Constraining Future Projections of Freezing Level Height and Equilibrium‐Line Altitudes in the Tropical Andes Based on CMIP6

Abstract Over the past several decades, glacier retreat in the tropical Andes has accelerated. Given the role glacier melt plays for water supply, ecosystem integrity and glacier‐related natural hazards, improving projections of glacier changes in the region is critical. The accuracy of global climate models in this region remains an issue as the complex terrain and climate characteristics are difficult to realistically simulate. Here, we examine historical changes of freezing level height (FLH) on four tropical Andean glaciers: Antisana 15 glacier in Ecuador, Artesonraju glacier and Quelccaya ice cap in Peru, and Zongo glacier in Bolivia. The changes in FLH at each site are estimated based on ERA5 reanalysis data and then compared with historical simulations from 35 different CMIP6 models. Constraints are then placed on future projections via correction of model bias, selection of “best‐performing” models, and excluding models with an equilibrium climate sensitivity outside the Intergovernmental Panel on Climate Change AR6 likely range. By utilizing the significant empirical linear relationship observed between FLH and glacier equilibrium‐line altitude, we estimate the future shrinkage of the glaciers' accumulation zone under two emissions scenarios, SSP2‐4.5 and SSP5‐8.5. By the year 2100, the Quelccaya ice cap will likely have passed a point of no return, committing to losing its entire accumulation zone, regardless of emission pathway. The same is true for Antisana 15‐alpha glacier under SSP5‐8.5 while a small accumulation zone remains under SSP2‐4.5. Thanks to their higher accumulation area, Zongo and Artesonraju glaciers are more likely to survive beyond 2100, albeit in a strongly reduced extent.