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Mio‐Pliocene sedimentary archives of the East Asian summer monsoon (EASM) in NE Tibet record a monotonic response to orbital forcing, dominated by eccentricity. By contrast, Pleistocene archives display a more stochastic response that varies regionally and temporally. When and why this response changed is poorly understood. Here, we present a new high‐resolution Rb/Sr ratio data set of EASM intensity from the Sanmenxia Basin, North China, that spans the Plio‐Pleistocene transition. Our results indicate decreased monsoonal rainfall in the late Pliocene, dated at 2.75–2.6 Ma, associated with an intensified response to obliquity and enhanced climate stochasticity. This transition is attributed to the increase of Northern Hemisphere ice volume. Quaternary monsoons display a sensitivity unique to the modern icehouse with large bipolar ice sheets, while pre‐Quaternary monsoons were solely impacted by Antarctic ice sheet dynamics on orbital time‐scales. Plain Language Summary: The late Pliocene‐early Pleistocene transition (LP/EP) is a period of global cooling and the intensification of Northern Hemisphere glaciation. Yet, the evolution of the East Asian summer monsoon (EASM) during this time remains a topic of ongoing discussion and debate. Here we present high‐resolution paleoclimate data from a record of lake sediments across the Pliocene‐Pleistocene transition period. We show that there was a reorganization of the East Asian monsoon system 2.75–2.6 million years ago, associated with an increase in climate forcing caused by cyclic changes in Earth's tilt and in climate stochasticity. This transition is attributed to the appearance of new high‐latitude forcing mechanisms on monsoon intensity caused by the growth of ice sheets in the Northern Hemisphere. Key Points: Cyclic patterns in East Asian summer monsoonal intensity during the Pliocene were mainly driven by Antarctic ice sheet variationsA major reorganization of the East Asian climate system occurred at 2.75–2.6 Ma in response to Northern Hemisphere glaciationQuaternary East Asian summer monsoon on orbital scales displays a sensitivity that is unique to the modern icehouse with bipolar ice sheets [ABSTRACT FROM AUTHOR]

Orbital-scale East Asian summer monsoon (EASM) variability has been extensively investigated in Pliocene-Quaternary loess and marine sediments. In recent years, middle-late Miocene orbital-scale EASM variability has been depicted in lacustrine deposits from NE Tibetan basins, showing a dominant cyclicity of ∼100 kyr eccentricity. However, EASM variability during the early Miocene time remains poorly understood. Here, we investigate the sedimentary record of the early Miocene Xianshuihe Formation, Lanzhou Basin, in order to further constrain the NE Tibetan hydrological evolution and link it to orbital-scale monsoonal variability. By analyzing the lithology, magnetostratigraphy and cyclostratigraphic records of a 270-m section, we show that Lanzhou lake expansion cycles were consistently dominated by ∼100 kyr eccentricity forcing in early Miocene, except for the ∼20.8-19.8 Ma interval showing dominant obliquity control. These records are in agreement with deep-sea oxygen isotopic records, and are potentially linked to Antarctic ice sheet modulations related the precession amplitude modulation by eccentricity. A weakening of weathering intensity in Lanzhou Basin after 20.8 Ma may be attributed to orogenic barriers for westerly moisture and weakened surface temperature resulted from the rapid uplift of NE Tibet and surrounding mountains in the time interval of late Oligocene-early Miocene. • Eccentricity forcing Lanzhou lake evolution of ∼21.6-20.8 Ma and ∼17.2-19.8 Ma. • Obliquity forcing Lanzhou lake evolution between ∼19.8 and 20.8 Ma. • Weakening of the weathering intensity in Lanzhou Basin after 20.8 Ma. [ABSTRACT FROM AUTHOR]