Permafrost Thawing

What Is Permafrost

Permafrost forms over thousands of years in regions where average annual temperatures remain below freezing. It ranges from a few meters thick at its southern margins to over 1,500 meters in parts of Siberia. Above the permafrost lies the "active layer" that thaws each summer and refreezes in winter, typically 0.5 to 3 meters deep depending on climate and soil properties. Vegetation, peat, and soil in the active layer insulate underlying permafrost, moderating the effect of surface temperature changes.

During previous cold periods, vast quantities of plant material accumulated faster than it could decompose in frozen, waterlogged soils. This organic carbon has been preserved in a frozen state for thousands to tens of thousands of years. Particularly carbon-rich deposits include yedoma (ice-rich Pleistocene-age sediments in Siberia containing 2 to 5 percent carbon by weight) and peatlands built up over the Holocene.

Observed Thawing

Permafrost temperatures have increased by 0.5 to 2 degrees Celsius across the Arctic since the 1970s, with the largest warming in the coldest permafrost regions. Active layer depth is increasing at many monitoring sites. In warm permafrost regions near the southern margin (temperatures just below 0), outright thawing and disappearance is occurring. Infrastructure built on permafrost is experiencing damage as ground ice melts, causing subsidence, cracking, and structural failure.

Abrupt thaw processes including thermokarst (land collapse from ground ice melt), coastal erosion, and retrogressive thaw slumps can expose deep permafrost carbon rapidly. These processes are difficult to represent in models but may account for a significant fraction of total carbon release. Thermokarst lakes that form in collapsed permafrost produce methane from anaerobic decomposition of exposed organic material in their sediments.

Carbon Release Pathways

As permafrost thaws, previously frozen organic matter becomes available to microbial decomposition. In aerobic conditions (drained soils), decomposition produces primarily CO2. In anaerobic conditions (waterlogged soils, lake bottoms), it produces methane, which has roughly 80 times the warming potential of CO2 over 20 years. The ratio of CO2 to methane release depends on hydrology, which itself changes as permafrost degrades and alters drainage patterns.

Current estimates suggest permafrost regions could release 30 to 150 billion tonnes of carbon by 2100 under moderate to high warming scenarios, equivalent to 3 to 15 years of current fossil fuel emissions. However, these estimates have large uncertainties related to the pace of abrupt thaw, the depth of thawing, microbial response rates, and changes in vegetation that could partially offset carbon losses through increased plant growth in warming Arctic regions.

Feedback Dynamics

Permafrost carbon release creates a positive feedback: warming thaws permafrost, releasing greenhouse gases that cause additional warming, which thaws more permafrost. Unlike fossil fuel emissions which could theoretically be stopped by human choice, permafrost emissions are determined by physical processes that will continue as long as Arctic warming persists. This makes them a "committed" emission source that cannot be directly controlled.

The feedback is expected to be gradual rather than abrupt at the global scale, adding 0.05 to 0.15 degrees of additional warming by 2100 beyond what emissions scenarios alone predict. However, this cumulative addition continues for centuries as deep permafrost slowly warms, meaning the total eventual contribution could be substantially larger than the 2100 increment suggests.