Protective forests are crucial for maintaining slope stability and play a vital role in mitigating natural hazards in mountain environments. Forest gaps influence shallow landslide occurrences and significantly impact forests, shaping their development, resistance, resilience, overall ecology, and biodiversity. Understanding the extent of forest gaps is thus essential for effective forest management, balancing protective effects with ecological functions and the promotion of regeneration. We determine practice-oriented gap dimension thresholds (length, width, area), assess how slope gradient and vegetation influence these dimensions, and discuss the trade-offs between protective capacity and adaptive management. We compiled a comprehensive dataset from landslide inventories and remote sensing data across six landslide-prone regions in Switzerland and statistically analysed 632 forest gaps. The probability of landslide occurrence increases with gap length, width, and area. This is consistent across different slope gradients. For gaps with more than three trees and a vegetation cover percentage exceeding 25 %, the dimensions of landslide-associated gaps are significantly larger than those without landslides. We suggest applying logistic regression to identify thresholds that indicate landslide occurrence probabilities, thus improving awareness of the related risk levels. Based on 25 % and 33 % probability levels, we recommend a maximum gap length and width of 22 m, with a gap area of 500 m2; where natural regeneration is ensured, these parameters could be increased to 30 m by 32 m and 1000 m2, respectively. The suggested threshold values should be regarded as references and require on-site evaluation. Our outlined approach relies on dialogue and close cooperation with forest management practitioners.

Dramatic changes have occurred over the past century on many parts of the planet due to natural factors and intensifying human activities. Understanding these changes is critical for quantifying long-term environmental trends and modeling future conditions. A vast underexploited resource for such analysis lies in historical aerial and satellite stereo imagery captured with analog cameras from the early 1900s to the early 2000s. Originally acquired for military and mapping purposes, the stereoscopic nature of historical images offers unique potential to reconstruct 3D Earth surface changes across the 20th century. Recent algorithmic advances in photogrammetry and computer vision have greatly enhanced this potential. Despite the early recognition of their value, these datasets remain underexploited due to challenges related to 1) fragmented and inaccessible archives, 2) digitization and associated costs, and 3) a lack of scalable automated processing solutions. This review article addresses these challenges by analyzing 198 studies that digitally process historical aerial and satellite stereo imagery scanned from film. As the primary focus of this article is to review the processing of historical imagery, the greater emphasis on 3D reconstruction, such as the generation of digital elevation models (DEMs), and on vertical accuracy reflects both the focus of most published studies and the types of metrics commonly reported in literature. However, we also review work on 2D products, such as orthophotos, when photogrammetric processing is involved. We provide an overview of the accessed archives, processing strategies, and software pipelines. We discuss emerging tools and advances in image matching algorithms and georeferencing solutions, and we highlight how historical imagery can support a wide range of geoscientific applications, from climate change to urban development. Finally, we emphasize the urgent need to unlock these archives and develop efficient reproducible workflows to preserve and exploit this irreplaceable remote sensing dataset before physical degradation or institutional neglect make it inaccessible.

Abstract Global warming is amplified in the Arctic, accelerating glacier melt and freshwater runoff. At tidewater glaciers, runoff typically enters fjords at depth and generates buoyancy‐driven circulation that enhances glacier‐ocean exchanges of energy and matter, influencing macronutrient delivery and marine primary production. However, most studies lack the temporal resolution to capture low‐frequency, high‐magnitude events, leaving their impacts poorly understood. Here, we combine glacier observations with high‐frequency fjord and glacier‐lake sampling to examine the 2021 glacier lake outburst flood (GLOF) from Lake Setevatnet into Kongsfjorden (Svalbard). We show how evolving subglacial conditions before and during the GLOF shaped macronutrient supply to the inner fjord through both direct runoff and entrainment of bottom waters. Early in summer, nutrient delivery was dominated by direct runoff, supplying nitrate (NO 3 − ) and silicate via an inefficient drainage system. As the melt increased, an efficient system formed, generating a subglacial plume and initiating buoyancy‐driven circulation that entrained nutrient‐rich deep water. Despite high NO 3 − lake concentrations, the flood barely affected fjord NO 3 − levels. Instead, it produced a seasonal maximum in nitrite (NO 2 − ). Comparisons with conservative mixing estimates and nitrogen budget analyses reveal a non‐conservative nutrient signal. Although sedimentary sources cannot be excluded, the timing and spatial pattern of the NO 2 − anomaly suggest subglacial modification during floodwater transit. These findings indicate that Kongsfjorden functions as a summer nitrogen sink, partly shaped by subglacial transformations. Overall, nutrient delivery from tidewater glaciers depends not only on runoff volume but also on the subglacial drainage system characteristics, which evolve during high‐magnitude events such as GLOFs.

Historical aerial images, captured by film cameras in the previous century, are valuable resources for quantifying Earth's surface and landscape changes over time. In the post-war period, these images were often acquired to create topographic maps, resulting in the acquisition of large-scale aerial photographs with stereo coverage. Photogrammetric techniques applied to these stereo images enable the extraction of 3D information to reconstruct digital surface models (DSMs) and orthoimages. Here, we present a highly automated photogrammetric approach for generating countrywide DSMs of Switzerland, at a 1 m resolution, from approximately 32 000 scanned aerial stereo images acquired between 1979 and 2006, with known exterior and interior orientation. We derived four countrywide DSMs for the epochs 1979-1985, 1985-1991, 1991-1998, and 1998-2006. From the DSMs, we generated corresponding countrywide vegetation height models (VHMs). We assessed the quality of the historical DSMs at the country scale and within six representative study sites, evaluating the vertical accuracy and the completeness of image matching across different land cover types. Mean completeness ranged from 64 % for "glacial and perpetual snow" to 98 % for "sealed surfaces", with a value of 93 % for the "closed forest" class. Across Switzerland, the median elevation accuracy of the historical DSMs compared with a reference digital terrain model (DTM) on sealed surface points ranged from 0.08 to 0.16 m, with a normalized median absolute deviation (NMAD) of around 0.8 m and a maximum root mean square error (RMSE) of 1.20 m. Similar accuracies are obtained when comparing historical DSMs with measured geodetic points. The VHMs generated in this study enabled the detection of major changes in forest areas due to windstorm damage, forest dynamics, and growth. This work demonstrates the feasibility of generating accurate, very-high-resolution DSM time series (spanning three decades) and VHMs from historical aerial images of the entire surface of Switzerland in a highly automated manner. The VHMs are already being used to estimate countrywide biomass changes. The countrywide DSMs and VHMs for the four epochs, along with auxiliary data, are available online at https://doi.org/10.16904/envidat.528 (Marty et al., 2024) and can be used to quantify long-term elevation changes and related processes across different surfaces.

Glacial lake outburst floods (GLOFs) from ice-dammed lakes are frequent in Svalbard, impacting local ice dynamics, and subglacial hydrological systems, causing geomorphological changes, and posing flooding hazards. Additionally, GLOFs can influence nutrient dynamics in the fjord of tidewater glaciers, affecting the local ecosystem. In this study, we use high-resolution topographic data to monitor the formation of an ice-dammed lake and identify the drainage mechanisms of a GLOF that occurred in the summer of 2021 on the Kongsvegen glacier, a surge-type tidewater glacier located in Kongsfjorden (Svalbard). Additionally, seismometers were deployed to monitor the subglacial dynamics at the kilometre scale. Over the 2.5-month-long process starting in early June, terrestrial laser scanning (TLS) data and drone images were acquired at nearly daily intervals to monitor the ice-dammed lake formation and drainage. A time-lapse camera and pressure logger installed at the border of the ice-dammed lake allowed us to estimate the drainage timing, occurring from July 23 to July 26, resulting in a total drainage duration of 77 hours. To reconstruct the lake volume, the lake extension was manually digitized from the TLS data and drone orthophotos. Elevation information of the corresponding lake outlines was extracted from a 1 m resolution Digital Elevation Model (DEM) generated from Pléiades stereo satellite images acquired on 20 September 2020, at the end of the thaw season. This DEM serves as bathymetric data, representing the lake bottom. The extracted water level was used to calculate the stage-volume curve. The lake's maximum volume reached approximately 7.17 million m3 with an average discharge rate of 26 m3/s. Analyzing seismic data allowed for monitoring of the development of the subglacial drainage, assessing the transition from an inefficient to an efficient system. This study highlights the importance of very high spatial and temporal resolution data for accurate lake volume quantification and a better understanding of the link between GLOF and subglacial system.

Historical aerial images, captured by film cameras in the previous century, have emerged as valuable resources for quantifying Earth's surface and landscape changes over time. In the post-war period, historical aerial images were often acquired to create topographic maps, resulting in the acquisition of large-scale aerial photographs with stereo coverage. Using photogrammetric techniques on stereo-images enables extracting 3D information to reconstruct Digital Surface Models (DSMs), and orthoimages. This study presents a highly automated photogrammetric approach for generating nationwide DSMs for Switzerland at 1 m resolution using aerial stereo-images acquired between 1979 and 2006. The 8-bit scanned images, with known exterior and interior orientation, were processed using BAE Systems' SocetSet (v5.6.0) with the "Next-Generation Automatic Terrain Extraction" (NGATE) package for DSM generation. The primary objective of the study is to derive four nationwide DSMs for the epochs 1979-1985, 1985-1991, 1991-1998, and 1998-2006. The study assesses DSM quality in terms of vertical accuracy and completeness of image matching across different land cover types, with a focus on forest dynamics and management research. The elevation accuracy of the generated DSMs was assessed using two reference datasets. Firstly, the elevation differences between a nationwide reference Digital Terrain Model (DTM - swissAlti3d 2017 by Swisstopo) and the generated DSMs were calculated on points classified as "sealed surface". Secondly, elevation values of the DSMs were compared to approximately 500 independent geodetic points distributed across the country. Six study areas were chosen to assess completeness, and it was calculated as the percentage of successfully matched points to the potential total number of matched points within a predefined area. This assessment was conducted for six land cover classes based on the land cover/land-use statistics dataset from the Federal Office of Statistics. Across the entire country, the median elevation accuracy of the DSMs on sealed points ranges between 0.28 to 0.53 m, with a Normalized Median Absolute Deviation (NMAD) of around 1 m (maximum 1.41 m) and an RMSE of a maximum of 3.90 m. The elevation differences between geodetic points and DSMs show higher accuracy, with a median value of a maximum of 0.05 m and an NMAD smaller than 1 m. Completeness results reveal mean completeness between 64 % to 98 % for the classes "glacial and perpetual snow" and "sealed surfaces," respectively and 93 % specifically for the “closed forest” class. This work demonstrates the feasibility of generating accurate DSM time series (spanning four epochs) from historical scanned images for the entire Switzerland in a highly automated manner. The resulting DSMs will be available upon publication, providing an excellent opportunity to detect major surface changes, such as forest dynamics.

Accurate estimation of glacier elevation change is crucial for long-term geodetic mass balance and assessing glacier response to climate change. This study introduces an automated pipeline for generating Digital Elevation Models (DEMs) from satellite stereo imagery to quantify glacier elevation changes. We focus on SPOT-5 High-Resolution Stereoscopic (HRS) imagery, recently freely accessible through the SPOT World Heritage program by CNES. SPOT-5 is an underutilized archive with global coverage from 2002 to 2015 and stereoscopic capabilities, making it valuable for reconstructing glacier elevation changes and complementing stereo imageries from more recent satellites. However, its inherent challenges, such as limited radiometric resolution, rectangular pixel geometry, and absence of camera's Rational Polynomial Coefficient model, require specific attention. We apply our workflow to Hofsjökull, Iceland’s third-largest ice cap, because of extensive SPOT-5 temporal coverage, further complemented by ArcticDEM, SPOT-6 and Pleiades for recent years. Our workflow addresses key steps in DEM generation such as stereo pair selection, bundle adjustment, stereo correlation, noise filtering, point cloud gridding, void filling, and co-registration. Each of these steps significantly affects DEM quality and glacier elevation change estimates. Therefore, we compare and evaluate various approaches to identify optimal solutions for automation. We benchmark open-source photogrammetry tools, including Ames Stereo Pipeline and MicMac, and geospatial libraries like xDEM, GeoUtils, and OPALS, integrating them for interoperability. We tested different stereo-matching algorithms and found that the More Global Matching algorithm performs best for SPOT-5 data under diverse illumination and viewing conditions. For DEM gridding and void filling, we use a Robust Moving Planes fitting method in OPALS. Co-registration is performed using the globally available Copernicus DEM (GLO-30) as reference, using appropriate masks to exclude glaciers, forests, water bodies and steep areas. The least-squares template matching algorithm implemented in OPALS enhances alignment accuracy by estimating full affine transformations, while sub-pixel refinement is achieved with the Nuth and Kääb method. Finally, we derive elevation-band-based trends from spaceborne DEM time series to extrapolate elevation changes over decadal intervals. This enables us to calculate area-weighted mean elevation change estimates for each glacier and the entire ice cap over defined periods. This study contributes to the Glacier Mass Balance Intercomparison Exercise (GlaMBIE) by advancing scalable, open-access methodologies for glacier elevation change assessments. Additionally, our systematic comparison and integration of algorithms and techniques for each stage ensures optimized performance, making the pipeline reproducible across regions, temporal scales, and satellite platforms.

Understanding vertical forest structure is essential for forest management and is strongly linked to ecosystem functioning. In countries with protective forests, sustainable management is key to protecting humans and infrastructure from natural hazards. Well-structured forests usually feature multiple canopy layers, enhancing resistance and resilience to natural disturbances, reducing vulnerability to natural hazards, and improving adaptation to environmental changes. Forests with a single canopy layer often require prioritised and targeted management to increase their structural complexity. Hence, mapping forest canopy layering is crucial for effective forest management and risk mitigation.We developed a reproducible method for large-scale mapping of forest canopy layering to support sustainable forest management. Using airborne laser scanning data, we combined tree- and area-based methods to distinguish between single- and multi-layered forest areas in Switzerland. Our study encompasses four forest ecoregions, seven altitudinal vegetation belts, and a variety of topographic and climatic conditions. The resulting canopy layering map (resolution 10 x 10 m) was validated against two forest structure data sets, one from the Swiss National Forest Inventory (279 plots) and an Independent Forester Assessment (198 plots), yielding overall accuracies of 68 % and 81 %, respectively. Our study provides a robust and scalable method for mapping forest canopy layering across large mountain areas, which is transferable to other regions. Validation across various environmental settings demonstrates the suitability of the approach for nationwide use. Our forest canopy layering map supports sustainable, climate change-adapted forest management decisions, helping to set management priorities that promote well-structured forests for the future.

Digital elevation models (DEMs) from the spaceborne interferometric radar mission TanDEM-X hold a large potential for glacier elevation change assessments and monitoring. However, a bias is potentially introduced through the penetration of the X-band signal into snow and firn that can be substantial. The magnitude of this bias has been analysed in some glaciarized regions of the world; still, the knowledge about X-band penetration of TanDEM-X in the European Alps is limited.In this study, we investigated the unique situation of almost synchronous acquisition of TanDEM-X and Pléiades DEMs over the Grosser Aletschgletscher, complemented with in-situ observations (ground penetrating radar, snow cores, snow pits), all within a four-day period in late winter 2021. The comparison of the TanDEM-X and Pléiades DEM revealed an elevation bias due to radar penetration of up to 8 m above 3400 m. Further, the concurrent in-situ measurements reveal that the signal is not obstructed by the last summer horizon but reaches into perennial firn.Our study improves our understanding about the magnitude of X-band penetration of TanDEM-X in the Alps and the underlying process with a relevance for glaciology, snow science, remote sensing and the wider geoscience community.

We show a long-term erosion monitoring of several geomorphologically active gully systems on Little Ice Age lateral moraines in the European Central–Eastern Alps, covering a total time period from 1953 to 2019 and including several survey periods in order to identify corresponding morphodynamic trends. For the implementation, DEM (digital elevation model) of Differences (DoDs) were calculated, based on multitemporal high-resolution digital elevation models from historical aerial images (generated by structure from motion photogrammetry with multi-view stereo) and light detection and ranging from airborne platforms. Two approaches were implemented to achieve the corresponding objectives. First, by calculating linear regression models using the accumulated sediment yield and the corresponding catchment area (on a log–log scale), the range of the variability in the spatial distribution of erosion values within the sites. Second, we use volume calculations to determine the total and the mean sediment yield (as well as erosion rates) of the entire sites. Subsequently, both the sites and the different time periods of both approaches are compared. Based on the slopes of the calculated regression lines, it can be shown that the highest variability in the sediment yield at the sites occurs in the first time period (mainly 1950s to 1970s). This can be attributed to the fact that within some sites the sediment yield per square metre increases clearly more strongly (regression lines with slopes up to 1.5). In contrast, in the later time periods (1970s to mid-2000s and mid-2000s to 2017/2019), there is generally a decrease in 10 out of 12 cases (regression lines with slopes around 1). However, even at sites with an increase in the variability in the sediment yield over time, the earlier high variabilities are no longer reached. This means that the spatial pattern of erosion in the gully heads changes over time as it becomes more uniform. Furthermore, using sediment volume calculations and corresponding erosion rates, we show a generally decreasing trend in geomorphic activity (amount of sediment yield) between the different time periods in 10 out of 12 sites, while 2 sites show an opposite trend, where morphodynamics increase and remain at the same level. Finally, we summarise the results of long-term changes in the morphodynamics of geomorphologically active areas on lateral moraines by presenting the “sediment activity concept”, which, in contrast to theoretical models, is based on actually calculated erosion. The level of geomorphic activity depends strongly on the characteristics of the sites, such as size, slope length, and slope gradient, some of which are associated with deeply incised gullies. It is noticeable that especially areas with influence of dead ice over decades in the lower slope area show high geomorphic activity. Furthermore, we show that system internal factors, as well as the general paraglacial adjustment process, have a greater influence on long-term morphodynamics than changing external weather and climate conditions, which, however, had a slight impact mainly in the last, i.e. most recent, time period (mid-2000s to 2017/2019) and may have led to an increase in erosion at the sites.

Observations of glacier mass changes are key to understanding the response of glaciers to climate change and related impacts, such as regional runoff, ecosystem changes, and global sea level rise. Spaceborne optical and radar sensors make it possible to quantify glacier elevation changes, and thus multi-annual mass changes, on a regional and global scale. However, estimates from a growing number of studies show a wide range of results with differences often beyond uncertainty bounds. Here, we present the outcome of a community-based inter-comparison experiment using spaceborne optical stereo (ASTER) and synthetic aperture radar interferometry (TanDEM-X) data to estimate elevation changes for defined glaciers and target periods that pose different assessment challenges. Using provided or self-processed digital elevation models (DEMs) for five test sites, 12 research groups provided a total of 97 spaceborne elevation-change datasets using various processing approaches. Validation with airborne data showed that using an ensemble estimate is promising to reduce random errors from different instruments and processing methods but still requires a more comprehensive investigation and correction of systematic errors. We found that scene selection, DEM processing, and co-registration have the biggest impact on the results. Other processing steps, such as treating spatial data voids, differences in survey periods, or radar penetration, can still be important for individual cases. Future research should focus on testing different implementations of individual processing steps (e.g. co-registration) and addressing issues related to temporal corrections, radar penetration, glacier area changes, and density conversion. Finally, there is a clear need for our community to develop best practices, use open, reproducible software, and assess overall uncertainty to enhance inter-comparison and empower physical process insights across glacier elevation-change studies.

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 Br & oslash;ggerhalv & oslash;ya, 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 Br & oslash;ggerhalv & oslash;ya 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.

Abstract. We show a long-term erosion monitoring of several geomorphologically active gully systems on Little Ice Age lateral moraines in the central Eastern Alps covering a total time period from 1953 to 2019 including several survey periods in order to identify corresponding morphodynamic trends. For the implementation, DEM of Differences were calculated based on multitemporal high-resolution digital elevation models from historical aerial images (generated by structure-from-motion photogrammetry with multi-view-stereo) and light detection and ranging from airborne platforms. Two approaches were implemented to achieve the corresponding objectives. First, by calculating linear regression models using the accumulated sediment yield and the corresponding catchment area (on a log-log scale), the range of the variability of the spatial distribution of erosion values within the areas of interest is shown. Secondly, we use volume calculations to determine the total/mean sediment output (and erosion rates) of the entire areas of interest. Subsequently, a comparison is made between the areas of interest and the epochs of both approaches. Based on the slopes of the calculated regression lines, it could be shown that the highest range of the variability of sediment yield within all areas of interest is in the first epoch (mainly 1950s to 1970s), as in some areas of interest sediment yield per square metre increases clearly more (regression lines with slopes up to 1.5), which in the later epochs (1970s to mid-2000s and mid-2000s to 2017/2019) generally decreases in 10 out of 12 cases (regression lines with slopes around 1). However, even in the areas of interest with an increase in the variability of sediment yield over time, the earlier high variabilities are no longer reached. This means that the spatial pattern of erosion in the gully heads changes over time as it becomes more uniform. Furthermore, using sediment volume calculations and corresponding erosion rates, we show a generally decreasing trend in geomorphic activity (amount of sediment yield) between the different epochs in 10 out of 12 areas of interest, while 2 areas of interest show an opposite trend where morphodynamics increase and remain at the same level. Finally, we summarise the results of long-term changes in the morphodynamics of geomorphologically active areas on lateral moraines by presenting the "sediment activity concept", which, in contrast to theoretical models, is based on actually calculated erosion. The level of geomorphic activity depends strongly on the characteristics of the areas of interest, such as size, slope length and slope gradient, some of which are associated with deeply incised gullies. It is noticeable that especially areas with decades of dead ice influence in the lower slope area show high geomorphic activity. Furthermore, we show that system-internal factors as well as the general paraglacial adjustment process have a greater influence on long-term morphodynamics than changing external weather and climate conditions, which, however, had a slight impact mainly in the last, i.e. most recent epoch (mid-2000s to 2017/2019) and may have led to an increase in erosion at the areas of interest.

Climate change and the associated glacier retreat lead to considerable enlargement and alterations of the proglacial systems. The colonisation of plants in this ecosystem was found to be highly dependent on terrain age, initial site conditions and geomorphic disturbances. Although the explanatory variables are generally well understood, there is little knowledge on their collinearities and resulting influence on proglacial primary succession. To develop a sphere-spanning understanding of vegetation development, a more interdisciplinary approach was adopted. In the proglacial areas of Furkeleferner, Zufallferner and Langenferner (Martell Valley, Eastern Italian Alps), in total 65 plots of 5x2 m were installed to perform the vegetation analysis on vegetation cover, species number and species composition. For each of those, 39 potential explanatory variables were collected, selected through an extensive literature review. To analyse and further avoid multicollinearities, 33 of the explanatory variables were clustered via principal component analysis (PCA) to five components. Subsequently, generalised additive models (GAMs) were used to analyse the potential explanatory factors of primary succession. The results showed that primary succession patterns were highly related to the first component (elevation and time), the second component (solar radiation), the third component (soil chemistry), the fifth component (soil physics) and landforms. In summary, the analysis of all explanatory variables together provides an overview of the most important influencing variables and their interactions; thus it provides a basis for the debate on future vegetation development in a changing climate.

<p>Quantifying glacier elevation and volume changes is critical to understanding the response of glaciers to climate change and related impacts, such as regional runoff and global sea-level rise. Spaceborne remote sensing techniques enable the quantification of spatially distributed glacier elevation changes at regional and global scales using multi-temporal digital elevation models (DEMs). A growing number of spaceborne studies exist to assess glacier elevation changes but they show widespread differences often beyond the error bars. Here, we present the results of a community-based inter-comparison experiment using spaceborne optical (ASTER) and radar (TanDEM-X) sensors to assess elevation changes for selected individual glaciers and regional glacier samples. Using a predefined set of DEMs, participating groups provided their own estimates using various processing strategies.  </p> <p>For the selected individual glaciers, the results were validated using airborne data. The validation shows that the median of the spaceborne ensemble is biased by a few decimetres per year with a standard deviation of about half a meter per year. An interesting finding is that no sensor and no processing strategy perform significantly better for all experiment sites. At the regional scale, we find that the co-registration of DEMs is the most relevant processing step for an accurate assessment of elevation change. Other corrections such as gap filling, filtering, and radar penetration have less impact in general but can be essential for individual cases. Temporal corrections (i.e. seasonal and annual) can have a great impact; however, they are not yet well resolved by the remote sensing community.</p> <p>Our study confirms that the currently available spaceborne geodetic assessments result in relatively widespread glacier elevation changes. Therefore, we recommend an ensemble approach of observations from multiple observational sources. Furthermore, there is a need to establish best practices for related uncertainty estimates.</p>

<p>This work aims to develop a highly automated workflow for generating a forest cover map and detecting forest gaps at the countrywide level (i.e. Switzerland) using the alpha shape approach (Edelsbrunner et al. 1983).</p> <p>Forest provides society with several functions. In Switzerland e.g., more than 50% of the forests have a protection function and mitigate or prevent the impact of a natural hazard. The accurate detection of forest gaps (openings in the forest canopy) is crucial for properly managing and planning protection forests. In addition, knowledge of the distribution of forest gaps is a useful indicator to assess forest structure and biodiversity. Although the required information is collected at the plot level within the framework of the National Forest Inventory (NFI), remote sensing allows us to derive spatially explicit and accurate products at the pixel level for the entire country.</p> <p>The countrywide available 1 m spatial resolution Vegetation Height Model (VHM) (Ginzler & Hobi, 2015) serves as a basis to extract forest cover and forest gaps. The VHM was generated from image-based point clouds acquired between 2013 and 2021 for the full coverage of Switzerland. In the first step, a forest cover map was derived using the VHM. In a second step, a dense forest cover map was generated and forest gaps were delineated taking into account the Swiss NFI forest definition criteria comprising minimum tree height and width, crown coverage, and land use. In summary, the overall workflow consists of extracting the tree top points from the VHM (FINT software). Erroneous tree tops were removed using the probability forest mask derived from Sentinel-1/-2 data (Rüetschi et al. 2021). We then derived forest area and non-forest area polygons from the filtered tree top points using alpha shape (lasboundary, LAStools from rapidlasso) that computes a boundary polygon that encloses the points.</p> <p>A dense forest cover map is calculated using a moving window approach and forest areas greater than 60% are extracted. The forest gaps detection within the dense forest cover map follows a similar approach adopted for the forest cover map, but the alpha shape polygons are extracted from the VHM which is converted to the las format. The entire workflow is developed in Python.</p> <p>Accuracy assessments of forest cover boundary and forest gaps based on terrestrial and stereo image-interpreted NFI plots are promising and reveal an overall agreement of more than 95% over the entire country.</p> <p><strong>Reference</strong></p> <p>Edelsbrunner, H., Kirkpatrick, D.G., Seidel, R., 1983. On the shape of a set of points in the plane. IEEE Transactions on Information Theory, 29(4), pp.551-559.</p> <p>Ginzler, C. and Hobi, M.L., 2015. Countrywide stereo-image matching for updating digital surface models in the framework of the Swiss National Forest Inventory. Remote Sensing, 7(4), pp.4343-4370.</p> <p>Rüetschi, M., Weber, D., Koch, T.L., Waser, L.T., Small, D. and Ginzler, C., 2021. Countrywide mapping of shrub forest using multi-sensor data and bias correction techniques. International Journal of Applied Earth Observation and Geoinformation, 105, 102613.</p>

Abstract. In many Arctic regions marine coastlines change rapidly in the currently warming climate. In contrast, coastal rock cliffs on Svalbard are considered to be comparably stable, based on previous studies that considered only a few years and limited coastal reaches. Long-term trends of coastal retreat rates in rock cliffs on Svalbard are unknown so far, but their quantification could improve the understanding of coastal dynamics on the Arctic archipelago. This study presents coastal retreat rates in rock cliffs along several kilometers of the Brøgger peninsula, Svalbard. The work is based on high-resolution orthoimages from 1970, 1990, 2010, and 2021, corroborated by high-precision dGNSS measurements along selected segments of the coastline and by rock surface temperature measurements during the period 2020–2021. Our analysis shows that coastal retreat rates accelerate statistically significant along the Brøgger peninsula in the time period of 2010 to 2021. This is true for both the northeast facing coastline, with retreat rates increasing from 0.04 ± 0.06 m/a (1970–1990) and 0.04 ± 0.04 m/a (1990–2010) to 0.07 ± 0.08 m/a (2010–2021) and the southwest facing coastline, where retreat rates of 0.26 ± 0.06 m/a (1970–1990), 0.24 ± 0.04 m/a (1990–2010) and 0.30 ± 0.08 m/a (2010–2021) are measured. Furthermore, the parts of the coastline affected by erosion increase along the northeast facing coastline from 47 % (1970–1990) to 65 % (2010–2021), while they stay consistently above 90 % along the southwest facing coastline. Measurements of rock surface temperature show mean annual values close to the thaw threshold with −0.49 °C at the southwest facing coastline, while records at the northeast facing coastline are lower with −1.64 °C. The recently accelerated retreat rates coincide with increasing storminess and retreating sea ice, together with increasing ground temperatures, all factors that can enhance coastal erosion.

Rapid warming in the Arctic leads to increased glacier melt and freshwater runoff, especially from tidewater glaciers. Here, runoff enters the fjord at depth; induces upwelling and enhances macronutrient delivery to the fjords. However, most studies have low temporal resolutions and so the effects of low-frequency, high-amplitude events on the marine environment remain poorly known. Here, we combine glacier observations with fjord and glacier lake sampling to describe the impact of the 2021 glacier lake outburst flood (GLOF) from lake Setevatnet into Kongsfjorden (Svalbard). We demonstrate the importance of changing subglacial conditions and examine their effects upon macronutrient availability in the inner fjord. Our observations reveal that direct nutrient subsidy from the glacier is most important in early summer, providing critical nitrate (NO3-) and silicate following the routing of meltwater through an inefficient drainage system. Increasing quantities of ice melt force the establishment of an efficient drainage system, creating a plume in the inner fjord, and resulting in upwelling of nutrient-rich bottom water. When the sudden drainage of a glacier lake with high NO3- concentrations occurred, it left little imprint on the NO3- content of the inner fjord, and instead induced seasonal maximum nitrite (NO2-) concentrations. This outcome implies that NO3- was removed by denitrification at the glacier bed and its product NO2- was discharged by the flood waters into the inner fjord. Our findings show that the delivery of key, productivity-limiting nutrients from tidewater glaciers not only depends on runoff, but also on characteristics of the glacier drainage system.

Alpine rivers have experienced considerable changes in channel morphology over the last century. Natural factors and human disturbance are the main drivers of changes in channel morphology thatmodify natural sediment and flow regimes at local, catchment, and regionalscales. In glaciated catchments, river sediment loads are likely to increase due to increasing snowmelt and glacier melt runoff, facilitated by climate change. Additionally, channel erosion and depositional dynamics andpatterns are influenced by sediment delivery from hillslopes and sedimentin the forefields of retreating glaciers. In order to reliably assess themagnitudes of the channel-changing processes and their frequencies due torecent climate change, the investigation period needs to be extended to thelast century, ideally back to the end of the Little Ice Age. Moreover, ahigh temporal resolution is required to account for the history of changesin channel morphology and for better detection and interpretation of relatedprocesses. The increasing availability of digitised historical aerial imagesand advancements in digital photogrammetry provide the basis forreconstructing and assessing the long-term evolution of the surface, interms of both planimetric mapping and the generation of historical digitalelevation models (DEMs). The main issue of current studies is the lack of information over a longerperiod. Therefore, this study contributes to research on fluvial sedimentchanges by estimating the sediment balance of a main Alpine river (Fagge) in a glaciated catchment (Kaunertal, Austria) over 19 surveyperiods from 1953 to 2019. Exploiting the potential of historicalmulti-temporal DEMs combined with recent topographic data, we quantify 66 years of geomorphic change within the active floodplain, including erosion,deposition, and the amounts of mobilised sediment. Our study focuses on aproglacial river that is undergoing a transition phase, resulting from anextensive glacier retreat of approximately 1.8 km. This has led to the formation of new channel networks and an overall negative cumulativesediment balance for the entire study area. We found that high-magnitudemeteorological and hydrological events associated with local glacierretreats have a significant impact on the sediment balance. The gauge recordindicates an increase in such events, as well as in runoff and probably insediment transport capacity. Despite this, the sediment supply has declinedin the last decade, which can be attributed to a lower contribution of thelateral moraines coupled to the channel network and less sediment sourcedfrom the melting Gepatsch Glacier as evidenced by roches moutonneesexposed in the current/most recent forefield. Nonetheless, we observedsignificant erosion in the tributary, leading to the transport of sedimentdownstream. Overall, this study enhances our understanding of the complexity of sediment dynamics in proglacial rivers across various spatial and temporal scales and their relationship to climate change factors.