Emergent patterns in nature and society

Weak monsoons have collapsed entire civilisations

Today NewScientist report yet another study relating changes on the strength of Monsoon systems and the collapse of ancient civilisations. This time is a paper in Geology entitled Abrupt weakening of the summer monsoon in northwest India ~4100 yr ago where the authors found out that the weakening of the monsoon lead to a drought that lasted 200yrs close to the paleo lake Kotla Dahar, which coincide with the abandonment of urban Indu centres. Here some of the relevant text:

Dixit found that Kotla Dahar was a deep freshwater lake between 4500 and 3800 BC. It then started getting shallower, until about 2200 BC, when the summer monsoon suddenly weakened for 200 years.

That would have been bad news for the people living there. The Indus Valley Civilization depended on the monsoons for their crops, says Dixit. “It is inevitable that they were affected by a pronounced drought of this kind.”

The dates of the drought do not match perfectly with the date of the collapse, but Dixit says both figures are quite uncertain.

By itself, the lake is not representative of the entire civilization, says Supriyo Chakraborty of the Indian Institute of Tropical Meteorology in Pune. “But the authors have compared their results with various other observations and found agreement, giving credence to their claim,” he says.

Global drying

It is not the first time shifts in the monsoon have been linked to the collapse of civilizations.

Around 900 AD one of China’s biggest empires, the Tang dynasty, collapsed. At the same time, halfway across the world, the Mayan civilisation in South America all but disappeared. Records from a lake in China show that stronger winds made the summer monsoon fail, causing widespread drought.

Dixit says the drying events at 900 AD and 2200 BC were both linked to shifts in the position of the Intertropical Convergence Zone, a band of cloud that runs east to west in the tropics and has a big influence on rainfall. “These climate phenomena were not regional but global in nature,” she says.

 

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