Peer review of: Fast response of cold ice-rich permafrost in northeast Siberia to a warming climate (2020)

Reviewers' comments: 
 
Reviewer #1 (Remarks to the Author): 
 
Review of Nitzbon et al. 
 
This manuscript describes a numerical model that represents thermokarst processes in ice-rich terrain 
common to northern Siberian lowlands. The model focuses on several thermokarst processes present 
in the study region, with a particular focus on ice wedge dynamics. This study is timely, particularly in 
light of the recent IPCC special report on oceans and the cryosphere. While an increasing number of 
Earth system models are beginning to incorporate permafrost into their frameworks, few if any 
address permafrost thaw initiated by thermokarst or thermoerosion. 
 
Overall this paper is well written, the approach is described well, and the results are interesting. It is 
clear that this modeling approach is an advancement over models that cannot simulate change in ice-
rich permafrost, including land surface subsidence. The style of the writing is quite dense – while all 
interesting it was difficult at times to discern the key findings that the authors wished to emphasize 
over others. The results largely focus on hydrophysical changes to the landscape and surface 
expressions that result from the thermokarst simulations. The paper also uses soil and sediment data 
from the region to estimate the stock of permafrost carbon exposed to thawed conditions. This is the 
weakest portion of the paper, and uses inappropriate terminology in several locations. 
 
This paper will be an important contribution to the permafrost and modeling communities but given 
the detail and complexity of the simulations, might be best received in a more disciplinary journal. I 
would stress, however, that a number of papers addressing thermokarst have been published in high 
profile journals (including Nature journals) in recent years, but these tend to focus on estimates of 
permafrost carbon release. This paper does not address carbon release associated with permafrost 
thaw, but does provide a new modeling framework that could be an important first step towards this 
goal. 
 
While there is a lot of detail and very nice graphics presented in this manuscript, the text raises a 
number of important issues and questions that should be addressed before this manuscript can be 
considered ready for publication: 
 
1) While the manuscript clearly describes (and visualizes!) which types of thermokarst and surface 
expression change are included in the simulations, there is no clear sense of whether these are the 
dominant types of thaw/thermokarst in the study region. Can the authors use remote sensing or other 
spatial data to provide an assessment of whether they are representing the dominant pathways ice-
rich permafrost thaw and resulting landscape change? What types of thaw/thermokarst are missing 
and how important are these trajectories in the study area? 
 
2) It is unclear to me whether existing (old) thaw lakes or alas are included in the simulations. These 
are major features on the landscape, presumably are important to water and nutrient flow, and 
certainly need to be taken into account when estimating potential carbon mobilization. 
 
3) Walter Anthony et al. (2018) found that RCP8.5 warming affected yedoma carbon release very 
differently than RCP4.5 warming because of more rapid thaw lake drainage under RCP8.5. This is a 
mechanism by which thaw lake carbon mobilization could be greater under mitigation scenarios 
relative to RCP8.5 warming scenarios. It is not clear to me whether this study is able to address this 
issue. If not, how much of the results of this study are likely to change if thaw lake drainage is 
included more explicitly? 
 
4) The authors present simulation results that in general conform to other modeling studies, but do 
not compare results from the model to any empirical information. I am not an expert in modeling 
myself so do not have an expert opinion on what would be considered standard. But it seems 
important to evaluate the model performance more fully than what is presented in the current 
manuscript. 
 
5) The authors need to use more appropriate terminology in the carbon section. The manuscript 
currently does a rough calculation of how much permafrost carbon (i.e., a C stock assessment) will be 
exposed to thaw under different scenarios. This really has nothing to do with mobilization and it is 
misleading for the authors to use terms like that. In my opinion, this entire section needs to be 
carefully re-thought. 
 
6) The authors make a number of simulations in their modeling framework and simulations, such as 
decisions regarding aerobic: anaerobic proportions. It seems important for this manuscript to provide 
an assessment of the sensitivity of model performance to these assumptions and to the suite of 
parameters involved in the simulations. 
 
7) As a more minor point, I do not think that the term “conservation” is appropriate for use in the title 
of this manuscript. To most scientists, conservation implies active protection or management of the 
land or resource. What the authors are really hinting at in the title is whether response of ice-rich 
permafrost will be a slow or fast response, relative to change of ice-poor permafrost. 
 
 
Reviewer #2 (Remarks to the Author): 
 
Major comments 
 
Developing a process-based understanding of polygonal tundra evolution and linking it to the 
permafrost carbon feedback is important piece of the permafrost puzzle that is currently missing from 
the global scale permafrost modeling picture. This work is timely and addressing one of the pressing 
issues of the Arctic, the fate of carbon from the organic rich frozen ground of the tundra type 
ecosystem. Below I have several questions that I hope authors will take in the positive spirit intended 
during revision of this important paper. 
 
1. The effective hillslope sediment transport model was adapted to tundra environment to model 
polygonal tundra evolution. Why did authors decide to adapt hillslope sediment transport model 
instead of adapting one of the existing erosion model by the corresponding rescaling of geological 
years? 
 
2. Currently, model simulates progressive changes leading transition from one state to another with 
the corresponding permafrost degradation. From field experiments, we know that LCP can also have 
small ice wedges in the middle (e.g. studies by Jastrow et al), suggesting changes from HCP to LCP 
and back. However, it is not clear under what conditions it is possible to cyclically develop a dynamical 
evolution of HCP to LCP and vice versa? Applying the RCP 2.6. scenario might be useful to illustrate 
how negative trend in the global temperatures during this century could affect changes in polygonal 
tundra landscapes. It also could be interesting to show that these transitions are possible. For 
example, Kleinen and Brovkin (2018) drew some insightful conclusions based RCP 2.6 suggesting that 
under this scenario almost double amount of carbon need to be removed from the atmosphere in 
order to reach the 1.5oC target. 
 
3. Finally, authors used simplified empirical relationship to address the amount carbon that could be 
released as a result of permafrost thawing. Recent studies show that observation from flux towers 
indicate that most of this release is not captured by the near-by flux towers, suggesting significant 
lateral transport of the carbon in the form of the dissolved organic carbon (Plaza et al. 2019). Taking 
this into account how it might change the current numbers on Figure 4. 

Minor comments 
 
Authors claim that there is such a strong dependence between subsidence, amount of ground ice, and 
carbon emissions. I suggest to include plots illustrating changes in the carbon emission with respect to 
subsidence and ground ice. This might to conceptualize these relationships and benefit future studies 
working on similar topic. 
 
From page 8. “Locally observed degradation ….” It is not clear does the model address the effect of 
the anthropogenic origins or not? 
 
It would be nice to know the percent difference between projected carbon release by global models 
from Russian permafrost and current study only for tundra over 21st century. Showing this difference 
can make a greater impact and better convince readers about importance of the tundra (yedoma) 
permafrost thaw. 
 
References 
Kleinen T and Brovkin V 2018 Pathway-dependent fate of permafrost region carbon Environ. Res. Lett. 
13 094001 
Plaza C, Pegoraro E, Bracho R, Celis G, Crummer KG, Hutchings JA, Pries CEH, Mauritz M, Natali SM, 
Salmon VG, Schädel C, Webb EE, Schuur EAG (2019) Direct observation of permafrost degradation 
and rapid soil carbon loss in tundra. Nat Geosci 12:627–631. https://doi.org/10.1038/s41561-019-
0387-6