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


RR2
 REVIEWERS' COMMENTS:
Reviewer #1 (Remarks to the Author):
  This is the second time I have reviewed this manuscript and I found the revised manuscript to be
much improved as a result of the revision process. The authors should be commended for taking
  reviewer comments seriously and addressing issues both through text revisions but also in some cases
 by including new supplemental information.
 After evaluating the individual responses to reviewer comments, I do note that the authors spent
 considerable effort responding to each comment, but some of this information remained in the
 responses rather than being incorporated into the manuscript text. I do think the revised manuscript
overall is acceptable for publication, but there are some interesting points that did not actually get
  incorporated into the manuscript. I would encourage the authors to reread the response document
 and consider adding some additional information from these responses into the manuscript or the
 supplemental file. A reader is very well likely to benefit from this information.
 A few minor points regarding the treatment of carbon stocks:
 1) at the bottom of page 16, "substantially larger amounts of carbon could thaw". While I understand
the key point here, I wanted to point out that it's not the carbon per se that is thawing. The authors
  could clarify that the carbon rich soils are thawing, or that permafrost organic matter is thawing,
 exposing carbon to mineralization.
 2) Somewhere perhaps on page 18 or page 19, I think the authors need to point out that carbon
 emissions to the atmosphere are not determined solely by thaw and mineralization of soil/sediment
 carbon. Rather measurements of ecosystem carbon exchange also need to include assessments of
carbon uptake by vegetation. Not only would this more accurately reflect the
  measurements/assessments that are required for models to address carbon balance/exchange from
 changing permafrost environments, but it will help to reinforce/remind readers that they cannot
 assume that the carbon stock estimates presented here will all wind up in the atmosphere. A well
 placed sentence or two could solidify this concept.
 3) Finally, there is a running debate on social media between climate and energy experts regarding
 RCP8.5. The argument has gone back and forth for months, but boils down to a segment of the
 community arguing that we cannot and should not refer to RCP8.5 as "business-as-usual", which the
 authors do at the end of the paper (and perhaps in other places?). The debate is too lengthy and
 nuanced to summarize here. In the end, the authors have done a nice job in this study comparing
 different RCP scenarios here, and this is exactly what is needed. However, I would encourage the
 authors to strictly refer to the scenarios by name, and avoid terms like "business-as-usual".
Reviewer #2 (Remarks to the Author): Major
   The current version of the manuscript was significantly improved. The overall mechanics of the ice-
 wedge dynamics require some further clarification. In particular, it was not clear to me, how does the
 thermodynamics between air and surface water work? Once the ice-wedge is melted and water starts
ponding at the surface. If the water body completely freezes over the winter and snow depth is
  shallow then it could lead to colder subsurface contributing to the retardation of the permafrost
 thawing. Also, when the standing water is accumulated, how exactly does CryoGrid model treat the
 standing water physics? Does the water layer has water properties or properties of subsurface
 modified to represent fully saturated soil with thermal properties corresponding to water?

 I apologize if I missed that but the paper is refereeing to rapid thaw process/features without explicitly
 defining it. To me instead of calling it rapid thaw, which I found confusing, it would be better to say
 that these processes are micro-scale processes and build your story of that, instead of referring to
 “rapid thaw”. The micro-scale processes make a perfect sense to readers and it is clear that they are
not represented in any LSM or GCM type models. The introduction of the micro-scale process (ice-
  wedge dynamics) allows us to better address subsurface carbon dynamics in the tundra. In addition, it
 would be useful talking about when the tundra would switch from carbon sink to carbon source. Using
 the language that is more in-line with the PCN language (e.g. McGuire et al., 2018). That said, I would
 encourage authors not to refer to the “rapid thaw” process. Instead, the scale is the most important
 feature here that is not represented in global-scale models.
 Minor
Why the authors decided to average over 11 years? Why not 5 or 10? Need to explain that SI. P10. L6 change 1999 to 2099
Main. P3.L2. change “very” to “high”.
How exactly lateral heat and fluid flow happening between adjacent polygons (SI Fig 3a)?
    Consider changing Fig 7b with the one that is not from the coast because the current one illustrates
 coastal erosion and could be confusing. Since the story has nothing to do with coastal erosion.
  Also, I would suggest focusing on the SI and add more details to the description of the thermo-hydro-
mechanical processes.