Description du sujet de thèse

Climate change poses significant scientific and societal challenges, particularly regarding the evolution of the cryosphere, which is a major contributor to sea-level rise. Uncertainties in sea-level projections primarily stem from the evolution of the Antarctic ice sheet, the largest ice reservoir on Earth, with a potential total contribution of 58 meters to sea-level rise. In this region, glacier mass losses occur almost entirely due to increased ice discharge into the ocean, driven by glacier flow acceleration. In Antarctica, glaciers have the unique characteristic of flowing from the interior of the ice sheet toward the ocean, where they begin to float, forming vast floating ice platforms known as ice shelves. Ice shelves play a crucial role in buttressing the ice flux from the ice sheet. Their weakening has a direct impact on the discharge of ice into the ocean (Rignot et al., 2019). Collapse of ice shelves, observed since 2000s in the Antarctic peninsula and North Greenland have led to a tripling or quadrupling of the discharge of ice in the ocean (Scambos et al., 2004; Millan et al., 2023). Therefore, understanding the evolution of ice shelves is critical to better constrain future sea level rise projections, and is one of the main sources of uncertainties in the future contribution of Antarctica to sea level.
Enhanced damage through fracturing has recently been shown to play a critical role in ice shelf weakening, reducing its buttressing capacity and potentially accelerating their collapse. Despite its significance, the processes governing ice shelf damage remain poorly understood and represented in ice sheet models. One of the main sources of ice shelf weakening, that can potentially lead to enhanced fracturing and collapse, is the increase in basal melting rates, attributed to the advection of circumpolar deep waters on the continental shelf that penetrate inside cavities, enhancing the melting of basal ice. Previous studies measuring melting rates (Adusumili et al., 2022) have low spatial resolutions (several kilometers), revealing significant spatial differences compared to high-resolution satellite-based local studies (Shean et al., 2019). This may directly affect the ability to map and track the evolution of localized basal melting channels that weaken the ice shelf locally, potentially leading to fracture/rifts opening. This increase in melting is often most pronounced at the point where the glacier begins to float (the grounding line), directly influencing its position and having a direct impact on glacier acceleration, ice discharge, and consequently on deformation rates, which further weaken the ice shelf by increasing fracturing. Recent studies have highlighted the complexity of these processes, which can operate on multiple timescales (from daily variations to several decades) and remain poorly understood (Rignot et al., 2024).
Other processes can also weaken ice shelves, such as hydrofracturing driven by increased surface melting and the drainage of water ponds as well as changes in sea ice, which is thought to play a key role in shielding ice shelves from excessive ocean swells, and ice mélange. Today, there are few, if any, studies that have comprehensively demonstrated the multi-temporal and interconnected evolution of these processes, as well as the relationships and feedback loops linking them in the weakening of ice shelves. Therefore, the goal of this PhD project is to understand the processes that weaken ice shelves, as well as the spatial and temporal scales in which they operate and contribute to the increase in damage across the Antarctic coast.
The specific objectives of the thesis include:
1. Quantify ice shelf thickness changes and their spatio-temporal evolution to understand the long-term trends in ice shelf thinning and their implications for the dynamic stability of the ice sheet. This will involve the processing and analysis of high-resolution digital elevation models from GeoEye/WorldView (Maxar satellite imagery), ASTER (NASA/METI), Pléiades (CNES/Airbus), and TanDEM-X (DLR) from the 2000s onwards. It will include the development of a framework to vertically and horizontally register DEMs to high-precision laser altimetry data from satellites (NASA's ICESat-1/2, ESA's Cryosat-2, CNES's SWOT) and airborne platforms (NASA Airborne Topographic Mapper and the Land, Vegetation, and Ice Sensor).
2. Reconstruct basal melting rates of Antarctic ice shelves using mass conservation methods to assess the spatial and temporal variability of basal melt channels. This objective will integrate surface flow velocity data, surface mass balance models, and firn air content estimates to better constrain uncertainties in basal melt rates and their role in ice shelf dynamics.
3. Investigate the evolution of key ice shelf processes, including grounding line dynamics (via quadruple differential SAR interferometry, QDInSAR, Rignot et al., 2011) from daily to decadal time-scales, surface hydrology (meltwater ponding, lake drainages, hydrofracturing), sea ice concentrations, and ocean swell. This will involve the reanalysis of existing products and the generation of new distributed variables.
4. Evaluate the interplay and timing of processes measured above in relation to the progression of ice shelf damage throughout the study period (fracture maps obtained through deep learning from collaborators). This includes quantifying a parameterization of damage based on the different processes studied to determine which contribute most to large-scale ice shelf weakening.
The candidate will join a motivated team of researchers within the IceDaM project (Ice Shelf Damage Characterization and Monitoring around Antarctica), funded by the European Research Council (ERC). This PhD project will be at the heart of national and international collaborations (LEGOS, University of Copenhagen, University of California at Irvine, Dartmouth College, …) and may involve travel to the project's partner institutions. Throughout the thesis, the candidate will also be expected to present their work at international conferences (e.g., European Geophysical Union, American Geophysical Union, International Glaciological Society, ...), publish the findings in peer-reviewed scientific journals and mentor Master's students on research projects related to the PhD. Depending on the work progress, a field campaign will be considered in East Antarctica to compare satellite observations with ground data from local ice shelves.

Contexte de travail

The Institute of Environmental Geosciences (IGE) is a public research laboratory affiliated with CNRS, IRD, the University of Grenoble Alpes (UGA), and Grenoble-INP. It focuses on climate change and human impact on our planet in polar, mountain, and intertropical regions, which are particularly sensitive areas with significant societal implications.

The laboratory has an average staff of about 330 people, including 190 permanent members (researchers, teacher-researchers, engineers, technicians, and administrative personnel) and approximately 140 doctoral students, postdoctoral researchers, and fixed-term contract personnel. Each year, the laboratory hosts around 120 interns and scientific visitors. IGE is located in four buildings on the Grenoble University campus (Glaciology Building, OSUG-B, Climate and Planet House, and INRAE-Grenoble in Saint-Martin-d'Hères).

IGE is one of the primary laboratories of the Grenoble Observatory for Earth Sciences (OSUG), a federative structure of the National Institute for Earth Sciences and Astronomy (INSU).

IGE actively contributes to the national strategy for research in polar regions in collaboration with national and international partners. It is involved in numerous national and international projects.

The recruited individual will carry out their mission within the Cryodyn team of IGE, as part of the European OCEAN:ICE project (WP3), and will report to Romain Millan.

Contraintes et risques

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