In 774 AD, the atmospheric level of carbon-14 jumped by 1.2% over the course of no longer than a year, about 20 times more than the normal rate of variation. There is no theory based on any kind of evidence on why this occurred. So I read that period in history. Europe became formally united under the Holy Roman Empire and the Mideast loosely united as Muslims. The climax of bloodshed put an end to what civilization was left for a millennium. Also, the founders of the religion in Japan were born. I did not find anything that would shed light on the actual events.