Ancient Harappan Civilization Collapsed Because of Changes in the Climate
The study also resolves the debate on the source and identity of the Sarasvati, the sacred river of Hindu mythology.
At its height, the Indus civilization contained up to 10% of the world’s population, extending over 1 million square kilometers — from across the plains of the Indus river, to the Arabian sea, to the Ganges; over what is now Pakistan, India, and Afghanistan.
It was the largest of the first urban cultures, but is also the least known now. These first urban cultures included Egypt and Mesopotamia, and like them, the Harappans lived next to rivers, depending on them for agriculture.
“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,” said Liviu Giosan, a geologist with Woods Hole Oceanographic Institution (WHOI) and lead author of the study published the week of May 28 in the Proceedings of the National Academy of Sciences. “Until now, speculations abounded about the links between this mysterious ancient culture and its life-giving mighty rivers.”
Today, the remnants of the Harappan civilization lie in the middle of a vast desert region, far from any flowing rivers. Whereas Egypt and Mesopotamia have been a part of Western historical memory for as long as they’ve been around, the Harappan civilization was completely forgotten until the 1920’s.
Since then, archaeological work has uncovered a complex urban culture, with myriad internal trade routes and dealings with Mesopotamia. They had standards for building construction, sanitation systems, arts and crafts, and a writing system that isn’t yet understood.
“We considered that it is high time for a team of interdisciplinary scientists to contribute to the debate about the enigmatic fate of these people,” added Giosan.
The research work was done between 2003 and 2008 in Pakistan, from the coasts of the Arabian sea to the valleys of Punjab to the northern Thar desert. The international team included scientists from the U.K., the U.S., Pakistan, India, and Romania; with specialties in geology, geomorphology, mathematics, and archaeology.
They combined satellite photos with topographic data collected by the Shuttle Radar Topography Mission (SRTM), then used this to prepare maps of landforms built by the Indus and neighboring rivers, which were then investigated in the field by drilling, voting, and manually-dug trenches. The samples we’re used to determine the origin of the sediments in them; looking at whether they got there from wind or rivers, and their age. The researchers did this to develop a chronology of the landscape changes there.
“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,” said co-author 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 research suggests that the decline in monsoon rains led to weaker river dynamics, and that this played a key role in the development and collapse of the Harappan culture, which relied on river floods for their agriculture.
“From the new research, a compelling picture of 10,000 years of changing landscapes emerges. Before the plain was massively settled, the wild and forceful Indus and its tributaries flowing from the Himalaya cut valleys into their own deposits and left high ‘interfluvial’ stretches of land between them. In the east, reliable monsoon rains sustained perennial rivers that crisscrossed the desert leaving behind their sedimentary deposits across a broad region.”
One of the most striking things the researchers found was a mounded plain 10-20 meters high and 100 kilometers wide, running almost 1000 km along the Indus, built from sediment the Indus was carrying.
“At this scale, nothing similar has ever been described in the geomorphological literature,” said Giosan. “The mega-ridge is a surprising indicator of the stability of Indus plain landscape over the last four millennia. Remains of Harappan settlements still lie at the surface of the ridge, rather than being buried underground.”
Mapping all of the archaeological and geological data together shows that the Harappan culture began to form only after the monsoons weakened and run-off from the mountains was reduced enough to enable agriculture along the river.
“The Harappans were an enterprising people taking advantage of a window of opportunity — a kind of “Goldilocks civilization,” said 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.”
Another major finding of the research, is that the researchers believe that they have discovered the fate of the mythical river, Sarasvati. In the Vedas, the Sarasvati is described as “surpassing in majesty and might all other waters” and “pure in her course from mountains to the ocean.” In modern times, the Ghaggar is the only river that matches the location descriptions of the Sarasvati, but it only flows during strong monsoons and dissipates in the desert along it’s course. The new evidence strongly suggests that the Sarasvati was a large monsoon-fed river that has simply been greatly reduced from aridification.
By 3900 years ago, as the rivers were drying up, the Harappans had an easy escape to the east, near the Ganges, where the monsoons were still occurring regularly.
“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,” said Fuller. “This may have produced smaller surpluses, and would not have supported large cities, but would have been reliable.”
Such a system is obviously not favorable for civilizations that depend on bumper crops to support their urban environments.
“Thus 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,” said Fuller.
“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,” said Giosan. “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.”
Source: Woods Hole Oceanographic Institute
Image Credits: Liviu Giosan, Woods Hole Oceanographic Institution; Stefan Constantinescu, University of Bucharest; James P.M. Syvitski, University of Colorado