Starburst: P1 int

The Earthwide cataclysm was started by a solar flare.

“So unlikely, almost impossible” is the unofficial or semi-official statement you would have gotten from scientists, were you to ask them how likely it was to happen, that is before it happened. let alone twice. I suppose we should be grateful that it wasn't a direct strike. if it were, the surface of the planet would have been instantly turned to molten rock. Instead, it stripped essential parts of our atmosphere, which protected us from radiation. 

But this was the end of the earth, but civilization and life on it.

in that order...

Cosmic radiation sickness, 

famine 

rapid mutation across an array of organisms causing birth defects in mammals, reptiles and most marine life. 

Initially these effects were concentrated on and around the Australian continent where it became so bad; its government destabilized and its surviving population of 9 million had to be completely evacuated to Asia. Which they had just barely managed to succeed in doing, just before the second solar flare, which was thought "Astronomical in probability" struck.