by Andy Mayhew
Changes in insolation (the amount of sunlight the Earth receives, determined in part by small variations in our orbit and axial tilt) in the Northern Hemisphere are believed to be the ultimate cause of this, initiating feedback mechanisms which caused changes in global ocean circulation and the locations of predominant weather systems. There have also been suggestions of a change in solar output around this time.
Meanwhile, perhaps as as consequence of one of these feedback mechanisms, the variations in ocean currents in the Pacific, known as the El Nino Southern Ocillation, also became established at this same time with it's modern frequency (Rodbell etal 1999). Every few years the distribuution of warm and cold waters in the Pacific changes, and upswelling of warm waters off the coast of Peru indicates the start of El Nino. In some parts of the world, rainfall intensifies and cyclones ravage regions not normally affected by tropical storms; elsewhere the monsoon rains fail completely and drought afflicts the land. A reversal of the warm water pattern - known as La Nina, results in the opposite effect and often follows on directly after a major El Nino event. Every few hundred years a more serious event - a Mega El Nino - occurs. Whilst a normal El Nino causes hardship, a Mega El Nino is a destroyer of civilisations.
All of which leads to a small bit of speculation.
Whilst we can not be sure of the exact sequence or timing of events, it does seem reasonable to suppose that a steadily deteriorating climate was followed by the first major El Nino event to have been experienced by modern man. Whether this itself brought the deluge, or simply compounded the drought, to be followed hot on its heels by a La Nina induced deluge would depend on the exact location where these events occurred. But such a sequence does match very well with that given in the earliest written account of what is now commonly known as Noah's Flood - the Summerian story of Atrahasis - in which years of drought, famine and disease (major epidemics are another frequent consequence of El Nino events) were followed by a sudden, massive, storm and flood. Is this just coincidence?
Historically, major El Nino events have seen the fall of civilisations, provoked civil wars and started mass migration as people flee the drought or flood afflicted regions. Evidence of a diaspora of peoples from the Middle East, around the world, circa 3,000bce would perfectly match this known human response to such climatic events.
Or maybe, that too, is just coincidence