Did Climate Change Kill The Indus Civilisation?

Artboard 3 Created with Sketch.

The Times of India tells us that it’s time to rewrite the history books. New evidence has surfaced which claims that the Indus Valley Civilisation is much older than we initially thought. Even older than the Egyptian and Mesopotamian ones. More than 8,000 years old.

The Times of India should be complimented for spotting this valuable research story, published in Scientific Reports, a journal published by Nature publications. Perhaps, it was the first newspaper to cover the story.

However, the story has some false claims.

First, its not ‘new evidence’. It’s an old study. The authors of the paper themselves have cited Rao, L. S., Sahu, N. B., Sahu, P., Shastry, U. A. & Diwan, S. New light on the excavation of Harappan settlement at Bhirrana. Puratattva 35, 67–75 (2005). 2005, that’s 11 years old news.

In fact, their central idea in the paper is about the possibility of a connect between climate, agriculture and subsistence pattern during the Harappan civilization.

Also, the claim that scientists have said climate change killed the Indus civilisation is plain wrong. This is not the impression one gets when reading the original research paper. Things are more complicated than that. The authors of the research paper say this in the abstract itself:

Our study suggests that other cause like change in subsistence strategy by shifting crop patterns rather than climate change was responsible for Harappan collapse.

The research paper, published on 25 May is attributed to Anindya Sarkar, Arati Deshpande Mukherjee, M. K. Bera, B. Das, Navin Juyal, P. Morthekai, R. D. Deshpande, V. S. Shinde, L. S. Rao.

Anyway, we will come to the topic of “what caused the downfall of Indus civilisation” (according to the authors) later. First, let’s discuss the finding that’s going viral, courtesy Times of India’s catchy headline.

Establishing the antiquity of Harappa Civilisation

Based on radiocarbon dating obtained from more than hundred Harappan and nearby sites, Harappan cultural levels have been classified into four phases:

- Early Ravi Phase (~5700 years–4800 years Before Present [BP])

- Transitional Kot Diji phase (~4800 years–4600 years BP)

- Mature phase (~4600 years–3900 years BP)

- and Late declining phase (3900 years–3300 years BP).

The Indus civilisation evolved from one being representative of pastoral and arable farming to a highly urbanised one which boasted of rich art and culture, a functioning currency system and vibrant trade relations with regions as far as Mesopotamia and Arabia. This prosperous phased was then followed by de-urbanisation and decline.

One of the leading experts on Indus Valley, late Gregory Possehl, challenged this conventional chronology and advocated a much older chronology. Based on the spatio-temporal distribution of the archaeological remains spread throughout the subcontinent, the paper says, the time spans of the above four phases were revised (not by them, Times Of India) to ~9000–6300 years BP, 6300–5200 years BP, 5200–3000 years BP and 3000–2500 years BP respectively. Here is the carbon dating evidence that supports this classification.

A large number (~70) of conventional and AMS radiocarbon dates indeed support the antiquity of this phase in different parts of the Indus-Ghaggar Hakra river belts viz. Girawad (Pit-23, 6200 years BP), Mithathal (Trench A-1, 8200 years BP), Kalibangan (sample TF-439, 7600 years BP). The recent excavations at Rakhigarhi suggest hitherto unknown largest Harappan settlement in India preserving all the cultural levels including the Hakra phase (sample S-4173, 6400 years BP.

The carbon dating of charcoal samples and that of pottery using a technique called Optically Stimulated Luminescence (OSL) reveals:

“At Bhirrana the earliest level has provided mean 14C age of 8350 ± 140 years BP (8597 to 8171 years BP). The successive cultural levels at Bhirrana, as deciphered from archeological artefacts along with these 14C ages, are Pre-Harappan Hakra phase (~9500–8000 years BP), Early Harappan (~8000–6500 years BP), Early mature Harappan (~6500–5000 years BP) and mature Harappan (~5000–2800 years BP.)”

Bhirrana is a small village in the Fatehabad district of Haryana. Located near the Ghaggar-Hakra river basin, this Harappan site is currently the oldest site of the civilisation and has retained all the cultural levels.

How did the scientists confirm the antiquity of the Bhiranna settlement?

To check the validity of radiocarbon dates (which already establish that Bhirrana is an 8,000+ years old site) and the antiquity of the settlement, the team of scientists isotopically analysed teeth and bone phosphates excavated from one of the trenches of the settlement. They dated pottery fragments from mature and early mature phases by the OSL method. This is what they found:

The pottery at 42 cm, identified as mature Harappan level yielded mean 4800 ± 300 (1σ) years BP age (range 5120 to 4520 year BP) while the pottery from deeper level corresponding to early mature Harappan at 143 cm yielded 5900 ± 250 (1σ) years BP age (range 6185 to 5695 year BP).

In the Hakra level (pre-harappan), at 300cm and more depth, the age is found to be 8384 years BP.

“Isotope based paleoclimatic information also lends supports to the antiquity of Harappan settlements at Bhirrana,” the scientists claim.

(For complete methodology and the explanation of the processes, please read the original research paper.)

Did Climate Change lead to the “collapse” of civilisation?

First, scientists don’t agree with the assumption that the Indus civilisation suddenly collapsed. The evidence doesn’t support this hypothesis, they say. Why exactly?

“Although the collapse of the Harappan as well as several contemporary civilisations like Akkadian (Mesopotamia), Minoan (Crete), Yangtze (China) has been attributed to either weakening of monsoon or pan-Asian aridification (drought events) at ~4100 years BP, the evidence is both contradictory and incomplete. Either the climatic events and cultural levels are asynchronous or the climate change events themselves are regionally diachronous,” they reason.

There is no continuous climate record for the particular sites of the civilisation. Plus, the climate reconstructions made for sites like Thar desert or Arabian sea to show a relation between the collapse of Harappa and weakening monsoon cannot give a true representation since these sites are far from the Harappan ones. Climate in those areas could have been influenced by other factors than what might have affected Harappan sites.

With the help of isotope base paleoclimatic information, scientists found that the weak monsoon phase existed before 9000 years BP too, Not just in India but throughout Asia. It only intensified from 9000 years BP to 7000 years BP. This intensified monsoon period transformed the Ghaggar-Hakra into mighty rivers. And settlements on their banks became the cradle of civilisation. As the low monsoon phase returned from mature Harappan period onwards, these rivers lost their mojo.

Using a simple moisture flux method, the scientists have estimated that the monsoon precipitation during the intensified monsoon period (9000-7000 years BP) was ~100-150 mm higher than today. During the weak monsoon phase (7000 years BP to mature Harappan phase), the mean annual rainfall was similar to present day non-monsoon months. According to another paper published in 2014, this drought phase existed for over 200 years (100 years give and take).

This converted the perennial rivers into dry ones. It definitely hurt the civilisation. But it didn’t collapse. People evolved. Settlements survived at most of the sites, including Bhirrana. The Indus civilisation continued to thrive despite weak monsoon phase. So, climate change cannot alone be attributed to the fall of civilisation. Scientists say:

It is difficult to point to one single cause that drove the Harappan decline.……..The continued survival of Harappans at Bhirrana suggests adaptation to at least one detrimental factor that is monsoon change.

Archeobotanical data suggests that people changed the crop pattern and subsistence strategy. Crop patterns in and around Bhirrana indicate that people shifted from the large-grained cereals like wheat and barley to drought-resistant species of small millets and rice. People moved from large storage system (one of the features of Harappan sites) to an individual household one.

This, scientists believe acted as catalyst for the de-urbanisation and later decline of the Harappan civilization. There was no sudden collapse. And multiple factors (agriculture, change in crop pattern etc) other than climate change did the civilisation in.

But we shouldn’t ignore the fact that the 200-year drought started this chain of events which culminated in the end of the civilisation. These new findings have major lessons for us. We are still overdependent on monsoon for our agriculture. As far as our water management is concerned, less said the better.

Even today, two years of weak monsoons impact the country so adversely.