A new study by Earth scientists in the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) offers an explanation for one of Earth’s great climate puzzles: how the Sturtian glaciation, an ancient ice age when the planet was nearly entirely frozen, could have lasted 56 million years – far longer than standard climate models have predicted. This lengthy freeze took place during Earth’s Cryogenian period, roughly 717 to 660 million years ago, predating dinosaurs and complex plant life.

The research is published in Proceedings of the National Academy of Sciences and led by graduate student Charlotte Minsky, who is advised by co-author Robin Wordsworth, the Gordon McKay Professor of Environmental Science and Engineering and Professor of Earth and Planetary Sciences. Co-authors are David T. Johnston and Andrew H. Knoll.

Using a coupled model of the ancient climate and the global carbon cycle, the researchers make the case that Earth may not have been locked in a single, unbroken “Snowball Earth” state, or period when the entire planet was frozen. Instead, they find that the planet likely oscillated between fully ice-covered “snowball” conditions and ice-free “hothouse” intervals throughout the Sturtian period.

The team’s simulations suggest that intense weathering of basalt in the Franklin Large Igneous Province, a vast volcanic region located in northern Canada and believed to have erupted just before the onset of the Sturtian glaciation, drew down atmospheric carbon dioxide enough to trigger multiple global glaciations.

As volcanoes and other processes slowly rebuilt atmospheric carbon dioxide, the climate warmed, the ice retreated, and large areas of fresh basalt were again exposed to the atmosphere. Renewed breakdown from weathering then pulled carbon dioxide back down, pushing the climate into another Snowball phase. This repeating cycle of carbon dioxide-driven freezing and thawing, the authors argue, could naturally sustain glacial–interglacial swings over tens of millions of years.

The mechanisms revealed by the Harvard study resolve several longstanding paradoxes, most notably the previously inexplicable length of the Sturtian when compared with physical climate models. The study also matches observed sedimentary patterns from that time period and explains how atmospheric oxygen levels could have remained stable despite extreme climate upheavals.

Repeated returns to warmer, ice-free conditions may have helped prevent a complete collapse of atmospheric oxygen, the study further suggests. “This could help explain how aerobic life persisted through such an extreme interval,” Minsky said.

Learn more: "Repeated Snowball-hothouse cycles within the Neoproterozoic Sturtian glaciation."