the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 24 Aug 2023
Persistent warming of the ground on the Earth’s Third Pole
Abstract. A continuously increasing ground temperature in the Tibetan Plateau (TP) can result in permafrost degradation and impact regional climate through land-atmosphere interactions. However, systematic knowledge of spatiotemporal dynamics and regulatory mechanisms of ground temperature changes in the region is limited. Here, we quantify the thermal status and trends of both soil temperatures measured at depth and typical permafrost profile temperatures. We show that, shallow soil layers (0–40 cm) in most TP regions experienced significant warming from 1981 to 2021, with lower rates at greater depths. Ground surface warming trends aligned with air temperature, but the timing of the maximum warming trend was progressively delayed with depth. Cold seasons exhibited the largest warming trends for air and ground surface, while warm seasons saw greater warming trends at 5–40 cm soil depths. Regionally, warming trends were larger at sites with lower mean annual temperature and higher elevation. Out of the factors tested in addition to air temperature, only snow-cover days and downward longwave radiation were significantly related to the soil warming rate trends. Further analysis reveals that the persistent shallow ground temperature increase over decades is linked to the Atlantic Multidecadal Oscillation's warm phase, impacting near-surface air temperature through teleconnections. Additionally, we present spatially heterogeneous observations of continuous warming in permafrost profiles, which show intense warming in the surface layers, and minimal warming at 40 m depth. Permafrost profile warming magnitude and depth-dependent variation are influenced by local climate and elevation. This study provides a comprehensive view of persistent warming in the Tibetan Plateau across surface and deep layers.
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CC1: 'Comment on tc-2023-125', Zhiyong Ding, 06 Sep 2023
In this paper, the author quantified the thermal status and trends of both soil temperatures measured at depth and typical permafrost profile temperatures on the Earth’s Third Pole. The topic selection has important research value. They show shallow soil layers in most study regions experienced significant warming from 1981 to 2021, with lower rates at greater depths. Moreover, the deep ground temperature variation at the typical permafrost borehole sites were also analyzed. This study provide a comprehensive understanding of the implications of recent variations in ground temperature across the Tibetan Plateau. However, there are still obvious drawbacks in this study. It is recommended to minor revision the manuscript before publication.
Major comments:
In the introduction section, after providing an overview of previous research, the authors should introduce the core scientific questions to be addressed in this study. Subsequently, they should outline the aspects from which this study intends to make improvements. Authors need to refine this part.
In the Discussion section, there have been previous studies on soil temperature in the Tibetan Plateau as well. The authors should extensively reference relevant literature and highlight the novel findings and innovations of this study.
Ground ice content is mentioned as a crucial element in the discussion but no information on ice content is provided in the analysis. The author needs to explain this.
The authors used some grid climate variables and other variables in attributing the increase in soil temperature. The resolution of these variables is relatively rough, and whether they match the soil temperature observed at the site. Before making correlation analysis, the author needs to first explore the reliability of these grid dataset.
The author believes that some atmospheric circulation plays an important role in the attribution of the interdecadal increase in soil temperature, but the mechanism behind it is not discussed enough. The author should add relevant references for further explanation.
Minor comments:
Some of the legends in the figures need more explanation, and the symbols used in the figures are not clearly described. The formatting of some figures and tables could be improved. For example, Figure 3 should give the literature source of the permafrost distribution map and indicate what figures (a)-(f) represent respectively. The color of soil at different depths in Figure 4 should be consistent with other figures in the paper. Some of the information in Figure 8 partially overlaps with previously published literature, and relevant references should be explicitly cited.
Some descriptions in the conclusion part lack support from data analysis and constitute speculative conclusions. These should be removed. For example, “The extent and variation of profile warming with depth are influenced by multiple factors, including local climate, lithology, and elevation”. The results regarding the lithology of the observation sites are not presented in the text.
The grammar and logical coherence of this article are still lacking. The authors should seek a native English-speaking scholar for comprehensive language refinement.
Citation: https://doi.org/10.5194/tc-2023-125-CC1 -
RC1: 'Comment on tc-2023-125', Anonymous Referee #1, 12 Oct 2023
Comments to the Authors
The manuscript submitted by Wang et al. investigates ground temperature changes over four decades in the permafrost regions of the Tibetan Plateau (TP). The analysis focusses mainly on shallow soil temperatures which tend to respond to short-term fluctuations of climate compared to deeper ground temperatures (which get limited attention in the MS), which filter out these short-term variations and preserve the longer-term changes. The overall topic of the research is relevant to understand the impact of climate change on permafrost regions and the authors appear to have put much effort into the correlation analysis that includes several variables. However, the MS falls short in terms of advancing knowledge relative to several recent studies in the TP region and elsewhere. Much of the information provided in the discussion and conclusions are general statements and for the most part are not unexpected or new. Some statements also lack evidence from the results presented. The MS requires considerable revision for it to be acceptable for publication.
The authors refer to the TP as the largest permafrost region outside of the Arctic and mention the importance of changes in permafrost conditions in the Introduction and Conclusion. Despite the main rationale for the research being the implications of warming on permafrost, there is very little about permafrost throughout the rest of the MS. Although the meteorological stations are distributed throughout the permafrost regions as shown in Fig. 1 this does not necessarily mean that the station is underlain by frozen ground as permafrost may be discontinuous. The figures showing time series (e.g. Fig. 2) of shallow soil temperatures (averages of all site) also provide no information on whether permafrost is present (at the depths considered temperature does not stay below 0°C the whole year based on average temperature). The only sites where the reader can determine whether permafrost is present (at least at some sites) is from the deeper ground temperature profiles acquired from the borehole sites (Fig. 8 and 9). The authors should identify the sites underlain by permafrost and provide some information about the permafrost characteristics especially in the discussion of shallow ground temperature – e.g. what is the active layer thickness or seasonal frost depth. It may have been beneficial to compare permafrost and non permafrost sites in the analysis rather than group all sites together. Ground ice content is mentioned as an important factor in the discussion but no information on ice content is provided in the analysis.
There are several correlations done between shallow soil temperatures and climate and other variables. Although some of the data used is acquired at the same site as the ground temperature data (e.g. air temperature, precipitation), other data is derived from low resolution remote sensing data or national scale gridded databases such as information on vegetation and soils including moisture content. These characteristics can be spatially variable and the conditions at the soil temperature site may be different than that derived from these databases. This may be part of the reason the correlation between soil temperature and these variables is generally low as evidenced by the large scatter shown in the figures. The utility of these correlations is therefore questionable. For meteorological stations in other countries the surface conditions are generally the same for all stations (i.e. open area with similar ground vegetation). It is not clear if conditions are similar for meteorological stations in the TP region and therefore it is difficult to determine whether the correlation analysis between vegetation (NDVI) and soil temperature is that useful.
There are many figures presented in the MS. Some of these require more explanation in the legend and the symbols used are not always adequately described in the legend. The format for some of the figures could be improved. All the figures are probably not necessary as they are presenting the same information, some of which is summarized in tables. Although I have made some comments on figures the authors should consider what is the most effective way to present their results to support the conclusions that are made. The text could also be improved as there are many unclear statements, some of these are identified below but these are only examples.
The MS could be more focussed as it currently tries to cover many things. It is not clear to the reader how this study differs from other similar analysis done in the region using the same data sets, some of which are cited in the MS and also others (e.g. Wang et al. 2021; Zhou et al. 2020; Li et al. 2022 and others), or the new knowledge generated. The authors should consider re-defining their objective to focus on existing knowledge gaps (rather than repeat previous studies), perhaps those identified in studies previously done. This would help in producing an improved MS that would be acceptable for journal publication.
Additional Comments
L30 – Are you referring to air or ground temperature?
L52 – What do you mean by “regulatory mechanisms”? Are you referring to the factors controlling the response of soil temperatures to changes in climate?
L54-55 – This is more a consequence of poor understanding rather than the reason for poor understanding. It is suggested that the comment on oversimplified representation be deleted here.
L66 – Unclear
L67-68 – Several 10s of metres? Most of these studies generally refer to temperatures in the upper 20-30 m.
L71 – How deep? 20 m? 100 m?
L80-81 – This isn’t quite true as conditions in the circumarctic (high latitude) permafrost region vary from relatively thin permafrost at temperatures that are >-1 to -2 °C. Also, not all permafrost is ice-rich. Conditions in some areas maybe quite similar to the TP region.
L84-85 –You should provide a reference for this amplification with elevation such as Wang et al. (2016).
L97 – Here and elsewhere in text - What is meant by layered soil temperature? Usually, we just refer to soil or ground temperatures measured at depth.
L110-112 – Information on accuracy and precision of instrumentation should be provided.
L150-152 – You should have a brief description of instrumentation (including accuracy and precision) used and length of record here.
L190-191 – Unclear
L192-193 – This result is not unexpected and is shown by temperature envelopes in many other publications.
L200 – Do you mean “correlation coefficients”?
L205-207 – Unclear
L209 –It is unclear what you mean in this sentence. Are you referring to the depth at which there is no diurnal variation?
L229-234 – This lag is not unexpected or unknown.
L237-242 – This reversal of gradient is not unknown. There should be some discussion of latent heat effects associated with soil freezing and thawing.
L269 – It is not clear what you mean by middle of the surface soil layer. You seem to be assuming soil is 40 cm thick everywhere which may not necessarily be the case.
L273 – This may be related to changes in thermal conductivity.
L275-276 – If there is less snow then there is less insulation and a more direct link between changes in air and ground temperature.
L279-280 – The type of vegetation is important as it can have an effect on snow cover as well as shading. Do the meteorological stations all have similar vegetation and surface conditions? If so, it is difficult to investigate impacts of variations in vegetation conditions.
L287 – Isn’t the effect on ground temperature indirect as it is related to air temperature and precipitation variation that is influenced by regional atmospheric circulation patterns?
L312-313 – Are these the same 2 sites mentioned above?
L315-319 – Isn’t this just repeating what you have essentially shown for the two sites above?
L318 – Why/how are the trends anomalous?
L322 – Haven’t these results (2001-2010) already been shown in papers such as Zhao et al. (2010 – in your ref. list). Zhao et al. (2020) provide a synthesis and analysis of a longer time-series. No new information on trends appears to be provided here.
L325 – It isn’t really useful to conclude that there were record high temperatures in 2010 given there are more recent data such as that presented in Zhao et al. (2020) or annual State of Climate updates (e.g. Noetzli et al. 2022). Based on longer time-series, record high values occurred more recently.
L326 – What evidence do you have that snow cover is important here? Do you have snow cover data?
L327-329 – What evidence do you have that these are the influences on ground temperature variability. There has not been any information provided regarding some of these variables such as lithology or ice content. These seem to be general statements.
L330-331 – Are you referring to the depth of zero annual amplitude (depth at which there is no seasonal variation), below which temperatures increase with depth according to the geothermal gradient.
L333-335 – Sentence unclear.
L337-339 – Are you referring to latent heat effects here?
L340 – Discussion section. There are several general statements and much of what is presented is not unexpected or unknown. What is the unique contribution of your analysis. What is the new knowledge?
L342-348 – This seems to be a repetition of results. How are these new compared to other studies in the region?
L349-352 – Isn’t TP included in some of these publications? There have been other recent global syntheses with more up to date data (some are cited in the ref list) that provide a clear indication of global warming including Noetzli et al. (2022), Smith et al. (2022) and most recent IPCC report (e.g. Gulev et al. 2021).
L352-365 – What is the new information here as much of this is already known from other studies.
L368-371 – What do you mean by the deepest soil layers. Are you referring to the near-surface temperatures or depths of several to 10s of metres? It isn’t clear what you mean here. The deeper ground temperatures do respond to air temperature, but they respond to the longer-term variation rather than the higher frequency fluctuations. The lag time increases with depth and this could also be the reason for poorer correlation if you compare air and ground temperatures measured at the same time.
L377 – I didn’t see anything about the onset of snow cover in the results, only snow duration (which does not give timing of onset). It isn’t clear how you arrive at this conclusion. Other studies have examined the role of timing of snow cover on ground thermal regime such as Zhang et al. (2019), Noetzli et al. (2020); Palmer et al. (2012); Morse et al. (2012).
L389-391 – What type of vegetation. What about shading of the ground (cooling effect) or effects vegetation has on snow cover? Are you referring to radiation absorption by vegetation/canopy? See earlier comment regarding vegetation and surface conditions at meteorological sites.
L393 – You refer to permafrost here and briefly elsewhere, but you don’t say anything in the MS about the permafrost conditions. Is permafrost present at all sites? This is not clear from the information provided. What about ground ice conditions?
L394-399 – This is an issue when using site specific data and national scale databases together. Soil conditions including organic content are spatially variable so without site-specific data this analysis isn’t very useful.
L401 – See earlier comments – Isn’t this really related to the effect of air temperature and precipitation on ground temperature since atmospheric circulation is related to these meteorological variables (i.e. the effect on ground temperature is indirect).
L402-403 – Are you referring to elevational amplification? See earlier comment for reference.
L405-406 – Does this have to do with exposure to wind?
L428 – Permafrost conditions have not really been mentioned much throughout the MS (see earlier comment).
L429 – “Seasonal permafrost” is incorrect terminology as permafrost is not seasonal. Are you referring to seasonal frost? Revision is required.
L429-448 – There is much repetition here of material presented earlier. There are also many general statements and not much new presented.
L438-439 – The deeper temperatures preserve the longer-term trends in temperatures at the ground surface with the lag time increasing with depth.
L444-445 – Are you referring to an isothermal temperature profile due to the phase change?
L449-452 – There is no information presented on permafrost properties such as ice-content and nothing about the deeper material properties. General statements are being made here with no evidence from your results.
L459 – Soil temperatures are ground temperatures.
L474-478 – Some explanation is required here. Is there even permafrost under the glaciers. Isn’t this warming of the glacier due more to geothermal heat flux as well as changes in air temperature and snowfall?
Figure 1 – The colour scale for elevation is the reverse of what is normally used. It might be good to identify which study sites are underlain by permafrost.
Figure 2 – Couldn’t the information on trends be summarized better in a table rather than in the figures on the right side. Information on trends at individual sites for various depths is already shown in figure 3.
Figure 3 – Reduction of categories would be helpful as would using red for warming and blue for cooling. It isn’t easy for the reader to distinguish between the symbols the way the information is presented.
References
Gulev SK, Thorne PW, Ahn J, Dentener FJ, Domingues CM, Gerland S, Gong D, Kaufman DS, Nnamchi HC, Quaas J, Rivera A, Sathyendranath S, Smith SL, Trewin B, von Schuckmann K, Vose RS (2021) Changing State of the Climate System. In: Masson-Delmotte V, Zhai P, Pirani A et al. (eds) Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp 287-422. doi:10.1017/9781009157896.004
Li, Y. et al. 2022 Analysis on the temporal and spatial characteristics of the shallow soil temperature of the Qinghai‑Tibet Plateau. Scientific Reports 12:19746 https://doi.org/10.1038/s41598-022-23548-4
Morse PD, Burn CR, Kokelj SV (2012) Influence of snow on near-surface ground temperatures in upland and alluvial environments of the outer Mackenzie Delta, Northwest Territories. Canadian Journal Earth Sciences 49:895-913. doi:10.1139/E2012-012
Noetzli J, Christiansen HH, Guglielmin M, Hrbáček F, Isaksen K, Smith SL, Zhao L, Streletskiy DA (2022) [Global Climate] Permafrost Thermal State [in "State of the Climate in 2021"]. Bulletin of the American Meteorological Society 103 (8):S41-S43
Noetzli J, Christiansen HH, Isaksen K, Smith S, Zhao L, Streletskiy DA (2020) [Global Climate] Permafrost Thermal State [in "State of the Climate in 2019"] Bulletin of the American Meteorological Society 101 (8):S34-S36
Palmer MJ, Burn CR, Kokelj SV (2012) Factors influencing permafrost temperatures across tree line in the uplands east of the Mackenzie Delta, 2004–2010. Canadian Journal of Earth Sciences 49:877-894. doi:10.1139/E2012-002
Smith SL, Romanovsky VE, Isaksen K, Nyland KE, Kholodov AL, Shiklomanov NI, Streletskiy DA, Drozdov DS, Malkova GV, Christiansen HH (2022) [Arctic] Permafrost [in "State of the Climate in 2021"]. Bulletin of the American Meteorological Society 103 (8):S286-S290. doi:10.1175/BAMS-D-22-0082.1
Wang Q, Fan X, Wang M (2016) Evidence of high-elevation amplification versus Arctic amplification. Scientific Reports 6:19219. doi:0.1038/srep19219
Wang X, Chen R, Han C, et al. Response of shallow soil temperature to climate change on the Qinghai–Tibetan Plateau. Int J Climatol. 2021;41:1–16. https://doi.org/10.1002/joc.6605
Zhang G, Nan Z, Wu X, Ji H, Zhao S (2019) The role of winter warming in permafrost change over the Qinghai-Tibet Plateau. Geophysical Research Letters 46:11261-11269. doi:10.1029/2019GL084292
Zhao L, Zou D, Hu G, Du E, Pang Q, Xiao Y, Li R, Sheng Y, Wu X, Sun Z, Wang L, Wang C, Ma L, Zhou H, Liu S (2020) Changing climate and the permafrost environment on the Qinghai–Tibet (Xizang) plateau. Permafrost and Periglacial Processes 31:396-405. doi:10.1002/ppp.2056
Citation: https://doi.org/10.5194/tc-2023-125-RC1 -
RC2: 'Comment on tc-2023-125', Meven Philippe, 07 Nov 2023
General comments
This manuscript investigates warming trends in ground temperatures over the Tibetan Plateau, and their relationships with various environmental factors. In this respect, the amount of data collected and the effort put into the evaluation of statistical correlations between the variables has to be praised.
However, the manuscript currently shows significant flaws that will require consequent reflexion and revision before publication. Overall, it would benefit from a review of previous studies done on the TP, that will help identifying knowledge gaps, defining scientific objectives and focussing the study. Methodological processes and figure/table captions must be detailed more rigorously.
Specific comments
Major comments
- As stated above, this study lacks a well-defined scientific objective. It is reflected throughout the manuscript with sections (Introduction, Results and Discussion) that cover a wide variety of subjects (atmospheric circulation, vegetation, organic carbon content…), making the whole manuscript to lack consistency. This is also reflected by some statements repeated two or three times among the Results & Discussion sections. Therefore, I suggest to add a paragraph at the end of the Introduction, detailing the state-of-the-art concerning the ground thermal regime in the TP. This will lead to the identification of current knowledge gaps, and the definition of one (or several) specific scientific objectives at the end of that section. I believe this will also help maintaining the manuscript focus overall.
- The Method section needs to be refined as is currently lacks explanation and details on the data acquisition and processing. For instance, the specific temperature measurement method is not specified (sensors in boreholes? In fractures?); biased are identified in the section (e.g. inconsistent consideration of the 0cm-elevation) but it is not specified how they were accounted for; the “strict quality control assessments” (l. 118) are not explained; the time series which the study is based on was selected as it has “relatively little missing data”, without quantification; the reason why increasing measurement depth reduces the availability of shallow data (l. 120-121) is not specified.
- Presented equipment and data also lack associated accuracy and errors. The models and accuracy of the thermal sensors must be stated, and the associated error on data must be shown in Tables (± X) and in Figures (with error bars).
- Some interpretations made in the Discussion are global knowledge on the thermal regime of ground or permafrost ground (e.g. delayed warming with depth, higher-amplitude fluctuations at shallower depths). Overall it is not clearly highlighted what this manuscript brings new – which is also a point where a comparison of this study with the state-of-the-art would be helpful.
- Figures and Tables need their captions to be refined. Overall, explanations lack about the significance of some values (for tables), symbols and colours (for figures), which makes the understanding sometimes challenging. Some examples are identified in ‘Minor comments’, but this list is not comprehensive.
- Supplementary Material should give accessibility to the raw data used to make figures and tables.
- An overall lack of references leaves some statements without justification. Some examples are identified in ‘Minor comments’, but this list is not comprehensive.
Minor comments
Overall, descriptions are too qualitative (e.g. l. 438 “the warming trend (…) becoming quite weak”; l. 114-115 “relatively little missing data”). It is generally good to avoid such statements, or to justify them with quantitative arguments.
Further minor comments (in reading order):
24-25: “typical” from the TP? From permafrost ground in general?
27-28: statement here is general knowledge
35-38: general knowledge
72-74: this statement is even fairly certain, also it lacks references
82: “cold” or “periglacial” instead of “frigid”; also be more specific about the process(es)
83-85: lack of references
88-89: more specific about the process(es)
Section 2.1.: accuracy of data and sensors is lacking
109-110: was this bias accounted for? If yes, how?
114-115: quantify “relatively little missing data”
118-120: those “strict quality control assessments” must be detailed, as they seem to be part of the methodological process. Also quantitatively define “high-quality and long-term sites”
120-121: explanation lacks
122-123: accuracy lacks; also specify what is “daily”? One measurement/day? An average of several measurements?
122-140: details lack on how the meteorological parameters are calculated, and on their accuracy/error
125: how do you calculate MAST?
126-127: how do you calculate the “annual mean of daily snow depth”?
141-148: what are those indices? Why those nine were selected specifically?
149-152: where are those borehole sites? How many are they? How is ground temperature calculated, as a mean of temperature at various depth?
170-171: unclear statement
202-204: statements are not quantitatively assessed
271-273: if non-significant, why mentioning the results?
Section 3.6.: describe how measurements were performed, with what accuracy and with which equipment
Discussion: some statements are general knowledge
345-346: aren’t higher elevations linked with overall lower mean temperatures?
398-399: unclear – explain
402-403: see comment for lines 345-346
412-427: any previous study on the impact of those atmospheric circulation on TP? If yes, to be included
414: why “Ts_20cm” only?
419-420: what is this “crucial impact” for your study zone?
433: “ice content” not “ice, content”
436: “the filtering and ‘memory’ functions of the ground” – unclear, I suggest using actual scientific terms to better identify the processes
438: “quite weak” – unclear, also should be quantified
459-461: isn’t it general knowledge on climate warming?
464-467: accurate, but not specific to the TP – what would be the specific impact on your study zone, from the literature?
470-471: quantify “large amount”
482: I suggest to remove that summary and add a short one at the beginning of the conclusions
Conclusion: some statements are general knowledge
Fig. 1: what is the source for the distribution of permafrost?
Fig. 2: clarify what each panel and symbol is in the caption
Fig. 3: idem
Fig. 5: describe each panel
Fig. 6: describe what values in brackets are, and what “*” means
Fig. 7: describe all panels
Fig. 8: describe panels/units of values; also why are there different number of sites on the different panels?
Fig. 9: common horizontal and vertical scales for all panel would make comparison easier
Fig. 10: not called out in the manuscript at all
Table 1: unclear – clarify in caption what all values refer to
Table 2: clarify how you obtain the correlation values
Table 3: how are those values calculated?
Technical corrections
As a non-native English speaker I found the manuscript to be well written overall, but I lack competence to provide a comprehensive checking of spelling, grammar and sentence construction.
Citation: https://doi.org/10.5194/tc-2023-125-RC2
Status: closed
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CC1: 'Comment on tc-2023-125', Zhiyong Ding, 06 Sep 2023
In this paper, the author quantified the thermal status and trends of both soil temperatures measured at depth and typical permafrost profile temperatures on the Earth’s Third Pole. The topic selection has important research value. They show shallow soil layers in most study regions experienced significant warming from 1981 to 2021, with lower rates at greater depths. Moreover, the deep ground temperature variation at the typical permafrost borehole sites were also analyzed. This study provide a comprehensive understanding of the implications of recent variations in ground temperature across the Tibetan Plateau. However, there are still obvious drawbacks in this study. It is recommended to minor revision the manuscript before publication.
Major comments:
In the introduction section, after providing an overview of previous research, the authors should introduce the core scientific questions to be addressed in this study. Subsequently, they should outline the aspects from which this study intends to make improvements. Authors need to refine this part.
In the Discussion section, there have been previous studies on soil temperature in the Tibetan Plateau as well. The authors should extensively reference relevant literature and highlight the novel findings and innovations of this study.
Ground ice content is mentioned as a crucial element in the discussion but no information on ice content is provided in the analysis. The author needs to explain this.
The authors used some grid climate variables and other variables in attributing the increase in soil temperature. The resolution of these variables is relatively rough, and whether they match the soil temperature observed at the site. Before making correlation analysis, the author needs to first explore the reliability of these grid dataset.
The author believes that some atmospheric circulation plays an important role in the attribution of the interdecadal increase in soil temperature, but the mechanism behind it is not discussed enough. The author should add relevant references for further explanation.
Minor comments:
Some of the legends in the figures need more explanation, and the symbols used in the figures are not clearly described. The formatting of some figures and tables could be improved. For example, Figure 3 should give the literature source of the permafrost distribution map and indicate what figures (a)-(f) represent respectively. The color of soil at different depths in Figure 4 should be consistent with other figures in the paper. Some of the information in Figure 8 partially overlaps with previously published literature, and relevant references should be explicitly cited.
Some descriptions in the conclusion part lack support from data analysis and constitute speculative conclusions. These should be removed. For example, “The extent and variation of profile warming with depth are influenced by multiple factors, including local climate, lithology, and elevation”. The results regarding the lithology of the observation sites are not presented in the text.
The grammar and logical coherence of this article are still lacking. The authors should seek a native English-speaking scholar for comprehensive language refinement.
Citation: https://doi.org/10.5194/tc-2023-125-CC1 -
RC1: 'Comment on tc-2023-125', Anonymous Referee #1, 12 Oct 2023
Comments to the Authors
The manuscript submitted by Wang et al. investigates ground temperature changes over four decades in the permafrost regions of the Tibetan Plateau (TP). The analysis focusses mainly on shallow soil temperatures which tend to respond to short-term fluctuations of climate compared to deeper ground temperatures (which get limited attention in the MS), which filter out these short-term variations and preserve the longer-term changes. The overall topic of the research is relevant to understand the impact of climate change on permafrost regions and the authors appear to have put much effort into the correlation analysis that includes several variables. However, the MS falls short in terms of advancing knowledge relative to several recent studies in the TP region and elsewhere. Much of the information provided in the discussion and conclusions are general statements and for the most part are not unexpected or new. Some statements also lack evidence from the results presented. The MS requires considerable revision for it to be acceptable for publication.
The authors refer to the TP as the largest permafrost region outside of the Arctic and mention the importance of changes in permafrost conditions in the Introduction and Conclusion. Despite the main rationale for the research being the implications of warming on permafrost, there is very little about permafrost throughout the rest of the MS. Although the meteorological stations are distributed throughout the permafrost regions as shown in Fig. 1 this does not necessarily mean that the station is underlain by frozen ground as permafrost may be discontinuous. The figures showing time series (e.g. Fig. 2) of shallow soil temperatures (averages of all site) also provide no information on whether permafrost is present (at the depths considered temperature does not stay below 0°C the whole year based on average temperature). The only sites where the reader can determine whether permafrost is present (at least at some sites) is from the deeper ground temperature profiles acquired from the borehole sites (Fig. 8 and 9). The authors should identify the sites underlain by permafrost and provide some information about the permafrost characteristics especially in the discussion of shallow ground temperature – e.g. what is the active layer thickness or seasonal frost depth. It may have been beneficial to compare permafrost and non permafrost sites in the analysis rather than group all sites together. Ground ice content is mentioned as an important factor in the discussion but no information on ice content is provided in the analysis.
There are several correlations done between shallow soil temperatures and climate and other variables. Although some of the data used is acquired at the same site as the ground temperature data (e.g. air temperature, precipitation), other data is derived from low resolution remote sensing data or national scale gridded databases such as information on vegetation and soils including moisture content. These characteristics can be spatially variable and the conditions at the soil temperature site may be different than that derived from these databases. This may be part of the reason the correlation between soil temperature and these variables is generally low as evidenced by the large scatter shown in the figures. The utility of these correlations is therefore questionable. For meteorological stations in other countries the surface conditions are generally the same for all stations (i.e. open area with similar ground vegetation). It is not clear if conditions are similar for meteorological stations in the TP region and therefore it is difficult to determine whether the correlation analysis between vegetation (NDVI) and soil temperature is that useful.
There are many figures presented in the MS. Some of these require more explanation in the legend and the symbols used are not always adequately described in the legend. The format for some of the figures could be improved. All the figures are probably not necessary as they are presenting the same information, some of which is summarized in tables. Although I have made some comments on figures the authors should consider what is the most effective way to present their results to support the conclusions that are made. The text could also be improved as there are many unclear statements, some of these are identified below but these are only examples.
The MS could be more focussed as it currently tries to cover many things. It is not clear to the reader how this study differs from other similar analysis done in the region using the same data sets, some of which are cited in the MS and also others (e.g. Wang et al. 2021; Zhou et al. 2020; Li et al. 2022 and others), or the new knowledge generated. The authors should consider re-defining their objective to focus on existing knowledge gaps (rather than repeat previous studies), perhaps those identified in studies previously done. This would help in producing an improved MS that would be acceptable for journal publication.
Additional Comments
L30 – Are you referring to air or ground temperature?
L52 – What do you mean by “regulatory mechanisms”? Are you referring to the factors controlling the response of soil temperatures to changes in climate?
L54-55 – This is more a consequence of poor understanding rather than the reason for poor understanding. It is suggested that the comment on oversimplified representation be deleted here.
L66 – Unclear
L67-68 – Several 10s of metres? Most of these studies generally refer to temperatures in the upper 20-30 m.
L71 – How deep? 20 m? 100 m?
L80-81 – This isn’t quite true as conditions in the circumarctic (high latitude) permafrost region vary from relatively thin permafrost at temperatures that are >-1 to -2 °C. Also, not all permafrost is ice-rich. Conditions in some areas maybe quite similar to the TP region.
L84-85 –You should provide a reference for this amplification with elevation such as Wang et al. (2016).
L97 – Here and elsewhere in text - What is meant by layered soil temperature? Usually, we just refer to soil or ground temperatures measured at depth.
L110-112 – Information on accuracy and precision of instrumentation should be provided.
L150-152 – You should have a brief description of instrumentation (including accuracy and precision) used and length of record here.
L190-191 – Unclear
L192-193 – This result is not unexpected and is shown by temperature envelopes in many other publications.
L200 – Do you mean “correlation coefficients”?
L205-207 – Unclear
L209 –It is unclear what you mean in this sentence. Are you referring to the depth at which there is no diurnal variation?
L229-234 – This lag is not unexpected or unknown.
L237-242 – This reversal of gradient is not unknown. There should be some discussion of latent heat effects associated with soil freezing and thawing.
L269 – It is not clear what you mean by middle of the surface soil layer. You seem to be assuming soil is 40 cm thick everywhere which may not necessarily be the case.
L273 – This may be related to changes in thermal conductivity.
L275-276 – If there is less snow then there is less insulation and a more direct link between changes in air and ground temperature.
L279-280 – The type of vegetation is important as it can have an effect on snow cover as well as shading. Do the meteorological stations all have similar vegetation and surface conditions? If so, it is difficult to investigate impacts of variations in vegetation conditions.
L287 – Isn’t the effect on ground temperature indirect as it is related to air temperature and precipitation variation that is influenced by regional atmospheric circulation patterns?
L312-313 – Are these the same 2 sites mentioned above?
L315-319 – Isn’t this just repeating what you have essentially shown for the two sites above?
L318 – Why/how are the trends anomalous?
L322 – Haven’t these results (2001-2010) already been shown in papers such as Zhao et al. (2010 – in your ref. list). Zhao et al. (2020) provide a synthesis and analysis of a longer time-series. No new information on trends appears to be provided here.
L325 – It isn’t really useful to conclude that there were record high temperatures in 2010 given there are more recent data such as that presented in Zhao et al. (2020) or annual State of Climate updates (e.g. Noetzli et al. 2022). Based on longer time-series, record high values occurred more recently.
L326 – What evidence do you have that snow cover is important here? Do you have snow cover data?
L327-329 – What evidence do you have that these are the influences on ground temperature variability. There has not been any information provided regarding some of these variables such as lithology or ice content. These seem to be general statements.
L330-331 – Are you referring to the depth of zero annual amplitude (depth at which there is no seasonal variation), below which temperatures increase with depth according to the geothermal gradient.
L333-335 – Sentence unclear.
L337-339 – Are you referring to latent heat effects here?
L340 – Discussion section. There are several general statements and much of what is presented is not unexpected or unknown. What is the unique contribution of your analysis. What is the new knowledge?
L342-348 – This seems to be a repetition of results. How are these new compared to other studies in the region?
L349-352 – Isn’t TP included in some of these publications? There have been other recent global syntheses with more up to date data (some are cited in the ref list) that provide a clear indication of global warming including Noetzli et al. (2022), Smith et al. (2022) and most recent IPCC report (e.g. Gulev et al. 2021).
L352-365 – What is the new information here as much of this is already known from other studies.
L368-371 – What do you mean by the deepest soil layers. Are you referring to the near-surface temperatures or depths of several to 10s of metres? It isn’t clear what you mean here. The deeper ground temperatures do respond to air temperature, but they respond to the longer-term variation rather than the higher frequency fluctuations. The lag time increases with depth and this could also be the reason for poorer correlation if you compare air and ground temperatures measured at the same time.
L377 – I didn’t see anything about the onset of snow cover in the results, only snow duration (which does not give timing of onset). It isn’t clear how you arrive at this conclusion. Other studies have examined the role of timing of snow cover on ground thermal regime such as Zhang et al. (2019), Noetzli et al. (2020); Palmer et al. (2012); Morse et al. (2012).
L389-391 – What type of vegetation. What about shading of the ground (cooling effect) or effects vegetation has on snow cover? Are you referring to radiation absorption by vegetation/canopy? See earlier comment regarding vegetation and surface conditions at meteorological sites.
L393 – You refer to permafrost here and briefly elsewhere, but you don’t say anything in the MS about the permafrost conditions. Is permafrost present at all sites? This is not clear from the information provided. What about ground ice conditions?
L394-399 – This is an issue when using site specific data and national scale databases together. Soil conditions including organic content are spatially variable so without site-specific data this analysis isn’t very useful.
L401 – See earlier comments – Isn’t this really related to the effect of air temperature and precipitation on ground temperature since atmospheric circulation is related to these meteorological variables (i.e. the effect on ground temperature is indirect).
L402-403 – Are you referring to elevational amplification? See earlier comment for reference.
L405-406 – Does this have to do with exposure to wind?
L428 – Permafrost conditions have not really been mentioned much throughout the MS (see earlier comment).
L429 – “Seasonal permafrost” is incorrect terminology as permafrost is not seasonal. Are you referring to seasonal frost? Revision is required.
L429-448 – There is much repetition here of material presented earlier. There are also many general statements and not much new presented.
L438-439 – The deeper temperatures preserve the longer-term trends in temperatures at the ground surface with the lag time increasing with depth.
L444-445 – Are you referring to an isothermal temperature profile due to the phase change?
L449-452 – There is no information presented on permafrost properties such as ice-content and nothing about the deeper material properties. General statements are being made here with no evidence from your results.
L459 – Soil temperatures are ground temperatures.
L474-478 – Some explanation is required here. Is there even permafrost under the glaciers. Isn’t this warming of the glacier due more to geothermal heat flux as well as changes in air temperature and snowfall?
Figure 1 – The colour scale for elevation is the reverse of what is normally used. It might be good to identify which study sites are underlain by permafrost.
Figure 2 – Couldn’t the information on trends be summarized better in a table rather than in the figures on the right side. Information on trends at individual sites for various depths is already shown in figure 3.
Figure 3 – Reduction of categories would be helpful as would using red for warming and blue for cooling. It isn’t easy for the reader to distinguish between the symbols the way the information is presented.
References
Gulev SK, Thorne PW, Ahn J, Dentener FJ, Domingues CM, Gerland S, Gong D, Kaufman DS, Nnamchi HC, Quaas J, Rivera A, Sathyendranath S, Smith SL, Trewin B, von Schuckmann K, Vose RS (2021) Changing State of the Climate System. In: Masson-Delmotte V, Zhai P, Pirani A et al. (eds) Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp 287-422. doi:10.1017/9781009157896.004
Li, Y. et al. 2022 Analysis on the temporal and spatial characteristics of the shallow soil temperature of the Qinghai‑Tibet Plateau. Scientific Reports 12:19746 https://doi.org/10.1038/s41598-022-23548-4
Morse PD, Burn CR, Kokelj SV (2012) Influence of snow on near-surface ground temperatures in upland and alluvial environments of the outer Mackenzie Delta, Northwest Territories. Canadian Journal Earth Sciences 49:895-913. doi:10.1139/E2012-012
Noetzli J, Christiansen HH, Guglielmin M, Hrbáček F, Isaksen K, Smith SL, Zhao L, Streletskiy DA (2022) [Global Climate] Permafrost Thermal State [in "State of the Climate in 2021"]. Bulletin of the American Meteorological Society 103 (8):S41-S43
Noetzli J, Christiansen HH, Isaksen K, Smith S, Zhao L, Streletskiy DA (2020) [Global Climate] Permafrost Thermal State [in "State of the Climate in 2019"] Bulletin of the American Meteorological Society 101 (8):S34-S36
Palmer MJ, Burn CR, Kokelj SV (2012) Factors influencing permafrost temperatures across tree line in the uplands east of the Mackenzie Delta, 2004–2010. Canadian Journal of Earth Sciences 49:877-894. doi:10.1139/E2012-002
Smith SL, Romanovsky VE, Isaksen K, Nyland KE, Kholodov AL, Shiklomanov NI, Streletskiy DA, Drozdov DS, Malkova GV, Christiansen HH (2022) [Arctic] Permafrost [in "State of the Climate in 2021"]. Bulletin of the American Meteorological Society 103 (8):S286-S290. doi:10.1175/BAMS-D-22-0082.1
Wang Q, Fan X, Wang M (2016) Evidence of high-elevation amplification versus Arctic amplification. Scientific Reports 6:19219. doi:0.1038/srep19219
Wang X, Chen R, Han C, et al. Response of shallow soil temperature to climate change on the Qinghai–Tibetan Plateau. Int J Climatol. 2021;41:1–16. https://doi.org/10.1002/joc.6605
Zhang G, Nan Z, Wu X, Ji H, Zhao S (2019) The role of winter warming in permafrost change over the Qinghai-Tibet Plateau. Geophysical Research Letters 46:11261-11269. doi:10.1029/2019GL084292
Zhao L, Zou D, Hu G, Du E, Pang Q, Xiao Y, Li R, Sheng Y, Wu X, Sun Z, Wang L, Wang C, Ma L, Zhou H, Liu S (2020) Changing climate and the permafrost environment on the Qinghai–Tibet (Xizang) plateau. Permafrost and Periglacial Processes 31:396-405. doi:10.1002/ppp.2056
Citation: https://doi.org/10.5194/tc-2023-125-RC1 -
RC2: 'Comment on tc-2023-125', Meven Philippe, 07 Nov 2023
General comments
This manuscript investigates warming trends in ground temperatures over the Tibetan Plateau, and their relationships with various environmental factors. In this respect, the amount of data collected and the effort put into the evaluation of statistical correlations between the variables has to be praised.
However, the manuscript currently shows significant flaws that will require consequent reflexion and revision before publication. Overall, it would benefit from a review of previous studies done on the TP, that will help identifying knowledge gaps, defining scientific objectives and focussing the study. Methodological processes and figure/table captions must be detailed more rigorously.
Specific comments
Major comments
- As stated above, this study lacks a well-defined scientific objective. It is reflected throughout the manuscript with sections (Introduction, Results and Discussion) that cover a wide variety of subjects (atmospheric circulation, vegetation, organic carbon content…), making the whole manuscript to lack consistency. This is also reflected by some statements repeated two or three times among the Results & Discussion sections. Therefore, I suggest to add a paragraph at the end of the Introduction, detailing the state-of-the-art concerning the ground thermal regime in the TP. This will lead to the identification of current knowledge gaps, and the definition of one (or several) specific scientific objectives at the end of that section. I believe this will also help maintaining the manuscript focus overall.
- The Method section needs to be refined as is currently lacks explanation and details on the data acquisition and processing. For instance, the specific temperature measurement method is not specified (sensors in boreholes? In fractures?); biased are identified in the section (e.g. inconsistent consideration of the 0cm-elevation) but it is not specified how they were accounted for; the “strict quality control assessments” (l. 118) are not explained; the time series which the study is based on was selected as it has “relatively little missing data”, without quantification; the reason why increasing measurement depth reduces the availability of shallow data (l. 120-121) is not specified.
- Presented equipment and data also lack associated accuracy and errors. The models and accuracy of the thermal sensors must be stated, and the associated error on data must be shown in Tables (± X) and in Figures (with error bars).
- Some interpretations made in the Discussion are global knowledge on the thermal regime of ground or permafrost ground (e.g. delayed warming with depth, higher-amplitude fluctuations at shallower depths). Overall it is not clearly highlighted what this manuscript brings new – which is also a point where a comparison of this study with the state-of-the-art would be helpful.
- Figures and Tables need their captions to be refined. Overall, explanations lack about the significance of some values (for tables), symbols and colours (for figures), which makes the understanding sometimes challenging. Some examples are identified in ‘Minor comments’, but this list is not comprehensive.
- Supplementary Material should give accessibility to the raw data used to make figures and tables.
- An overall lack of references leaves some statements without justification. Some examples are identified in ‘Minor comments’, but this list is not comprehensive.
Minor comments
Overall, descriptions are too qualitative (e.g. l. 438 “the warming trend (…) becoming quite weak”; l. 114-115 “relatively little missing data”). It is generally good to avoid such statements, or to justify them with quantitative arguments.
Further minor comments (in reading order):
24-25: “typical” from the TP? From permafrost ground in general?
27-28: statement here is general knowledge
35-38: general knowledge
72-74: this statement is even fairly certain, also it lacks references
82: “cold” or “periglacial” instead of “frigid”; also be more specific about the process(es)
83-85: lack of references
88-89: more specific about the process(es)
Section 2.1.: accuracy of data and sensors is lacking
109-110: was this bias accounted for? If yes, how?
114-115: quantify “relatively little missing data”
118-120: those “strict quality control assessments” must be detailed, as they seem to be part of the methodological process. Also quantitatively define “high-quality and long-term sites”
120-121: explanation lacks
122-123: accuracy lacks; also specify what is “daily”? One measurement/day? An average of several measurements?
122-140: details lack on how the meteorological parameters are calculated, and on their accuracy/error
125: how do you calculate MAST?
126-127: how do you calculate the “annual mean of daily snow depth”?
141-148: what are those indices? Why those nine were selected specifically?
149-152: where are those borehole sites? How many are they? How is ground temperature calculated, as a mean of temperature at various depth?
170-171: unclear statement
202-204: statements are not quantitatively assessed
271-273: if non-significant, why mentioning the results?
Section 3.6.: describe how measurements were performed, with what accuracy and with which equipment
Discussion: some statements are general knowledge
345-346: aren’t higher elevations linked with overall lower mean temperatures?
398-399: unclear – explain
402-403: see comment for lines 345-346
412-427: any previous study on the impact of those atmospheric circulation on TP? If yes, to be included
414: why “Ts_20cm” only?
419-420: what is this “crucial impact” for your study zone?
433: “ice content” not “ice, content”
436: “the filtering and ‘memory’ functions of the ground” – unclear, I suggest using actual scientific terms to better identify the processes
438: “quite weak” – unclear, also should be quantified
459-461: isn’t it general knowledge on climate warming?
464-467: accurate, but not specific to the TP – what would be the specific impact on your study zone, from the literature?
470-471: quantify “large amount”
482: I suggest to remove that summary and add a short one at the beginning of the conclusions
Conclusion: some statements are general knowledge
Fig. 1: what is the source for the distribution of permafrost?
Fig. 2: clarify what each panel and symbol is in the caption
Fig. 3: idem
Fig. 5: describe each panel
Fig. 6: describe what values in brackets are, and what “*” means
Fig. 7: describe all panels
Fig. 8: describe panels/units of values; also why are there different number of sites on the different panels?
Fig. 9: common horizontal and vertical scales for all panel would make comparison easier
Fig. 10: not called out in the manuscript at all
Table 1: unclear – clarify in caption what all values refer to
Table 2: clarify how you obtain the correlation values
Table 3: how are those values calculated?
Technical corrections
As a non-native English speaker I found the manuscript to be well written overall, but I lack competence to provide a comprehensive checking of spelling, grammar and sentence construction.
Citation: https://doi.org/10.5194/tc-2023-125-RC2
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