Scientists now know how much carbon dioxide was present in the atmosphere as far back as 800,000 years ago. How did they obtain this data?

Ice core records.

Explanation:

The origin of man took place about 200,000 years ago and looking at the records that are stored in the ice cores before the existence of man which were found by the deep drilling of the antarctic and Greenland shows that how the temperatures have changed and as the example of the carbon in the air. As in the 800,000 years go the Carbon dioxide levels were on average of 170 to 280 ppm and now they have crossed the 400 ppm mark also at that time the sea levels were high as 80 feet and now are more than 100 feet.