Climate change and collapse of early civilisations

About 4200 years ago early civilisations of the Old World underwent decline and collapse. Examples are the Akkadian civilisation in the upper Tigris and Euphrates basins, famed for Hammurabi’s Hanging Gardens of Babylon, the Harappan of the Indus Valley (Mohenjodaro), the phaoronic Old Kingdom and the Minoan of Crete. This period of the Bronze Age has been thought by some to have experienced either massive volcanism – the explosion of Santorini – or even a comet strike. Others have correlated collapses of city states with Biblical events. Whatever happened, its outcome spanned a vast area of western Asia and north-eastern Africa, so another candidate is climatic drying leading to drought and famine. That is perhaps not such a spectacular fate as near-instant environmental upheavals, but probably just as effective for societies dependant on regular agriculture production or, in the case of Crete, on wide-ranging trade.

Detecting climate change is now well established on proxy records of one kind or another, such as those based on isotopes and sedimentation changes from sea-floor sediments and flowstone (speleothem) in caves, and dust records in ice cores. Such time-series from the mid- to late Holocene are increasing in number, with particular interest growing in records from speleothem now that precise age sequences are possible using uranium-series dating. A flowstone record from a cave in northern Italy, has helped link other time series ranging from the North Atlantic floor, in the Middle East and East Africa (Drysdale, R. et al. 2006. Late Holocene drought responsible for the collapse of Old World civilizations is recorded in an Italian cave flowstone. Geology, v. 34, p. 101-104). A team of geochemists ad environmental scientists from Australia, Italy and the UK has shown a remarkable coincidence among these widely different records, centred on 3900-4200 b.p.. From the North Atlantic at high latitudes is an upsurge in fragments deposited by ice rafting, while mean sea-surface temperatures swung downwards. Kilimanjaro ice shows a marked peak in atmospheric dustiness. Carbonate deposition peaked in the Gulf of Oman. Finally, the Italian flowstone shows peaks in d18O, d13C and the magnesium:calcium ratio of its carbonates. The conclusion is a period of climatic cooling and drying that spanned 40 degrees of latitude over a period of several hundred years. This is not the signature likely to have been associated with instantaneous catastrophes. Yet nor is it typical of the episodic climate shifts of the order of a few thousand years, which were now well known features of the last glacial period and the current interglacial. It was certainly sufficiently prolonged and large enough to have wrought havoc on early civilisations, and throughout the Old World it clearly did.

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