What Is Permafrost? The Frozen Ground That Is Starting to Thaw
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Under a fifth of the land in the northern hemisphere the ground is frozen solid, in places to a depth of a kilometre and for tens of thousands of years, and it holds, by the best estimates, about twice as much carbon as the entire atmosphere. Permafrost is not ice; it is soil, rock and gravel held together by ice, and buildings, roads and pipelines across Siberia, Alaska and Canada stand on it as if it were bedrock. It is warming faster than any other part of the climate system, and as it thaws it sinks, slumps, and gives back what it has kept.
What it is and where
The definition is thermal, not material: permafrost is any ground that has stayed at or below zero degrees for at least two consecutive years. Most of it has been frozen since the last ice age, and the deepest, in northern Siberia, goes down 1,500 metres and has been frozen for over half a million years. Above it is the active layer, a few tens of centimetres to a few metres of soil that thaws each summer and refreezes each winter, and it is in that layer that the tundra's plants root. Permafrost underlies nearly all of the Arctic land, about 85 percent of Alaska, half of Canada and Russia, the Tibetan plateau, and patches of high mountains as far south as the Alps and the Andes, and it also extends under the shallow seabed off Siberia, drowned when sea levels rose.
What is locked in it
Frozen ground is a freezer, and the contents have been accumulating since the Pleistocene. Every summer for tens of thousands of years, plants grew on the surface and their remains were buried before they could rot, because decomposition stops in ice, and the result is a store of about 1,500 billion tonnes of organic carbon, roughly twice what the atmosphere holds and more than all the world's forests. The ice also holds the animals that died on it, which is why frozen mammoths, woolly rhinos, cave lion cubs and a 46,000-year-old worm that revived when thawed in 2023 keep emerging from the Siberian riverbanks. It holds mercury, deposited from the air over millennia and now the largest reservoir of the metal on Earth, and it holds, in principle, whatever bacteria and viruses were frozen with everything else.
How it thaws
The Arctic has warmed about four times faster than the global average, and permafrost temperatures measured in boreholes have risen by around half a degree per decade since the 1980s. Thaw does not happen evenly:
- •The active layer deepens year by year, so that a little more of the frozen store thaws each summer and less refreezes
- •Where the ground held large ice wedges or lenses, thaw removes the ice and the ground collapses into pits, ponds and hummocks called thermokarst, which can drop a landscape by metres in a decade
- •Lakes that form in the hollows warm the ground beneath them and thaw it faster, and drain suddenly when they breach
- •Wildfires, more common as the tundra dries, strip the insulating moss and expose the frozen soil to summer heat
- •On slopes and coasts, thawed ground slumps and slides into rivers and the sea, and Arctic coastlines are retreating by metres a year
The carbon feedback
When frozen organic matter thaws, the microbes in it wake and eat, and they exhale carbon dioxide where there is oxygen and methane, a far stronger greenhouse gas, where the ground is waterlogged. That carbon warms the climate, which thaws more permafrost, which releases more carbon: a feedback loop that runs on its own once started and that the climate models of the 2000s did not include. Current estimates put the emissions from thawing permafrost this century at somewhere between the annual output of a large country and something considerably larger, spread over decades, with the uncertainty depending mostly on how fast the abrupt thaw of ice-rich ground proceeds. It is not a runaway bomb, as it is sometimes described; it is a large, slow, one-way leak that makes every other emissions target harder to meet.
Living on it
About 35 million people live on permafrost, in cities such as Yakutsk, Norilsk and Fairbanks, and their buildings were designed for ground that did not move. Thaw is now buckling roads, cracking foundations, tilting apartment blocks and bending pipelines; in 2020 a fuel tank in Norilsk collapsed on thawing ground and spilled 20,000 tonnes of diesel into the rivers. The engineering answer, known for decades, is to keep the ground frozen: buildings on piles with an air gap beneath, so that their heat does not reach the soil, and thermosiphons, passive pipes that pump winter cold into the ground, which the Trans-Alaska pipeline has used since 1977. Those measures were designed for a climate that stayed put. Whole villages on the Alaskan coast are being moved, and the cost across the Arctic of repairing what the thaw breaks is estimated in the tens of billions.
The takeaway
Permafrost is ground that has stayed frozen for at least two years, underlying a quarter of the northern hemisphere's land to depths of up to 1,500 metres and holding about twice the atmosphere's carbon in undecayed plant matter, along with ancient animals, mercury and microbes. As the Arctic warms it thaws from the top and collapses where its ice was, releasing carbon dioxide and methane in a slow feedback and undermining the towns, roads and pipelines built on it.