The three-century climatic upheaval of c. 2000 BC, and regional radiocarbon disparities

Abstract

Several researchers have previously identified a severe climatic upheaval in tropical North Africa that began just over 4000 years ago and lasted for about three centuries. The upheaval is known to have occurred shortly after a volcanic eruption, and companion work proposes that this eruption was colossal. Here, we suggest how the eruption acted as a trigger for the upheaval: by forcing changes in ocean circulation; although the initial (atmospheric) forcing lasted only a few years, the ocean required three centuries to regain equilibrium. The suggested triggering mechanism is supported by palaeoceanographic, palaeoecological, and archaeo-historical data and by related experiments with a (coupled general-circulation) climate model. We argue that the changes in ocean circulation forced changes in sea-surface temperatures that led to a weakening of the south-west North African monsoon. The upheaval has been proposed to have also encompassed south-western Asia. We argue that it encompassed most of the Northern Hemisphere: we present a variety of palaeoecological and palaeoceanographic evidence and describe the principal underlying climatology. In some areas the upheaval was the most severe since the ice ages. The full scope of the upheaval has previously been missed in part because radiocarbon dates from some areas are centuries too early: palaeoclimatic events in different areas thus appeared asynchronous. (The erroneous radiocarbon dates also misled searches seeking ice-core and tree-ring evidence of the eruption.) The cause of the radiocarbon-dating disparities is identified as a regional deficiency in 14C, and we locate the region's source of 14C-deficient carbon.