Romain Hugonnet

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Hugonnet

First Name

Romain

ORCID

0000-0002-0955-1306

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Publications 1 - 10 of 20

Divergent causes of terrestrial water storage decline between drylands and humid regions globally
An, Linli; Wang, Jida; Huang, Jianping; et al. (2021)
Geophysical Research Letters
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.
Landslide activation during deglaciation in a fjord-dominated landscape: observations from southern Alaska (1984-2022)
Walden, Jane; Jacquemart, Mylène; Higman, Bretwood; et al. (2025)
Natural Hazards and Earth System Sciences
A consequence of the current global glacier mass loss is the destabilization of valley walls as the support provided by the glacier evolves and eventually vanishes. In this work, we examined the evolution of eight large, active landslides in southern coastal Alaska, a region experiencing some of the fastest glacier mass loss worldwide. Additionally, many glaciers in this area are retreating out of glacially carved fjords, leaving landslides in contact with deep waterbodies that can substantially increase the reach of a catastrophic failure through displacement waves or hazard cascades. We used automatic and manual feature tracking of optical imagery to derive slope movement from the 1980s to the present and compared this with glacier terminus retreat and thinning, precipitation, and seismic energy, paying particular attention to landslides in contact with lake or ocean water. We found that the majority of landslides underwent a pulse of accelerated motion during the studied time period. In four cases, landslide movement coincided with the rapid retreat of a lake- or marine-terminating glacier past the instability. At these sites and during these accelerations, the glacier retreat rates were up to 7 times higher than average, while the landslides reached velocities that were up to 9 times higher than their long-term average. At two sites where the landslides are still in contact with the ice, above-average precipitation and increased glacier thinning were found to coincide with accelerated motion, though conclusive causal links could not be drawn and the effect of short-term precipitation could not be ruled out. In two other cases, the landslides showed little to no movement, indicating that slopes may have complex and varied responses to large environmental changes. Our results suggest that landslides adjacent to lakes or fjords may be especially susceptible to sudden activation, which we propose is due to the particularly rapid retreat rates of water-terminating glaciers as well as mechanical and hydrological changes resulting from the replacement of ice with water at the landslide toe in relatively short timescales. By showing that glacier mass loss is associated with increased landslide movement across various settings in Alaska, we suggest that glacier-landslide interactions in coastal settings deserve special attention and further substantiate the need for establishing broader and more systematic paraglacial hazard monitoring in a warming world.
Less extreme and earlier outbursts of ice-dammed lakes since 1900
Veh, Georg; Lützow, Natalie; Tamm, Jenny; et al. (2023)
Nature
Episodic failures of ice-dammed lakes have produced some of the largest floods in history, with disastrous consequences for communities in high mountains1–7. Yet, estimating changes in the activity of ice-dam failures through time remains controversial because of inconsistent regional flood databases. Here, by collating 1,569 ice-dam failures in six major mountain regions, we systematically assess trends in peak discharge, volume, annual timing and source elevation between 1900 and 2021. We show that extreme peak flows and volumes (10 per cent highest) have declined by about an order of magnitude over this period in five of the six regions, whereas median flood discharges have fallen less or have remained unchanged. Ice-dam floods worldwide today originate at higher elevations and happen about six weeks earlier in the year than in 1900. Individual ice-dammed lakes with repeated outbursts show similar negative trends in magnitude and earlier occurrence, although with only moderate correlation to glacier thinning8. We anticipate that ice dams will continue to fail in the near future, even as glaciers thin and recede. Yet widespread deglaciation, projected for nearly all regions by the end of the twenty-first century9, may bring most outburst activity to a halt.
Uncertainty Analysis of Digital Elevation Models by Spatial Inference From Stable Terrain
Hugonnet, Romain; Brun, Fanny; Berthier, Etienne; et al. (2022)
IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing
The monitoring of Earth’s and planetary surface elevations at larger and finer scales is rapidly progressing through the increasing availability and resolution of digital elevation models (DEMs). Surface elevation observations are being used across an expanding range of fields to study topographical attributes and their changes over time, notably in glaciology, hydrology, volcanology, seismology, forestry, and geomorphology. However, DEMs frequently contain large-scale instrument noise and varying vertical precision that lead to complex patterns of errors. Here, we present a validated statistical workflow to estimate, model, and propagate uncertainties in DEMs. We review the state-of-the-art of DEM accuracy and precision analyses, and define a conceptual framework to consistently address those. We show how to characterize DEM precision by quantifying the heteroscedasticity of elevation measurements, i.e., varying vertical precision with terrainor sensor-dependent variables, and the spatial correlation of errors that can occur across multiple spatial scales. With the increasing availability of high-precision observations, our workflow based on independent elevation data acquired on stable terrain can be applied almost anywhere on Earth. We illustrate how to propagate uncertainties for both pixel-scale and spatial elevation derivatives, using terrain slope and glacier volume changes as examples. We find that uncertainties in DEMs are largely underestimated in the literature, and advocate that new metrics of DEM precision are essential to ensure the reliability of future land elevation assessments.
Automated Processing of Declassified KH-9 Hexagon Satellite Images for Global Elevation Change Analysis Since the 1970s
Dehecq, Amaury; Gardner, Alex S.; Alexandrov, Oleg; et al. (2020)
Frontiers in Earth Science
Observing changes in Earth surface topography is crucial for many Earth science disciplines. Documenting these changes over several decades at regional to global scale remains a challenge due to the limited availability of suitable satellite data before the year 2000. Declassified analog satellite images from the American reconnaissance program Hexagon (KH-9), which surveyed nearly all land surfaces from 1972 to 1986 at meter to sub-meter resolutions, provide a unique opportunity to fill the gap in observations. However, large-scale processing of analog imagery remains challenging. We developed an automated workflow to generate Digital Elevation Models and orthophotos from scanned KH-9 mapping camera stereo images. The workflow includes a preprocessing step to correct for film and scanning distortions and crop the scanned images, and a stereo reconstruction step using the open-source NASA Ames Stereo Pipeline. The processing of several hundreds of image pairs enabled us to estimate reliable camera parameters for each KH-9 mission, thereby correcting elevation biases of several tens of meters. The resulting DEMs were validated against various reference elevation data, including snow-covered glaciers with limited image texture. Pixel-scale elevation uncertainty was estimated as 5 m at the 68% confidence level, and less than 15 m at the 95% level. We evaluated the uncertainty of spatially averaged elevation change and volume change, both from an empirical and analytical approach, and we raise particular attention to large-scale correlated biases that may impact volume change estimates from such DEMs. Finally, we present a case study of long-term glacier elevation change in the European Alps. Our results show the suitability of these historical images to quantitatively study global surface change over the past 40–50 years.
Observing glacier elevation changes from spaceborne optical and radar sensors - an inter-comparison experiment using ASTER and TanDEM-X data
Piermattei, Livia; Zemp, Michael; Sommer, Christian; et al. (2024)
The Cryosphere
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.
Accelerated global glacier mass loss in the early twenty-first century
Hugonnet, Romain; McNabb, Robert; Berthier, Etienne; et al. (2021)
Nature
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.
The unquantified mass loss of Northern Hemisphere marine-terminating glaciers from 2000–2020
Kochtitzky, William; Copland, Luke; Van Wychen, Wesley; et al. (2022)
Nature Communications
In the Northern Hemisphere, ~1500 glaciers, accounting for 28% of glacierized area outside the Greenland Ice Sheet, terminate in the ocean. Glacier mass loss at their ice-ocean interface, known as frontal ablation, has not yet been comprehensively quantified. Here, we estimate decadal frontal ablation from measurements of ice discharge and terminus position change from 2000 to 2020. We bias-correct and cross-validate estimates and uncertainties using independent sources. Frontal ablation of marine-terminating glaciers contributed an average of 44.47 ± 6.23 Gt a−1 of ice to the ocean from 2000 to 2010, and 51.98 ± 4.62 Gt a−1 from 2010 to 2020. Ice discharge from 2000 to 2020 was equivalent to 2.10 ± 0.22 mm of sea-level rise and comprised approximately 79% of frontal ablation, with the remainder from terminus retreat. Near-coastal areas most impacted include Austfonna, Svalbard, and central Severnaya Zemlya, the Russian Arctic, and a few Alaskan fjords.
Halving of Swiss glacier volume since 1931 observed from terrestrial image photogrammetry
Schytt Holmlund, Erik Karl Eldar; Dehecq, Amaury; Hugonnet, Romain; et al. (2022)
The Cryosphere Discussions
The monitoring of glaciers in Switzerland has a long tradition, yet glacier changes during the 20th century are only known through sparse observations. Here, we estimate a halving of Swiss glacier volumes between 1931 and 2016 by mapping historical glacier elevation changes at high resolution. Our analysis relies on a terrestrial image archive known as TerrA, which covers about 86 % of the Swiss glacierised area with 21,703 images acquired during the period 1916–1947 (1931 on average). We developed a semi-automated workflow to generate digital elevation models (DEMs) from these images, resulting in a 45 % total glacier coverage. Using the geodetic method, we estimate a Swiss-wide glacier mass balance of –0.52 ± 0.09 m w.e. a−1 between 1931 and 2016. This equates to a 51.5 ± 6.1 % loss in glacier volume. We find that low elevation, high debris cover, and gently sloping glacier termini are conductive to particularly high mass losses. In addition to these glacier-specific, quasi- centennial elevation changes, we present a new inventory of glacier outlines with known timestamps and complete attributes from around 1931. The fragmented spatial coverage and temporal heterogeneity of the TerrA archive are the largest sources of uncertainty in our glacier-specific estimates, reaching up to 0.50 m w.e. a−1. We suggest that the high-resolution mapping of historic surface elevations could unlock great potentials also for research fields other than glaciology.
Everest South Col Glacier did not thin during the period 1984-2017
Brun, Fanny; King, Owen; Réveillet, Marion; et al. (2023)
The Cryosphere
The South Col Glacier is a small body of ice and snow (approx. 0.2 km(2)) located at the very high elevation of 8000ma.s.l. (above sea level) on the southern ridge of Mt. Everest. A recent study by Potocki et al. (2022) 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 Pleiades satellite acquisition from March 2017, from which we estimate a mean elevation change of 0.01 +/- 0.05m 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.

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Romain Hugonnet

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