Accelerated old basal ice loss in Tibetan glaciers after the 1990s

Abstract Surface meltwater that penetrates glacier interiors to the beds can warm the ice and enhance basal mass loss, exerting critical control over glacier dynamics. However, these processes remain poorly constrained in Tibetan Plateau glaciers, particularly those in the western sector, which have long been regarded as cold-based. Here, we used pollen radiocarbon ages from sediments in seven proglacial lakes, spanning catchments with 0.5–55.0% glacier cover, to distinguish old terminal and/or basal ice melt from young surface snow/ice melt of upstream glaciers previously assumed to be cold-based (four lakes) and polythermal (three lakes) over the past 150 years. We show that after the 1990 s, the melting of Tibetan glaciers shifted into a new regime, marked by accelerated mass loss of old basal ice under intensified surface melt conditions. These findings underscore the urgent need for integrated observations of both supra- and subglacial processes to resolve the evolving hydrothermal regimes in Tibetan glaciers under ongoing climate warming.