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 542my). For periods with sufficient CO 2 coverage, all cool events are associated with CO 2 levels below 1000ppm. A CO 2 threshold of below ∼500ppm 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 ∼1000ppm is proposed for the initiation of cool non-glacial conditions. A pervasive, tight correlation between CO 2 and temperature is found both at coarse (10my 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.