Climate at 4.2ka BP and deterioration of ancient civilisations

The deterioration of three ancient civilisations: The Egyptian Old Kingdom, The Harappans of the Indus River Valley and the Akkadian Empire of Mesopotamia, are thought to be linked to a climate event at 4.2ka BP in which rapid aridification occurred (Cookson et al., 2019). To evaluate this theory, proxy data was collated and climate data between 4.5-3.6ka was simulated through the HadCM3 model to gauge precipitation, evaporation, temperature, river output and monsoon strength. The archaeological record was referenced to give context and evaluate non-environmental causes of collapse. A discrete ‘4.2ka event’ was not evident for any of the three regions. In Egypt, there was a general weakening of the East African Monsoon and a more rapid weakening ~4.5-3.5ka which led to low Nile river levels that remained due to the monsoon strength not recovering post 4ka. Consequently famine occurred, partly due to social/political issues and when the Pharaoh Pepi II died, the Egyptian Old Kingdom came to an end (Butzer, 2012). The Indian Summer Monsoon also decayed in strength between 6-3ka but more rapidly between both 4.75-4.5ka and 4.2-4.2ka. Regions of the Indus River Valley closer to the Thar desert thus experienced diminished precipitation below the threshold for drought resistant crops, which led to mass migration beginning ~4.4ka and advancing further ~4.2ka (Petrie, 2017; Petrie et al., 2017; Green and Petrie, 2018). Mesopotamia also experienced two main climate excursions ~4.4ka and 4.1ka but this appeared to be part of a larger scale climate shift between 6-3ka. The North Atlantic Oscillation was neither positive nor negative and therefore was not the driving force of climate change. However, as the civilisation urbanised massively ~4.6ka, settlements pushed further into drier regions which would then be more susceptible to drought (Lawrence et al., 2021).