Future glacier and runoff evolution in the Tien Shan mountains

Glaciers and ice caps in the Tien Shan mountains play a crucial role in terms of water supply for irrigation, industry and drinking. The retreat of these ice masses can consequently have a major impact on downstream densely populated dry lowland areas. Therefore, it is crucial to understand how ice masses in the Tien Shan are reacting to climate change and how they will evolve in the future. In this study, we model the future evolution of all glaciers and ice caps in the Tien Shan mountains under CMIP6 SSP climate scenarios using the large-scale GloGEMflow model. The model is calibrated to match glacier-specific geodetic mass balances while accounting for debris cover under recent climatic conditions (downscaled climate reanalysis, ERA5). In our modelling framework, we rely on a total of six independent ice thickness datasets, of which the effect on the modelled future glacier evolution is analysed in detail. Our results reveal an exceptionally pronounced retreat of most of the ice masses under all climate scenarios (vs. other regions), which can be related to the specific climate regime. Since most of the precipitation on Tien Shan glaciers occurs in spring and early summer, temperature increases not only increase melt (as is the case in most regions around the world), but additionally strongly influence the precipitation type (solid vs. liquid). In all scenarios, the total runoff for the major river catchments in the Tien Shan is projected to drastically reduce by the end of the 21st century. Further, peak water is modelled to be reached before the middle of the century and the annual runoff peak is anticipated to shift from early summer towards late spring.