Seasonal trajectories of Snow and Ice as Diagnostics of Climate-Driven Cryosphere Change: An Earth Observation Approach to Address Strategic Polar Research Priorities Leveraging current and next Copernicus capabilities

Climate change is accelerating cryosphere transformation across polar regions, affecting icesheet melt dynamics, snow seasonality, and ecosystem functioning. In response, the European Union and the Copernicus Programme have identified polar monitoring as a strategic priority within the Copernicus Polar Roadmap for Service Evolution and the ESA Earth Observation Science Strategy, highlighting the need for improved monitoring of snow, ice, and climate-driven surface processes at high latitudes. The Copernicus Data Space Ecosystem and downstream services now provide an unprecedented volume of temporally dense, multi-source observations across polar regions; yet existing Earth Observation approaches have predominantly exploited this resource through state-based monitoring, characterising cryosphere variables at discrete observation times, thereby limiting the capacity to resolve process transitions, regime shifts, and the impacts of weather extremes across the full seasonal cycle. Within this framework, this thesis proposes and validates a seasonal trajectory approach, defined here as the analysis of the full temporal evolution of a given variable across the seasonal cycle, as a process-oriented diagnostic framework. Rather than characterising surface states at isolated observation times, this approach analyses how cryosphere variables evolve throughout the seasonal cycle, with the objective of unlocking the diagnostic information already embedded within current Copernicus observations. The first case study investigates the evolution of supraglacial lakes on the Nansen Ice Shelf (Antarctica) during the anomalously warm 2024–2025 austral melt season, using Sentinel-2 multispectral time series, PRISMA hyperspectral imagery, UAV LiDAR observations, and field measurements. Analysis of the seasonal near-infrared reflectance evolution enabled the identification of spatial and temporal transitions between snow-covered surfaces, scattering layers formed by blue ice disaggregation, and liquid meltwater throughout the melt season. The results reveal strong spatial heterogeneity in hydrological activity, highlighting areas characterised by enhanced meltwater persistence and more intense surface evolution in response to weather extremes and local topographic controls. The second case study investigates the impact of winter heatwaves and rain-on-snow events on snow optical properties in an Arctic periglacial environment at Ny-Ålesund, Svalbard. By integrating Sentinel-2 observations with continuous ground-based optical measurements from the Reflectance Box (RoX) instrument, the study demonstrates that the full seasonal temporal evolution of snow reflectance provides diagnostic information on snow metamorphism and the impacts of weather extremes on snow-covered surfaces. The results identify distinct seasonal patterns associated with mid-winter melt events and rain-on-snow processes, highlighting the potential of seasonal snow-signature analysis for detecting and monitoring climate-driven snow disturbances. The third case study analyses long-term variability in snow-depth seasonal evolution across Finland using ERA5 climate reanalysis from the Copernicus Climate Change Service, MODIS vegetation productivity records, and Finnish Meteorological Institute ground observations. The integration of long-term field measurements and climate datasets enabled the identification of emerging seasonal snow-depth signatures consistent with those observed during weather-extreme events in the second case study. The results indicate that, following the 2018 climatic tipping point, northern Europe has experienced a progressive emergence of weather-extreme-driven snow regimes, accompanied by a systematic northward displacement of snow seasonality patterns and enhanced vegetation productivity linked to Arctic greening dynamics. Overall, the thesis demonstrates that the seasonal evolution of cryosphere variables provides an effective and spatially scalable framework for identifying climate-driven process transitions across polar environments, linking weather extremes, surface hydrology, snow metamorphism, and ecosystem response. The results highlight both the current potential and the limitations of the Copernicus ecosystem for monitoring cryosphere change and demonstrate the added scientific value of integrating in situ observations, proximal sensing, and hyperspectral missions such as PRISMA. Situated within the user-driven development paradigm promoted by the Copernicus User Forum, the thesis contributes science-policy relevant evidence supporting the evolution of Copernicus polar services, and discusses how forthcoming Next Generation Missions, including CRISTAL, CIMR, ROSE-L, and CHIME, could substantially enhance the capability to monitor climate-driven surface processes and emerging cryosphere dynamics at high latitudes.