For data going further back, we have deep antarctic ice sheet, ice core samples of atmospheric co2: https://www.bas.ac.uk/data/our-data/publication/ice-cores-a

For data going further back, we have deep antarctic ice sheet, ice core samples of atmospheric co2: https://www.bas.ac.uk/data/our-data/publication/ice-cores-and-climate-change/ https://www.bas.ac.uk/data/our-data/publication/ice-cores-an... https://climate.mit.edu/ask-mit/how-do-we-know-how-much-co2-was-atmosphere-hundreds-years-ago https://climate.mit.edu/ask-mit/how-do-we-know-how-much-co2-... At about 1:50 in the video linked above, when it zooms out, notice the citations to the ice cores.