Past East Asian monsoon evolution controlled by paleogeography, not CO2

The East Asian monsoonplays an integral role in human society, yet its geological history and controlling processes are poorly understood. Using a general circulation modeland geological data, we explore the drivers controlling the evolution of the monsoonsystem over the past 150 million years. In contrast to previous work, we find that the monsoonis controlled primarily by changes in paleogeography, with little influence from atmospheric CO2. We associate increased precipitation since the Late Cretaceouswith the gradual uplift of the Himalayan-Tibetan region, transitioning from an ITCZ-dominated monsoonto a sea breeze–dominated monsoon. The rising region acted as a mechanical barrier to cold and dry continental air advecting into the region, leading to increasing influence of moist air from the Indian Ocean/South China Sea. We show that, apart from a dry period in the middle Cretaceous, a monsoonsystem has existed in East Asia since at least the Early Cretaceous.