These days I am thinking that global cataclysms occur every 5000 years or so, alternating between major ones and minor ones - evidence for the last minor one:

Glaciologist Lonnie Thompson worries that he may have found clues that show history repeating itself, and if he is right, the result could have important implications to modern society. ...A professor of geological sciences at Ohio State and a researcher with the Byrd Polar Research Center, Thompson points to markers in numerous records suggesting that the climate was altered suddenly some 5,200 years ago with severe impacts. Evidence shows that around 5,200 years ago, solar output first dropped precipitously and then surged over a short period. It is this huge solar energy oscillation that Thompson believes may have triggered the climate change he sees in all those records.

Source: Eureka Alert

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I would like everyone to visit peterelee.tripod.com. This book is science-fiction based on the Dec 12, 2012 Mayan timeline.