Unprecedented recent summer warming and cross-sphere hydrological coupling in Asian Water Towers

The Asian Water Towers play a crucial role by storing and releasing vast amounts of freshwater, thereby sustaining the base flow of major Asian rivers and water security for billions of people at sub-continent to hemispheric scales. Instrumental records, though spatially and temporally limited, indicate rapid warming in High Asia. However, the sensitivity and long-term resilience of these Water Towers remain uncertain. Here, we use an 814-year-long tree-ring record (including tree-ring width and maximum latewood density) from Picea likiangensis on the eastern Tibetan Plateau to develop a summer (June-September) temperature reconstruction. Our reconstruction reveals that the series has warmed by 1.5 °C during the modern observational period (1970–2023), which is 0.5 ± 0.4 °C above the pre-industrial baseline (1210–1850 or 1850-1900), making the summer of 2024 the warmest in the past eight centuries. This unprecedented warming amplifies winter runoff in the Brahmaputra, Indus, and Salween headwaters through a cascade of atmosphere-cryosphere feedbacks: enhanced meltwater and spring soil-moisture persistence promote earlier and lusher vegetation growth, which reduces summer albedo and further accelerates regional warming. Detection and attribution analyses identify volcanic and solar forcing as the main drivers of natural, pre-industrial variability before 1850 CE, whereas anthropogenic forcing is detected with high confidence (exceeding the 99% confidence level) after 2020 CE.