PALEOCLIMATE: An Appealing Snowball Earth That's Still Hard to Swallow

The startling claim that Earth has frozen over from pole to pole for millions of years at a time, a hypothesis widely known as the "snowball Earth" scenario, has intrigued many earth scientists but as yet convinced few. But the theory, which would explain some previously inexplicable observations from the Neoproterozoic era 600 million to 700 million years ago, is gaining adherents. Last year, for example, most researchers agreed that one part of the sweeping hypothesis--the claim that glaciers once flowed into ice-covered tropical seas--is correct, even though this idea had been rebuffed for more than 30 years.