forest fires don't typically survive cold, wet winters. But "zombie fires" buck the mold.

In boreal forests just below the Arctic Circle, these rare blazes travel and persist underground, deep beneath the winter snow cover. They bide their time until the snow melts and spring begins, then reignite on the surface and begin to wreak havoc again, starting right where they left off.

Zombie fires can be devastating: In 2008, one such fire was responsible for 38% of the burned land in Alaska alone, scorching an area the size of San Francisco, according to a study published Wednesday in the journal Nature. That research predicts these fires will become more common as the Earth continues to warm.

"It is possible that we may see more zombie fires in the future," Rebecca Scholten, a climate researcher at Vrije Universiteit Amsterdam who co-authored the study, told Insider. "We do see an upward trend in summer temperatures in boreal regions, and this goes in line with increases in annual burned area."

Scholten's team found that zombie fires were, unsurprisingly, more frequent after hotter summers in which large fires burned across wide areas. The higher summer temperatures climb, the drier the subterranean vegetation and soil become - and that's what zombie fires consume as they hibernate. The bigger the fire, the deeper its flames can penetrate underground in the summer. That makes them more likely to survive the winter.

Burn. Sleep. Repeat.

Scholten's team looked at reports from local fire managers and firefighters, as well as satellite imagery of Alaska and Canada's Northwestern Territories captured between 2002 and 2018. They found 74 zombie fires in those 16 years.

"We can identify zombie fires from satellites because they appear close to an old fire scar," Scholten said.

In Canada, they found that fires pulled through the winter following the six hottest summers in the study's time frame. The analysis suggested that zombie flames can spread up to 650 feet (200 meters) underground. But no zombie fires survived the winter after the seven coolest summers.

The scientific term for zombie fires is "overwintering," since the blazes hibernate underground for up to eight months like bears, then awaken four weeks after the snow starts melting. But Scholten said the colloquial moniker works.

"I like the term - it's a really visual and engaging description," she said.

Overwintering fires require a specific habitat. They happen in the Arctic and sub-Arctic regions of North America and Siberia because the deepest soil layers there, called peat, are rich with organic matter. The smoldering flames can devour that matter, thereby staying alive even when the surrounding temperatures drop to minus 40 degrees Fahrenheit.

Overall, zombie fires are still rare: The new research suggests they accounted for just 0.8% of the total burned area in Alaska and the Northwestern Territories during the 16 years studied. But because climate change makes both hot summers and large, intense wildfires more likely, zombie blazes may become more common, too.

Atmospheric carbon-dioxide concentrations hit a record high last year, and the last seven years have been the seven warmest ever recorded, according to NASA. The Arctic, in particular, is warming faster than the rest of the Earth.

A vicious cycle

Perhaps the worst part of the zombie fire phenomenon is its self-perpetuating nature. When a fire burns through trees and vegetation, that emits carbon dioxide, exacerbating the climate problem.

A zombie fire is double trouble: It burns through flora in the summer before its hibernation and during the spring after. In between, the peat it burns underground emits methane, a greenhouse gas with 80 times the warming potential of carbon dioxide.

"What's special about fires in arctic and boreal regions is that the largest part of carbon emissions comes from the soils," Scholten said.

Her group found that large overwintering fires in Alaska and the Northwest Territories emitted 3.5 million metric tons of carbon between 2002 and 2018.

More emissions means more warming, which increases the likelihood of more zombie fires, which in turn create more emissions, and so on.

It's possible to hunt down zombie fires

Most fires are caused by people or lightning strikes. In Alaska and Canada, lightning season begins in June, which kicks off fire season.

But zombie fires don't follow that schedule. They start "as soon as the snow melts and dry fuel is available," Scholten said.

So the new study suggests that by keeping tracking of summer temperatures and recording where the largest fires were each summer, firefighters might be able to predict and suppress zombie fires before they fully reignite.

Doing so would be cheaper than fighting a full-blown fire, the study authors wrote, and would also limit the blaze's greenhouse-gas emissions.