Interactive comment on: Recent degradation of Interior Alaska permafrost mapped with ground surveys, geophysics, deep drilling, and repeat airborne LiDAR by Douglas et al. (ms no. tc-2021-47)

The paper by Douglas et al. represents a permafrost monitoring study that combines drilling, thermometry, geophysical and remote sensing approaches to detect permafrost dynamics over seven years in Central Alaska near Fairbanks in an area of discontinuous permafrost distribution. The combination of multiple belowground and aboveground observations allows the authors deducing significant interactions and trends between permafrost and surface features such as vegetation and disturbances over the observation period 2013-20. The permafrost response to current warming is clearly seen in the presented data and comprehensively interpreted by the authors. Thus, the vulnerability of discontinuously distributed permafrost to thaw under current warming is convincingly presented by Douglas et al. The impressive monitoring setup of drilling, ground temperature and thaw depth measurements combined with ERT, and satellite imagery and airborne LiDAR imagery at four different transects is, to my knowledge, exceptional in permafrost research.

The paper is clearly written and scientifically sound. To my opinion, the study by Douglas et al. represents a valuable contribution to The Cryosphere.

However, I suggest to restructure slightly the paper by adding a section 2 Study area (presently it is subsection 2.1) and a section 3 Material and methods. According changes throughout the manuscript are indicated below. Further, I noticed some redundancy in the paper and ask the authors to carefully shorten the text where appropriate. I have some further minor recommendations which are outlined below.

p1 ln14: The abstract reads very detailed presenting much of the results of the study. I’d recommend some shortening while focusing on the main outcome of the study.

p1 ln24: Add space between ‘500’ and ‘m’.

p1 ln26: Define ‘LiDAR’ as your defined ‘ERT’ at first occurrence in the ms.

p1 ln30: Is ‘… active layer depth measurements must be made …’ meant here?

p2 ln38: Use subscripted number for ‘CO₂’ here and elsewhere in the ms.

p3 ln46: Specify ‘Mean annual air temperatures …’

p3 ln53-54: Better use ‘ice wedges’ instead of ‘massive ice bodies’ since wedge ice is characteristic for Yedoma and you use this term anyway later on while massive ice is more general and might imply other origin.

p5 ln102-103: Do the area calculations refer to Strauss et al. (2016)? Add reference.

p5 ln 114: I’d suggest to omit section heading ‘2. Field measurements’ and differentiate the second section as ‘2. Study area’. Consequently, the third section would start as ‘3. Material and methods’ including the sub-sections ‘3.1 Satellite and LiDAR imagery’, ‘3.2 Field survey measurements, coring and meteorology’, ‘3.3 Electrical Resistivity Tomography’.

p6 ln130: Add space between ‘400’ and ‘m’.

p6 ln145-146: The statement is also given on p8 ln212 and could be omitted either here or there.

p6 ln147: Insert section ‘3. Material and Methods‘.

p6 ln147: Rename sub-section heading to ‘3.1 …’.

p6 ln156: Define ‘DSM’.

p7 ln162: Define ‘DEM’.

p7 ln169: Rename sub-section heading to ‘3.2 …’.

p7 ln176: Delete space after ‘August’.

p8 ln191: Rename sub-section heading to ‘3.3 …’.

p8 ln194: Delete dot after ‘4 m’.

p8 ln208: Rename section heading to ‘4. …’.

p8 ln209: Rename sub-section heading to ‘4.1 …’.

p8 ln212: The statement is also given on p6 ln145-146 and could be omitted either here or there.

p9 ln216-217: I assume what you see in the LiDAR imagery and describe here are high-centre polygons that remain when the surrounding wedge ice melts. Consider omitting the term baydzherakh here and elsewhere in the manuscript. The correct term however would be thermokarst mound to skip the ‘graveyard’ nomenclature. Commonly acknowledged definitions on permafrost terminology can be found in van Everdingen, R.O. (1998) Multi-Language Glossary of Permafrost and Related Ground- Ice Terms; https://globalcryospherewatch.org/reference/glossary_docs/Glossary_of_Permafrost_and_Ground-Ice_IPA_2005.pdf. Here, under no. 256 high-centre polygons and under no. 560 thermokarst mounds are defined.

p9 ln224: Add space between ‘310’ and ‘m’.

p9 ln225: Change to ‘P. mariana’.

p9 ln226: Change to ‘B. neoalaskana’ and ‘P. glauca’.

p9 ln229: Change to ‘P. mariana’.

p9 ln243: Delete second dot at the end of the sentence.

p10 ln244: Rename sub-section heading to ‘4.2 …’.

p10 ln245-246: Consider rephrasing to ‘The mid-June and early August seasonal thaw depth measurements and those in October 2014 show …’.

p10 ln257: Replace ‘fifty’ with ‘50’.

p10 ln272: Add ‘at’ to ‘358 m’.

p11 ln278: Add space between ‘50’ and ‘m’.

p11 ln287-290: Add a sub-section on radiocarbon dating to section 3 ‘Material and methods’ and a table with the dating results (probably to the supplement) including sample ID, lab ID, sample depth, material, δ¹³C, sample mass, and radiocarbon age although some of this information is given as description in Table S1. Please, further consider common nomenclature and calibartion to give ages as years before present (yr BP) or calibrated years before present (cal yr BP).

p12 ln316: Rename sub-section heading to ‘4.3 …’.

p14 ln358: Rename section heading to ‘5. …’.

p14 ln370: Add space between ‘1.2’ and ‘m’.

p14 ln375: Consider rephrasing to ‘… have established that vegetation provides …’.

p14 ln384: Add space between ‘2’ and ‘m’.

p14 ln384: Delete space before ‘Infrastructure …’.

p15 ln393: Add dot after ‘al’.

p16 ln430: Because ground subsidence and thermokarst are no synonyms, I’d suggest to remove the term thermokarst given in brackets after (ground subsidence).

p16 ln444: Consider rephrasing to ‘… measurements show that the thermokarst …’.

p16 ln445: Add space between ‘100’ and ‘m’.

p17 ln447: Should read ‘108 m’ instead of ’10.8m’.

p17 ln460: Add space between ‘10’ and ‘m’.

p17 ln468: Delete ‘T. Douglas, unpublished’.

p17 ln471: Rename section heading to ‘6. …’.

p18 ln485: Use subscripted number for ‘CO₂’ here and elsewhere in the ms.

p18 ln496-497: Please, consider the data policy of the Copernicus journals: https://www.the-cryosphere.net/policies/data_policy.html

p27 Table 1: Add ‘cm’ as depth unit to the ‘mean’ column.

p30 Figure 1: Consider rephrasing to ‘Worldview 2 (© Digital Globe) satellite image of the area around Fairbanks, Alaska (red dot) identifying the field site sites (colored regions) and transects (white lines) in this study.

p31 Figure 2, p32 Figure 3, p33 Figure 4 and p34 Figure 5, p35 Figure 6: Since the ‘white line’ refers to the transect shown in Figure 1, add ‘white line in Fig. 1’ in each of these captions. Sub-figures in the caption are identified by capitalized letters and ‘non-capitalised’ letters in the figure. Please adjust. 

p38 Figure 9: Consider fixed scale for the y-axes (temperatures) ranging from –12 °C to 12 °C to enhance the comparability of the nine T plots over time presented here.

p39 Figure 10: In the caption, do you mean ‘500 km²‘or ‘500,000 km²‘ as on p5 ln 103? Differentiate the combined Figure 10 into (a) and (b) instead of ‘left and ‘right’. Add scale and coordinates to the Alaska map (a). Here, location names whose data are presented in (b) are barely seen. In (b), consider one y-axis for thaw depth covering all locations presented here and ranging accordingly from 30 to 170 cm thaw depth.

Table S1: Add coordinates of the drill locations. Unfortunately, cryostructures are not described, but probably beyond the focus of this study.

The article by Douglas et al. incorporates long-term monitoring of ground temperatures and thaw depths with LiDAR-based analyses, radiocarbon dating, and permafrost coring. This is an extremely well-planned and thorough investigation of permafrost near Fairbanks, Alaska. The study is clearly presented and explained.

The study is appropriate for The Cryosphere, and will likely be of interests to scientists and policymakers across multiple disciplines.  In general, the conclusions are supported by the presented data, and the thawing and changes to the permafrost are clearly presented in many of the figures and tables. A very thorough, interesting and important study of Alaskan permafrost.

The only conclusion that seems like a reach is the calculation of potential carbon release for all of the yedoma-type permafrost in central Alaska. Given the many variables that can impact permafrost thaw and active layers (as well as variations in carbon-content even across yedoma-type permafrost), I question whether the authors can actually constrain that the total thaw of permafrost carbon in central Alaska to 0.44 Gt.

Line 16 – long description is a bit confusing “ice rich high carbon content syngenetic yedoma permafrost”

Line 24 – 500 m needs space between

Line 30 – I think “made” is the wrong word here. Confusing sentence.

Line 38 – CO₂ subscript. Spell out carbon. “7-year”

Line 40 – comma after “cover”

Line 129 – maybe write (n=3) and (n=1)

Line 260 – where are the cores located on Figure 2–6. Can you include labeled sample circles?

Line 287 – are these radiocarbon years or calibrated years? Also, need to include the radiocarbon methods earlier in the article. Maybe include a table of the results.

Line 367 – “deep end?” What do you mean here?

Line 370 – What is the importance of 1.2 m? Why is this the depth that many of the measurements are from?

Line 375 – add “that” between established and vegetation, if you want.

Line 378–9 – this sentence reiterates ideas that have already been stated. You could delete.

Line 384 – Just during this period of time (after 2013)? Were active layers deepening before this?

Line 390 – “would increase” instead of increases.

Line 403 – what is “cookie cut”? Please explain.

Line 407–411 – Given the small region of the study and the variability in thaw dynamics across the region, I question whether the authors can constrain the organic carbon pool and potential loss from thawing across all of central Alaska. I think a better way to discuss this would be to calculate the total thawed permafrost and the organic carbon release from the study sites. They could then discuss how much more yedoma-type permafrost exists in Alaska – eluding to the potential magnitude of permafrost thaw, but not directly calculating it.

Line 439 – does this decrease in elevation refer to all troughs? Is this the average? Was this calculated with LiDAR? Based on Fig. 7, it looks like trough subsidence only occurred near the lake/river – is there spatial variability?

Line 442 – delete comma between May 2020

Line 447 – 108 or 10.8 or 10 8?

Line 487–480 – This sentence is confusing.

Figure 2–6 – the caption references “white line” but there is no white line in the image.

Figure 2–6 – please show core sites

Figure 9 – in caption put space between October 1

Figure 10 – try to make the locations and labels easer to see on the map of Alaska.