Décapitation glaciaire et trajectoires comportementales divergentes : segmentation U-Net multi-capteurs de 22,155 glaciers dans l’Ouest de l’Amérique du Nord (1984–2025)

Abstract

The long-term fate of a glacier depends not only on its remaining area but on the functional persistence of its accumulation zone. This thesis presents two complementary contributions addressing this problem at continental scale, for 22,155 glaciers in Alaska and western North America over 42 years (1984–2025). Chapter 2 (submitted to Nature Climate Change, tracking number NCLIM-26062362-T) introduces the concept of glacier decapitation — the persistent, satellite-detectable absence of end-of-season snow over the entire visible glacier surface — as an observable signature of a non-functional accumulation zone. Four decades of satellite observations resolve three trajectory groups : Resilient (n = 10,708, 48.3 %) glaciers that maintain their accumulation zone and buffer interannual variability ; Declining (n = 3209, 14.5 %) glaciers in structural decline since 1984; and Tippers (n = 7831, 35.3 %) that followed resilient trajectories before abruptly tipping between 2013 and 2020. Decapitation already affects 9.1% of glaciers, is accelerating, and shows strong reversibility asymmetry across trajectory groups. Glacier fate is structured by morpho-topographic setting and climatic exposure —not size— enabling identification of at-risk glaciers before snow loss is directly observable. It relies on a four-class segmentation U-Net pipeline (Snow, Ice, Cloud, Other), covering five sensors (1984–2025) and validated under strict glacier-level spatial split — the only reliable protocol for continental deployment—achieving mIoU = 0.705 on the held-out test set. Applied to 22,155 glaciers, this pipeline produces 786,014 glacier-year observations of Fsnow over 1984– 2025, with a documented applicability envelope. Appendices I through V, written in French, complement chapter 2 on points not developed within its editorial format : interannual forcing of Fsnow by subgroup (CatBoost, SHAP, glacier memory vs. thermal forcing) ; supplementary external validation against WGMS AMCE geodetic mass balance, including candidate selection and decomposition by trajectory group ; technical complements on the Visible Glacier Surface, the unassignable cohort, bidirectional reconstruction, spatial fragmentation and altitudinal dynamics, centroid retreat trajectories, and two complementary SGV metrics (FSGVref snow and FSGVref SGV ) ; regional climate forcing and climate-event context (1977–2025) ; and a complete inventory of exogenous data sources and digital elevation models mobilised in chapter 2.

Date	7 Jul 2026
Original language	French
Awarding Institution	École de technologie supérieure
Supervisor	Michel Baraër (Supervisor) & Caroline Aubry-Wake (Co-supervisor)