Permafrost and temperature targets
Permafrost, the permanently frozen ground found in large parts of the Arctic and boreal regions, holds enormous amounts of carbon, roughly twice as much as is currently in the atmosphere. As permafrost thaws, it releases carbon dioxide and methane to the atmosphere and adds significantly to further warming. Permafrost thaw is happening faster and faster and is no longer an issue of the distant future. The potential magnitude of permafrost carbon emissions means they directly affect estimates of remaining carbon budgets needed to keep Earth’s temperature increase below the 1.5° and 2° Celsius thresholds established by the Paris Agreement.
Most importantly, actions to keep global temperatures below any threshold need an accurate target, which requires that all major sources of carbon emissions are included in the accounting framework. Carbon budgets are critical for these frameworks because increases in global surface air temperature scale linearly with cumulative carbon emissions. The more carbon dioxide emitted, the warmer it gets. This relationship allows remaining carbon budgets to be used as a tool for assessing how much additional carbon can be emitted while still meeting temperature thresholds.
The motivation behind our work is that missing critical data on global emissions in carbon budget estimates will lead to inaccurate and misinformed policy temperature goals. It is like planning a meal without understanding all the ingredients, you will end up with a meal that does not taste right. Unlike a meal however, missing a temperature target has global consequences that are felt for decades.
What are models capable of?
Earth System Models, our best tools to simulate future climate change, have so far only included one aspect of permafrost thaw, the gradual, top-down thawing that occurs due to increased temperatures. The 6th assessment of the Intergovernmental Panel on Climate Change (IPCC) suggested that gradual, top-down permafrost thaw could release tens to over one hundred billion tons of carbon by 2100.
However, in recent years, research is increasingly recognizing the importance of abrupt thaw processes, which occur in ice-rich permafrost landscapes and are characterized by rapid rates of thawing and severe ground collapse. These processes (e.g., retrogressive thaw slumps, active layer detachments, and other thermo-erosional features ) expose deep layers and highly organic material to much warmer conditions and can mobilize large amounts of previously frozen carbon over short timescales. Even though individual features are relatively small (10s-100s of meters), their impact on the permafrost carbon feedback is expected to be substantial.
Wildfires emit carbon to the atmosphere at the time of burning and further accelerate permafrost thaw by removing the insulating organic layer at the soil surface, increasing active layer depth, and triggering thermokarst development. The impact of wildfires and abrupt thaw processes are both largely missing from Earth System Models. This contributes to lower confidence in estimates of remaining carbon budgets and limits our ability to fully assess mitigation pathways consistent with temperature targets. In our research, we focused on a modeling approach that can address this gap and inform carbon budgets on policy-relevant timescales.
How much do remaining carbon budgets shrink when permafrost emissions are accounted for?
In this study, we provide a first estimate of these missing emissions using a reduced-complexity Earth System Model called OSCAR. OSCAR is designed to emulate the large-scale behavior of more complex, process-based models. Gradual permafrost thaw was already included in OSCAR based on earlier work, and we added new modules representing abrupt thaw and permafrost-wildfire interactions.
Our results show that accounting for abrupt permafrost thaw and wildfire-related emissions significantly reduces the remaining carbon budgets for keeping global temperatures below 1.5° and 2° Celsius. Including these processes reduces the carbon budgets remaining from 2025 onward by about one quarter for 1.5° Celsius and by nearly one fifth for 2° Celsius, compared with simulations that do not include permafrost carbon.
While total human emissions remain much larger on an annual basis, permafrost carbon emissions represent a large and irreversible contribution over the coming decades. These emissions must be accounted for to accurately assess whether the world is on track to meet its temperature goals set under the Paris Agreement.
How can disturbance processes be included?
A lack of data and the absence of processes included in complex Earth System Models meant we had to make assumptions. For example, we assumed that rates of abrupt thaw increase with warming in a manner broadly consistent with gradual thaw. We also followed the approach that large-scale wildfire activity and post-fire ground heating can be related to global temperature change. Although these assumptions might not capture the full complexity and heterogeneity of permafrost landscapes, they provide a transparent and conservative way to explore the potential magnitude of permafrost carbon emissions in the absence of better data and fully mechanistic representation in Earth System Models.
What’s next
While fully resolving abrupt thaw and wildfire–permafrost interactions in future simulations will ultimately require large, process-based Earth System Models, that level of complexity is not yet feasible on policy-relevant timescales. In this context, our approach provides a valuable first-order assessment of how these under-represented processes affect carbon budgets and temperature targets.