The flood in the bible was bastardized to include "The entire world" - but we all know telephoning stories will change reality. Add to the fact that people 4,000 years ago had no idea how big the world was, so to them, a flood was "world-wide". It is clearly, in my mind, that The Younger Dryas, which had massive, world-wide effect, is one of the most well-known examples of abrupt change that could have easily be the source of these stories passed on from generation to generation, which is why it is found is almost all religions.