When glaciers covered the entire Earth.

Climate scientists get a chill when they think of an ice age blanketing the entire globe. Once totally frozen, a planet should, in theory, remain trapped in ice forever because its blinding white surface would reflect almost all incoming sunlight. Evidence from the distant past, however, suggests that Earth has somehow managed to thaw itself from global ice ages.

A team of geologists studying rocks from South Africa has discovered signs that ice reached all the way from the poles to the tropics 2.2 billion years ago, during Earth's Proterozoic era. This globe-girdling glacier was far larger than the ones in the recent geologic past, which extended only as far south as the Ohio River valley in North America and the Black Sea in Europe.

David A. Evans of the California Institute of Technology in Pasadena and his colleagues determined the extent of the ancient ice ages by studying glacial deposits in South Africa. Directly above those rocks are lava flows that erupted during the ice age or immediately thereafter. From the orientation of magnetic grains in the lava, the scientists could tell that this region sat close to the equator, at about 11#161# latitude, during the Proterozoic ice age. "This would be the equivalent of finding glaciers in Panama today," says Evans, whose team reported its findings in the March 20 Nature.

The South African rocks are not the first known indications of tropical ice sheets. Geologists have discovered signs of glacial deposits even closer to the equator in rocks roughly 700 million years old. The new evidence deepens the climatic enigma, says Evans, because it shows that Earth has escaped from global ice ages at least twice. He calls these episodes "snowball Earths."

Alan J. Kaufman of Harvard University says that a lack of carbon dioxide may have pushed Earth into such deep freezes. If the atmospheric concentration of this greenhouse gas dropped, the surface temperature might have plummeted enough to permit a global ice age.

Only a truly catastrophic event could have thawed the planet from such a frosty state, he speculates. Some possibilities are a huge volcanic eruption, a comet impact, or the sudden release of frozen methane deposits in the ocean floor (SN: 11/9/96, p. 298; 3/22/97, p. 181). Such kicks to the planet would have flooded the atmosphere with heat-trapping carbon dioxide, thereby warming Earth's surface enough to end the ice ages, Kaufman hypothesizes.