Around the Miocene–Pliocene boundary (MPB) at ~5.3 Ma, a late Miocene cooling trend gave way to early-to-middle Pliocene warming, coinciding with a rise in atmospheric CO₂ concentration of ~100–250 ppm, an increase that is similar in amplitude (albeit not in rate) to the current rise due to anthropogenic emissions. The early-to-middle Pliocene Warm Period was the most recent period of persistently warmer-than-present conditions and was characterized by high atmospheric CO₂ concentrations and a mostly ice-free northern hemisphere, similar to future conditions expected under sustained anthropogenic forcing. Detailed analysis of the MPB, therefore, allows us to better understand the large-scale climate response to natural atmospheric CO₂ increases and global warming over equilibrium timescales for comparison with present-day rapid out-of-equilibrium responses, serving as a likely template for future climate change with sustained global warming. Compared with relatively extensive studies of Quaternary and modern climates, however, little is known about terrestrial climate and oceanic changes related to this largely unexplored global warming event.
    Here, we use new high-resolution palaeoclimate records spanning continuously from 8.1 to 3.5 million years ago from Chinese Loess Plateau to document that the Asian summer monsoon intensified prominently across the MPB, with an orbital expression transitioning from a late Miocene mode dominated by 100-kyr cycles to an early-to-middle Pliocene state dominated by 405-kyr cycles. We further synthesize new and existing records and combine detailed land-sea correlations and climate model simulations with realistic late Neogene boundary conditions to demonstrate that CO₂-induced MPB warming increased summer monsoon moisture transport over East Asia, and simultaneously enhanced aridification over arid Central Asia by increasing evaporation, accentuating spatial hydrological gradients within Asia in the early-to-middle Pliocene Warm Period. Our results offer strong palaeoclimate support for the “wet gets wetter, dry gets drier” climate model projections that have been made for Earth’s climate system under sustained anthropogenic forcing.
 

Miocene–Pliocene boundary,Asian summer monsoon,Orbital cycles