Ancient Climate Change Rocked Tibetan Cultures, Research Suggests

John Roach
for National Geographic News
May 22, 2007
An abrupt change in weather 700 years ago may have forced people on the Tibetan plateau to abandon their farms and reorganize their society, an anthropologist says.

Mark Aldenderfer of the University of Arizona is leading a research project in far western Tibet to piece together how the Asian monsoon—a system of summer winds that brings heavy rain—shifted and how the culture adapted.

His project's preliminary findings suggest that an abrupt shift in the monsoon caused famine, population movements, and political reorganization.

The research project started last year and will continue with at least two more field seasons.

The results of the research could hold lessons for modern societies faced with the likelihood of a changing climate, Aldenderfer added.

"Can we learn anything from what they did at 1300 A.D. that would help us understand how people are going to have to deal with any kinds of similar sorts of changes that might take place in the Asian monsoon during the process of global warming?" he asked.

Crucial Rain

Scientists credit the Asian monsoon with bringing much-needed summer rains to nearly half the world's population.

The seasonal shift in wind usually brings moisture-laden clouds to India, Bangladesh, China, and other countries in southeast Asia (see map of Asia).

Carrie Morrill at the National Climatic Data Center in Boulder, Colorado, has led research into past changes in the monsoon in southeastern Asia. Her team's findings suggest an abrupt shift in the monsoon around A.D. 1300.

"The shift had an interesting spatial pattern: It got drier in some areas, and it got wetter in other areas," Morrill said.

The shift over far western Tibet was probably to a drier phase, she added (see a printable map of the Tibetan plateau).

Aldenderfer said scientists are still working out the exact nature of the shift—for example, whether bouts of rainfall became harder but shorter, or whether it didn't rain at all.

Morrill's research does not distinguish between those scenarios. "All I can say is that overall it became drier in India and Tibet," she said.

Either way, the change would have altered the way springs channel water off the Tibetan plateau, according to Aldenderfer.

"This has an effect on the irrigation systems," he added, explaining that the springs were the primary means for watering crops in the region.

And whatever the exact changes entailed, Aldenderfer continued, his Tibet project is beginning to reveal how the culture high up on the plateau responded.

For example, Guge, an ancient kingdom in western Tibet, shifted inland at about the time of the suggested shift in the monsoon, he noted.

He and his colleagues also have some evidence of large-scale abandonment of agricultural fields.

The team will return to Tibet this summer to continue their investigations.

Lessons for the Future

While climate experts are just now figuring out what will happen to the Asian monsoon as a result of global warming, Aldenderfer said intensity is likely to increase and thus cause more erosion.

(Learn about the causes and effects of global warming.)

"That would make planting of agricultural fields really a very chancy thing," he said.

The Tibetan plateau, like all mountainous areas, is also likely to experience earlier snowmelt. As a result, people who rely on snowmelt for irrigation will be forced to plant their crops earlier, before the start of the traditional growing season.

"There's going to be some serious issues with agricultural shortfalls," Aldenderfer said.

He added that governments must prepare now to regulate access to water.

"Water conflicts are likely to increase in the future as problems of this kind worsen," he noted.

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