Droughts often lasted for several generations, creating havoc with food supplies in large cities.
The more scientists learn about ancient civilizations, the more they realize that a changing climate often precipitated their collapse. Both the Khmer Empire in Southeast Asia and the Mayan civilization in Mesoamerica were largely undone, after centuries of flourishing, by periods of intense climate variablity.
The same, scientists say, applies to the seminal Indus civilization in what is now India and Pakistan, which experienced a two-hundred-year period of increased aridity from around 4,200 years ago, including several prolonged droughts that lasted between 25 years and 90 years.
This change in climate spelled the end of this once great civilization, according to experts at the University of Cambridge who have published their findings in a study.
“We find clear evidence that this interval was not a short-term crisis but a progressive transformation of the environmental conditions in which Indus people lived,” explains Cameron Petrie, a professor at Cambridge’s Department of Archaeology.
To chart the variablity of rainfall during the period, the scientists examine growth layers in a stalagmite collected from a cave near Pithoragarh in India and measured such environmental tracers in deposits as oxygen, carbon and calcium isotopes to determine relative rainfall during various seasons.
“Multiple lines of evidence allow us to piece together the nature of these droughts from different angles and confirm they are in agreement,” notes Alena Giesche, a scientist Cambridge’s Department of Earth Sciences who participated in the research.
The use of high-precision uranium-series dating helped the experts better understand the length of droughts back in those days. From these they learned that periods of below-average rainfall occured both in summer and winter, which placed stresses on local agricultural production.
Worse yet for locals, droughts often lasted for several generations, creating havoc with food supplies in large cities within the Indus Valley civilization during the Bronze Age.
“The evidence for drought affecting both cropping seasons is extremely significant for understanding the impact of this period of climate change upon human populations,” Petrie says.
The new study, adds Giesche, now explains that the extent of droughts proved critical to the collapse of this great civilization. “That extra detail is really important when we consider cultural memory and how people make adaptations when faced with environmental change,” the researcher says,
Based on the archaeological evidence, the inhabitants of the region tried their best to weather the prolonged droughts for two centuries. The measures included leaving larger urban areas to smaller rural settlements that could be more self-sufficient.
“At the same time, agriculture shifted towards reliance on summer-crops, especially drought-tolerant millets, and the population transitioned to a lifestyle that appears to have been more self-reliant,” the scientists explain.
The scientists are now exploring the climatic conditions at the time elsewhere in the region.
“Currently, we have a huge blind spot on our maps extending across Afghanistan and Pakistan where the Indian summer monsoon and the Westerlies interact,” stresses Sebastian Breitenbach, a professor of palaeoclimatology at Northumbria University.