First of all, the measurement of the CO2 hundreds of thousands of years ago is done using methods of looking back in the past. The current measurements are done using completely different methods. In order to compare measurements, it would be best to use the same methods to get results that are consistent.
The measurements are consistent up until about 1970. Ice core sampling. Current measurements are done by sampling the atmosphere. Do you have reason to think these methods don't work?
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