Pan‐glacial—a third state in the climate system

Radiative energy‐balance models reveal that Earth could exist in any one of three discrete climate states—‘non‐glacial’ (no continental ice‐sheets), ‘glacial‐interglacial’ (high‐latitude ice‐sheets) or ‘pan‐glacial’ (ice‐sheets at all latitudes)—yet only the first two were represented in Phanerozoic time. There is mounting evidence that pan‐glacial states existed at least twice in the Cryogenian (roughly 750–635 Ma), the penultimate period of the Neoproterozoic. Consensus is lacking on whether the world ocean was fully glaciated (‘snowball’ model) or largely unglaciated (‘slushball’ model). The first appearances of multicellular animal fossils (diapause eggs and embryos in China, and sponge‐specific biomarkers in Oman), being closely associated with the last pan‐glacial state, revive speculation that environmental forces had a hand in the origin of metazoa.