Rise of Tibet and Rockies set ice-age stage.

Rise of Tibet and Rockies set ice-age stage

Ever since 19th-century geologists discovered that huge ice sheets covered parts of Europe and North America at several times in the past, scientists have wondered what causes ice ages. Computer experiments now suggest the rise of the two great plateaus on either side of the Northern Hemisphere primed the planet for entering the current glacial cycle, which began 2.5 million years ago. Moreover, these plateaus may explain other geologically recent climate changes, such as the development of the Gobi Desert and the drying out of the Mediterranean and California.

According to William F. Ruddiman from Columbia University's Lamont-Doherty Geological Observatory in Palisades, N.Y., and his colleagues, both the Tibetan plateau and the high country of western North America rose at about the same time, mostly during the last 10 million years. To learn how this uplift changed the world's climate, Ruddiman's group turned to an atmospheric general-circulation model run on a supercomputer at the National Center for Atmospheric Research in Boulder, Colo.

The results show that the plateaus may go a long way toward explaining the modern climate of the Northern Hemisphere. "Uplift with remarkable accuracy explains the direction of climate change in most areas," says Ruddiman.

He and John E. Kutzbach from the University of Wisconsin-Madison simulated three worlds with the climate model: one without the Tibetan and North American plateaus, one with the full plateaus and one with the plateaus at half-height. Because the climate model's limited resolution could not include individual mountains or ranges, the model tested only how extensive areas of high topography affect climate.

Researchers have long known that mountainous regions divert wind streams and create "rain shadows" on their leeward sides. But the computer model demonstrates that plateaus cause other profound changes in weather patterns, Ruddiman reported last week at a meeting of the American Geophysical Union in Baltimore.

Without uplift, according to the model, winds tend to travel from west to east over most of the Northern Hemisphere. Plateaus divert the flow as expected, but they also create thermal effects that produce monsoonal circulation patterns. During summer, heat radiating from the high plateaus creates a low-pressure system that draws air toward the uplifted region. Because of the Earth's Coriolis force, this converging air curves to the right, causing winds to move counter-clockwise around the plateau. In winter, the opposite thermal effect drives wind in a clockwise circulation pattern around the plateau, the researchers say.

This combination of diversion and monsoonal circulation exerts strong climate effects. According to the model, the plateaus cause drier summers in both the Mediterranean and the west coast of North America. The interiors of Eurasia and North America also dry out, and northern regions experience colder winters and summers. The summer cooling is important because it should help snow survive year-round in the high latitudes, allowing ice sheets to develop. Ruddiman says the model results match the pattern of changes occurring over the last 10 million years or so.

These results will not alter scientist's prevailing belief that variations in Earth's orbit set the pace of the present glacial cycle. Rather, says Ruddiman, uplift played a key role in setting the stage for this cycle. He is planning future computer experiments that should better test how plateaus affect climate, especially in the oceans.

Eric J. Barron, a climate researcher at Pennsylvania State University in University Park, calls these experiments an important contribution. "No one had really modeled how changes in topography might actually affect the evolution of climate toward glaciation," he says.