For some 10 million years, the Indian subcontinent has steadily muscled in on Asia, forcing upward the mighty Himalayas and raising the Tibetan Plateau by as much as two miles, according to some models. A new study suggests that this drive did more than change the earth's topography. Armed with rich geologic records and a sophisticated computer-driven climate model, an international team of scientists from China and the U.S. report in today's Nature that the mountains' rise may have also started Asia's monsoons about 8 million years ago and contributed to several Ice Ages beginning 2.5 million years ago.

An Zhisheng of the Chinese Academy of Sciences, John E. Kutzbach of the University of Wisconsin-Madison, Warren L. Prell of Brown University and Stephen C. Porter of the University of Washington used a computer climate model to demonstrate that the mountain and plateau uplift enhanced both the winter and summer Asian monsoons and gave rise to a drying trend in central Asia, which may have helped form the Gobi and Mongolian deserts. So, too, the drying trend may have carried eastward fine-grained particles of dust, which have collected in huge loess deposits in western China.

The dust can be read "like the pages of a book. You can dig down into these deposits and read the story of past climate," Kutzbach says. "The base of these loess sediments has now been dated to 8 million years ago, thereby providing evidence of the timing of uplift that is independent of that obtained from tectonic models." The Himilayas have trapped precipitation to the south and east of the Tibetan Plateau, the researchers say, explaining why the monsoons are so large and why some areas nearby are arid, while others stay wet.

In addition, the loess deposits indicate that between 3.6 and 2.6 million years ago, more dust followed the prevailing westerlies, possibly intensifying the winter monsoons. This shift in turn may have "signaled a dustier phase in the earth's atmosphere," Kutzbach says, "and, at the same time, glacial cycles intensified." Although other factors may be to blame, the researchers say that the rise of the Tibetan Plateau may have caused the increase in dust. "It's the chicken and the egg scenario," Kutzbach adds. "We don't know what came first, but it is possible that the continued uplift of the Tibetan Plateau at its northern and eastern margins may have helped set the stage for the transition to a colder climate with more intense glacial cycles."

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