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Images: Climate Change Spurred Fall of Ancient Culture

Climate changes that weakened the Indian monsoon caused one ancient culture to collapse and another to advance. About 4,500 years ago (top), strong monsoon rains allowed the Harappans to farm the floodplains of the Indus River and its tributaries and to build large cities. At the same time, lush vegetation farther south on the Indian peninsula provided enough food for people there to survive as hunter-gatherers with no permanent settlements. When the monsoons weakened (bottom), the Harappans’ agriculture failed, and they abandoned their cities and clustered in the foothills of the Himalayas. The peninsula became more arid and less lush, prompting the people there to settle in towns and become farmers. (Illustration by Amy Caracappa-Qubeck, Woods Hole Oceanographic Institution)

(Illustration by Amy Caracappa-Qubeck, Woods Hole Oceanographic Institution)

MIT/WHOI Joint Program student Camilo Ponton and his advisor, WHOI geologist Liviu Giosan, examine a sediment core taken from the Bay of Bengal near the mouth of the Godavari River. The core yielded shells of Globigerinoides ruber, a planktonic foraminiferan, and plant waxes that had been flushed into the river and then settled to the seafloor, preserving a record of conditions in the bay and on land in ages past. Ponton found evidence that the Indian monsoon has lessened and become less predictable over the past 5,000 years, resulting in extended periods of drought and changes in vegetation on the Indian peninsula. Ponton, Giosan, and their colleagues correlated those environmental changes with changes in human cultures of the region. (Photo by Tom Kleindinst, Woods Hole Oceanographic Institution)

The "Dancing Girl of Mohenjo-Daro," one of the most famous pieces of art created by the Harappan civilization that thrived along the Indus River and its tributaries about 4,500 years ago. The Harappans practiced flood agriculture, relying on gentle yearly floods fed by monsoons to recharge the groundwater their crops needed. When the amount of monsoon rain declined, their crops failed. (Joe Ravi, CC-BY-SA 3.0)

The ruins of Mohenjo-Daro, a major Harappan city of about 4,500 years ago and now a World Heritage Site. The Harappans traded extensively with foreign lands, developed sophisticated architecture, building techniques, arts, and symbolic language, and were probably the most egalitarian of ancient societies. When the declining monsoon caused their crops to fail starting around 4,000 years ago, the Harappans migrated to the foothills where smaller-scale farming was still possible. Their great cities, including Mohenjo-Daro, were abandoned and the civilization eventually disappeared. (Public domain)

WHOI geologist Liviu Giosan (in blue) and Florin Filip of the University of Bucharest collect sediment from a trench in the Indus River floodplain in 2002. Analysis of these and other sediment samples indicated that annual floods of the Indus and its tributaries have become sporadic and less predictable over the past few thousand years, due to weakening of the Indian monsoon. Loss of predictable floods led to collapse of the ancient Harappan civilization, which depended on the floods for irrigation. (Photo by Peter Clift)

WHOI geologist Liviu Giosan (red shirt) steadies a percussion corer as it drives into the sediment on the Indus River floodplain. Giosan and his colleagues collected sediments along the Indus and its tributaries to learn how the rivers' flood behavior had changed during the past 5,000 years. (Photo by Peter Clift)

WHOI geologist Liviu Giosan and his colleagues determined the age of ancient river sediments using both radiocarbon dating of organic material and a technique called optically-stimulated luminescence using grains of quartz sand from the sediment. Quartz crystals absorb ionizing radiation that can be released as light. Exposure to sunlight “bleaches” the crystal, halting the emission of light. If a crystal is buried, such as by being covered by flood-borne sediment, the ionizing radiation can build up again and restore the crystal’s ability to luminesce. Scientists can tell how long ago the crystals were buried by unearthing them and exposing them to specific wavelengths of light that cause them to luminesce. The longer the crystal has been in the dark, the more light it emits. Each new exposure to sunlight resets the crystal’s “clock” to zero, allowing researchers to discover when the mineral was last on the surface. (Public domain)