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500,000,000 years of my understanding of atmospheric CO2 concentration is what I was referring to as "Historic".

(Since there's no other definition of historic that I know of (500M years seems as arbitrary as any other number), I think the person was referring to the literal definition of historic, namely, written records. Not that I think this is a very fruitful argument to make, or whether weather recorded in shells could be considered written is perhaps also debatable)

Technically that's prehistoric. History conventionally starts with the invention of writing

Not sure that's conventional. Looking up the dictionary definition, it's clearly being used generally. It's the definition I was taught in school and probably it's used in this technical way in some fields or perhaps in science generally, but commenting on someone casually using "historic values" and going "I don't think you know what that means!" is the best way to help anyone get a better understanding of anything

("conventional" in meaning 1 of the word as per Wordnik, namely that it's in accordance with general practice, before we start another such discussion...)

You’re right that "history" starts with writing, but "prehistoric" refers to the human period before written records, starting with the use of stone tools around 2.5 million years ago.

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