| 11 Sep 2025
Hysteresis and irreversibility in permafrost physical response to increase and decrease of CO2 emissions
Abstract. Boreal permafrost over the Northern Hemisphere high latitudes, defined as areas where the ground temperature is below 0 °C for two or more years, stores more than twice as much carbon as the atmosphere. Therefore, thawing of the permafrost, an important tipping element, due to global warming may lead to additional carbon emissions and accelerate the warming. To investigate the permafrost response to increase and decrease of CO2 emissions, we conducted a series of numerical experiments using an emission-driven Earth System Model, MIROC-ES2L, and adopting idealized overshooting scenarios in which a prescribed CO2 emission of 10 PgC is given until the global warming level reaches different values between 2 and 8 °C followed by the negative emission until the cumulative emission becomes zero.
We found that the response of permafrost area to surface warming and cooling is reversible but has hysteresis for all the emission scenarios. Furthermore, the permafrost property was shown to have irreversibility in the deep soil layer; part of the frozen area in the initial condition was replaced by a mixed water-ice area in the final state despite ground temperature turned almost to the initial condition. Sensitivity experiments reveal that the hysteresis and irreversibility are attributed to the delay of the soil freezing and melting associated with the soil heat conductivity and specific heat of water phase change. This result indicates that once permafrost thaws with warming it will continue for decades after warming diminishes and the delay in the permafrost recovery is larger at global warming levels greater than 2 °C. An offline calculation shows that the additional CO2 emission during the permafrost hysteresis cycle accounts for about 0.6–41 % of the prescribed cumulative carbon emission.
Review of “Hysteresis and irreversibility in permafrost physical response to increase and decrease of CO2 emissions” by Watanabe et al. 2025.
The authors present new emissions driven simulations of the MIROC-ES2L earth system model, incorporating developments to the land surface model since the CMIP6 configuration, and accompanying offline simulations of the PDGEM GHG model, focussing on research questions related to the physical response of permafrost and quantifying the additional CO2 and CH4 emissions from thawing permafrost. The authors also perform a perturbed parameter ensemble to diagnose the drivers of hysteresis in the physical soil properties.
The idealised experimental design and land surface model developments are well suited to answering these relevant research questions, although the analysis could be improved with several clarifying points, and further detail on certain features of the results, listed below. In general, the figures should be bigger, including a larger font size.
Major comments:
In addition, the bulk of the explanation of the permafrost hysteresis is attributed to the presence of a soil condition referred to as “sherbet”. I cannot find any reference to this in the literature, and it appears to be a new metric introduced in section 3.2.2, therefore it needs defining in much greater detail. If this is referring to a certain proportion (%) of water in a grid cell below 0 C being frozen, then this proportion should be clearly defined, and if available, a reference should be provided.
Minor comments:
Technical corrections:
Line 35: replace “10 Pc G” with “10 Pc G yr-1”.
Line 65: “IPCC, 2021” use reference for the specific chapter.
Line 163: First sentence too similar to the subtitle, so this sentence is unnecessary.
Line 155: Unclear use of “overshoot simulations”, clarify if these are the NEC experiments.
Line 176: replace “he” with “the”.
Line 217: replace “cumulative emission” with “cumulative emissions”.
Lines 251, 252 and 256: measures of area are incorrectly referred to in “m3”, and I recommend referring to permafrost area in units of “106 km2”, instead of “1013 m2”, throughout the manuscript.
Line 298: replace “keeps 0C” with “remains at 0C”.
Line 456: remove “about”.
Line 520: replace “become” with “reaches”.
Line 566: replace “ground models” with “land surface models”
References:
Burke, E. J., Zhang, Y., and Krinner, G.: Evaluating permafrost physics in the Coupled Model Intercomparison Project 6 (CMIP6) models and their sensitivity to climate change, The Cryosphere, 14, 3155–3174, https://doi.org/10.5194/tc-14-3155-2020, 2020.
So-Won Park et al. Continued permafrost ecosystem carbon loss under net-zero and negative emissions, Sci. Adv. 11, eadn8819(2025). DOI:10.1126/sciadv.adn8819.
Schleussner, CF., Ganti, G., Lejeune, Q. et al. Overconfidence in climate overshoot. Nature 634, 366–373 (2024). https://doi.org/10.1038/s41586-024-08020-9
Norman J Steinert et al 2024, Evaluating permafrost definitions for global permafrost area estimates in CMIP6 climate models, Environ. Res. Lett. 19 014033
Yokohata, T., Saito, K., Takata, K. et al. Model improvement and future projection of permafrost processes in a global land surface model. Prog Earth Planet Sci 7, 69 (2020a). https://doi.org/10.1186/s40645-020-00380-w
Yokohata, T., Saito, K., Ito, A. et al. Future projection of greenhouse gas emissions due to permafrost degradation using a simple numerical scheme with a global land surface model. Prog Earth Planet Sci 7, 56 (2020b). https://doi.org/10.1186/s40645-020-00366-8