Abstract

Glacier melt sustains water discharge from mountain basins during droughts, but ongoing glacier retreat threatens this fundamental capacity. Here, we assess the response of glaciers in the Southern Andes to one of the most severe, persistent, and extensive droughts on record in South America (2010-present), and to projected end-of-century megadroughts. Using glacio-hydrological numerical simulations, we show that despite a mean annual precipitation deficit of 36%, glacier runoff in 2010-2019 remained almost unaltered compared to the preceding decade (2000-2009), sustained by a 10% loss of total ice volume. However, simulations of future glacier evolution indicate that annual and summer glacier runoff could decline by up to 20 ± 11% and 48 ± 6%, respectively, during end-of-century megadroughts compared to pre-2010 levels. Our results project a weakening of the glacier’s buffering role against precipitation deficits during extreme droughts, increasing water scarcity for ecosystems and livelihoods in the mountain regions of South America. (Figure presented.) © The Author(s) 2025.