Snowball versus slushball Earth: Dynamic versus nondynamic sea ice?

Modeling studies of the Neoproterozoic snowball Earth offer two variations for snowball conditions, the original “hard” snowball Earth where the ocean is completely covered by sea ice, and an alternate slushball Earth or “soft” snowball, where there is an equatorial oasis of open water. We use the University of Victoria Earth System Climate Model to show that the soft snowball result is only possible when dynamics are excluded from the sea ice component of the model. Using a purely thermodynamic sea ice component the soft snowball condition is stable, whereas with a dynamic and thermodynamic sea ice component it is not. As the behavior of dynamic sea ice largely depends on wind stress, we compare simulations using two different wind fields: a zonally averaged present‐day wind field and a wind field derived by a general circulation model, the Fast Ocean Atmosphere Model, using Neoproterozoic conditions. Another consequence of using dynamic sea ice is that the sea ice does not become sufficiently thick to flow under its own weight when there is open water; this suggests that sea glacier dynamics are not important for snowball inception.