At its zenith, the Indus Valley Civilisation, spanning much of present-day Pakistan and northwest India, was characterised by advanced urban planning, including gridded streets, multi-storey brick homes, and sophisticated sanitation systems boasting flush toilets. Beliefs suggest that the downfall of the Indus Valley Civilisation was due to a single mysterious or catastrophic reason, but a recent study has claimed that it was actually a series of prolonged droughts that led to the decline of the civilisation.
A study published in the journal Communications Earth & Environment by an international research team utilised paleoclimate data and computer modelling to analyse the climate from 3000 to 1000 BCE. The findings highlighted that the downfall of Harappa, one of the most significant urban centres of the Indus Valley Civilisation, was not due to a singular catastrophic event, but rather a series of prolonged droughts that lasted for centuries, leading to desiccation of rivers and soils.
As lead author, Hiren Solanki, from the Indian Institute of Technology at Gandhinagar elucidated, these recurring droughts likely prompted the Harappans to frequently relocate in search of more viable living conditions. Co-author Balaji Rajagopalan, a hydrology researcher at the University of Colorado Boulder, emphasised that although droughts played a critical role, they were compounded by factors such as diminished food supply and a fragile governance structure, which collectively pushed the society towards decline and dispersal.
Despite these daunting challenges, the Indus Valley Civilisation demonstrated resilience over approximately two millennia. The researchers noted that the Harappans adapted by switching agricultural practices, diversifying trade, and strategically relocating settlements closer to reliable water sources, particularly along the Indus River and its tributaries. This adaptability hints at crucial lessons about proactive planning, diversified water management, and sustainable agricultural systems – insights that remain pertinent for contemporary societies grappling with climate change. To simulate the climate conditions of that era, the study integrated model outputs with environmental indicators, such as stalactites and stalagmites from Indian caves and water level data from several Indian lakes, providing a comprehensive understanding of the factors influencing the decline of one of history’s most remarkable civilisations.
Between approximately 3000 and 2475 BCE, a significantly vigorous monsoon season spurred by cooler tropical Pacific Ocean conditions led to increased rainfall across the region, resulting in wetter conditions than those observed today. Rajagopalan emphasised that this La Niña-like climate allowed for the establishment of settlements near areas with ample rain. However, as the tropical Pacific began to warm in the following centuries, the region experienced drier conditions characterised by reduced rainfall and rising temperatures, culminating in periods of drought.
Major droughts
The research team identified four major drought events, each spanning over 85 years, occurring between 2425 and 1400 BCE. Notably, the third severe drought peaked around 1733 BCE, which lasted approximately 164 years and impacted nearly the entirety of the region. The analysis revealed an overall temperature increase of 0.5 degree Celsius (0.9 degree Fahrenheit), along with a rainfall decrease of 10 per cent to 20 per cent.
The resultant hydrological changes had significant consequences, leading to the shrinkage of lakes and shallow water bodies (playas), diminished river flows, and soil desiccation, as articulated by co-author Vimal Mishra and his team. These shifts hindered trade activities reliant on river navigation and made agriculture more challenging, particularly in areas distant from waterways. This forced populations to migrate, which may have contributed to the decline of their societies.
The findings represent a meaningful advancement in understanding the relationship between hydroclimate dynamics and the evolution of ancient civilisations, as noted by geoscientist Liviu Giosan from the Woods Hole Oceanographic Institution. Previous studies relied on limited geological data to evaluate rainfall patterns, whereas this comprehensive study integrates diverse records to illustrate the water cycle’s broader effects. This methodology could also provide insights into other ancient river-dependent cultures, such as those in Mesopotamia, Egypt, and China.
Giosan highlighted the unforeseen implications of droughts on settlement patterns within the Indus territory, suggesting a framework for archaeological testing. He reflected on the resilience of these civilisations during sustained climatic challenges, noting the relevance of these findings to contemporary society, which may face similar climatic stresses. Rajagopalan noted that understanding the future fluctuations of tropical Pacific Ocean temperatures will have critical implications for regional rainfall patterns, posing significant questions for future climate research.