Climate change, landscapes and human impacts during the 4.2kyr event in Mesopotamia and beyond

The 4.2kyr event saw major changes in civilizations across the world, often attributed to climate change and widespread drought. Indeed, there are high quality palaeohydrological proxies that seem to document a significant drying at this time in Egypt, the Indus Valley and in the Near East. In Mesopotamia, settlements were abandoned, agricultural practices shifted and the Akkadian Empire ended. Climate model simulations covering 5000-3000 years ago are able to reproduce much of the evidence for Holocene climate change and some of the key reductions in mid-late Holocene precipitation. However, unlike some other regions, the annual mean rainfall in Mesopotamia at 4.2kyr is similar to the adjoining millennia. The climate model simulates significant droughts during this time, but these are within the range seen both before and after.

The end of the middle Holocene saw rapid population growth in Mesopotamia, with significant urban centres putting pressure on local resources. Although the models suggest no significant change in climate, the archaeological evidence shows the exploitation of a dryer landscape, both in existing sites and new urban centres in dryer regions. This time also sees changing agricultural practices and the loss of available sources of wood. Vegetation models suggest little impact of climate change on the natural landscape, although drought years bring major losses in net primary productivity. However, human activities in Mesopotamia could have affected the landscape and exacerbated the impacts of existing drought cycles. We suggest that an unexceptional drought, combined with a large population and anthropogenic impacts on the landscape contributed to significant societal change in Mesopotamia at 4.2kyr.