Deep below the surface of some of the planet’s northernmost wilderness, the soil and rock has been frozen for millions of years.

It’s called permafrost, and in central Alaska just south of the Arctic circle, this ice-cold layer reaches hundreds of feet below the tundra.

Global climate change is causing this layer, and equivalent ecosystems near the poles, to melt for the first time in ages, releasing carbon dioxide into the atmosphere at alarming rates.

It would be safe to assume research would be focused on keeping the permafrost frozen, but for one group of researchers working out of the Permafrost Research Tunnel, their goal is the opposite.

Carbon emissions from permafrost are driven by ancient microbes that are awakened and then begin to break down organic material, releasing carbon dioxide in the process, according to a study published Sept. 23 in the peer-reviewed journal JGR Biogeosciences.

What scientists don’t know is how long it takes for permafrost microbes to reactivate once the warming begins.

“It’s one of the biggest unknowns in climate responses,” study author Sebastian Kopf said in an Oct. 2 news release from the University of Colorado Boulder. “How will the thawing of all this frozen ground, where we know there’s tons of carbon stored, affect the ecology of these regions and the rate of climate change?”

To answer this question, Kopf and the rest of an international research team traveled to Fox, Alaska, where the U.S. Army Corps of Engineers’ Permafrost Research Tunnel cuts 350 feet into the ground, according to the release.

It’s the size of a mine shaft, and when study author Tristan Caro walked into the tunnel for the first time, he saw fossilized bones of a bison and mammoth in the stone walls.

“The first thing you notice when you walk in there is that it smells really bad. It smells like a musty basement that’s been left to sit for way too long,” Caro said in the release. “To a microbiologist, that’s very exciting because interesting smells are often microbial.”

Decked out in Tyvek suits and N95 respirators, the researchers collected meter-long cores from the permafrost layer, according to the study.

The cores were then transferred to a lab where they were saturated with water and warmed to between 39 and 54 degrees Fahrenheit, researchers said.

Caro noted that while there is no indication that the microbes could infect humans, they are kept in sealed chambers to be safe.

Then, researchers tracked how fast the microbes grew.

“Exceptionally slow rates of microbial growth are observed after 7 and 30 days incubation post-thaw,” researchers said, but that changed when the microbe “incubations of 180 days experienced dramatic changes,” including high enrichment from hydrogen, something the microbes received from the water.

At the beginning, the cells would only replace about one in every 100,000 cells each day, a significantly slower pace than average bacterial colonies that can completely replicate in lab settings in the same period, according to the release.

But the longer the microbes had to warm, the faster their replacement rates became, meaning as warm periods last longer, the microbes are more likely to be “resurrected” and then active.

This means a single hot spell in the Arctic is likely not enough to warm carbon dioxide-releasing microbes, but a warm season is a different story.

“You might have a single hot day in the Alaskan summer, but what matters much more is the lengthening of the summer season to where these warm temperatures extend into the autumn and spring,” Caro said in the release. “There’s so much permafrost in the world — in Alaska, Siberia and in other northern cold regions. We’ve only sampled one tiny slice of that.”

Fox is in central Alaska, about a 10-mile drive north from Fairbanks.

The research team includes Caro, Kopf, J.M. McFarlin, A.E. Maloney, S.D. Jech, A.J. Barker, T.A. Douglas and R.A. Barbato.

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