Unprecedented recent summer warming and cross-sphere hydrological 1 coupling in Asian Water Towers

The Asian Water Towers play a crucial role in the hydrological cycle at sub-continent to hemispheric scales. Although temperature measurements show rapid warming in High Asia, the sensitivity and resilience of the large Water Towers are uncertain, because observational data are limited in space and time. Here, we use 814-year-long tree-ring width (TRW) and maximum latewood density (MXD) chronologies from Picea likiangensis from the eastern Tibetan Plateau to reconstruct June–September mean temperatures. Our reconstruction reveals the series warms by 1.5°C during the modern observation window (1970–2023), which is 0.5±0.4°C above the pre-industrial baseline (1210–1850), making 2024 the warmest summer in the past eight centuries. The winter runoff in the Brahmaputra, Indus and Salween headwaters amplifies this unprecedented warming due to atmosphere-cryosphere feedbacks in which enhanced meltwater, spring soil‑moisture persistence and reduced summer albedo accelerate 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 exceeds the 99 % detection threshold after 2020 CE.亚洲水塔（Asian Water Towers）在次大陆至半球尺度的水文循环中发挥着至关重要的作用。尽管气温观测数据显示高亚洲（High Asia）区域正经历快速变暖，但由于观测数据在时空维度上均存在局限，大型水塔系统的敏感性与恢复力仍不明朗。本研究利用采自青藏高原东部的丽江云杉（Picea likiangensis）的814年树轮宽度（TRW）与最大晚材密度（MXD）年表，重建了6-9月的平均气温。重建结果显示，在现代观测窗口（1970-2023年）内，该区域升温幅度达1.5℃，较工业化前基线（1210-1850年）偏高0.5±0.4℃，据此判定2024年为过去8个世纪以来夏季气温最高的年份。受大气-冰冻圈反馈作用影响，雅鲁藏布江、印度河与萨尔温江上游的冬季径流加剧了此次前所未有的变暖——具体表现为融水量增加、春季土壤湿度持续时间延长以及夏季反照率降低，进而加速区域升温。检测与归因分析表明，公元1850年之前的自然、工业化前气候变率主要受火山活动与太阳辐射强迫驱动；而公元2020年之后，人为辐射强迫已超过99%的检测阈值。