Enhanced warming of European mountain permafrost in the early 21st century

Domain: earth_science
Published: 2024-12-10
Editorial outcome: major_revision

Abstract
Mountain permafrost, constituting 30% of the global permafrost area, is sensitive to climate change and strongly impacts mountain ecosystems and communities. This study examines 21st century permafrost warming in European mountains using decadal ground temperature data from sixty-four boreholes in the Alps, Scandinavia, Iceland, Sierra Nevada and Svalbard. During 2013–2022, warming rates at 10 metres depth exceed 1°C dec−1 in cases, generally surpassing previous estimates because of accelerated warming and the use of a comprehensive dataset. Substantial permafrost warming occurred at cold and ice-poor bedrock sites at high elevations and latitudes, at rates comparable to surface air temperature increase. In contrast, latent heat effects in ice-rich ground near 0°C reduce warming rates and mask important changes of mountain permafrost substrates. The warming patterns observed are consistent across all sites, depths and time periods. For the coming decades, the propagation of permafrost warming to greater depths is largely predetermined already.

Introduction
While the retreat of glaciers in the 21st century has been widely recognised1,2, permafrost changes in cold regions worldwide are far less visible despite being similarly important3–12. Permafrost is a key component of the cryosphere and is defined thermally and temporally as ground with a maximum temperature of 0°C over at least two consecutive years. Mountain permafrost constitutes 30% of the global permafrost area13,14 and spans low to high latitudes in both hemispheres and all continents. Complex topography leads to large environmental gradients in these mountain areas and controls the permafrost distribution e.g., refs. 15,16. Mountain permafrost is highly sensitive to the pronounced atmospheric warming observed at high elevations17. This permafrost warming and degradation has implications for the stability of steep, perennially frozen mountain slopes18–24 with potentially far-reaching impacts and risks to both human safety25–27 and infrastructure28, as well as effects on ecosystems29–32 and hydrological processes33–36.

The assessment of permafrost changes relies on long-term records of ground temperatures measured in boreholes, which are the direct thermal observations defined as one of the products of the Essential Climate Variable (ECV) permafrost37–39. Temperatures near the ground surface follow the fluctuations in atmospheric conditions. These variations are increasingly attenuated and delayed with depth. At the Depth of Zero Annual Amplitude (DZAA) and below, seasonal temperature changes become negligible and variations result from long-term climate-related changes. Near the DZAA, Biskaborn et al.3 reported global permafrost warming of 0.29°C dec−1 and of 0.19°C dec−1 for mountain regions during the period 2007–2016. The study included 28 mountain permafrost sites, of which a large part is located in Europe. This is where the most densely populated permafrost landscapes are found and where – motivated by the growth of tourism and infrastructure development – systematic research on mountain permafrost commenced in the late 1970s40–42 for a review see ref. 43. Climate-related European permafrost monitoring was initiated in the late 1990s with a borehole transect from high Arctic Svalbard to the Mediterranean Sierra Nevada Massif7,44. Although several national and regional permafrost observation programs have been established in recent decades45–51 and activities in the framework of the Global Terrestrial Network for Permafrost (GTN-P) have increased37, there is no pan-European coordination and synthesis of mountain permafrost temperature data to date. Regional and local studies point to relevant differences in permafrost warming rates due to the high variability in topographic and (sub-)surface conditions in mountain regions7–9,11,52–54. However, they are typically restricted to single sites or regions and are not directly comparable.

In this study, we present warming patterns of mountain permafrost across Europe up to the year 2022. We compiled a consistent dataset of 64 ground temperature time series measured for at least one decade in or near permafrost areas. Data are obtained in boreholes extending at least 10m in depth and the sites cover a wide range of landforms, elevations and latitudinal zones from high Arctic Svalbard and Scandinavia to Iceland, the Alps and the Sierra Nevada. To assess the spatial and temporal variability of warming patterns, we derive warming rates based on annual mean values of ground temperatures (MGT1yr) for recent 10- and 20-year periods (2013–2022 and 2003–2022) at 5, 10 and 20m depth. We find evidence for permafrost warming for all considered regions, depths, and time periods. The warming rates identified on the basis of the comprehensive and current data set exceed previous estimates for mountain permafrost3 and match the levels observed in the permafrost of the Arctic hotspot regions4. The warming patterns are primarily dependent on the depth of observation and the exchange of latent heat for phase change. In ice-bearing ground, the latter can significantly reduce warming rates when temperatures approach 0°C thereby masking important changes such as increasing water content or ground ice melt. Warming rates are highest at cold permafrost sites with low ground ice content, where latent heat effects are largely absent. Further, warming rates in the uppermost 10m are higher than at greater depths, where temperatures react with delay. These patterns prove consistent across all depths and time periods considered and surpass regional differences.

Results
Measured ground temperatures in European mountains
Our data compilation provides decadal time series of ground temperatures measured in 64 boreholes at 39 sites in European mountain permafrost regions between 37 and 78° N and at elevations between 275 and 3800 masl., and in nine countries55 (Fig. 1, Table 1, Supplementary Figs. S1–S2, for detailed data origin see Supplementary Table S1). Two boreholes are situated on Svalbard, 13 in Scandinavia (Norway, Sweden), four on Iceland, 15 in the Eastern and Central Alps (Austria, Germany, Switzerland, Italy), 29 in the Western Alps (Switzerland, Italy, France) and one in the Spanish Sierra Nevada. Data from additional sites in these regions exist, but they do not meet the minimum length or observation depth. No data are available yet from other European mountain regions such as the Pyrenees. Sites are in landforms typically found in mountain permafrost such as talus or bedrock slopes, rock crests, plateau blockfields, rock glaciers, or moraines. Nine boreholes are not in permafrost from the start of observations but are located close by. They are included in the analyses for comparison. The longest time series is measured in rock glacier Murtèl-Corvatsch and covers 35 years. Four time series from the Western and Central Alps only cover a decade.

Permafrost temperatures recorded in European mountains range from cold permafrost with a minimum MGT1yr of −6.2°C at 10m depth to very warm permafrost with MGT1yr at or close to 0°C (Fig. 2). MGT1yr below −4°C are measured at the highest latitudes and elevations in High Arctic Svalbard (Breinosa, Janssonhaugen) or in North-facing bedrock slopes at elevations above 3500 masl. in the Western Alps (Jungfrau Ridge, Aiguille du Midi). MGT1yr below −12°C were measured near the surface on northern slopes in the Western Alps at elevations well above 4000 masl.52, indicating very cold permafrost conditions. However, long-term observations at depth from such locations are not available to date. Permafrost with temperatures close to 0°C is found in all European mountain regions, mainly in rock glaciers and unconsolidated sediments at elevations around 2500 masl. in the Alps, or in blockfields and debris at elevations around 1500 masl. in Scandinavia (e.g., at Bellecombes, Gentianes, Les Attelas, Dovrefjell, Juvvasshoe, or Tron). Permafrost temperatures measured in the Central and Eastern Alps are not as low as in the Western Alps. This is due to the lower elevations in the former regions and the lack of observations >3000m asl., and less due to climatic differences. MGT1yr at the non-permafrost sites attain >3°C (Veleta Peak and the south-exposed rock face at Gemsstock).

42 of the 57 time series (74%) with available MGT1yr data at 10m depth for one of the past three years (2020–2022) reached their maximum value in these 3 years (Fig. 2). Similarly, 27 time series out of 36 (75%) reached their maximum MGT1yr at 20m depth between 2020 and 2022 (Supplementary Fig. S3). The two sites on Svalbard (the time series with the lowest temperatures in Fig. 2a) experienced their maximum before 2020 due to the extremely warm years 2016–201856. MGT1yr at 10m depth increased to above 0°C during the measurement period for 10 time series (17%), i.e. permafrost is no longer present at these sites and this depth.

Permafrost warming rates
Temperature change rates at 10m depth for the period 2013–2022 (or for the latest decade if time series extend at least to the year 2018, which is the case for 9 time series) range from −0.10 to 1.77°C dec−1 (Table 1, n = 59, 58% of p-values < 0.05). The mean warming rate for all sites where permafrost is present at this depth for at least part of the decade is 0.41°C dec−1 (n = 50, median 0.31°C dec−1, standard error of 0.05°C dec−1, 60% of p-values < 0.05). For 42% of the permafrost time series, the warming rate is higher than 0.5°C dec−1 (n = 21, 86% of p-values < 0.05). The highest 20% of the warming rates are above 0.7°C dec−1 and are typically measured in bedrock slopes or where permafrost is cold or absent (n = 10, 90% of p-values < 0.05). Warming rates can also be high in non-bedrock slopes where permafrost has disappeared during the 10-year period. In fact, the maximum value of 1.77°C dec−1 was observed for Dovrefjell DB6 in southern Norway. Warming rates lower than 0.5°C dec−1 are typically observed in ice-bearing talus slopes and blockfields or in rock glaciers (n = 25, 44% of p-values < 0.05). Cooling rates were obtained for 4 locations in warm or near-zero permafrost in ice-bearing ground. They are, however, lower than 0.1°C dec−1 (p-value < 0.05 only for one time series).

At depths greater than 10m and over a longer time period, calculated permafrost warming rates are generally lower and statistically more robust: at 20m depth they range up to 0.71°C dec−1 for 2013–2022 (mean 0.24°C dec−1, n = 33, 73% of p-values < 0.05) and for the 20-year period 2003–2022 at 10m depth up to 0.85°C dec−1 (mean 0.39°C dec−1, n = 16, 94% of p-values < 0.05). A non-parametric Wilcoxon signed rank test on paired samples of warming rates at 10m and 20m depth shows a significant difference (0.05 level) for both 10-year (n = 35, mean difference 0.13°C dec−1) and 20-year periods (n = 13, mean difference 0.12°C dec−1). This shows a delayed propagation of the recent acceleration of the warming to greater depths resulting from slow heat conduction. For the time period 2007–2016, Biskaborn et al.3 found in the global-scale analysis an average mountain permafrost warming rate of 0.19°C dec−1 at the DZAA (mean of 14m). Based on the time series in our dataset that cover the period 2007–2016, we obtain a mean warming rate of 0.39°C dec−1 at 10m depth (n = 26, 65% of p-values < 0.05) and of 0.24°C dec−1 at 20m depth (n = 20, 85% of p-values < 0.05) for 2007–2016, and slightly higher values of 0.40 and 0.26°C dec−1 for 10 and 20m depth (64% and 84% of p-values < 0.05) for 2013–2022, respectively.

Warming rates vary between time periods depending on the interannual fluctuations of MGT1yr. This variability is shown by warming rates over a moving window of 10 and 20 years (Fig. 3, Supplementary Fig. S4). At 5m depth, the warming rates can fluctuate by more than 1.5°C dec−1 when moving a 10-year averaging window by 5 years. They are thus not suited to describe long-term changes in the permafrost. Decadal warming rates for 10 and 20m depth exhibit lower variation and higher significance, but can still vary up to nearly 1.0 and 0.5°C dec−1, respectively. Considering 20-year periods, the variability is clearly lower than for decadal warming rates. Figure 4 shows the most recent values (cf. diamonds) in the context of the variability of decadal warming rates at 10m depth for all boreholes of the dataset. This variability is however influenced by the length of the time series. At the coldest sites and for some sites in Svalbard, for example, most recent decadal warming rates are lower than those obtained for earlier 10-year periods.

Permafrost warming patterns
For the latest available decade of data, mean permafrost warming rates at 10m depth are 0.36°C dec−1 in the European Alps (n = 34), 0.40°C dec−1 for Svalbard (n = 2), 0.63°C dec−1 for Scandinavia (n = 11), and 0.2°C dec−1 for Iceland (n = 3). Differences related to the thermal conditions (cf. permafrost classes), and ground ice content surpass these regional differences (Fig. 5). We observe a general decrease in warming rates with ground temperatures increasing towards 0°C (Fig. 6). Cold permafrost warms at a mean rate of 0.67°C dec−1 (n = 9), compared to 0.35°C dec−1 for warm permafrost (n = 13) and 0.13°C dec−1 for near-zero permafrost (n = 16). Ground temperatures at sites where permafrost is absent or has disappeared during the considered time period are increasing by 0.60°C dec−1 on average (n = 21). Warming rates in cold permafrost, at ice-poor bedrock sites and at non-permafrost sites are in a similar range and correspond to warming rates of surface air temperature (SAT) over the past 30 years (Fig. 5, cf. 'Methods' for details).

The classification by ground ice content (cf. ‘Methods’) confirms this pattern: For ice-rich rock glaciers, warming rates at 10m depth are very low (0.05°C dec−1, n = 6, 17% of p-values < 0.05) and ground temperatures can remain just below 0°C for many years until the ground ice has melted at the respective depth (e.g., Ritigraben, Schafberg, Bellecombes). For ice-bearing sites like talus slopes or blockfields, the mean warming rate is 0.34°C dec−1 (n = 14, 43% of p-values < 0.05). Once the ground ice has melted, warming rates substantially increase as the buffering effect of latent heat is no longer effective (cf. thawed sites in Fig. 4 and sites which originally contained ice with MGT10/20yr > 0°C in Fig. 6). Sites with little or no ground ice typically warm at rates of >0.5°C dec−1 (the mean warming rate for ice-poor sites is 0.51°C dec−1 with n = 30 and 77% of p-values < 0.05 and the mean warming rate for the sites without ice is 0.55 with n = 9 and 44% of p-values < 0.05). The classification following surface cover characteristics also yields a similar pattern. Surface cover characteristics are typically related to the type of landform and, hence, to the ground ice content.

The patterns described above refer to the latest decade and a depth of 10m but prove consistent across all considered depths, time periods and time spans (cf. Supplementary Figs. S5–S9).

Seasonal distribution of permafrost warming
Examination of monthly ground temperatures at 10m depth reveals distinct differences in warming rates over the course of the year. They are characterized by prominent months for both the highest and lowest values and show a typical spread of 0.2–0.4°C dec−1 (Fig. 7). The patterns observed for annual values based on the classification by permafrost conditions, surface cover and ground ice content also match with those derived for monthly warming rates. On average, the maximum monthly warming rate in cold permafrost was 0.74°C dec−1. The highest monthly warming rates are observed at 10m depth at Aiguille du Midi NE in the Western Alps (monthly warming rate of 1.1°C dec−1 in November, with a phase lag of five months with respect to the ground surface, cf. Supplementary Table S1) and at Janssonhaugen BH10 in Svalbard (monthly warming rate of 0.9°C dec−1 in June, with a phase lag of six months with respect to the ground surface).

Methods
Measurement of permafrost temperatures
Ground temperature time series in mountain permafrost are obtained in boreholes of around 10–100m depth, which are drilled vertically in most cases, but also horizontally through crests or perpendicular to the surface in near-vertical bedrock slopes. Temperatures are continuously and automatically logged at different depths with permanently installed multi sensor strings and at intervals of 1–24h. Thermistors are the most widely used sensors for permafrost temperature measurements. Temperature sensor strings are typically calibrated in an ice-water bath at 0°C with the logging system prior to installation in the borehole. The drilling and instrumentation of boreholes in harsh, cold environments with difficult access, and their long-term operation over decades are technically and logistically challenging. Guidelines on the measurement procedure were recently published38,78,79.

Typical measurement accuracies reported for permafrost temperature measurements vary depending on the sensor types and logging system between ±0.01 and ±0.25°C3,38,78. Gaps in the time series can occur due to power failure or damage from moisture intrusion, natural hazards (storms, lightning, avalanches, rockfalls), or disruption by animals. Potential data inconsistencies may result from changes in the measurement setup (such as replacement of sensor strings and loggers, or change of maintenance procedures), sensor drift over time, moisture or water intrusion into the borehole or instruments (particularly for sites with temperatures close to 0°C with higher unfrozen water contents), or damage due to ground deformation. Re-calibration of the system to assess sensor drift is often not possible as sensor strings can be stuck in the borehole, particularly in ice-rich deforming ground such as rock glaciers83. Data quality control primarily relies on value-range and consistency checks, as well as on plausibility assessment based on data from nearby sensors or stations. A single drifting sensor is detectable through its anomalous temporal trend compared to neighbouring sensors.

Data collection and processing
Ground temperature time series measured in boreholes in European mountain permafrost regions were collected from national networks such as the Swiss Permafrost Monitoring Network (PERMOS, www.permos.ch), the Norwegian Permafrost Monitoring Network (cryo.met.no and sios-svalbard.org), and the Réseau français d’observation du permafrost (PermaFrance, permafrance.osug.fr). Additional collected data from regions without a national repository were maintained on the long-term by research institutions (such as the Universities of Barcelona, Insubria, Oslo and Stockholm) and regional environment agencies (such as GeoSphere Austria, the Regional Environmental Protection Agencies in Northern Italy or the Bavarian Environment Agency). Many of these networks and institutions contribute to the Global Terrestrial Network for Permafrost (GTN-P, gtnp.arcticportal.org). Detailed data sources for each time series are provided in Supplementary Table S1.

The criteria to include a ground temperature time series in this study were: (i) the borehole is in permafrost region in mountain terrain (MGT1yr ≤ 0°C for two consecutive years), (ii) reaches a depth of at least 10m, and (iii) the time series covers at least one decade until 2022. For comparison to changes in non-permafrost terrain, we additionally considered data from boreholes located in close vicinity of permafrost.

We performed basic quality checks on daily ground temperature time series and consistently aggregated them to monthly (MGTm) and annual mean values (MGT1yr). Due to the decreasing temporal variability of ground temperatures with depth, we defined depth-dependent criteria for data completeness: depth ranges with daily fluctuations (above 2m depth, 12 MGTm values required to calculate MGT1yr), with seasonal fluctuations (2–17m depth, 10 MGTm required for MGT1yr) or with only intra-annual fluctuations (below the DZAA at ca. 15m, 4 MGTm required).

For the analyses of warming patterns, borehole sites were classified based on region, surface cover, permafrost conditions and ground ice content (cf. Table 1, Supplementary Table S1). For the latter, usually only little and qualitative information is available from drilling logs or semi-direct geophysical soundings. This means that only a basic classification related to the amount of ground ice is possible. However, this can be used for a general assessment of the influence of latent heat exchange during warming. We therefore considered whether ground ice is only present in rock fissures and pores as is the case in bedrock, if there is considerable ground ice as is possible in unconsolidated sediments, or if excess ice is present, as in rock glaciers. We distinguished the four classes no ice (referring here to boreholes that are not in permafrost), ice-poor (ground ice only in pores or clefts; ground ice content up to around 10 Vol.%), ice-bearing (considerable and varying ground ice in unconsolidated sediments, ground ice content about 10–50 Vol.%), and ice-rich (ice supersaturation in rock glaciers; ground ice content up to 100 Vol.%).

While surface cover and ground ice content were considered a constant characteristic for a borehole site, permafrost conditions were assigned for each period and depth. In addition to the classical distinction between warm and cold permafrost4, we considered that the portion of unfrozen water, and hence the exchanged amount of latent heat, increases exponentially when the ground temperature increases towards 0°C e.g., refs. 84,85. Most of the phase change takes place at temperatures very close to 0°C. We distinguished the following four classes: cold permafrost with a mean ground temperature for the 10-year or 20-year period (MGT10y or MGT20y) below −2°C (negligible latent heat effects expected), warm permafrost with MGT10y/20yr ≥ −2°C and < −0.5°C (small latent heat effects, depending on ground freezing characteristics), and near-zero permafrost with MGT10y/20yr > −0.5°C and maximum MGT1yr ≤ 0°C (considerable latent heat effects during phase change expected for locations containing substantial ground ice). We distinguished no permafrost from near-zero permafrost when the maximum MGT1yr is > 0°C. That is, time series for which permafrost was present at the beginning but not at the end of the considered period, were classified as no permafrost (and as thawed permafrost in Fig. 4).

Calculation of permafrost warming rates
With increasing depth, the evolution of the permafrost thermal regime integrates temperature changes registered over a longer time period, over a larger surface area and with increasing delay. In previous work, permafrost warming rates were often derived at the Depth of Zero Annual Amplitude DZAA e.g., refs. 3,4. This depth is particularly important when relying on irregular and infrequent manual temperature readings6,78,86. However, modern logging systems record quasi-continuous temperature time series allowing for seasonal fluctuations to be factored out. In addition, the DZAA varies between sites and over time with changing surface temperatures and some boreholes, e.g. in near-vertical bedrock, do not reach this depth. We analysed warming rates at three key depths defined for climate related long-term permafrost observation: 5, 10, and 20m78. Ground temperatures closely follow surface temperature variations at 5m depth and are characterized by seasonal variations at 10m depth. The latter are no longer observed at 20m depth, which is generally below the DZAA. Due to different sensor depths in the individual boreholes, time series were assigned to three depth classes using the sensors closest to the key depths. This assignment was manually adapted for a few boreholes because of longer gaps at the closest sensor or specific site characteristics (e.g., to correct for differences between the borehole depth and the actual distance to the surface in steep terrain). Horizontal boreholes that completely pierce a ridge (i.e., the boreholes on Zugspitze and Gemsstock) were considered as two boreholes, one half on each side.

We calculated warming rates for the three depth classes for each time series of MGT1yr using ordinary least squares (OLS) linear regression in the R-environment. We did this for the latest available 10-year and 20-year period (2013–2022 and 2003–2022). To assess how much the obtained warming rates depend on the time period and relate them to values from previous time periods, we further calculated a running warming rate over a 10- and 20-year moving window starting from the first available year. Thus, we obtained warming rates for all available 10-year and 20-year time periods and for 1–3 depth classes for each borehole. For a non-parameteric data distribution, the Theil-Sen Slope (TSS) is often preferred over the OLS method as it is less sensitive to outliers. The comparison of the results from applying both methods, however, showed marginal differences (mean difference for all warming rates < 0.02°C dec−1, median difference < 0.001°C dec−1). Exceptions are ice-bearing sites with a sudden temperature increase above 0°C after the ground ice has melted and the latent heat effect ceases to play a role. In such a case, the OLS method is preferred since such a temperature increase is not an outlier but rather a relevant signal. This approach has also been widely applied to determine permafrost temperature warming rates3,4,7,8.

Following Biskaborn et al.3, we applied the following data completeness criteria to calculate a warming rate for a given 10-year or 20-year period: one MGT1yr value at the start of the period (±1 year), one at the end of the period (±1 year) and a minimum of 5 (10 years) or 10 values (20 years). It is important to note that while the number of MGT1yr values used for a linear regression is relatively small (n of 5–10 for 1 decade, 10–20 for 2 decades), these are not independent individual observations. We argue that the aggregated annual data allow for the application of a linear regression to time series covering one decade. Indeed, each value aggregates daily observations over one year and short-term variations are filtered with increasing depth. Further, due to slow thermal diffusion, ground temperatures at depth are the result of a signal emerging over a longer period of time. While the temporal aggregation has little effect on the obtained warming rates, it can reduce the statistical significance of the results, which should be considered when interpreting p-values. To test this, we additionally derived decadal warming rates based on monthly and daily time series for several locations with p-value > 0.05, which yielded significant p-values for most tested time series using monthly data, and for all time series using daily data.

To better understand the seasonal distribution of warming, we examined linear change rates in monthly temperatures for the latest 10-year and 20-year periods (2013–2022 and 2003–2022) near the ground surface (at ca. 0.2m depth), at 5m and at 10m depth. Because some locations lacked data at 5m depth, 10m depth was selected for subsequent analyses and the calculation of intra-annual amplitudes. The maximum and minimum monthly values were calculated, along with the corresponding months in which they occurred. To identify the month with the highest change rate near the ground surface, we analyzed the month with the maximum warming rate at 10m depth and corrected for the corresponding phase lag in relation to the surface (cf. Supplementary Table S1). To do so, the signal speed of the annual temperature wave was determined by calculating the time difference between the average monthly maximum temperature at approximately 0.2m and 10m depth.

Time series of annual mean surface air temperature (SAT1yr) from weather stations located at or close to several permafrost boreholes were used to derive SAT change rates. To avoid effects of local topography or valleys (such as temperature inversions), we used data from three high-elevation weather stations of the Federal Office for Meteorology and Climatology MeteoSwiss (Piz Corvatsch at 3294m asl., Jungfraujoch at 3571m asl. and Weissfluhjoch at 2691m asl.). In addition, we used three mountain weather stations of the Norwegian Meteorological Institute (Juvvasshøe at 1894m asl., Fokstugu at 973m asl. and Iskoras at 591m asl.) and one station on Svalbard from The University Centre in Svalbard (Janssonhaugen at 275m asl.). The sites are considered representative for three larger regions: the European Alps, Scandinavian mountains and Svalbard. The high-resolution Norwegian reanalysis named NORA387 was employed to extend the Scandinavian and Svalbard SAT datasets (except Fokstugu) to cover 30 years. NORA3 was selected as it is currently the only high-resolution hindcast available to cover both the entire Scandinavian Peninsula and Svalbard up to 2023. Additionally, it was shown to outperform both the reanalysis ERA5 and the earlier hydrostatic 10-km Norwegian Hindcast Archive (NORA10), especially in complex terrain87. For direct comparison with the original instrumental observations and to derive a value at the station locations, the NORA3 data series were adjusted by regression analysis (R2 0.95–0.97). We applied linear regression using the OLS method as for permafrost time series to derive SAT change rates, but for a period of three decades (1993–2022) due to the higher interannual variability of SAT compared to the naturally filtered ground temperatures. The application of the TSS method led to negligible differences for SAT change rates compared to the OLS method.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "General comments to authors and editor\nIn the submitted manuscript by Noetzli et al. titled “Enhanced warming of European mountain permafrost in the\nearly 21st century” the authors do a nice job of highlighting a rich and diverse dataset in multiple European regions\nto show that permafrost and depth temperatures have warmed in the early part of this century. The manuscript is a\ngood study with a list of notable and quality authors well known in the permafrost community. Although the paper\ndoes not report on anything too surprising in many ways it articulates the impact and magnitudes of warming in\nthe mountains of Europe very well. The collection of data and the way it is presented is a strength of this\nmanuscript. Additionally, the manuscript is well written and easy to read. I feel that the manuscript is suitable for\npublication pending some minor revisions. I reviewed this manuscript with my PhD student and am happy to look\nat a revised version of the editors feel this is required. A list of more specific comments can be seen below.\nSpecific comments\nIntroduction –\nLines 82-109 - seems like a lengthy version of an abstract to me where they highlight their main findings. Is this out of place?\nRather should they just make their objective clearers with a brief mention of their methodology?\nLine 85 – Change “Altitudinal” to “Elevational” these are different things.\nLine 99 – ground is misspelled.\nLine 103-106 – Seems out of place in the introduction, definitely should be a discussion point though\nResults\nSection 2.1 had some very interesting findings. Technically this section has results in areas that do not have permafrost 10\nm deep. Should the section heading be different in this case as it is not just permafrost temperatures you are looking at?\nLine 127 – What is the significance of only having one borehole so far south in the Sierra Nevada mountains? Expand on\nthis and why is this the case?\nLine 131-132 – “Four time series from the Western and Central Alps just cover a decade.” Something does not sound right\nwith this sentence. Change ‘just’ to ‘only’?\nLine 135 – Why the melting point of ice rather than 0 °C? this is an odd way to state this, consider revising.\nLine 136 – “high Arctic” should be written as “High Arctic”.\nLine 143 – Can you write the for example in short form? (e.g.).\nLine 144-146 – Your use of low temperatures and low mountains in this sentence convolutes the message. Is there a way to\nreword this sentence?\nLine 146-147 – What do you mean by ranges? Is this a trend or variability between sites?\nLine 149-150 – Say “reached their maximum MAGT at 20 m depth” here as it is not entirely clear as is.\nLine 152 – Reference for source listing extreme warmth in Svalbard in 2016 to 2018?\nSection 2.2 – This section has some really interesting findings. I think they are presented well. My main note is how does\nremoving the trends that are not statistically significant effect your results? Are some of the patterns you identify being at\nleast in part shaped by non-significant results?\nLine 174 – Are there any significant cooling trends (p-value <0.05)?\nSection 2.2 – If you removed the non-significant trends how does this change the results? Do you see similar patterns in\ntrends based on locations and substrate types?\nLine 176 – Is there way to quantify and statistically test at how many of the sites the trends were lower and more robust at the\n20 m depth?\nFigure 5 – The 10 and 20y labels are a little hard to read as is. Green and red may be tricky for colour blind individuals,\nplease consider revising.\nLines 236-239 – Very interesting result. Shows that local heterogeneity of ice content has more of an influence on warming\ntrends of mountain permafrost than regional location.\nLine 245-247 – Again, a very interesting result that ground temperatures outside of the ice bearing or rich permafrost follow\nmore closely the trends in surface air temperature. One note here, do you need to define the acronym SAT? It is not written\nout before this.\nLine 249 – Well done covering a variety of different combination of depths, time periods, and time spans.\nFigure S5 – Why does this figure have a half eye showing the distribution of individual warming trends while your other\nfigures have violins?\nSection 2.4 – Clear section to me. Seasonal patterns of warming are discussed and result support a summer/autumn\nconcentration of warming in the most recent decade while longer term it is based in winter.\nFigure 7 – in the caption Colour is spelled “Color” when the UK/Can spelling is used before. Check the MS for consistency.\nLine 295 to 297 – The final sentence of this paragraph should be revisited for clarity.\nLine 299-301 – I think this is a good conclusion to make as their data is unique in including warmer ground temperature\nboreholes and cold permafrost boreholes in alpine regions.\nLine 307-309 – Has warming been homogeneous in mountainous regions? Elevation dependent warming?\nLine 313-314 – Have you given any examples of air temperature trends compared to ground temperature trends? Could you\nquantify the difference between these trends?\nLine 316-317 – Would it be better to say “frozen and unfrozen water content” rather than ‘(un)frozen’?\n352-355 – You use various/varies several times in this sentence. Look for possible synonyms.\n370 – Spelling error, should be ‘unevenly’.\nMethods\nLine 434 – I checked your supplementary table (S1) and found that none of the boreholes were deployed later than 2013. Is\nit meant to be 2013 on this line rather than 2014?\n4.3 Authors used the ordinary least squares method to calculate their moving 10- and 20-year trends. Alternatively, they\ntested the non-parametric method called the Theil-Sen Slope but found very little difference in the results. Should these be in\nthe supplemental information? Authors also made the minimum data requirements 5 years for a decade and 10 years for 2\ndecades. Authors argue that inter seasonal and annual variability are not relevant to skewing the trends as the depths they\nare working at has very little signal from those. For SAT trends they took nearby air temperature stations at high elevations to\nreduce the effect of topography. Authors used the NORA3 reanalysis dataset to extend their Scandinavian and Svalbard air\ntemperature datasets for trends. Please address these decisions in the revised MS.", "reviewer_2": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature\nCommunications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career\nResearchers who co-review manuscripts.", "reviewer_3": "The paper submitted by Noetzli et al. is a compilation of ground temperature records from boreholes located in European\nmountains. The records are overwhelmingly located in the European Alps but also include records from Sweden, Norway\nand Spain. The original component of the paper is to present a compilation that includes a much greater number of records\nfrom European mountains and to extend the record until the year 2022 (The records presented by Biskaborn et al. stopped in\n2016). This dataset allows the authors to capture the great diversity of settings inherent to mountain topography and to\ndiscuss the role of ground ice in mitigating some of the temperature changes close to 0°C. They also discuss the “lag”\ninherent with ground temperature change in relation to surface temperature and its implications for future ground temperature\nwarming.\nIn my view, this paper presents a unique set of ground temperature records that fully warrants its publication in Nature\nCommunications. These datasets are collected over decades in difficult conditions and provide invaluable information and\nvalidation for engineers, modelers, and ultimately decision makers in mountain areas. These datasets are, however, seldom\npulled together in synthesis papers and I applaud the authors for doing so. These papers have lasting impacts in the\nscientific community, as we have seen from the paper by Biskaborn et al. (2019).\nI have suggested major revisions but the authors will appreciate that my concerns are mostly related to the quality of the\nwriting and not to the quality of the scientific work. The manuscript is somewhat challenging to read for several reasons: 1.\nThere is substantial redundancy that could be reduced 2. Some words are poorly chosen and could lead to misleading\nunderstanding of some of the processes being discussed 3. Some sections, particularly the ones on the history and\nnecessity of observing networks, are superfluous or redundant with the conclusion.\nI have provided detailed comments below on the manuscript and the figures.\nPage 2, line 31: “Constituting”: Throughout the manuscript, the authors use present participles, often in convoluted\nsentences. I would strongly recommend to get rid of all present participles and simplify the sentences. I’ll therefore pass on\nthe notes I made on the use of present participles in the manuscript (e.g. Page 2, lines 48 and 51), hoping that the authors\ncan introduce these changes.\nPage 2, line 32: “influences”. This is quite vague. “Impacts” might be a better option\nPage 2, line 36: “and employing a more comprehensive data set”. I think I understand what the author mean, but it will be\nunclear to the reader why the data set is “more comprehensive data set” without mentioning Biskaborn et al. I would suggest\nto remove these words\nPage 2, line 39: “reduce”. The authors used the past tense to report on the results in the abstract and now switch to present\nPage 2, line 39: “obscure important transformations….” I don’t think that it is possible to understand this sentence without\nhaving read the manuscript. The wording is convoluted and… …obscure. I suggest to reword.\nPage 2, line 44: “While the ….” This sentence is trying to say too many things at the same time. I would suggest to\nsimplify and simply say that the warming of permafrost has not garnered the same attention.\nPage 3, line 58: Is Permafrost temperature really the “primary” product of the ECV? I do not recall exactly the last iteration of\nthe GTN-P or the ECV literature, but I do not remember it being made “primary” and Active layer depth and rockglacier\nkinematics secondary. De Facto it is, I agree with the authors, but since we are referring to the official networks here, we\nmight as well be consistent with the official nomenclature.\nPage 3, line 63 to line 69: This section is problematic in my view. It attempts to justify the originality of this paper compared to\nBiskaborn et al. 2019. The language, however, is very vague and convoluted: “were considered in a lump manner, etc.”. I\nwould recommend to focus on simple factual information: 1. There are more records than in Biskaborn et al. 2019 (give\nnumbers) 2. They cover more settings 3. They extend the record by six more years. The text written here is not convincing. I\nalso think you should clearly introduce the year 2022 here. I found the first mention of the last measurement year at page 9\n(line 163)\nPage 3, line 70 to 81: The history section is lengthy and I think quite unnecessary. To some extent, some of the points made\nin this section (e.g. “there is no specific Europe-wide coordination of permafrost data to date”) should be addressed in the\nconclusion of the manuscript, but not to this section\nPage 4, line 83: “distinct”. Why say “distinct”? This is somewhat misleading and gives the feeling that is a dataset conceived\n“out of the system”. I would suggest using “new”. It is simpler and reflects the reality. This is a dataset with new records from\nboreholes that are not new.\nPage 4, line 83: “next”. This is also somewhat misleading. Intuitively, the reader is led to think that the permafrost areas were\nidentified first, and that boreholes were then drilled “next” to those on purpose. One could introduce the term “recently\nthawed” to better describe the strategy.\nPage 4, line 84: The dataset is now “new” and not “distinct”\nPage 4, line 87: “Some of these….” This is very vague wording and there is no explanation of what is meant precisely with\nthe “length” “enabling the evaluation”. Since this is explained in the methods section, I would strongly advise to remove this\nsentence not to confuse the reader.\nPage 4, line 88: Here, MAGT is introduced for the first time. This is reasonable, but until this section, the manuscript only\nintroduced DZAA (Page 3, line 60). It is somewhat confusing to hear that MAGT will be used as the main indicator of change\nwhen DZAA was mentioned prominently earlier on. I would suggest adding a quick line after the introduction of MAGT to\nexplain the main difference between MAGT and DZAA or to refer to section 4.3\nPage 4, line 90: “running temperature trends”. The term “running” disappears from the rest of the manuscript. Why using it\nhere?\nPage 4, line 94: “and on depth range”. I am not exactly sure what the authors mean with this. Why not making another\nsentence to explain this precisely.\nPage 4, line 95: “Warming rates are highest….”: This is a sentence describing and explaining too many processes. The\nauthors could possibly break it in two sentences\nPage 4, line 103: “Permafrost warming…” This sentence is problematic. In the current context, it is true, but in absolute terms,\nit is not. I would suggest rewording.\nPage 4, line 104: “Thermal disequilibrium”. This is also a problematic term. I assume you mean that there is a\n“disequilibrium” between temperatures at the surface and temperatures at depth. It took me a while to understand what is\nmeant and I started wondering what the equilibrium would be, if we are now in disequilibrium. I am not sure the use of\ndisequilibrium helps here.\nPage 5, line 109: “need to be endorsed”. I would argue that these efforts are actually “endorsed” at the national and\ninternational levels. What is lacking is the capacity to perform all these tasks.\nFigure 1: Very nice figure, it took me a while to find the Spanish borehole, but I assume it is difficult to improve the figure to\naddress this.\nPage 7, line 142: The order of the sites is a mystery to me. Alphabetical? By country? I am not sure this is consistent with the\nrest of the manuscript\nPage 7, line 144: “permafrost temperatures in the Central….”: The wording is awkward: “do not reach as low “ could be\nsimplified to “are not as low”. “Which is due to the lower mountains” is also awkward. “which is due to the lower elevations”\nwould be more scientific.\nPage 9, line 164: Here the change rate as two decimals (0.10), in most other places it has only one (0.1). Check through the\nmanuscript.\nPage 9, line 177: “more robust”: Do you mean statistically? If yes, mention it\nPage 9, line 184: “for the same group”: I think you mean the same group of records as in Biskaborn et al. It is not obvious\nhere. I would mention it directly\nPage 10, line 190: Link to atmospheric conditions: It would help to see one example where atmospheric temperature, SAT\nand MAGT are shown on the same chart for a season or a decade to illustrate this statement.\nPage 11, line 210. Caption Figure 4: typo with “calculated”\nPage 11, line 213: The use of MGT to describe mean ground temperature is somewhat challenging to understand in the\nmanuscript. The way I understand it is that it can use to describe means for ten or twenty years, but used primarily for ten\nyear means. I do not understand why MAGT is used for annual means and not MDGT for decadal means. The rather loose\nuse of MGT introduces some confusion. Later in the manuscript (Line 220), MGT is used single-handedly. I am left\nwondering if it is the decadal or 20-year mean, even though the authors mention the “corresponding decadal warming rates”.\nIt could be useful to introduce two acronyms for the decadal (MDGT) and the 20-year mean (MTGT).\nPage 12, Figure 5: Minor comment: One could capitalize LOESS so that it clearly refers to the statistical method and not to\nthe sediment.\nPage 12, line 241: “Picture”. Too colloquial. Replace with “pattern”?\nPage 13, line 246: As far as I could tell, this is the first time SAT is introduced, spell it out\nPage 13, Figure 6: Inset (b): Why labelling it “no” when it was labelled “no pf” in Figure 4. “No pf” seems like a more\nunderstandable label\nPage 13, Figure 6: The caption indicates that air temperature warming rates are shown in comparison to permafrost warming\nrates in (a). I looked at the chart several times, but could not see the air temperature warming rates. This is too bad because\nit would help to contextualize all the discussion points made about the lag between atmospheric warming and ground\ntemperature warming (see my earlier comment about Page 10, line 190). I might not understand the chart well enough to see\nit though, but will probably not be the only one to do so.\nPage 14: Generally, the paragraph from line 271 to 282 is very wordy and long.\nPage 15, line 291: The term “Thermal monitoring” is awkward. It could be interpreted in many different ways. I would\nrecommend changing it to “ground temperature monitoring”.\nPage 15, line 293 to line 295: The point made of snow here comes out of nowhere. It sounds quite specific and I do not\nunderstand why the authors make this point in the very first sentences of the discussion.\nPage 15, figure 7: Small detail: The reader is referred to figure 6 for the box plot labelling, but in figure 6, the “number of time\nseries” is labelled in black, whereas here it is in color.\nPage 16, line 299: say “these studies” instead of “earlier studies” so that one does not have to cite them again here (since\nthey are cited in the previous sentence).\nPage 16, line 302: “only just reached lengths”. Put the required length between brackets\nPage 16, line 307: This refers to my previous comment on Figure 6: I don’t see the SAT change on figure 6. Also, in figure 6, it say “air temperature” and here it says SAT. Is it the same? Are those used interchangeably? I would recommend only use\none consistent wording. I\nPage 16, line 308: “comparatively homogeneous”: This is awkward wording. Say “consistent”?\nPage 17, line 327: Give a number for the ground ice content, either gravimetric or volumetric\nPage 17, line 353 “and contribute to understand”. You mean “contribute to explain”?\nPage 18, line 355: Here is the right location to discuss the role of snow\nPage 18, line 356: “which”: replace with “since it can temporarily…”\nPage 18, line 370: Typo in unevenly\nPage 18, line 374: “While a considerable number of time series”. This sentence is very difficult to understand. Possibly break\nit in two sentences and simplify\nPage 19, line 382: Here is the term robust is used in a non statistical sense, which strengthens my previous comment on the\n“statistically robust”\nPage 19, line 386: Typo in maintenance\nPage 19, line 386: Maintenance work WILL likely increase\nPage 19, line 393: One could add an “s” to framework\nPage 20, line 427, typo in agencies\nPage 20, line 432: the borehole is in THE permafrost region\nPage 24, line 519: “high elevation”. Are these sites, or stations?", "reviewer_4": "The manuscript focused on the variability and trends in mountain permafrost temperature in Europe over the last thirty years.\nBy compiling available observations on permafrost temperature from Arctic Svalbard to France, the authors demonstrate that\npermafrost in Europe is warming and especially so in the last decade. However, the rates of warming are not uniform as cold\nand ice-poor permafrost reacts on atmospheric warming stronger due to heat conduction, while ice-rich and warm permafrost\nshow less warming due to latent heat effects associated with melting of ground ice.\nThese results are significant as have substantial implications on alpine ecosystems, infrastructure and slope hazards in the\nregion. The results are generally consistent with other permafrost regions; however, it looks like that due to absence of\nstrong buffers like thick snow and vegetation, mountain permafrost is stronger coupled to atmospheric climate and therefore\nless resilient compared to no-alpine permafrost. The authors show that the atmospheric signal will further propagate to the\nground therefore affecting deeper permafrost with further consequences for slope stability and hydrology in the region.\nI believe that this manuscript will be of great interest to diverse readership of the journal, as it provides (to my best\nknowledge) the first synthesis of permafrost data from boreholes in the entire Europe using uniform methodology. While the\nduration of records varies by region, in my opinion the methodology is explicit enough to address the sensors accuracy and\ndrifts, record length, significance levels and use of OSL to demonstrate the individual trends. Then I am a bit concerned with\naggregation of data, I guess geographic location and surface cover make sense. However, I find that classification of\npermafrost based on ice content and temperature somewhat arbitrary, so it would be great if authors clarify why warm vs cold\npermafrost at -2 C? Why not -3 C? Why ice bearing 10-50% vs ice poor (<10%) vs ice-rich (50-100%). Why not low ice\ncontent (0-10%), medium (10-20%), and high (>20%). To my knowledge these classes are not uniform across countries and\nconstruction manuals on permafrost so require some additional explanation or just a simple reference if available as the way\nyou group your data may influence the results. Even from figure 5 it seems that rates can be quite variable both within and\nbetween proposed classes. For example, from figure 6c ice-poor and no-ice seem quite similar, does it mean that if\npermafrost has <10% of ice content than ice can be neglected as factor in warming trend? So ice becomes important at\n10%? Does it really matter if it is 40 or 80% of ice or the same? Explaining a bit more on how/why you group classes will be\nvery helpful as it will not just report results by classes but help in process understanding and will be very valuable input to\nmodeling communities. Similar with temperature is warm vs cold really maters? If it does matter where is the threshold (what\nhappens at -2 C?). Interesting find that cold permafrost and no permafrost warm at the same rated based on Figure 6b,\nsomething that probably not the case in non-mountain permafrost regions where heat conduction of frozen vs non-frozen\nsoils is quite different and maybe requires a bit of further explanation. Specifically, you stated “Differences in the observed\nwarming patterns depend in the first place on the amount of latent heat exchanged in the ground, which is related to the\ntemperature range and ground ice content, and on the depth” so can you elaborate more on this based on your observational\ndata.\nOverall, I found the text informative and easy to follow with figures very helpful addition to the text. I recommend to accept the\nmanuscript pending clarifications (see above)."}, "author_response": "Enhanced warming of European mountain permafrost in the early 21st century\nPoint-by-point reply to four reviewers’ comments\nWe would like to thank the four reviewers for taking the time to provide thorough reviews with many\nhelpful comments and suggestions, which have significantly helped to improve our manuscript. Our point-\nby-point reply is presented below. The reviewers’ comments are printed in black, while our replies to them\nare in blue. Relevant text parts from the manuscript are given in italic.\nReviewer #1\nGeneral comments to authors and editor\nIn the submitted manuscript by Noetzli et al. titled “Enhanced warming of European mountain permafrost in the\nearly 21st century” the authors do a nice job of highlighting a rich and diverse dataset in multiple European regions\nto show that permafrost and depth temperatures have warmed in the early part of this century. The manuscript is a\ngood study with a list of notable and quality authors well known in the permafrost community. Although the paper\ndoes not report on anything too surprising in many ways it articulates the impact and magnitudes of warming in\nthe mountains of Europe very well. The collection of data and the way it is presented is a strength of this\nmanuscript. Additionally, the manuscript is well written and easy to read. I feel that the manuscript is suitable for\npublication pending some minor revisions. I reviewed this manuscript with my PhD student and am happy to look\nat a revised version of the editors feel this is required. A list of more specific comments can be seen below.\nReply to Reviewer #1\nWe thank Reviewer#1 very much for the positive and supportive comments and the thorough review, including the\nvaluable input by a young researcher in the field (Reviewer #2). We are pleased that the value of the compiled data\nset of permafrost temperatures in European mountains was recognized. We addressed all comments raised by\nReviewers #1 and 2 and adapted the manuscript accordingly (see detailed response below). We believe that the\ncomments helped to considerably improve our manuscript and hope that the reviewers agree with our revisions.\nWe have particularly reworked the introduction and discussion with some restructuring and moving of statements\nfor a better focus on the scope and novelty of the study. To answer the comments by Reviewer #1, the influence of\nthe non-significant warming rates and the use and comparison with surface air temperature are pointed out more\nclearly. We further included four additional time series obtained in mountain permafrost in Iceland, which comple-\nment the data set and the spatial representativeness of the study. Our detailed point-by-point reply is given below.\nSpecific comments\nWhere Reviewer comment Author reply\nIntroduction\n82-109 Seems like a lengthy version of an abstract to We thank the reviewer for pointing this out. We have revised\nme where they highlight their main findings. Is the introduction, additionally addressing the comment\nthis out of place? Rather should they just make regarding line 103–106 (below) and several related comments\ntheir objective clearers with a brief mention of by another reviewer.\ntheir methodology?\nWe have shortened the text of the introduction (particularly the\nhistorical part) to more clearly present the novelty relative to\n1\nearlier studies and our objective. In the last paragraph of the\nintroduction we briefly summarize the key results and conclu-\nsions (according to the formatting instructions of Nature Com-\nmunications https://www.nature.com/documents/ncomms-\nformatting-instructions.pdf)\ndifferent things.\n99 ground is misspelled. Corrected.\n103– Seems out of place in the introduction, Yes, we agree with this comment. We moved the three state-\n106 definitely should be a discussion point though. ments of this paragraph to the discussion (cf. reply to the first\ncomment above).\nResults\nSection Section 2.1 had some very interesting findings. Yes, this is a good point. We adapted the heading for a more\n2.1 Technically this section has results in areas that general wording because also results from non-permafrost\ndo not have permafrost 10 m deep. Should the sites are presented in this section:\nsection heading be different in this case as it is\nMeasured ground temperatures in European mountains\nnot just permafrost temperatures you are\nlooking at?\nborehole so far south in the Sierra Nevada exists in the Spanish mountains to date. This was again confir-\nmountains? Expand on this and why is this the med by our Spanish co-author. No permafrost borehole data\ncase? are available from the Pyrenees.\nWe aim to collect a data set that is as comprehensive as\npossible for European mountain regions. The borehole at\nVeleta Peak is considered an important complement as it is the\nonly one in the region and it has been installed as part of a\nborehole transect in the framework of the EU-funded\npermafrost monitoring project PACE (cf., Harris et al. 2001).\nWe clarify in the text:\nNo data are available yet from other European mountain\nregions such as the Pyrenees.\n131-132 “Four time series from the Western and Central Corrected as suggested, we changed just to only.\nAlps just cover a decade.” Something does not\nsound right with this sentence. Change ‘just’ to\n‘only’?\nthis is an odd way to state this, consider\nrevising.\n136 “high Arctic” should be written as “High Corrected.\nArctic”.\n(e.g.).\n144-146 Your use of low temperatures and low moun- We reworded this and split it into two sentences to make it\ntains in this sentence convolutes the message. easier to read.\nIs there a way to reword this sentence?\nPermafrost temperatures measured in the Central and Eastern\nAlps are not as low as in the Western Alps. This is due to the\nlower elevations in the former regions and the lack of obser-\nvations >3000 m asl., and less due to climatic differences.\n146-147 What do you mean by ranges? Is this a trend or Here, we refer to the observed values of annual mean ground\nvariability between sites? temperatures MGT at the non-permafrost sites. We replaced\n1yr\nrange up to with attain.\n2\n149-150 Say “reached their maximum MAGT at 20 m We agree and have corrected the sentence as suggested.\ndepth” here as it is not entirely clear as is.\nSvalbard in 2016 to 2018?\nSection This section has some really interesting fin- We thank the reviewer for this positive comment and the rele-\n2.2 dings. I think they are presented well. vant question on the influence of non-significant warming\nrates on the resulting pattern.\nMy main note is how does removing the trends\nthat are not statistically significant effect your To look at this question in detail, we calculated the mean\nresults? Are some of the patterns you identify values of all classes using only warming rates with p-\nbeing at least in part shaped by non-significant values≤0.05. This results in a smaller n per calculated mean\nresults? value of the distinguished classes, but the observed patterns\nremain the same. That is, they are not shaped by non-\nIf you removed the non-significant trends how\nsignificant values:\ndoes this change the results? Do you see similar\npatterns in trends based on locations and The number of warming rates with p-values>0.05 is generally\nsubstrate types? higher for shorter time periods (10 years) and for shallower\ndepths (5 or 10 m).\nFor 20-year time periods, >80% of the warming rates at 5 m\ndepth have p-values≤0.05 and >90% for the lower depths. Re-\nmoving non-significant values has hardly any influence on the\npattern and a small effect on the presented mean warming rates\n(<0.1 °C dec–1).\nFor 10-year periods, the significant portion of warming rates\nis lower, particularly at 5 m depth (ca. 40%). We stated in our\npaper, that this depth is not suitable to derive decadal warming\nrates. If non-significant values are excluded, the mean\nwarming rates of all time series is 0.17 °C dec–1 higher at 10\nm depth and 0.06 °C dec–1 higher at 20 m depth. Results show\nhigher mean values if warming rates with p-values>0.05 are\nexcluded parti-cularly for low warming rates (i.e., for ice-\nbearing sites or at sites with temperatures close to 0 °C). This\nis caused by the fact that p-values are generally higher for\nlower warming rates (see Figure A) => Excluding non-\nsignificant values would therefore lead to an overestimation of\nmean warming rates for the distinguished classes.\nFigure A: Most recent warming rates of permafrost\ntemperature time series plotted against the p-value of the\nlinear regression. Each point is the warming rate of one time\nseries with the three different depths distinguished by colour\nand the length of the period by the filling of the point: 10-year\nperiods are shown with filled points and drawn lines, 20-year\nperiods with circles and dashed lines.\nThe statistical significance of the warming rates and its inter-\npretation is discussed in the 5th paragraph of the discussion.\nfollowing text to the end of the paragraph:\nExcluding warming rates with p-values>0.05 would therefore\nnot only reduce the number of observations, but result in an\noverestimation of mean warming rates, particularly for lower\nvalues. We also tested the resulting warming pattern based on\nwarming rates with p-values<0.05 only and found that they\nare not controlled by non-significant values.\nWe also address the interpretation of the statistical significance\nin the methods part (this text was not adapted during revision):\nWhile the temporal aggregation has little effect on the\nobtained warming rates, it can reduce the statistical\nsignificance of the results, which should be considered when\ninterpreting p-values. For a test, we additionally derived\ndecadal warming rates based on monthly and daily time series\nfor several locations with p-value>0.05, which yielded\nsignificant p-values for most tested time series using monthly\ndata, and for all using daily data.\nvalue <0.05)? decade has a p-value<0.05. This information was added to the\ntext in brackets:\nCooling rates were obtained for 4 locations in warm or near-\nzero permafrost in ice-bearing ground. They are, however,\nlower than 0.1 °C dec–1 (p-value<0.05 only one time series).\nhow many of the sites the trends were lower and input. We have analysed this question for the time series for\nmore robust at the 20 m depth? which recent warming rates are available for both,10 m and 20\nm depth.\nFirst, we calculated the difference of the warming rates at the\ntwo depths. A density plot shows that calculated differences\nare approximately symmetrical (see Figure B). Based on this,\nwe performed a non-parametric Wilcoxon signed rank test on\npaired samples (i.e., warming rates at two different depths) to\ntest if there is a significant difference.\nFigure B: Density plot (red line) and histogram for the\ndifference in most recent warming rates at 10 and 20 m depth.\nFor 20-year periods, all warming rates considered have p-\nvalues ≤ 0.05 for both depths. The mean difference between 10\nm and 20 m depth is 0.12 °C dec–1 (0.41 vs. 0.29 °C dec–1,\nn=13). The Wilcoxon signed rank test yields a p-value of\n0.040, indicating significance.\nFor 10-year periods, statistical robustness of the warming rates\nconsidered in the test is higher at 20 m depth than at 10 m depth\n(47% vs. 30%). The mean difference in warming rates is 0.13\n°C dec–1 (0.38 vs. 0.25 °C dec–1, n=35). The Wilcoxon signed\nrank test provides a p-value of 0.007, indicating significance.\nThis information was added to the paper at this place and as\nfollows:\nof warming rates at 10 m and 20 m depth shows a significant\ndifference (0.05 level) for both 10-year (n=35, mean difference\n0.13 °C dec–1) and 20-year periods (n=13, mean difference\n0.12 °C dec–1).\nFigure 5 The 10y and 20y labels are a little hard to read We thank the reviewer for noticing. For better readability, we\nas is. Green and red may be tricky for colour changed the background of the labels to darker grey and\nblind individuals, please consider revising. printed the labels in bold with years written out.\nWe further adapted the figure to avoid the combination of red\nand green. The red was changed to yellow and the red LOESS\nline to dark grey. Note: Figure 5 is Figure 6 in the revised\nmanuscript.\nFor consistency between figures, we adapted the colours,\nlabels and background for all figures showing facets (incl.\nSupplementary Figures).\n236-239 Very interesting result. Shows that local This is well summarized.\nheterogeneity of ice content has more of an\ninfluence on warming trends of mountain\npermafrost than regional location.\n245-247 Again, a very interesting result that ground We thank the reviewer for this comment and agree that this is\ntemperatures outside of the ice bearing or rich a key result of our study. Following a comment by another\npermafrost follow more closely the trends in reviewer, we added running warming rates of SAT to Figure 3\nsurface air temperature. for comparison with ground temperature warming rates.\nOne note here, do you need to define the SAT is now introduced at this place as it is the first occurrence\nacronym SAT? It is not written out before this. of the acronym in the text.\ncombination of depths, time periods, and time\nspans.\nFigure Why does this figure have a half eye showing We originally included the half eye in Supplementary Fig. 5\nS5 the distribution of individual warming trends because of the smaller number of categories shown in the plot\nwhile your other figures have violins? and to show more detail. But we agree, this is not necessary,\nand we changed the figure to be consistent with the other\nfigures.\nSection Clear section to me. Seasonal patterns of We thank the reviewer for this comment!\n2.4 warming are discussed and result support a\nsummer/autumn concentration of warming in\nthe most recent decade while longer term it is\nbased in winter.\nFigure 7 In the caption Colour is spelled “Color” when We checked the text and adapted the spelling to consistently\nthe UK/Can spelling is used before. Check the follow the Oxford spelling.\nMS for consistency.\nDiscussion\n295 to The final sentence of this paragraph should be We reformulated the final sentence of this paragraph, splitting\n297 revisited for clarity. it in two parts:\nObserved warming patterns depend mainly on the depth of\nobservation and on the amount of latent heat exchanged in the\nground. The latter depends on the ground temperature and the\nground ice/water content.\n299-301 I think this is a good conclusion to make as their We thank the reviewer for this comment.\ndata is unique in including warmer ground\ntemperature boreholes and cold permafrost\nboreholes in alpine regions.\n5\n307-309 Has warming been homogeneous in moun- Recent studies (e.g., Pepin et al. 2022; Beaumet et al. 2021)\ntainous regions? Elevation dependent suggest that the occurrence of elevation dependent warming\nwarming? is contingent upon the period, season, or region being\nanalysed. We acknowledge that the sentence was somewhat\nunclear and included updated references to cover more recent\nclimate development:\nSince the 1980s, when the first mountain permafrost data\nbecame available, there was a marked increase in atmospheric\nwarming in the Alps, which appears to have become more\npronounced in recent years (Isotta et al. 2019). The highest\nrates of SAT increase are observed in summer, particularly at\nhigh elevations (>2000 m asl.) (Isotta et al. 2019; Beaumet et\nal. 2021).\n313-314 Have you given any examples of air SAT warming rates are presented in Figure 5a together with\ntemperature trends compared to ground ground temperature warming rates (SAT warming rates are\ntemperature trends? Could you quantify the given for 30-year periods for the three regions Svalbard,\ndifference between these trends? Scandinavia, Alps). We additionally revised Figure 3 and\nadded SAT warming rates for direct comparison with ground\ntemperature warming rates.\nFor a detailed qualitative comparison at the site scale, we lack\nlong-term SAT data at the borehole locations for all sites.\nHowever, we compare the magnitude of the changes in the\nresults section (Section on «Permafrost warming rates»).\nWarming rates in cold permafrost, at ice-poor bedrock sites\nand at non-permafrost sites are in a similar range and\ncorrespond to warming rates of surface air temperature (SAT)\nover the past 30 years (cf. methods for details on the\ncalculation).\nFurther, the third and fourth paragraph in the discussion\nsection deals with the comparison and drivers of change rates\nin the ground and in the air.\n316-317 Would it be better to say “frozen and unfrozen Yes, we agree. The sentence was corrected accordingly.\nwater content” rather than ‘(un)frozen’?\n352-355 You use various/varies several times in this The sentence was adapted to avoid the repetition of\nsentence. Look for possible synonyms. various/varies:\nThe aforementioned effects influence the sites to varying\ndegrees and contribute to explain the differences in warming\nrates observed across the surface covers and slope expositions\nof our monitoring sites.\nMethods\nfound that none of the boreholes were deployed hole time series in the study. Since we allow for one missing\nlater than 2013. Is it meant to be 2013 on this year at the beginning or end of the time series (cf. methods), a\nline rather than 2014? time series can start in 2014 to be considered. Therefore, 2014\nis correct in the text. However, the borehole with the shortest\ntime series included starts in 2013.\nSection Authors used the ordinary least squares method We calculated all warming rates using both, the ordinary least\n4.3 to calculate their moving 10- and 20-year squares (OLS) and the Theil-Sen Slope (TSS) methods.\ntrends. Alternatively, they tested the non- Because the differrences of the resulting values between the\nparametric method called the Theil-Sen Slope two methods are marginal, we think that presenting them in the\nbut found very little difference in the results. Supplementary Information does not add relevant content. We\nShould these be in the supplemental therefore suggest not to include it. To underline this, we calcu-\ninformation? lated the mean and median difference of the warming rates\nbased on the two methods and added this information in\nbrackets at the respective place in the methods section:\nshows marginal differences (mean difference for all calculated\nwarming rates <0.02 °C dec–1, median difference <0.001 °C\ndec–1).\nSection Authors also made the minimum data Here, we are not sure we understand the reviewer’s request, as\n4.3 requirements 5 years for a decade and 10 years no specific question or suggestion was formulated. We suspect\nfor 2 decades. Authors argue that inter seasonal the comment is triggered by the choices for data completeness\nand annual variability are not relevant to made in our approach and thus take the opportunity to clarify\nskewing the trends as the depths they are them:\nworking at has very little signal from those.\nThe criteria for the minimum number of values to calculate a\ndecadal warming rate follow those used by Biskaborn et al.\n(2019) and were adapted for 20-year periods.\nWe argue that the following two points justify for the applica-\ntion of a linear regression on annual values: 1) annual values\nare aggregated data of continuous daily measurements, and 2)\ntemperature variations are dampened with depth and\ninfluenced by longer time periods.\nSeasonal patterns are analysed and presented in the results\nsection and discussed later in the text. The influence of inter-\nannual variability is shown in Figure 3 with the running war-\nming rates.\nSection For SAT trends they took nearby air SAT obtained at high elevation stations are highly correlated\n4.3 temperature stations at high elevations to across the Alps. This correlation is much higher than with SAT\nreduce the effect of topography. Authors used from stations located nearby but in valley bottoms. The latter\nthe NORA3 reanalysis dataset to extend their are influenced by local or topographic effects such as tempe-\nScandinavian and Svalbard air temperature rature inversions. In contrast, high elevation stations measure\ndatasets for trends. Please address these the temperature of the larger air mass and are hardly influenced\ndecisions in the revised MS. by local effects. We added the example of temperature\ninversion as a local effect to give some more information in\nthe text:\nTo avoid effects of local topography or valleys (such as tempe-\nrature inversions), we used data from three high elevation\nweather stations of the Federal Office for Meteorology and\nClimatology MeteoSwiss (...).\nFurther, a sentence was included to provide clarification on the\nselection of the NORA3 data set:\nNORA3 was selected as it is currently the only high-resolution\nhindcast available to cover both the entire Scandinavian\nPeninsula and Svalbard up to 2023. Additionally, it was shown\nto outperform both the host reanalysis ERA5 and the earlier\nhydrostatic 10-km Norwegian Hindcast Archive (NORA10),\nespecially in complex terrain (Haakenstad & Breivik 2022).\nReviewer #2\nI co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the\nNature Communications initiative to facilitate training in peer review and to provide appropriate recognition\nfor Early Career Researchers who co-review manuscripts.\nReply to Reviewer #2\nWe thank Reviewer #2 very much for taking the time and providing useful comments, which helped to im-\nprove our work. We highly appreciate the initiative for and the input from Early Career Researchers. Our\ndetailed replies to the comments raised by Reviewer #1 and Reviewer #2 are included above.\nThe paper submitted by Noetzli et al. is a compilation of ground temperature records from boreholes located\nin European mountains. The records are overwhelmingly located in the European Alps but also include\nrecords from Sweden, Norway and Spain. The original component of the paper is to present a compilation\nthat includes a much greater number of records from European mountains and to extend the record until the\nyear 2022 (The records presented by Biskaborn et al. stopped in 2016). This dataset allows the authors to\ncapture the great diversity of settings inherent to mountain topography and to discuss the role of ground ice\nin mitigating some of the temperature changes close to 0°C. They also discuss the “lag” inherent with ground\ntemperature change in relation to surface temperature and its implications for future ground temperature\nwarming.\nIn my view, this paper presents a unique set of ground temperature records that fully warrants its publication\nin Nature Communications. These datasets are collected over decades in difficult conditions and provide\ninvaluable information and validation for engineers, modelers, and ultimately decision makers in mountain\nareas. These datasets are, however, seldom pulled together in synthesis papers and I applaud the authors for\ndoing so. These papers have lasting impacts in the scientific community, as we have seen from the paper by\nBiskaborn et al. (2019).\nI have suggested major revisions, but the authors will appreciate that my concerns are mostly related to the\nquality of the writing and not to the quality of the scientific work. The manuscript is somewhat challenging\nto read for several reasons: 1. There is substantial redundancy that could be reduced 2. Some words are poorly\nchosen and could lead to misleading understanding of some of the processes being discussed 3. Some\nsections, particularly the ones on the history and necessity of observing networks, are superfluous or\nredundant with the conclusion.\nI have provided detailed comments below on the manuscript and the figures.\nReply to Reviewer #3\nWe are grateful for the positive and constructive review and the many useful and detailed comments by\nReviewer #3. They helped to considerably improve our manuscript and the readability of the text. We are\npleased that Reviewer #3 emphasizes the importance and challenges of collecting long, consistent time series\nof permafrost data and acknowledges the effort to combine them in a synthesis paper on European mountains.\nWe addressed all comments raised and adapted the manuscript accordingly. Our point-by-point reply to the\ndetailed comments is given below. We answer the three main points mentioned as challenge for reading the\nmanuscript as follows:\n1) Redundancy: we checked the text for redundancy and removed text where it is not necessary. We shorte-\nned the introduction to avoid duplicate statements and particularly revised the results section on perma-\nfrost warming pattern: statements on the same pattern were combined and the part was structured follo-\nwing the different classifications. Figures 5 and 6 were swapped due to the changed order of reference in\nthe text.\n2) Choice of words: we adapted the manuscript according to the detailed comments, which improved the\nwording and clarity of the text. Additional comments on the wording by another reviewer also helped to\nimprove readability.\n3) Unnecessary sections: we reworked the introduction and particularly removed most of the historical para-\ngraph. We included the points we think are most relevant for our study into other paragraphs of the intro-\nduction. The necessity of observing networks and their long-term support is important to mention in our\nview, but this now only appears in the discussion part. This reduces redundancy and makes for a more\nconcise introduction.\nWhere Reviewer #3 Comment Author reply\nAbstract\nPage 2, “Constituting”: Throughout the manuscript, the We checked the text for the use of present participles and\nline 31 authors use present participles, often in convo- simplified it where appropriate. Indeed, the readability\nluted sentences. I would strongly recommend to and clearness of several sentences could be improved by\nget rid of all present participles and simplify the reformulating them.\nsentences. I’ll therefore pass on the notes I made\non the use of present participles in the manuscript\n(e.g. Page 2, lines 48 and 51), hoping that the\nauthors can introduce these changes.\nPage 2, “influences”. This is quite vague. “Impacts” We changed the word as suggested.\nline 32 might be a better option:\nPage 2, “and employing a more comprehensive data set”. We adapted the sentence to avoid using «more compre-\nline 36 I think I understand what the author mean, but it hensive»:\nwill be unclear to the reader why the data set is\n… because of accelerated warming and the use of a\n“more comprehensive data set” without men-\ncomprehensive data set.\ntioning Biskaborn et al. I would suggest to remo-\nve these words:“\nPage 2, “reduce”. The authors used the past tense to We agree on the use of the tenses and changed the\nline 39 report on the results in the abstract and now abstract to present, except for where it concerns past\nswitch to present:\nevents.\nPage 2, “obscure important transformations….” I don’t We changed obscure to mask.\nline 39 think that it is possible to understand this\nsentence without having read the manuscript. The\nwording is convoluted and… …obscure. I\nsuggest to reword\nIntroduction\nPage 2, “While the ….” This sentence is trying to say too We agree that the start of the introduction was cumber-\nline 44 many things at the same time. I would suggest to some to read and simplified the sentence as follows:\nsimplify and simply say that the warming of\nWhile the retreat of glaciers in the 21st century has been\npermafrost has not garnered the same attention\nwidely recognised (Zemp et al. 2015; Hugonnet et al.\n2021), permafrost changes in cold regions worldwide\nare far less visible despite being similarly important\n(Biskaborn et al. 2019; Smith et al. 2022; Isaksen, Lutz,\net al. 2022; Romanovsky et al. 2010; Etzelmüller et al.\n2020; Haberkorn et al. 2021; PERMOS 2023; Zhao et\nal. 2020; Magnin et al. 2023; Hock et al. 2019).\nPage 3, Is Permafrost temperature really the “primary” We agree with this comment. There is no «official»\nline 58 product of the ECV? I do not recall exactly the ranking of the three products of the ECV permafrost. We\nlast iteration of the GTN-P or the ECV literature, adapted the text as follows:\nbut I do not remember it being made “primary”\nThe assessment of permafrost changes relies on long-\nand Active layer depth and rockglacier\nterm records of ground temperatures measured in bore-\nkinematics secondary. De Facto it is, I agree with\nholes, which are the direct thermal observations defined\nthe authors, but since we are referring to the\nas one of the products of the Essential Climate Variable\nofficial networks here, we might as well be\n(ECV) permafrost (Streletskiy et al. 2021; Noetzli et al.\nconsistent with the official nomenclature\n2021; GCOS 2022).\nPage 3, This section is problematic in my view. It We reworked the introduction of the paper with\nline 63 attempts to justify the originality of this paper particular attention to present the novelty and key results\nto line compared to Biskaborn et al. 2019. The language, of the paper more clearly, to simplify sentences and to\n69 however, is very vague and convoluted: “were introduce the time-period considered.\nconsidered in a lump manner, etc.”. I would re-\ncommend to focus on simple factual information:\n9\n1. There are more records than in Biskaborn et al.\n2019 (give numbers) 2. They cover more settings\n3. They extend the record by six more years. The\ntext written here is not convincing. I also think\nyou should clearly introduce the year 2022 here.\nI found the first mention of the last measurement\nyear at page 9 (line 163)\nPage 3, The history section is lengthy and I think quite We included this paragraph to justify and describe our\nline 70 unnecessary. To some extent, some of the points focus on European mountains. However, we agree that\nto 81 made in this section (e.g. “there is no specific the section is somewhat long for an introduction and not\nEurope-wide coordination of permafrost data to all information is needed to follow our work. We\ndate”) should be addressed in the conclusion of therefore removed this section and included the\nthe manuscript, but not to this section\ninformation we consider most relevant into the other\nparagraphs of the introduction.\nHowever, we chose to keep the sentence that there was\nno organized European collection of permafrost data to\ndate. This was a motivation for our work and is now an\nimportant result of it.\nPage 4, “distinct”. Why say “distinct”? This is somewhat The formatting instructions of Nature Communications\nline 83 misleading and gives the feeling that is a dataset state that phrases like novel, new, unprecedented should\nconceived “out of the system”. I would suggest be avoided. To follow this and at the same time make it\nusing “new”. It is simpler and reflects the reality. clearer for the reader that the dataset was compiled\nThis is a dataset with new records from boreholes particularly for this study, we adapted the sentence as\nthat are not new follows (including a reference to the detailed description\nof the data collection in the methods section).\nWe compiled a consistent data set of 64 ground\ntemperature time series measured for at least one decade\nin or near permafrost areas. Data are obtained in bore-\nholes extending at least 10 m in depth and the sites cover\na wide range of landforms, elevations and latitudinal\nzones from high Arctic Svalbard and Scandinavia to\nIceland, the Alps and the Sierra Nevada.\nPage 4, “next”. This is also somewhat misleading. Yes, the sentence was not clear. Next to was meant\nline 83 Intuitively, the reader is led to think that the spatially, not temporally. We therefore changed it to\npermafrost areas were identified first, and that near.\nboreholes were then drilled “next” to those on\npurpose. One could introduce the term “recently\nthawed” to better describe the strategy.\nPage 4, The dataset is now “new” and not “distinct” We deleted the word new at this place to avoid the use of\nline 84 new (cf. comment above on Page 4, Line 83).\nPage 4, “Some of these….” This is very vague wording We agree and have deleted this sentence.\nline 87 and there is no explanation of what is meant\nprecisely with the “length” “enabling the\nevaluation”. Since this is explained in the\nmethods section, I would strongly advise to\nremove this sentence not to confuse the reader.\nPage 4, Here, MAGT is introduced for the first time. This The DZAA needs to be introduced above, because we\nline 88 is reasonable, but until this section, the manu- refer to earlier studies reporting on warming rates for this\nscript only introduced DZAA (Page 3, line 60). It depth. We follow the suggestion of the reviewer to refer\nis somewhat confusing to hear that MAGT will at this place to the methods section for more explana-\nbe used as the main indicator of change when tions on the calculation of the trends.\nDZAA was mentioned prominently earlier on. I\nwould suggest adding a quick line after the\nintroduction of MAGT to explain the main\ndifference between MAGT and DZAA or to refer\nto section 4.3:\nline 90 “running” disappears from the rest of the in the results section. We therefore prefer to keep it at\nmanuscript. Why using it here?: this place. However, we changed it to running warming\nrates to be consistent with how the term is used further\ndown in the text and to avoid confusion.\nPage 4, “and on depth range”. I am not exactly sure what The differences in the warming rates mainly relate to two\nline 94 the authors mean with this. Why not making another factors: 1) the amount of exchanged latent heat and 2)\nsentence to explain this precisely.: the considered depth. We changed the sentence and split\nit into two parts for more clarity:\nThe warming patterns are primarily dependent on the\ndepth of observation and the exchange of latent heat for\nphase change. In ice-bearing ground, the latter can\nsignificantly reduce warming rates when temperatures\napproach 0 °C thereby mask important changes such as\nincreasing water content or ground ice melt.\nPage 4, “Warming rates are highest….”: This is a We agree and have made two sentences out of this rather\nline 95 sentence describing and explaining too many complicated one:\nprocesses. The authors could possibly break it in\nWarming rates are highest at cold permafrost sites with\ntwo sentences\nlow ground ice content, where effects of latent heat are\nlargely absent. Further, warming rates in the uppermost\n10 meters are higher than at greater depths, where\ntemperatures react with delay.\nPage 4, “Permafrost warming…” This sentence is We agree with the reviewer that also fast permafrost war-\nline problematic. In the current context, it is true, but ming processes exists (such as for example via advective\n103 in absolute terms, it is not. I would suggest heat transport by water). Such processes are typically\nrewording. active in the uppermost metres. To be more general we\nchanged the beginning of the sentence to\nPermafrost warming at depth …\nNote that this paragraph was moved to the end of the dis-\ncussion section to address another reviewer’s comment.\nPage 4, “Thermal disequilibrium”. This is also a Yes, this is what we mean. Temperatures at depth are\nline problematic term. I assume you mean that there reflecting surface temperatures of past time periods.\n104 is a “disequilibrium” between temperatures at the When the surface temperatures change fast, the differ-\nsurface and temperatures at depth. It took me a rences to temperatures at depth become larger as the\nwhile to understand what is meant and I started latter cannot adapt fast enough. This is what we mean\nwondering what the equilibrium would be, if we when speaking of a thermal disequilibrium. We argue\nare now in disequilibrium. I am not sure the use that this is an appropriate term to describe the current\nof disequilibrium helps here. and particularly the future subsurface temperature field\nin permafrost regions. The term is also used in the con-\ntext of inversion modelling in geothermal studies to\nreconstruct ground surface temperature histories. That\nsaid, we agree that a thermal equilibrium is a rather\ntheoretical state. To clarify what we mean, we adapted\nthe text as follows:\nThe higher warming observed in the upper 10 metres of\nthe ground compared to lower depths points to an in-\ncreasing thermal disequilibrium between the uppermost\nmetres in the permafrost and at depths below the DZAA.\nNote that this paragraph was moved to the end of the dis-\ncussion section to address another reviewer’s comment.\nPage 5, “need to be endorsed”. I would argue that these We adapted endorsed to supported and conducted. The\nline efforts are actually “endorsed” at the national and capacity to perform all the tasks is indeed what is mostly\n109 international levels. What is lacking is the lacking, we agree. However, we believe that continued\ncapacity to perform all these tasks and further endorsement at all levels is important to\nincrease this capacity.\nFigure Very nice figure, it took me a while to find the We agree that the light-yellow colour on white\nimprove the figure to address this. to spot. We improved the visibility of the boreholes/\npoints with thicker contour lines. In addition, we added\nlabels for the regions considered in the study. That way,\nthe Sierra Nevada appears more prominently on the map.\nPage 7, The order of the sites is a mystery to me. We assume the reviewer refers to the list of examples on\nline Alphabetical? By country? I am not sure this is line 143, which indeed did not follow a clear order. The\n142 consistent with the rest of the manuscript: sites are now ordered first by region (Alps, Scandinavia),\nthen alphabetically, and with three examples for each\nregion. Schafberg was mentioned twice by mistake.\nPermafrost with temperatures just close to 0 °C is found\nin all European mountain regions, mainly in rock gla-\nciers and unconsolidated sediments at elevations around\n2500 m asl. in the Alps, or in blockfields and debris at\nelevations around 1500 m asl. in Scandinavia (e.g., at\nBellecombes, Gentianes, Les Attelas, Dovrefjell,\nJuvvasshoe, or Tron).\nPage 7, “permafrost temperatures in the Central….”: The The sentence was adapted for clarification:\nline wording is awkward: “do not reach as low “ could\nPermafrost temperatures measured in the Central and\n144 be simplified to “are not as low”. “Which is due\nEastern Alps are not as low as in the Western Alps. This\nto the lower mountains” is also awkward. “which\nis due to the lower elevations in the former regions and\nis due to the lower elevations” would be more\nthe lack of observations >3000 m asl., and less due to\nscientific.\nclimatic differences.\nPage 9, Here the change rate as two decimals (0.10), in The manuscript was checked to present all change rates\nline most other places it has only one (0.1). Check with two decimals throughout, unless the values are\n164 through the manuscript given as a comparison/order of magnitude such as\n«greater than 0.5 °C dec-1».\nPage 9, “more robust”: Do you mean statistically? If yes, Yes, we mean robust in the sense of more statistically\nline mention it significant. The text was adapted accordingly.\nline group of records as in Biskaborn et al. It is not late the change rates for the decade 2007–2016 in our\n184 obvious here. I would mention it directly data set, which we compare to the values provided by\nBiskaborn et al. (2019). This is mentioned because our\ndataset includes additional time series to calculate the\n2013–2022 warming rates (but they do not start early\nenough to calculate the 2007–2016 warming rate). We\nadapted the wording to make this clearer:\nBased on the time series in our dataset that cover the\nperiod 2007–2016, we obtain a mean warming rate of\n0.39 °C dec–1 at 10 m depth (n=26, 65% of p-\nvalues<0.05) and of 0.24 °C dec–1 at 20 m depth (n=20,\n85% of p-values<0.05) for 2007–2016, and slightly\nhigher values of 0.40 and 0.26 °C dec–1 for 10 and 20 m\ndepth (64% and 84% of p-values<0.05) for 2013–2022,\nrespectively.\nPage Link to atmospheric conditions: It would help to Yes, we agree. SAT is shown together with permafrost\n10, line see one example where atmospheric temperature, temperature change rates later in the manuscript in\na season or a decade to illustrate this statement warming rates of SAT also to Figure 3 for comparison\nwith ground temperature change rates.\nPage Caption Figure 4: typo with “calculated” Corrected.\n11, line\n210\n11, line temperature is somewhat challenging to the use of the acronyms for annual or decadal aggre-\n213 understand in the manuscript. The way I under- gations can be confusing. Indeed, MAAT or MAGT are\nstand it is that it can use to describe means for ten often used for mean values of longer periods than one\nor twenty years, but used primarily for ten year year, such as for 30-year periods and climate normals.\nmeans. I do not understand why MAGT is used So we should choose different acronyms in our text.\nfor annual means and not MDGT for decadal\nWe changed the naming of the aggregated temperatures\nmeans. The rather loose use of MGT introduces\nthroughout the manuscript. We now use mean ground\nsome confusion. Later in the manuscript (Line\ntemperature (MGT) together with a subscript to indicate\n220), MGT is used single-handedly. I am left\nthe period of averaging: MGT , MGT , MGT . We\nwondering if it is the decadal or 20-year mean, 1yr 10yr 20yr\nhope that way it is clear if we consider an annual mean\neven though the authors mention the\nvalue, a decadal or 20-year mean value.\n“correspondding decadal warming rates”. It\ncould be useful to introduce two acronyms for the\ndecadal (MDGT) and the 20-year mean (MTGT)\nPage Minor comment: One could capitalize LOESS so The figure legend and caption were adapted accordingly.\n12, that it clearly refers to the statistical method and Note: Figure 5 is Figure 6 in the revised manuscript.\nFigure not to the sediment.\n12, line “pattern”?\n13, line introduced, spell it out we therefore spell it out.\n13, labelled “no pf” in Figure 4. “No pf” seems like of Figure 4 (please note that Figure 6 is Figure 5 in the\nFigure a more understandable label revised version).\n6:\nPage The caption indicates that air temperature Indeed, SAT change rates are shown for three regions in\n13, warming rates are shown in comparison to Figure 5a to the right in red colour (please note that\nFigure permafrost warming rates in (a). I looked at the Figure 6 is Figure 5 in the revised version). We renamed\n6 chart several times, but could not see the air the labels to SAT for consistency and mentioned the SAT\ntemperature warming rates. This is too bad because change more explicitly in the caption to help the reader\nit would help to contextualize all the see this information.\ndiscussion points made about the lag between\nPanel (a) shows recent decadal warming rates (2013–\natmospheric warming and ground temperature\n2022 or <4 years earlier) at 10 m depth for 4 major\nwarming (see my earlier comment about Page 10,\nEuropean mountain regions (Svalbard, Scandincavia\nline 190). I might not understand the chart well\nNorth, Scandinavia South, Iceland, Eastern Alps and\nenough to see it though, but will probably not be\nWestern Alps) and compared to surface air temperature\nthe only one to do so.\nSAT warming rates for the 30-year period 1993–2022 in\nSvalbard (SAT Sv.), Scandinavia (SAT Sc.) and the\nEuropean Alps (SAT Alps).\nIn addition, SAT change rates are added to Figure 3 in\nthe revised manuscript.\nPage Generally, the paragraph from line 271 to 282 is We agree, the paragraph was too wordy and long. We\n14: 271 very wordy and long. reduced the length by about 50 words and adapted it as\nto 282 follows for better readability:\nNotable differences emerge between the most recent\ndecade 2013–2022 and the 20-year period 2003–2022\n(Fig. 7, left). For the latest decade, the highest monthly\nwarming rates at 10 m depth originate from increased\nground surface temperatures in summer and autumn,\nwhile for the 20-year period the highest monthly war-\nming rates are primarily influenced by winter warming\n(Figure 7, right). For the 10-year period, summer and\nautumn contribute equally to warming for cold and\nwarm permafrost, while for near-zero permafrost,\nautumn has a more significant impact. In areas without\n13\npermafrost, summer is the dominant warming season.\nOver the 20-year period, winter emerges as the\ndominant warming season for cold-, warm and near-\nzero permafrost. Regionally, early winter appears as the\npeak warming time in Scandinavia and Svalbard, while\nlate winter is most dominant in the Alps. Finally, non-\npermafrost areas experience the highest warming during\nsummer and autumn.\nDiscussion\nPage The term “Thermal monitoring” is awkward. It The term was changed as suggested.\n15, line could be interpreted in many different ways. I\n291 would recommend changing it to “ground\ntemperature monitoring”\nPage The point made of snow here comes out of We agree. We moved this point on snow to the snow-\n15, line nowhere. It sounds quite specific and I do not specific section later in the discussion (cf. comment\n293 to understand why the authors make this point in the related to Page 18, line 355 below).\nline very first sentences of the discussion.\n15, the box plot labelling, but in figure 6, the well as the style and layout of Figure 7 to make it\nfigure 7 “number of time series” is labelled in black, consistent with the other figures.\nwhereas here it is in color.:\nPage say “these studies” instead of “earlier studies” so Corrected.\n16, line that one does not have to cite them again here\n299 (since they are cited in the previous sentence)\nPage “only just reached lengths”. Put the required We added a decade in brackets:\n16, line length between brackets\nSeveral additional boreholes have recently been drilled\n302\ninto high-elevation bedrock, but the related time series\nare yet too short (< 1 decade) to evaluate long-term\nchanges. The information thus remains thus very scarce\nfor the highest elevations (e.g. 4000 m asl. and higher in\nthe Alps), where permafrost temperatures are likely a few\ndegrees lower than those available to date.\nPage This refers to my previous comment on Figure 6: Surface air temperature (SAT) is shown for three regions\n16, line I don’t see the SAT change on figure 6. Also, in (Svalbard, Scandinavia, Alps) in Figure 5a to the right in\n307 figure 6, it say “air temperature” and here it says red colour (see also our reply to the comments on page\nSAT. Is it the same? Are those used 13 above; note that Figure 6 is Figure 5 in the revised\ninterchangeably? I would recommend only use manuscript). We adapted the labelling to SAT to make it\none consistent wording. I\nclearer and mentioned this more explicitly in the caption\n(see reply to the earlier comment above).\nYes, air temperature and SAT were used interchangeably\nin the submitted text. We have revised the manuscript to\nconsistently use only one term (SAT) throughout the\ntext.\nPage “comparatively homogeneous”: This is awkward We rewrote this sentence for a better wording and to con-\n16, line wording. Say “consistent”? sider another reviewer's comment:\navailable in the Alps, there was a marked atmospheric\nwarming, which appears to have become more pronoun-\nced in recent years (Isotta et al. 2019). The highest rates\nof SAT increase are detected in summer, particularly in\nhigh elevations (>2000 masl) (Isotta et al. 2019;\nBeaumet et al. 2021).\n17, line gravimetric or volumetric typically available from semi-direct geophysical\n327 soundings or documentation during drilling. That is,\nprecise quantitative information is hardly available, but\na basic classification is possible based on this infor-\nmation. We assess in the applied classification if ground\nice is present in rock fissures and pores only (as in\nbedrock), if there is considerable ground ice (as it is\npossible in unconsolidated sediments), or if excess ice\nand ice lenses are present (as in rock glaciers). This clas-\nsification is qualitative, and therefore the thresholds\ngiven in the methods text are used to describe the cate-\ngories rather than to classify the time series.\nWe elaborate on this in the methods section and rewor-\nded this part to be clearer (see also reply to Reviewer\n#4).\nFor the latter, usually only little qualitative information\nis available from drilling logs or semi-direct geophysical\nsoundings. This means that only a basic classification on\nthe amount of ground ice is possible. However, this is\nsufficient for a general assessment of the influence of\nlatent heat exchange during warming. We therefore\nconsidered if ground ice is present in rock fissures and\npores only as in bedrock, if there is considerable ground\nice as it is possible in unconsolidated sediments, or if\nexcess ice is present as in rock glaciers. We distinguished\nthe four classes …\nPage “and contribute to understand”. You mean Yes. The text was adapted accordingly.\n17, line “contribute to explain”?\n18, line snow:” discussion was relocated and integrated into this section\n355 in the revised manuscript (cf. also comment on Page 15,\nline 293 to line 295 above).\nPage “which”: replace with “since it can temporarily… Corrected.\n18, line\n18, line\n18, line This sentence is very difficult to understand. content. We adapted the text for better readability:\nSeveral additional boreholes have recently been drilled\ninto high-elevation bedrock, but the related time series\nare yet too short (< 1 decade) to evaluate long-term\nchanges. The information thus remains very scarce for\nthe highest elevations (e.g. 4000 m asl. and higher in the\nAlps), where permafrost temperatures are likely a few\ndegrees lower than those available to date.\nPage Here is the term robust is used in a non statistical Yes, you are right. Here the term robust it is not meant\n19, line sense, which strengthens my previous comment in a statistical sense. We use it to refer to the quality of\n382 on the “statistically robust” the data.\nPage Typo in maintenance: Corrected.\n19, line\n386\n19, line\n19, line\nPage typo in agencies Corrected.\n20, line\n20, line\n(i) the borehole is located in permafrost.\n24, line missing and is now included.\n519\nThe manuscript focused on the variability and trends in mountain permafrost temperature in Europe over the last\nthirty years. By compiling available observations on permafrost temperature from Arctic Svalbard to France, the\nauthors demonstrate that permafrost in Europe is warming and especially so in the last decade. However, the rates\nof warming are not uniform as cold and ice-poor permafrost reacts on atmospheric warming stronger due to heat\nconduction, while ice-rich and warm permafrost show less warming due to latent heat effects associated with\nmelting of ground ice.\nThese results are significant as have substantial implications on alpine ecosystems, infrastructure and slope hazards\nin the region. The results are generally consistent with other permafrost regions; however, it looks like that due to\nabsence of strong buffers like thick snow and vegetation, mountain permafrost is stronger coupled to atmospheric\nclimate and therefore less resilient compared to no-alpine permafrost. The authors show that the atmospheric signal\nwill further propagate to the ground therefore affecting deeper permafrost with further consequences for slope\nstability and hydrology in the region.\nI believe that this manuscript will be of great interest to diverse readership of the journal, as it provides (to my best\nknowledge) the first synthesis of permafrost data from boreholes in the entire Europe using uniform methodology.\nWhile the duration of records varies by region, in my opinion the methodology is explicit enough to address the\nsensors accuracy and drifts, record length, significance levels and use of OSL to demonstrate the individual trends.\nThen I am a bit concerned with aggregation of data, I guess geographic location and surface cover make sense.\nHowever, I find that classification of permafrost based on ice content and temperature somewhat arbitrary, so it\nwould be great if authors clarify why warm vs cold permafrost at -2 C? Why not -3 C? Why ice bearing 10-50%\nvs ice poor (<10%) vs ice-rich (50-100%). Why not low ice content (0-10%), medium (10-20%), and high (>20%).\nTo my knowledge these classes are not uniform across countries and construction manuals on permafrost so require\nsome additional explanation or just a simple reference if available as the way you group your data may influence\nthe results. Even from figure 5 it seems that rates can be quite variable both within and between proposed classes.\nFor example, from figure 6c ice-poor and no-ice seem quite similar, does it mean that if permafrost has <10% of\nice content than ice can be neglected as factor in warming trend? So ice becomes important at 10%? Does it really\nmatter if it is 40 or 80% of ice or the same?\nExplaining a bit more on how/why you group classes will be very helpful as it will not just report results by classes\nbut help in process understanding and will be very valuable input to modeling communities. Similar with temperature\nis warm vs cold really maters? If it does matter where is the threshold (what happens at -2 C?). Interesting find that\ncold permafrost and no permafrost warm at the same rated based on Figure 6b, something that probably not the case\nin non-mountain permafrost regions where heat conduction of frozen vs non-frozen soils is quite different and maybe\nrequires a bit of further explanation. Specifically, you stated “Differences in the observed warming patterns depend\nin the first place on the amount of latent heat exchanged in the ground, which is related to the temperature range\nand ground ice content, and on the depth” so can you elaborate more on this based on your observational data.\nOverall, I found the text informative and easy to follow with figures very helpful addition to the text. I recommend\nto accept the manuscript pending clarifications (see above).\nReply to Reviewer #4\nWe thank Reviewer #4 very much for the positive and supporting evaluation of our manuscript. We’d like to take\nthe opportunity to further explain here our classification regarding permafrost conditions (which is based on the\ntemperature) and ground ice content (which is mainly based on the landform).\nfollow a pattern, which is primarily driven by the exchange of latent heat during warming. The amount of latent\nheat exchanged depends on 1) how much ice/water is present in the ground and 2) the ground temperature.\nClassification by ground ice content\nInformation on ground ice content at the drill sites is typically available from semi-direct geophysical soundings\nor observations during drilling. That is, precise quantitative information is hardly available, but a basic classifi-\ncation is possible based on this information. We assess in our approach if ground ice is present only in rock fissures\nand pores as in bedrock (ice-poor), if there is considerable ground ice as often found in permafrost in uncon-\nsolidated sediments (ice-bearing), or if excess ice is present as in rock glaciers (ice-rich). This classification is\nrather qualitative, and the thresholds indicated in the methods text are used to describe the categories rather than\nfor the classification itself. Therefore, they have little effect on the resulting pattern of warming rates. The classi-\nfication by ground ice content is considered constant for a site for our study and purpose.\nThe methods text was adapted to be clearer on this:\nFor the latter (i.e. ground ice content), usually only little qualitative information is available from drilling logs or\nsemi-direct geophysical soundings. This means that only a basic classification on the amount of ground ice is\npossible. However, this is sufficient for a general assessment of the influence of latent heat exchange during\nwarming. We therefore considered whether ground ice is only present in rock fissures and pores as is the case in\nbedrock, if there is considerable ground ice as is possible in unconsolidated sediments, or if excess ice is present,\nas in rock glaciers. We distinguished the four classes no ice (referring here to boreholes that are not in permafrost),\nice-poor (ground ice only in pores or clefts; ground ice content up to around 10 Vol.%), ice-bearing (considerable\nand varying ground ice in unconsolidated sediments, ground ice content about 10–50 Vol.%), and ice-rich (ice\nsupersaturation in rock glaciers; ground ice content up to 100 Vol. %).\nClassification by permafrost conditions\nThe permafrost classes are assigned for each time series, depth and period based on the mean ground temperature\nof the 10-year (MGT ) or 20-year period (MGT ). The classification into cold and warm permafrost is widely\n10yr 20yr\nused in the scientific literature. We use the threshold of –2 °C, which is often used in the literature to distinguish\nbetween the two classes (e.g., Smith et al. 2022). In cold permafrost hardly any unfrozen water is available\n(Williams & Smith 1989). With increasing permafrost temperature the amount of unfrozen water exponentially\nincreases towards 0 °C (see for example Williams & Smith 1989; Lunardini 1991; Mottaghy & Rath 2006). To\naccount for the fact that most of the phase transition takes place at ground temperatures just below 0 °C we further\ndistinguish between warm and near-zero permafrost. Here, latent heat effects are largest and temperature changes\nare very small. We chose 0.5 °C as the threshold for «just below 0 °C». However, using a threshold ±0.1 °C has\nonly little effect on the result (see Figure C).\nFigure C: Histogram of mean ground tempe-\nratures MGT and the two depths 10 and 20 m\n10yr\nfor all time series and the most recent decade\n2013–2022. The red lines indicate the thresholds\nused to define the permafrost classes, the\nnumbers in rectangles are the number of time\nseries in each class.\nFinally, the distinction between near-zero and no permafrost based on MGT and MGT was the most dis-\n10yr/20yr 1yr\ncussed among the authors. The key question was whether time series with positive maximum MGT during the\n1yr\nperiod but with MGT ≤0 °C should be considered «no» or «near zero» permafrost. Several approaches were\n10yr/20yr\ntested and resulting pattern generally remained unchanged. Finally, we agreed to only consider time series where\n1y\nMGT does not exceed 0 °C as near-zero permafrost. Time series with any MGT >0 °C during the period is\n1y\nclassified as no permafrost because of the changing thermal processes when ground temperatures rise above 0 °C.\nThe relation between calculated warming rates and MGT with the derived LOESS fit shown in Figure 6\n10yr/20yr\nconfirms this classification. In this figure MGT is shown as a continuous variable without classification into\n10yr/20yr\npermafrost conditions. Smaller changes in the applied thresholds have only little effect on the resulting pattern and\non the mean values of the permafrost condition classes.\nWe adapted the text in the methods section to better explain the classification by permafrost condition and the\nthresholds applied:\nWhile surface cover and ground ice content were considered a constant characteristic for a borehole site, perma-\nfrost conditions were assigned for each period and depth. In addition to the classical distinction between warm\nand cold permafrost4, we considered that the portion of unfrozen water, and hence the exchanged amount of latent\nheat, increases exponentially when the ground temperature increases towards 0 °Ce.g., 83,84. Most of the phase\nchange takes place at temperatures very close to 0 °C. We distinguished the following four classes: cold permafrost\nwith a mean ground temperature for the 10-year or 20-year period (MGT or MGT ) below –2 °C (negligible\n10y 20y\nlatent heat effects expected), warm permafrost with MGT ≥–2 °C and <–0.5 °C (small latent heat effects,\n10y/20yr\ndepending on ground freezing characteristics), and near-zero permafrost with MGT >–0.5 °C and maximum\n10y/20yr\nMGT ≤ 0 °C (considerable latent heat effects during phase change expected for locations containing substantial\n1yr\nground ice). We distinguished non-permafrost from near-zero permafrost when the maximum MGT is >0°C. That\n1yr\nis, time series for which permafrost was present at the beginning but not at the end of the considered period, were\nclassified as no permafrost (and as thawed permafrost in Fig. 4).\nReferences\nBeaumet, J., Ménégoz, M., Morin, S., Gallée, H., Fettweis, X., Six, D., Vincent, C., Wilhelm, B. & Anquetin, S. 2021.\nTwentieth century temperature and snow cover changes in the French Alps. Regional Environmental Change 21: 114.\nDOI: 10.1007/s10113-021-01830-x.\nBiskaborn, B.K., Smith, S.L., Noetzli, J., Matthes, H., Vieira, G., Streletskiy, D.A., Schoeneich, P., Romanovsky, V.E.,\nLewkowicz, T., Abramov, A., Allard, M., Boike, J., Cable, W.L., Christiansen, H.H., Delaloye, R., Diekmann, B.,\nDrozdov, D., Etzelmuller, B., Grosse, G., Guglielmin, M., Ingeman- Nielsen, T., Isaksen, K., Ishikawa, M., Johansson,\nM., Johannsson, H., Joo, A., Kaverin, D., Kholodov, A., Konstantinov, P., Kroger, T., Lambiel, C., Lanckman, J.-P.,\nLuo, D., Malkova, G., Meiklejohn, I., Moskalenko, N., Oliva, M., Phillips, M., Ramos, M., Sannel, A.B.K., Sergeev,\nD., Seybold, C., Skryabin, P., Vasiliev, A., Wu, Q., Yoshikawa, K., Zheleznyak, M. & Lantuit, H. 2019. Permafrost is\nwarming at a global scale. Nature Communications 10: 264. DOI: 10.1038/s41467-018-08240-4.\nEtzelmüller, B., Guglielmin, M., Hauck, C., Hilbich, C., Hoelzle, M., Isaksen, K., Noetzli, J., Oliva, M. & Ramos, M. 2020.\nTwenty years of European mountain permafrost dynamics—the PACE legacy. Environmental Research Letters 15 :\n104070. DOI: 10.1088/1748-9326/abae9d.\nGCOS 2022. The 2022 GCOS Implementation Plan, Geneva: World Meteorological Organization.\nHaakenstad, H. & Breivik, Ø. 2022. NORA3. Part II: Precipitation and Temperature Statistics in Complex Terrain Modeled\nwith a Nonhydrostatic Model. Journal of Applied Meteorology and Climatology 61: 1549–1572. DOI: 10.1175/JAMC-\nD-22-0005.1.\nHaberkorn, A., Kenner, R., Noetzli, J. & Phillips, M. 2021. Changes in Ground Temperature and Dynamics in Mountain\nPermafrost in the Swiss Alps. Frontiers in Earth Science 9: 626686. DOI: 10.3389/feart.2021.626686.\nHarris, C., Haeberli, W., Vonder Mühll, D. & King, L. 2001. Permafrost monitoring in the high mountains of Europe: the\nPACE Project in its global context C. Harris, ed. Permafrost and Periglacial Processes 12: 3–11. DOI:\n10.1002/ppp.377.\nHock, R., Alder, C., Caceres, B., Gruber, S., Hirabayashi, Y., Jackson, M., Kääb, A., Kang, S., Kutuzov, S., Milner, A., Molau,\nU., Morin, S., Orlove, B. & Stelzer, H. 2019. High mountain areas. In IPCC Special Report on the Ocean and\nCryosphere in a Changing Climate. pp. 131–202.\nHugonnet, R., McNabb, R., Berthier, E., Menounos, B., Nuth, C., Girod, L., Farinotti, D., Huss, M., Dussaillant, I., Brun, F. &\nKääb, A. 2021. Accelerated global glacier mass loss in the early twenty-first century. Nature 592: 726–731. DOI:\n10.1038/s41586-021-03436-z.\nIsaksen, K., Lutz, J., Sørensen, A.M., Godøy, Ø., Ferrighi, L., Eastwood, S. & Aaboe, S. 2022. Advances in operational perma-\nfrost monitoring on Svalbard and in Norway. Environmental Research Letters 17: 095012. DOI: 10.1088/1748-\n326/ac8e1c.\nBenestad, R.E., Hanssen-Bauer, I., Brækkan, R., Sviashchennikov, P., Demin, V., Revina, A. & Karandasheva, T. 2022.\nExceptional warming over the Barents area. Scientific Reports 12: 9371. DOI: 10.1038/s41598-022-13568-5\nIsotta, F.A., Begert, M. & Frei, C. 2019. Long‐Term Consistent Monthly Temperature and Precipitation Grid Data Sets for\nSwitzerland Over the Past 150 Years. Journal of Geophysical Research: Atmospheres 124: 3783–3799. DOI:\n10.1029/2018JD029910.\nLunardini, V.J. 1991. Heat transfer with freezing and thawing. Developments in Geotechnical Engineering, CRREL, U.S.\nArmy Corps of Engineers, Hanover, NH, USA.\nMagnin, F., Ravanel, L., Bodin, X., Deline, P., Malet, E., Krysiecki, J. & Schoeneich, P. 2023. Main results of permafrost\nmonitoring in the French Alps through the PermaFrance network over the period 2010–2022. Permafrost and\nPeriglacial Processes ppp.2209. DOI: 10.1002/ppp.2209.\nMottaghy, D. & Rath, V. 2006. Latent heat effects in subsurface heat transport modelling and their impact on palaeotemperature\nreconstructions. Geophysical Journal International 164: 236–245. DOI: 10.1111/j.1365-246x.2005.02843.x.\nNoetzli, J., Arenson, L.U., Bast, A., Beutel, J., Delaloye, R., Farinotti, D., Gruber, S., Gubler, H., Haeberli, W., Hasler, A.,\nHauck, C., Hiller, M., Hoelzle, M., Lambiel, C., Pellet, C., Springman, S.M., Vonder Muehll, D. & Phillips, M. 2021.\nBest Practice for Measuring Permafrost Temperature in Boreholes Based on the Experience in the Swiss Alps. Frontiers\nin Earth Science 9: 607875. DOI: 10.3389/feart.2021.607875.\nPepin, N.C., Arnone, E., Gobiet, A., Haslinger, K., Kotlarski, S., Notarnicola, C., Palazzi, E., Seibert, P., Serafin, S., Schöner,\nW., Terzago, S., Thornton, J.M., Vuille, M. & Adler, C. 2022. Climate Changes and Their Elevational Patterns in the\nMountains of the World. Reviews of Geophysics 60: DOI: 10.1029/2020RG000730.\nPERMOS 2023. Swiss Permafrost Bulletin 2022, Fribourg and Davos: Swiss Permafrost Monitoring Network PERMOS.\nRomanovsky, V.E., Smith, S.L. & Christiansen, H.H. 2010. Permafrost thermal state in the polar Northern Hemisphere during\nthe international polar year 2007–2009: a synthesis. Permafrost and Periglacial Processes 21 : 106–116. DOI:\n10.1002/ppp.689.\nSmith, S.L., O’Neill, H.B., Isaksen, K., Noetzli, J. & Romanovsky, V.E. 2022. The changing thermal state of permafrost.\nNature Reviews Earth and Environment 3: 10–23. DOI: doi.org/10.1038/s43017-021-00240-1.\nStreletskiy, D., Noetzli, J., Smith, S.L., Vieira, G., Schoeneich, P., Hrbacek, F. & Irrgang, A.M. 2021. Strategy and Implemen-\ntation Plan for the Global Terrestrial Network for Permafrost (GTN-P) 2021–2024. DOI: 10.5281/ZENODO.6075468.\nWilliams, P.J. & Smith, M.W. 1989. The Frozen Earth, Cambridge: Cambridge University Press.\nZemp, M., Frey, H., Gaertner-Roer, I., Nussbaumer, S.U., Hoelzle, M., Paul, F., Haeberli, W., Denzinger, F., Ahlstrom, A.P.,\nAnderson, B., Bajracharya, S., Baroni, C., Braun, L.N., Caceres, B.E., Casassa, G., Cobos, G., Davila, L.R., Delgado\nGranados, H., Demuth, M.N., Espizua, L., Fischer, A., Fujita, K., Gadek, B., Ghazanfar, A., Hagen, J.O., Holmlund,\nP., Karimi, N., Li, Z., Pelto, M., Pitte, P., Popovnin, V.V., Portocarrero, C.A., Prinz, R., Sangewar, C.V., Severskiy, I.,\nSigurosson, O., Soruco, A., Usubaliev, R., Vincent, C. & Correspondents, W.N. 2015. Historically unprecedented\nglobal glacier decline in the early 21st century. Journal of Glaciology 61: 745-+. DOI: 10.3189/2015JoG15J017\nZhao, 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,\nH. & Liu, S. 2020. Changing climate and the permafrost environment on the Qinghai–Tibet (Xizang) plateau.\nPermafrost and Periglacial Processes 31: 396–405. DOI: 10.1002/ppp.2056\nPoint-by-point reply to final comments by reviewers\nWe would like to thank the four reviewers for taking the time to assess our revision and replies. We are\npleased that we were able to answer the comments and questions satisfactorily.\nHere, we provide the reply to the remaining comments by Reviewer #4, which we addressed with the final\nrevision. The reviewers’ comments are printed in black, while our replies to them are in blue. Reviewer #4\nThe authors addressed comments and concerns raised by the reviewers and I believe that the revised\nmanuscript improved significantly and is ready for the publication. I believe that this manuscript is a valuable\ncontribution and will be received well by diverse readership of NComm.\nFew minor suggestions:\nL327 is awkward at the end and can it be rephrased without DZAA “and deeper permafrost that is not\naffected by seasonal variability” or something that is not too technical.\nWe agree with this comment. We have rephrased the sentence as follows:\nThe higher warming observed in the upper 10 metres of the ground compared to 20 m depth points to an\nincreasing thermal disequilibrium between the uppermost metres and permafrost at depth.\nTable 1: Lat without long makes little sense, please add lon column to the table.\nYes, you are right. To avoid making the large table even larger, we have inserted the region of the borehole site\ninstead of the coordinates. The coordinates (Lat/Lon) can be found in the Supplementary Table 1.\nFigures are good quality. I suggest to change 0/12 to just 12 in figure 7 (right, both panels)\nWe have adapted Figure 7 accordingly.\n1"}, {"round": 2, "reviewer_comments": {"reviewer_1": "Review has satisfied my concerns and the MS looks good.", "reviewer_2": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature\nCommunications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career\nResearchers who co-review manuscripts.", "reviewer_3": "The authors have done a great job of reorganizing the paper and addressing my comments. I have no additional request.", "reviewer_4": "The authors addressed comments and concerns raised by the reviewers and I believe that the revised manuscript improved\nsignificantly and is ready for the publication. I believe that this manuscript is a valuable contribution and will be received well\nby diverse readership of NComm.\nFew minor suggestions:\nL327 is awkward at the end and can it be rephrased without DZAA “and deeper permafrost that is not affected by seasonal\nvariability” or something that is not too technical.\nTable 1: Lat without long makes little sense, please add lon column to the table.\nFigures are good quality. I suggest to change 0/12 to just 12 in figure 7 (right, both panels)"}, "author_response": ""}]

Decision letter