So, I've been considering the Norse legend of Ragnarök, I'm considering how the legend could come true in a purely scientific setting. In other words what science-based explanation would best fit the old Viking legends. For this question I'm looking at the "signs" section, the prelude to the actual event. A follow on question will deal with Ragnarök itself.
There will be some signs if Ragnarok "the end of the world" is coming. The murder of the god Balder, the son of Odin and Frigg.
We will have a long and cold winter that, will last for three years with no summer in between. The name of these uninterrupted winters are called "Fimbulwinter" During these three years, the world will be plagued by wars, and brothers will kill brothers.
A beautiful red rooster "Fjalar" which name means the "All knowner", will warn all the giants that the beginning of Ragnarok has begun. At the same time in Niflheim, will a red rooster warn all the dead, that the war was started. And also in Asgard, will a red rooster “Gullinkambi” warn all the gods.
Heimdall will blow his horn as loud as he can and that will be the warning for all the einherjar in Valhalla that the war has started.
Obviously to be science-based we will need to take it that some or all of the legend is metaphorical (especially when wolves start eating the sun). However answers should stay as close to the legend as possible while being as scientifically plausible as possible.