CR6.1

In the Arctic, the spatial distribution of boreal forest cover and soil profile transition characterizing the taiga-tundra ecological transition zone (TTE) is experiencing an alarming transformation. The SIBBORK-TTE model provides a unique opportunity to predict the spatiotemporal distribution patterns of vegetation heterogeneity, forest structure change, arctic-boreal forest interactions, and ecosystem transitions with high resolution scaling across broad domains. Within the TTE, evolving climatological and biogeochemical dynamics facilitate moisture signaling and nutrient cycle disruption, i.e. permafrost thaw and nutrient decomposition, thereby catalyzing land cover change and ecosystem instability. To demonstrate these trends, in situ ground measurements for active layer depth were collected to cross-validate below-ground-enhanced modeled simulations from 1980-2017. Shifting trends in permafrost variability (i.e. active layer depth) and seasonality were derived from model results and compared statistically to the in situ data. The SIBBORK-TTE model was then run to project future below-ground conditions utilizing CMIP6 scenarios. Upon visualization and curve-integrated analysis of the simulated freeze-thaw dynamics, the calculated performance metric associated with annual active layer depth rate of change yielded 76.19%. Future climatic conditions indicate an increase in active layer depth and shifting seasonality across the TTE. With this novel approach, spatiotemporal variation of active layer depth provides an opportunity for identifying climate and topographic drivers and forecasting permafrost variability and earth system feedback mechanisms.

How to cite: Gay, B., Armstrong, A., Osmanoglu, B., Montesano, P., Ranson, K., and Epstein, H.: Examination of Current and Future Permafrost Dynamics Across the North American Taiga-Tundra Ecotone, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3066, https://doi.org/10.5194/egusphere-egu21-3066, 2021.

Winter warming events (WWE) in the Swedish subarctic are abrupt and short-lasting (hours-to-days) events of positive air temperature that occur during wintertime, sometimes accompanied by rainfall (rain on snow; ROS). These events cause changes in snow properties, which affect the below-ground thermal regime that, in turn, controls a suite of ecosystem processes ranging from microbial activity to permafrost and vegetation dynamics. For instance, winter melting can cause ground warming due to the shortening of the snow cover season, or ground cooling as the reduced snow depth and the formation of refrozen layers of high thermal conductivity at the base of the snowpack facilitate the release of soil heat. Apart from these interacting processes, the overall impacts of WWE on ground temperatures may also depend on the timing of the events and the preceding snowpack characteristics. The frequency and intensity of these events in the Arctic, including the Swedish subarctic, has increased remarkably during the recent decades, and is expected to increase even further during the 21st Century. In addition, snow depth (not necessarily snow duration) is projected to increase in many parts of the Arctic, including the Swedish subarctic. In 2005, a manipulation experiment was set up on a lowland permafrost mire in the Swedish subarctic, to simulate projected future increases in winter precipitation. In this study, we analyse this 15-year record of ground temperature, active layer thickness, and meteorological variables, to evaluate the short- (days to weeks) and long-term (up to 1 year) impacts of WWE on the thermal dynamics of lowland permafrost, and provide new insights into the influence of the timing of WWE and the underlying snowpack conditions on the thermal response of permafrost. On the short-term, the thermal responses to WWE are faster and stronger in areas with a shallow snowpack (5-10 cm), although these responses are more persistent in areas with a thicker snowpack (>25 cm), especially after ROS events. On the long term, permafrost in areas with a thicker snowpack exhibit a more durable warming response to WWE that results in thicker active layers at the end of the season. On the contrary, we do not observe a correlation between WWE and end of season active layer thickness in areas with a shallow snowpack. 

How to cite: Pascual Descarrega, D. and Johansson, M.: Contrasting thermal responses of permafrost to winter warming events under different snow regimes in the subarctic, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5788, https://doi.org/10.5194/egusphere-egu21-5788, 2021.

In the last two decades, there were registered record high permafrost temperatures promoting permafrost thawing and leading to additional CO₂ and CH₄ emissions. It is crucial to assess the amount of C that is mineralized to CH₄, due to its higher global warming potential (GWP) compared to CO₂. The role of CH₄ in the total C emissions is mainly governed by the hydrological patterns of ecosystems. CH₄ oxidation is another critical process and is largely controlled by vegetation. The soil CO₂:CH₄ production ratio shows the contribution of CH₄ to the C emission budget of a determined area. Few studies evaluated in situ CO₂:CH₄ production ratios. Our objective was to assess CH₄ emissions and the heterotrophic CO₂:CH₄ production ratios in the Siberian tundra during the growing season. To accomplish these goals, we measured CH₄ and CO₂ fluxes using the chamber technique in the polygonal tundra of Samoylov Island in the Lena River Delta, Northeastern Siberia. The plant-mediated CH₄ transport and the heterotrophic respiration (R_h) were determined by comparing plots with and without vegetation through a trenching experiment. To account for the differences between wet and dry tundra, one representative polygon was selected, measurements were made at its water-saturated center and at its drained rim. We also estimated the C budget of the polygonal tundra of Samoylov Island during the measurement period. This is the first study measuring and calculating in situ CO₂:CH₄ ratios from the R_h of the soil. The CH₄ emissions at the polygon center were much higher than the rim and showed evident seasonality. The polygon center median CH₄ flux of 26 mg.m^-2.d^-1 decreased by 80% when the vegetation was removed, indicating the relevance of plant-mediated CH₄ transport in these emissions. This was not detected at the polygon rim that had much lower emissions (1.8 mg.m^-2.d^-1). The heterotrophic CO₂:CH₄ ratios varied from 1 to 100 at the polygon center, and from 100 to 1000 at the polygon rim, showing the greater importance of CH₄ production to the heterotrophic C release at the polygon center. The polygonal tundra on Samoylov Island was a C sink during the measurement period. The wet tundra had a CO₂-C sequestration rate (-23 kg CO₂-C.ha^-1.d^-1) more than 3 times higher than the dry tundra (-7 kg CO₂-C.ha^-1.d^-1). Overall, the CH₄ emissions represent a decrease of just 5% in the total CO₂-e offset of the tundra in Samoylov during the growing season. The CH₄ emissions measured in this study were low. However, it is important to point out that only the growing season is considered, and the off-season and winter C emissions might be significant. Our results stress the high microscale variability of emissions of CO₂ and CH₄, specially related to hydrology, topography, and vegetation.

How to cite: Galera, L. D. A., Knoblauch, C., Eckhardt, T., Beer, C., and Pfeiffer, E.-M.: CH4 and CO2 fluxes at sites with different hydrological patterns in the polygonal tundra of Samoylov Island, Northeastern Siberia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-137, https://doi.org/10.5194/egusphere-egu21-137, 2021.

Warming-induced shrub expansion on Arctic tundra is generally thought to warm up permafrost, as shrubs trap blowing snow and increase the thermal insulation effect of snow, limiting permafrost winter cooling. We have monitored the thermal regime of permafrost on Bylot Island, 73°N in the Canadian high Arctic at nearby herb tundra and shrub tundra sites. Once adjusted for differences in air temperature, we find that shrubs actually cool permafrost by 0.6°C over November-March 2019, despite a snowpack twice as insulating in shrubs. By simulating the rate of propagation of thermal perturbations and using finite element calculations, we show that heat conduction through frozen shrub branches have a winter cooling effect of 1.5°C which compensates the warming effect induced by the more insulating snow in shrubs. In spring shrub branches under snow absorb solar radiation and accelerate permafrost warming. Over the whole snow season, simulations indicate that heat and radiation transfer through shrub branches result in a 0.3°C cooling effect. This is contrary to many previous studies, which concluded to a warming effect, sometimes based on environmental manipulations that may perturb the natural environment. The impact of shrubs on the permafrost thermal regime may need to be re-evaluated.

How to cite: Dominé, F., Fourteau, K., and Picard, G.: Shrubs covered by snow in the high Arctic cool down permafrost in winter by thermal bridging through frozen branches, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7222, https://doi.org/10.5194/egusphere-egu21-7222, 2021.

Different approaches exist for a satellite-based estimation of mean annual ground temperature (MAGT). Landsurface temperature can be ingested by transient models. Surface status information (frozen/unfrozen days) has been shown to be applicable for the estimation of ground temperature as well. Such approaches are based on an empirically defined relationship. Both approaches have been evaluated with in situ bore hole measurements, but not yet compared with each other.

A comparison between yearly arctic mean temperatures, derived from the advanced scatterometer (ASCAT) and data from ESA’s CCI+ Permafrost project was carried out. The used ASCAT record is available from 2008 (first full year) onwards while the latest CCI+ Permafrost data is available from 1997 to 2018. The ASCAT data was recorded by satellites whose measurements are only intermittently available as one flyover over the whole arctic north of 60°N takes two days on average. To fill in the missing values exponentially weighted moving averages (EWMA) were used. From the number of frozen days an expected average temperature was derived based on Kroisleitner et al. (2018).

The CCI+ Permafrost data incorporates modelled MAGT for depths between the surface down to a depth of 10 meters. These data points were extracted from the raster files (~1km resolution) and averaged over polygons representing an approximation of the ASCAT grid (footprint approximation). Single polygon areas range from 150-160 km². Only footprints for which data is available in both records (and thus permafrost presence) have been eventually compared.

The CCI+ Permafrost data shows an average surface temperature of -1.42 °C for the areas analyzed between 2008 and 2018 while the statistically padded ASCAT data suggests a mean temperature of -1.18 °C over the same time period. The ASCAT retrieval corresponds to a general MAGT whereas CCI+ Permafrost values are available for certain depths. Water fraction within ASCAT footprint also affect the quality of the derivation of frozen days. New calibration considering certain depths and water fraction is suggested.

How to cite: Jakober, D., Bergstedt, H., Kroisleitner, C., and Bartsch, A.: Comparison of permafrost mean annual ground temperature derived from two different satellite-based schemes: land surface temperature based (ESA CCI+ Permafrost) versus surface status (Metop ASCAT), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9824, https://doi.org/10.5194/egusphere-egu21-9824, 2021.

Computed X-ray Tomography is a non-destructive technique that allows three-dimensional imaging of soil samples' internal structures, determined by variations in their density and atomic composition. This study's objective was to develop an image processing workflow for the quantitative analysis of ice cores using high-resolution CT in order to determine the volume fraction and vertical distribution of ice, mineral, gas, and organic matter in permafrost cores. We analyzed a 155 cm permafrost core taken from a Yedoma permafrost upland on Kurungnakh Island in the Lena River Delta (northeast Siberia). The obtained results were evaluated and compared with the results of detailed, but sample-destructive laboratory analysis. The frozen permafrost core was subjected to a computerized X-ray imaging procedure with a resolution of 50 micrometers. As a result, we obtained 31000 images. Noise in the raw images is removed with a non-local means denoising filter. We chose multilevel thresholding method for the image segmentation step. Threshold values were determined based on the histograms of the images. We measured the volumetric ice content (VIC) using Java-based image processing software (ImageJ). In addition, the vertical profiles were analyzed in 1-2cm intervals. We received bulk densities and VIC by freeze-drying and standard laboratory analysis. From the top of the core and until roughly 86 cm, it mainly consists of ice and organic, with an average of 67% and 30% results, respectively. The rest of the volume is divided almost equally between air and mineral parts. Below 86 cm, it consists almost entirely of pure ice. The ice content constitutes around 97% of the composition, and air rises to roughly 3%, while mineral and organic are almost equal to zero. The difference between VIC derived through CT scan and laboratory-derived VIC lies within the range of -37% to 25%. However, the vast majority of values lie within the range of -10% to 10%. This image processing technique to quantify VIC provides a non-destructive analog to traditional laboratory analysis that could help increasing the vertical resolution for quantifying mineral, ice, gas, and organic components in permafrost cores as well as enhance the volumetric estimate.

How to cite: Gadylyaev, D., Nitzbon, J., Schlüter, S., Köhne, J. M., Grosse, G., and Boike, J.: Applying Computed Tomography (CT) scanning for segmentation of permafrost constituents in drill cores, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11395, https://doi.org/10.5194/egusphere-egu21-11395, 2021.

The Mackenzie-Delta region is known for widespread permafrost and the association of different landforms, which are characteristic of a periglacial landscape development. Especially the density of closed-system pingos is nowhere on earth higher than in the area of the Tuktoyaktuk Peninsula. This type of pingos is common only in the continuous permafrost zone and is very sensitive to changing thermal conditions. In this study, we investigated the surface and subsurface conditions in the area of such a closed-system Pingo near Parsons Lake in the southern part of the Tuktoyaktuk Peninsula to study its internal structure and evolutional state. Therefore, we used a combined approach of electrical resistivity tomography (ERT), ground-penetrating radar (GPR) and manual frost probing. In addition, a high-resolution digital elevation model and an orthophoto were generated using in situ drone acquisitions. These enabled a detailed and areawide mapping of surface characteristics (e.g. vegetation height or type) and should contribute to the investigation of linkages between surface and subsurface characteristics.

Such a linkage could be observed comparing the mapped vegetation type and heights with active layer depths derived from manual frost probing and GPR measurements. Both parameters show a significant zonation in the area of the pingo and its surrounding. In addition, the results of the quasi three-dimensional ERT measurements could deliver new insights into the three-dimensional internal structure of the pingo and a massive ice core could be detected. However, the shape as well as the position of the massive ice core in relation to the elevated surface of the pingo differ from the previous theory of closed-system pingo formation and therefore raises some questions. Also the existence of a talik could be confirmed, but its position beside the ice core within the eastern flank of the pingo and not below the massive ice core also differs from the theoretical models and should be discussed.

How to cite: Kunz, J. and Kneisel, C.: Multi-methodological three-dimensional investigation of a closed-system Pingo in Northwestern Canada, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11801, https://doi.org/10.5194/egusphere-egu21-11801, 2021.

Global scale warming has led to permafrost thaw, which may release large amounts of carbon to the atmosphere as CO₂ and CH₄, potentially accelerating global warming (i.e. a positive feedback). However, uncertainty in the mechanisms controlling carbon mineralization is compounded by concurrent changes in soil hydrology associated with permafrost thaw. Thawing permafrost can lead to surface water accumulation in some areas and seasonal or permanent soil drying in areas where permafrost thaw opens up new channels of water to penetrate into the groundwater system. The complexity of the hydrologic response to permafrost thaw increases the challenge in generating reliable estimates of the permafrost carbon climate feedback. Furthermore, limited observational data exist to i) quantify the effects of permafrost thaw on net tundra carbon budgets, particularly on an annual basis, and ii) as well as constrain the underlying processes governing carbon release under aerobic and anaerobic conditions.

Here, we investigated how changes in local hydrology affects CO₂ and CH₄ release from permafrost soils by establishing a field gradient study in northern Norway (69ᵒ N), where recent abrupt degradation of permafrost created thaw ponds in palsa-mire ecosystems.  The site exhibits a natural gradient of permafrost thaw, which also corresponds to a strong hydrological gradient (i.e. dry palsas with intact permafrost, seasonally inundated thaw slumps, and thaw ponds). Since 2017, we have used a range of manual and automated techniques to measure changes in vegetation, soil and water microclimate, biogeochemistry, and soil CO₂ and CH₄ concentrations and efflux across the permafrost thaw gradient.

Our preliminary results show that abrupt permafrost thaw and landscape subsidence – both intermediate slumping and thaw pond formation – increase net annual carbon loss from this type of subarctic wetland. Permafrost thaw approximately doubles CO₂ emissions from thaw slumps compared to vegetated or soil palsas. Furthermore, CH₄ release greatly increased across the permafrost thaw gradient. While vegetated palsas were small sinks of atmospheric CH₄ during the growing season, permafrost thaw slumping and pond formation led to a dramatic increase in CH₄ efflux compared to bare palsas. In contrast, bare soil palsas on were the most important source of N₂O. Soil profile CO₂ and CH₄ concentrations in thawed permafrost plots were overall highly enriched relative to palsa profiles, reflecting soil conditions with inundated pore space and low oxygen availability along the permafrost thaw gradient. We therefore conclude that abrupt thaw will increase annual carbon loss in subarctic palsa wetlands. 

How to cite: Althuizen, I., Christiansen, C., Dörsch, P., Kjær, S., Michelsen, A., Risk, D., Westermann, S., and Lee, H.: Abrupt thaw enhances annual global warming potential of an actively degrading permafrost peatland, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12231, https://doi.org/10.5194/egusphere-egu21-12231, 2021.

Difficulties to quickly reduce carbon emissions to levels compatible with the long-term goal of the Paris Agreement increase the likelihood of scenarios that temporarily overshoot the respective climate targets. We used simulations with JSBACH, the land surface component of the Max-Planck-Institute for Meteorology’s Earth system model MPI-ESM1.2 to investigate the long-term response of the terrestrial Arctic to climate stabilization at such a climate target. In particular, we seek to answer the question whether the state of permafrost-affected soils and the Arctic carbon cycle could converge to different equilibria depending on the climate trajectory that precedes climate stabilization at 1.5°C above pre-industrial levels. To this end, we compare simulations that are forced with the same non-transient atmospheric conditions – corresponding to the 1.5°C-target --, but started from different initial conditions. One simulation was initialized with the conditions before and one simulation with the conditions after a temperature overshoot which follows SSP5-8.5 until the year 2100 subsequent to which the atmospheric conditions are reversed to the 1.5°C-target. Our results reveal that feedbacks between water-, energy- and carbon cycles allow for path-dependent steady-states in permafrost-affected regions. These depend on the soil organic matter content at the point of climate stabilization, which is significantly affected by the soil carbon loss resulting from overshooting the climate target. Here, the simulated steady-states do not only differ with respect to the amount of carbon stored in the frozen fraction of the soil, but also with respect to soil temperatures, the soil water content and even net primary productivity and soil respiration.

How to cite: de Vrese, P. and Brovkin, V.: Legacy effects of climate overshoot scenarios in permafrost-affected regions, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13099, https://doi.org/10.5194/egusphere-egu21-13099, 2021.

Retrogressive thaw slumps (RTS) are a common thermokarst landform along arctic coastlines with an increasing thermoerosional activity. They underlay a rapid change in topographical as well as internal structures due to various external factors, e.g. changing climate conditions.

In 2011 and 2019 electrical resistivity tomography (ERT) measurements were carried during field campaigns to Herschel Island (Yukon Territory, Canada). Transects crossing Herschel Islands largest slump were performed, as well as quasi 3D-ERT-profiles. For better understanding these changes we compared the datasets focusing on the internal structures just as variations in the topography.

The aim for our study is gaining an impression of structural and topographical changes over several years, leading towards a better comprehension of long-term processes in retrogressive thaw slumps.

How to cite: Eppinger, S. and Krautblatter, M.: How retrogressive thaw slumps change over time - a study from Herschel Island (Canada) using 3D electrical resistivity tomography (ERT), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14989, https://doi.org/10.5194/egusphere-egu21-14989, 2021.

Cryoplanation terraces (CTs) are large, staircase-like erosional features found in upland periglacial environments throughout the circum-Arctic region. They are ubiquitous in unglaciated Beringia. This presentation summarizes recent research on these features conducted in interior and western Alaska and northwestern British Columbia. The work falls into several categories:

(1) Relative dating: Relative weathering indices (fracture counts, Cailleux roundness and flatness, Krumbein sphericity, rebound, and weathering rind thickness) were measured at a series of sites extending across eastern Beringia. Patterns of these indices indicate that inner treads were more recently exposed than distal locations. A model of time-transgressive CT development through nivation-driven scarp retreat addresses the removal of weathered material from terrace treads down side slopes through piping and gravity-driven mass-wasting processes.

(2) Absolute dating: Several ¹⁰Be and ³⁶Cl Terrestrial Cosmogonic Nuclide ages reveal that terrace scarps in the Alaskan Yukon-Tanana Upland were last actively eroding during the last glacial maximum (LGM). CT treads exhibit time-transgressive development. Boulder exposure ages and distances between sampled boulder locations were used to estimate rates of scarp retreat. The numerical exposure ages demonstrate that CTs are diachronous surfaces actively eroding during multiple cold intervals.

(3) Landscape evolution: The unusual deglaciation history of “Frost Ridge” in northwestern British Columbia facilitates estimation of long-term denudation attributable to nivation processes since the LGM. Snowbanks accumulated and persisted in marginal drainage features on the ridge’s north-facing ridge flank, creating a series of CTs through nivation. Data obtained from an unmanned aerial vehicle were used to estimate the volumes of eroded material. Estimated erosion rates are comparable to short-term nivation rates reported from Antarctica and mid-latitude alpine periglacial areas.

(4) Process monitoring: Soil thermal and moisture records, particle-size analysis, apparent thermal diffusivity calculations, and sediment-deposition patterns were used to examine periglacial processes operating on two active CTs. The coarse portions of sorted stripes function as underground channels (pipes) for sediment transportation across CT treads by flowing water. Late-lying snowbank environments are highly dynamic during warm weather, with large amounts of sediment transported over short periods.

(5) Geomorphometry: Semi- and fully automated recognition algorithms (CTAR) were applied to high-resolution DEMs to identify the locations of CTs. CTAR achieved an overall accuracy of 90 percent. A strong linear relation exists between the size of CTAR-delimited terraces and those identified in a previous study. Hypsometric analysis was applied over extensive areas of eastern Beringia. Glaciated areas have hypsometric signatures distinctly different than those of cryoplanated terrain, across a spectrum of geographical scale. Results from fluvial morphometric analysis of a sorted-stripe field verifies the origins of such networks and their effectiveness for transporting water and suspended sediment across CT surfaces.

(6) Climatic dependencies: Geospatial analysis involving nearly 700 CTs in eastern Beringia demonstrates that their elevation rises from Bering Sea islands to the Alaska-Canada border at rates nearly identical to those of Wisconsinan cirques, indicating close genetic links between the two classes of feature. Cryoplanation terraces can be considered the periglacial equivalent of glacial cirques.

How to cite: Mitchell, R., Nelson, F., Nyland, K., and Queen, C.: Advances in Cryoplanation Terrace Research: Recent Contributions, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13756, https://doi.org/10.5194/egusphere-egu21-13756, 2021.

To date, the treatment of permafrost in earth system models has been simplified due to the prevailing uncertainties in the processes involving frozen ground. In this study, we improved the modeling of permafrost physical processes in a state-of-the-art earth system model (MIROC) by taking into account some of the relevant physical properties of soil such as changes in the thermophysical properties due to freezing (https://doi.org/10.1186/s40645-020-00380-w). As a result, the improved version of the model was able to reproduce a more realistic permafrost distribution at the southern limit of the permafrost area by increasing the freezing of soil moisture in winter. The improved modeling of permafrost processes also had a significant effect on future projections. Using the conventional formulation, the predicted cumulative reduction of the permafrost area by year 2100 was approximately 60% (40–80% range of uncertainty from a multi-model ensemble) in the RCP8.5 scenario, while with the improved formulation, the reduction was approximately 35% (20–50%). Our results indicate that the improved treatment of permafrost processes in global climate models is important to ensuring more reliable future projections.

In addition, the processes of greenhouse gas (GHG) emissions due to permafrost degradation are not considered in many earth-system models. Therefore, we developed a model to diagnose that processes by using the output of earth system models (https://doi.org/10.1186/s40645-020-00366-8). The model called PDGEM (Permafrost Degradation and GHG Emission Model) describes the thawing of the Arctic permafrost including the Yedoma layer due to climate change and the GHG emissions. Our model simulations show that the total GHG emissions from permafrost degradation in the RCP8.5 scenario was estimated to be 31-63 PgC for CO2 and 1261-2821 TgCH4 for CH4 (68th percentile of the perturbed model simulations, corresponding to a global average surface air temperature change of 0.05–0.11 °C), and 14-28 PgC for CO2 and 618-1341 TgCH4 for CH4 (0.03–0.07 °C) in the RCP2.6 scenario. An advantage of PDGEM is that geographical distributions of GHG emissions can be estimated by combining a state-of-the-art land surface model featuring detailed physical processes with a GHG release model using a simple scheme, enabling us to consider a broad range of uncertainty regarding model parameters. In regions with large GHG emissions due to permafrost thawing, it may be possible to help reduce GHG emissions by taking measures such as restraining land development.

How to cite: Yokohata, T., Saito, K., Iwahana, G., Ito, A., and Tanaka, K.: Model improvement and projection of permafrost degradation and greenhouse gas emission, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13891, https://doi.org/10.5194/egusphere-egu21-13891, 2021.

Initialization (spin-up) of a numerical ground temperature model is a critical but often neglected step for solving heat transfer problems in permafrost. Improper initialization can lead to significant underlying model drift in subsequent transient simulations, distorting the effects on ground temperature from future climate change or applied infrastructure.  In a typical spin-up simulation, a year or more of climate data are applied at the surface and cycled repeatedly until ground temperatures are declared to be at equilibrium with the imposed boundary conditions, and independent of the starting conditions.

Spin-up equilibrium is often simply declared after a specified number of spin-up cycles. In few studies, equilibrium is visually confirmed by plotting ground temperatures vs spin-up cycles until temperatures stabilize; or is declared when a certain inter-cycle-temperature-change threshold is met simultaneously at all depths, such as ∆T ≤ 0.01^oC per cycle. In this study, we investigate the effectiveness of these methods for determining an equilibrium state in a variety of permafrost models, including shallow and deep (10 – 200 m), high and low saturation soils (S = 100 and S = 20), and cold and warm permafrost (MAGT = ~-10 ^oC and >-1 ^oC). The efficacy of equilibrium criteria 0.01^oC/cycle and 0.0001^oC/cycle are compared. Both methods are shown to prematurely indicate equilibrium in multiple model scenarios.  Results show that no single criterion can programmatically detect equilibrium in all tested models, and in some scenarios can result in up to 10^oC temperature error or 80% less permafrost than at true equilibrium.  A combination of equilibrium criteria and visual confirmation plots is recommended for evaluating and declaring equilibrium in a spin-up simulation.

Long-duration spin-up is particularly important for deep (10+ m) ground models where thermal inertia of underlying permafrost slows the ground temperature response to surface forcing, often requiring hundreds or even thousands of spin-up cycles to establish equilibrium. Subsequent transient analyses also show that use of a properly initialized 100 m permafrost model can reduce the effect of climate change on mean annual ground temperature of cold permafrost by more than 1 ^oC and 3 ^oC under RCP2.6 and RCP8.5 climate projections, respectively, when compared to an identical 25 m model. These results have important implications for scientists, engineers and policy makers that rely on model projections of long-term permafrost conditions.

How to cite: Ross, C., Beddoe, R., and Siemens, G.: Equilibrium Spin-up of Cold and Warm Permafrost Models, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13922, https://doi.org/10.5194/egusphere-egu21-13922, 2021.

The Vissátvuopmi palsa complex (N 68°74′50′′, E 21°11′30”) is the largest coherent palsa complex in Sweden (ca 274 ha). Aerial photo-interpretation over an area covered by plateau palsas showed a 30% decline in lateral area -- from ca 70 to 49 ha -- that occurred between 1955 to 2016 (Olvmo et al., 2020). Within Vissátvuopmi, we have more closely studied two single palsas, one dome-shaped and one ridge-shaped, for changes in extent, height and vegetation composition. Manual interpretation of aerial photography between 1955 and 2016 show lateral degradation of 35% and 54% for the dome and ridge palsas, respectively. Since 2018 we have monitored the palsas using images from drones as well as analysis of Planet Dove and Sentinel-2 satellite imagery. Photogrammetry is used to produce orthophotos as well as digital surface models (DSMs) from the drone images, and compared to earlier LiDAR and aerial photo DSMs, to study lateral and vertical degradation.

The drone-generated DSMs from 2018, 2019 and 2020 show further lateral degradation of the two large palsas. In 2020 a rapid change in vegetation composition was seen on the dome-shaped palsa, where a 250 m² area of Betula nana and Empetrum hermaphroditum transitioned to lichen. This vegetation change could be seen in spectral data from both drone and satellite platforms. The future development of this palsa, monitored annually using both fine and medium spatial resolution data, will give insight into the timing and signs of the individual palsas in stages of degradation.

How to cite: Reese, H., Olvmo, M., Thorsson, S., and Holmer, B.: Multi-scale remote sensing observations of a palsa in degradation phase, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14455, https://doi.org/10.5194/egusphere-egu21-14455, 2021.

The thermal regime in sediment below the ocean or lakes is mostly governed by the sea or lake bed temperature and by the geothermal heat flow. This thermal regime will determine whether permafrost beneath water bodies is preserved or how rapidly it thaws. Thermal modelling uses mean annual bottom water temperatures to calculate permafrost presence or absence, while predictions of shallow sediment thermal regimes must be forced with time series of changing bottom water temperatures that also account for freezeback of the water column to the bottom, forming bottom-fast ice. However, continuous, annual measurements of bottom water temperatures in Arctic lakes and coastal marine settings are hard to obtain and therefore scarce. Waves and sea ice movement make deployment and recovery of instruments difficult.

We provide a parameterization of the bottom water temperature function that relies on easier to obtain variables, such as the mean, minimum and maximum air temperature and the water depth, by comparing measured and modelled shallow sediment thermal regimes from the Arctic. We use a parameterization based on a simple cosine for the water temperature with mean temperature, amplitude and time shift and add the minimum water temperature to obtain a 4 parameter function. For shallow regions with bottom-fast ice, additionally the duration of the ice-growth and -melting period as well as the minimum air temperature are needed.

We test our parameterizations with a globally unique data set of 4 years of ground temperature data collected from the seabed to a depth of 10 m from the near shore zone of the Mackenzie Delta. At the instrumented sites, permafrost is present beneath mostly freshwater bottom-fast and floating ice. Forward modeling of sediment temperatures is performed using the 1D heat transfer model CryoGrid with depth dependent thermal properties. We neglect advective processes and long-term temperature trends in the bottom water temperatures.

Rough parameterization of the annual variation of water bottom temperatures reproduce measured water temperatures with a RMSE of 20-40 %. The resulting modeled sediment temperature field based on 10 years of repeated parameterized bottom water temperatures matches the modeled sediment temperature field based on measured water temperatures in terms of permafrost characteristics, including the depth of the active layer defined by the 0°C isotherm over the year. However, both modelled temperature fields yield significantly higher sediment temperatures than the measured sediment temperature field. This may be the result of choice of sediment thermal properties in the thermal model or shifts in the duration of bottom-fast ice contact or on-ice snow Since modelled temperature fields from both repeated measured and parameterized bottom water temperatures show the same deviation, it suggests that the bottom water temperatures were warmer during the measurement period than the average over the previous 10 years.

How to cite: Miesner, F., Overduin, P. P., and Stevens, C.: Seasonal Variations in Bottom Water Temperatures and their Influence on Subaquatic Permafrost Thermal Regimes, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14883, https://doi.org/10.5194/egusphere-egu21-14883, 2021.

Warming of permafrost regions with an associated increase in subsurface temperatures has been reported worldwide. Thus, long-term monitoring of the thermal state of permafrost and the associated ground ice contents has become an essential task also for the European Alps. Geophysical methods have proven to be well-suited to support and interconnect spatially sparse borehole data and investigate the distribution and temporal evolution of permafrost. In particular, electrical resistivity tomography (ERT) is a widely applied technique for permafrost characterization, commonly associated with a significant increase in the electrical resistivity upon freezing. However, air is also characterized by high electrical resistivity values complicating the interpretation of ERT results. Recent studies have revealed that the spectral induced polarization (SIP) response of frozen rocks is affected by the temperature-dependent polarization behaviour of ice at higher frequencies. Thus, the SIP or complex resistivity method offers potential for an improved characterization of permafrost sites.

We here present SIP imaging results conducted over a broad range of frequencies (0.1-225 Hz) at an operational long-term permafrost monitoring site covering a period of one and a half years. The selected study area Cervinia Cime Bianche (Italian Alps) is situated at an elevation of ~3100m and provides comprehensive geophysical, borehole temperature and water content data for validation. Shielded cables and an adequate measuring protocol were deployed to minimize the electromagnetic coupling in the SIP data. Data were collected as normal and reciprocal pairs for the quantification of data error, and we developed an analysis scheme for data quality that considers changes in time and in the frequency to remove spatial and temporal outliers and erroneous measurements. To understand the temperature dependence of the polarization response, we compare our field results with SIP laboratory measurements on water-saturated rock samples, collected in close proximity to the monitoring profile, in a frequency range of 10 mHz to 45 kHz during controlled freeze-thaw cycles (+20°C to -40°C).

Our field results show clear seasonal changes in the complex resistivity images. Resistivity magnitude shows an increase in winter and decrease in summer throughout the image plane, with most prominent changes at shallow depths, where also resistivity phase shows distinctly increased (absolute) values in winter for frequencies above 10 Hz. This region coincides with the active layer as monitored by borehole temperature logging, suggesting that especially the polarization response is indicative of the seasonal freezing and thawing of the ground. This interpretation is confirmed by the laboratory measurements on the rock samples from the site, which upon freezing and thawing exhibit an absolute phase increase with decreasing temperature at higher frequencies (above 10 Hz for temperatures down to -10°C), with the general spectral behaviour being consistent with the known polarization properties of ice. We conclude that with appropriate measurement and processing procedures, the characteristic dependence of the SIP response of frozen rocks on temperature, and thus ice content, can be utilized in field surveys for an improved assessment of thermal state and ice content at permafrost sites.

How to cite: Maierhofer, T., Limbrock, J. K., Katona, T., Drigo, E., Hilbich, C., Morra Di Cella, U., Kemna, A., Hauck, C., and Flores-Orozco, A.: Seasonal and annual dynamics of frozen ground at a mountain permafrost site in the Italian Alps detected by spectral induced polarization, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14598, https://doi.org/10.5194/egusphere-egu21-14598, 2021.

How to cite: Schroeder, T. and Krautblatter, M.: A high-resolution multi-phase thermo-geophysical permafrost rock model to verify long-term ERT monitoring at the Zugspitze (German/Austrian Alps), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15231, https://doi.org/10.5194/egusphere-egu21-15231, 2021.

According to consdidered influence of snow cover thickness and air temperature on variations of ground freezing depth at the site of meteorological observatory of Moscow State University and also according to the data of observatories in the Moscow region it is expected to make conclusions about the impact of the urban heat island to a ground freezing depth in Moscow region. For this purpose, the values of the maximum ground freezing depth were analyzed for MSU meteorological observatory and for the weather stations of the Moscow region: Kolomna, Mozhaisk and Sukhinichi. And since not always the data of actual observations are avaliable, for these weather stations the calculated values of the maximum ground freezing depth were obtained. The calculations were performed according to the previously developed calculation scheme, based on the problem of thermal conductivity of a three-layer medium (snow, frozen and thawed ground) with a phase transition at the boundary. The heat balance equation included the energy of the phase transition, the inflow of heat from the thawed ground and the outflow to the frozen ground and, in the presence of snow cover, through it to the atmosphere. The heat flow was calculated according to Fourier's law as the product of the thermal conductivity and the temperature gradient. It was assumed that the temperature in each medium varies linearly. For snow cover and frozen ground, the formula of thermal conductivity of a two-layer medium was used. The obtained calculated values were compared with the actual values of the ground freezing depth. The coefficients R² of the reliability of the linear trend line approximation when comparing the calculated and actual values for Moscow and the Moscow region were at the level of 0.6-0.7. The maximum ground freezing depth in Moscow and in the Moscow region in the same years may differ by an average of 10 cm. This confirms that the designed scheme well describes ground freezing depth based on data on air temperature and snow cover thickness and can be used to model the underground heat island of the Moscow region. In report it is also supposed to present the results of the recent years observations of snow cover and freezing depth variations in Moscow and the Moscow region. The past  2020 year is considered as the warmest in the entire history of observations according to the MSU Meteorological Observatory for Moscow, according to the Hydrometeorological Center of Russia for the whole of Russia and according to the Copernicus Climate Change Service (C3S) for the entire Globe. So the winter season of 2019/20 in Moscow region was also unusually warm, and therefore in the winter season of 2019/20 there was very little snow in the Moscow region. However, the warm summer of 2020 resulted in one of the lowest summer values of sea ice extent in the Arctic and, as a result, abnormally strong minimum temperatures and heavy snowfall in the winter of 2020/21 in Eurasia and Moscow. The work was done in a frame of state topic AAAA-A16-116032810093-2.

How to cite: Frolov, D.: Influence of snow cover and air temperature on variations of ground freezing depth in Moscow and the Moscow region, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4455, https://doi.org/10.5194/egusphere-egu21-4455, 2021.

Rock glaciers (RG) are visual evidence of mountain permafrost, and are one of the most important geomorphological features in the Peruvian Andes. The main objective of this research was to determine the spatial distribution of RG, their degree of activity, as well as their morphological and climatic characteristics in Cordilleras Vilcanota (Southeast), Ampato (Southwest) and La Viuda (Center). For this study, we used high-resolution images from Google Earth-Pro, SASPlanet and a DEM ALOS PALSAR (12.5 m) to identify and digitize the RG based on their geomorphological attributes, and we derived  the potential incoming solar radiation (PISR), based on the DEM . The WorldClim dataset (1970-2000) was used to determine the mean annual air temperature (MAAT) and the precipitation in the analyzed zones.

The Cordillera Ampato, with 139 RGs, presents the lowest minimum altitude of the RGs inventory (4537 m a. s. l.), the lowest MAAT (-0.4°C), lower slope (18°) and concentrates the highest PISR (1083 kWh/m²). The Cordillera Vilcanota concentrates a lower number of RGs (54), a higher minimum altitude of RGs (4733 m a. s. l.) and a relatively higher MAAT (1.9°C). Comparing both southern Cordilleras with respect to Cordillera central (La Viuda), it has the lowest amount of RG (8), a higher minimum altitude of RG (4747 m a. s. l.), higher slope (23°), higher MAAT (2.2°C) and lower persistence of snow cover. With regard to the RG activity, it was found that the quantity of active RG compared to inactive RG is in a proportion of 1.6 in Cordillera Ampato and 0.2 in Cordilleras Vilcanota and La Viuda.

Finally, the spatial distribution analysis shows that the greatest amount of RGs is located in the southern zone, decreasing towards the northern regions of Peru while the opposite occurs with the average MAAT of the RG, that is, the MAAT decreases as the RG moves to southern regions of Peru. On the other hand, the SW zone (dry climate) concentrates the largest amount of RG compared to the SE zone (wet climate). In addition, the topoclimatic parameters condition the formation of RG in the Cordilleras of study. 

How to cite: Medina, K., Loarte, E., Badillo-Rivera, E., León, H., Bodin, X., and Huggel, C.: Analysis of the spatial distribution and characteristics of the rock glaciers in the Ampato, Vilcanota and La Viuda Cordilleras southern and central Peru, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8466, https://doi.org/10.5194/egusphere-egu21-8466, 2021.

Permafrost has become thermally instable as a result of surface warming, which has an uncertain impact on future hydrogeological conditions and the associated mobilisation of carbon and release into the atmosphere. Numerical modelling can provide insights into future permafrost spatial and temporal dynamics. However, crucial observational data of permafrost active-layer thermal properties; thermal conductivity and heat capacity are sparse, resulting in a large uncertainty in forecasts of the future development of the active layer. Therefore, our study aims to develop a methodology to numerically determine the permafrost thermal and soil properties from observations of temperature time-series in the subsurface, in order to reduce the current model uncertainty.

We used an ensemble of 786 numerical 1D permafrost models fitted against observed active layer temperature data from the Qinghai-Tibetan Plateau (QTP)¹ to find the optimal values for the soil thermal conductivity, heat capacity and porosity. Optimal parameter values are determined by finding the minimum RMSE, KGE and using the Russell error measure. We find optimized values for bulk volumetric heat capacity 1.3-1.85 10⁶J/m³°C , bulk thermal conductivity 0.9-1.1 W/m°C and porosity between 0.25-0.35 (-), which are in agreement with typical ranges reported in literature for similar settings on the QTP. In a further sensitivity study, the 3 optimal parameter combinations were used to model the active layer thickness over a 100-year period with a gradual hypothetical air temperature increase of 5°C. The results indicate a substantial difference in rate of thawing and increase in depth of the active layer for these models, with a maximum time-lag of roughly 15 years in before the models reach the same active layer thawing depth. The study shows how numerical models can be applied to determine active layer thermal properties without the need for field samples, opening up new possibility for in-situ permafrost temperature observation.

1. Luo, D. L., Jin, H. J., He, R. X., Wang, X. F., Muskett, R. R., Marchenko, S. S., & Romanovsky, V. E. (2018). Characteristics of water-heat exchanges and inconsistent surface temperature changes at an elevational permafrost site on the Qinghai-Tibet Plateau. Journal of Geophysical Research: Atmospheres, 123, 10,057–10,075. https://doi.org/10.1029/2018JD028298

How to cite: de Bruin, J., Bense, V., and van der Ploeg, M.: Determining permafrost active layer thermal properties of the Qinghai–Tibet Plateau using field observations and numerical modelling, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9377, https://doi.org/10.5194/egusphere-egu21-9377, 2021.

Warming of permafrost in steep rock walls decreases their mechanical stability and could triggers rockfalls and rockslides. However, the direct link between climate change and permafrost degradation is seldom quantified with precise monitoring techniques and long-term time series. Where boreholes are not possible, laboratory-calibrated Electrical Resistivity Tomography (ERT) is presumably the most accurate quantitative permafrost monitoring technique providing a sensitive record for frozen vs. unfrozen bedrock. Recently, 4D inversions allow also quantification of frozen bedrock extension and of its changes with time (Scandroglio et al., in review).

In this study we (i) evaluate the influence of the inversion parameters on the volumes and (ii) connect the volumetric changes with measured mechanical consequences.

The ERT time-serie was recorded between 2006 and 2019 in steep bedrock at the permafrost affected Steintälli Ridge (3100 m asl). Accurately positioned 205 drilled-in steel electrodes in 5 parallel lines across the rock ridge have been repeatedly measured with similar hardware and are compared to laboratory temperature-resistivity (T–ρ) calibration of water-saturated samples from the field. Inversions were conducted using the open-source software BERT for the first time with the aim of estimating permafrost volumetric changes over a decade.

(i) Here we present a sensitivity analysis of the outcomes by testing various plausible inversion set-ups. Results are computed with different input data filters, data error model, regularization parameter (λ), model roughness reweighting and time-lapse constraints. The model with the largest permafrost degradation was obtained without any time-lapse constraints, whereas constraining each model with the prior measurement results in the smallest degradation. Important changes are also connected to the data error estimation, while other setting seems to have less influence on the frozen volume. All inversions confirmed a drastic permafrost degradation in the last 13 years with an average reduction of 3.900±600 m³ (60±10% of the starting volume), well in agreement with the measured air temperatures increase.

(ii) Average bedrock thawing rate of ~300 m³/a is expected to significantly influence the stability of the ridge. Resistivity changes are especially evident on the south-west exposed side and in the core of the ridge and are here connected to deformations measured with tape extensometer, in order to precisely estimate the mechanical consequences of bedrock warming.

In summary, the strong degradation of permafrost in the last decade it’s here confirmed since inversion settings only have minor influence on volume quantification. Internal thermal dynamics need correlation with measured external deformation for a correct interpretation of stability consequences. These results are a fundamental benchmark for evaluating mountain permafrost degradation in relation to climate change and demonstrate the key role of temperature-calibrated 4D ERT.

Reference:

Scandroglio, R. et al. (in review) ‘4D-Quantification of alpine permafrost degradation in steep rock walls using a laboratory-calibrated ERT approach’, Near Surface Geophysics.

How to cite: Offer, M., Scandroglio, R., Draebing, D., and Krautblatter, M.: 4D-Quantification of alpine permafrost degradation in a bedrock ridge using multiple inversion schemes and deformation measurements, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13419, https://doi.org/10.5194/egusphere-egu21-13419, 2021.