Extreme climates and current analogues : from the last millennia to the Holocene optimum (5)

Abstract

Mesopotamia is considered the cradle of civilization, with cuneiform writing dating back to more than 3 000 years BC. Around 2300 BC, Sargon forcibly united Upper and Lower Mesopotamia, founding the Akkad Empire. This brilliant civilization traded with other peoples from the Mediterranean to the Indus Valley. There is no certainty as to the location of Akkad's capital, but several fortified cities have been excavated, illustrating the prosperity of this society, and studying its abrupt decline.

Archaeologists have highlighted a temporal hiatus in occupation (2200-3900 BC) for certain regions of Mesopotamia, accompanied by a migration of populations to wetter refuge areas on the banks of the Tigris and Euphrates rivers and their tributaries. The texts also tell of warlike conflicts with the peoples of the Zagros, who finally invaded the Akkad empire around 2200 BC, leading to the Gutis dynasty.

Evidence of decline exists in archaeological sites in the form of unfinished constructions and changes in sedimentary facies marking the drying up of the environment. It is also possible to reconstruct regional hydrological variations from various archives located around Mesopotamia : mineral dust fluxes in marine sediments in the Gulf of Oman, increased surface salinity in the Red Sea leading to a drastic change in deep sedimentation to the north of this enclosed sea, geochemical records of a period of drought in stalagmites in caves to the east and west of Mesopotamia.

The speed of the collapse of the Akkadian empire and the representativeness of the sites studied to reconstruct paleoenvironments are the subject of lively scientific debate. Nevertheless, the climatic event of 4.2 k led to an undeniable drying out of Mesopotamia, which inevitably affected these populations, whose prosperity was based on agriculture optimized by sophisticated irrigation.

There is currently no scientific consensus on the origin of the 4.2 k event. Records of external forcings (insolation and solar irradiance, volcanic activity, atmospheric greenhouse gas concentrations) show no significant anomalies for this period. Climate model simulations have been carried out for the entire Holocene, but they do not show a climate anomaly corresponding to the 4.2 k event.

It should be noted, however, that these simulations do show climate variation for a more recent event centred around 700 yr BC. In this case, a transient drop in solar irradiance affects the northern seas of the Atlantic, leading to an expansion of pack ice and a southward shift of the convection zone from surface to deep waters. The result is a reduction in thermohaline circulation, with global repercussions. As precipitation at low latitudes and in the Asian monsoon zone is sensitive to variations in this thermohaline circulation, this type of causal chain could explain certain transient reductions in precipitation during the Holocene. However, these effects are likely to be amplified by as yet poorly identified mechanisms, and these hypotheses are unlikely to explain the 4.2 k event, given the current state of knowledge on climate forcings.