A few miles north of Fairbanks, Alaska, you can step inside the permafrost. The Army Corps of Engineers tunneled into a hillside here in the 1960s, yielding a passageway lined with ancient soils, ice wedges, and fossils. Scientists use the tunnel to study everything from blasting techniques to sublimation.

I sought out the tunnel because I wanted to get a better grasp of the carbon cycle in the Arctic. What I didn’t expect was the smell.

In this multi-part series, filmmaker Ian Cheney examines the scale of climate change, in all its many dimensions

In this multi-part series, filmmaker Ian Cheney examines the scale of climate change, in all its many dimensions. Click the play button in the video above to view the short documentary.

“Mammoth poop,” joked Matthew Sturm, a geophysicist who showed me around. I never did get a clear answer on the dairy barn odor, but Sturm explained that the smell tells you that something is coming out of the permafrost. And that’s the kind of sensory experience that is hard to come by with the carbon cycle.

The carbon cycle describes how carbon moves through Earth’s systems. As a result, it determines the fate of our fossil fuel emissions — and the future of our climate. But like the climate itself, it can feel a little abstract, or hard to pin down.

It’s partly a conceptual challenge. The quintessential currency of the carbon cycle – the carbon atom – is not exactly something you can hold in your palm. Although of course, you wouldn’t have a palm without carbon atoms. And that’s the other thing: it’s everywhere, in many different forms. Carbon is suffused through the air as invisible carbon dioxide; it’s the cellular scaffolding of plants and bugs and politicians; it’s the rock cementing itself together in the depths of the sea. To grasp the carbon cycle, you have to think microscopically on a very macro scale.

For scientists, there’s an added logistical challenge: How do you track the carbon moving through Earth’s lithosphere, cryosphere, biosphere, hydrosphere, and atmosphere? It’s massive, measured in incomprehensible units like petatons. And as MIT/WHOI doctoral candidate Sarah Rosengard pointed out to me, it’s four-dimensional. First you have to follow carbon across Earth’s great longitudes and latitudes, then up into the air and down into the Earth — and then of course it’s all changing across time. So any given measurement, in any given place, at any given time, is only a snapshot of an ever-shifting system.

Which brings me back to the permafrost tunnel. No one knows precisely how much carbon is locked up in the permafrost, but it’s clearly a massive amount, and scientists are seeking more measurements to determine its fate. The concern is that if the permafrost’s carbon escapes into the atmosphere in substantial amounts, it will accelerate global warming. And more warming leads to more melting permafrost, which leads to more carbon being released into the atmosphere, and thus more warming, more melting, more permafrost emissions — you get the idea. Scientists call this “positive feedback,” but for coastal dwellers worried about sea level rise, it might be more appropriate to call it a “vicious cycle.”

Measuring carbon flows in the Arctic is challenging, given the remote terrain, but Sturm also pointed out the conceptual challenge of permafrost feedback: How do you grasp the importance of tiny microbes, in thawing northern soils, emitting invisible carbon dioxide (CO2) and odorless methane (CH4) into the atmosphere?

Smelling the permafrost helped.