Factors that inhibit snowball Earth simulation

A coupled ocean-atmosphere general circulation model with a thermodynamic sea-ice model, the Fast Ocean Atmosphere Model version 1.5, is used to investigate the factors that inhibit the simulation of global sea ice. In the control experiment with reduced solar luminosity (93% of modern), low atmospheric pCO(2) (140 ppm), and an idealized tropical continent, the sea-ice margin equilibrates at similar to27degrees latitude. A series of experiments was completed to systematically test the influence of deep-ocean circulation, wind-driven ocean circulation, convective mixing, sea-ice treatment, and radiative-cloud forcing, on the sea-ice extent. Model results indicate that both wind-driven circulation and cloud-radiative forcing are critical factors that inhibit sea-ice advance into the low latitudes. The wind-driven ocean circulation transports heat to the sea-ice margin, stabilizing the sea-ice margin. Clouds yield a positive radiative forcing over ice, warming the air overlying sea ice and decreasing sensible heat loss at the sea-ice margin. In the absence of either factor, sea ice expands to the equator within 15 model years, yielding a snowball Earth. We also find that intensification of the Hadley circulation as sea ice enters the Hadley domain promotes the climate instability that leads to global sea-ice cover. Results from this study help explain the wide disparity in conditions necessary to simulate global ice cover in previous climate models of the Neoproterozoic.