The picture that is emerging, drawn with the help of newly analyzed geologic records and a sophisticated computer-driven climate model, portrays the rise of the towering Himalayas and the adjacent Tibetan Plateau, the world's largest, as the primary driver of the onset of Asian monsoons about 8 million years ago, and hints that the rise of the world's tallest mountains and plateau may also have helped set the stage for the Ice Ages that began about 2.5 million years ago.

An international team of scientists from China and the United States now has documented the profound influence the Himalaya-Tibetan Plateau has on climate over a broad swath of the world.

"The work further defines the effects of mountain and plateau uplift on climate change," says John E. Kutzbach, a climatologist at the Center for Climatic Research and a co-author of the paper published in the May 3 edition of Nature by An Zhisheng of the Chinese Academy of Sciences, lead author, and Warren L. Prell of Brown University and Stephen C. Porter of the University of Washington.

In 1989, Kutzbach, Prell, and Columbia University's William R. Ruddiman were among the first to suggest that the uplift of mountains and large plateaus could significantly affect climate.

In recent years, geologists have developed tectonic models of how, over the past 10 million years, the Tibetan Plateau has risen as much as two miles as the Indian subcontinent has continued to plow into Asia. To assess the effects of the rise of the Himalayas and the Tibetan Plateau on climate, the team used a computer model of world climate to show that the mountain and plateau uplift enhanced both the winter and summer Asian monsoons and gave rise to a drying trend in central Asia.

This drying trend may have helped create the Gobi and Mongolian deserts and caused a very fine dust to be carried on prevailing westerly winds from the deserts east to China and beyond where it left a record in both land and ocean sediments.

In western China, the fine-grained particles of dust collected in huge loess deposits providing a record of climate that can be read "like the pages of a book. You can dig down into these deposits and read the story of past climate, " says Kutzbach. "The base of these loess sediments has now been dated to eight million years ago, thereby providing evidence of the timing of uplift that is independent of that obtained from tectonic models."

The Chinese loess deposits, together with the records from Indian Ocean sediments that indicate onset of the Indian summer monsoon at about the same time, provide physical evidence that is consistent with the computer model's picture of the evolution of Asia's climate.

For eight million years, the Himalayas have trapped and diverted precipitation to the south and east of the Tibetan Plateau, preventing moisture from reaching what are now the Mongolian and Gobi deserts.

"The uplift of the mountains has diverse regional effects," says Kutzbach. "It's drier in some regions and wetter in others, giving us a satisfactory explanation of why there are large Asian monsoons and why some areas are arid."

Moreover, climate signals found in the Chinese loess deposits suggest that between 3.6 and 2.6 million years ago dust deposition increased, suggesting that the winter monsoons of eastern Asia intensified. This further climate transition, perhaps caused by continued uplift of the Tibetan Plateau along its northern and eastern margins, "signaled a dustier phase in the Earth's atmosphere," Kutzbach says, "and, at the same time, glacial cycles intensified."

It is possible, Kutzbach says, that other factors led to the intensification of glacial cycles and, in turn, increased polar aridity, increased intensity of earth-scouring winds, and a dustier atmosphere.

"It's the chicken and the egg scenario. 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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