Changes in insolation — the amount of solar energy received by the Earth from the sun — triggered the collapse.
The key culprit in the mysterious fall of the world’s largest and earliest urban civilization nearly 4,000 years ago in what is now India, Pakistan, Nepal and Bangladesh appears to have been changes in insolation — changes in the amount of solar energy received by the Earth from the sun, says this article by Charles Choi.
“Our research provides one of the clearest examples of climate change leading to the collapse of an entire civilization,” said Liviu Giosan, a geologist at Woods Hole Oceanographic Institution in Massachusetts.
The largest of the ancient great urban cultures was the Indus or Harappan civilization, which once extended some 386,000 sq miles (1 million sq km) across the plains of the Indus River from the Arabian Sea to the Ganges. The civilization developed about 5,200 years ago, then slowly disintegrated between 3,900 and 3,000 years ago, when residents largely abandoned its cities and migrated east.
Nearly a century ago, researchers began discovering numerous remains of Harappan settlements along the Indus River and its tributaries, as well as in a vast desert region at the border of India and Pakistan. Evidence was uncovered for sophisticated cities, sea links with Mesopotamia, internal trade routes, arts and crafts, and as-yet undeciphered writing.
“They had cities ordered into grids, with exquisite plumbing, which was not encountered again until the Romans,” Giosan told LiveScience.
Now Giosan and his colleagues have reconstructed the landscape where this long-forgotten Harappan civilization developed.
Initially, its the monsoon-drenched rivers were prone to devastating floods. Over time, the monsoons weakened, enabling agriculture and civilization to flourish along the riverbanks for nearly 2,000 years.
But then, the amount of insolation — the amount of solar energy received by the Earth from the sun — changed.
Insolation varies in cycles, which can impact monsoons, Giosan pointed out. “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.”
Did you catch that? “All climate on Earth is driven by the sun.”
Eventually, these monsoon-based rivers held too little water and dried, and the Harappans apparently fled east toward the Ganges basin, where monsoon rains remained reliable.
This change would have spelled disaster for the cities of the Indus, which were built on the large surpluses seen during the earlier, wetter era.
The scientists detailed their findings online May 28 in the journal Proceedings of the National Academy of Sciences.
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