Captain Nemo wrote on May 17
th, 2020 at 2:05pm:
The ice-core data does NOT show CO2 atmospheric concentration being above 300 PPM going back 800,000 years.
That's only in the Antarctic Ice Cores petal.
For Greenland Ice Core (GRIP) -
"CO2 record between 40 and 8 kyr B.P. from the
Greenland Ice Core Project ice core "
" We present CO2 measurements performed on an ice core from central Greenland, drilled during the Greenland Ice Core Project (GRIP). This CO2 profile from GRIP confirms the most prominent CO2 increase from the LGM, with a mean concentration of 200 ppmv, to the early Holocene with concentrations between
290 and 310 ppmv. Some structures of the new CO2 record are similar to those previously obtained from the Dye 3 ice core (Greenland), which indicated a dilemma between Greenland and Antarctic CO2 records [Oeschger et al., 1988]. Both Greenland cores show high CO2 values for rather mild climatic periods during the last glaciation, whereas CO2 records from Antarctica do not show such high CO2 variations during the glaciation and, furthermore, the CO2 values in the early Holocene are about 20-30 ppmv higher in the GRIP record than in Antarctic records. There is some evidence that the difference could be due to chemical reactions between impurities in the ice leading to an increase of the CO2 concentration under certain conditions. If in situ processes can change the CO2 concentration in the air bubbles, the question arises about how reliably do CO2 records from ice cores reflect the atmospheric composition at the time of ice formation."
https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/97JC00182I believe between 8K and 40K years is shorter than 800K years.