Climate change may have determined the fate of the ancient world’s most expansive civilization. A new study suggests that the waning of monsoons spurred both the rise and fall of the Harappans, who flourished in the floodplains of the Indus Valley thousands of years ago.
Small floods driven by the rains nourished the crops of early cities but proved unreliable generations later, researchers report online the week of May 28 in the Proceedings of the National Academy of Sciences.
“The Indus people were a Goldilocks civilization,” says Liviu Giosan, a geologist at the Woods Hole Oceanographic Institution in Massachusetts. “They settled when the floods were just right, and then they disappeared when that window of opportunity disappeared.”
Giosan and his colleagues mapped the parts of western Asia inhabited by the Harappans using measurements made by the space shuttle Endeavour in 2000. An instrument onboard bounced radar signals off Earth’s surface and revealed subtle valleys, mounds and channels difficult to see from the ground, including the long-dry bed of a monsoon-fed river in desert that was once the heart of the Indus civilization.
Sediments unearthed and dated by the researchers showed a remodeling of the landscape over time. About 10,000 years ago, flooding rivers in parts of India, Pakistan and Afghanistan changed behavior as monsoon rains weakened. Instead of dumping loads of sediment that built up the land, the rivers began to cut out valleys.
New valleys next to increasingly mild rivers would have been fertile ground for human settlement. Starting about 5,200 years ago, cities sprang up in spots where small, regular floods would have watered farmlands.
“They were smart in picking sites with just enough water,” says Peter Clift, a sedimentologist at Louisiana State University in Baton Rouge and coauthor of the new study.
Conditions continued to dry, though, which would have made floods less frequent and therefore less reliable. That could explain why cities began declining about 3,900 years ago, says Clift. He and his colleagues report a trend toward smaller settlements developing after this time farther east, where monsoons are stronger.
But archaeologist Rita Wright of New York University cautions that blaming monsoons for the decline of the Harappans leaves out a very important element: the Harappans themselves.
“The Harappan farmers were well aware that the climate fluctuated,” Wright says. “Over time, they developed a variety of ways to cope with their environment.”
Farmers in some cities might have found ways to deal with changing climate. Though the Indus people didn’t build irrigation systems like their contemporaries in ancient Egypt and Mesopotamia, they did cultivate a variety of different crops, some of which were more resistant to drought. They also raised animals for food and built massive water reservoirs.
To really understand how climate change played out in each city, says Wright, archaeologists will need to move beyond the big picture and dig up new clues.
