Knowledge of when and how a specific climatic threshold is crossed is one key to an understanding of the evolution of Earth's climate system. We have observed a simultaneous intensification of the ~400 k.y. cycle, in both proxy records for global ice volume and for East Asian monsoon precipitation on the Chinese Loess Plateau at ca. 4 Ma, indicating that important climatic thresholds were crossed. The 400 k.y. climatic cycles can be substantially intensified by clipped responses to insolation. We argue that Tibetan uplift accelerated at ca. 4 Ma and triggered such responses. The weaker orbital-band climatic signal during the interval 4–3.6 Ma indicates that the thresholds were only crossed slightly.

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