Permafrost is ground that has remained at or below freezing for at least two consecutive years, and across large parts of Siberia, Alaska, and northern Canada, it has stayed frozen continuously for thousands of years, in some places tens of thousands. Roughly a quarter of land in the Northern Hemisphere sits on permafrost of some kind. Buried within that frozen ground is an enormous quantity of ancient organic material — the remains of plants and animals that died in past millennia and, because the ground never fully thawed, never fully decomposed. Scientific estimates suggest permafrost soils hold roughly twice as much carbon as is currently present in the entire atmosphere, frozen in place and effectively removed from the active carbon cycle for as long as it stays frozen.
What Happens When It Thaws
As Arctic regions warm — significantly faster than the global average, a phenomenon called Arctic amplification — permafrost that has remained solid for millennia is beginning to thaw. Once thawed, the organic material inside it becomes available to microorganisms again, which begin decomposing it and releasing the carbon it contains, primarily as carbon dioxide in drier, oxygen-rich conditions and as methane in wetter, oxygen-poor conditions such as thawing wetlands and lake beds. Methane is a substantially more potent greenhouse gas than carbon dioxide over shorter timescales, meaning the specific pathway by which permafrost carbon re-enters the atmosphere matters considerably for how quickly its warming effect is felt.
A Feedback Loop, Not Just an Emission Source
What makes permafrost thaw particularly significant in climate science is that it represents a feedback loop rather than a simple, fixed emission source. Human-caused warming triggers permafrost thaw; permafrost thaw releases additional greenhouse gases; those additional gases contribute to further warming, which accelerates further thaw. This is one of several climate feedback loops that were not fully accounted for in early climate models, and its inclusion in more recent projections has generally pushed expected warming trajectories higher, since it represents a source of emissions largely outside direct human control once triggered — unlike fossil fuel combustion, which can in principle be stopped through policy, thawing permafrost continues releasing carbon as long as the ground stays warm enough, regardless of any subsequent emissions reductions elsewhere.
Sudden Collapse, Not Just Gradual Melting
Some of the most consequential permafrost loss does not happen gradually. Abrupt thaw events, in which ice-rich ground collapses suddenly rather than thawing evenly from the surface downward, can transform stable land into slumping terrain, thermokarst lakes, or landslides within a single season, releasing stored carbon far more rapidly than steady, top-down thawing would suggest. These abrupt processes remain harder to model and predict precisely than gradual thaw, and researchers now believe they may account for a disproportionate share of total permafrost carbon release, despite affecting a relatively small fraction of total permafrost area.
Watching Frozen History Wake Up
Long-term monitoring networks, including research coordinated by organisations such as the US Geological Survey, track permafrost temperature at boreholes across the Arctic to measure how quickly the thaw is progressing at different depths and locations. Permafrost carbon has been safely out of circulation for longer than human civilisation has existed. Whether it stays that way is now, unusually for a geological process, something being decided on a human timescale rather than a geological one.
Thawing permafrost is not only a climate concern. Infrastructure built decades ago on the assumption of permanently frozen, stable ground — roads, pipelines, airport runways, entire buildings across parts of Siberia and Alaska — is beginning to buckle and subside as the ground beneath it softens, requiring costly reinforcement or relocation. Some thawing ground has also released long-dormant pathogens and revealed remarkably preserved Ice Age animal remains, a reminder that permafrost has been acting as a deep freezer for more than just carbon, and that what gets released as it thaws is not always predictable in advance.