The uplift of the Tibet Plateau (TP) during the Miocene is crucial to understanding the evolution of Asian monsoon regimes and alpine biodiversity. However, the northern Tibet Plateau (NTP) remains poorly investigated. We use pollen records of montane conifers ( Tsuga , Podocarpus , Abies , and Picea ) as a new paleoaltimetry to construct two parallel midrange paleoelevation sequences in the NTP at 1332 ± 189 m and 433 ± 189 m, respectively, during the Middle Miocene [~15 million years ago (Ma)]. Both midranges increased rapidly to 3685 ± 87 m in the Late Miocene (~11 Ma) in the east, and to 3589 ± 62 m at ~7 Ma in the west. Our estimated rises in the east and west parts of the NTP during 15 to 7 Ma, together with data from other TP regions, indicate that during the Late Miocene the entire plateau may have reached a high elevation close to that of today, with consequent impacts on atmospheric precipitation and alpine biodiversity.

The modern pattern of the Asian monsoon is thought to have formed around the Oligocene/Miocene transition and is generally attributed to Himalaya–Tibetan Plateau (H–TP) uplift. However, the timing of the ancient Asian monsoon over the TP and its response to astronomical forcing and TP uplift remains poorly known because of the paucity of well-dated high-resolution geological records from the TP interior. Here, we present a precession-scale cyclostratigraphic sedimentary section of 27.32 to 23.24 million years ago (Ma) during the late Oligocene epoch from the Nima Basin to show that the South Asian monsoon (SAM) had already advanced to the central TP (32°N) at least by 27.3 Ma, which is indicated by cyclic arid–humid fluctuations based on environmental magnetism proxies. A shift of lithology and astronomically orbital periods and amplified amplitude of proxy measurements as well as a hydroclimate transition around 25.8 Ma suggest that the SAM intensified at ~25.8 Ma and that the TP reached a paleoelevation threshold for enhancing the coupling between the uplifted plateau and the SAM. Orbital short eccentricity-paced precipitation variability is argued to be mainly driven by orbital eccentricity-modulated low-latitude summer insolation rather than glacial-interglacial Antarctic ice sheet fluctuations. The monsoon data from the TP interior provide key evidence to link the greatly enhanced tropical SAM at 25.8 Ma with TP uplift rather than global climate change and suggest that SAM’s northward expansion to the boreal subtropics was dominated by a combination of tectonic and astronomical forcing at multiple timescales in the late Oligocene epoch.

Dramatic warming in the late Oligocene occurred after 8 Ma, characterized by ocean warming and a collapse of the Antarctic ice sheet (the late Oligocene warming event [LOWE]); however, few terrestrial records have yet been employed to verify the LOWE and investigate its associated driving forces. The Nima Basin, located in the central Tibetan Plateau contains thick Oligocene sediments, allowing its ecological record to be used as verification of the LOWE. Here we used the reconstructed vegetation record from the Nima Basin in the 1,100‐m‐thick DZC section, aided with zircon U‐Pb dating of the interbedded tuffite layers. Two tuffite layers yielded ages of about 23.4 and 24.8 Ma; thus, the sedimentary age of the whole section is predicted to be 27.5–22.4 Ma. Sporopollen analysis revealed that the dominant taxa throughout the whole section were conifers (over 85%), including Piceapollis, Pinuspollenites, and Abietpollenites. One notable feature is that the broad‐leaved trees (e.g., Quercoidites and Meliaceoidites) increased obviously (averaging from 0.9% to 6.6%, max 41.2%) after 25.6 Ma. This change could be interpreted as a response to late Oligocene global warming, which changed the forest line and corresponding plant species composition. The mean average precipitation of 400–850 and 800–1,000 mm, respectively, inferred from the above vegetation types in the Nima Basin, both exceed the present 150 mm. This indicates that the south Asian monsoon had already developed in the Oligocene and that the LOW event accompanied the global warming.