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Warming Permafrost in the Western Alps: A Further Evidence of Elevation Dependent Warming?

Paolo Pogliotti, Edoardo Cremonese et Umberto Morra di Cella
Traduction(s) :
Réchauffement du permafrost dans les Alpes occidentales: une nouvelle preuve d’un réchauffement qui dépend de l’altitude? [fr]

Résumé

The present study aims at characterising the recent thermal evolution of permafrost in the Aosta Valley region (Italy, Western European Alps) using active layer thickness anomalies and warming trends of both surface and deep temperatures as indicators. The dataset includes monitoring sites located at different altitudes and geomorphological contexts such as rock walls, high-altitude plateau and proglacial margins.
The results show that the thickness of the active layer is increasing everywhere and that anomalies are consistent between sites despite their different characteristics and distances. Significant warming trends of about +0.2°C/10y are observed at 15 m depth in sites around 3000 m of elevation.
In steep rockwalls warming trends at the rock surface are significant only on the north-facing faces where values are on average +0.42°C/10y at elevations above 4000 m.
The present study aims
to provide a snapshot on the current thermal evolution of permafrost in the north-western Alps as well as consistent and useful information for risk management. Taking advantage of this analysis, the study also aims to highlight as the temperature measures in permafrost could contribute to the elevation-dependent-warming debate.

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Introduction

1Permafrost is a globally widespread thermal phenomenon that affects about 15% of the exposed land surface (Gruber, 2012) and occurs essentially in the coldest places of the Earth: at high latitudes or at high elevations. Being a purely thermal phenomenon, permafrost is a valuable climate indicator, useful for quantifying the effects of global warming (Hock et al., 2019; Smith et al., 2022). The understanding of the thermal state and the evolution of permafrost in mountain regions is crucial since the ongoing degradation caused by global warming poses significant problems in terms of slope and infrastructure stability (e.g., Gruber & Haeberli 2007; Haeberli & Gruber, 2009; Duvillard et al., 2021).

2In mountain regions, such as the Alps, the distribution and thermal characteristics of permafrost are strongly influenced by the complex topography and the extreme spatial variability of snow and debris covers (Guodong & Dramis, 1992; Noetzli et al., 2007; Gruber & Haeberli, 2009) making its survey challenging. To address this complexity at regional level, it is necessary to measure permafrost temperature in different morphological contexts, making the evidences arising from single Alpine sites difficult to generalise. In this sense, looking at anomalies and trends can be a useful approach to compare different sites and to infer overarching signals at massif or regional scales.

3Permafrost temperatures are closely related to mean annual air temperatures (MAAT) and mean annual ground surface temperatures (MAGST), in fact, the amplitude of ground temperature variations declines with distance below the surface, converging at depth on a value close to the mean annual temperatures of the air and ground above (Brown, 1973). Such a depth is usually known as ZAA (zero annual amplitude) that is the depth beyond which it is no longer possible to observe seasonal variations; in deep boreholes, mean annual ground temperature (MAGT) at or below the ZAA is commonly used to assess the thermal regime of the ground (Gold & Lachenbruch, 1973). Nevertheless, the match between permafrost temperatures and MAAT or MAGST is not exact and the difference between them is mainly controlled by two factors: the “surface offset” (SO) and “thermal offset” (TO) (Smith and Riseborough, 2002). The SO is due to the insulating effect of winter snow cover and is negligible in sites where snow is scarce or absent; TO is due to the different thermal conductivities of the active layer when frozen or thawed and it is greatest in wet materials (e.g., fine-grained mineral soils) and negligible in dry materials (e.g., gravelly soils or bedrock) (Burn and Smith, 1988). As a consequence, if in sites where SO and TO are low or negligible the MAAT or MAGST can be used to estimate deep ground temperatures it is also possible the vice versa: deep temperatures below the ZAA or MAGST can be considered a good proxy of the air temperature course above the site location.

4With elevation dependent warming (EDW) it is indicated the process driven by the amplification of the rate of warming with elevation, that results in high-mountain environments experiencing more rapid changes in temperature than those at lower elevations (Pepin et al. 2022). Such a difference not always imply that warming is more rapid in mountains compared to lowlands. In fact, despite many model simulations project an acceleration of warming rates at high elevations (Kotlarski et al., 2012; Rupp et al., 2017; Palazzi et al., 2019), the analyses of air temperature observations available in mountain regions (Ceppi et al., 2012; Isotta et al., 2019) or at global level (Pepin et al., 2022) do not always agree, with the exception of the Tibetan Plateau (e.g., You et al., 2020). This is attributed to the inherent complexity of mountain climate dynamics (Pepin et al., 2022) but also to the imbalance towards low elevation of the current monitoring networks that rarely have stations above the 2000 or 3000 m a.s.l. (Pepin et al., 2015). In this context, the measure of MAGST in steep (and north facing) rockwalls, that is conducted by permafrost researchers for many years (e.g., Gruber et al., 2003) at elevations often above the 3000 m a.s.l. could represent a valuable dataset for contributing to the debate on the EDW.

5The present study aims at providing a snapshot of the current thermal state and recent evolution of permafrost in the north-western Alps looking at trends and anomalies in (i) Permafrost ECV time series (essential climate variables) that is active layer thickness (ALT) and permafrost temperatures (PT) as well as in (ii) MAGST on steep high-mountain rockwalls. The dataset includes observation sites located at different altitudes and heterogeneous geomorphological contexts like rock walls, high-altitude plateau and proglacial margins, the main observation areas are located in the Matterhorn and Mont Blanc regions. Furthermore, the study also aims to highlight as the temperature measures in permafrost could contribute to the elevation-dependent-warming debate.

Sites Description

6In the Aosta Valley (North-Western European Alps, Italy) permafrost is present above 2500 m a.s.l. and has been systematically monitored since the beginning of the 2000s in the Matterhorn area. Previous studies have described the presence of permafrost in Aosta Valley and neighboring regions, with varying degrees of accuracy and spatial distribution (Cremonese et al., 2011, Boeckli et al., 2012). For example, Magnin et al. (2015) estimated the presence of permafrost in the Mont Blanc massif at altitudes above 2800 m a.s.l., while Giaccone et al. (2015) reported the presence of permafrost in the near Sabbione basin at altitudes above 2800 m. Other studies have focused on the thermal state of permafrost in the North Western Alps, reporting a general warming trend in recent decades (e.g. Bodin et al., 2015, Pogliotti et al., 2015) and possible relations with slope stability (e.g. Palomba et al., 2015).

7The monitoring sites considered in this study are located in two distinct geographical areas. The first area is located at the head of the Valtournenche Valley on the Italian side of Matterhorn and includes three main locations: Cime Bianche, Carrel’s Hut on the south-west ridge of Matterhorn (Lion Ridge) and summit of Matterhorn. The second study area is located in the eastern part of the Mont Blanc massif and includes two sites: the Grandes Jorasses north face and a periglacial area in front of the Grapillon Glacier at the head of the Val Ferret. These two areas (Matterhorn and Mont Blanc) are about 50 km apart (Fig. 1).

8The analysis is based on two types of observation points: (i) GST (ground surface temperature) in shallow boreholes (0.55 m depth) and (ii) BH (deep boreholes) (see section “ALT Anomaly” for details).

Figure 1. Overview of the monitoring site

Figure 1. Overview of the monitoring site

Matterhorn area: (a) Cime Bianche, (b) Carrel’s Hut, (c) Matterhorn summit.
Mont Blanc area: (d) Grapillon and (e) Grandes Jorasses north face.
The map shows the Valle d’Aosta region with positions and relative distances between the sites.

Matterhorn Sites

Cime Bianche (3100 m a.s.l.)

9The site consists of a small plateau located at an altitude of 3100 m a.s.l. slightly sloping towards west, characterised by small terraces, convexities, and depressions which, during the winter, generate a strong spatial variability of snowpack thickness (Figure 1a). The lithology is homogeneous, mainly consisting of granatiferous micaschists and calcschists belonging to the upper part of the Zermatt—Saas ophiolitic complex (Dal Piaz, 1992). The substrate is strongly altered and fractured and, in general, appears as a widespread covering of rotated and disarticulated blocks of various sizes interspersed with very pervasive fractures. The thickness of this cover varies from a few decimetres to a few metres. On this site there are two boreholes (BH), 41 and 6 m deep, located in a short distance (about 30 m) and equipped since 2006 (see Table 1 and Pogliotti et al., 2015, for further details).

Carrel Hut (3850 m a.s.l.)

10The monitoring site is located in the median sector of the south-west ridge of Matterhorn, about 30 m below the Carrel Hut (Figure 1b) on the south face. The sector is characterised by vertical rock faces, modelled in basement rocks belonging to the Austro-Alpine geological unit of the Dent Blanche (gneiss of the Arolla series), interrupted by small terraces generated by the main families of discontinuities that characterise the rock mass. The substrate is compact and little fractured. The two measurement points, namely a GST site equipped since 2005 and a 9 m deep BH equipped since 2011 are located on the southern slope at an altitude of approximately 3850 m a.s.l.

Matterhorn summit (4450 m a.s.l.)

11The site is a very steep and sharp rocky ridge running east-west which separates the north and south faces of the Matterhorn pyramid (Figure 1c). The summit is made up of compact and slightly fractured basement rocks still belonging to the Dent Blanche geological unit here represented by the Kinzigites of the Valpelline series (high-grade metamorphic gneiss). The measurement points are two GST sites located about 30 m below the summit, on both the north and south face of the mountain at an elevation of about 4450 m. The measures are available since 2011.

Mont Blanc sites

Grapillon (3100 m a.s.l.)

12The Grapillon is a small glacier located at the head of Val Ferret on the Italian side of the Mont Blanc massif. Like all the glaciers in the area, it has undergone a heavy retreat over the last decades (Figure 1d). The monitoring site is located in the neighboring area in front of the current position of the glacier body on a bedrock slope gently degrading towards west. The bedrock substrate (Mont Blanc granite) is compact and well smoothed by the action of the glaciers with almost no debris cover. During winter the site is usually covered by a thick snowpack (> 4 m) that persists until late summer.

13On this site, there are two 20 m deep boreholes (BH) placed at an increasing distance from the glacier front. Temperature profiles in both boreholes highlight the absence of permafrost, with deep temperatures (19 m) of 0.4°C and 1.4°C for the closer and the farther borehole respectively. Nevertheless, in both boreholes mean annual ground temperatures decrease with depth, suggesting the former presence of permafrost conditions and probably their actual presence at greater depths.

14Despite the absence of permafrost in the range of the borehole measures, these data are considered highly significant because they probably represent an advanced stage of permafrost degradation, something that many monitoring sites in the Alps will face in the coming years. Given that, in this study, the Grapillon data are used for looking at both warming trends and also, rearranging the data as explained in the paragraph 3.2, at the active layer thickness.

15Since the long-term thermal signal in the two boreholes is very similar, only the colder one is used for the present study because much closer to permafrost conditions. This borehole is located at 3100 m of elevation and data are available since 2013.

Grandes Jorasses (4100 m a.s.l.)

16The Grandes Jorasses are a peak of 4208 m a.s.l., located in the eastern part of the Mont Blanc massif. The mountain is characterised by very steep and compact slopes shaped in the Mont Blanc granite. On the southern slope, there is a hanging glacier that extends from the summit up to an altitude of about 4000 m, while the northern slope consists of a vertical wall about 900 m high (Figure 1e). The measuring point used for this study is a GST site located on the north face, a hundred metres below the summit at an altitude of approximately 4100 m. The measures are available since 2011.

Methods

Boreholes (GST & BH)

17GST (ground surface temperature) sites consist of small boreholes 55-cm-deep and 10 mm in diameter made by a manual battery-powered driller. Once the thermometric chain is inserted, the perforation entrance is sealed with silicone to prevent the circulation of air and water. These measuring points are located on vertical rockwalls and equipped with a thermometric chain consisting of 3 sensors placed at different depths (10, 30 and 55 cm) connected to a battery-powered Geoprecision D-Log12 datalogger with half-hourly acquisition frequency. Only the sensors at 55 cm of depth are used for this study.

18BH (boreholes) sites consist of boreholes of variable diameter (on average 6/7 cm) and depth carried out by professional companies with core destruction method and equipped with a PVC pipe for housing the thermistor chain. The top of the borehole is sealed with expanded foam to prevent the circulation of air and water in the cavity. Once the thermometric chain is inserted, the PVC pipe is sealed by means of a cap. The BH sites used in the study are equipped with thermometric chains of variable length and sensor spacing.

19Most of the sites are equipped with sensors of type Dallas (digital chip) while Cime Bianche boreholes are equipped with thermistors of type YSI-44031. In both cases the sensors resolution is 0.01°C and an accuracy of +/- 0.1°C in the range -40/+85°C.

Table 1. Characteristics of the monitoring sites

Type

Site name

Morphology

Surface

Elev.

Slope

Aspect

Depth

Sensor

Datalogger

Freq.

Boreholes

BH

MH_Cime Bianche

plateau

debris

3100

5

west

41

YSI 44031

CS_cr800

10 min

BH

MH_Cime Bianche

plateau

debris

3100

5

west

6

YSI 44031

CS_cr800

10 min

BH

MH_Carrel

rock wall

bedrock

3850

60

south

9

digital chip

GP_D-Log12

30 min

BH

MB_Grapillon

slope

bedrock

3100

30

west

19

digital chip

GP_D-Log12

60 min

Ground Surface Temperatures

GST

MH_Carrel

rock wall

bedrock

3850

90

south

0,55

digital chip

GP_D-Log12

30 min

GST

MH_Summit

rock wall

bedrock

4450

90

south

0,55

digital chip

GP_D-Log12

10 min

GST

MH_Summit

rock wall

bedrock

4450

90

north

0,55

digital chip

GP_D-Log12

10 min

GST

MB_Gr. Jorasses

rock wall

bedrock

4100

90

north

0,55

digital chip

GP_D-Log12

30 min

Type: BH = boreholes, GST = ground surface temperature.
Site name: MH = Matterhorn area, MB = Mont Blanc area.
Units: Elev. [m], Slope [°], Drill depth [m].
The slope values
are referred to the average of the surroundings not to the exact point where the sensor is installed.

Data Analysis

20The methods used to calculate permafrost ECVs, anomalies and trends are summarised below.

21—ALT and ALT anomaly. ALT is the thickness of the active layer, defined as the maximum depth (m) reached annually by the 0°C isotherm (Muller, 1943). It is calculated considering the maximum daily temperatures at each depth and linearly interpolating between the deepest sensor with positive values and the underlying sensor. ALT can be calculated only in BH sites. The ALT anomaly of each time series is the difference between the annual ALT value and its average.

22As introduced above, the Grapillon monitoring site is actually in no-permafrost condition with positive temperatures until the bottom of the borehole (Fig. 2A). Negative temperatures penetrate during winters at depths between 3 and 6 metres (blue colours) while positive ones (orange/red colours) penetrate during summer at different depths depending on the years. In order to also include this site in the analysis of the ALT variability, 4°C has been subtracted to the temperature time series at all depth obtaining the fake dataset of Figure 2B, where ALT is clearly visible during summer periods. As the borehole is drilled in massive bedrock and conduction is the predominant mechanism of heat transfer, the subtracting of 4°C is like lifting up the site of about 1000 m of elevation. Bearing in mind the aims of the work and the added value of including a site from the Mont Blanc area, we considered this way of rearranging the data acceptable for analysing the long-term signal of ALT variability in the region.

Figure 2: contour plot of the Grapillon monitoring site

Figure 2: contour plot of the Grapillon monitoring site

(A) original dataset, (B) fake dataset obtained by subtracting 4°C to the temperature time series at all depths. Colours represent the variation of subsurface temperature in depth and time. The black line is the 0°C isotherm.

23—Trends on borehole temperatures and GST. The analysis is aimed at verifying the presence of significant linear trends within the temperature time series using two non-parametric methods: Mann-Kendall test (MK) (Mann, 1945; Kendall, 1948) and the Sen slope estimator (SS) (Sen, 1968). The temperature time series at each depth are aggregated at monthly steps then detrended for removing the seasonal component. The trend analysis is then performed and tested on the resulting time series. The details of this procedure are reported in Pogliotti et al. (2015). For the Grapillon site this trend analysis is conducted on the original dataset not on the fake one (see the ALT paragraph above).

Results and Discussion

ALT Anomaly

24Figure 3 shows the ALT anomalies for the two boreholes of Cime Bianche (debris-covered plateau), Matterhorn (bedrock rockwall) and Grapillon (bedrock slope).

25Despite the differences in terms of elevation (about 700 m), morphological context and distances of the monitoring areas (~50 km), the temporal courses of the anomalies look very similar between the sites and show a consistent ALT deepening, with the most abrupt changes recorded in summer 2015 and 2022.

26At Cime Bianche the anomaly lines are highly correlated from the beginning of the measure up to 2015 while, from 2016, a strong decoupling occurs: the shallower borehole (CimeBianche_BH_6m, red line) shows a bigger interannual variations compared to the deeper borehole (CimeBianche_BH_41m, orange line). In fact, the two boreholes, although located at short distances, are characterised by significant differences in bedrock fracturation hence in ice content and water/air circulation.

27On Matterhorn, since summer 2015, the ALT has always been deeper than the previous year with the exception of the years 2017 and 2021 both characterised by poor winter precipitations and summer with temperatures on average.

28At Grapillon the interannual variability is greater than other sites, probably due to the strong influence that thick snow cover plays in this sites. In general the course of the anomaly seems in accordance with the sites in the Matterhorn area with exceptions of the years 2016 and 2020 when the snow cover disappeared later than usual in summer reducing the snow-free days window thus the ground heating period.

29Looking at trends (dashed lines) on Matterhorn the ALT deepening calculated over the period 2012–2022 is from 2 to 3 times lower than the other sites. This difference is probably due to the topographic effect of the Lion ridge that separates the hot south face from the colder west face inducing a strong 3D transient thermal effect that in such a case could induce a cooling of the subsurface thermal regime. At Cime Bianche the difference between the two boreholes is probably due to the variation in ice contents or water circulation. The Grapillon trend is very similar to those observed at Cime Bianche.

30These results are in agreement with observations in other alpine sites and mountain regions and confirm an unquestionable trend of active layer deepening at regional level, with very similar and consistent signal regardless of the elevation or morphological context. Other studies have highlighted similar results. Recently, a general deepening of ALT during the last 2 decades has been reported in 7 sites of the swiss alps by PERMOS, 2023 with peaks in the years 2015 and 2022. Similar observations have been also reported by Etzelmuller et al. 2020 which analysed the 20-year dataset of the PACE project (Harris et al., 2001) that includes a south—north transect of deep boreholes in mountain permafrost from Sierra Nevada (37°N) to Svalbard (78°N). Their results show as the active layer thickness (ALT) increased at all sites between c. 10% and 200% with respect to the start of the study period, with the largest changes observed in the European Alps. At larger scale, Smith et al., 2022 analysed the thermal state of permafrost in the northern hemisphere including decadal observations of ALT. Results show that ALT is thickening almost everywhere since the 1990s especially in the Russian Arctic and in the Scandinavian regions (that include mountain permafrost). On the Tibetan Plateau, ALT has tended to increase at a rate of about 0.2 m per decade since the 1980s. In the European Alps the study reports a strong thickening with a doubling of ALT since about 2000s and rates of about 10 cm/year that is, on average, 10 times faster than in the rest of the hemisphere. These changes have generally been lower at sites with ice-rich permafrost than in bedrock sites.

Figure 3: active layer thickness anomaly in four boreholes: Cime Bianche (6 m and 41 m, red and orange lines respectively, 3100 m a.s.l.), Matterhorn Carrel Hut (blue line, 3850 m a.s.l.) and Grapillon in the Mont Blanc area (grey line, 3100 m a.s.l.)

Figure 3: active layer thickness anomaly in four boreholes: Cime Bianche (6 m and 41 m, red and orange lines respectively, 3100 m a.s.l.), Matterhorn Carrel Hut (blue line, 3850 m a.s.l.) and Grapillon in the Mont Blanc area (grey line, 3100 m a.s.l.)

Dashed coloured lines represent linear temporal trends of ALT anomalies fitted over the period 2012–2022 that is common to all time-series. Mean ALT is calculated as the average of all measures available in each site. In the years 2019, 2020, and 2022, the ALT in CimeBianche_BH_6m exceeded the maximum borehole depth.

Permafrost Temperatures

31Figure 4 compares the variation of the ground temperature warming trend with depths in the BH sites of Cime Bianche, Carrel Hut and Grapillon. Despite the differences in terms of morphological context and the distances between the Matterhorn and Mont Blanc area, the resulting warming trends are strikingly consistent, in particular the two bedrock sites (Matterhorn and Grapillon), in the depth range 5–10 m, show very similar values. On the other hand, the more fractured and weathered site (Cime Bianche) shows values lower than bedrock ones. This difference is probably caused by a much more efficient heat transfer in compact and slightly fractured bedrock (i.e., Matterhorn and Grapillon) than in highly fractured or debris-covered surface (i.e., Cime Bianche) (Gruber & Haeberli, 2009). The differences between the warming trends in the 3 sites decrease progressively with depth, becoming very close (difference lower than 0.1°C/10y) around 15/20 m and probably converging at greater depth.

32At Cime Bianche, 15.3 m corresponds to the ZAA (Pogliotti et al., 2015) where ground temperature should converge towards MAAT value. Below this depth the warming trend at Cime Bianche results on average +0.2°C/10y, a value that is in agreement with both the permafrost warming (Biskaborn et al., 2019) and the air warming (IPCC, 2018) rates observed at a global scale. In fact, Smith et al., 2022 underline that trends in permafrost warming are consistent with trends in air temperature but also that local conditions including snow and debris cover modulate the response of permafrost to a warming climate. In the time window from the beginning of the borehole measure (2006) and 2022 the mean MAAT at Cime Bianche is -2.34°C while the mean permafrost temperature around the ZAA depth is about -1.1°C, therefore an off-set of about 1.2°C exists between the air temperature and the permafrost temperature, due to the combined effect of snow cover (SO) and active layer (TO). As a consequence it is possible to assume that the warming trend observed on permafrost at Cime Bianche is probably lower than the warming trend present in the air above the monitoring site. In fact, the warming trend calculated on the air temperature data from the on-site automatic weather station (Pogliotti et al., 2015) in the time window 2007–2022 results to be 1°C/10y in accordance to the recent study of Nigrelli&Chiarle 2023 that analysing a dataset of air temperature data from weather stations located between 1500 and 3500 m a.s.l. in the Alps, found an average warming trend of 0.5°C/10y with a range that can varies from 0.3 to 0.9°C/10y.

Figure 4: variation of ground temperature warming trend vs. depth in three boreholes: Cime Bianche (orange, 3100 m a.s.l.), Matterhorn Carrel Hut (blue, 3850 m a.s.l.) and Grapillon in the Mont Blanc area (grey, 3100 m a.s.l.)

Figure 4: variation of ground temperature warming trend vs. depth in three boreholes: Cime Bianche (orange, 3100 m a.s.l.), Matterhorn Carrel Hut (blue, 3850 m a.s.l.) and Grapillon in the Mont Blanc area (grey, 3100 m a.s.l.)

The warming trends are calculated over the maximum time window available for each borehole. Trends are expressed as °C per decade. Vertical lines represent no trend (zero). Horizontal lines are the 10th and 90th percentile of the fitted trends at each depth.

Ground Surface Temperatures

33Figure 5 shows the warming trends of the rock surfaces temperatures at two elevations on the south face of Matterhorn (Fig.4a) and on the north faces of Matterhorn and Grandes Jorasses above 4000 m of elevation (Fig.4b). These warming trends are calculated using temperature observations at 55 cm below the rock surface. Looking at the figure, the data seems consistent with each other. All series show quite similar courses over time and an evident warming trend that is higher on the southern face.

34Starting from the Fig.4a the two sites are located on the south face of Matterhorn, about 30 m below the summit (yellow line) and about 30 m below the Carrel Hut (red line) with an elevation difference of about 600 m each other. In terms of mean annual temperatures, the two sites differ of about 4°C with, obviously, the sensor at the summit that is colder. In terms of warming trend, the summit site presents a value stronger than the Carrel site despite being not statistically significant (p> 0.05, Moore, 2009) probably because of the very high fluctuations resulting from the interplay of surface temperatures and solar radiation, compared to time series length. For this reason the values of warming rates on the south faces must be considered with caution.

35On the opposite, the temperatures on the two north-facing slopes in Fig.4b, being driven exclusively by the air temperature, are characterised by very significant (p <0.01) warming rates. The two sites have an elevation difference of 350 m and a distance of about 50 km with a temperature difference of about 3°C. Moreover the sensor on Matterhorn shows a greater inter-annual temperature variability. In terms of warming trend, the Grandes Jorasses summit, despite being at lower elevation, it shows a trend of 0.46°C/10y that is higher than that on the Matterhorn summit (0.38°C/10y). On average a warming rate of about 0.42°C/10y can be estimated and considered representative of the north faces above 4000 m a.s.l. in the region. It is not possible to compare these results with other studies because to the knowledge of the authors there are no trends calculated on similar data or at such high altitudes.

36Nevertheless, given that MAGST on the north facing steep rockwall can be considered a very good proxy of MAAT at the measuring location it is also possible to consider the time series of rock temperature measure a good proxy of the air temperature record. As a consequence the warming rate of air temperature should reasonably be equal or very close to the one observed in rock surface (0.42°C/10y). Comparing this value with the warming trend of the at the Cime Bianche (1°C/10 a) it seems that in the study area the trend above 4000 m is lower than the one observed at 3000 m. However we are conscious that the two monitoring sites, Cime Bianche (an high-mountain plateau with both surface and thermal offset) and north-facing rockwalls (direct coupling between rock surface and atmosphere without the effect of direct solar radiation) are too different contexts for making robust comparisons as well as direct comparisons between air temperature and rock temperature measures. On the contrary a systematic analysis on GST records at different elevations in steep north facing slopes could be a valuable and robust dataset to investigate EDW mechanisms at very high-elevation in mountain regions.

Figure 5: temperature warming trend at ground surface in south (a) and north (b) steep rock faces

Figure 5: temperature warming trend at ground surface in south (a) and north (b) steep rock faces

Continuous lines are the seasonal detrended time series of sensors at 0.55 m of depth; dashed lines are the trending lines.

Conclusions

37The study was conducted with the aim of describing and characterising the recent evolution of the thermal state of permafrost in north-western Alps, based on the historical monitoring series currently available. The analysis focused on the substrate temperature measurements conducted in deep and shallow boreholes in the Matterhorn and Mont Blanc areas over the last 10 to 15 years and compared the results with other studies in the Alps and in other mountain regions. As indicators of the thermal state of the permafrost, we used the active layer thickness anomaly and the warming trends of both deep and surface rock temperatures.

38The results show that in the region (i) the thickness of the active layer is increasing with an average deepening trend of about +2m per decade; (ii) the permafrost is warming at all observed depths with stronger rates in compact than in fractured/weathered rock masses; (iii) the warming rates detected at depths greater than 15 m at 3000 m of elevation are around +0.2°C/10y in agreement with those observed globally; (iv) rock surface temperatures on the north faces above 4000 m of elevation show an average warming rate of +0.42°C/10y.

39Taking advantage of this analysis the authors tried to underline the link between permafrost temperature and air temperatures highlighting as data collected in permafrost monitoring sites could contribute to the elevation-dependent-warming debate that suffers of lacking measures at high elevations. In particular GST records on steep north-facing slopes at different elevations could be a valuable dataset to explore, with the purpose of investigating EDW mechanisms at very high-elevation in mountain regions.

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Bibliographie

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Table des illustrations

Titre Figure 1. Overview of the monitoring site
Légende Matterhorn area: (a) Cime Bianche, (b) Carrel’s Hut, (c) Matterhorn summit. Mont Blanc area: (d) Grapillon and (e) Grandes Jorasses north face. The map shows the Valle d’Aosta region with positions and relative distances between the sites.
URL http://journals.openedition.org/rga/docannexe/image/11784/img-1.png
Fichier image/png, 902k
Titre Figure 2: contour plot of the Grapillon monitoring site
Légende (A) original dataset, (B) fake dataset obtained by subtracting 4°C to the temperature time series at all depths. Colours represent the variation of subsurface temperature in depth and time. The black line is the 0°C isotherm.
URL http://journals.openedition.org/rga/docannexe/image/11784/img-2.png
Fichier image/png, 136k
Titre Figure 3: active layer thickness anomaly in four boreholes: Cime Bianche (6 m and 41 m, red and orange lines respectively, 3100 m a.s.l.), Matterhorn Carrel Hut (blue line, 3850 m a.s.l.) and Grapillon in the Mont Blanc area (grey line, 3100 m a.s.l.)
Légende Dashed coloured lines represent linear temporal trends of ALT anomalies fitted over the period 2012–2022 that is common to all time-series. Mean ALT is calculated as the average of all measures available in each site. In the years 2019, 2020, and 2022, the ALT in CimeBianche_BH_6m exceeded the maximum borehole depth.
URL http://journals.openedition.org/rga/docannexe/image/11784/img-3.png
Fichier image/png, 375k
Titre Figure 4: variation of ground temperature warming trend vs. depth in three boreholes: Cime Bianche (orange, 3100 m a.s.l.), Matterhorn Carrel Hut (blue, 3850 m a.s.l.) and Grapillon in the Mont Blanc area (grey, 3100 m a.s.l.)
Légende The warming trends are calculated over the maximum time window available for each borehole. Trends are expressed as °C per decade. Vertical lines represent no trend (zero). Horizontal lines are the 10th and 90th percentile of the fitted trends at each depth.
URL http://journals.openedition.org/rga/docannexe/image/11784/img-4.png
Fichier image/png, 197k
Titre Figure 5: temperature warming trend at ground surface in south (a) and north (b) steep rock faces
Légende Continuous lines are the seasonal detrended time series of sensors at 0.55 m of depth; dashed lines are the trending lines.
URL http://journals.openedition.org/rga/docannexe/image/11784/img-5.png
Fichier image/png, 208k
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Paolo Pogliotti, Edoardo Cremonese et Umberto Morra di Cella, « Warming Permafrost in the Western Alps: A Further Evidence of Elevation Dependent Warming? »Journal of Alpine Research | Revue de géographie alpine [En ligne], 111-2 | 2023, mis en ligne le 24 octobre 2023, consulté le 11 décembre 2023. URL : http://journals.openedition.org/rga/11784

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Auteurs

Paolo Pogliotti

Environmental Protection Agency of Valle d’Aosta

Edoardo Cremonese

Environmental Protection Agency of Valle d’Aosta

Umberto Morra di Cella

Environmental Protection Agency of Valle d’Aosta

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