Sebastian Westermann’s research while affiliated with University of Oslo and other places

Interferometric Synthetic Aperture Radar (InSAR) remote sensing of surface displacement in permafrost environments has the potential to resolve ground ice dynamics and potentially active layer thickness, yet field validation is sparse. Here we present a comparison between in-situ ground ice contents and the seasonal InSAR displacements of the following thawing season at 12 coring sites in Adventdalen, Svalbard. The study is focused on the year 2023, where frozen sediment cores were collected at the end of spring from the active layer and the uppermost permafrost. The sediment cores were analyzed with high resolution for volumetric ground ice and excess ice contents. The active layer thickness was estimated by probing the thaw depth at the end of the thawing season 2023, allowing to estimate the amount of expected subsidence from seasonal ground ice melt. The InSAR vertical displacements for the thawing season were derived from Small Baseline Subset (SBAS) processing of Sentinel-1 imagery. The expected subsidence from ground ice melt within the measured active layer thickness aligned well with the seasonal InSAR maximum vertical displacement. Monte Carlo simulations were performed to include uncertainties in the expected and measured InSAR subsidence, leading to a mean coefficient of determination of 0.68 and a mean absolute error of 15 mm for the correlation between InSAR subsidence and expected subsidence from in-situ ground ice melt. Excess ice is highly variable and is the main source of the expected subsidence during this thawing season, which was exceptionally warm. The expected subsidence and active layer thickness show only a weak relationship due to the observed complex ice content distribution in the active layer and uppermost permafrost. Our results show the significant potential of InSAR for mapping ground ice variability; however, they also suggest that estimating active layer thickness using InSAR requires careful consideration of the complex occurrence of both pore and excess ice in the active layer and uppermost permafrost.

Grazing by livestock can alter the surface conditions at grassland sites, impacting the transfer of energy between the atmosphere and ground and consequentially ground temperatures. In this study, we investigate surface cover in summer and winter and measure ground surface temperatures over 14 months at sites in central Mongolia that feature different grazing intensities (intensely and ungrazed) and topographic aspects (north- and south-facing). Overall, intense grazing leads to a substantially reduced vegetation cover, altered snow conditions, and lack of surface litter accumulation. Comparing intensely grazed and ungrazed plots shows large seasonal differences in ground surface temperatures, with grazed plots being up to +5.1 °C warmer in summer and -5.4 °C colder in winter at a south-facing site. We also find that the effect of grazing intensity depends on topographic aspect, with smaller seasonal differences of +1.4 °C and -2.5 °C found between grazed and ungrazed plots at a north-facing site. This relates to the lower available solar radiation at north-facing sites, which reduces the differences in vegetation cover between open and fenced plots. For both aspects, the seasonal differences largely offset each other, with both a small net cooling and warming depending on effects in spring and autumn. Our study suggests that livestock management could be used to modify the annual ground temperature dynamics, possibly even influencing local permafrost dynamics.

Permafrost soils are undergoing rapid thawing due to climate change and global warming. Permafrost peatlands are especially vulnerable, as they are located near the southern margin of the permafrost domain in the zones of discontinuous and sporadic permafrost. They store large quantities of carbon (C) which, upon thawing, may be decomposed and released as carbon dioxide (CO2), methane (CH4) and dissolved organic carbon (DOC). This study characterises patterns of potential C degradation and mobilisation within an area with sporadic permafrost by evaluating C degradation in three permafrost peatland ecosystems in Finnmark, Norway, under laboratory conditions. Active-layer, transition zone and permafrost samples from distinct cores were thawed under controlled conditions and incubated for up to 350 d under initially oxic or anoxic conditions while measuring CO2, CH4 and DOC production. Carbon degradation varied among the three peat plateaus but showed a similar trend over depth, with the largest CO2 production rates in the upper active layer and the top of the permafrost. Despite marked differences in peat chemistry between the layers, post-thaw CO2 production of permafrost peat throughout the first 350 d reached 67 %–125 % of that observed in samples from the top of the active layer. De novo CH4 production occurred after prolonged anoxic incubation in samples from the transition zone and permafrost, but it was not found in active-layer samples. CH4 production was highest in incubations from thermokarst peat sampled next to decaying peat plateaus. DOC production by active-layer samples throughout 350 d incubation exceeded gaseous C loss by up to 23-fold under anoxic conditions, whereas production by permafrost peat was small. Taken together, the results of our study suggest that permafrost peat in thawing Norwegian peat plateaus degrades at rates similar to those of active-layer peat, while the highest CH4 production can be expected after the inundation of thawed permafrost material in thermokarst ponds.

In Central Asia, the ground thermal regime is strongly affected by the interplay between topographic factors and ecosystem properties. In this study, we investigate the governing factors of the ground thermal regime in an area in Central Mongolia, which features discontinuous permafrost and is characterized by grassland and forest ecosystems. Miniature temperature dataloggers were used to measure near-surface temperatures at c. 100 locations throughout the 6 km² large study area, with the goal to obtain a sample of sites that can represent the variability of different topographic and ecosystem properties. Mean annual near-surface ground temperatures showed a strong variability, with differences of up to 8 K. The coldest sites were all located in forests on north-facing slopes, while the warmest sites are located on steep south-facing slopes with sparse steppe vegetation. Sites in forests show generally colder near-surface temperatures in spring, summer and fall compared to grassland sites, but they are warmer during the winter season. The altitude of the measurement sites did not play a significant role in determining the near-surface temperatures, while especially solar radiation was highly correlated. In addition, we investigated the suitability of different hyperspectral indices calculated from Sentinel-2 as predictors for annual average near-surface ground temperatures. We found that especially indices sensitive to vegetation properties, such as the Normalized Difference Vegetation Index (NDVI), show a strong correlation. The presented observations provide baseline data on the spatiotemporal patterns of the ground thermal regime which can be used to train or validate modelling and remote sensing approaches targeting the impacts of climate change.

In many Arctic regions, marine coastlines change rapidly in our currently warming climate. In contrast, coastal rock cliffs on Svalbard are considered to be relatively stable. Long-term trends of coastal-retreat rates for rock cliffs on Svalbard remain unknown, but quantifying them could improve our understanding of coastal dynamics in the Canadian Arctic Archipelago. This study presents coastal-retreat rates for rock cliffs along several kilometres of Brøggerhalvøya, Svalbard. The analysis relies on high-resolution orthoimages from 1970, 1990, 2010, and 2021. The data are corroborated by high-precision dGNSS (differential Global Navigation Satellite System) measurements obtained along selected segments of the coastline. Our analysis reveals statistically significant acceleration in coastal-retreat rates across Brøggerhalvøya between 2010 and 2021. The northeast-facing coastline features fairly stable conditions, with retreat rates of 0.04 ± 0.06 ma-1 (1970–1990; calculated retreat rate ± the corresponding measurement uncertainty), 0.04 ± 0.04 ma-1 (1990–2010), and 0.06 ± 0.08 ma-1 (2010–2021). Along the southwest-facing coastline, higher retreat rates of 0.26 ± 0.06 ma-1 (1970–1990), 0.24 ± 0.04 ma-1 (1990–2010), and 0.30 ± 0.08 ma-1 (2010–2021) were calculated. For the most recent decade, this corresponds to an increase of 50 % for the northeast-facing coastline and an increase of 25 % for the southwest-facing coastline. Furthermore, for the northeast-facing coastline, the proportion of the coastline affected by erosion increased from 47 % (1970–1990) to 65 % (2010–2021), while it stayed consistently above 90 % for the southwest-facing coastline. The recent acceleration in retreat rates coincides with increasing storminess and retreating sea ice, factors that can enhance coastal erosion.

The Arctic is changing rapidly due to the amplification of global temperature trends, causing profound impacts on the ice sheet in Greenland, glaciers, frozen ground, ecosystems, and societies. Here, we focus on impacts that atmospheric circulation causes in addition to the climate warming trends. We combine time series of glacier mass balance from temporal satellite gravimetry measurements (GRACE/GRACE-FO; 2002–2023), active layer thickness in permafrost areas from ESA’s Climate Change Initiative remote sensing and modelling product (2003–2019), and field measurements of the Circumpolar Active Layer Monitoring Network (2002–2023). Despite regional and system-related complexities, we identify robust covariations between these observations, which vary asynchronously between neighbouring regions and synchronously in regions antipodal to the North Pole. We reveal a close connection with dominant modes of atmosphere circulation, controlling about 75% of the common pan-Arctic impact variability (2002–2022), also affecting the Greenland Ice Sheet. We emphasize that it is necessary to consider such atmospheric driving patterns when projecting impacts, particularly caused by extremes, in an increasingly warmer Arctic.

Permafrost warming has been observed all around the Arctic, however, variations in temperature trends and their drivers remain poorly understood. We present a comprehensive analysis of climatic changes spanning 25 years (1998–2023) at Bayelva (78.92094°N, 11.83333°E) on Spitzbergen, Svalbard. The quality controlled hourly data set includes air temperature, radiation fluxes, snow depth, rainfall, active layer temperature and moisture, and, since 2009, permafrost temperature. Our Bayesian trend analysis reveals an annual air temperature increase of 0.9 ± 0.5°C/decade and strongest warming in September and October. We observed a significant shortening of the snow cover by −14 ± 8 days/decade, coupled with reduced winter snow depth. The active layer simultaneously warmed by 0.6 ± 0.7°C/decade at the top and 0.8 ± 0.5°C/decade at the bottom. While the soil surface got drier, in particular during summer, soil moisture below increased in accordance with the longer unfrozen period and higher winter temperatures. The thawed period prolonged by 10–15 days/decade at different depths. In contrast to earlier top‐soil warming, we observed stable temperatures since 2010 and only little permafrost warming (0.14 ± 0.13°C/decade). This is likely due to recently stable winter air temperature and continuously decreasing winter snow depth. This recent development highlights a complex interplay among climate and soil variables. Our distinctive long‐term data set underscores (a) the changes in seasonal warming patterns, (b) the influential role of snow cover decline, and (c) that air temperature alone is not a sufficient indicator of change in permafrost environments, thereby highlighting the importance of investigating a wider range of parameters, such as soil moisture and snow characteristics.

Reconstructing historical climate change from deep ground temperature measurements in cold regions is often complicated by the presence of permafrost. Existing methods are typically unable to account for latent heat effects due to the freezing and thawing of the active layer. In this work, we propose a novel method for reconstructing historical ground surface temperature (GST) from borehole temperature measurements that accounts for seasonal thawing and refreezing of the active layer. Our method couples a recently developed fast numerical modeling scheme for two‐phase heat transport in permafrost soils with an ensemble‐based method for approximate Bayesian inference. We evaluate our method on two synthetic test cases covering both cold and warm permafrost conditions as well as using real data from a 100 m deep borehole on Sardakh Island in northeastern Siberia. Our analysis of the Sardakh Island borehole data confirms previous findings that GST in the region have likely risen by 5–9°C between the pre‐industrial period of 1750–1855 and 2012. We also show that latent heat effects due to seasonal freeze‐thaw have a substantial impact on the resulting reconstructed surface temperatures. We find that neglecting the thermal dynamics of the active layer can result in biases of roughly −1°C in cold conditions (i.e., mean annual ground temperature below −5°C) and as much as −2.6°C in warmer conditions where substantial active layer thickening (>200 cm) has occurred. Our results highlight the importance of considering seasonal freeze‐thaw in GST reconstructions from permafrost boreholes.

Grazing by livestock can alter the surface conditions at grassland sites, impacting the transfer of energy between the atmosphere and ground and consequentially ground temperatures. In this study, we investigate surface cover in summer and winter and measure ground surface temperatures over 14 months at sites in Central Mongolia that feature different grazing intensities (intensely and ungrazed) and topographic aspects (north- and south-facing). Overall, intense grazing leads to a substantially reduced vegetation cover, altered snow conditions and lack of surface litter accumulation. Comparing intensely grazed and ungrazed plots shows large seasonal differences in ground surface temperatures, with grazed plots being up to +5.1 °C warmer in summer and −5.4 °C colder in winter at a south-facing site. We find smaller seasonal differences of +1.4 °C and −2.5 °C between grazed and ungrazed plots at a north-facing site which receives less solar radiation and where differences in vegetation cover between open and fenced plots are smaller. For both aspects, the seasonal differences largely offset each other, with both a small net cooling and warming depending on effects in spring and autumn. Our study suggests that livestock management might be used to modify the annual ground temperature dynamics, possibly even influencing local permafrost dynamics.