Somehow I’ve got the idea that this is just one piece of data. Totally uncorrellated against other, different pieces, of data. Somehow I’ve got the idea that things like accurate sunlight levels, temperatures, and more are rather missing. Quick: what was the mean summer temperature in South Africa in AD 1000? How much rainfall was there in Europe in AD 1420?
Somehow I’ve got the idea that we were coincidentally (?) coming out of the Little Ice Age.
Somehow I get the impression that this is all very much guesswork. That doesn’t make it wrong, but it makes me very skeptical.
And in ancient times the Sun was cooler; it’s been slowly warming since it ignited. And as pointed out, there weren’t cities and nations in low lying areas or a human civilization dependent on a stable climate for agriculture back then. So, the levels of CO2 in ancient times don’t matter much. They won’t have the same effect now as then, and we weren’t there back then anyway.
And the problem isn’t just the world being warmer, but the process of changing to one climate to another. There’s no magic “switch to hot climate mode” switch for civilization; changing will require money, sacrifice, and time; and will result in hardship, suffering and death. We can expect famines and mass migrations of refugees, at the least.