Variations in the strength and duration of the Asian Monsoon affect more than half the population of the planet, and there is intense interest in where, and how much, rainfall amounts will change regionally in response to warmer climate conditions. The modern monsoon region is divided into distinct subsystems, so the response may be regionally heterogeneous. Northern Hemisphere monsoon systems intensified during the early to mid-Holocene, increasing summer rainfall and creating wetter conditions across much of eastern Asia. In this study we use the area encompassed by high shorelines of early Holocene paleolakes in Tibet to reconstruct paleorainfall (and hence paleomonsoon) patterns during this very wet period. We found that the early Holocene relative paleolake expansions of 130 closed-basin lake systems in the central Tibetan Plateau display a strong east-west gradient. Paleolake areas expanded by approximately fourfold in the western plateau, compared to approximately twofold expansion in the eastern region. This early Holocene pattern mirrors the modern west-east climate division on the plateau: rainfall in the west is closely tied to the Indian summer monsoon (ISM) subsystem, and in the east is closely tied to a probable mix of ISM and East Asian summer monsoon (EASM) subsystems. Our results suggest that these modern climate divisions are an enduring feature of the plateau and that ISM rainfall increased much more than EASM rainfall in response to the same insolation forcing.

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