Pump up a plateau to make a monsoon.

The Tibetan Plateau, an area in central Asia half the size of the United States and with an average altitude of more than 5 kilometers, has a powerful effect on climates in surrounding regions. Now, scientists have used computer models to show that both the onset of Asian monsoons and their strengthening over millions of years are strongly linked to various stages in the uplift of the plateau.

Sediments drilled from beneath the Arabian Sea show that the summertime onshore winds in the region gained strength between 8 million and 9 million years ago, says John E. Kutzbach, a climatologist at the University of Wisconsin-Madison. This airflow, the Indian monsoon, brings heavy rains to the Indian subcontinent.

Changes in vegetation during that period in Pakistan indicate that summertime precipitation south of the Tibetan plateau was increasing. Sediments drilled from the North Pacific show increases in dust deposits at that same time, suggesting that central Asia became more arid overall. Kutzbach and his colleagues report their analysis in the May 3 NATURE.

The team's computer modeling shows that the rapid uplift of the Tibetan Plateau can explain these changes, says Kutzbach. Stronger summertime heating atop the elevated region boosted the strength of humid winds from the ocean. This monsoonal air flow dumped its rainfall in the Himalayan foothills. It continued inland with less moisture, so the plateau got little rain.

Changes in sediments laid down in eastern Asia between 3.6 million and 2.6 million years ago suggest that the northern and eastern borders of the Tibetan Plateau grew even higher during that period. Simulations show that this uplift and expansion of the region wouldn't have affected the Indian monsoon but would have strengthened the winds that blow from Asia into the Pacific Ocean--a trend reflected in dusty North Pacific sediments.