Paleogeography strongly influences CO2 threshold for Sturtian Snowball Earth initiation

Neoproterozoic “snowball Earth” refers to extreme glaciations when sea ice extended from the poles to the tropics and perhaps to the equator. Despite decades of study, the mechanisms that triggered global glaciation are still debated, although many mechanisms link their onset to reductions in atmospheric CO₂ concentration. We use a coupled general circulation model and two geologically constrained paleogeographic reconstructions to re-examine the CO2 threshold for the initiation of the Sturtian snowball Earth (~717 Ma). With modern landmasses, a hard-Snowball transition occurs at 95±5 ppm CO₂, consistent with prior estimates. In contrast, one 720 Ma reconstruction, resists global glaciation down to 6±1 ppm CO₂ – a threshold so low that initiation via CO₂ drawdown might be challenging – while maintaining an "oasis climate" with a small, zonally asymmetric region of open tropical ocean. A second 720 Ma reconstruction glaciates at 110±10 ppm, similar to modern. We show that the oasis climate is possible because the former continental configuration inhibits ocean heat transport out of a small, tropical ocean basin, allowing it to maintain above-freezing sea surface temperatures. While the "oasis climate" lacks the hysteresis expected for snowball glaciations in our climate model, hysteresis might be supplied by land ice sheets. The apparent sensitivity of Earth's snowball glaciation behavior to subtle changes in continental geometry points to a need for better-constrained paleogeographic reconstructions for understanding snowball Earth events and highlight potential challenges to CO₂ drawdown mechanisms for snowball initiation.