The ‘4.2 ka drought event’ and the fall of the Harappan Civilization: A critical review

The role of climate variability in the expansion and collapse of the Harappan Civilization in South Asia has been a subject of intense research and debate among archaeologists, Quaternary climate scientists, meteorologists, climatologists, historians, geographers, archaeobotanists, and anthropologists. Moreover, high-resolution palaeoclimatic studies have challenged the previous perspectives attributing societal collapses solely to socio-political and economic factors. These studies have revealed that periods of abundant rainfall, resulting from climate amelioration and/or due to seasonality: Winter vs. Summer precipitation, often coincided with the peak of socio-economic, cultural, and technological advancements in various civilizations. Conversely, abrupt climatic changes or rapid climatic changes, such as prolonged droughts, led to the destruction of irrigation systems, agricultural devastation, and widespread famines. These climatic disruptions triggered cultural adaptations, migrations, and sometimes even the collapse of entire civilizations. Climate forcing, particularly solar variability, played a primary role in repeated societal displacements, migrations, and deurbanization across different regions in the past. Additionally, the intensified El Niño-Southern Oscillation, a shift of the Indian Ocean Dipole to a strong negative state, and the southward migration of the Inter-Tropical Convergence Zone contributed to an extended period of weakened Summer Monsoons of South Asia and increased aridity. This depletion of water resources triggered the general demographic shift vis-à-vis the 4.2 ka event. The displacement was a response to a weakened Summer Monsoons of South Asia. In this research review article, we aim to understand the role of climate variability and explore the complex socio-climatological linkages in the mysterious displacement of once-thriving, highly advanced, widespread, and urbanized Harappan Civilization at the 4.2 ka.