Although it sounds far-fetched, a small group of geoscientists has revived the hypothesis that Earth froze from pole to pole some 600 million years ago. Other researchers objected that 10 million years of "snowball Earth" would have caused a mass extinction (Science, 10 March, p. 1734 ). Now climate modelers say their most realistic simulations of weather patterns show that isolated regions of relative warmth could have harbored survivors.
Some 600 million years ago, glaciers covered much of the world. In the late 1960s, climate modeler Mikhail Budyko proposed an explanation: If the planet lost some of its carbon dioxide insulation, then white, highly reflective snow and ice would creep toward the equator. The larger the ice sheet grew, the more solar radiation it would reflect back into space, further cooling the planet. Add to the mix the fact that our sun was 6% dimmer then, and the effect could snowball into a snowball.
Newer models, however, suggest that Earth never completely froze over. William Hyde of Texas A&M University in College Station and colleagues simulate complex feedback loops between carbon dioxide, ice shelves, cloud cover, and ocean currents. Although some of the virtual continents still freeze, open water remains in tropical oceans, the researchers report in the 25 May issue of Nature. Using a more complex model, Raymond Pierrehumbert and Christopher Poulsen of the University of Chicago found that the model's ocean is crucial. Currents move enough heat to the edges of ice sheets and keep them from encroaching on the open ocean.
However, snowball proponents caution that the open oceans models shouldn't be carried too far. Geochemist Daniel Schrag of Harvard University says, "there's good evidence in the geological record that the ocean was sealed off or close to it."