Its emergence, its influence, its sudden decline… Many question marks remain about the Indus Valley Civilisation. Also known as the Harappan Civilisation, it ruled over the region of present-day Pakistan 4,000 years ago. Largely unknown, because it was only discovered in 1920, it ended for obscure reasons in 1,900 BC.
Yet, at its peak, this civilisation had no fewer than 5 million people and experienced the greatest geographical expansion of its time. It's therefore difficult to understand why it died so abruptly. According to a recent study published in the journal Climate of the Past, its decline was in fact largely due to climate change.
Pushed towards migration
After analysing marine sediments, researchers noted a sudden change in temperatures in 2,500 BC. According to scientists, it was on this date that the decline of the Harappan Civilisation began. Contrary to the current trend, the inhabitants of the valley experienced a “mini ice age” instead of rising temperatures.
This would have caused changes in the thermal balance between the hemispheres, leading to a gradual drying of summer monsoons but an increase in bad weather in winter, conditions which made agriculture more difficult near cities. The inhabitants would then have been forced to migrate and leave the floodplains where they thrived. They left their cities for small villages at the foot of the Himalayas.
Dr. Liviu Giosan, geologist at Woods Hole Institute of Oceanography (WHOI) in the United States, and lead author of the study of sediments found in the Arabian Sea, said:
We do not know if this migration took place over the course a few months or several centuries. But we do know is that when this civilisation ended, their urban way of life died out.
An important lesson
At the beginning of their settlement at the foot of the Himalayas, the rains caused by winter storms would have enabled the civilisation to continue farming. “Compared to the monsoon floods that the Harappans were used to seeing in the Indus, it would have been relatively little water, but at least it would have been reliable”, said Dr. Giosan.
However, after about a century, the storms seemed to stop, bringing about the civilisation’s extinction. “This is an important lesson for us today”, said the researcher, whose study is published in the journal Climate of the Past, “In Syria, in Africa, climate change is forcing people to migrate outside their country.”
But he continued, “this is only the beginning: rising sea levels, as a result of global warming, can lead to high levels of migration in regions such as Bangladesh, or the southern United States which is threatened by hurricanes”. He concluded, “at the time, the Harappan Civilisation was able to adapt by moving. But today, we face all kinds of barriers, political and social”.