Shifting monsoon altered early cultures in China, study says
The annual summer monsoon that drops rain onto East Asia, an area with about a billion people, has shifted dramatically in the distant past, at times moving northward by as much as 400 kilometers and doubling rainfall in that northern reach. The monsoon's changes over the past 10,000 years likely altered the course of early human cultures in China, say the authors of a new study.
"I think it is important to emphasize that these spatial fluctuations in the monsoon drive large changes in northern China," said Yonaton Goldsmith, a graduate student at Lamont-Doherty Earth Observatory and lead author of the paper. "When the monsoon is strong, it shifts northward and northern China becomes green. When the monsoon is weak, the monsoon stays in the south and northern China dries out. Such large fluctuations must have altered the ecosystems in northern China dramatically."
Proceedings of the National Academy of Sciences, also ties the shifting monsoon to changes in the earth's orbit and other periodic changes in the climate system. The study should help scientists understand how the monsoon is affected by those natural cycles, and how a changing climate today might influence the monsoon in the future.
Goldsmith said it's still unclear how the monsoon will react to global warming. One view is that the monsoon should grow stronger, but the area studied has been drying out over recent decades, he said, "so there is still a lot that needs to be done in that region before we can get definitive answers."
|East Asian modern rainfall distribution. A doubling of rainfall during the Early and Middle |
Holocene (from 400 to 800 mm/year) requires an ∼400 km northward shift
[Credit: Goldsmith et al., PNAS 2017]
They found that the lake reached peak levels around 123,000 years ago, again around 58,000 years ago, and once more between 11,000 and 5,500 years ago. They tie the periodic increases in rainfall to the range of the monsoon shifting north by as much as 400 kilometers. The lake record is "highly correlated" with measurements taken earlier from cave deposits in both northern and southern China.
|Dotted lines represent past highstands of Lake Dali |
[Credit: Yonaton Goldsmith]
"These findings show that climate change can have dramatic effects on human societies and highlight the necessity to understand the effect of global warming on rainfall patterns in China and all over the world," the authors write.
Intense variations in rainfall may have played a role in the collapse of other civilizations. A study led by Lamont scientist Brendan Buckley, published several years ago, suggested that extended drought coupled with changes in the monsoon could have doomed Cambodia's ancient Khmer civilization at Angkor nearly 600 years ago. Drought is thought to have played a role in the decline of the Classic Maya civilization, too, though in that case, another Lamont study suggests that the Maya themselves contributed to the drought by clearing forests for cities and crops.
Author: David Funkhouser | Source: Columbia University [February 06, 2017]