Akkadian Empire
Natural causes: drought, seasonal weather patterns

One theory, which remains controversial, associates regional decline at the end of the Akkadian period (and of the First Intermediary Period following the Old Kingdom in Ancient Egypt) with rapidly increasing aridity, and failing rainfall in the region of the Ancient Near East, caused by a global centennial-scale drought, sometimes called the 4.2 kiloyear event. Harvey Weiss has shown that

rchaeological and soil-stratigraphic data define the origin, growth, and collapse of Subir, the third millennium rain-fed agriculture civilization of northern Mesopotamia on the Habur Plains of Syria. At 2200 BC, a marked increase in aridity and wind circulation, subsequent to a volcanic eruption, induced a considerable degradation of land-use conditions. After four centuries of urban life, this abrupt climatic change evidently caused abandonment of Tell Leilan, regional desertion, and the collapse of the Akkadian empire based in southern Mesopotamia. Synchronous collapse in adjacent regions suggests that the impact of the abrupt climatic change was extensive.

Peter B. de Menocal has shown "there was an influence of the North Atlantic Oscillation on the streamflow of the Tigris and Euphrates at this time, which led to the collapse of the Akkadian Empire". More recent analysis of simulations from the HadCM3 climate model indicate that there was a shift to a more arid climate on a timescale that is consistent with the collapse of the empire.

Excavation at Tell Leilan suggests that this site was abandoned soon after the city's massive walls were constructed, its temple rebuilt and its grain production reorganized. The debris, dust, and sand that followed show no trace of human activity. Soil samples show fine wind-blown sand, no trace of earthworm activity, reduced rainfall and indications of a drier and windier climate. Evidence shows that skeleton-thin sheep and cattle died of drought, and up to 28,000 people abandoned the site, presumably seeking wetter areas elsewhere. Tell Brak shrank in size by 75%. Trade collapsed. Nomadic herders such as the Amorites moved herds closer to reliable water suppliers, bringing them into conflict with Akkadian populations. This climate-induced collapse seems to have affected the whole of the Middle East, and to have coincided with the collapse of the Egyptian Old Kingdom.

This collapse of rain-fed agriculture in the Upper Country meant the loss to southern Mesopotamia of the agrarian subsidies which had kept the Akkadian Empire solvent. Water levels within the Tigris and Euphrates fell 1.5 meters beneath the level of 2600 BC, and although they stabilized for a time during the following Ur III period, rivalries between pastoralists and farmers increased. Attempts were undertaken to prevent the former from herding their flocks in agricultural lands, such as the building of a 180 km (112 mi) wall known as the "Repeller of the Amorites" between the Tigris and Euphrates under the Ur III ruler Shu-Sin. Such attempts led to increased political instability; meanwhile, severe depression occurred to re-establish demographic equilibrium with the less favorable climatic conditions.

Richard Zettler has critiqued the drought theory, observing that the chronology of the Akkadian empire is very uncertain and that available evidence is not sufficient to show its economic dependence on the northern areas excavated by Weiss and others. He also criticizes Weiss for taking Akkadian writings literally to describe certain catastrophic events.

According to Joan Oates, at Tell Brak, the soil "signal" associated with the drought lies below the level of Naram-Sin's palace. However, evidence may suggest a tightening of Akkadian control following the Brak 'event', for example, the construction of the heavily fortified 'palace' itself and the apparent introduction of greater numbers of Akkadian as opposed to local officials, perhaps a reflection of unrest in the countryside of the type that often follows some natural catastrophe. Furthermore, Brak remained occupied and functional after the fall of the Akkadians.

In 2019, a study by Hokkaido University on fossil corals in Oman provides an evidence that prolonged winter shamal seasons led to the salinization of the irrigated fields; hence, a dramatic decrease in crop production triggered a widespread famine and eventually the collapse of the ancient Akkadian Empire.
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