A new study suggests that climate change may have led to the decline of the Indus Valley Civilisation. First, a wetter winter monsoon may have led to urban Harappan society turning into a rural one, as inhabitants migrated from a summer flood-deficient river valley to the Himalayan plains. Later, a decline in the winter monsoon could have played a role in the demise of the rural late Harappans, the study states.
Titled ‘Neoglacial climate anomalies and the Harappan metamorphosis’, the study was conducted by an international team of scientists. They looked at sediments from the Arabian Sea from the continental margin of Pakistan, reconstructed the Indian winter monsoon for the last 6,000 years, and examined undersea fossils and marine DNA.
Lead author Liviu Giosan, a geologist at the Woods Hole Oceanographic Institution in the US, said a shift in temperatures and weather patterns over the Indus Valley caused summer monsoon rains to gradually dry up, making agriculture difficult or impossible near Harappan cities.
In 2012, his team had showed that floods in the Indus and tributary rivers became less severe and probably less predictable. “That affected the Indus people who depended a lot on inundation agriculture. We also showed that Ghaggar-Hakra, which is the probable course of the Saraswati river, dried at the same time,” he told The Indian Express.
The study finds that between 4,500 and 3,000 years ago, strong winter monsoons were characterised by “early neoglacial anomalies (ENA)” — changes in wind and precipitation patterns that are evident across the eastern Northern Hemisphere and tropics. It was this coordinated climate reorganisation that “may have helped trigger the metamorphosis of the urban Harappan civilisation into a rural society”.
The decline in the winter monsoon took place between 3300 and 3000 years ago at the end of these anomalies. This “could have played a role in the demise of the rural late Harappans during that time as the first Iron Age culture established itself on the Ghaggar-Hakra interfluve”.
Indus Valley & monsoons
The civilisation developed on the Indus alluvial plain and adjacent regions. At the time, what is now a largely defunct and smaller drainage system, the Ghaggar-Hakra, which lies between the Indus and Ganga watersheds, was also heavily populated. Though the Harappans were agrarians, they reached their “urban peak” or the ‘Mature Phase’ between 4,500 and 3,900 years ago, building architecturally complex urban centres.
“Harappans appear to have invested less effort to control water resources by largescale canal irrigation near cities but relied primarily on fluvial inundation for winter crops and additionally on rain for summer crops,” the study states.
Though the trigger for the urban Harappan collapse was probably the decline of the summer monsoon, the study also points out that the agricultural Harappan economy showed a large degree of adaptation to water availability. The long survival of the Late Harappan cultures until 3,200 years ago under a drier climate and less active fluvial network, in fact, is the subject of ongoing studies.
Giosan points to the difficulty in finding evidence from soil samples to establish the shift in seasonal rainfall. “We could not use soil samples from Haryana or Punjab because you need to have in those soils a signal that can separate summer vs winter conditions. There is no type of mineral or rock that would form or be transported there only in winter or only in summer,” he said.
The team of researchers focused on sediments from the seafloor near the mouth of Indus, a very low-oxygen environment, so whatever grows or dies is very well preserved in sediments. “Sample collection was done by coring at strategic locations that were chosen using a “chirp”, which is an acoustic sub-bottom profiler that images the sediments on the seafloor. A piston corer extracts a cylinder of mud from the seafloor and we study its layers that get older and older from the top to the bottom of the core. We sieve the mud to extract foraminifera (small shells of calcium carbonate) and count how many of those are typical for winter conditions,” he said.
Scope & limitations
While the study finds broad spatial and temporal patterns of variability for summer and winter precipitation across the Harappan settlements, it acknowledges that it does not fully consider “local hydroclimate aspects”.
“For example, did the increase in winter rain during ENA lead to more snow accumulation in the Himalayas that affected the frequency and magnitude of floods along the Indus and its tributaries? Or did settlements in Kutch and Saurashtra, regions of relatively dense habitation during Late Harappan times, also benefit from increases in winter rains despite the fact that modern climatologies suggest scarce local precipitation?” the study asks.
“The Indus story is important today because it provides us with a vivid example of what climate change could do to people. The Indus people were smart and had ways to cope with climate. They did migrate and adapt, but what did they sacrifice for that?,” Giosan said.