Gradual Climate Change Led to Collapse of Indus Civilization

From Popular Archaeology.  This article includes many graphics and images that you should take a look at to get a full picture.

March 2012
Vol. 6 March 2012 - Print the March 2012 Issue

Climate Change Contributed to Ancient Indus Civilization Demise, Researchers Say

Using archaeological data and geoscience technology, an international team of scientists has concluded a study that shows that the great Indus Valley civilization, otherwise known as the Harappan civilization, declined and disappeared in large measure due to climatic and landscape changes. The study results suggest that a major, gradual decline in monsoon rains led to a weakened river system, adversely affecting the Harappan culture and leading to its collapse. The ancient culture relied on river floods to sustain its system of 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 geologist Liviu Giosan of the Woods Hole Oceanographic Institution (WHOI). "Until now, speculations abounded about the links between this mysterious ancient culture and its life-giving mighty rivers." Giosan is also the lead author of the study report now published in the Proceedings of the National Academy of Sciences.

The Harappan civilization was the largest of the "big three" early urban cultures of the world (the others being ancient Egypt and Mesopotamia), but less is known about it. Archaeological exploration over the past century has shed much more light on the culture. Its remains extend more than 1 million square kilometers across the plains of the Indus River from the Arabian Sea to the Ganges River, over what is now Pakistan, northwest India and eastern Afghanistan. Much like ancient Egypt and Mesopotamia, the Harappan people built and sustained their urban society along the recurring highs and lows of flowing rivers that provided the basis for the production of agricultural surpluses, vitally important for the development and sustenance of great urban centers.

Where the Harappan civilization thrived over 4,000 years ago, one now mostly sees arid desolation. The remains of many of its settlements and urban centers now dot a vast desert, far from flowing rivers. But in its heyday, it boasted a sophisticated urban culture with vaious trade routes and maritime connections with Mesopotamia, standards for building construction and sanitation systems, the arts, and a writing system that still eludes epigraphers.

Says Giosan, "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".

The team conducted the research between 2003 and 2008 in Pakistan, from the coast of the Arabian Sea into the fertile irrigated valleys of the Punjab and the northern Thar Desert. The project included scientists from the U.S., U.K., India, Pakistan, and Romania, consisting of expertise in geology, geomorphology, archaeology, and mathematics. Using satellite photos and topographic information from NASA's Shuttle Radar Topography Mission (SRTM), they devloped and analyzed digital landform maps of the Indus Valley area. Using this information, they probed the area by drilling, coring, and digging test trenches, collecting samples that would help them determine sediment origins and age. They were able to develop a 10,000-year chronology of landscape change.

"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," says Dorian Fuller, an archaeologist with the University College London and also co-author of the report. "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."

At first, as the story goes based on the research, the declining monsoon rains actually played a salient role in the rise of the Harappan civilization. Adds Giosan: "The Harappans were an enterprising people taking advantage of a window of opportunity – a kind of "Goldilocks" civilization. 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."

By about 3900 years ago the river system had dried to the point where the Harappans were compelled to move and disperse eastward toward the Ganges basin, where the monsoon rains were still plentiful and more reliable. The great urbanized Indus civilization, having relied on the bumper crop surpluses along the Indus and the Ghaggar-Hakra rivers to develop and sustain their cities and towns during the earlier, wetter period, thus no longer had the workforce concentration needed to support urbanism.

"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. 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."

In addition, the researchers believe they have discovered the fate of the mythical river, the Sarasvati. The ancient Sanskrit Vedas scriptures portrayed the Sarasvati as "surpassing in majesty and might all other waters", but it has been long considered lost to history. They have uncovered evidence that they suggest supports the current Ghaggar-Hakra river as ancient Sarasvati, based on sedimentary, topographical, and archaeological data of settlement near the river during the Harappan era. Moreover, the findings suggest that the ancient river was actually fed by perennial monsoons, not Himalayan glaciers, as was previously supposed, and that the increasingly arid climate had reduced it to the short seasonal flows of today.