What it is
Ground at or below freezing for at least two consecutive years. That is the definition. Not necessarily ice: dry rock can be permafrost. Most of what matters, though, is sediment bound with ice, sometimes more ice than soil.
On top, the active layer. Thaws in summer, refreezes in winter. Roots, water, life live here. Below it, the permafrost table, then frozen ground reaching down tens or hundreds of metres in the coldest regions, thinning towards the southern margin. In the oldest deposits, the ground has been frozen since the last ice age and holds the remains of the grassland that grew on it: roots, bones, peat, all suspended.
Two things change as summers lengthen. The active layer deepens, reaching material that has not thawed in a very long time. And the permafrost table itself sinks, year on year, at the margins first.
Zones
Mapped by how much of the landscape is underlain. Continuous in the far north: almost everything frozen, only lakes and rivers breaking the pattern. Discontinuous further south: frozen on north-facing slopes and under peat, thawed on sunny ground and under water. Sporadic and isolated at the edge: patches in the coldest, wettest pockets. The boundary between these zones is moving north.
| Zone | Coverage | Typical ground | Sensitivity |
|---|---|---|---|
| Continuous | Nearly all of the land | Cold, deep, often ice-rich | Thaws slowly from the top; abrupt where ice-rich |
| Discontinuous | Roughly half | Warm, shallow, patchy | Already near thawing; small warming tips it |
| Sporadic | A minority of the land | Under peat and shade | Disappearing at the margins |
| Isolated | Scattered pockets | Relic patches | Close to gone |
Carbon
Frozen soil is a freezer. Plant material that fell in over thousands of years never fully decomposed. Thaw it and the microbes wake. Two products. Where the ground is dry and oxygen reaches in: carbon dioxide. Where it is waterlogged, under thaw lakes and in wet lowlands: methane, weaker per year but far stronger per molecule while it lasts.
Two speeds. Gradual thaw: the active layer deepens a little each year, a broad, slow release across the whole region. Abrupt thaw: ice-rich ground collapses, lakes form, slopes fail, and deep, ancient carbon meets the air in a single season. Abrupt thaw affects a small fraction of the land but punches above its area.
Yedoma. The Siberian and Alaskan ice-age silt, very ice-rich, very carbon-rich. When yedoma thaws it does not sink quietly; it slumps. The Batagaika crater in Yakutia is the largest known example of the result, a widening scar in the taiga growing back into the hillside season after season.
Plants push the other way. Shrubs spread north, tundra greens, more carbon is drawn down. Not enough to balance the books, and the shrubs trap snow that insulates the ground and warms it further.
Thermokarst
The land surface reorganising itself as ground ice goes. Pits and hollows where ice wedges melt. Polygon patterns sharpening into troughs. Lakes appearing where none were, then draining suddenly when a channel cuts through. Slopes creeping, then failing in retrogressive thaw slumps that eat back into hillsides. Rivers running brown with thawed silt.
Fire accelerates all of it. A burned surface loses its insulating moss and peat, the ground beneath warms, and thaw follows within a few summers. Boreal and tundra fire seasons are lengthening.
What stands on it
Everything built in the north assumes the ground holds. Buildings stand on piles driven into permafrost so the heated interior does not thaw the foundation. Roads sit on gravel pads. The Trans-Alaska Pipeline runs above ground for long stretches on vertical supports fitted with heat pipes that pull warmth out of the soil. Airstrips, rail lines, fuel tanks, cemeteries, all resting on frozen ground that is no longer reliably frozen.
The failure mode is differential. One pile settles, the next does not. Walls crack, floors tilt, pipes shear. Ice cellars that stored meat and fish for generations fill with water. Repair means refreezing the ground, jacking the structure, or moving. Each is expensive. Moving is the theme of the communities note.
Coasts
Much of the Siberian and Alaskan shoreline is frozen mud held together by ice. While sea ice presses against it, it is protected from waves. As the open-water season lengthens, storms reach cliffs that were never exposed. Warm water thaws the base, the cliff undercuts and topples. Whole blocks of tundra slide into the sea. The Laptev and East Siberian coasts and the Beaufort coast of Alaska are the textbook cases: among the fastest-retreating shorelines on Earth.
Below the waterline, subsea permafrost: drowned tundra flooded when sea level rose after the ice age, still thawing slowly under the shelf seas.
Watching
Borehole temperatures, active layer probes, satellite surface deformation, aerial repeat photography of coastlines. Slow science, long records.
Borehole and active layer records: Global Terrestrial Network for Permafrost. Synthesis and outreach: Permafrost Carbon Network.