Climate change isn’t new. A recent study found that it destroyed an ancient civilization approximately 4,000 years ago. The gradual eastward movement of monsoons across Asia at first supported the formation of the Harappan civilization in the Indus Valley by allowing large-scale agricultural production, then wiped out the civilization as water supplies disappeared. This the initial reasoning behind why the Indus valley flourished for 2,000 years, became home to large cities and an empire the size of modern Egypt and Mesopotamia, then dwindled to small villages and isolated farms.
The Harappan civilization, named after its largest city, Harappa, evolved approximately 5,200 years ago and reached its pinnacle between 4,500 and 3,900 years ago, occupying what is now Pakistan, northwest India and Eastern Afghanistan. An urban society with major cities, a distinctive style of writing and extensive trade, the society accounted for roughly 10 percent of the world’s population at its height and equaled Egypt in its power. The Harappans’ downfall came because they did not attempt to develop irrigation to support agriculture but relied on the yearly monsoons. The civilization was largely forgotten until the 1920s when researchers began studying it in depth.
“Antiquity knew about Egypt and Mesopotamia, but the Indus civilization, which was bigger than these two, was completely forgotten until the 1920s,” said Liviu Giosan, a geologist at Woods Hole Oceanographic Institution in Massachusetts. “There are still many things we don’t know about them.”
Nearly 100 years ago, researchers found many remains of Harappan settlements along the Indus River and its tributaries and in a vast desert region. There were signs of sophisticated cities, sea links with Mesopotamia, internal trade routes, arts and crafts, and writing that has not yet been deciphered. “They had cities ordered into grids, with exquisite plumbing, which was not encountered again until the Romans,” Giosan said. “They seem to have been a more democratic society than Mesopotamia and Egypt — no large structures were built for important personalities like kings or pharaohs. Until now, speculations abounded about the links between this mysterious ancient culture and its life-giving mighty rivers,” Giosan said.
“Our research provides one of the clearest examples of climate change leading to the collapse of an entire civilization,” Giosan said. The researchers first analyzed satellite data of the landscape influenced by the Indus and neighboring rivers. Between 2003 and 2008, the researchers gathered samples of sediment from the Arabian Sea coast, the irrigated valleys of Punjab and the northern Thar Desert to find their source and ages and create a timeline of landscape changes. “It was challenging working in the desert — temperatures were over 110 degrees Fahrenheit all day long,” Giosan said.
After collecting the necessary data, “we could reexamine what we know about settlements, what crops people were planting and when, and how both agriculture and settlement patterns changed,” said researcher Dorian Fuller, an archaeologist with University College London. “This brought new insights into the process of eastward population shift, the change towards many more small farming communities, and the decline of cities during late Harappan times.”
“The insolation — the solar energy received by the Earth from the sun — varies in cycles, which can impact monsoons,” Giosan said. “In the last 10,000 years, the Northern Hemisphere had the highest insolation from 7,000 to 5,000 years ago, and since then insolation there decreased. All climate on Earth is driven by the sun, and so the monsoons were affected by the lower insolation, decreasing in force. This meant less rain got into continental regions affected by monsoons over time.”
For the next several centuries, Harappans seem to have fled along an escape route toward the Ganges basin, where monsoon rains remained reliable. “We can envision that this eastern shift involved a change to more localized forms of economy — smaller communities supported by local rain-fed farming and dwindling streams,” Fuller said. “This may have produced smaller surpluses, and would not have supported large cities, but would have been reliable.”