Waste disposal in the Arctic involves numerous challenges due to the region's harsh, remote environment, permafrost conditions, climate change, and the fragility of the Arctic ecosystem. In 2017, 66.1% of Arctic communities were situated on permafrost. Waste from these communities is still primarily disposed of in landfills. To ensure environmentally safe and sustainable waste management, and ultimately achieve the permanent closure of Arctic landfills, it is essential to carefully evaluate factors such as the thermal state of the ground and the effects of climate change.

We examine a solid waste landfill situated in the Advent Valley on Svalbard, approximately nine kilometers southeast of Longyearbyen. The landfill is placed directly on the ground, with no constructed sealing layer separating the original soil from the waste and is entirely located below the marine limit. We analyze five years of ground temperature data, down to depths of 11-12 meters, both around and within the landfill, in relation to recent temperature trends in a deep borehole in Longyearbyen, which was established in 1985.

Borehole temperatures in all five boreholes outside the landfill suggest a zero annual amplitude slightly below 11 m and mean annual ground temperatures in the range of -1 to -2.5 °C, with maximum active layer depths at undisturbed locations reaching down to 1 to 2 m. This is in the same range as reported for other sites near Longyearbyen, but warmer than the mean annual ground temperature (at 10 meters depth) of -2.8 °C, measured in the deep borehole at the beach (Gisnås et al., 2023). Near the landfill, our results point to permafrost degradation due to water accumulation at a concrete wall intended to mitigate effects of water-flow. Borehole temperature measurements at the landfill reveal, furthermore, that there are no permafrost conditions below the landfill today.

The thermal state of permafrost and the thickness of the active layer have attracted a huge interest over recent decades because climate changes have provoked worldwide permafrost warming and active-layer thickening, with potentially severe consequences for nature and society. Permafrost and active-layer monitoring is therefore of great importance.

Besides temperature, geophysical and/or manual measurements, a variety of models have been developed for estimating the mean annual permafrost table temperature (MAPT) and active-layer thickness (ALT). These tools typically require at least a few ground physical properties as input parameters in addition to temperature variables, which are, however, unavailable or unrepresentative at most sites. Ground physical properties are therefore commonly estimated, which may yield model outputs of unknown validity.

Here, we present two novel and simple analytical−statistical models (ASMs) for estimating MAPT and ALT, which are driven solely by pairwise combinations of thawing and freezing indices in the active layer; no ground physical properties are required. ASMs reproduced MAPT and ALT well in most numerical validations, which corroborated their theoretical assumptions under idealized scenarios. Under field conditions of Antarctica and Alaska, the mean ASMs deviations in MAPT and ALT were less than 0.03 °C and 5 %, respectively, which is similar or better than other analytical or statistical models. This suggests that ASMs can be useful tools for estimating MAPT and ALT under a wide range of environmental conditions.

INTERACT (International Network for Terrestrial Research and Monitoring in the Arctic) is an Arctic network of 74 terrestrial field bases (with an additional 21 research stations in Russia on pause) that aims at building capacity for research and monitoring in the Arctic. Alfred Wegener Institute Helmholtz Center for Polar and Marine Research (AWI) was the responsible partner for the Research Station Samoylov Island in the Siberian Lena Delta and coordinated access to the station and the long-term observatories on Samoylov Island as well as to the wider Lena Delta region for international research teams until 2021.

Since the cooperation stop with Russia, we have intensified the compilation and publication of data, which had been acquired in the Lena Delta region for the past >20 years, according to the ‘FAIR Guiding Principles for scientific data management and stewardship’ through the INTERACT Virtual Access program. These include compilations of:

- thaw depth (TD) measurements of the active layer (AL) based on published and unpublished field data collected during Russian and joint German-Russian expeditions from 1998 to 2022,

- metadata on greenhouse gas (GHG) measurements (in situ, in the lab) based on field work between 1998 and 2021,

- published and unpublished data related to ground ice (ice-wedge ice, intra-sedimentary ice).

The datasets are unique in their spatial and temporal coverage and will be, once published in PANGAEA (https://pangaea.de/), highly valuable sources for future studies related to permafrost thaw, ecosystem changes, climate feedbacks, etc. in the Lena Delta region and beyond.

Acknowledgement

This work was funded by INTERACT III under EU-H2020 Grant Agreement No. 871120.

Permafrost is observed globally as an Essential Climate Variable (ECV) of the Global Climate Observation System (GCOS). Mountain permafrost constitutes about a third of the global permafrost area and exists at low to high latitudes on both hemispheres. It is characterized by high spatial variability in (sub)surface and atmospheric conditions and large environmental gradients. Mountain permafrost warming and thawing strongly impacts mountain ecosystems and communities.

Our study examines 21st century permafrost warming patterns in European mountains based on a compilation of decadal ground temperature data from sixty-four boreholes collected in the Alps, Scandinavia, Iceland and Svalbard, spanning one to three decades until 2022. The data set has unique spatial coverage with a latitudinal range from 45 to 78 °N and elevations between 275 and 3850 m asl. Observation sites are located in dry bedrock slopes or plateaus, ice-bearing talus slopes and ice-rich rock glaciers.

Measured annual ground temperatures at 10 m depth vary between nearly 0 °C and below –6 °C. Calculated warming rates at 10 meters depth for the period 2013–2022 exceed 1 °C per decade in some cases, generally surpassing previous estimates because of accelerated warming and employing a more comprehensive data set. Substantial permafrost warming occurred at cold and ice-poor bedrock sites at high elevations and latitudes (e.g., mountain sites on Svalbard or above 3500 m asl. in the European Alps), at rates comparable to surface air temperature increase. In contrast, latent heat effects in ice-rich ground close to the lower permafrost boundary (such as in many rock glaciers in the Alps) reduce warming rates and obscure important changes of mountain permafrost substrates. The observed warming patterns are consistent across all regions, depths and time periods considered. The future propagation of warming and thawing permafrost to greater depths is largely predetermined already for the coming decades.

Snow cover duration and timing of snowmelt are important factors in periglacial environments: The snowpack affects heat flux between the subsurface and atmosphere due to its insulating properties and acts as water storage and supply influencing liquid water availability. As snow cover duration usually shows high spatial heterogeneity, in situ observations have limited capabilities to monitor variations on larger scales with high spatial coverage. Well established optical remote sensing approaches experience challenges due to frequent cloud coverage and, at high latitudes, polar night. Further, optical sensors are insensitive to the liquid water content and, therefore, cannot differentiate between wet and dry snow. However, several studies have shown that SAR remote sensing is capable of detecting wet snow but comparative analyses with in-situ observations so far remain limited.

Therefore, we compare Sentinel-1 derived snow melt phases (runoff onset and melt-out) with the start and the end of the zero curtain phase observed in the temperature logger time series. Results show that the start of the zero curtain phase (runoff onset) can be detected with an RMSE of 12 days and the end of the zero curtain phase (melt-out) with an RMSE of 20 days. The decreased accuracy for the latter can be explained with effects of microtopography and different spatial resolutions of the satellite data (20 m pixel size) and in-situ measurements as especially loggers located on elevated terrain often detect the end of the zero curtain phase around 40-60 days earlier than the SAR-derived melt-out date. This preliminary analysis indicates high potential of SAR time series data for the detection of snow melt phases.

More information about snowpack properties could further increase the understanding, how the SAR signal reacts to changes in the snowpack and how SAR could be used to monitor snowmelt phases at larger scale.

The “Fjellviten” (Mountain Knowledge) project, initiated by the Norwegian Mountain Center in partnership with national research institutions, seeks to actively engage the Norwegian public, especially young people, in the monitoring of mountain permafrost. The project focuses on enhancing real-time permafrost and climate monitoring in the high-altitude regions of Norway, particularly in Jotunheimen, and aims to foster greater public involvement in understanding climate change impacts in these areas.

Over four years, it will establish digital and physical platforms for data sharing, with a strong emphasis on involving high school students in developing knowledge and get a broader understanding of how climate research is carried out. These efforts will benefit students across Norway, particularly those near Jotunheimen, while also broadening public access to permafrost and climate data on both national and international levels through modern communication tools.

The project area in Jotunheimen is a high-altitude plateau situated at approximately 1800 masl, surrounded by Norway's highest peaks, which reach up to 2500 masl. Jotunheimen has been the focus of extensive permafrost research over the past 50 years and is home to the deep borehole (129 meters) at Juvvasshøe established by the PACE (Permafrost and Climate in Europe) project in 1999.

As part of the project, new boreholes were drilled to a depth of 45 m adjacent to the University of Oslo's 10 m deep boreholes Juv-BH1 on Juvflye (1851 masl) and Juv-BH3 at Dugurdskampen (1546 masl) with continuous data dating back to 2008 and selected observations dating back to 1982.

The boreholes are upgraded with new thermistor cables and data loggers for real-time data transmission, integrating them into the operational permafrost monitoring program at the Norwegian Meteorological Institute to ensure long-term operation. Daily updates and data visualization products will be available on https://cryo.met.no/permafrost, supporting the digitalization strategy in the project.