[Lurkinanloomin]

280 parts per million before the dawn of the industrial era
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Below 180 parts per million mass extinction begins.

[ProtectOurFreedom]

Excellent point. You got me curious, so I looked that up and found: What's the (or some of the) minimum(s) amount of atmospheric carbon dioxide needed by plants?

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.