Andreas Max Kääb

• Strozzi, Tazio; Mannerfelt, Erik Schytt; Cartus, Oliver; Santoro, Maurizio; Schellenberger, Thomas & Kääb, Andreas (2026). Glacier surge activity over Svalbard from 1992 to 2025 interpreted using heritage satellite radar missions and Sentinel-1. The Cryosphere. ISSN 1994-0416. 20(3). doi: 10.5194/tc-20-1679-2026. Fulltekst i vitenarkiv Vis sammendrag

  Based on massive processing of heritage radar data from the satellite missions ERS-1/2, JERS-1, ENVISAT ASAR, ALOS PALSAR and Radarsat-2, and in combination with data from the current Sentinel-1 and ALOS-2 PALSAR-2 missions, we compiled a ∼ 30-year time series of radar backscatter over Svalbard. We exploited this data to detect glacier surges by using changes in backscatter as an indicator of increased or decreased surge-related crevassing. In this way, we reconstructed an as consistent as possible time series of surge activity on Svalbard for 1992 to 2025. We recorded 24 surge-type events during the pre Sentinel-1 period 1992–2014 (23 years) and 34 surge-type events during the post Sentinel-1 period 2015–2025 (11 years). This time series shows an approximately threefold increase in surges since 2015, from an average of about one surge per year to more than three surges per year. We show that this increase is unlikely to be explained alone by the better resolution, coverage and quality of the Sentinel-1 data compared to the data from the earlier SAR heritage missions. Simulation results indicate that the observed increase is extremely unlikely to be attributed to random perturbations in surge cyclicity, and instead suggest the influence of an external forcing mechanism. The number of surges during the recent decade seems high, but due to uncertainties in historical records, it remains unclear whether this frequency is exceptional or if earlier decades were unusually quiet. The cause of the observed threefold increase in surge activity also remains uncertain, given our incomplete understanding of surge initiation in relation to climate variability and non-climatic surge controls.

• Lovell, Harold; Benn, Douglas I.; Jiskoot, Hester; Stokes, Chris R.; Flowers, Gwenn E. & Guillet, Gregoire [Vis alle 15 forfattere av denne artikkelen] (2026). Glacier surging and surge-related hazards in a changing climate. Nature Reviews Earth & Environment. doi: 10.1038/s43017-025-00757-9. Fulltekst i vitenarkiv Vis sammendrag

  Glacier surges are ice flow instabilities characterized by periods of acceleration, during which mass is rapidly transferred from high to low elevations and the glacier front often advances. In this Review, we outline global trends in glacier surging and the influence of climate on the distribution and behaviour of surge-type glaciers and surge-related hazards. Glaciers exhibit diverse surging behaviours and typically recur at intervals that range from 5 years to over 100 years, with all surges largely driven by a reduction in basal friction. Most of the ~3,100 surge-type glaciers identified globally are clustered in the Arctic and Subarctic (48.3%) and High Mountain Asia (50.5%), where climate conditions are conducive to the development of surge instabilities, and there is emerging evidence that climate warming is changing surge behaviour. At least 81 surge-type glaciers globally have caused hazards such as ice-dammed glacial lake outburst floods, which can lead to infrastructure damage and loss of life. Future research should acquire spatiotemporally high-resolution remote-sensing data and direct observations of basal processes during all stages of surges, develop numerical models to better capture surge mechanisms and diversity, and project the impact of future climate warming on surge-type glacier behaviour and distribution.

• Cusicanqui, Diego; Lacroix, Pascal; Bodin, Xavier; Robson, Benjamin Aubrey; Kääb, Andreas Max & MacDonell, Shelley (2025). Detection and reconstruction of rock glacier kinematics over 24 years (2000-2024) from Landsat imagery. The Cryosphere. ISSN 1994-0416. 19(7), s. 2559–2581. doi: 10.5194/tc-19-2559-2025. Fulltekst i vitenarkiv Vis sammendrag

  Rock glacier velocity is now widely acknowledged as an Essential Climate Variable for permafrost. However, representing decadal regional spatiotemporal velocity patterns remains challenging due to the limited availability of high-resolution (<5 m) remote sensing data. In contrast, medium-resolution satellite data (10–15 m) covering several decades are globally available but have not been widely used for rock glacier kinematics. This study presents a robust methodological approach combining pairwise feature-tracking image correlation with medium-resolution Landsat 7/Landsat 8 optical imagery, surface displacement time-series inversion and the automatic detection of persistent moving areas (PMAs). Applied to rock glacier monitoring in the semiarid Andes of South America, this methodology enables the detection and quantification of the surface kinematics of 153 rock glaciers, 124 landslides and 105 unclassified landforms over 24 years across a 2250 km2 area. This is the first time that Landsat images have been used to quantify rock glacier displacement time series. The study estimates an average velocity of 0.30±0.07 m yr−1 for all PMAs, with rock glaciers moving 23 % faster (0.37 m yr−1) over the 24-year period. Some large rock glaciers and debris-frozen landforms exhibit surface velocities exceeding 2 m yr−1. The results align well with high-resolution imagery, recent Global Navigation Satellite System measurements and previous inventories. However, the Landsat 7/Landsat 8 (L7/8) imagery-derived velocities are underestimated by approximately 20 %–30 % on average. High uncertainties between consecutive image pairs limit the reliability of interpreting annual velocity variations. However, decadal velocity changes exceeding the uncertainties were observed in only 2 % of PMAs, with two (one) rock glaciers exhibiting significant acceleration (deceleration) over the past two decades. Our calculations show that decadal velocity changes <0.4 m yr−1 are generally within the uncertainty range when using L7/8 data, with sensitivity depending on the reference period. Despite these limitations, our results highlight the correlation between velocity trends and topographic parameters such as PMA size, orientation, slope and elevation. These relationships suggest that permafrost thaw may influence the occurrence of high-altitude landslides. Overall, this study demonstrates the feasibility of using medium-resolution optical satellite imagery for monitoring rock glacier velocity over several decades.

• Feng, Min; Xu, Jinhao; Zhang, Xuefei; Yan, Dezhao; Li, Xin & Kääb, Andreas Max [Vis alle 8 forfattere av denne artikkelen] (2025). Underestimated number and significance of rock glaciers on the Tibetan Plateau. Science Bulletin. ISSN 2095-9273. 71(2), s. 434–443. doi: 10.1016/j.scib.2025.12.004. Fulltekst i vitenarkiv
• D., Harcourt, William; Pearce, Danni M; Wojciech, Gajek,; Lovell, Harold; Luckman, Adrian & Douglas, Benn, [Vis alle 9 forfattere av denne artikkelen] (2025). Surging glaciers in Svalbard: Current knowledge and perspectives for monitoring (SvalSurge). I Neuber, R.; Łupikasza, E.; Hübner, C. & Holmén, K. (Red.), The State of Environmental Science in Svalbard – an annual report. Svalbard Integrated Arctic Earth Observing System. ISSN 9788293871149. s. 84–105. doi: 10.5281/zenodo.14425522. Fulltekst i vitenarkiv Vis sammendrag

  Glacier surges are cycles of long-lasting slow flow followed by a relatively short phase of fast flow where a glacier undergoes a 10/100-fold increase in velocity. These cycles are driven by internal ice dynamics and are not related to external forcing. In this chapter, we review the benefits and limitations of different approaches for monitoring and detecting glacier surges in Svalbard. Since 2000, satellite monitoring has enabled detection of most large surges, and the launch of the Copernicus Sentinels since 2014 has further enhanced our monitoring capabilities. In the satellite era (1970s onwards), surges have been detected based upon tracking the speed of glaciers over time, elevation changes, and more recently automatically detecting surface changes related to a surge such as increased crevassing. Geophysical sensors are critical for observing subglacial conditions and further work is required to improve deployment strategies on heavily crevassed glaciers. Furthermore, we have compiled a new and updated database of surge-type glaciers in Svalbard and find that 36% of glaciers display surge-type behaviour, rising to 51% when removing glaciers smaller than 1 km2. Only 9% of glaciers have been directly observed to surge. Surges have a significant impact on glacier mass balance, but their impact is variable in time. Therefore, we need to closely monitor when surges occur and how much ice is discharged downglacier and subsequently lost. This has implications for regional mass balance, sea level rise, and the influx of freshwater into the ocean which alters ocean circulation patterns and marine ecology.

• Dunn, R. J. H.; Blannin, J.; Willett, K. M.; Gobron, N.; Morris, G. A. & Ades, Melanie [Vis alle 254 forfattere av denne artikkelen] (2025). State of the Climate in 2024: Global Climate. Bulletin of The American Meteorological Society - (BAMS). ISSN 0003-0007. 106(8), s. 1–162. doi: 10.1175/bams-d-25-0102.1. Fulltekst i vitenarkiv Vis sammendrag

  For the second year in a row, record-high global surface temperatures were set in 2024, according to all six global temperature datasets assessed in this report (Berkeley Earth, GISTEMP, HadCRUT5, the NOAA Merged Land Ocean Global Surface Temperature Analysis [NOAAGlobalTemp], ERA5, and the Japanese Reanalysis for Three Quarters of a Century [JRA-3Q]). The last time consecutive years set records was in 2015 and 2016 when a strong El Niño similarly boosted global temperatures. The last 10 years (2015–24) are now the warmest 10 in the instrumental record—warmer than the 2011–20 average—and hence “more likely than not warmer than any multi-century period after the last interglacial period, roughly 125,000 years ago” (Gulev et al. 2021). The increased energy within the climate system is detectable at the top of the atmosphere, with the outgoing longwave radiation anomaly continuing to be above the range of natural variability. During 2024, El Niño conditions that had been present since the middle of 2023 faded to neutral by the end of the year. The warm conditions observed around the globe over the last two years had impacts across the climate system, as demonstrated by many of the metrics presented in this chapter. Other temperature metrics also reached record levels over the instrumental periods assessed in this chapter: over the oceans at night, on the surfaces of lakes, and in the lower troposphere as well as measures of equivalent temperature (which considers the moisture contribution to heat), and high and low temperature extremes. The frozen parts of Earth responded with permafrost temperatures continuing to reach record-high levels in many locations, and the active-layer thickness (the portion that melts and refreezes annually) also increasing at most sites. Repeated high temperatures over the European Alps during recent summers has led to large increases in rock glacier velocities in that region. The Great Lakes had much-below-average ice cover over the 2023/24 winter, and there was below-average snow cover extent in the Northern Hemisphere. All 58 reference glaciers across five continents lost ice during 2024, resulting in the greatest average ice loss in the record, which began in 1970. One more glacier was also declared extinct during 2024. Higher global temperatures impacted the water cycle. Although lower than 2023 values, water evaporation from land in the Northern Hemisphere reached one of the highest annual values on record, in line with the long-term increasing trend. Specific humidity reached record levels over land and ocean, and relative humidity over both domains was higher than 2023. There was little relief from high humid-heat conditions, with the frequency of high humid-heat days at a record level and intensity at the second-highest level in the record—only a fraction of a degree cooler than that of 2023. The global atmosphere contained the greatest amount of water vapor in the record, and over one-fifth of the globe recorded their highest values. This far exceeded 2023, where only one-tenth of the globe experienced record-high total column water vapor. Rainfall was globally high; 2024 was the third-wettest year since records began in 1983. However, rainfall over land was close to average, while over the ocean it was the fourth-wettest year on record (following 2015, 2016, and 1998). Extreme rainfall, as characterized by the annual maximum daily rainfall over land, was the wettest on record. Averaged globally (4190 lakes), lakes had a small increase in water storage, and regionally, over 40% of monitored lakes showed significant changes in storage and level. The effects of ongoing droughts in southern Africa and in North and South America can be seen in the soil moisture and water storage patterns. They are also apparent in the river discharge and runoff levels, which are topics that will be covered in the chapter after a few years of absence. Globally, however, drought severity and extent decreased from the record set in 2023. Atmospheric concentrations of the three main greenhouse gases (carbon dioxide [CO2], methane [CH4], nitrous oxide [N2O]) again all reached record levels, with a record-equal annual increase in the annual change of CO2 concentrations. However, concentrations of ozone-depleting substances continued to decline, corroborated by stratospheric ozone columns well above the 1998–2008 average, especially in the Northern Hemisphere. In contrast, stratospheric aerosols remained high because of the Ruang eruption in April 2024, affecting the atmospheric transmission of solar radiation over Hawaii later in the year, and the ongoing effects from the Hunga eruption in 2022. The latter eruption also caused the ongoing elevated stratospheric water vapor concentrations. Our planet’s surface albedo continued to darken with increased plant growth and decreased snow and ice cover. Plants responded to the warmer temperatures with some of the earliest starts to spring in the record over Europe—one to two weeks earlier than the 2000–20 baseline—and a warm autumn resulted in a much longer leaf-on season. Severe wildfire seasons occurred in South America (the worst since 2010), Canada (for the second consecutive year), and the Arctic, contributing to the second-highest atmospheric carbon monoxide concentrations since 2003 and the highest tropospheric aerosol optical depth since 2019, at 550 nm. This year’s iteration of the Global Climate chapter features two Sidebars, both of which present new topics that have not yet been explored in the report. The first covers the ability of satellite products to monitor changes in land surface temperature extremes and identify hotspots where regions of Earth are becoming uninhabitable. This Sidebar also discusses the importance of dataset stability for climate studies, as well as the correlation of land surface temperature and air temperature anomalies. The second Sidebar complements the section on greenhouse gas concentrations by examining short-lived climate forcers—compounds that have lifetimes ranging from a few hours to a few decades. As usual in the Global Climate chapter, Plate 2.1 shows maps of global annual anomalies for many of the variables and metrics presented herein. Many of these variables are also presented as time series in Plate 1.1. Most sections now use the 1991–2020 climatological reference period, in line with the World Meteorological Organization’s (WMO) recommendations, although this reference period is not possible for all datasets due to their length or legacy processing methods.

• Hu, Yan; Arenson, Lukas U.; Barboux, Chloé; Bodin, Xavier; Cicoira, Alessandro & Delaloye, Reynald [Vis alle 16 forfattere av denne artikkelen] (2025). Rock Glacier Velocity: An Essential Climate Variable Quantity for Permafrost. Reviews of Geophysics. ISSN 8755-1209. 63(1). doi: 10.1029/2024RG000847. Fulltekst i vitenarkiv Vis sammendrag

  Rock glaciers are distinctive debris landforms found worldwide in cold mountainous regions. They express the long-term movement of perennially frozen ground. Rock Glacier Velocity (RGV), defined as the time series of the annualized surface velocity of a rock glacier unit or a part of it, has been accepted as an Essential Climate Variable Permafrost Quantity in 2022. This review aims to highlight the relationship between rock glacier velocity and climatic factors, emphasizing the scientific relevance of interannual rock glacier velocity in generating RGV products within the context of observed rock glacier kinematics. Under global warming, rock glacier velocity exhibits widespread (multi-)decennial acceleration. This acceleration varies regionally in onset timing (from the 1950s to the 2010s) and magnitude (up to a factor of 10), and has been observed in regions such as the European Alps, High Mountain Asia, and the Andes. Despite different local conditions, a synchronous interannual velocity pattern prevails in the European Alps since the 2000s, highlighting the primary influence of climate. A common pattern is the seasonal velocity rhythm, which peaks in late summer to autumn and declines in spring. RGV assesses permafrost evolution via (multi-)decennial and interannual changes in rock glacier velocity, influenced by air temperature shifts with varying time lags and snow cover effects. Although not integrated into the RGV products, seasonal variations should be examined. This rhythmic behavior is attributed to alterations in pore water pressure influenced by air temperature, snow cover, and ground water conditions.

• Mannerfelt, Erik Schytt; Schellenberger, Thomas & Kääb, Andreas (2025). Tracking glacier surge evolution using interferometric SAR coherence-examples from Svalbard. Journal of Glaciology. ISSN 0022-1430. 71. doi: 10.1017/jog.2025.27. Fulltekst i vitenarkiv Vis sammendrag

  We present a practically simple methodology for tracking glacier surge onset and evolution using interferometric synthetic aperture radar (InSAR) coherence. Detecting surges early and monitoring their build-up is interesting for a multitude of scientific and safety-related aspects. We show that InSAR coherence maps allow the detection of surge-related instability on Svalbard many years before being detectable by, for instance, feature tracking or crevasse detection. Furthermore, we present derived data for two types of surges; down- and up-glacier propagating, with interestingly consistent surge propagation and post-surge relaxation rates. The method works well on Svalbard glaciers, and the data and core principle suggest a global applicability.

• Aga, Juditha; Piermattei, Livia; Girod, Luc; Aalstad, Kristoffer; Eiken, Trond & Kääb, Andreas [Vis alle 7 forfattere av denne artikkelen] (2024). Acceleration of coastal-retreat rates for high-Arctic rock cliffs on Brøggerhalvøya, Svalbard, over the past decade. Earth Surface Dynamics. ISSN 2196-6311. 12(5), s. 1049–1070. doi: 10.5194/esurf-12-1049-2024. Fulltekst i vitenarkiv Vis sammendrag

  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 m a−1 (1970–1990; calculated retreat rate ± the corresponding measurement uncertainty), 0.04 ± 0.04 m a−1 (1990–2010), and 0.06 ± 0.08 m a−1 (2010–2021). Along the southwest-facing coastline, higher retreat rates of 0.26 ± 0.06 m a−1 (1970–1990), 0.24 ± 0.04 m a−1 (1990–2010), and 0.30 ± 0.08 m a−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.

• Piermattei, Livia; Zemp, Michael; Sommer, Christian; Brun, Fanny; Braun, Matthias H. & Andreassen, Liss M. [Vis alle 35 forfattere av denne artikkelen] (2024). Observing glacier elevation changes from spaceborne optical and radar sensors - an inter-comparison experiment using ASTER and TanDEM-X data. The Cryosphere. ISSN 1994-0416. 18(7), s. 3195–3230. doi: 10.5194/tc-18-3195-2024. Fulltekst i vitenarkiv Vis sammendrag

  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.

• Milkoreit, Manjana; Boyd, Emily; Constantino, Sara M.; Hausner, Vera Helene; Hessen, Dag Olav & Kääb, Andreas [Vis alle 15 forfattere av denne artikkelen] (2024). Governance for Earth system tipping points – A research agenda. Earth System Governance. 21. doi: 10.1016/j.esg.2024.100216. Fulltekst i vitenarkiv Vis sammendrag

  Tipping points in the Earth system could be passed within the Paris Agreement's temperature goal range (1.5°C–2°C). Tipping processes are a feature of complex Earth system dynamics that present major governance challenges not addressed by existing global governance institutions. The common governance toolkit is a poor match for dealing with tipping processes, especially non-linear change, and radical intertemporality. To support the development of effective responses to anticipated, rapid state changes in the Earth system, there is an urgent need for new interdisciplinary research programs focused specifically on tipping-point governance. We distinguish two domains of action in a multi-phase framework - prevention and impact governance - and identify key research areas and questions that need to be addressed. These include developing governance principles, identifying actors and institutions that should be involved or need to be created, and determining the appropriate temporal and spatial scales for governance efforts.

• Dunn, Robert J.H.; Blannin, Josh; Gobron, Nadine; Miller, John B.; Willett, Kate M. & Ades, Melanie [Vis alle 52 forfattere av denne artikkelen] (2024). State of the Climate in 2023 : Global Climate. Bulletin of The American Meteorological Society - (BAMS). ISSN 0003-0007. 105(8), s. S12–S155. doi: 10.1175/BAMS-D-24-0116.1. Fulltekst i vitenarkiv
• Kääb, Andreas & Røste, Julie (2024). Rock glaciers across the United States predominantly accelerate coincident with rise in air temperatures. Nature Communications. 15. doi: 10.1038/s41467-024-52093-z. Fulltekst i vitenarkiv Vis sammendrag

  Despite their extensive global presence and the importance of variations in their speed as an essential climate variable, only about a dozen global time series document long-term changes in the velocity of rock glaciers – large tongue-shaped flows of frozen mountain debris. By analysing historical aerial photographs, we reconstruct here 16 new time series, a type of data that has not previously existed for the North American continent. We observe substantial accelerations, as much as 2–3 fold, in the surface displacement rates of rock glaciers across the mountains of the western contiguous United States over the past six to seven decades, most consistent with strongly increasing air temperatures in that region. Variations between individual time series suggest that different local and internal conditions of the frozen debris bodies modulate this overall climate response. Our observations indicate fundament

• Kääb, Andreas; Mouginot, Jeremie; Prats-Iraola, Pau; Rignot, Eric; Rabus, Bernhard T & Benedikter, Andreas [Vis alle 10 forfattere av denne artikkelen] (2024). Potential of the Bi-Static SAR Satellite Companion Mission Harmony for Land-Ice Observations. Remote Sensing. 16(16). doi: 10.3390/rs16162918. Fulltekst i vitenarkiv Vis sammendrag

  The EarthExplorer 10 mission Harmony by the European Space Agency ESA, scheduled for launch around 2029–2030, consists of two passive C-band synthetic-aperture-radar companion satellites flying in a flexible constellation with one Sentinel-1 radar satellite as an illuminator. Sentinel-1 will serve as transmitter and receiver of radar waves, and the two Harmonys will serve as bistatic receivers without the ability to transmit. During the first and last year of the 5-year mission, the two Harmony satellites will fly in a cross-track interferometric constellation, such as that known from TanDEM-X, about 350 km ahead or behind the assigned Sentinel-1. This constellation will provide 12-day repeat DEMs, among other regions, over most land-ice and permafrost areas. These repeat DEMs will be complemented by synchronous lateral terrain displacements from the well-established offset tracking method. In between the cross-track interferometry phases, one of the Harmony satellites will be moved to the opposite side of the Sentinel-1 to form a symmetric bistatic “stereo” constellation with ±~350 km along-track baseline. In this phase, the mission will provide opportunity for radar interferometry along three lines of sight, or up to six when combining ascending and descending acquisitions, enabling the measurement of three-dimensional surface motion, for instance sub- and emergence components of ice flow, or three-dimensional deformation of permafrost surfaces or slow landslides. Such measurements would, for the first time, be available for large areas and are anticipated to provide a number of novel insights into the dynamics and mass balance of a range of mass movement processes.

• Kääb, Andreas & Girod, Luc (2023). Brief communication: Rapid ∼ 335 × 106 m3 bed erosion after detachment of the Sedongpu Glacier (Tibet). The Cryosphere. ISSN 1994-0416. 17(6), s. 2533–2541. doi: 10.5194/tc-17-2533-2023. Fulltekst i vitenarkiv Vis sammendrag

  Following the 130 ± 5 × 106 m3 detachment of the Sedongpu Glacier, south-eastern Tibet, in October 2018, the Sedongpu Valley, which drains into the Yarlung Tsangpo (Brahmaputra) River, underwent rapid large-volume landscape changes. Between December 2018 and 2022 and in particular during summer 2021, an enormous volume of in total ∼ 335 ± 5 × 106 m3 was eroded from the former glacier bed, forming a new canyon of up to 300 m depth, 1 km width, and almost 4 km length. The 2021 erosion peak happened through massive but still gradual retrogressive erosion into the former glacier bed. Several rock–ice avalanches of in total ∼ 150 ± 5 × 106 m3 added to the total rock, sediment, and ice volume of over 600 × 106 m3 (0.6 km3) that has been exported from the basin since around 2017. The recent erosion volumes at Sedongpu are by order of magnitude equivalent to the average annual denudation volume of the entire Brahmaputra basin upstream of the location where the river leaves the Himalayas. This high-magnitude low-frequency event illustrates the potential for rapid post-glacial landscape evolution and associated hazards that has rarely been observed and considered at such high intensity so far.

• Dunn, Robert J. H.; Miller, John B.; Willet, Kate M.; Gobron, Nadine; Ades, Melanie & Adler, Robert [Vis alle 56 forfattere av denne artikkelen] (2023). State of the Climate in 2022: 2. Global Climate. Bulletin of The American Meteorological Society - (BAMS). ISSN 0003-0007. 104(9), s. S11–S145. doi: 10.1175/BAMS-D-23-0090.1. Fulltekst i vitenarkiv
• Berthier, Etienne; Floriciou, Dana; Gardner, Alex S.; Gourmelen, Noel; Jakob, Livia & Paul, Frank [Vis alle 16 forfattere av denne artikkelen] (2023). Measuring glacier mass changes from space-a review. Reports on progress in physics (Print). ISSN 0034-4885. 86(3). doi: 10.1088/1361-6633/acaf8e. Fulltekst i vitenarkiv Vis sammendrag

  Abstract Glaciers distinct from the Greenland and Antarctic ice sheets are currently losing mass rapidly with direct and severe impacts on the habitability of some regions on Earth as glacier meltwater contributes to sea-level rise and alters regional water resources in arid regions. In this review, we present the different techniques developed during the last two decades to measure glacier mass change from space: digital elevation model (DEM) differencing from stereo-imagery and synthetic aperture radar interferometry, laser and radar altimetry and space gravimetry. We illustrate their respective strengths and weaknesses to survey the mass change of a large Arctic ice body, the Vatnajökull Ice Cap (Iceland) and for the steep glaciers of the Everest area (Himalaya). For entire regions, mass change estimates sometimes disagree when a similar technique is applied by different research groups. At global scale, these discrepancies result in mass change estimates varying by 20%–30%. Our review confirms the need for more thorough inter-comparison studies to understand the origin of these differences and to better constrain regional to global glacier mass changes and, ultimately, past and future glacier contribution to sea-level rise.

• Kääb, Andreas; Bazilova, Varvara; Leclercq, Paul & Mannerfelt, Erik Schytt (2023). Global clustering of recent glacier surges from radar backscatter data, 2017-2022. Journal of Glaciology. ISSN 0022-1430. 69(277), s. 1515–1523. doi: 10.1017/jog.2023.35. Fulltekst i vitenarkiv Vis sammendrag

  Abstract Using global Sentinel-1 radar backscatter data, we systematically map the locations of glaciers with surge-type activity during 2017–22. Patterns of pronounced increases or decreases in the strongest backscatter between two winter seasons often indicate large changes in glacier crevassing, which we treat here as a sign of surge-type activity. Validations against velocity time series, terminus advances and crevassing found in optical satellite images confirm the robustness of this approach. We find 115 surge-type events globally between 2017 and 2022, around 100 of which on glaciers already know as surge-type. Our data reveal a pronounced spatial clustering in three regions, (i) Karakoram, Pamirs and Western Kunlun Shan (~50 surges), (ii) Svalbard (~25) and (iii) Yukon/Alaska (~9), with only a few other scattered surges elsewhere. This spatial clustering is significantly more pronounced than the overall global clustering of known surge-type glaciers. The 2017–22 clustering may point to climatic forcing of surge initiation.

• Westermann, Sebastian; Ingeman-Nielsen, Thomas; Scheer, Johanna; Aalstad, Kristoffer; Aga, Juditha & Chaudhary, Nitin [Vis alle 24 forfattere av denne artikkelen] (2023). The CryoGrid community model (version 1.0) - a multi-physics toolbox for climate-driven simulations in the terrestrial cryosphere. Geoscientific Model Development. ISSN 1991-959X. 16(9), s. 2607–2647. doi: 10.5194/gmd-16-2607-2023. Fulltekst i vitenarkiv Vis sammendrag

  The CryoGrid community model is a flexible toolbox for simulating the ground thermal regime and the ice–water balance for permafrost and glaciers, extending a well-established suite of permafrost models (CryoGrid 1, 2, and 3). The CryoGrid community model can accommodate a wide variety of application scenarios, which is achieved by fully modular structures through object-oriented programming. Different model components, characterized by their process representations and parameterizations, are realized as classes (i.e., objects) in CryoGrid. Standardized communication protocols between these classes ensure that they can be stacked vertically. For example, the CryoGrid community model features several classes with different complexity for the seasonal snow cover, which can be flexibly combined with a range of classes representing subsurface materials, each with their own set of process representations (e.g., soil with and without water balance, glacier ice). We present the CryoGrid architecture as well as the model physics and defining equations for the different model classes, focusing on one-dimensional model configurations which can also interact with external heat and water reservoirs. We illustrate the wide variety of simulation capabilities for a site on Svalbard, with point-scale permafrost simulations using, e.g., different soil freezing characteristics, drainage regimes, and snow representations, as well as simulations for glacier mass balance and a shallow water body. The CryoGrid community model is not intended as a static model framework but aims to provide developers with a flexible platform for efficient model development. In this study, we document both basic and advanced model functionalities to provide a baseline for the future development of novel cryosphere models.

• Paul, Frank; Piermattei, Livia; Treichler, Désirée; Gilbert, Lin; Girod, Luc & Kääb, Andreas [Vis alle 10 forfattere av denne artikkelen] (2022). Three different glacier surges at a spot: What satellites observe and what not. The Cryosphere. ISSN 1994-0416. 16(6), s. 2505–2526. doi: 10.5194/tc-16-2505-2022. Fulltekst i vitenarkiv Vis sammendrag

  In the Karakoram, dozens of glacier surges occurred in the past two decades, making the region one of its global hot spots. Detailed analyses of dense time series from optical and radar satellite images revealed a wide range of surge behaviour in this region: from slow advances longer than a decade at low flow velocities to short, pulse-like advances over one or two years with high velocities. In this study, we present an analysis of three currently surging glaciers in the central Karakoram: North and South Chongtar Glaciers and an unnamed glacier referred to as NN9. All three glaciers flow towards the same region but differ strongly in surge behaviour. A full suite of satellite sensors and digital elevation models (DEMs) from different sources are used to (a) obtain comprehensive information about the evolution of the surges from 2000 to 2021 and (b) to compare and evaluate capabilities and limitations of the different satellite sensors for monitoring relatively small glaciers in steep terrain. A strongly contrasting evolution of advance rates and flow velocities is found, though the elevation change pattern is more similar. For example, South Chongtar Glacier had short-lived advance rates above 10 km y−1, velocities up to 30 m d−1 and surface elevations increased by 200 m. In contrast, the neighbouring and three times smaller North Chongtar Glacier had a slow and near linear increase of advance rates (up to 500 m y−1), flow velocities below 1 m d−1 and elevation increases up to 100 m. The even smaller glacier NN9 changed from a slow advance to a full surge within a year, reaching advance rates higher than 1 km y−1. It seems that, despite a similar climatic setting, different surge mechanisms are at play and a transition from one mechanism to another can occur during a single surge. The sensor inter-comparison revealed a high agreement across sensors for deriving flow velocities, but limitations are found on small and narrow glaciers in steep terrain, in particular for Sentinel-1. All investigated DEMs have the required accuracy to clearly show the volume changes during the surges and elevations from ICESat-2 ATL06 data fit neatly. We conclude that the available satellite data allow for a comprehensive observation of glacier surges from space when combining different sensors to determine the temporal evolution of length, elevation and velocity changes.

• Gartner-Roer, Isabelle; Brunner, Nina; Delaloye, Reynald; Haeberli, Wilfried; Kääb, Andreas & Thee, Patrick (2022). Glacier-permafrost relations in a high-mountain environment: 5 decades of kinematic monitoring at the Gruben site, Swiss Alps. The Cryosphere. ISSN 1994-0416. 16, s. 2083–2101. doi: 10.5194/tc-16-2083-2022. Fulltekst i vitenarkiv Vis sammendrag

  Digitized aerial images were used to monitor the evolution of perennially frozen debris and polythermal glacier ice at the intensely investigated Gruben site in the Swiss Alps over a period of about 50 years. The photogrammetric analysis allowed for a compilation of detailed spatio-temporal information on flow velocities and thickness changes. In addition, high-resolution GNSS (global navigation satellite system) and ground surface temperature measurements were included in the analysis to provide insight into short-term changes. Over time, extremely contrasting developments and landform responses are documented. Viscous flow within the warming and already near-temperate rock glacier permafrost continued at a constant average but seasonally variable speed of typically decimetres per year, with average surface lowering limited to centimetres to a few decimetres per year. This constant flow causes the continued advance of the characteristic convex, lava-stream-like rock glacier with its oversteepened fronts. Thawing rates of ice-rich perennially frozen ground to strong climate forcing are very low (centimetres per year) and the dynamic response strongly delayed (timescale of decades to centuries). The adjacent cold debris-covered glacier tongue remained an essentially concave landform with diffuse margins, predominantly chaotic surface structure, intermediate thickness losses (decimetres per year), and clear signs of down-wasting and decreasing flow velocity. The former contact zone between the cold glacier margin and the upper part of the rock glacier with disappearing remains of buried glacier ice embedded on top of frozen debris exhibits complex phenomena of thermokarst in massive ice and backflow towards the topographic depression produced by the retreating glacier tongue. As is typical for glaciers in the Alps, the largely debris-free glacier part shows a rapid response (timescale of years) to strong climatic forcing with spectacular retreat (>10 m a−1) and mass loss (up to >1 m w.e. specific mass loss per year). The system of periglacial lakes shows a correspondingly dynamic evolution and had to be controlled by engineering work for hazard protection.

• Gilbert, Adrien; Gimbert, Florent; Thøgersen, Kjetil; Schuler, Thomas & Kääb, Andreas (2022). A Consistent Framework for Coupling Basal Friction With Subglacial Hydrology on Hard-Bedded Glaciers. Geophysical Research Letters. ISSN 0094-8276. 49(13). doi: 10.1029/2021GL097507. Fulltekst i vitenarkiv Vis sammendrag

  Below hard-bedded glaciers, both basal friction and distributed subglacial drainage are thought to be controlled by a network of cavities. Previous coupled hydro-mechanical models, however, describe cavity-driven friction and hydraulic transmissivity independently, resulting in a physically inconsistent cavity evolution between the two components of the models. Here, we overcome this issue by describing the hydro-mechanical system using a common cavity-evolution description, that governs both transient friction and hydraulic transmissivity. We show that our coupling approach is superior to previous formulations in explaining a unique observation record of glacier sliding speed from the French Alps. We find that, at multi-day to multi-decadal timescales, sliding speed can be expressed as a direct function of basal shear stress and water discharge, without accounting for water pressure, which simply adjusts to maintain the cavitation ratio needed to accommodate the water supply.

• Dunn, Robert J.H.; Aldred, Freya; Gobron, Nadine; Miller, John B.; Willett, Kate M. & Ades, Melanie [Vis alle 195 forfattere av denne artikkelen] (2022). State of the Climate in 2021: Global Climate. Bulletin of The American Meteorological Society - (BAMS). ISSN 0003-0007. 103(8), s. S11–S142. doi: 10.1175/BAMS-D-22-0092.1. Fulltekst i vitenarkiv
• Vries, Maximillian Van Wyk De; Bhushan, Shashank; Jacquemart, Mylène; Deschamps-Berger, Cesar; Berthier, Etienne & Gascoin, Simon [Vis alle 9 forfattere av denne artikkelen] (2022). Pre-collapse motion of the February 2021 Chamoli rock-ice avalanche, Indian Himalaya. Natural Hazards and Earth System Sciences. ISSN 1561-8633. 22(10), s. 3309–3327. doi: 10.5194/nhess-22-3309-2022. Fulltekst i vitenarkiv Vis sammendrag

  Abstract. Landslides are a major geohazard that cause thousands of fatalities every year. Despite their importance, identifying unstable slopes and forecasting collapses remains a major challenge. In this study, we use the 7 February 2021 Chamoli rock–ice avalanche as a data-rich example to investigate the potential of remotely sensed datasets for the assessment of slope stability. We investigate imagery over the 3 decades preceding collapse and assess the precursory signs exhibited by this slope prior to the catastrophic collapse. We evaluate monthly slope motion from 2015 to 2021 through feature tracking of high-resolution optical satellite imagery. We then combine these data with a time series of pre- and post-event digital elevation models (DEMs), which we use to evaluate elevation change over the same area. Both datasets show that the 26.9×106 m3 collapse block moved over 10 m horizontally and vertically in the 5 years preceding collapse, with particularly rapid motion occurring in the summers of 2017 and 2018. We propose that the collapse results from a combination of snow loading in a deep headwall crack and permafrost degradation in the heavily jointed bedrock. Despite observing a clear precursory signal, we find that the timing of the Chamoli rock–ice avalanche could likely not have been forecast from satellite data alone. Our results highlight the potential of remotely sensed imagery for assessing landslide hazard in remote areas, but that challenges remain for operational hazard monitoring.

• Altena, Bas; Kääb, Andreas & Wouters, Bert (2022). Correlation dispersion as a measure to better estimate uncertainty in remotely sensed glacier displacements. The Cryosphere. ISSN 1994-0416. 16(6), s. 2285–2300. doi: 10.5194/tc-16-2285-2022. Fulltekst i vitenarkiv Vis sammendrag

  In recent years a vast amount of glacier surface velocity data from satellite imagery has emerged based on correlation between repeat images. Thereby, much emphasis has been put on the fast processing of large data volumes and products with complete spatial coverage. The metadata of such measurements are often highly simplified when the measurement precision is lumped into a single number for the whole dataset, although the error budget of image matching is in reality neither isotropic nor constant over the whole velocity field. The spread of the correlation peak of individual image offset measurements is dependent on the image structure and the non-uniform flow of the ice and is used here to extract a proxy for measurement uncertainty. A quantification of estimation error or dispersion for each individual velocity measurement can be important for the inversion of, for instance, rheology, ice thickness and/or bedrock friction. Errors in the velocity data can propagate into derived results in a complex and exaggerating way, making the outcomes very sensitive to velocity noise and outliers. Here, we present a computationally fast method to estimate the matching precision of individual displacement measurements from repeat imaging data, focusing on satellite data. The approach is based upon Gaussian fitting directly on the correlation peak and is formulated as a linear least-squares estimation, making its implementation into current pipelines straightforward. The methodology is demonstrated for Sermeq Kujalleq (Jakobshavn Isbræ), Greenland, a glacier with regions of strong shear flow and with clearly oriented crevasses, and Malaspina Glacier, Alaska. Directionality within an image seems to be the dominant factor influencing the correlation dispersion. In our cases these are crevasses and moraine bands, while a relation to differential flow, such as shear, is less pronounced on the correlation spread.

• Bazilova, Varvara & Kääb, Andreas (2022). Mapping Area Changes of Glacial Lakes Using Stacks of Optical Satellite Images. Remote Sensing. 14(23). doi: 10.3390/rs14235973. Fulltekst i vitenarkiv Vis sammendrag

  Glacial lakes are an important and dynamic component of terrestrial meltwater storage, responding to climate change and glacier retreat. Although there is evidence of rapid worldwide growth of glacial lakes, changes in frequency and magnitude of glacier lake outbursts under climatic changes are not yet understood. This study proposes and discusses a method framework for regional-scale mapping of glacial lakes and area change detection using large time-series of optical satellite images and the cloud processing tool Google Earth Engine in a semi-automatic way. The methods are presented for two temporal scales, from the 2-week Landsat revisit period to annual resolution. The proposed methods show how constructing an annual composite of pixel values such as minimum or maximum values can help to overcome typical problems associated with water mapping from optical satellite data such as clouds, or terrain and cloud shadows. For annual-resolution glacial lake mapping, our method set only involves two different band ratios based on multispectral satellite images. The study demonstrates how the proposed method framework can be applied to detect rapid lake area changes and to produce a complete regional-scale glacial lake inventory, using the Greater Caucasus as example.

• Kääb, Andreas; Jacquemart, Mylene; Gilbert, Adrien; Leinss, Silvan; Girod, Luc & Huggel, Christian [Vis alle 15 forfattere av denne artikkelen] (2021). Sudden large-volume detachments of low-angle mountain glaciers – more frequent than thought? The Cryosphere. ISSN 1994-0416. 15(4), s. 1751–1785. doi: 10.5194/tc-15-1751-2021. Fulltekst i vitenarkiv
• Kääb, Andreas; Strozzi, Tazio; Bolch, Tobias; Caduff, Rafael; Trefall, Håkon & Stoffel, Markus [Vis alle 7 forfattere av denne artikkelen] (2021). Inventory and changes of rock glacier creep speeds in Ile Alatau and Kungöy Ala-Too, northern Tien Shan, since the 1950s. The Cryosphere. ISSN 1994-0416. 15(2), s. 927–949. doi: 10.5194/tc-15-927-2021. Fulltekst i vitenarkiv
• Leclercq, Paul; Kääb, Andreas & Altena, Bas (2021). Brief communication: Detection of glacier surge activity using cloud computing of Sentinel-1 radar data. The Cryosphere. ISSN 1994-0416. 15(10), s. 4901–4907. doi: 10.5194/tc-15-4901-2021. Fulltekst i vitenarkiv
• Shugar, D.H.; Jacquemart, M.; Shean, D.; Bhushan, S.; Upadhyay, K. & Sattar, A. [Vis alle 53 forfattere av denne artikkelen] (2021). A massive rock and ice avalanche caused the 2021 disaster at Chamoli, Indian Himalaya. Science. ISSN 0036-8075. 373(6552), s. 300–306. doi: 10.1126/science.abh4455. Fulltekst i vitenarkiv
• Jawak, Shridhar D.; Andersen, Bo Nyborg; Pohjola, Veijo A; Godøy, Øystein; Hübner, Christiane & Jennings, Inger [Vis alle 32 forfattere av denne artikkelen] (2021). SIOS’s Earth Observation (EO), Remote Sensing (RS), and operational activities in response to COVID-19. Remote Sensing. 13(4). doi: 10.3390/rs13040712. Fulltekst i vitenarkiv Vis sammendrag

  Svalbard Integrated Arctic Earth Observing System (SIOS) is an international partnership of research institutions studying the environment and climate in and around Svalbard. SIOS is developing an efficient observing system, where researchers share technology, experience, and data, work together to close knowledge gaps, and decrease the environmental footprint of science. SIOS maintains and facilitates various scientific activities such as the State of the Environmental Science in Svalbard (SESS) report, international access to research infrastructure in Svalbard, Earth observation and remote sensing services, training courses for the Arctic science community, and open access to data. This perspective paper highlights the activities of SIOS Knowledge Centre, the central hub of SIOS, and the SIOS Remote Sensing Working Group (RSWG) in response to the unprecedented situation imposed by the global pandemic coronavirus (SARS-CoV-2) disease 2019 (COVID-19). The pandemic has affected Svalbard research in several ways. When Norway declared a nationwide lockdown to decrease the rate of spread of the COVID-19 in the community, even more strict measures were taken to protect the Svalbard community from the potential spread of the disease. Due to the lockdown, travel restrictions, and quarantine regulations declared by many nations, most physical meetings, training courses, conferences, and workshops worldwide were cancelled by the first week of March 2020. The resumption of physical scientific meetings is still uncertain in the foreseeable future. Additionally, field campaigns to polar regions, including Svalbard, were and remain severely affected. In response to this changing situation, SIOS initiated several operational activities suitable to mitigate the new challenges resulting from the pandemic. This article provides an extensive overview of SIOS’s Earth observation (EO), remote sensing (RS) and other operational activities strengthened and developed in response to COVID-19 to support the Svalbard scientific community in times of cancelled/postponed field campaigns in Svalbard. These include (1) an initiative to patch up field data (in situ) with RS observations, (2) a logistics sharing notice board for effective coordinating field activities in the pandemic times, (3) a monthly webinar series and panel discussion on EO talks, (4) an online conference on EO and RS, (5) the SIOS’s special issue in the Remote Sensing (MDPI) journal, (6) the conversion of a terrestrial remote sensing training course into an online edition, and (7) the announcement of opportunity (AO) in airborne remote sensing for filling the data gaps using aerial imagery and hyperspectral data. As SIOS is a consortium of 24 research institutions from 9 nations, this paper also presents an extensive overview of the activities from a few research institutes in pandemic times and highlights our upcoming activities for the next year 2021. Finally, we provide a critical perspective on our overall response, possible broader impacts, relevance to other observing systems, and future directions. We hope that our practical services, experiences, and activities implemented in these difficult times will motivate other similar monitoring programs and observing systems when responding to future challenging situations. With a broad scientific audience in mind, we present our perspective paper on activities in Svalbard as a case study.

• Hugonnet, Romain; McNabb, Robert Whitfield; Berthier, Etienne; Menounos, Brian; Nuth, Christopher & Girod, Luc Maurice Ramuntcho [Vis alle 11 forfattere av denne artikkelen] (2021). Accelerated global glacier mass loss in the early twenty-first century. Nature. ISSN 0028-0836. 592, s. 726–731. doi: 10.1038/s41586-021-03436-z. Fulltekst i vitenarkiv Vis sammendrag

  Glaciers distinct from the Greenland and Antarctic ice sheets are shrinking rapidly, altering regional hydrology1, raising global sea level2 and elevating natural hazards3. Yet, owing to the scarcity of constrained mass loss observations, glacier evolution during the satellite era is known only partially, as a geographic and temporal patchwork4,5. Here we reveal the accelerated, albeit contrasting, patterns of glacier mass loss during the early twenty-first century. Using largely untapped satellite archives, we chart surface elevation changes at a high spatiotemporal resolution over all of Earth’s glaciers. We extensively validate our estimates against independent, high-precision measurements and present a globally complete and consistent estimate of glacier mass change. We show that during 2000–2019, glaciers lost a mass of 267 ± 16 gigatonnes per year, equivalent to 21 ± 3 per cent of the observed sea-level rise6. We identify a mass loss acceleration of 48 ± 16 gigatonnes per year per decade, explaining 6 to 19 per cent of the observed acceleration of sea-level rise. Particularly, thinning rates of glaciers outside ice sheet peripheries doubled over the past two decades. Glaciers currently lose more mass, and at similar or larger acceleration rates, than the Greenland or Antarctic ice sheets taken separately7,8,9. By uncovering the patterns of mass change in many regions, we find contrasting glacier fluctuations that agree with the decadal variability in precipitation and temperature. These include a North Atlantic anomaly of decelerated mass loss, a strongly accelerated loss from northwestern American glaciers, and the apparent end of the Karakoram anomaly of mass gain10. We anticipate our highly resolved estimates to advance the understanding of drivers that govern the distribution of glacier change, and to extend our capabilities of predicting these changes at all scales. Predictions robustly benchmarked against observations are critically needed to design adaptive policies for the local- and regional-scale management of water resources and cryospheric risks, as well as for the global-scale mitigation of sea-level rise.

• How, Penelope; Messerli, Alexandra; Mätzler, Eva; Santoro, Maurizio; Wiesmann, Andreas & Caduff, Rafael [Vis alle 11 forfattere av denne artikkelen] (2021). Greenland-wide inventory of ice marginal lakes using a multi-method approach. Scientific Reports. 11. doi: 10.1038/s41598-021-83509-1. Fulltekst i vitenarkiv Vis sammendrag

  Ice marginal lakes are a dynamic component of terrestrial meltwater storage at the margin of the Greenland Ice Sheet. Despite their significance to the sea level budget, local flood hazards and bigeochemical fluxes, there is a lack of Greenland-wide research into ice marginal lakes. Here, a detailed multi-sensor inventory of Greenland’s ice marginal lakes is presented based on three well-established detection methods to form a unified remote sensing approach. The inventory consists of 3347 (±8%) ice marginal lakes (>0.05 km 2) detected for the year 2017. The greatest proportion of lakes lie around Greenland’s ice caps and mountain glaciers, and the southwest margin of the ice sheet. Through comparison to previous studies, a ∼75% increase in lake frequency is evident over the west margin of the ice sheet since 1985. This suggests it is becoming increasingly important to include ice marginal lakes in future sea level projections, where these lakes will form a dynamic storage of meltwater that can influence outlet glacier dynamics. Comparison to existing global glacial lake inventories demonstrate that up to 56% of ice marginal lakes could be unaccounted for in global estimates of ice marginal lake change, likely due to the reliance on a single lake detection method.

• Altena, Bas & Kääb, Andreas (2021). Quantifying river ice movement through a combination of European satellite monitoring services. International Journal of Applied Earth Observation and Geoinformation. ISSN 1569-8432. 98. doi: 10.1016/j.jag.2021.102315. Fulltekst i vitenarkiv Vis sammendrag

  Every spring the mechanical river ice break-up and associated ice-runs or flooding pose a threat to communities at Northern latitudes. Monitoring and mitigation efforts along remote Arctic rivers are possible but logistically complex. In recent years, Earth observation programs have emerged based on spaceborne sensors that record large parts of the Earth’s surface at a regular interval and with fast downlink. Most optical satellites have a similar sun-synchronous orbit, and have thus an akin ground track. When different sun-synchronous missions are combined this results in near-simultaneous acquisitions, which make it possible to monitor fast displacements that occur at or near the Earth’s surface over large scales. Hence, it becomes possible to generate a new monitoring system; one of observing river ice movement. In this study we demonstrate the feasibility of a multi-satellite monitoring system by combining data from freely available medium- and coarse-resolution satellites, in this study that is Sentinel-2 and PROBA-V. Velocities of floating river ice during the spring of 2016 are estimated over a more than 700 km long reach of the Lena River in Russia. In order to achieve automatic velocity estimates at such scales, efficient and river-ice specific processing steps are included. Entropy filters are used to detect regions of high contrast and neglects open water or an intact ice cover, and also help the image matching. Post-processing is done through filtering on the general flow direction, stemming from a global river mask dataset. In all, this study shows the potential of extracting river ice movement from a combination of low and medium resolution satellite sensors in sun-synchronous orbit.

• Strozzi, Tazio; Caduff, Rafael; Jones, Nina; Barboux, Chloe; Delaloye, Reynald & Bodin, Xavier [Vis alle 9 forfattere av denne artikkelen] (2020). Monitoring Rock Glacier Kinematics with Satellite Synthetic Aperture Radar. Remote Sensing. 12(3). doi: 10.3390/rs12030559. Fulltekst i vitenarkiv Vis sammendrag

  Active rock glaciers represent the best visual expression of mountain permafrost that can be mapped and monitored directly using remotely sensed data. Active rock glaciers are bodies that consist of a perennially frozen ice/rock mixture and express a distinct flow-like morphology indicating downslope permafrost creep movement. Annual rates of motion have ranged from a few millimeters to several meters per year, varying within the annual cycle, from year to year, as well as at the decennial time scale. During the last decade, in situ observations in the European Alps have shown that active rock glaciers are responding almost synchronously to inter-annual and decennial changes in ground temperature, suggesting that the relative changes of their kinematics are a general indicator of the evolution of mountain permafrost conditions. Here, we used satellite radar interferometry (InSAR) to monitor the rate of motion of various active rock glaciers in the Swiss Alps, Qeqertarsuaq (Western Greenland), and the semiarid Andes of South America. Velocity time series computed with Sentinel-1 SAR images, regularly acquired since 2014, every six days over Europe and Greenland and every 12 days over the Andes, show annual fluctuations, with higher velocities at the end of the summer. A JERS-1 image pair of 1996 and stacks of very high-resolution SAR images from TerraSAR-X and Cosmo-SkyMed from 2008 to 2017 were analyzed using InSAR and offset tracking over the Western Swiss Alps in order to extend the main observation period of our study. A quantitative assessment of the accuracy of InSAR and offset tracking was performed by comparison with in situ methods. Our results for the three different study regions demonstrate that Sentinel-1 InSAR can complement worldwide in situ measurements of active rock glacier kinematics.

• Haga, Odin Næss; McNabb, Robert Whitfield; Nuth, Christopher; Altena, Bas; Schellenberger, Thomas & Kääb, Andreas (2020). From high friction zone to frontal collapse: dynamics of an ongoing tidewater glacier surge, Negribreen, Svalbard. Journal of Glaciology. ISSN 0022-1430. 66(259), s. 742–754. doi: 10.1017/jog.2020.43. Fulltekst i vitenarkiv Vis sammendrag

  Abstract Negribreen, a tidewater glacier located in central eastern Svalbard, began actively surging after it experienced an initial collapse in summer 2016. The surge resulted in horizontal surface velocities of more than 25 m d −1 , making it one of the fastest-flowing glaciers in the archipelago. The last surge of Negribreen likely occurred in the 1930s, but due to a long quiescent phase, investigations of this glacier have been limited. As Negribreen is part of the Negribreen Glacier System, one of the largest glacier systems in Svalbard, investigating its current surge event provides important information on surge behaviour among tidewater glaciers within the region. Here, we demonstrate the surge development and discuss triggering mechanisms using time series of digital elevation models (1969–2018), surface velocities (1995–2018), crevasse patterns and glacier extents from various data sources. We find that the active surge results from a four-stage process. Stage 1 (quiescent phase) involves a long-term, gradual geometry change due to high subglacial friction towards the terminus. These changes allow the onset of Stage 2, an accelerating frontal destabilization, which ultimately results in the collapse (Stage 3) and active surge (Stage 4).

• Daiyrov, Mirlan; Narama, Chiyuki; Kääb, Andreas & Tadono, Takeo (2020). Formation and Outburst of the Toguz-Bulak Glacial Lake in the Northern Teskey Range, Tien Shan, Kyrgyzstan. Geosciences. 10(11). doi: 10.3390/geosciences10110468. Fulltekst i vitenarkiv Vis sammendrag

  In Kyrgyzstan, outburst flood disasters from glacial lakes are increasing. An example is the sudden drainage on 8 August 2019 of the Toguz-Bulak glacial lake in the Tosor river basin of the northern Tien Shan region. In this study, we used remote sensing and field surveys to examine the reasons for the outburst. We found that the lake area changed from 0.021 km² to 0.002 km2 due to the outburst, in which most of the initial 130,000 m3 of water discharged within four hours. In examining the longer-term behavior of this lake, we found that from 2010 through 2019, it appears in June and disappears in September every year. Its maximum area occurs in late July and early August. With the expansion of the lake basin between 2010 and 2019, the lake also increased greatly in size, particularly so in the three years before the outburst, linked to high summer temperatures and the resulting higher inflow of glacier meltwater, finally leading to the sudden drainage in 2019. Before this outburst, a 2-m high moraine dam retained the lake. Continuously inflowing meltwater and the related increasing pressure by the lake water mass eventually broke the moraine dam. Satellite radar interferometry revealed active displacement fringes in the lake basin and moraine dam due to the melting and subsidence of buried ice. An analysis using digital elevation models from 1964 and 2010 also confirms the surface lowering in the lake basin by up to 8.5 m and on the moraine dam by 2 m. Such lowering of the proglacial moraine complex destabilized the moraine dam.

• Altena, Bas & Kääb, Andreas (2020). Ensemble matching of repeat satellite images applied to measure fast-changing ice flow, verified with mountain climber trajectories on Khumbu icefall, Mount Everest. Journal of Glaciology. ISSN 0022-1430. 66(260), s. 905–915. doi: 10.1017/jog.2020.66. Fulltekst i vitenarkiv Vis sammendrag

  Abstract Velocities within an icefall are typically the fastest within a glacier system and experience complex flow. The combination of convergent and fast flow, and steep slope generate a quickly changing and intensely fractured surface. This complicates velocity extraction from repeat satellite images, especially when common pattern matching procedures are used. In this study, we exploit the high temporal revisit of medium-resolution satellite images using a novel image matching technique, ensemble matching, making it possible to generate a high-resolution (30 m) velocity field from high-repeat image sequences despite challenging image conditions. We demonstrate this technique for the first time in the glaciology domain using repeat Sentinel-2 optical data over the famous Khumbu icefall, situated on the southern slopes of Mount Everest. Estimates of velocity go just over 1 m d −1 , which is slower than summer velocities from noisy single pair image matching. This icefall is frequently crossed by high-altitude mountaineers who use a route confined by fixed ropes and ladders set out every season. The mountain climbers typically record their trajectory on their personal satellite navigation device. We use such volunteered geographic information to verify our velocity estimates, confirming our underestimation with ensemble matching. Besides unprecedented remotely sensed surface velocities over the icefall, we also note that the generated velocity field can aid with the planning of a safe passage through this icefall.

• Obu, Jaroslav; Westermann, Sebastian; Vieira, Gonçalo; Abramov, Andrey; Balks, Megan Ruby & Bartsch, Annett [Vis alle 9 forfattere av denne artikkelen] (2020). Pan-Antarctic map of near-surface permafrost temperatures at 1 km2 scale. The Cryosphere. ISSN 1994-0416. 14(2), s. 497–519. doi: 10.5194/tc-14-497-2020. Fulltekst i vitenarkiv Vis sammendrag

  Permafrost is present within almost all of the Antarctic's ice-free areas, but little is known about spatial variations in permafrost temperatures except for a few areas with established ground temperature measurements. We modelled a temperature at the top of the permafrost (TTOP) for all the ice-free areas of the Antarctic mainland and Antarctic islands at 1 km2 resolution during 2000–2017. The model was driven by remotely sensed land surface temperatures and downscaled ERA-Interim climate reanalysis data, and subgrid permafrost variability was simulated by variable snow cover. The results were validated against in situ-measured ground temperatures from 40 permafrost boreholes, and the resulting root-mean-square error was 1.9 ∘C. The lowest near-surface permafrost temperature of −36 ∘C was modelled at Mount Markham in the Queen Elizabeth Range in the Transantarctic Mountains. This is the lowest permafrost temperature on Earth, according to global-scale modelling results. The temperatures were most commonly modelled between −23 and −18 ∘C for mountainous areas rising above the Antarctic Ice Sheet and between −14 and −8 ∘C for coastal areas. The model performance was good where snow conditions were modelled realistically, but errors of up to 4 ∘C occurred at sites with strong wind-driven redistribution of snow.

• Schuler, Thomas Vikhamar; Kohler, Jack; Elagina, Nelly; Hagen, Jon Ove Methlie; Hodson, Andrew & Jania, Jacek A. [Vis alle 13 forfattere av denne artikkelen] (2020). Reconciling Svalbard Glacier Mass Balance. Frontiers in Earth Science. 8. doi: 10.3389/feart.2020.00156. Fulltekst i vitenarkiv Vis sammendrag

  Since the first estimates of Svalbard-wide glacier mass balance were made in the early 2000s, there has been great progress in remote sensing and modeling of mass balance, existing field records have been extended, field records at new locations have been added, and there has been considerable environmental change. There is a wide spread in the available estimates of both total mass balance and surface or climatic mass balance, but there is overall agreement that the glaciers on Svalbard have been losing mass since the 1960s, with a tendency toward more negative mass balance since 2000. We define criteria to select data that are representative and of high credibility; this subset shows a more coherent evolution and reduced spread. In addition, we combine individual field mass balance records collected by different groups into a single dataset that samples glaciers across Svalbard and a range of different size classes. We find a close relationship between measured specific surface mass balance and size of the glacier, in such a way that smaller glaciers experience more negative surface mass balances. A qualitatively similar relationship between the accumulation area ratio and glacier area is found for all glaciers in the Svalbard, suggesting that the relation derived from glaciological records is not only an artifact caused by the limited number of samples (n = 12). We apply this relation to upscale measured surface mass balance for a new estimate for all glaciers of Svalbard. Our reconciled estimates are −7 ± 4 Gt a–1 (2000–2019) for the climatic mass balance, and −8 ± 6 Gt a–1 for the total mass balance. The difference between the two represents the sum of frontal ablation and the combined uncertainty, which together amount to ca. −2 ± 7 Gt a–1. While this is consistent with a previous estimate of Svalbard-wide frontal ablation, the uncertainties are large. Furthermore, several large and long-lasting surges have had considerable and multi-year impact on the total mass balance, and in particular on calving rates, emphasizing the need for better-resolved and more frequently updated estimates of frontal ablation.

• Alexander, Andreas; Obu, Jaroslav; Schuler, Thomas; Kääb, Andreas & Christiansen, Hanne H (2020). Subglacial permafrost dynamics and erosion inside subglacial channels driven by surface events in Svalbard. The Cryosphere. ISSN 1994-0416. 14( 11), s. 4217–4231. doi: 10.5194/tc-14-4217-2020. Fulltekst i vitenarkiv Vis sammendrag

  Abstract. Cold glacier beds, i.e., where the ice is frozen to its base, are widespread in polar regions. Common theories state that stable permafrost should exist under glacier beds on shorter timescales, varying from years to decades. Presently, only a few direct measurements of both subglacial permafrost and the processes influencing its thermal regime exist. Here, we present subglacial permafrost and active layer measurements obtained from within the basal drainage systems of two cold-based glaciers on Svalbard during the summer melt season. Temperature observations were obtained from subglacial sediment that was accessed through the drainage systems of the two glaciers in the previous winters. The temperature records cover the periods from spring to autumn in 2016 and 2019 at the glaciers Larsbreen and Tellbreen in central Svalbard. The ground temperature below Larsbreen indicates colder ground conditions, whereas the temperatures of the Tellbreen drainage system show considerably warmer conditions, close to the freezing point. We suggest the latter is due to the presence of liquid water all year round inside the Tellbreen drainage system. Both drainage systems investigated show an increase in subglacial sediment temperatures after the disappearance of snow bridges and the subsequent connection to surface meltwater supply at the start of the summer melt season. Temperature records show influence of sudden summer water supply events, when heavy melt and rain left their signatures on the thermal regime and the erosion of the glacier bed. Observed vertical erosion can reach up to 0.9 m d−1 at the base of basal drainage channels during summer. We also show that the thermal regime under the subglacial drainage systems is not stable during summer but experiences several freeze–thaw cycles driven by weather events. Our results show the direct importance of heavy melt events and rain on the thermal regime of subglacial permafrost and the erosion of the glacier bed in the vicinity of subglacial drainage channels. Increased precipitation and surface melt, as expected for future climate, will therefore likely lead to increased degradation of subglacial permafrost, as well as higher subglacial erosion of available sediment around the preferential hydrological paths. This in turn might have significant impacts on proglacial and fjord ecosystems due to increased sediment and nutrient input.

• Alexander, Andreas; Obu, Jaroslav; Schuler, Thomas; Kääb, Andreas & Christiansen, Hanne H (2020). Subglacial permafrost dynamics and erosion inside subglacial channels driven by surface events in Svalbard. The Cryosphere Discussions. doi: 10.5194/tc-2020-124. Fulltekst i vitenarkiv
• Alexander, Andreas; Kruusmaa, Maarja; Tuhtan, Jeffrey; Hodson, Andrew; Schuler, Thomas & Kääb, Andreas (2020). Pressure and inertia sensing drifters for glacial hydrology flow path measurements. The Cryosphere. ISSN 1994-0416. 14(3), s. 1009–1023. doi: 10.5194/tc-14-1009-2020. Fulltekst i vitenarkiv Vis sammendrag

  Glacial hydrology plays an important role in the control of glacier dynamics, of sediment transport, and of fjord and proglacial ecosystems. Surface meltwater drains through glaciers via supraglacial, englacial and subglacial systems. Due to challenging field conditions, the processes driving surface processes in glacial hydrology remain sparsely studied. Recently, sensing drifters have shown promise in river, coastal and oceanographic studies. However, practical experience with drifters in glacial hydrology remains limited. Before drifters can be used as general tools in glacial studies, it is necessary to quantify the variability of their measurements. To address this, we conducted repeated field experiments in a 450 m long supraglacial channel with small cylindrical drifters equipped with pressure, magnetometer, acceleration and rotation rate sensors and compared the results. The experiments (n=55) in the supraglacial channel show that the pressure sensors consistently yielded the most accurate data, where values remained within ±0.11 % of the total pressure time-averaged mean (95 % confidence interval). Magnetometer readings also exhibited low variability across deployments, maintaining readings within ±2.45 % of the time-averaged mean of the magnetometer magnitudes. Linear acceleration measurements were found to have a substantially higher variability of ±34.4 % of the time-averaged mean magnitude, and the calculated speeds remained within ±24.5 % of the time-averaged mean along the flow path. Furthermore, our results indicate that prominent shapes in the sensor records are likely to be linked to variations in channel morphology and the associated flow field. Our results show that multimodal drifters can be a useful tool for field measurements inside supraglacial channels. Future deployments of drifters into englacial and subglacial channels promise new opportunities for determining hydraulic and morphologic conditions from repeated measurements of such inaccessible environments.

• Sterckx, Sindy; Brown, Ian; Kääb, Andreas; Krol, Maarten; Morrow, Rosemary & Veefkind, Pepijn [Vis alle 10 forfattere av denne artikkelen] (2020). Towards a European Cal/Val service for earth observation. International Journal of Remote Sensing. ISSN 0143-1161. 41(12), s. 4496–4511. doi: 10.1080/01431161.2020.1718240. Fulltekst i vitenarkiv Vis sammendrag

  Societal dependence on, and commercial and scientific exploitation of Earth-Oriented remote sensing from satellites is growing at an exponential rate. The comprehensive EU Copernicus programme provides a major contribution to the global effort, but even so, to achieve the necessary global and temporal coverage requires synergistic cooperation and associated interoperability of the Worlds sensors. For a user to exploit Earth Observation (EO) data there must exist confidence in data characteristics, quality and reliable delivery. Although long-term data records for climate may be the most demanding in nature, generation of analysis-ready operational data sets for applications, as diverse as food security to pollution monitoring, all require the user to have some quantitative level of confidence in the data and derived information. A long-term Calibration/Validation (Cal/Val) vision necessitates clear ownership and long-term funding. Delineating the roles of the European Commission (EC), space agencies and member states in long-term Cal/Val would provide clarity. It is clear that the space agencies have the responsibility to meet the mission requirement of their spaceborne instruments but long-term validation is often entrusted to interested parties bringing their own resources to the task. Furthermore, there is a critical need for Fiducial Reference Measurements (FRMs), acquired in operational mode, and comprehensive in coverage both spatially and temporally, to assure that the satellite product accuracies are met. This paper discusses the current status, gaps and challenges regarding long-term Cal/Val of EO satellites and recommends the creation of a European coordinating entity for satellite product calibration and validation. The proposed entity would be an integrative organization coordinating the European Cal/Val activities in partnership with the member states and the space agencies and working together with existing data providers to secure access to satellite and in-situ data of traceable FRM standards.

• Nuth, Christopher; Gilbert, Adrien; Köhler, Andreas; McNabb, Robert Whitfield; Schellenberger, Thomas & Sevestre, Heidi [Vis alle 10 forfattere av denne artikkelen] (2019). Dynamic vulnerability revealed in the collapse of an Arctic tidewater glacier. Scientific Reports. 9. doi: 10.1038/s41598-019-41117-0. Fulltekst i vitenarkiv Vis sammendrag

  Glacier flow instabilities can rapidly increase sea level through enhanced ice discharge. Surge-type glacier accelerations often occur with a decadal to centennial cyclicity suggesting internal mechanisms responsible. Recently, many surging tidewater glaciers around the Arctic Barents Sea region question whether external forces such as climate can trigger dynamic instabilities. Here, we identify a mechanism in which climate change can instigate surges of Arctic tidewater glaciers. Using satellite and seismic remote sensing observations combined with three-dimensional thermo-mechanical modeling of the January 2009 collapse of the Nathorst Glacier System (NGS) in Svalbard, we show that an underlying condition for instability was basal freezing and associated friction increase under the glacier tongue. In contrast, continued basal sliding further upstream increased driving stresses until eventual and sudden till failure under the tongue. The instability propagated rapidly up-glacier, mobilizing the entire 450 km2 glacier basin over a few days as the till entered an unstable friction regime. Enhanced mass loss during and after the collapse (5–7 fold compared to pre-collapse mass losses) combined with regionally rising equilibrium line altitudes strongly limit mass replenishment of the glacier, suggesting irreversible consequences. Climate plays a paradoxical role as cold glacier thinning and retreat promote basal freezing which increases friction at the tongue by stabilizing an efficient basal drainage system. However, with some of the most intense atmospheric warming on Earth occurring in the Arctic, increased melt water can reduce till strength under tidewater glacier tongues to orchestrate a temporal clustering of surges at decadal timescales, such as those observed in Svalbard at the end of the Little Ice Age. Consequently, basal terminus freezing promotes a dynamic vulnerability to climate change that may be present in many Arctic tidewater glaciers.

• Kääb, Andreas; Altena, Bas & Mascaro, Joseph (2019). River-ice and water velocities using the Planet optical cubesat constellation. Hydrology and Earth System Sciences (HESS). ISSN 1027-5606. 23(10), s. 4233–4247. doi: 10.5194/hess-23-4233-2019. Fulltekst i vitenarkiv Vis sammendrag

  Abstract. The PlanetScope constellation consists of ∼150 optical cubesats that are evenly distributed like strings of pearls on two orbital planes, scanning the Earth's land surface once per day with an approximate spatial image resolution of 3 m. Subsequent cubesats on each of the orbital planes image the Earth surface with a nominal time lag of approximately 90 s between them, which produces near-simultaneous image pairs over the across-track overlaps of the cubesat swaths. We exploit this short time lag between subsequent Planet cubesat images to track river ice floes on northern rivers as indicators of water surface velocities. The method is demonstrated for a 60 km long reach of the Amur River in Siberia, and a 200 km long reach of the Yukon River in Alaska. The accuracy of the estimated horizontal surface velocities is of the order of ±0.01 m s−1. The application of our approach is complicated by cloud cover and low sun angles at high latitudes during the periods where rivers typically carry ice floes, and by the fact that the near-simultaneous swath overlaps, by design, do not cover the complete Earth surface. Still, the approach enables direct remote sensing of river surface velocities for numerous cold-region rivers at a number of locations and occasionally several times per year – which is much more frequent and over much larger areas than currently feasible. We find that freeze-up conditions seem to offer ice floes that are generally more suitable for tracking, and over longer time periods, compared with typical ice break-up conditions. The coverage of river velocities obtained could be particularly useful in combination with satellite measurements of river area, and river surface height and slope.

• Treichler, Désirée Silvana; Kääb, Andreas Max; Salzmann, Nadine & Xu, Chong-Yu (2019). Recent glacier and lake changes in High Mountain Asia and their relation to precipitation changes. The Cryosphere. ISSN 1994-0416. 13(11), s. 2977–3005. doi: 10.5194/tc-13-2977-2019. Fulltekst i vitenarkiv Vis sammendrag

  Abstract. We present an updated, spatially resolved estimate of 2003–2008 glacier surface elevation changes for the entire region of High Mountain Asia (HMA) from ICESat laser altimetry data. The results reveal a diverse pattern that is caused by spatially greatly varying glacier sensitivity, in particular to precipitation availability and changes. We introduce a spatially resolved zonation where ICESat samples are grouped into units of similar glacier behaviour, glacier type and topographic settings. In several regions, our new zonation reveals local differences and anomalies that have not been described previously. Glaciers in the Eastern Pamirs, Kunlun Shan and central TP were thickening by 0.1–0.7 m a−1, and the thickening anomaly has a crisp boundary in the Eastern Pamirs that continues just north of the central Karakoram. Glaciers in the south and east of the TP were thinning, with increasing rates towards southeast. We attribute the glacier thickening signal to a stepwise increase in precipitation around ∼1997–2000 on the Tibetan Plateau (TP). The precipitation change is reflected by growth of endorheic lakes in particular in the northern and eastern TP. We estimate lake volume changes through a combination of repeat lake extents from Landsat data and shoreline elevations from ICESat and the Shuttle Radar Topography Mission (SRTM) digital elevation model (DEM) for over 1300 lakes. The rise in water volume contained in the lakes corresponds to 4–25 mm a−1, when distributed over entire catchments, for the areas where we see glacier thickening. The precipitation increase is also visible in sparse in situ measurements and MERRA-2 climate reanalysis data but less visible in ERA-Interim reanalysis data. Taking into account evaporation loss, the difference between average annual precipitation during the 1990s and 2000s suggested by these datasets is 34–100 mm a−1, depending on region, which can fully explain both lake growth and glacier thickening (Kunlun Shan) or glacier geometry changes such as thinning tongues while upper glacier areas were thickening or stable (eastern TP). The precipitation increase reflected in these glacier changes possibly extended to the northern slopes of the Tarim Basin, where glaciers were nearly in balance in 2003–2008. Along the entire Himalaya, glaciers on the first orographic ridge, which are exposed to abundant precipitation, were thinning less than glaciers in the dryer climate of the inner ranges. Thinning rates in the Tien Shan vary spatially but are rather stronger than in other parts of HMA.

• Altena, Bas; Scambos, Ted A; Fahnestock, M & Kääb, Andreas (2019). Extracting recent short-term glacier velocity evolution over southern Alaska and the Yukon from a large collection of Landsat data. The Cryosphere. ISSN 1994-0416. 13(3), s. 795–814. doi: 10.5194/tc-13-795-2019. Fulltekst i vitenarkiv Vis sammendrag

  The measurement of glacier velocity fields using repeat satellite imagery has become a standard method of cryospheric research. However, the reliable discovery of important glacier velocity variations on a large scale is still problematic because time series span different time intervals and are partly populated with erroneous velocity estimates. In this study we build upon existing glacier velocity products from the GoLIVE dataset (https://nsidc.org/data/golive, last access: 26 February 2019) and compile a multi-temporal stack of velocity data over the Saint Elias Mountains and vicinity. Each layer has a time separation of 32 days, making it possible to observe details such as within-season velocity change over an area of roughly 150 000 km2. Our methodology is robust as it is based upon a fuzzy voting scheme applied in a discrete parameter space and thus is able to filter multiple outliers. The multi-temporal data stack is then smoothed to facilitate interpretation. This results in a spatiotemporal dataset in which one can identify short-term glacier dynamics on a regional scale. The goal is not to improve accuracy or precision but to enhance extraction of the timing and location of ice flow events such as glacier surges. Our implementation is fully automatic and the approach is independent of geographical area or satellite system used. We demonstrate this automatic method on a large glacier area in Alaska and Canada. Within the Saint Elias and Kluane mountain ranges, several surges and their propagation characteristics are identified and tracked through time, as well as more complicated dynamics in the Wrangell Mountains.

• McNabb, Robert Whitfield; Nuth, Christopher; Kääb, Andreas & Girod, Luc Maurice Ramuntcho (2019). Sensitivity of glacier volume change estimation to DEM void interpolation. The Cryosphere. ISSN 1994-0416. 13(3), s. 895–910. doi: 10.5194/tc-13-895-2019. Fulltekst i vitenarkiv Vis sammendrag

  Glacier mass balance has been estimated on individual glacier and regional scales using repeat digital elevation models (DEMs). DEMs often have gaps in coverage (“voids”), the properties of which depend on the nature of the sensor used and the surface being measured. The way that these voids are accounted for has a direct impact on the estimate of geodetic glacier mass balance, though a systematic comparison of different proposed methods has been heretofore lacking. In this study, we determine the impact and sensitivity of void interpolation methods on estimates of volume change. Using two spatially complete, high-resolution DEMs over southeast Alaska, USA, we artificially generate voids in one of the DEMs using correlation values derived from photogrammetric processing of Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) scenes. We then compare 11 different void interpolation methods on a glacier-by-glacier and regional basis. We find that a few methods introduce biases of up to 20 % in the regional results, while other methods give results very close (<1 % difference) to the true, non-voided volume change estimates. By comparing results from a few of the best-performing methods, an estimate of the uncertainty introduced by interpolating voids can be obtained. Finally, by increasing the number of voids, we show that with these best-performing methods, reliable estimates of glacier-wide volume change can be obtained, even with sparse DEM coverage.

• Altena, Bas; Haga, Odin Næss; Nuth, Christopher & Kääb, Andreas (2019). Monitoring sub-weekly evolution of surface velocity and elevation for a high-latitude surging glacier using sentinel-2. International Archives of Photogrammetry, Remote Sensing and Spatial Information Sciences (ISPRS Archives). ISSN 1682-1750. 42(2), s. 1723–1727. doi: 10.5194/isprs-archives-XLII-2-W13-1723-2019. Fulltekst i vitenarkiv Vis sammendrag

  Currently, the Sentinel-2 twin satellite constellation of the Copernicus program is in operational mode and generates high repeat acquisitions at high-latitudes during polar day. These pushbroom satellites have a large field-of-view and are therefore ideal for simultaneous extraction of glacier displacement and elevation data. In this study we showcase the capabilities of this system set-up by generating time-series of glacier flow and elevation change over Negribreen, a tidewater glacier in Svalbard which nowadays is in its surge phase.

• Obu, Jaroslav; Westermann, Sebastian; Bartsch, Annett; Berdnikov, Nikolai M.; Christiansen, Hanne H & Dashtseren, Avirmed [Vis alle 21 forfattere av denne artikkelen] (2019). Northern Hemisphere permafrost map based on TTOP modelling for 2000–2016 at 1 km2 scale. Earth-Science Reviews. ISSN 0012-8252. 193, s. 299–316. doi: 10.1016/j.earscirev.2019.04.023. Fulltekst i vitenarkiv Vis sammendrag

  Permafrost is a key element of the cryosphere and an essential climate variable in the Global Climate Observing System. There is no remote-sensing method available to reliably monitor the permafrost thermal state. To estimate permafrost distribution at a hemispheric scale, we employ an equilibrium state model for the temperature at the top of the permafrost (TTOP model) for the 2000–2016 period, driven by remotely-sensed land surface temperatures, down-scaled ERA-Interim climate reanalysis data, tundra wetness classes and landcover map from the ESA Landcover Climate Change Initiative (CCI) project. Subgrid variability of ground temperatures due to snow and landcover variability is represented in the model using subpixel statistics. The results are validated against borehole measurements and reviewed regionally. The accuracy of the modelled mean annual ground temperature (MAGT) at the top of the permafrost is ±2 °C when compared to permafrost borehole data. The modelled permafrost area (MAGT <0 °C) covers 13.9 × 106 km2 (ca. 15% of the exposed land area), which is within the range or slightly below the average of previous estimates. The sum of all pixels having isolated patches, sporadic, discontinuous or continuous permafrost (permafrost probability >0) is around 21 × 106 km2 (22% of exposed land area), which is approximately 2 × 106 km2 less than estimated previously. Detailed comparisons at a regional scale show that the model performs well in sparsely vegetated tundra regions and mountains, but is less accurate in densely vegetated boreal spruce and larch forests.

• Falaschi, Daniel; Kääb, Andreas; Paul, Frank; Tadono, Takeo; Rivera, Juan Antonio & Lenzano, Luis Eduardo (2019). Brief communication: Collapse of 4Mm3 of ice from a cirque glacier in the Central Andes of Argentina. The Cryosphere. ISSN 1994-0416. 13(3), s. 997–1004. doi: 10.5194/tc-13-997-2019. Fulltekst i vitenarkiv Vis sammendrag

  Abstract. Among glacier instabilities, collapses of large parts of low-angle glaciers are a striking, exceptional phenomenon. So far, merely the 2002 collapse of Kolka Glacier in the Caucasus Mountains and the 2016 twin detachments of the Aru glaciers in western Tibet have been well documented. Here we report on the previously unnoticed collapse of an unnamed cirque glacier in the Central Andes of Argentina in March 2007. Although of much smaller ice volume, this 4.2±0.6×106 m3 collapse in the Andes is similar to the Caucasus and Tibet ones in that the resulting ice avalanche travelled a total distance of ∼2 km over a surprisingly low angle of reach (∼5∘).

• McNabb, Robert Whitfield; Nuth, Christopher; Kääb, Andreas & Girod, Luc Maurice Ramuntcho (2018). Sensitivity of geodetic glacier mass balance estimation to DEM void interpolation. The Cryosphere Discussions. doi: 10.5194/tc-2018-175. Fulltekst i vitenarkiv
• Gilbert, Adrien; Leinss, Silvan; Kargel, Jeffrey S.; Kääb, Andreas; Gascoin, Simon & Leonard, Gregory [Vis alle 9 forfattere av denne artikkelen] (2018). Mechanisms leading to the 2016 giant twin glacier collapses, Aru Range, Tibet. The Cryosphere. ISSN 1994-0416. 12(9), s. 2883–2900. doi: 10.5194/tc-12-2883-2018. Fulltekst i vitenarkiv Vis sammendrag

  In north-western Tibet (34.0∘ N, 82.2∘ E) near lake Aru Co, the entire ablation areas of two glaciers (Aru-1 and Aru-2) suddenly collapsed on 17 July and 21 September 2016. The masses transformed into ice avalanches with volumes of 68 and 83×106 m3 and ran out up to 7 km in horizontal distance, killing nine people. The only similar event currently documented is the 130×106 m3 Kolka Glacier rock and ice avalanche of 2002 (Caucasus Mountains). Using climatic reanalysis, remote sensing, and three-dimensional thermo-mechanical modelling, we reconstructed the Aru glaciers' thermal regimes, thicknesses, velocities, basal shear stresses, and ice damage prior to the collapse in detail. Thereby, we highlight the potential of using emergence velocities to constrain basal friction in mountain glacier models. We show that the frictional change leading to the Aru collapses occurred in the temperate areas of the polythermal glaciers and is not related to a rapid thawing of cold-based ice. The two glaciers experienced a similar stress transfer from predominant basal drag towards predominant lateral shearing in the detachment areas and during the 5–6 years before the collapses. A high-friction patch is found under the Aru-2 glacier tongue, but not under the Aru-1 glacier. This difference led to disparate behaviour of both glaciers, making the development of the instability more visible for the Aru-1 glacier through enhanced crevassing and terminus advance over a longer period. In comparison, these signs were observable only over a few days to weeks (crevasses) or were absent (advance) for the Aru-2 glacier. Field investigations reveal that those two glaciers were underlain by soft, highly erodible, and fine-grained sedimentary lithologies. We propose that the specific bedrock lithology played a key role in the two Tibet and the Caucasus Mountains giant glacier collapses documented to date by producing low bed roughness and large amounts of till, rich in clay and silt with a low friction angle. The twin 2016 Aru collapses would thus have been driven by a failing basal substrate linked to increasing pore water pressure in the subglacial drainage system in response to increases in surface melting and rain during the 5–6 years preceding the collapse dates.

• Girod, Luc Maurice Ramuntcho; Nielsen, Niels Ivar; Couderette, Frédérique; Nuth, Christopher & Kääb, Andreas (2018). Precise DEM extraction from Svalbard using 1936 high oblique imagery. Geoscientific Instrumentation, Methods and Data Systems. ISSN 2193-0856. 7, s. 277–288. doi: 10.5194/gi-7-277-2018. Fulltekst i vitenarkiv Vis sammendrag

  Stretching time series further in the past with the best possible accuracy is essential to the understanding of climate change impacts and geomorphological processes evolving on decadal-scale time spans. In the first half of the twentieth century, large parts of the polar regions were still unmapped or only superficially so. To create cartographic data, a number of historic photogrammetric campaigns were conducted using oblique imagery, which is easier to work with in unmapped environments as collocating images is an easier task for the human eye given a more familiar viewing angle and a larger field of view. Even if the data obtained from such campaigns gave a good baseline for the mapping of the area, the precision and accuracy are to be considered with caution. Exploiting the possibilities arising from modern image processing tools and reprocessing the archives to obtain better data is therefore a task worth the effort. The oblique angle of view of the data is offering a challenge to classical photogrammetric tools, but the use of modern structure-from-motion (SfM) photogrammetry offers an efficient and quantitative way to process these data into terrain models. In this paper, we propose a good practice method for processing historical oblique imagery using free and open source software (MicMac and Python) and illustrate the process using images of the Svalbard archipelago acquired in 1936 by the Norwegian Polar Institute. On these data, our workflow provides 5 m resolution, high-quality elevation data (SD 2 m for moderate terrain) as well as orthoimages that allow for the reliable quantification of terrain change when compared to more modern data.

• Winsvold, Solveig Havstad; Kääb, Andreas; Nuth, Christopher; Andreassen, Liss Marie; Pelt, Ward J.J. Van & Schellenberger, Thomas (2018). Using SAR satellite data time series for regional glacier mapping. The Cryosphere. ISSN 1994-0416. 12(3), s. 867–890. doi: 10.5194/tc-12-867-2018. Fulltekst i vitenarkiv Vis sammendrag

  With dense SAR satellite data time series it is possible to map surface and subsurface glacier properties that vary in time. On Sentinel-1A and RADARSAT-2 backscatter time series images over mainland Norway and Svalbard, we outline how to map glaciers using descriptive methods. We present five application scenarios. The first shows potential for tracking transient snow lines with SAR backscatter time series and correlates with both optical satellite images (Sentinel-2A and Landsat 8) and equilibrium line altitudes derived from in situ surface mass balance data. In the second application scenario, time series representation of glacier facies corresponding to SAR glacier zones shows potential for a more accurate delineation of the zones and how they change in time. The third application scenario investigates the firn evolution using dense SAR backscatter time series together with a coupled energy balance and multilayer firn model. We find strong correlation between backscatter signals with both the modeled firn air content and modeled wetness in the firn. In the fourth application scenario, we highlight how winter rain events can be detected in SAR time series, revealing important information about the area extent of internal accumulation. In the last application scenario, averaged summer SAR images were found to have potential in assisting the process of mapping glaciers outlines, especially in the presence of seasonal snow. Altogether we present examples of how to map glaciers and to further understand glaciological processes using the existing and future massive amount of multi-sensor time series data.

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• Aalstad, Kristoffer; Alonso-González, Esteban; Fiddes, Joel; Kääb, Andreas; Pirk, Norbert & Treichler, Désirée [Vis alle 9 forfattere av denne artikkelen] (2026). Global ensemble-based mountain snow reanalysis. doi: https:/snowhydro.unirioja.es/public/files/Snowhydro_abstracts.pdf. Fulltekst i vitenarkiv Vis sammendrag

  Snow is an essential climate variable that regulates the global cycles of energy, carbon, and water. However, the state of the snowpack is only partially observable by satellites due to gaps, noise, and indirect retrieval algorithms. The fusion of uncertain snow models and noisy observations through data assimilation presents a natural solution to this problem. Nonetheless, data assimilation has received relatively little attention from the snow remote sensing community despite its potential to serve as a generalized retrieval method that adds value by inferring gap-free hidden snow state variables with uncertainty quantification. We present ongoing research on applying ensemble-based data assimilation techniques to produce a global mountain snow reanalysis constrained by satellite data. These efforts are motivated by the challenge of estimating seasonal snow mass in mountainous terrain which remains a major unsolved problem in snow hydrology. Taking a step towards solving this problem, we aim to generate the first global, consistent, ensemble-based, daily, kilometer-scale reanalysis of seasonal snow mass (i.e. snow water equivalent) and fractional snow-covered area dynamics for all mountains outside Antarctica. To produce our reanalysis, we assimilate fractional snow-covered area satellite retrievals from ESA Snow_cci into a simple snow model run at a relatively high spatio-temporal resolution. Given the efficiency of our model, we are able to leverage promising iterative ensemble-based schemes while ensuring that the problem remains computationally feasible. Specifically, we build on developments hybridizing ensemble Kalman and particle methods for robust and tractable Bayesian inference. We also demonstrate how to hierarchically infer hyperparameters related to snow climatology. The new reanalysis approach is evaluated using independent spaceborne, airborne, and in-situ validation data. Finally, we benchmark the performance of our reanalysis by comparing it to other global mountain snow products. Unlike existing global snow mass products, our product is uncertainty-aware and is tailored to address a key knowledge gap concerning mountain snow mass. We highlight that, by combining observations with models, data assimilation can transform largely untapped climate data into actionable climate information on essential climate variables. The uncertainty quantification in this probabilistic climate information enables informed decision making in response to climate change and its impacts.

• David, Harcourt, William; Pearce, Danni M; Wojciech, Gajek,; Harold, Lovell,; Mannerfelt, Erik Schytt & Kääb, Andreas [Vis alle 10 forfattere av denne artikkelen] (2025). Svalbard Surge Database 2024 (RGI2000-v7.0-G-07). doi: 10.5281/zenodo.15240754. Fulltekst i vitenarkiv Vis sammendrag

  We have developed a new database of surge-type glaciers in Svalbard by combining existing compilations and reviewing studies examining their dynamics. Our database is based upon the Global Land and Ice Measurements from Space (GLIMS) database (König et al. 2014), which is now incorporated into RGI 7.0 (RGI 7.0 Consortium 2023) and consists of 1,583 glaciers in Svalbard. Therefore, the first five fields come from the RGI 7.0 database: rgi_id: Glacier ID from RGI database. glims_id: Glacier ID from GLIMS database. cenlon: Longitude of glacier centre point. cenlat: Latitude of glacier centre point. glac_name: Name of glacier. Our compilation of existing Svalbard-wide glacier surge databases is sourced from several studies: Lefauconnier and Hagen (1991) [LH1991]; Hagen et al. (1993) [H1993]; Sevestre and Benn (2015) [SB2015]; Farnsworth et al. (2016) [F2016]; Kääb et al. (2023) [KA2023]; and Koch et al. (2023) [KO2023]. The compilation of LH1991 only covers eastern Svalbard and is focused on marine-terminating glaciers but is included as it contains several important details on surge characteristics. H1993 is the original database of glaciers across Svalbard and similarly contains details of historical surges. The current RGI 7.0 database defines the “surge status” of each glacier according to Sevestre and Benn (2015): no evidence of surging (0); possible surge (1), probably surge (2), and observed surge (3). Where the SB2015 database does not have corresponding evidence from one of the other compilations, we determine the glaciers surge status to be ‘undefined’ and do not include it in the S_All field. The F2016 compilation was manually translated into the RGI 7.0 database. The glacier names described in F2016 often referred to tributaries which are now combined into single glacier catchments (e.g., Nuddbreen / Strongbreen), hence we manually combined these entries. The recent compilations from KA2023 and KO2023 were manually transcribed from tables in PDF files. The subsequent eight fields document each compilation: SB2015: Surge database from Sevestre and Benn (2015). [0-3] F2016: Surge database from Farnsworth et al. (2016). [0-1] H1993: Surge database from Hagen et al. (1993). [0-1] LH1991: Surge database from Lefauconnier and Hagen (1991). [0-1] KA2023: Surge observations from Kääb et al. (2023). This data set is based on manual surge identification in annual Sentinel-1 interferometric wide-swath (IW) satellite radar backscatter differences between 2017 and 2022 (Kääb et al. 2023). This has been updated in this database (version 3) by mapping more recent surges from winter-to-winter differences 2022-2023, 2023-2024, and 2024-2025 using new IW data. Before 2017, no Sentinel-1 IW data are available over Svalbard, and we use 2015-2016 and 2016-2017 extended wide-swath data (EW) instead, acknowledging that these coarser data (compared to IW) might lead to less detailed surge identification, or overlooking of surges of small glaciers or surges accompanied by only limited backscatter changes. Based on these additional data, we are also able to update some surge information contained in the original KA2023, for instance concerning surge start and end years, and by adding the last year of strongly enhanced backscatter (before backscatter reduction). The new 2015-2025 backscatter-derived surge inventory over Svalbard contains now 40 surging glaciers (the 2017-2022 KA2023 contained 26 surging glaciers). [0-1] KO2023: Surge observations from Koch et al. (2023). [0-1] Other: Surge observations from other literature sources. [0-1] S_Direct: All surges that have been directly observed. [0-1] S_Indirect: All surges that have been indirectly observed e.g. from palaeo-glaciological analysis. [0-1] S_All: All surges that have been either directly observed or inferred from the palaeo-glaciological record. [0-1] Contemporary and palaeo-glaciological evidence of surges is generally limited to the period ~1850–present, which broadly corresponds to the end of the LIA through to the modern-day. Where multiple surges have been recorded, we separate these using “;” in the database, and use “n/a” where the details of the surge have not been recorded. Surges have been classified as a binary 0 (not surge-type) or 1 (surge-type), with the exception of the Sevestre and Benn (2015) database as described above. The subsequent eight fields document (if known) the following characteristics for each glacier in Svalbard: S_Onset: Surge Onset (Year) S_Term: Surge Termination (Year) S_Act_Vel: Max Active-Phase Velocity (m/d) S_Qui_Vel: Mean Quiescence Velocity (m/d) S_Term_Ch: Terminus Change (m) Here, we use 'n/a' for glaciers with no evidence of surging, whilst 'Not observed' is used where we have not observed any of the above characteristics for a glacier with evidence of surging. The final column contains references to where surges have been reported. Included in this version is also a version of the RGI for Svalvard with the new database included.  

• Lappe, Ronja; Enzenhofer, Ursula; Egli, Pascal Emanuel; Andreassen, Liss Marie; Calovi, Martina & Kääb, Andreas (2025). Remote sensing capabilities of monitoring a periodically outbursting ice-dammed glacier lake: a case study at Nedre Demmevatnet, Hardangerjøkulen ice cap, Norway. Fulltekst i vitenarkiv Vis sammendrag

  Glacial lake outburst floods (GLOFs) are sudden drainage events from glacier-dammed lakes that can cause significant downstream damage (Taylor et al., 2023). In Norway, such events have been documented for centuries, with one of the most well-known sites being Nedre Demmevatnet, dammed by the retreating Rembesdalskåka glacier at Hardangerjøkulen. Historical records describe destructive GLOFs as early as the 18th century. GLOFs in 1893 and 1937 caused large damage to houses, bridges and cropland and resulted in construction of artificial drainage tunnels in 1899 and 1938. Since 2014 new outbursts have occurred (Kjøllmoen et al., 2024). This study investigates the potential of combining open-access satellite data with field observations to detect and characterize GLOFs at Nedre Demmevatnet. Sentinel-1 radar, and Sentinel-2 optical imagery were processed in Google Earth Engine using a Random Forest classifier to reconstruct lake area dynamics and constrain drainage timings between 2014 and 2025. Preliminary results show that GLOF events can be identified within ±2 days of dates reported in the official NVE inventory (NVE, 2024), while seasonal lake evolution is reliably captured despite cloud cover challenges. Complementary UAV photogrammetry from 2022 provided high-resolution bathymetry of the drained lake basin, enabling annual volume reconstructions of up to 3.6 million m3 in peak years. Validation with in-situ lake level logging between 2022-2025 confirmed the ability of satellite data to track both gradual refilling and sudden drainage. Future work will link these observations to climatic drivers (e.g. temperature, precipitation or snow melt) to assess variability in outburst mechanisms and improve understanding of GLOF triggers. This study shows how freely available Earth observation data can support long-term GLOF monitoring in remote alpine environments. With refinements in classifier robustness and expanded validation, such workflows can contribute to hazard assessment and risk reduction at glacier-dammed lakes in Norway and worldwide (Emmer et al., 2022; Wangchuk and Bolch, 2020). References Emmer, A., Wood, J.L., Cook, S.J., Harrison, S., Wilson, R., Diaz-Moreno, A., Reynolds, J.M., Torres, J.C., Yarleque, C., Mergili, M., Jara, H.W., Bennett, G., Caballero, A., Glasser, N.F., Melgarejo, E., Riveros, C., Shannon, S., Turpo, E., Tinoco, T., Torres, L., Garay, D., Villafane, H., Garrido, H., Martinez, C., Apaza, N., Araujo, J., Poma, C., 2022. 160 glacial lake outburst floods (GLOFs) across the Tropical Andes since the Little Ice Age. Global and Planetary Change (208), 103722. Kjøllmoen, B. (Ed.), L.M. Andreassen, H. Elvehøy. 2024. Glaciological investigations in Norway 2023. NVE Rapport 22-2024, 98 pp +app. NVE, 2024. Glacier lake outburst floods. Taylor, C., Robinson, T.R., Dunning, S., Rachel Carr, J., Westoby, M., 2023. Glacial lake outburst floods threaten millions globally. Nat Commun (14), 487. Wangchuk, S., Bolch, T., 2020. Mapping of glacial lakes using Sentinel-1 and Sentinel-2 data and a random forest classifier: Strengths and challenges. Science of Remote Sensing (2), 100008.

• Aalstad, Kristoffer; Alonso-Gonzalez, Esteban; Fiddes, Joel; Guillet, Gregoire; Kääb, Andreas Max & Mazzolini, Marco [Vis alle 11 forfattere av denne artikkelen] (2025). Ensemble-based cryospheric reanalysis to infer global snow mass. Fulltekst i vitenarkiv Vis sammendrag

  Snow, glaciers, and permafrost are essential climate variables (ECV) that regulate the global cycles of energy, carbon, and water. At the same time, these cryospheric ECVs are only partially observable by satellites due to gaps, noise, and indirect retrieval algorithms. Data assimilation (DA), namely the Bayesian fusion of uncertain models and noisy observations, presents a natural solution to the problem of partial observability. Nonetheless, unlike in numerical weather prediction, DA has received relatively little attention from the Earth observation (EO) community despite its potential as a generalized retrieval framework that adds value by filling gaps and inferring latent ECVs with uncertainty quantification. This contribution presents active research on applying ensemble-based DA techniques to carry out cryospheric reanalysis constrained by satellite data. Our future goals with these efforts are to generate consistent global high resolution reanalyses of seasonal snow mass (also known as snow water equivalent or simply SWE), glacier mass balance, and the thermal state of permafrost. In doing so, we seek to leverage a myriad of data streams including Earth observing satellites, global atmospheric reanalyses, airborne retrievals, and in-situ observations. Here we focus on seasonal snow since accurate global snow mass estimation, particularly in mountainous terrain, remains a major unsolved problem in snow hydrology. To help tackle this problem, we are developing a global scale ensemble-based snow reanalysis product by assimilating ESA Snow_cci fractional snow-covered area retrievals. Using a simple snow model we are able to devote considerable computational resources to generate a global snow reanalysis at daily temporal resolution and kilometric spatial resolution while still being able to afford using promising iterative ensemble-based DA schemes. For the latter, we explore recent developments in hybridizing iterative ensemble Kalman and particle methods to provide robust Bayesian posterior inference. In particular, we demonstrate how these schemes can be used as nested smoothers to hierarchically infer prior hyperparameters related to snow climatology. The new reanalysis approach is evaluated using independent spaceborne, airborne, and in-situ validation data by comparing its performance to state-of-the-art regional snow reanalyses and existing global snow mass products in the form of ERA5-Land reanalysis data and ESA Snow_cci SWE retrievals. Unlike existing global snow products, this new snow reanalysis product is uncertainty-aware, assimilates snow satellite data, and specifically targets a key knowledge gap concerning mountain snow mass. Concurrent efforts to adapt this DA framework towards the generation of global ensemble-based reanalyses for glaciers and permafrost will also be showcased to emphasize synergies across these terrestrial cryospheric reanalysis efforts. We highlight that, by combining EO with cryospheric models, ensemble-based DA can transform largely untapped climate data into actionable climate information on cryospheric ECVs. The baked-in uncertainty quantification in this probabilistic climate information empowers us to make decisions in response to climate change and its perilous impacts on the mountain cryosphere.

• Jawak, Shridhar D.; Pohjola, Veijo; Kääb, Andreas Max; Andersen, Bo Nyborg; Błaszczyk, Małgorzata & Salzano, Roberto [Vis alle 12 forfattere av denne artikkelen] (2023). Status of Earth Observation and Remote Sensing Applications in Svalbard. Remote Sensing. 15(2). doi: 10.3390/rs15020513. Fulltekst i vitenarkiv
• Paul, Frank; Piermattei, Livia; Treichler, Désirée; Gilbert, Lin; Wuite, Jan & Strozzi, Tazio [Vis alle 7 forfattere av denne artikkelen] (2022). Three different glacier surges at a spot: What satellites observe and what not. Fulltekst i vitenarkiv
• Rouyet, Line; Lilleøren, Karianne Staalesen; Etzelmüller, Bernd; Kääb, Andreas Max; Christiansen, Hanne H. & Humlum, Ole [Vis alle 9 forfattere av denne artikkelen] (2022). SAR satellite remote sensing for mapping and monitoring Norwegian rock glaciers. Fulltekst i vitenarkiv
• Truffer, M; Kääb, Andreas; Harrison, W.D.A.; Ospisova, G.; Nosenko, G. & Espizua, Lydia [Vis alle 11 forfattere av denne artikkelen] (2021). Chapter 13 - Glacier surges. I Haeberli, Wilfried (Red.), Snow and Ice-Related Hazards, Risks, and Disasters (Second Edition). Elsevier. ISSN 9780128171295. s. 417–466. doi: 10.1016/B978-0-12-817129-5.00003-2. Fulltekst i vitenarkiv
• Treichler, Désirée; Piermattei, Livia; Paul, Frank; Gilbert, Lin; Kääb, Andreas & Libert, Ludivine [Vis alle 9 forfattere av denne artikkelen] (2021). Observing small surging glaciers from space. Fulltekst i vitenarkiv
• Andreassen, Liss Marie; Moholdt, Geir & Kääb, Andreas (2021). Satellitter overvåker norske breer. Det er det gode grunner til. [Journal]. Aftenposten. Fulltekst i vitenarkiv
• Leclercq, Paul Willem; Kääb, Andreas; Altena, Bas & Bazilova, Varvara (2021). Detection of glacier surge activity using cloud computing of Sentinel-1 radar data. Fulltekst i vitenarkiv
• Messerli, A.; Andreassen, Liss M.; Moholdt, G.; Winsvold, S. H.; Nagy, T. & Kääb, A. [Vis alle 7 forfattere av denne artikkelen] (2021). Glacier snowlines and snowzones 2015-2019. Fulltekst i vitenarkiv
• Hugonnet, Romain; McNabb, Robert Whitfield; Berthier, Etienne; Menounos, B; Nuth, Christopher & Girod, Luc Maurice Ramuntcho [Vis alle 11 forfattere av denne artikkelen] (2020). Accelerated global glacier mass loss in the early twenty-first century. Fulltekst i vitenarkiv
• Hugonnet, Romain; McNabb, Robert Whitfield; Berthier, Etienne; Menounos, B.; Nuth, Christopher & Girod, Luc Maurice Ramuntcho [Vis alle 11 forfattere av denne artikkelen] (2020). A globally complete, spatially and temporally resolved estimate of glacier mass change: 2000 to 2019. Fulltekst i vitenarkiv
• McNabb, Robert Whitfield; Girod, Luc Maurice Ramuntcho; Nuth, Christopher & Kääb, Andreas (2020). An open-source toolset for automated processing of historic spy photos: sPyMicMac. Fulltekst i vitenarkiv
• Alexander, Andreas; Obu, Jaroslav; Schuler, Thomas; Kääb, Andreas & Christiansen, Hanne H (2020). Subglacial permafrost dynamics and erosion inside subglacial channels driven by surface events in Svalbard. Fulltekst i vitenarkiv
• Kääb, Andreas & Treichler, Désirée Silvana (2019). High Mountain Asia glacier changes in the early 21st century. Fulltekst i vitenarkiv
• Kääb, Andreas (2019). The Ocean and Cryosphere in a Changing Climate. Fulltekst i vitenarkiv
• Kääb, Andreas (2019). The Ocean and Cryosphere in a Changing Climate. Fulltekst i vitenarkiv
• Kääb, Andreas (2019). The Ocean and Cryosphere in a Changing Climate. Fulltekst i vitenarkiv
• Kääb, Andreas (2019). The Ocean and Cryosphere in a Changing Climate. Fulltekst i vitenarkiv
• Kääb, Andreas (2019). A new era for observing glaciers from space. Fulltekst i vitenarkiv
• Kääb, Andreas (2019). Collapsing Glaciers. Fulltekst i vitenarkiv
• Altena, Bas; Armstrong, William & Kääb, Andreas (2019). On the origin of ogives. Fulltekst i vitenarkiv
• Alexander, Andreas; Kruusma, Maarja; Tuhtan, Jeffrey; Hodson, Andrew; Schuler, Thomas & Kääb, Andreas (2019). Sensing drifters as tool for glacial hydrology measurements. Fulltekst i vitenarkiv
• Alexander, Andreas; Kruusma, Maarja; Tuhtan, Jeffrey; Hodson, Andrew; Schuler, Thomas & Kääb, Andreas (2019). Sensing drifters as tool for glacial hydrology measurements. Fulltekst i vitenarkiv
• Altena, Bas; Schellenberger, Thomas & Kääb, Andreas (2019). Bringing together remote sensing data to produce region-wide glacier velocities. Fulltekst i vitenarkiv Vis sammendrag

  The recent increase of medium resolution satellite programs that consistently senses the Earth surface has brought great potential for glaciology. Satellites like the Sentinel-1&2 tandems, Landsat 7&8 duo and Rapideye constellation sense mountain ranges almost without missing a day. Consequently, high resolution time-series can be constructed, for snow-cover or crack propagation, or other single scene information. Properties derived from combinations of acquisitions, such as, glacier velocity is more challenging. Velocity estimation is based upon image matching, which can be hindered by coherence loss or cloud cover. Consequently, generated velocity data have gaps and have scattered coverage at different time intervals. This steady data stream has resulted in several initiatives to use image matching techniques in batch processing mode. Resulting in glacier velocity services being set-up (e.g.: cryoportal, GoLIVE, Geodetic data portal), but these initiatives are satellite specific. If such products are merged it is possible to increase temporal and spatial coverage and increase the reliability of the velocity estimates. We developed such a workflow that merges glacier velocity products of different time spans and type. This combined dataset covers several mountain ranges and has a regular time step at a monthly pace. This consistent time-series make it possible to observe glacier dynamics that are of short nature. As the satellite recordings already span several years, the dynamics of tidewater glaciers or (partial) surges can be observed and analyzed. This study therefor shows the great potential of bringing together different satellite systems.

• Altena, Bas & Kääb, Andreas (2019). River ice velocity from Sentinel-twogether and PROBA-VELOCITY. Fulltekst i vitenarkiv Vis sammendrag

  Most optical satellite systems are set in a sun-synchronous orbit. Because of such similar configurations, orbits-of-opportunity arise. These are combinations of satellites that sense near-simultaneously. Consequently, more information can be generated from a single satellite mission when it is orchestrated with another. Hence, one tactic is to let a small-satellite piggyback with a monitoring program. In this study we demonstrate such a strategy where we make use of the PROBA-V small-satellite in combination with Sentinel-2. The inclination of both satellites are very similar, as well as, their overpass time. This makes large scale extraction of displacements possible. One such application that can make use of this recording sequence is the monitoring of river ice break-up. In spring the melting season sets in and Arctic rivers increase in flow discharge. Eventually the water is able to break the ice cover, which will then float along. However, congestion of this ice results in jamming, associated flooding and ice run, causing damage to infrastructure and river banks. The long stretch of rivers make monitoring difficult, thus satellite remote sensing can be a means for identifying bottlenecks. The resolution of Proba-V is 100 meters, but the time separation results in drift of the floating ice in the order of 1 km. Hence, from data of this satellite pair we are able to generate a dense velocity field over the Lena River in Russia, which stretches more than 300 km. This study demonstrates the potential of the Sentinels as an umbrella for other smaller missions. This near-simultaneous configuration has not been envisioned as a mission objective, but is coincidence. However for a future small-satellite design through clever orbit configurations can easily enhance a satellites product portfolio.

• Altena, Bas & Kääb, Andreas (2019). Techniques to reveal inter-annual glacier flow from optical remote sensing. Fulltekst i vitenarkiv
• Altena, Bas & Kääb, Andreas (2019). Shifts in surface, speed, or shadows: What do we measure with different time-of-day overpasses of Sentinel-2. Fulltekst i vitenarkiv
• Altena, Bas; Wouters, Bert; Broeke, Michiel Van den; Kääb, Andreas; Raup, Bruce & Scambos, Ted A [Vis alle 7 forfattere av denne artikkelen] (2019). Eratosthenes. Fulltekst i vitenarkiv
• Nuth, Christopher; Kohler, Jack; Köhler, Andreas; Lefeuvre, Pierre-Marie; Schuler, Thomas & Pelt, Ward Van [Vis alle 13 forfattere av denne artikkelen] (2018). The different dynamic personalities of two neighboring tidewater glaciers. Fulltekst i vitenarkiv
• Girod, Luc Maurice Ramuntcho; Nielsen, Niels Ivar; Couderette, Frédérique; Hoslt, Marion Dominique; Lazizi, Laetitia Alice & Nuth, Christopher [Vis alle 7 forfattere av denne artikkelen] (2018). Extraction of glacier thickness change data and communication material from historical oblique imagery. Fulltekst i vitenarkiv
• Gilbert, Adrien; Thøgersen, Kjetil; Kääb, Andreas; Nuth, Christopher; Schuler, Thomas & Gimbert, Florent (2018). Stable and unstable glacier flows: Role of friction law and feedback mechanisms at the basal interface. Fulltekst i vitenarkiv
• Altena, Bas & Kääb, Andreas (2018). Doves & displacements. Fulltekst i vitenarkiv
• Altena, Bas & Kääb, Andreas (2018). High-resolution surface and upper fjord circulation of Greenland fjords from optical remote sensing. Fulltekst i vitenarkiv Vis sammendrag

  Water within Greenland's fjords functions as conduit between the ice sheet and the Atlantic ocean. Exchange of energy and mass in the proglacial fjords is mainly driven by tides, wind, density fluctuations, glacial discharge and calving. All these forces change or alternate at different time scales and/or places. Measuring related (sub)surface fluxes in these fjords is complicated as fjord-ice hampers logistics and iceberg migration is able to damage measurement installations. However, an increasing fleet of optical satellites are sensing the Earth surface and can in this way observe Greenland fjords at multiple times a day. In this study we introduce a novel type of surface velocity extraction over even short time scales (minutes and hours apart), which exploits the multitude of satellites images taken daily over specific sites and is able to separate the surface velocities from the flows occurring lower in the water column. The resulting velocity fields are able to capture small scale features (±10m) over the full range of large fjords (±100km). Imagery from the Planet constellation, Sentinel-2 and Landsat 8 are used to extract the (sub)surface flow regime of Sermilik fjord. This can be estimated for several days within a week, making it possible to capture whole fjord variability and the mode of fjord circulation. Such detailed and complete circulation information will help to complement in-situ measurements and improve the modelling of ice-ocean interaction.

• Brun, Fanny; Berthier, Etienne; Wagnon, Patrick; Kääb, Andreas & Treichler, Désirée Silvana (2018). Author Correction: A spatially resolved estimate of High Mountain Asia glacier mass balances from 2000 to 2016. Nature Geoscience. ISSN 1752-0894. s. 1–1. doi: 10.1038/s41561-018-0171-z. Fulltekst i vitenarkiv
• Altena, Bas & Kääb, Andreas (2018). Sensing a moving Arctic from space. Fulltekst i vitenarkiv
• Altena, Bas & Kääb, Andreas (2018). ICEFLOW: short-term movements in the cryosphere. Fulltekst i vitenarkiv
• Brun, Fanny; Berthier, Etienne; Wagnon, Patrick; Kääb, Andreas & Treichler, Désirée Silvana (2018). A spatially resolved estimate of High Mountain Asia glacier mass balances from 2000 to 2016: changes on debris-covered glaciers and comparison to ICESat and SPOT5-SRTM estimates. Fulltekst i vitenarkiv
• Andreassen, Liss Marie; Moholdt, Geir; Kääb, Andreas; Messerli, Alexandra; Nagy, Teodor & Winsvold, Solveig Havstad (2021). Monitoring glaciers in mainland Norway and Svalbard using Sentinel. Norges vassdrags- og energidirektorat. ISSN 9788241021039. Fulltekst i vitenarkiv Vis sammendrag

  In this report we evaluate the suitability of data from the Sentinel 1&2 satellites for monitoring glaciers in mainland Norway and Svalbard.

• Wolken, Gabriel J.; Liljedahl, Anne K.; Brubaker, Michael; Coe, J.A.; Fiske, G. & Christiansen, Hanne H [Vis alle 13 forfattere av denne artikkelen] (2021). Arctic Report Card 2021: Glacier and Permafrost Hazards. NOAA. Fulltekst i vitenarkiv
• Bergaas, Julie R.; Kääb, Andreas & Strout, James Michael (2021). Optical remote sensing and change detection for landslide mapping in a humid climate. Universitetet i Oslo. Fulltekst i vitenarkiv Vis sammendrag

  Previous landslides can be important indicators for where to expect future landslide activity and under which conditions. Unfortunately, information concerning previous landslides is often insufficient and landslide databases are incomplete. Only a subset of landslides is registered, in most cases those who interact or affect human life and infrastructure. However, landslides often appear in remote areas, and several landslides, therefore, remain unknown and unnoticed. This study proposes the use of free accessible optical satellite data, produced by moderate resolution sensors in combination with vegetation indices to map and detect previous landslides. The focus is change detection analysis using images from the Sentinel 2 and Landsat 8 optical satellites, which are post-processed to calculate various vegetation indices. Primarily two vegetation indices are used, Atmospherically Resistant Vegetation Index (ARVI) and Normalized Difference Vegetation Index (NDVI). The mapping of landslides is performed in ArcMap, where two different mapping approaches are undertaken. First, a manual mapping where the difference between pre-and post-image is studied. Secondly, a semi-automatic mapping approach in the Raster Calculator. These mapping approaches are applied to Jølster municipality for detecting landslides after the Jølster landslide event in 2019. After the event, 18 landslides were defined within Jølster municipality and registered in the NVEs landslide database. In this study, a total of 108 landslides were identified. A comparative analysis in ArcMap, between ARVI and NDVI, reveals that ARVI detected changes caused by landslides better than NDVI. To verify the method, it has been applied to a second study site; Oso located in the state of Washington, USA. At this case site as well, ARVI maps more accurately than NDVI for landslide recognition. This study suggests that moderate resolution, optical satellite images used in the study of detecting previous landslides increases the number of detected landslides. Further, the vegetation index ARVI should be used when detecting landslides in humid climates. Throughout the research conducted, this study contributes to an improved understanding of the Jølster event, as well as a comparison of different methods for remote landslide detection. The research contributes supplementary landslide information to the database of already known landslides. Furthermore, testing and comparison of methods contribute towards an understanding of best-practice.

• Altena, Bas; Kääb, Andreas; Leclercq, Paul Willem & Nuth, Christopher (2018). Observing change in glacier flow by using optical satellites. Universitetet i Oslo. Fulltekst i vitenarkiv Vis sammendrag

  In the last couple of years many Earth observation satellites with optical instruments have been set in space. These satellites generate an enormous amount of data and give us a image of different landforms on Earth. The data are available for researchers in Earth Science, though efficiently transforming this imagery data to glaciological information has been a challenge. The work in this dissertation presents modern day techniques to extract glacier velocity information from the satellite imagery. Now it is possible to extract reliable displacement measurements from any satellite independent of its flight path. In this way extracting reliable decadal changes of glacier velocity is finally possible. Moreover, by recent development in technology and clever algorithms developed in this PhD work, extracting short term velocity changes are one of the possibilities. So the timing of sliding of a glacier due to melt water can be observed and located. Lastly, methods for data reduction of big data volumes are exploited to develop a discovery tool that is able to observe glacier dynamics over several large mountain ranges. This research might be the first step towards transforming large data volumes into useful information for worldwide glacier monitoring.

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