Rapid Arctic warming has intensified northern wildfires and is thawing carbon-rich permafrost. Carbon emissions from permafrost thaw and Arctic wildfires, which are not fully accounted for in global emissions budgets, will greatly reduce the amount of greenhouse gases that humans can emit to remain below 1.5 °C or 2 °C. The Paris Agreement provides ongoing opportunities to increase ambition to reduce society’s greenhouse gas emissions, which will also reduce emissions from thawing permafrost. In December 2020, more than 70 countries announced more ambitious nationally determined contributions as part of their Paris Agreement commitments; however, the carbon budgets that informed these commitments were incomplete, as they do not fully account for Arctic feedbacks. There is an urgent need to incorporate the latest science on carbon emissions from permafrost thaw and northern wildfires into international consideration of how much more aggressively societal emissions must be reduced to address the global climate crisis.
The summer of 2020 saw a record-breaking Siberian heat wave during which temperatures reached 38 °C, the highest ever recorded temperature within the Arctic Circle. During the same year, unprecedented Arctic wildfires released 35% more CO2 than in 2019 (the previous record high for Arctic wildfire emissions since 2003), and Arctic sea ice minimum was the second lowest on record. These are clear reminders of the extreme and accelerating effects of climate change in northern regions. The Arctic has already warmed to more than 2 °C above the preindustrial level, and this rapid warming is expected to double by midcentury (1). Climate-driven changes are having transformative consequences for northern communities and ecosystems (1⇓–3). Furthermore, because of greenhouse gas emissions from thawing permafrost and wildfire, rapid Arctic warming threatens the entire planet and complicates the already difficult challenge of limiting global warming to 1.5° C or 2 °C.
The permafrost region contains a massive frozen store of ancient organic carbon (4), totaling approximately twice the amount of carbon as is in Earth’s atmosphere. This carbon accumulated over tens of thousands of years when cold and frozen conditions protected the carbon-rich organic material (derived from dead plants and animals) from microbial decomposition. However, warming and thawing of permafrost promotes decomposition of this once frozen organic matter, threatening to turn the Arctic carbon sink into a net source of greenhouse gases to the atmosphere (5, 6). Permafrost thaw, which can proceed as a gradual, top-down process, can also be greatly exacerbated by abrupt, nonlinear thawing events that cause extensive ground collapse in areas with high ground ice (Fig. 1). These collapsed areas can expose deep permafrost, which, in turn, accelerates thaw. Extreme weather, such as the recent Siberian heat wave, can trigger catastrophic thaw events, which, ultimately, can release a disproportionate amount of permafrost carbon into the atmosphere (7). This global climate feedback is being intensified by the increasing frequency and severity of Arctic and boreal wildfires (8, 9) that emit large amounts of carbon both directly from combustion and indirectly by accelerating permafrost thaw. Fire-induced permafrost thaw and the subsequent decomposition of previously frozen organic matter may be a dominant source of Arctic carbon emissions during the coming decades (9).
Abrupt permafrost thaw on the Peel Plateau in Canada. Thawing of ice-rich permafrost can cause abrupt ground collapse, which can further accelerate thaw and amplify permafrost carbon emissions. For scale, the lake length parallel to the headwall of the thaw feature is 150 m. Image credit: Scott Zolkos (photographer).
Despite the potential for a strong positive feedback from permafrost carbon on global climate, permafrost carbon emissions are not accounted for by most Earth system models (ESMs) or integrated assessment models (IAMs), including those that informed the last assessment report of the Intergovernmental Panel on Climate Change (IPCC) and the IAMs which informed the IPCC’s special report on global warming of 1.5 °C (10, 11). While a modest level of permafrost carbon emissions was mentioned in these reports, these emissions were not then accounted for in the reported remaining carbon budgets. Within the subset of ESMs that do incorporate permafrost, thawing is simulated as a gradual top-down process, ignoring critical nonlinear processes such as wildfire-induced and abrupt thaw that are accelerating as a result of warming.
These nonlinear processes are particularly relevant when considering the pathway to 2 °C—that is, whether mitigation keeps global average temperature increase below 2 °C (“avoidance”) or causes an “overshoot” in temperature before stabilizing. Permafrost emissions from gradual thaw alone are highly dependent on both the extent and duration of the temperature overshoot (12). For example, for a 1.5 °C or 2 °C target, an overshoot of 0.5 °C leads to a twofold increase in permafrost emissions, and an overshoot of 1.5 °C leads to a fourfold increase (Fig. 2A) (12). The impact on carbon budgets of exceeding a given temperature target will only be amplified when accounting for abrupt thaw and wildfire, both of which will have long-lasting impacts, even if global temperatures are reduced (e.g., through negative emissions) following the period of temperature overshoot.
Carbon emissions from Arctic permafrost thaw and the impact on global carbon budgets for 2 °C. (A) Cumulative carbon emissions from gradual permafrost thaw (solid line) and abrupt thaw, fire-induced thaw, and other nonlinear processes (dashed line) are expected to increase as temperatures “overshoot” the 2 °C temperature goal. (B) Permafrost emissions (only gradual thaw shown here) will therefore require additional negative emissions to draw temperatures back down to 2 °C. Figure combines modeled data (12) (black points), estimated trend based on these data (red solid line), and conceptual trend added for illustrative purposes (red dashed line).
A comprehensive understanding of the impacts of these pathways on permafrost carbon emissions—including from abrupt thaw and wildfire-induced thaw—and the implications for global emission budgets is urgently needed in order to motivate and guide mitigation decisions that will impact the state of the Arctic and the planet. Developing such an estimate is a critical next step for pinning down and communicating the relevance of permafrost carbon emissions to decision makers in order to support increased ambition to reduce fossil fuel emissions.
Scientists are aware of the risks of a rapidly warming Arctic, yet the potential magnitude of the problem is not fully recognized by policy makers or the public. Carbon emissions from thawing permafrost and intensifying wildfire regimes present a major challenge to meeting the Paris Agreement’s already difficult goal of holding the global average temperature increase to well below 2 °C above preindustrial levels—and an even bigger challenge to meet the aspirational goal of limiting the temperature increase to 1.5 °C. There is an urgent need for an accelerated scientific effort to more accurately estimate and communicate the likely magnitude of increased carbon dioxide and methane emissions from a warming Arctic to better inform decisions about the “increased ambition” that is needed to keep the global temperature increase well below 2 °C. At present, not even the current scientific understanding of future emissions from a warming Arctic is reflected in most climate policy dialogue and planning. That should be remediated without delay.