The current CO2 density in the atmosphere is ~420 ppm, although it has been as high as 4,000 ppm during the Cambrian period. How was that not a problem back then, but it is now? - TL Winslow, Pansophist master of STEM and world history

https://www.quora.com/The-current-CO2-density-in-the-atmosphere-is-420-ppm-although-it-has-been-as-high-as-4-000-ppm-during-the-Cambrian-period-How-was-that-not-a-problem-back-then-but-it-is-now/answer/TL-Winslow