Tuesday, May 29, 2012

Climate change wiped out one of the world's first, great civilisations more than 4,000 years ago

Climate change led to the collapse of the ancient Indus civilization more than 4,000 years ago, archaeologists believe.

The Indus civilization was the largest - but least known - of the first great urban cultures that also included Egypt and Mesopotamia.

The empire stretched over more than a million square kilometers across the plains of the Indus River from the Arabian Sea to the Ganges, over what is now Pakistan, northwest India and eastern Afghanistan.

Now for the first time scientists believe they have discovered that climate change was a key ingredient in the collapse of the civilisation.

The study also resolves a long-standing debate over the source and fate of the Sarasvati, the sacred river of Hindu mythology, the authors believe.

Dr Liviu Giosan, a geologist with Woods Hole Oceanographic Institution (WHOI) and lead author of the study, said: 'We reconstructed the dynamic landscape of the plain where the Indus civilization developed 5200 years ago, built its cities, and slowly disintegrated between 3900 and 3000 years ago.

'Until now, speculations abounded about the links between this mysterious ancient culture and its life-giving mighty rivers.'

Like their contemporaries, the Harappans, who may have made up 10 per cent of the world's population, the group lived next to rivers, owing their livelihoods to the fertility of annually watered lands.

But the remains of their settlements are located in a vast desert region far from any flowing river.

The civilisation was forgotten until the 1920s. But since then, a flurry of research has uncovered a sophisticated urban culture with myriad internal trade routes and well-established sea links with Mesopotamia,

Archaeologists have also discovered building constructions, sanitation systems, arts and crafts, and a yet-to-be deciphered writing system.

Over five years an international team has been combining satellite photos and topographic data to make digital maps of landforms constructed by the Indus and neighboring rivers, which were then probed in the field by drilling, coring, and even manually-dug trenches and samples were tested.

Co-author Dorian Fuller, an archaeologist with University College London, said: 'Once we had this new information on the geological history, we could re-examine what we know about settlements, what crops people were planting and when, and how both agriculture and settlement patterns changed.

'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 study suggests the decline in monsoon rains led to weakened river dynamics, and played a critical role both in the development and the collapse of the Harappan culture, which relied on river floods to fuel their agricultural surpluses.

The research provides a picture of 10,000 years of changing landscapes and the researchers identified a striking mounded plain, 10 to 20 meters high, over 100 kilometers wide, and running almost 1000 kilometers along the Indus, they call the 'Indus mega-ridge,' built by the river as it purged itself of sediment along its lower course.

'The Harappans were an enterprising people taking advantage of a window of opportunity - a kind of 'Goldilocks civilization,' said Dr Giosan.

'As monsoon drying subdued devastating floods, the land nearby the rivers - still fed with water and rich silt - was just right for agriculture. This lasted for almost 2,000 years, but continued aridification closed this favorable window in the end.'

The researchers believe they have uncovered the fate of a mythical river, the Sarasvati, described in The Vedas, ancient Indian scriptures composed in Sanskrit over 3000 years ago, which it is believed was fed by perennial glaciers in the Himalayas.

Today, the Ghaggar, an intermittent river that flows only during strong monsoons and dissipates into the desert along the dried course of Hakra valley, is thought to best approximate the location of the mythic Sarasvati, but its Himalayan origin and whether it was active during Vedic times remain controversial.

By 3900 years ago, their rivers drying, the Harappans had an escape route to the east toward the Ganges basin, where monsoon rains remained reliable.

'We can envision that this eastern shift involved a change to more localised forms of economy: smaller communities supported by local rain-fed farming and dwindling streams,' said Dr Fuller.

'This may have produced smaller surpluses, and would not have supported large cities, but would have been reliable.

'Cities collapsed, but smaller agricultural communities were sustainable and flourished. Many of the urban arts, such as writing, faded away, but agriculture continued and actually diversified.'

Dr Giosan added: 'An amazing amount of archaeological work has been accumulating over the last decades, but it's never been linked properly to the evolution of the fluvial landscape. We now see landscape dynamics as the crucial link between climate change and people.

'Today the Indus system feeds the largest irrigation scheme in the world, immobilizing the river in channels and behind dams. If the monsoon were to increase in a warming world, as some predict, catastrophic floods such as the humanitarian disaster of 2010, would turn the current irrigation system, designed for a tamer river, obsolete.'


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