Climate Number: 1810 Years
In northern China, how much rain falls each year is controlled by the strength of the Asian Monsoon – the system of seasonal winds that bring moist marine air onto land during the summer and cold dry air out to sea during the winter. When the monsoon system is strong, there is ample rainfall and the region’s rice crops do well. When the monsoon is weak, summertime rainfall decreases and rice production suffers. Researchers recently discovered an over 11-foot-long stalagmite in Wanxiang Cave, located in a semi-arid area of northern China. Stalagmites form on cave floors as water seeps through the soil and picks up minerals along the way. Channels in the ground flow and drip water into subterranean caves, depositing the minerals into icicle-like formations. Because the monsoon is an annual cycle, there are distinct layers like tree rings that mark each year of deposits. The bigger the layer, the more minerals were deposited that year and the more rainfall there was. These deposits also accumulate large amounts of uranium, which decays into thorium at a uniform pace. Analysis of the ratio of uranium to thorium in each of the different layers can give an absolute (within 2.5 years) date when the deposit was laid down. This method tells us that the 11-foot stalagmite in Wanxiang Cave has been growing for 1810 years. Not surprisingly, periods of thin deposits (indicating a weak monsoon and low rainfall) corresponded to historical periods of hunger and social unrest while periods of thick deposits (strong monsoon, ample rainfall) corresponded to historic periods of good crop production and abundant food, population growth, and social stability.
For comparison: When the stalagmite began to grow, the Roman Empire was beginning its long process of decline. In modern day California, the Grizzly Giant Sequoia tree in Yosemite National Park was just sprouting. Modern Hinduism was established in India around this time.
Seasons: Winter, Spring, Summer, Fall
Source: Zhang, P et al. “A Test of Climate, Sun and Culture Relationships from an 1810-Year Chinese Cave Record.” Science 322 (2008): 940-942 and Kerr, RA. “Chinese Cave Speaks of a Fickle Sun Bringing Down Ancient Dynasties.” Science 322 (2008): 837-838.
