Permafrost is ground that has stayed frozen for at least two consecutive years, and across much of the far north it is far older than that. Its thaw changes habitat physically before it changes anything biological.
What permafrost actually is
Permafrost is defined by temperature rather than by ice content, though most of it contains substantial ice bound into soil, sediment and buried organic material.
Above it sits an active layer that thaws each summer and refreezes each winter. Plant roots, burrows and drainage all operate within that shallow band.
As the active layer deepens, the working space for roots and animals changes, and the frozen barrier that once held water near the surface retreats downward.
Why the ground collapses
Where permafrost is ice-rich, thawing removes volume. The ice becomes water, the water drains, and the surface subsides into pits, troughs and slumps.
This uneven collapse produces a distinctive hummocked terrain. Roads buckle, trees tilt into what northern residents call drunken forest, and hillslopes fail in long scars.
The changes are abrupt rather than gradual. A slope can remain stable for decades and then lose material over a single warm summer once a threshold is crossed.
What thaw does to water
Frozen ground is effectively waterproof, which is why the Arctic holds so many shallow lakes and wetlands despite low rainfall.
When permafrost thaws through, that seal breaks and lakes drain into the subsurface, sometimes emptying within days. Waterbird nesting habitat disappears with them.
Elsewhere subsidence creates new ponds, so the regional pattern is churn rather than simple loss. Habitat is being reshuffled at a pace that favours mobile species.
How vegetation shifts follow
Deeper thaw releases nutrients previously locked in frozen soil, and shrubs respond faster than the lichens and low herbs of open tundra.
Taller shrubs then trap snow, which insulates the ground, keeps winter soil warmer and encourages further thaw. The process reinforces itself.
For caribou and reindeer this is a direct problem, because the lichen they depend on in winter grows slowly and is easily overtopped.
Why the carbon feedback matters here
Permafrost holds an enormous quantity of ancient organic matter that never decomposed because it was frozen rather than because it was resistant.
Thawing hands that material to microbes, which release carbon dioxide in drained soils and methane in waterlogged ones. The gas balance therefore depends on how the ground drains.
The habitat consequence is that northern warming is partly self-driven, so the changes described above continue even if emissions elsewhere are reduced.