What Is Permafrost? The Frozen Ground That Is Starting to Thaw

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.

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, reported 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.

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

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.

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.

Permafrost leaves its mark on the landscape in patterns you can see from an aircraft. In winter the frozen ground contracts in the extreme cold and cracks, much as mud cracks when it dries. In spring meltwater trickles into the cracks and freezes there, and because the crack reopens in the same place the following winter, a vertical wedge of ice grows a little every year. Over centuries these ice wedges join up into a network, and the ground above them forms a honeycomb of polygons, often tens of metres across, that covers large areas of the Arctic tundra.

Pingos are more dramatic. A pingo is a hill with a core of ice, formed when water in the ground is squeezed upward and freezes, lifting the soil above it into a dome that can grow to tens of metres high over many centuries. The Mackenzie Delta region of Arctic Canada, around Tuktoyaktuk, has one of the largest concentrations in the world. Similar polygon patterns photographed on Mars have been read as a sign of ground ice there too, and permafrost scientists and planetary scientists have compared notes, using the Arctic to help interpret what orbiting spacecraft see.

Air temperature alone does not decide whether ground stays frozen, because the surface is wrapped in insulation that changes with the seasons. Snow is the most important layer. Fresh snow is mostly trapped air, and a deep blanket that settles early in autumn keeps the bitter winter cold from reaching the soil, so the ground beneath can stay several degrees warmer than the air above it. That is why permafrost can be patchy or absent under deep drifts, yet survive on exposed ridges where the wind scours the snow away and the cold reaches straight down.

Peat works in the opposite direction, and it explains why permafrost lingers at the southern edge of its range. In summer the top of a peat bog dries out, and dry peat is an excellent insulator, so little warmth gets through. In winter the same peat is wet and frozen, and ice conducts heat far better than dry plant fibre, so the cold passes down easily. The ground ends up colder on average than the air above it, so islands of permafrost survive under raised peat plateaus where the climate alone would not support them, while a mild winter with deep snow can warm the ground considerably.

In north-eastern Siberia, the Batagaika crater shows thawing permafrost on a huge scale. It began as a gully after forest was cleared in the area in the twentieth century, letting the sun warm the ground. As the ice-rich ground beneath thawed, the slopes collapsed, and the scar has kept widening ever since, so that it is now around a kilometre across.

Local people call it a gateway to the underworld, and it has drawn scientists because its exposed walls reveal frozen soil layers built up over hundreds of thousands of years, containing ancient pollen, wood and animal remains. It is one of the largest examples of a thaw slump, a landform expected to become more common as Arctic warming continues.

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.