In Pragati: What caused the decline of Harappa?

(This was originally published in Pragati)
In The Wonder That Was India, A L Basham presented a dramatic picture of the decline of the Harappan civilisation. According to him, from 3000 BCE, invaders were present in the region. After conquering the outlying villages, they made their move on Mohenjo-daro. The people of Mohenjo-daro fled, but were cut down by the invaders; the skeletons that were discovered proved this invasion. Basham concluded that the Indus cities fell to barbarians “who triumphed not only through greater military prowess, but also because they were equipped with better weapons, and had learnt to make full use of the swift and terror-striking beats of the steppes.” Sir R Mortimer Wheeler claimed these horse riding invaders were none other than Aryans and their war-god Indra destroyed the forts and citadels at Harappa. But Basham was not that certain of the identity of the charioteers; he stated that they could be non-Aryans as well.
Basham wrote his book in early 1950s and a lot has changed after that. The decline of the Harappan civilisation is no longer attributed to “invading Aryans”, though that theory is still kept alive by political parties in South India. Even the non-Aryan invasion theory has been refuted as there is no trace in the archaeological record for such a disruptive event or the arrival of a new culture from Central Asia. The skeletons, which were touted as evidence for the invasion, were found to belong to different cultural phases thus nullifying the theory of a major battle. Due to all this, historians like Upinder Singh categorically state that the Harappan civilisation was not destroyed by an Indo-Aryan invasion. Instead of blaming the decline of the civilisation to invading or migrating population, the end is now attributed to environmental changes and whims and fancies of rivers.
From the late 1950s, historians believed that Mohenjo-daro was destroyed due to tectonic shifts in the region. According to one version, tectonic movements blocked the course of lower Indus river which must have caused floods that submerged the city. An opposing and the currently favoured theory suggests that instead of submerging in water, the city was starved of water. This happened because Indus shifted away from Mohenjo-daro, thus disrupting the crop cycle as well as the river-based communication network.
While Sindh, where Mohenjo-daro and Harappa are located, has just 9 percent of the 1140 Mature Harappan sites, the Ghaggar-Hakra basin has 32 percent of them; Archaeologists like S P Gupta and J M Kenoyer identify Ghaggar-Hakra with Sarasvati river. Around 1900 BCE, Kalibangan, located on the left bank of Ghaggar, was abandoned. Between the Mature and Late Harappan period, the number of sites along the river reduced considerably implying that the some hydrological change stopped the river from flowing.
One theory suggests that declining monsoons impacted water availability in Ghaggar-Hakra and that in turn caused the societal changes. Around 4000 years back, a dramatic climate change happened across North Africa, the Middle East, the Tibetan Plateau, southern Europe and North America. In India, during that period, there was an abrupt shift in monsoons, which lasted two centuries. In general, if you observe the patterns of recent years, monsoons have strong years and weak years, but they rarely deviate far away from the mean due to the dynamic feedback systems. It is a self-regulating system, but there have been occasions when the anomaly has lasted for few decades.
But what happened 4,000 years back was truly unusual; it was an anomaly larger than anything the subcontinent had faced since in the last 10,000 years. A paper published recently by Berkelhammer was able to narrow down the exact time frame during which this shift happened and it coincides with the decline of the Harappan civilization. This new study does not depend on indirect proxies (like pollen data), but uses a direct terrestrial climate proxy from the Mawmluh Cave in Cherrapunji and hence was able to show an unprecedented age constraint.
According to the paper, the most dramatic change occurred between 4071 (+/- 18) years and 3888 (+/- 22 years) Before Present (BP) for a period of 183 years. First there was a small rise between 4315 and 4303 years and a more precipitous one between 4071 and 4049 years BP. Once this change — which was earlier onset of monsoons or earlier withdrawal — happened, the monsoons stayed in this state for around 180 years before returning to normal values. Earlier monsoon withdrawal suggests that monsoon, which is tied to ocean-atmosphere dynamics and influences from the land surface, was weakened. For the Ghaggar-Hakra, which was fed by the monsoons, the impact was quite serious as it affected the habitability along its course. The study is quite interesting because it provides precise numbers for the duration and onset time for this climactic event. The previous studies did not have proper age constraints and some of them depended on factors (pollen, sedimentation rates) which could be influenced by external natural and man-made causes
Thus when one study claims that Ghaggar was a monsoon fed river and hence was easily susceptible to the vagaries of declining rainfall, there is another which shows that Sarasvati was a glacier-fed river and climate is not the only cause for changes. A paper in Current Science by K S Valdiya published in January of this year, titled The river Sarasvati was a Himalayan-born river, provides numerous counter arguments. First, the Sarasvati flowed through Western Rajasthan, which is one of the dustiest places on earth. 3500 years of dust storms have altered the landscape so much that the landforms created by the river would not be visible today. Second, the river ran through a region which saw tectonic upheavals and that would have altered the course of the river, like what happened to Indus. Third, the dimensions of paleochannels in the upper reaches of the river show that it was created by a large long-lived system. The paper strongly states that it was not a weakened monsoon, but the deflection of rivers by powerful tectonic activities which caused the decline of the Harappan civilisation along the Ghaggar river. Around 3,750 years Before Present, the Tamasa river joined Yamuna and a millennia later the Sutlej joined Beas. Due to this, the discharge of water in the Ghaggar was reduced and forced the Harappans to migrate elsewhere.
This is a contentious issue among academics; arguments and counter-arguments arrive sooner than you can digest them. While one controversy is over if tectonics or monsoon was responsible for the drying up of the river, there is another one over the climatic conditions during the Mature Harappan period. Some papers claim that Mature Harappan period occurred in a wetter phase and there are several others which show that Harappan urbanism rose in an arid phase. Paleoclimatology is a complicated field and more studies will give clarity to this controversy. But there is one certainty: the decline of the Harappan civilisation was not caused by invading Aryans or non-Aryans.
References:

1. Singh, Upinder. A History of Ancient and Early Medieval India: From the Stone Age to the 12th Century. 1st ed. Prentice Hall, 2009.
2. Basham, AL The Wonder That Was India;: A Survey of the Culture of the Indian Sub-continent Before the Coming of the Muslims. 21st ed. Evergreen, 1977.
3. Danino, Michel. Lost River: On The Trail of the Sarasvati. Penguin Books India, 2010.
4. Berkelhammer, M, A Sinha, L Stott, H Cheng, F S R Pausata, and K Yoshimura (2012), An abrupt shift in the Indian monsoon 4000 years ago, in Climates, Landscapes, and Civilizations, Geophys. Monogr. Ser., vol. 198, edited by L. Giosan et al., 75–87, AGU, Washington, D. C., doi:10.1029/2012GM001207.
5. Valdiya, KS “The River Saraswati Was a Himalayan-born River.” CURRENT SCIENCE 104, no. 1 (2013): 42–54.
6. Giosan, Liviu, Peter D Clift, Mark G Macklin, Dorian Q Fuller, Stefan Constantinescu, Julie A Durcan, Thomas Stevens, et al. “Fluvial Landscapes of the Harappan Civilization.” Proceedings of the National Academy of Sciences (May 29, 2012). doi:10.1073/pnas.1112743109.