GROWTH OF THE TIBETAN PLATEAU AND EVOLUTION OF EAST ASIAN CLIMATE

Although the presence of the Tibetan Plateau is widely believed to influence the present-day Asian monsoon, controversy surrounds both the date when in geologic time the monsoon developed and the general timing of the Tibet’s growth into a high plateau. Fueling this controversy, at least in part, are suggestions not only that the monsoon increased in strength at approximately 8 million years ago, but also that simultaneously, the plateau rose roughly 1000 to perhaps 2000 m. Although a 1000 m increase in elevation might seem small, compared with Tibet’s nearly 5000 m present-day mean elevation, such a change may have enabled Tibet’s effect on the monsoon to exceed a threshold necessary for a strong monsoon. When this possible cause-and-effect relationship between a 1000 m rise of the plateau surface and a strengthening of the Asian monsoon was proposed roughly 15 years ago, the few data supporting this suggestion gained strength from their diverse sources of information and consistency. The present paper presents an updated review of that evidence and shows that, on the one hand, some arguments for tectonic processes approximately 8 million years ago no longer seem convincing, but on the other hand, new evidence, for instance showing that deposition of wind-blown sediment became widespread 8 million years ago, can be interpreted as support for a pulse in Tibet’s growth near 8 million years ago and concurrent environmental changes. Thus, the possible connection of rapid upward and, perhaps more important, outward growth to regional environmental changes appears to remain speculative, but sufficiently plausible to stimulate some earth scientists.