Permafrost, ground that remains frozen for at least two consecutive years, covers approximately 24% of the Northern Hemisphere’s land surface, primarily across Siberia, Alaska, and Canada. This frozen ground contains vast quantities of ancient organic matter. As global temperatures rise, this frozen reservoir is beginning to thaw, initiating the permafrost feedback loop. This loop describes a self-reinforcing cycle where initial warming causes effects that accelerate the original warming, creating a cycle that is difficult to stop once fully underway.
Understanding the Carbon Reservoir in Frozen Ground
The frozen soils of the Arctic and boreal regions hold an enormous reserve of organic carbon. Estimates suggest that the northern permafrost region stores between 1,460 and 1,700 gigatons of organic carbon. This amount is roughly twice the carbon currently present in the Earth’s atmosphere.
This carbon is locked into the permafrost layer, which is defined by its temperature staying below freezing year-round. Above this permanently frozen layer is the active layer, a thin surface zone that thaws every summer and refreezes in the winter. The active layer typically supports plant life and is part of the regular seasonal carbon cycle.
The stability of the deep permafrost depends on the insulation provided by the active layer and surface vegetation. As air temperatures warm, the active layer thickens, allowing the seasonal thaw to penetrate deeper into the previously frozen ground. This exposes ancient organic material to decomposition for the first time in millennia, releasing the stored carbon.
A significant portion of this material, approximately 65 to 70% of the total, is contained within the top three meters of the permafrost layer, making it particularly vulnerable to near-term thaw. The stability of the deeper ground is also threatened by abrupt thaw events, which can rapidly expose material previously thought to be safe from warming.
The Core Mechanism of the Permafrost Feedback Loop
The mechanism of the permafrost feedback loop begins with the initial warming of the global climate, driven by human-caused greenhouse gas emissions. This warming trend is amplified in the Arctic, which is experiencing temperature increases at a rate significantly faster than the global average. As the air temperature rises, it causes the frozen ground to warm and the active layer to deepen.
Once the permafrost thaws, the previously inaccessible organic matter becomes available to dormant soil microbes. These microorganisms begin the process of decomposition.
The primary output of this microbial decomposition is the release of carbon as carbon dioxide and methane. These gases enter the atmosphere and trap heat, further increasing the global average temperature. This atmospheric warming then feeds back into the Arctic environment, leading to even more extensive permafrost thaw.
This cycle is self-reinforcing because the effect of the thaw—the release of greenhouse gases—acts to intensify the original cause—global warming. The increased warming triggers more thawing, making more carbon available for decomposition, which releases more gases, and so on. The process is a classic positive feedback loop, where each step magnifies the next.
Even if only a small fraction of the stored carbon is released, it represents a substantial addition to the atmospheric greenhouse gas load. Once initiated, the emissions can continue for centuries, even if human-caused emissions are reduced. This is because microbial activity does not stop until the available organic material is consumed or the ground refreezes.
The Role of Methane and Carbon Dioxide
The decomposition of the newly thawed organic matter produces two primary greenhouse gases, and the specific gas released depends heavily on the local environmental conditions. In well-drained, drier soils where oxygen is present, the microbes perform aerobic decomposition, which results in the release of carbon dioxide (CO2). This gradual release is the most common form of carbon emission from thawing permafrost across large areas.
Conversely, if the permafrost thaws in a water-saturated environment, such as a wetland or under a newly formed lake, oxygen is absent. In these anaerobic conditions, a different set of microbes produces methane (CH4). Methane is a potent greenhouse gas, and its release from thawing permafrost is a particular concern due to its strong short-term warming effect.
Over a 20-year period, methane is estimated to be about 80 times more effective at trapping heat than CO2, though its lifespan in the atmosphere is much shorter—around a decade. The high potency of methane means that even a smaller volume release can have a disproportionately strong and immediate impact on global temperature.
The release of these gases can occur either gradually over large areas of slow thaw or rapidly through abrupt thaw events, which involve the sudden collapse of ice-rich permafrost, leading to the formation of sinkholes and rapidly expanding lakes. This type of thaw can accelerate carbon release by up to 50% in some areas, quickly exposing deeper, carbon-rich layers to decomposition.
Physical Changes to the Arctic Landscape
The melting of ice within the permafrost is causing widespread physical instability across the Arctic landscape. The ground ice acts as a cement binding the soil and sediment together; when it melts, the ground loses its structural integrity and volume, leading to subsidence.
This process often results in the formation of thermokarst features. These include slumping hillsides, pitted ground, and the formation of numerous new lakes and ponds. Thermokarst development threatens ecosystems by altering drainage patterns and changing the hydrology of entire regions.
These physical changes pose a significant threat to human infrastructure. Foundations of buildings, roads, pipelines, and airstrips rely on the support of the permafrost. As the ground subsides and shifts, this infrastructure buckles, cracks, and collapses.
Observed damage is already substantial, with damage reported in up to 80% of buildings in some northern Russian cities and significant portions of transportation routes in permafrost zones. Projections indicate that between 30% and 70% of circumpolar infrastructure could be at high risk of damage by the middle of the century, leading to substantial economic costs for repair and replacement.