What is a Glacier? Assessing Ice Dynamic Thresholds

The current global Randolph Glacier Inventory (RGI V6) minimum area cutoff is 0.01 km². Including features this small empowers comprehensive assessments of global glacier water resources. It also enables high-resolution glacier hindcasts, ensuring that sites where modern glacier extent is now diminutive are charted and not overlooked. Yet the automated and manual mapping techniques used to generate RGI glacier outlines do not necessarily discriminate based on ice motion. There is currently no RGI mask that discerns between glaciers that likely still deform under their own weight (classic glacier) versus glaciers that are unlikely to satisfy this criterion (stagnant ice patch). Here is a highly simplified, data-driven attempt to develop a globally complete ice dynamic mask. Features are treated as simple slabs, with area given by the RGI database, order of magnitude thickness derived from volume-area power law scaling, and median surface slope derived from topography data (RGI-TOPO dataset, beta release). Driving stress is calculated using these inputs and assuming material density 900 kg m^-3. This is repeated using varying elevation data sources, the globally complete consensus ice thickness estimate, and sparse direct ice thickness measurements (GlaThiDa), to explore driving stress sensitivity to different slab representations. Slabs with driving stress less than 10⁵ Pa are interpreted as features where the ambient driving stress is insufficient to overcome the yield strength of ice. Uncertainty analysis and comparison against ice motion observations determines if these sub 10⁵ Pa slab features reliably mask RGI glaciers that are no longer in motion. This approach serves as a first cut at developing a reproducible, systematic way of discerning between classic glaciers (bodies of ice that move) versus other cryosphere features. This may enhance consistency across technical analyses within the glaciological research community and science communication with policy makers.