Irreversible glacier change and trough water for centuries after overshooting 1.5 °C

Exceeding 1.5 °C of global warming above pre-industrial levels has become a distinct possibility, yet the consequences of such an overshoot for mountain glaciers and their contribution to raising sea levels and impacting water availability are not well understood. Here we show that exceeding and then returning to below 1.5 °C will have irreversible consequences for glacier mass and runoff over centuries. Global climate and glacier simulations project that a 3.0 °C peak-and-decline scenario will lead to 11% more global glacier mass loss by 2500 compared with limiting warming to 1.5 °C without overshooting. In basins where glaciers regrow after peak temperature, glacier runoff reduces further than if the glaciers stabilize, a phenomenon we call ‘trough water’. Half the studied glaciated basins show reduced glacier runoff with overshoot compared with without for decades to centuries after peak warming. These findings underscore the urgency of near-term emissions reductions and limiting temperature overshoot.