At Chandalar Shelf in Alaska, a site about 200 kilometres (124 miles) north of the Arctic Circle, I kneeled on the damp, muddy ground, and plunged in a small serrated shovel. It easily penetrated the loamy top layer before I heard it thunk and scrape against the cold, frozen ground beneath—permafrost.
This permanently frozen ground, together with the layers of soil above it, stores vast amounts of carbon from organic material that's accumulated over millennia. Organic material decomposes slowly in the Arctic's cold, wet conditions. If it thaws, microbes in the soil break it down, emitting methane and carbon dioxide, and quickly releasing carbon that was stashed there over thousands of years.
"The permafrost is definitely getting warmer," said Alexander Kholodov from the University of Alaska Fairbanks, as he looked at the temperature readout from a probe buried in the ground. He was one of the lead researchers who I accompanied on this expedition.
While Kholodov recorded properties of the soil, I took soil samples and measured the depth of the permafrost. Nearby, Mike Loranty from Colgate University sat hunched over a small plot of earth. He was clipping off shrubs and grasses, sorting them into paper bags to identify what kinds of vegetation were growing and to estimate their relative abundance. We were compiling an inventory of all the components in the ecosystem to understand how the plants and the soil affect the thermal properties of the underlying permafrost.
Thawing permafrost is already wreaking havoc with Alaskan roads and buildings, but the potential for massive amounts of carbon to be released and contribute to a warming climate is a global concern.
The Alaskan landscape isn't uniform, so the way it's thawing isn't either. Since vegetation influences the temperature and rate of thaw, Loranty, Kholodov, and the rest of their team are trying to sort out how the variety of trees, shrubs, grasses, lichens, and mosses impact permafrost temperature in ecosystems across Alaska. It's part of a five-year project that spans from Alaska to Siberia and the data will help to predict how climate change will differentially influence the Arctic.
I always wondered how scientists study permafrost in an area as large and diverse as the north, so I joined this team of researchers for three weeks. We travelled over 800 kilometres (roughly 500 miles) from Fairbanks, up the Dalton Highway through mountain passes, to the open tundra at the end of the road at Prudhoe Bay on the Arctic Ocean.
Along the way, at 24 sites that represented 24 distinct ecosystems, the team took inventories of the variety and density of vegetation growing there, the characteristics of the soil, and measured the depth to the permafrost.
The ground above the permafrost that freezes and thaws on an annual cycle is called the active layer. The uppermost segment is organic soil, because it contains all the roots and decomposing vegetation from the surface. Beneath the organic layer is the moist, clay-like mineral soil, which sits directly on top of the permafrost. The types of vegetation will influence the contents of the soil—but in return, the soil determines what can grow there.
Kholodov inserted probes into the layers of soil and the permafrost to measure its temperature, moisture content, and thermal conductivity. The air-filled organic layer is a much better insulator than the waterlogged mineral soil. So an ecosystem with a thicker organic layer, where there's more vegetation, should provide better protection for the permafrost below.
On a warm morning in the boreal forests around Fairbanks, Loranty squeezed between two black spruce trees and motioned to all the woody debris scattered on the ground. "Here, where we have more trees and denser forests, we have shallower permafrost thaw depths."
He grabbed a T-shaped depth probe and shoved it into the ground. It only sank about a handspan before it struck permafrost. "When you have trees, they provide shade," he said, "and that prevents the ground from getting too warm in the summer." So here, the permafrost is shallow, right beneath the surface.
Other vegetation, like moss, can also protect permafrost. "It's fluffy, with lots of airspace, like a down coat," Loranty explained, "and heat can't move through it well, so it's a good insulator."
But 800km north on the tundra, close to the Arctic Ocean, there are no trees. It's a less productive ecosystem than the forest and provides little insulation to the frozen ground. Here, low-lying shrubs, grasses, and lichens dominate underfoot. When I grabbed the depth probe and pushed it in, it sunk down a meter before it bottomed out because the permafrost was much deeper.
"By going to all these different sites, we're trying to capture the range of variability," Loranty said.
Loranty watched me struggle to pull the depth probe out once it was sunk in right up to the handle. "It's like pulling the sword from the stone," he joked. I only had to take 60 more measurements at that site.
Even though this site represents just the tiniest fraction of land in the Arctic, they hope that the combined data will help to develop a model that will more accurately predict which parts of the Arctic will thaw faster than others. If the rate of warming continues, they'll soon need a longer depth probe.
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