Sufficiently bad and frequent plagues/famine could "help" extend that period, as it could serve to break apart polities, fragment trade routes, and make accumulation of knowledge more difficult.
Probably some sort of climate change based scenario could do it, but I am not sure what form exactly.
Basically you only need to make the period in question last twice as long, so that should be doable.
Probably some sort of climate change based scenario could do it, but I am not sure what form exactly.
Basically you only need to make the period in question last twice as long, so that should be doable.