From the PNAS
abstract
The rapid warming of the Arctic has intensified the forest fires in the north and is thawing carbon-rich permafrost. Carbon emissions from permafrost thaws and forest fires in the Arctic, which are not fully accounted for in global emissions budgets, will significantly reduce the amount of greenhouse gases humans can emit to stay below 1.5 ° C or 2 ° C. The Paris Agreement continues to provide opportunities to increase society’s ambitions to reduce greenhouse gas emissions, which will also reduce emissions from the thawing of permafrost. In December 2020, more than 70 countries announced more ambitious nationally set contributions as part of their Paris Agreement commitments. However, the carbon budgets informing these commitments were incomplete as they did not fully take into account the feedback from the Arctic. There is an urgent need to incorporate the latest scientific knowledge on carbon emissions from permafrost thaws and forest fires in the north into international consideration of how much more aggressively societal emissions need to be reduced in order to tackle the global climate crisis.
In the summer of 2020, there was a record-breaking Siberian heat wave, with temperatures reaching 38 ° C, the highest temperature ever recorded within the Arctic Circle. In the same year, unprecedented Arctic forest fires released 35% more CO2 than in 2019 (the previous record high for Arctic forest fire emissions since 2003), and the Arctic sea ice minimum was the second lowest on record. These are clear memories of the extreme and accelerating effects of climate change in the northern regions. The Arctic has already warmed more than 2 ° C above pre-industrial levels, and this rapid warming is expected to double by mid-century (1). Climate-induced changes have transformative consequences for northern communities and ecosystems (1⇓ – 3). Due to greenhouse gas emissions from thawing permafrost and wildfire, the rapid warming of the Arctic 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 reservoir of old organic carbon (4), which contains roughly twice as much carbon as the earth’s atmosphere. This carbon accumulated over tens of thousands of years as cold and frozen conditions protected carbon-rich organic matter (derived from dead plants and animals) from microbial degradation. However, warming and thawing permafrost promotes the breakdown of this once-frozen organic matter and threatens to turn the Arctic carbon sink into a net source of greenhouse gases into the atmosphere (5, 6). The thawing of permafrost, which can take place gradually from top to bottom, can also be significantly intensified by abrupt, non-linear thawing events that lead to severe soil collapse in areas with high ground ice (Fig. 1). These collapsed areas can expose deep permafrost, which in turn accelerates thawing. Extreme weather conditions like the recent Siberian heat wave can trigger catastrophic thaw events that can ultimately release a disproportionate amount of permafrost carbon into the atmosphere (7). This global climate feedback program is exacerbated by the increasing frequency and severity of arctic and boreal forest fires (8, 9), which emit large amounts of carbon both directly from combustion and indirectly by accelerating the thawing of permafrost. Fire-induced thawing of permafrost and the subsequent decomposition of previously frozen organic matter could be a dominant source of carbon emissions in the Arctic in the coming decades (9).
Fig. 1.
Abrupt thawing of the permafrost on the Peel Plateau in Canada. Thawing ice-rich permafrost can lead to a sudden collapse of the ground, which can further accelerate thawing and increase carbon emissions from the permafrost. For the scaling, the length of the lake parallel to the head wall of the thaw is 150 m.Photograph: Scott Zolkos (photographer).
Despite the potential for strong positive feedback from permafrost carbon on the global climate, permafrost carbon emissions are not accounted for by most Earth System Models (ESM) or Integrated Assessment Models (IAMs), including those that briefed the Intergovernmental Panel on Climate Change (IPCC) and its most recent Assessment Report the IAMs that posted the IPCC special report on global warming of 1.5 ° C (10, 11). While modest levels of permafrost carbon emissions were mentioned in these reports, those emissions were then not included in the reported remaining carbon budgets. Within the subset of ESMs that contain permafrost, thawing is simulated as a step-by-step top-down process, ignoring critical non-linear processes such as forest fire and abrupt thawing accelerating as a result of warming.
These non-linear processes are particularly relevant if the path to 2 ° C is considered – that is, whether the weakening keeps the global average temperature rise below 2 ° C (“avoidance”) or causes an “overshoot” of the temperature before stabilization. Permafrost emissions from gradual thawing alone depend heavily on the extent and duration of the excess temperature (12). For example, for a target of 1.5 ° C or 2 ° C, an overshoot of 0.5 ° C doubles permafrost emissions and an overshoot of 1.5 ° C quadruples it (Fig. 2A) (12). The impact of exceeding a certain temperature target on carbon budgets is only magnified when considering abrupt thaws and devastating fires, both of which have permanent effects even if global temperatures are lowered after the period of (e.g., from negative emissions) exceeding temperature .
Fig. 2.
Carbon emissions from the thawing of the Arctic permafrost and the impact on global carbon budgets for 2 ° C. (A) The cumulative carbon emissions are expected to be reduced by the gradual thawing of the permafrost (solid line) and abrupt thawing, fire-induced thawing and other non-linear processes ( dashed line) increase when temperatures “exceed” the temperature target of 2 ° C. (B) Permafrost emissions (here only gradual thawing) therefore require additional negative emissions in order to lower the temperatures back to 2 ° C. The figure combines modeled data (12) (black dots), the estimated trend based on this data (red solid line), and the conceptual trend added for illustration (red dashed line).
A thorough understanding of the impact of these pathways on carbon emissions from permafrost – including abrupt thaws and thaws caused by forest fires – and the impact on global emissions budgets is urgently needed to motivate and guide mitigation decisions that affect the state of the Arctic will affect and the planet. Developing such an estimate is a critical next step in establishing the relevance of permafrost carbon emissions and communicating it to decision makers to support the increased drive to reduce fossil fuel emissions.
Scientists are aware of the risks of a rapidly warming Arctic, but the potential scope of the problem is not fully recognized by policy makers or the general public. The carbon emissions from the thawing of permafrost and the tightening of the forest fire regime represent a major challenge in order to achieve the already difficult goal of the Paris Agreement of keeping the global average temperature rise well below 2 ° C above pre-industrial levels – and an even greater one Challenge to achieve the desired goal of limiting the temperature rises to 1.5 ° C. There is an urgent need for accelerated scientific efforts to more accurately estimate and communicate the likely extent of increased carbon dioxide and methane emissions from a warming Arctic to better inform decisions about the “heightened ambitions” needed to well sustain global temperature rise below 2 ° C. Currently, not even current scientific understanding of future emissions from a warming Arctic is reflected in most climate policy dialogues and planning. That should be fixed immediately.
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