The sun regularly has violent outbursts that almost kills off all life on the planet. This happens approximately every 12,000 years, and when it does, planet orbits are pushed very slightly outwards, and a small percentage of the atmosphere and water is lost.
We're about due for another smackdown. All we know as "history" has taken place in the lull since the last outburst.
There will soon be a world-wide natural disaster beyond our comprehension that sweeps away human civilization as we know it. A few human in protected areas in the mountains will survive, and a new era begins.
Now with a slightly longer year…