— The downfall of the one of the greatest Chinese dynasties may have been catalysed by severe changes in climate. The same climate changes may have simultaneously led to the end of the Maya civilisation depicted in Mel Gibson's new film Apocalypto.
So says Gerald Haug of the GeoForschungsZentrum in Germany and colleagues, who studied geological records of monsoons over the past 16,000 years. They have found a startling correlation between climate extremes and the fall of two great civilisations: the Tang dynasty in China and the Maya of South America. It blew me away," says Haug.
The records show that around the time that these civilizations went into decline, they experienced stronger than average winds in the winter and weaker summer monsoon rains. These weak rains would have reduced crop yields.
Records of monsoons beyond the last 50 years are difficult to obtain. Looking for signs of monsoon trends in geological records going back thousands of years can help solve this problem. In China, stalagmites provide the best available historical record of summer monsoon rains, says Haug, as more rain increases the amount of water dripping down from the roofs of caves. But until now, there has been no reliable estimate of winter winds.
Iron and titanium
Haug and his colleagues solved this problem by studying the sediments deposited at the bottom of Lake Hugauang Maar in southeastern China. The sediments are made up primarily of material deposited there by winter monsoon winds because the catchment area is small, meaning very few streams bring in sediments from other sources. As a result, the sediments provide an accurate historical record of the strength of the winter monsoon winds.
The researchers looked at iron and titanium levels in a sediment core that was extracted from the lake floor. The oxidation level of the iron told them how much oxygen was present in the lake waters when the sediments were deposited, and therefore how much wind was stirring up the lake surface. Titanium in particles is non-reactive and the quantities accumulated in the layers of sediment provided another measure of wind strength.
When they compared the 16,000 years represented by the mud core, the researchers found that years of strong winter winds corresponded very closely to strong summer rains and vice versa. "Our sediment data provides a mirror image to summer records in stalagmites," explains Haug.
The researchers believe the only coherent explanation for the summer and winter trends and is a shift in the position of a band of low-pressure that girdles the Earth, known as the inter-tropical convergence zone, or ITCZ.
They found that when warm temperatures in the Northern hemisphere indicated a northward shift of the ITCZ, summer monsoon rains were strong and winter monsoon winds were weak. "It seems possible that major shifts in ITCZ catalysed simultaneous events in civilisations on opposite sides of the Pacific Ocean," conclude the researchers in a paper in Nature.
Previously, Haug had shown that the repetitive periods of decline of the Mayan civilisation in Latin America corresponded to dry periods on that continent.
The Maya civilisation and Tang dynasty were contemporary and there is a striking similarity between the Chinese and Latin American climate data. These include a general shift towards a drier climate around AD 750 and three very dry periods between then and AD 910, the last of which coincides with both the Maya and the Tang collapse.
"I am not a historian," cautions Haug, but "there is a coincidence at least". He says his work is part of "a growing piece of evidence that climate has catalysing effect on societies".
Analysing historical monsoon records can be extremely useful in making future climate predictions. For instance, some researchers suggest that strong summer monsoon rains are preceded by weak winter winds. If true, this theory could prove extremely useful in preparing agriculture for a difficult year ahead.
Journal reference: Nature (vol 445, p 74)
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