claim: the concentration of carbon dioxide in Earth’s atmosphere has climbed to a level last seen more than 3 million years ago — before humans even appeared on the rocky ball we call home.
evidence: About 34 million years ago, the time of the Eocene–Oligocene extinction event and when the Antarctic ice sheet started to take its current form, CO 2 was about 760 ppm, and there is geochemical evidence that concentrations were less than 300 ppm by about 20 million years ago.