Shifting monsoons seen as key factor in Harappan decline
By Summit Voice
SUMMIT COUNTY — In what could be a warning sign for modern civilizations that sustain themselves with complex water diversion schemes, a new study of the Indus River Basin suggests that climate change led to the collapse Harappan Civilization almost 4000 years ago.
The Indus civilization was the largest —but least known — of the first great urban cultures that also included Egypt and Mesopotamia. At its height, the culture spread across about 600,000 square miles in what is now Pakistan, northwest India and eastern Afghanistan.
Like their contemporaries, the Harappans, named for one of their largest cities, lived next to rivers owing their livelihoods to the fertility of annually watered lands.
The new study suggests declining monsoons reduced the river flows and associated floodplain development that helped fuel the development of the Harappan culture by nurturing agricultural surpluses used to build wealth.
“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. “Until now, speculations abounded about the links between this mysterious ancient culture and its life-giving mighty rivers,” said Giosan, lead author of the study published the week of May 28 in the Proceedings of the National Academy of Sciences.
Today, numerous remains of the Harappan settlements are located in a vast desert region far from any flowing river. In contrast to Egypt and Mesopotamia, which have long been part of the Western classical canon, this amazingly complex culture in South Asia with a population that at its peak may have reached 10 percent of the world’s inhabitants, was completely forgotten until 1920′s. Since then, a flurry of archaeological research in Pakistan and India has uncovered a sophisticated urban culture with myriad internal trade routes and well-established sea links with Mesopotamia, standards for building construction, sanitation systems, arts and crafts, and a yet-to-be deciphered writing system.
An international team from the the U.S., U.K., Pakistan, India, and Romania studied the region between 2003 and 2008, across an area extending from he coast of the Arabian Sea into the fertile irrigated valleys of Punjab and the northern Thar Desert.
Combining geology, geomorphology, archaeology, and mathematics, along with radar data from space shuttle overflights, the researchers created digital maps of landforms constructed by the Indus and neighboring rivers. Those findings were ground-truthed by drilling, coring, and even with manually-dug trenches.The samples were analyzed to determined their origins and to establish a timeline of deposits and subsequent changes.
“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 resulting study paints a compelling picture of dynamically changing landscapes. 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.
Among the most striking features the researchers identified is a 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.
“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.”
Mapped on top of the vast Indo-Gangetic Plain, the archaeological and geological data shows instead that settlements bloomed along the Indus from the coast to the hills fronting the Himalayas, as weakened monsoons and reduced run-off from the mountains tamed the wild Indus and its Himalayan tributaries enough to enable agriculture along their banks.
“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.”
“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.”