For 56 million years, Earth appears to have lurched between two extremes that are hard to picture together: a planet sealed in global ice, then a world hot enough to strip carbon dioxide from the sky and set the stage for the next freeze. That is the case put forward by Earth scientists at Harvard’s John A. Paulson School of Engineering and Applied Sciences, who argue that the Sturtian glaciation was not one endless deep freeze. Instead, they say, the planet likely flipped back and forth between full Snowball Earth conditions and warm, ice-free intervals during the Cryogenian period, roughly 717 million to 660 million years ago. The idea offers a way through one of ancient climate science’s most stubborn problems. Standard physical models have long struggled to explain how the Sturtian glaciation could have lasted about 56 million years when a fully frozen Earth should, in theory, thaw much sooner as volcanic carbon dioxide builds up in the atmosphere. Graduate student Charlotte Minsky led the research with Robin Wordsworth, David T. Johnston and Andrew …