I just linked to it on the Zone.

My guess is that whatever lab or scientist did the research only had data going back that far. For example, if the data was developed by testing trapped air bubbles in ice cores from a Greenland glacier. Maybe they stopped drilling at that point. (Or they hit bedrock!)

I'm sure there are other sources of data, but its nice to display data from only one source, for consistency and comparison. You could probably go back a lot further if you included data from other sources (like petrified tree rings or deep ocean bottom sediments, or ancient rock strata, or even fossil Scandinavian bog pollen (TB actually published that one here on the Zone once), in an attempt to introduce evidence contradicting global warming. But that would be data from different sources, with different precision, from different physical processes, and involving different investigative techniques and sampling technologies.

What we can take away from that graph (if the data is to be trusted) is that over the last few hundred thousand years CO2 levels have oscillated roughly between 200 and 250 thousand ppm with a period of roughly 100k yr. This suggests some astronomical or geological driving mechanism, such as a variation in the solar constant
or an oscillation of earth's orbital parameters due to some gravitational resonance.

EXCEPT--sometime in the very recent past there was an extraordinary spike in CO2 levels which almost doubled the levels of the gas--abruptly. A reasonable explanation would be the sudden increase in atmospheric CO2 due to fossil fuel burning starting at the beginning of the Industrial Revolution about 1800.

Then again, it could be all lies by left wing Deep State conspirators who want to take away our Freedom because they hate America. Or maybe fraudulent climate scientists trying to advance their careers by ginning up funding for their bogus research?