[gatex]

"Point one: atmospheric CO2 concentration increase since the mid-1800s is due to fossil fuel burning and land-use changes. "

What caused the 304 ppmv concentrations 340,000 years ago (Vostok ice ) ?

[cogitator]

What caused the 304 ppmv concentrations 340,000 years ago (Vostok ice )?

Warmer global temperatures, obviously; same reason for the 280 ppmv concentrations at about 240K and 130K BP.

Time for my question in response. See where the red line is at the far right side of the graph? Notice anything unusual about that?