In this article, prominent scientist and global warming skeptic Princeton University physicist Dr. Will Happer says that average global CO2 levels have been 3 times the current level since the advent of multicellular life in the Cambrian era 500 million years ago.
According to Happer, "life on earth flourished with these higher levels of carbon dioxide".
“I have spent a long research career studying physics that is closely related to the greenhouse effect, for example, absorption and emission of visible and infrared radiation, and fluid flow,” Happer said this week. “Fears about man-made global warming are unwarranted and are not based on good science. The earth's climate is changing now, as it always has. There is no evidence that the changes differ in any qualitative way from those of the past,” he added.