Summer warming in the southeastern Tibetan Plateau reduces the discharge of the lower Brahmaputra River

Temperature changes on the Tibetan Plateau (TP) critically influence river discharge, impacting water supply in East and South Asia. Existing annually-resolved temperature reconstructions for the TP, which rely on tree-ring width and density, may not fully capture long-term trends of the past several centuries. Here, we present a regional tree-ring δ 18 O-based reconstruction of summer maximum temperatures for the southeastern TP from 1352 to 2010 AD. Notably, 66.7% of extreme high-temperature years occurred in the 20th and 21st centuries. A sustained and recognizable warming trend since the late 18th century predates 20th-century anthropogenic warming. Historical summer temperature increases in the upper and middle Brahmaputra basin reduced downstream discharge. Singular spectrum analysis and climate models project that, even under low-emission scenarios, this warming trend will persist until the 2070s, likely exacerbating discharge reductions. This poses significant challenges to water supply, resource management, and ecological sustainability in the lower Brahmaputra basin.