Beneath the ice: Unravelling the Topography of a Deglaciated Earth (TOPO-DE)

Glaciers distinct from the ice sheets in Greenland and Antarctica are experiencing accelerating retreat. Consequently, landscapes so-far hidden beneath the ice will emerge, yet their characteristics are poorly constrained by existing observations and models. At the same time, the subglacial topography itself controls retreat patterns and magnitudes. Hence, an improved representation of the glacier bed is key for improved future projections of glacier evolution. Here, we present a physically plausible map of subglacial topography for all >200,000 glaciers in the world, called Topography of a Deglaciated Earth (TOPO-DE). The map is constrained by an inverse modeling approach that relies on the higher-order Instructed Glacier Model (IGM), a wealth of surface observations, and automatic Bayesian calibration against thickness observations. We find a global glacier volume of 149±29 × 103 km3 (316±61 mm sea level equivalent) and a mean glacier thickness of 212 m. While the global total is consistent with previous work, regionally variable discrepancies highlight the differences between this study and previous reconstructions based on shallow ice-flow physics. The landscapes beneath the ice are characterized by a large potential to host future lakes, quantified as a combined potential lake volume of >3,000 km3, or 2% of the global glacier volume, and a potential areal lake coverage of the presently ice-covered lands of 6%. We show where previous studies produced unphysical bed features and compare that to solutions of our model. Our freely-available bed product offers new insights into landscapes emerging after glacier retreat and can serve as an input for future projections of glacier change and its consequences.