I think they use ice core samples. They can measure back way further than 800,000 years, millions of years. I'm not disbelieving either, but for every article like Will's, there is another argument saying that CO2 levels have been massively higher in the past, as has temperature. Equally both have been much lower. Sometimes they correlate, sometimes they don't. CO2 is absorbed into the deep oceans during cooler periods and released into the atmosphere during warmer times. CO2 comprises 0.04% of earth's atmosphere, humans add 0.4% to that 0.04%.Not disbelieving! - just wondering how they measured carbon dioxide levels for the planet 800,000 years ago.
The problem with climate change is that the debate is shut down and anyone who questions the narrative is labelled insane.