A runaway ice-albedo feedback at low latitudes has been proposed as an essential mechanism for achieving an ice-covered Earth. Previous simulations of the Neoproterozoic climate using a coupled ocean-atmosphere model have failed to trigger this feedback, but have also not simulated sea ice equatorward of 30°. To directly test for a runaway ice-albedo feedback, a 1000 yr simulation of the Neoproterozoic using a coupled ocean- atmosphere general circulation model was completed with sea ice specified to 10° latitude. In this experiment, low-latitude sea-surface temperatures remain above freezing, inhibiting sea-ice advance toward the equator. An ice-free ocean is maintained by a low latent-heat flux to the atmosphere and low ocean-heat transport from low latitudes. A Neoproterozoic climate with low-latitude sea ice is not stable. Once the sea-ice specification was released, allowing sea ice to melt, the sea-ice margin retreated poleward of 45° latitude. These climate simulations demonstrate the resilience of Earth's climate and emphasize the extreme conditions required to initiate a “hard” snowball Earth.

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