by Sam Lemonick Wednesday, May 17, 2017
The landscape near Lake Dali, with elevations of the lake surface during past highstands noted. Credit: Yonaton Goldsmith.
Rapid, climate-driven shifts in monsoon patterns may have shaped ancient Chinese societies, according to new research. And their history could be our future.
Today, Lake Dali, located 375 kilometers north of Beijing in arid Inner Mongolia, is only about 11 meters deep at its deepest point. But 6,000 years ago, the lake surface was 60 meters higher, and the region around it supported a rich culture. In a new study in Proceedings of the National Academy of Sciences, scientists point to fluctuations in the East Asian monsoon that they suggest influenced changes in the lake’s size over millennia.
Lake Dali is a closed basin, meaning there’s no stream or river draining it, so water only leaves by evaporation. Researchers first used satellite images to identify Lake Dali’s ancient shorelines. When waters recede from a former shoreline, shell fragments and other debris are left behind, like rings in a coffee cup, says Yonaton Goldsmith, a geochemistry graduate student at Columbia University and lead author of the new study.
After locating the paleoshorelines in the imagery, the researchers collected shell and charcoal samples from each one. They then radiocarbon-dated the samples to find out when they were deposited, thus reconstructing Lake Dali’s fluctuations dating back almost 16,000 years.
The team found that for several hundred years, about 6,000 years ago, the lake was about 60 meters higher than it is today. In contrast, 12,000 years ago, it may have been about the same height it is now.
Some changes in the lake’s level were rapid. The researchers reported that starting shortly after 12,000 years ago, a 60-meter increase occurred over just 400 years or so; meanwhile, between about 6,000 and 5,000 years ago, a 20-meter drop occurred. The fluctuations roughly correspond with changes in oxygen isotope ratios recorded in deposits in Chinese caves, which some scientists use as a proxy for past precipitation.
Since about 5,000 years ago, Lake Dali has slowly dropped about another 40 meters. Starting about the same time, the region’s dominant Neolithic Hongshan Culture, considered one of the most important early Chinese civilizations because it was among the first to build public buildings and make artistic jade carvings, began to collapse. Meanwhile, other cultures to the south began to grow.
Based on the position of the shoreline at 5,500 years ago, and given today’s rainfall at Lake Dali of about 400 millimeters per year, Goldsmith and his colleagues calculated that there was likely twice as much precipitation around that time. To account for the subsequent drying, Goldsmith says that the East Asian monsoon must have shifted about 400 kilometers south between then and now.
Using temperatures, ocean circulation patterns, radiation levels and other variables from 6,000 years ago, the group found that climate models accurately reproduced the monsoonal shift, although the models underestimated rainfall relative to what the team’s data suggested. Reconstructing past rainfall will improve modern climate models to better anticipate what might happen in the future, Goldsmith says.
There is still a lot of debate over what rainfall looked like at Lake Dali over the millennia. Barbara Maher, an environmental scientist at Lancaster University in England who has studied rainfall in the region but was not involved in the new study, says she largely agrees with the interpretation of Goldsmith’s team. But she notes that the group’s record of precipitation over the past 5,000 years or so is incomplete, including just two data points with a straight line plotted through them. “I suspect it’s not quite as cut and dried,” Maher says.
The study has limitations, but the data it does present are very robust, says Jay Quade, a geochemist at the University of Arizona. Quade says the climate changes described in the paper match what some researchers think will happen in the not-too-distant future. The connection between climate and rainfall patterns “is a huge issue for the coming centuries,” he says.
Investigations of past conditions at other lakes, which Goldmsith hopes to pursue, will help scientists better understand this connection, Goldsmith says. “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.”
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