The person who answered the question quoted a scientific paper that said probably around 170 ppm.
Another person responded "Prior to this Ice Age cycle that started about 3 1/2 million years ago the PPM Norm was between 900 and 1,120 ppm."
How did the world survive at those CO2 levels? Impossible.
How did the world survive at those CO2 levels? Impossible.
“It’s complicated.”