Oscillations between the dominance of aragonite and calcite in abiotic marine CaCO3 precipitates throughout Earth history are closely coupled with the evolution of Earth’s seawater composition and represent the environmental context in which organisms evolved their ability to biomineralize. The most important factor controlling these Phanerozoic oscillations in CaCO3 polymorph composition is the ratio of Mg:Ca in seawater, which is thought to separate aragonite and calcite precipitation along a distinct temperature-controlled threshold. A sharp threshold, however, is contradicted by overlapping aragonite and calcite precipitation fields at a range of experimental conditions. Here we present experimental data that enable us to quantify the proportions of CaCO3 polymorphs as a function of Mg:Ca ratio and temperature. This allows us to convert published Mg:Ca ratio proxy data and models of the Phanerozoic Mg:Ca ratio into proportions of abiotic CaCO3 polymorphs at a given temperature, and thus provides a temperature-corrected view of aragonite-calcite sea conditions. In this revised view, abiotic calcite precipitation was inhibited during aragonite sea intervals at temperatures above 20 °C, whereas calcite sea intervals were characterized by the co-precipitation of aragonite and calcite in environments above 20 °C. This continuous prominence of aragonite precipitation in Phanerozoic warm-water environments explains the Phanerozoic increase of aragonite over calcite skeletal composition in calcifying organisms.