4000 BP Climate Upheaval
Doug Keenan <email@example.com>
At the Cambridge conference of 1997 and at the NATO conference of 1994, the main question was this: was there a major climatic upheaval four millennia ago, and if so, what was the cause? The conferences, and several papers in the scientific literature, have led to much discussion. The general view seems to be that something happened--though it has been unclear what. The most popularly-suggested cause has been a comet.
Below is the abstract of a paper that I presented at the 1999 meeting of the IUGG (the main international conference for Earth scientists). I believe that the paper effectively answers the main question posed above. The case for a climatic upheaval now appears conclusive--and the upheaval was likely the largest climatic event since the ice ages. There is, however, only inconclusive/questionable evidence for a comet being the cause; rather, the upheaval appears to have been triggered by a colossal volcanic eruption.
The work also strongly supports proposals from Barbara Bell (Harvard), Harvey Weiss (Yale), and others, for a major climatic influence on civilisations. In each of the world's three regions of civilisation, the influence of the upheaval appears to have been dramatic:
- In the Ancient Near East, drought led to the collapse of the earliest civilisations--including the millennium-old Kingdom of Egypt.
- In the Indus Valley, the upheaval coincided with the transition of the civilisation to its depopulated Post Urban phase.
- In ancient China, the upheaval induced flooding, and the organisation to deal with this likely led to the founding of the first Chinese dynasty.
The upheaval was thus probably the biggest natural event to happen to humanity since the ice ages.
The Three-Century Climatic Upheaval of C. 2000 BC,
and Regional Radiocarbon Disparities
Douglas J. Keenan (The Limehouse Cut, London E14 6N, U.K.)
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.
The paper is available from http://www.informath.org/
Following are some brief technical remarks.
The initial (atmospheric) forcing was a cooling over the Labrador Sea and a warming over the Norwegian Sea. Such cooling/warming is caused by the intensification of the polar vortex that is induced by volcanogenic aerosols. The cooling/warming in the Labrador/Norwegian Sea increased/decreased deepwater production there. This, in turn, forced an extremely high phase of the North Atlantic Oscillation. The high NAO explains why, for example, some areas of Europe were cool/dry while others were warm/wet and still others experienced little change. Once disequilibrated, the oceans took centuries to recover: so did the climate.
The mechanism underlying the radiocarbon dating errors has been largely developed by others. Briefly, it is as follows. During the last ice age, the Black Sea was actually a freshwater lake. As the ice age ended, this freshwater flowed out of the Black Sea into the Mediterranean, which greatly altered Mediterranean circulation. In particular, Mediterranean subsurface waters stagnated for at least six millennia. During the stagnation, 14C in subsurface waters radioactively decayed and was not replenished. Later, as the Mediterranean circulation was restored - a process that took more millennia - the 14C-deficient subsurface waters circulated back to the surface. The 14C-deficient carbon was then degassed to the atmosphere: similar processes have been observed today off Ecuador, in the Arabian Sea, and possibly in the Weddell Sea. Thus many radiocarbon dates from samples that grew in or downwind from the Mediterranean are some centuries too old.
Cheers, Doug Keenan