The concept of climate tipping points—thresholds beyond which irreversible changes occur in the Earth's climate system—has been debated for decades. Recent research suggests these points might be closer than previously thought. This study uses the ESCIMO climate model to explore one such tipping point: the self-sustained thawing of permafrost. The importance of this research stems from the significant implications of permafrost thaw for global warming. Permafrost contains vast amounts of carbon, and its release as methane and carbon dioxide would further accelerate climate change, creating a positive feedback loop. The research question centers on whether the Earth's climate system has already passed a point of no return regarding permafrost thaw, even with immediate cessation of anthropogenic GHG emissions. The study's purpose is to report on the findings of the ESCIMO model regarding this question, encouraging wider investigation and validation by the scientific community. This study contributes significantly to the ongoing debate on climate tipping points and the urgency of climate action, particularly by providing concrete evidence from a reduced-complexity Earth system model.

The paper acknowledges previous work on climate tipping points and the potential for irreversible changes in the climate system. It cites studies emphasizing the risk of these tipping points and the potential for near-term thresholds. Specifically, it mentions the work of Russill & Nyssa (2009), Schneider et al. (Climate Change 2001), Smith et al. (2009), and Lenton et al. (2019), all highlighting the urgency and risks associated with these potential thresholds. The review sets the stage for the current study by emphasizing the lack of consensus on the timing and magnitude of these effects and the need for further investigation using diverse modeling techniques.

The research employs ESCIMO, a reduced-complexity Earth system dynamics model, to simulate global climate from 1850 to 2500. ESCIMO incorporates representations of the atmosphere, oceans, forests, other land types, and biomass, and their interactions. The model’s source code and documentation are publicly available. The study uses two scenarios: Scenario 1 assumes anthropogenic GHG emissions peak in the 2030s and decline to zero by 2100; Scenario 2 assumes a much faster reduction, with emissions reaching zero in 2020. In both scenarios, man-made emissions remain zero beyond the specified year. The model's output includes global temperature, sea level rise, GHG concentrations (CO2, CH4, H2O), and cumulative carbon release from permafrost. The study also conducts sensitivity analyses by varying fourteen uncertain parameters within ±10% of their standard values in 200 sensitivity runs. Additionally, three parameters crucial to permafrost thaw are varied to explore the robustness of the model's predictions. These parameters include: the fraction of carbon converted from methane to carbon dioxide before release from thawing permafrost; the slope of the permafrost thaw rate as a function of temperature; and the relationship between increased radiative forcing and increased atmospheric water vapor. The sensitivity analysis employs Latin-Hypercube sampling to explore the parameter space efficiently.

The core finding is that in both scenarios (emissions reaching zero in 2100 and 2020), global temperature continues to rise for centuries after man-made emissions cease. In Scenario 1, temperature reaches a peak of +2.3°C around 2075 before temporarily decreasing to +2°C by 2150. However, temperature increases again beyond 2150, reaching approximately +3°C by 2500. This secondary warming is attributed to a self-sustaining feedback loop involving declining surface albedo (due to melting ice and snow), increased atmospheric water vapor (a potent greenhouse gas), and the release of carbon (methane and CO2) from thawing permafrost. Sea level rises monotonically, reaching approximately +3m by 2500. The analysis of radiative forcing reveals that, after 2150, water vapor and CO2 become the primary drivers of warming, with albedo playing a significant role. The release of carbon from permafrost contributes substantially to the warming, though less than water vapor or CO2. Scenario 2, with emissions ceasing in 2020, demonstrates the same pattern of self-sustained warming, highlighting that early emission reduction alone is insufficient to prevent this effect. Sensitivity analysis shows that while parameter variation affects the magnitude of temperature changes, the fundamental pattern of self-sustained warming and permafrost thaw persists. Further experiments varying key parameters related to carbon conversion, permafrost thaw rate, and water vapor feedback confirm the robustness of the self-sustained thawing phenomenon. The model suggests that to avoid self-reinforcing thawing, all man-made emissions would have needed to cease between 1960 and 1970. Additionally, the model indicates that removing at least 33 GtCO2e per year (through measures like direct CO2 capture or biomass CCS) would be necessary to prevent self-sustained warming.

The study's findings challenge the assumption that halting anthropogenic GHG emissions is sufficient to prevent significant climate change. The model predicts a self-sustaining warming trend driven by physical feedbacks even in the absence of further emissions. The continued warming is primarily attributed to albedo reduction, water vapor feedback, and carbon release from thawing permafrost. These findings highlight the importance of these feedback mechanisms and underscore the potential for irreversible climate change. The comparison with other models shows some discrepancies, primarily concerning the magnitude of carbon release from permafrost thaw; ESCIMO projects a larger release. However, the study’s key finding – self-sustained warming – remains consistent, emphasizing the need for further research and validation. The implication for climate policy is that emission reduction alone may not be enough to avert dangerous climate change; more drastic mitigation efforts, including potentially large-scale carbon removal, are necessary. The model's limitations, however, necessitate further research with more comprehensive models to confirm these projections.

The ESCIMO model robustly demonstrates self-sustained thawing of permafrost, continuing even after cessation of anthropogenic GHG emissions. This self-sustaining warming is primarily driven by albedo reduction, water vapor feedback, and carbon release from thawing permafrost. The model suggests that immediate and drastic emission reduction is insufficient to prevent this, and large-scale carbon removal might be necessary. This highlights the urgency for a more comprehensive understanding of climate feedbacks and the need for immediate and concerted action to mitigate climate change. Future research should focus on validating these findings using more complex models with higher spatial resolution and enhanced representation of the complex physical processes involved in permafrost thaw and its feedback mechanisms. Improving the parametrization of key processes involved in permafrost thaw and their feedback to the global climate system is critical for refining future predictions.

The study relies on a reduced-complexity Earth system model (ESCIMO), which simplifies various complex processes. This simplification might affect the accuracy of the model's predictions, particularly regarding the magnitude of carbon release from thawing permafrost and regional variations in climate change. The model’s assumptions, particularly concerning parameters that control the relationship between temperature and thaw rate, and the conversion of methane to carbon dioxide, may also introduce uncertainty. The model does not capture all the complexities of the permafrost carbon feedback, such as the potential for carbon uptake by newly established vegetation on thawed ground. Further research is needed to refine parameterization and incorporate greater detail into the model.