Pervasive tropical-forced millennial monsoon variability over the past 3.5 million years.

Earth's climate has experienced abrupt millennial-scale climate variability (MCV) at least since the early Pleistocene, yet their evolution and drivers remain elusive. A 3.5-million-year decadal-to-centennial-resolved lake sediment sequence from the monsoonal Tibetan Plateau reveals a stepped evolution of MCV that differs in detail from that of North Atlantic records. Lithological and pollen changes show precursor millennial events during the interval ~ 3.5-2.7 million years ago (Ma), with the onset of multiple events at ~ 2.8 Ma. From ~ 2.7 Ma, MCV became a permanent feature of glacial periods, intensifying after ~ 1.5 Ma and extending over both glacial and interglacial periods. The embedded astronomical frequencies of MCV and their persistent linkage with precession indicate a tropical insolation origin, driven by changes in the latitudinal temperature gradient and propagated to the Tibetan Plateau via monsoonal circulation. These findings highlight that both tropical forcing and high-latitude ice-dynamic mechanisms should be considered as "dual engines" of MCV.