The correspondence between atmospheric CO 2 concentrations and globally averaged surface temperatures in the recent past suggests that this coupling may be of great antiquity. Here, I compare 490 published proxy records of CO 2 spanning the Ordovician to Neogene with records of global cool events to evaluate the strength of CO 2-temperature coupling over the Phanerozoic (last 542 my). For periods with sufficient CO 2 coverage, all cool events are associated with CO 2 levels below 1000 ppm. A CO 2 threshold of below ∼500 ppm is suggested for the initiation of widespread, continental glaciations, although this threshold was likely higher during the Paleozoic due to a lower solar luminosity at that time. Also, based on data from the Jurassic and Cretaceous, a CO 2 threshold of below ∼1000 ppm is proposed for the initiation of cool non-glacial conditions. A pervasive, tight correlation between CO 2 and temperature is found both at coarse (10 my timescales) and fine resolutions up to the temporal limits of the data set (million-year timescales), indicating that CO 2, operating in combination with many other factors such as solar luminosity and paleogeography, has imparted strong control over global temperatures for much of the Phanerozoic.