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Projecting the global evolution of glaciers is crucial to quantify future sea-level rise and changes in glacier-fed rivers. Recent intercomparison efforts have shown that a large part of the uncertainties in the projected glacier evolution is driven by the glacier model itself and by the data used for initial conditions and calibration. Here, we quantify the effect that mass balance observations, one of the most crucial data sources used in glacier modelling, have on glacier projections. For this, we model the 21st century global glacier evolution under Coupled Model Intercomparison Phase 6 project (CMIP6) climate scenarios with the Global Glacier Evolution Model (GloGEM) calibrated to match glacier-specific mass balance observations, as opposed to relying on regional mass balance observations. We find that the differences in modelled 21st century glacier changes can be large at the scale of individual glaciers (up to several tens of percent), but tend to average out at regional to global scales (a few percent at most). Our study thus indicates that the added value of relying on glacier-specific observations is at the subregional and local scale, which will increasingly allow projecting the glacier-specific evolution and local impacts for every individual glacier on Earth. To increase the ensemble of models that project global glacier evolution under CMIP6 scenarios, simulations are also performed with the Open Global Glacier Model (OGGM). We project the 2015–2100 global glacier loss to vary between 25 ± 15 % (GloGEM) and 29 ± 14 % (OGGM) under SSP1-2.6 to 46 ± 26 % and 54 ± 29 % under SSP5-8.5 (ensemble median, with 95 % confidence interval; calibration with glacier-specific observations). Despite some differences at the regional scale and a slightly more pronounced sensitivity to changing climatic conditions, our results agree well with the recent projections by Rounce et al. (2023), thereby projecting, for any emission scenario, a higher 21st century mass loss than the current community estimate from the second phase of the Glacier Model Intercomparison Project (GlacierMIP2). [ABSTRACT FROM AUTHOR]

Author(s): Romain Hugonnet [sup.1] [sup.2] [sup.3] , Robert McNabb [sup.4] [sup.5] , Etienne Berthier [sup.1] , Brian Menounos [sup.6] [sup.7] , Christopher Nuth [sup.5] [sup.8] , Luc Girod [sup.5] , [...], Glaciers distinct from the Greenland and Antarctic ice sheets are shrinking rapidly, altering regional hydrology.sup.1, raising global sea level.sup.2 and elevating natural hazards.sup.3. 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 patchwork.sup.4,5. 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 [plus or minus] 16 gigatonnes per year, equivalent to 21 [plus or minus] 3 per cent of the observed sea-level rise.sup.6. We identify a mass loss acceleration of 48 [plus or minus] 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 separately.sup.7-9. 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 gain.sup.10. 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. Analysis of satellite stereo imagery uncovers two decades of mass change for all of Earth's glaciers, revealing accelerated glacier shrinkage and regionally contrasting changes consistent with decadal climate variability.

Glaciers, distinct from the Greenland and Antarctic ice sheets, play a crucial role in Earth's climate system by affecting global sea levels, freshwater availability, nutrient and energy budgets and regional climate patterns. Accurate measurements of glacier mass changes are needed to understand and project glacier evolution and its related impacts on the climate system. Two distinct methods allow to measure glacier mass changes at high spatial resolution. Remotely sensed surface elevation data provides volume change estimates over large glacierized regions for multi-annual to decadal time periods. Field glaciological measurements provide annually to seasonally resolved information on glacier mass change for a small sample of the world's glaciers. By combining the two methods we provide annual time series of individual glacier mass changes and related uncertainties spanning the hydrological years from 1976 to 2023. The per-glacier time series can then be seamlessly integrated into annually resolved global regular grids of glacier mass changes at user-specified spatial resolution. Our results undergo a leave-one-out cross-validation confirming uncertainty estimates at the glacier level to be in the conservative side. Our dataset provides a new baseline for future glacier change modelling assessments and their impact on the world's energy, water, and sea-level budget. The present annual mass change time-series for the individual glaciers and the derived global gridded annual mass change product at a spatial resolution of 0.5° latitude and longitude will be made available from the WGMS webpage. During the review process, the dataset is temporarily available from URL: https://user.geo.uzh.ch/idussa/Dussaillant_etal_ESSD_data/. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 strategies. 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 in order to enhance inter-comparison and empower physical process insights across glacier elevation-change studies. [ABSTRACT FROM AUTHOR]

In Greenland, 87% of the glacierized area terminates in the ocean, but mass lost at the ice‐ocean interface, or frontal ablation, has not yet been fully quantified. Using measurements and models we calculate frontal ablation of Greenland's 213 outlet and 537 peripheral glaciers and find a total frontal ablation of 481.8 ± 24.0 for 2000–2010 and 510.2 ± 18.6 Gt a−1 for 2010–2020. Ice discharge accounted for ∼90% of frontal ablation during both periods, while mass loss due to terminus retreat comprised the remainder. Only 16 glaciers were responsible for the majority (>50%) of frontal ablation from 2010 to 2020. These estimates, along with the climatic‐basal balance, allow for a more complete accounting of Greenland Ice Sheet and peripheral glacier mass balance. In total, Greenland accounted for ∼90% of Northern Hemisphere frontal ablation for 2000–2010 and 2010–2020. Plain Language Summary: We estimate the mass of ice lost from all Greenland glaciers that entered the ocean during each of the last two decades. This ice loss at the front of these marine‐terminating glaciers is called frontal ablation and is approximately equal to the mass of icebergs entering the ocean. Frontal ablation is important because 87% of glacier area in Greenland ends in the ocean, through 750 outlets, and previous work has only approximated frontal ablation. This study quantifies it for the first time, helping to close the mass budget for the Greenland Ice Sheet and better partition its mass balance into components. We find that Greenland accounts for ∼90% of all Northern Hemisphere frontal ablation and, of that contribution, just 17 glaciers for 2000–2010 and 16 glaciers for 2010–2020 account for more than half of total Greenland frontal ablation. Key Points: Frontal ablation of the Greenland Ice Sheet averaged 510.2 ± 18.6 Gt a−1 for 2010–2020, ∼90% of which came from ice dischargeThe frontal ablation we measured is larger than the total mass loss from the ice sheet, indicating a positive climatic‐basal balanceOnly 16 glaciers account for 50% of the total frontal ablation from the Greenland Ice Sheet [ABSTRACT FROM AUTHOR]

The South Col Glacier is a small body of ice and snow (approx. 0.2 km 2) located at the very high elevation of 8000 m a.s.l. (above sea level) on the southern ridge of Mt. Everest. A recent study by proposed that South Col Glacier is rapidly losing mass. This is in contradiction to our comparison of two digital elevation models derived from aerial photographs taken in December 1984 and a stereo Pléiades satellite acquisition from March 2017, from which we estimate a mean elevation change of 0.01 ± 0.05 m a -1. To reconcile these results, we investigate some aspects of the surface energy and mass balance of South Col Glacier. From satellite images and a simple model of snow compaction and erosion, we show that wind erosion has a major impact on the surface mass balance due to the strong seasonality in precipitation and wind and that it cannot be neglected. Additionally, we show that the melt amount predicted by a surface energy and mass balance model is very sensitive to the model structure and implementation. Contrary to previous findings, melt is likely not a dominant ablation process on this glacier, which remains mostly snow-covered during the monsoon. [ABSTRACT FROM AUTHOR]

Knowledge of frontal ablation from marine-terminating glaciers (i.e., mass lost at the calving face) is critical for constraining glacier mass balance, improving projections of mass change, and identifying the processes that govern frontal mass loss. Here, we discuss the challenges involved in computing frontal ablation and the unique issues pertaining to both glaciers and ice sheets. Frontal ablation estimates require numerous datasets, including glacier terminus area change, thickness, surface velocity, density, and climatic mass balance. Observations and models of these variables have improved over the past decade, but significant gaps and regional discrepancies remain, and better quantification of temporal variability in frontal ablation is needed. Despite major advances in satellite-derived large-scale datasets, large uncertainties remain with respect to ice thickness, depth-averaged velocities, and the bulk density of glacier ice close to calving termini or grounding lines. We suggest ways in which we can move toward globally complete frontal ablation estimates, highlighting areas where we need improved datasets and increased collaboration. [ABSTRACT FROM AUTHOR]

Thousands of glacier lakes have been forming behind natural dams in high mountains following glacier retreat since the early 20th century. Some of these lakes abruptly released pulses of water and sediment with disastrous downstream consequences. Yet it remains unclear whether the reported rise of these glacier lake outburst floods (GLOFs) has been fueled by a warming atmosphere and enhanced meltwater production, or simply a growing research effort. Here we estimate trends and biases in GLOF reporting based on the largest global catalog of 1,997 dated glacier‐related floods in six major mountain ranges from 1901 to 2017. We find that the positive trend in the number of reported GLOFs has decayed distinctly after a break in the 1970s, coinciding with independently detected trend changes in annual air temperatures and in the annual number of field‐based glacier surveys (a proxy of scientific reporting). We observe that GLOF reports and glacier surveys decelerated, while temperature rise accelerated in the past five decades. Enhanced warming alone can thus hardly explain the annual number of reported GLOFs, suggesting that temperature‐driven glacier lake formation, growth, and failure are weakly coupled, or that outbursts have been overlooked. Indeed, our analysis emphasizes a distinct geographic and temporal bias in GLOF reporting, and we project that between two to four out of five GLOFs on average might have gone unnoticed in the early to mid‐20th century. We recommend that such biases should be considered, or better corrected for, when attributing the frequency of reported GLOFs to atmospheric warming. Plain Language Summary: Glacier lakes have been growing rapidly following atmospheric warming, glacier retreat, and the exposure of new storage space for meltwater. Many of these lakes have unstable dams and are exposed to impacts from rock/ice avalanches, raising concerns of a commensurate increase of glacier lake outburst floods (GLOFs). Their repeated catastrophic impacts motivate robust assessments that examine the extent to which the frequency of reported GLOFs has changed under ongoing atmospheric warming. Collating 2,000 cases between 1901 and 2017, we find little evidence for changes in the annual number of reported GLOFs, however, though air temperatures have increased markedly since the 1970s. Accordingly, the temperature‐driven increase in glacier lake area and number has unclear links with the frequency of reported GLOFs. Only glacier‐dammed lakes have had more reported outburst floods over the past five decades, as the stability of dams may have declined due to continued glacier downwasting. Changes in research activity have likely biased the actual regional rate of GLOFs, and we estimate that many hundreds of GLOFs went unnoticed in the early 20th century. We invite future research to learn more about the physical drivers of regional GLOF trends to improve projections of GLOF occurrence under ongoing atmospheric warming. Key Points: We present a global inventory of almost 2,000 glacier lake outburst floods (GLOFs) for the period 1901–2017Only a few regions had a growing number of reported GLOFs, likely more linked to increasing research activity than to atmospheric warmingWe estimate that on average two to four out of five GLOFs might have escaped human notice in the early to mid‐20th century [ABSTRACT FROM AUTHOR]

Declines in terrestrial water storage (TWS) exacerbate regional water scarcity and global sea level rise. Increasing evidence has shown that recent TWS declines are substantial in ecologically fragile drylands, but the mechanism remains unclear. Here, by synergizing satellite observations and model simulations, we quantitatively attribute TWS trends during 2002–2016 in major climate zones to three mechanistic drivers: climate variability, climate change, and direct human activities. We reveal that climate variability had transitory and limited impacts (<20%), whereas warming‐induced glacier loss and direct human activities dominate the TWS loss in humid regions (∼103%) and drylands (∼64%), respectively. In non‐glacierized humid areas, climate variability generated regional water gains that offset synchronous TWS declines. Yet in drylands, TWS losses are enduring and more widespread with direct human activities, particularly unsustainable groundwater abstraction. Our findings highlight the substantive human footprints on the already vulnerable arid regions and an imperative need for improved dryland water conservation. Plain Language Summary: Terrestrial water storage (TWS) losses are increasingly prominent in both global and regional scales, particularly in vulnerable drylands. An accurate attribution of TWS changes is essential for the sustainability and conservation of water resources. We provide a comprehensive interpretation of recent satellite‐observed TWS changes in each climate zone through quantitative attributions to natural and anthropogenic factors. We found climate change and direct human activities are the dominant drivers of zonal TWS changes but their impacts are considerably divergent between drylands and humid regions. In humid regions, TWS losses are primarily concentrated on glacierized regions and the net TWS budget in the non‐glacierized humid regions exhibits an equilibrium. Contrastively, TWS declines in drylands are more widespread and enduring with human groundwater depletion. Our analysis highlights a pressing need for improving water conservation strategies in global drylands. Key Points: Recent zonal terrestrial water storage (TWS) losses are primary results of climate change and human activities, instead of climate variabilityIn humid regions, glacier loss fully explains the net TWS decline, meaning a water budget equilibrium in non‐glacierized humid regionsIn drylands, widespread TWS losses are mainly attributed to direct human activities [ABSTRACT FROM AUTHOR]

Western North American (WNA) glaciers straddle the Alaska/Canada border and cover14,384 km2 of mountainous terrain. To better quantify the response of these glaciers to early21st century climate variability we generated over 15,000 multi-sensor digital elevationmodels from spaceborne optical imagery. Over the period 2000-2018, WNA glaciers lost 117± 42 Gigatons (Gt) of mass, which accounts for up to 0.32 ± 0.11 mm of sea level rise overthe full period of study. Using existing surface mass balance measurements for 14glaciers in WNA we estimate that these glaciers experienced an average mass changeof -874 ± 100 kg m−2 yr−1 over the period 2000-2017. When multiplied by thetotal glacierized area of WNA, this value yields an annual mass loss of 13.6 ±4.3 Gt yr−1, close to the value [14 ± 3 Gt yr−1] calculated previously for WNAglaciers. Both of these values are twice as large as those based on our trend analysis.The discrepancy between surface mass balance measurements and those obtainedfrom our geodetic surveys suggests that glaciers chosen for long-term monitoringprograms are losing mass more rapidly than the region as a whole. We also note afour-fold increase in mass loss rates between 2000-2009 [-2.9 ± 3.1 Gt yr−1] and2009-2018 [-12.3 ± 4.6 Gt yr−1] which we attribute to a shift in regional meteorologicalconditions driven by the location and strength of upper level zonal wind. Undermoderate emission scenarios, glaciers in both the conterminous US and westernCanada are expected to undergo continued mass loss throughout this century. Theseprojected changes will affect thermal- and flow-buffering capacity provided by glacierrunoff for many watersheds, with implications for downstream ecosystems and waterresources. Although an increase in thermal energy caused by increased greenhouse gasconcentration will drive widespread mass loss throughout the century decadal scaleclimate variability will likely modulate this long-term change in the years ahead. [ABSTRACT FROM AUTHOR]

Nowadays, geodetic methods are frequently used as a reference for local, regional or global glacier mass balances estimates, especially in remote areas of the globe. Data gaps in digital elevation models derived from remote sensing data are a very common occurrence over glacierized terrain. These can be due to clouds, failure of stereoscopic correlation where images lack contrast (snowfield, shadows,...), or simply out of instrument swath. These data gaps propagate in maps of elevation change and need to be filled when computing glacier volume change and, ultimately, glacier-wide and region-wide mass balances.Thus, the robustness of interpolation methods and their uncertainty for volume change estimates are key to an improved assessment of glacier mass change by allowing optimal use of limited coverage data. However, the dependence of these methods on the spatial distribution of data gaps has been studied to a limited extent.In this study, we simulate gaps over numerous glacier elevation change maps of different regions and with various characteristics to assess and improve the robustness of existing methods of void interpolation. We also investigate formal uncertainties related to these interpolation methods which are poorly known and conservatively estimated in the literature.While depending strongly on the distribution of data voids, glacier-wide hypsometric interpolation methods yield satisfying results given their simplicity. Other methods and related uncertainties are also investigated. [ABSTRACT FROM AUTHOR]

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., (© 2023 IOP Publishing Ltd.)