A brief history of Asian summer monsoon evolution in the Cenozoic era

The evolution of the Asian Summer Monsoon (ASM) over geological timescale remains uncertain^{1, 2–3} despite its fundamental role in shaping regional climate^4,5, ecosystems^6,7, and civilizations⁸. Using a series of time-slice simulations with a paleo-climate model, we assess how India–Eurasia collision tectonics^9,10, Tibetan Plateau (TP) uplift^11,12, and atmospheric CO₂ variability¹³ influenced ASM evolution through the Cenozoic. Our simulation-based results suggest that ASM intensification was contingent on the TP exceeding an areal mean elevation of ~ 3.5 km in the late Eocene–Oligocene (27–38 million years ago, Ma), which strengthened the upper-tropospheric temperature gradient, promoted the seasonal northward shift of the Intertropical Convergence Zone (ITCZ), and restructured atmospheric circulation. Initially confined to East Asia¹⁴, monsoonal rainfall expanded across South Asia by the Oligocene, coinciding with enhanced circulation and reversing the meridional relative sea surface temperature gradient in the Neotethys. While TP uplift played the primary role in early ASM evolution, declining atmospheric CO₂ levels became increasingly influential after the late Miocene. These findings, supported by sedimentary records of weathering and erosion^15,16, underscore the dominant role of TP in climate–tectonic interactions and ASM evolution over geological timescales.