Normal is around 250-275 ppm CO2. It was around 300ppm during the last interglacial. The sea level was higher than now. Apart from the usual academic sources, I'm not more than half a mile away from the old, last interglacial beach ridge and foreshore here at home. Most of it has been built over and you can't see it but there are spots where the topography and the soil type betrays it's presence. Not only that, but I've cored through it before and gotten sea shells
