"As far as I can tell, there's no other record of any other natural phenomenon on this Earth (climate change related or otherwise), that has been consistently recorded, in writing, more-or-less annually, for the past 1,000+ years. "

We don't need humans to record something in writing. Evidence from physical processes can be used to infer historical climate and temperature, see https://en.wikipedia.org/wiki/Paleothermometer as a starting point.