Wildfires and Permafrost Thaw in the Arctic-Boreal Region

While a significant threat to the global emission budget, the impact of wildfires on accelerated permafrost thaw is largely unaccounted for in research and, consequently, mitigation planning. Typically, permafrost thaw is predicted to increase linearly with warming temperatures. However, this does not include the significant emissions increase caused by wildfires in the Arctic-Boreal region, underlain by permafrost. Wildfire risk and its impact on permafrost must be further studied to inform mitigation and adaptation planning. 

Permafrost is ground that has been frozen continuously for at least two years and as many as hundreds of thousands. Its thickness can range from a few feet to over a mile. Above the permafrost is the organic soil/active layer composed of accumulated live and dead plant material. This can vary in thickness and acts as an insulation layer to frozen permafrost beneath it. Permafrost is estimated to contain around 1,400 gigatons of carbon which amounts to almost half of terrestrial carbon. This stored carbon equals almost 2x as much carbon as is currently in the atmosphere.

The Arctic-Boreal region collectively is heating up 1.5x faster than temperate zones and has warmed more than 2 degrees Celsius above pre-industrial levels. Click on the graphic below to watch the progressive increase in warmer temperature anomalies concentrated in the Northern hemisphere.

This increase in average air temperature has led to the thaw of permafrost. When permafrost thaws, the microbial decomposition of organic material releases carbon and methane stored in the soil for however long the ground was frozen. In the Arctic-Boreal region, permafrost can be tens of thousands of years old. The release of this carbon into the atmosphere at an unnatural rate increases the greenhouse effect driving climate change.

In addition to thaw from gradual warming, the increase in frequency and severity of wildfires due to climate change also accelerates permafrost thaw.

The surface organic horizon layer provides insulation to deeper soil from air temperature fluctuations. Fire-induced thaw occurs when wildfires burn the organic soil layer above the permafrost. This removes the insulating soil layer and exposes permafrost to warmer air temperatures, resulting in thaw. Furthermore, even when the soil remains, fires increase the temperature of the soil layer and lower the permafrost layer due to thawing.

The potential for large amounts of emissions due to wildfire-induced permafrost thaw calls for a better understanding of increased wildfire risk in the boreal forest region and arctic due to climate change.

Boreal forests are the most fire-prone zone in the northern hemisphere. Typically, fires in boreal forests burn below-ground organic soils, dead organic matter on the surface, and the vegetation above. During each fire, a portion of organic soil carbon does not burn, known as ‘legacy carbon,’ resulting in net accumulation of carbon over time (Walker et al. 2019). As a result, boreal forests tend to have a high concentration of organic matter accumulated in the soil.

However, the increased frequency and severity of wildfires in this region are estimated to cause a shift from net accumulation to a net loss of carbon (Walker et al. 2019). Specifically, deeper burns will continue to increase the combustion of legacy carbon.

Fire weather and increase in sparks

The effects of climate change have caused wildfire frequency and severity to increase due to more droughts, longer fire seasons, earlier snowmelt, and greater lightning frequency. Seasons in the Northern latitudes typically consist of long cold winters and warm dry summers. Climate change has shortened winters, which means melting snow two weeks earlier and a longer dry summer. Drier conditions in the vegetation and soil also allow for deeper burns into the ground, which increases soil temperature above permafrost. 

Lightning frequency has also increased due to climate change, with the most significant changes at higher latitudes. Specifically, in Alaska, 90% of the acreage burned in boreal forests is caused by lightning (Jandt and York 2021).

Fuel availability and wildfire severity

A study by Walker et al. (2020) indicates that fuel availability, not just fire weather, may control boreal wildfire severity and carbon emissions. This approach differs from the typical methods of evaluating wildfire risk based on top-down controls of fire weather. Instead, bottom-up controls of fuel characteristics and landscape qualities impact the potential for carbon emissions from wildfires. Gaining a more comprehensive understanding of available fuel and the frequency of fire weather conditions will be crucial in accounting for future emissions from wildfires and permafrost thaw.

The continued smolder of wildfires within the soil is also a significant threat to further carbon emissions and permafrost thaw. This is referred to as zombie or holdover fires, in which the smoldering of deep organic soils continues for weeks after a fire starts. In the case of the Swam Lake fire in Alaska in 2019, underground smoldering continued through the eight months of winter and then reignited in the spring. Understanding how wildfire severity is dependent on a variety of factors is vital to raising awareness about increased permafrost thaw.

Wildfire-induced permafrost thaw contributes to a positive feedback loop due to the release of carbon and methane into the atmosphere and increases the greenhouse effect. Furthermore, independent of wildfire-induced thaw, the increased warming also leads to nonlinear feedback in which warming thaws permafrost and reduces ice cover, which further increases the greenhouse effect (Ymashev et al. 2019).

Unlike tropical savanna forest fires, which are responsible for the higher proportion of global fire emissions and re-sequester carbon within months or years, boreal forests take a century to re-sequester. Consequently, the predicted shift from net accumulation to a net loss of carbon within the Arctic-Boreal region will be irreversible at centennial time scales (IPCC 2021). This means that action must be taken to better understand the risk of wildfire-induced permafrost thaw in order to update climate models and mitigation planning.

Currently, permafrost thaw is not included in most climate models. When emissions from permafrost are included, they are only evaluated as gradual thaw and not fire-induced. Estimates made to more accurately predict emissions from wildfires and permafrost thaw can be then used to inform policymaking.

Specifically, the emission estimations from permafrost thaw listed in the sixth assessment IPCC report do not account for wildfire-induced permafrost thaw. Instead, a wide range of estimates is provided based on levels of increased temperature dependent on intervention.

1. Strong climate intervention: 6 to 118 gigaton (Gt)C (22 to 432 Gt CO₂)
2. Weak intervention: 150 Gt C (550 Gt CO₂)

In comparison, the IPCC estimates the world's remaining carbon budget (the amount of CO2 that can still be emitted) for meeting the target of limiting warming below 1.5 degrees Celsius to be  290 to 440 Gt CO₂-e.

Considering current policies in place, limiting warming to 1.5 degrees is most likely unattainable. If we are on an overshoot pathway, the increased emissions from permafrost will have a compounding effect on overshoot magnitude and must be accounted for. It is essential to quantify the additional emissions from permafrost in models to emphasize the need for ambitious mitigation efforts from the power and transportation sectors.

Overall, permafrost thaw in the Arctic is less studied due to the ambiguity of which countries are responsible for the region. Furthermore, governments may also be less willing to quantify emissions from permafrost in their NDCs due to the impact it will have on their ability to meet emissions targets. However, if the goal is avoiding catastrophic damages from climate change, this must be done.

Permafrost carbon emissions must be accounted for in international decision-making. Countries like the United States, Russia, and Canada with large amounts of permafrost must increase the ambition of their NDCs to account for the positive feedbacks caused by rapid permafrost thaw.

Every additional ton of emissions corresponds with increased warming (IPCC 2021). So if permafrost thaw emissions are not accurately predicted - then the mitigation plans in place in Canada and the US will not be sufficient to prevent catastrophic warming. Significant increases in emissions from wildfires and permafrost thaw will restrict the available carbon emission budget and threaten the overshoot of goals.

Adaptation planning depends on the ability of governments to respond to wildfires and permafrost thaw and their impacts on communities in the Arctic-Boreal region. Wildfires on their own pose a threat due to unhealthy air quality, infrastructure damage, loss of vegetation, and increased emissions. The increased severity of permafrost thaw also has significant consequences for surrounding communities and ecosystems. 

As a consequence of damage from flooding and erosion, over 30 communities were at risk of displacement or relocation in Alaska as of 2017 (EPA). These threats disproportionately impact the health and practices of Indigenous communities in the Arctic. For instance, thaw in-ground ice-cellars used by Alaskan indigenous people can no longer be used. Furthermore, the health of surrounding communities could also be affected by the emergence of harmful bacteria or diseases from thawing animal carcasses in the permafrost. In order to prepare for these hazards, governments must focus on understanding the threat of wildfires and accelerated permafrost thaw.

In order to spur action from countries to update NDCs, the IPCC must include wildfire-induced thaw in their next report. This must be paired with more extensive data on thawing in the region and models for predicting wildfire frequency and severity. 

Overall, increased awareness of the reality of permafrost thaw and how it will impact emissions targets is essential for mitigation and adaptation planning.