HARVARD GAZETTE ARCHIVES

Professor Paul Hoffman (right) and student David Jones examine rocks in Namibia, Africa, which reveal that Earth was almost completely covered by snow and ice some 650 million years ago. (Photo by Francis MacDonald)

A theory that has been around since 1992 proposes that the Earth was like this at least twice between about 700 million and 600 million years ago. At first geologists dismissed it as a crazy idea, but for the past 10 years the idea of a snowball Earth has gained a lot of respect, if not snowballed.

Paul Hoffman, one of the pioneers of the theory and Sturgis Hooper Professor of Geology at Harvard University, recently uncovered some fascinating details of what happened to Earth's climate as its global ice covering gave way to rising temperatures. He and Phillip Allen, a geologist based in Switzerland, found evidence of huge rises in sea level coupled with extreme winds and waves. Such storms left giant ripples still visible in Australia, Brazil, Africa, Canada, and islands in the North Atlantic.

Tsunami, hurricanes, and ripples

Continents were not as spread out then as they are now. There may have been only one gigantic land mass, surrounded by oceans and located mainly in the Southern Hemisphere. Sustained winds of 40 mph and higher drove ocean water ahead of it and sent waves as high as 60 feet crashing on the shores. That piled loose sediments into giant ripples in the surrounding seabeds.

Those ripples are still preserved today in rocks that date back some 635 million years. In the eons since, their sharp crests and round troughs have been hardened into rocks and lifted above sea level by the movement of continents and ocean floors.

Were these waves like the tsunamis that recently drowned islands, shorelines, and hundreds of thousands of people in Southeast Asia? Hoffman says not. It typically takes about 15 minutes or more for one crest to follow another in a tsunami wave train. The fossil ripples show that waves generated as the snowball melted had periods of only 21-30 seconds. Quakes on the bottoms of oceans generate tsunamis; the waves Allen and Hoffman found came from winds blowing steadily across large stretches of water.

Glaciers and ice sheets are melting now as global warming gradually raises sea levels. Many researchers predict that the increasing warmth will bring us more severe hurricanes and typhoons. But things won't be like they were during the snowball melts. "Hurricanes are relatively small in size," Hoffman points out. "They would not generate the long 21- to 30-second periods we describe, which require strong winds that blow over vast stretches of ocean."

Icehouse to greenhouse

How could Earth have gotten into such a frozen state in the first place? Hoffman and other snowball proponents believe it happened as a sort of reverse-greenhouse effect. Today, carbon dioxide and other gases in our atmosphere roof us in a way that lets sunlight enter easily, but they partially block the heat from being reflected back into space.

Hundreds of millions of years ago, the theory goes, a lack of such greenhouse gases started a global cooling. To make matters worse, the sun was dimmer and colder then, so snow and ice built up on the surface. Once the accumulating white reached the subtropics, a runaway effect could occur. By the time snow was covering land that is now in the subtropics, most of the sunlight reaching Earth would be reflected back by the white planet.

Snowballers think this could have happened astonishingly fast, in decades rather than centuries. But fast or slow, this still leaves the problem of melting the glaring snow cover and getting the winds and waves that made those giant ripples.

Even on a frozen Earth, volcanoes still operated. They could exhale enough carbon dioxide to turn the planet back from an icehouse to a greenhouse. Again, this may have happened in tens instead of thousands of years.

Hoffman theorizes that this thaw would have been accompanied by a global change in temperature at the sea-ice boundary. Air above the ice would remain near freezing temperatures, while air above the water could become considerably warmer. Pressure differences due to such temperature changes would generate winds. High winds blowing continuously over long stretches of ocean could produce the aged ripples that Hoffman and Allen now find all over Earth.

Does all this have any significance for future weather and climate forecasting? Yes, indirectly. The researchers point out that forecasters need good models of what might possibly happen on Earth in conditions under which such unusual winds and waves might prevail.

"The best way to express the potential relevance of the new finding is that it challenges basic theories of how weather and climate dynamics work," Hoffman says. "If our theories are correct, the finding should be explainable in a way that makes geological sense, and can be tested with additional geological observations. If our theories cannot explain the new finding, then there is either something missing in our theories, or else I've made a dreadful mistake in the field."