Tree ring science
Ice cores are ice cores because they originate from a particular climate in the region. It's inherently imbalanced data to apply to the rest of the world. At best, ice cores are an indicator of CO2 dispersion in the polar regions. To take such data as the gospel as to the CO2 levels of the entire planet is preposterous.
It could be likened to using the same ice cores to tell us the relative salinity of the various oceans over the earth for the same period.
It's only when no other method has been developed yet for more accuracy that the data is paid attention to at all.
It's tree ring science. And all that implies.