Three-and-a-half million years of Tibetan Plateau vegetation dynamics in response to climate change.

The Tibetan Plateau supports the largest alpine meadow ecosystem globally. It is considered extremely vulnerable to global warming. Knowledge of past vegetation dynamics under similarly warm climates could shed insights into where the tipping point for regime shifts may lie. We report a continuous multicentennial-resolved pollen record for the last 3.5 Myr from a lake sediment core retrieved from the Zoige Basin (~3,350-3,450 m above sea level) on the eastern Tibetan Plateau. It reveals a detailed picture of the vegetation dynamics across several timescales using the approaches of biomization, numerical analysis, statistical modelling and vegetation simulations. These lines of evidence show that vegetation underwent transformation from stable forest in the mid-late Pliocene Period (3.5-2.73 million years ago (Ma)) to codominance of forest and steppe in the early Quaternary Period (2.73-1.54 Ma) and to a meadow-dominated ecosystem after ~1.54 Ma, along with glacial-interglacial and millennial-scale grassland-forest shifts. These vegetational changes were largely controlled by temperature change. A global warming of ~2-3 °C is the most important threshold for the forest expansion and meadow resilience loss on the Tibetan Plateau. By analogy to the past, we suggest that, without major reductions in greenhouse gas emissions, the current Tibetan Plateau meadow is at risk of major transformation.