Snowball Earth

Snowball Earth
by Paul F. Hoffman and Daniel P. Schrag

Ice entombed our planet hundreds of millions of years ago, and complex animals evolved in the greenhouse heat wave that followed

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ur human ancestors had it rough. Saber-toothed cats and woolly mammoths may have been dayto-day concerns, but harsh climate was a consuming long-term challenge. During the past million years, they facedone ice age after another. At the height of the last icy episode, 20,000 years ago, glaciers more than two kilometers thick gripped much of North America and Europe. The chill delivered ice as far south as New York City. Dramatic as it may seem, this extreme climate change pales in comparison to the catastrophic events that some of our earliest microscopic ancestors endured around 600 millionyears ago. Just before the appearance of recognizable animal life, in a time period known as the Neoproterozoic, an ice age prevailed with such intensity that even the tropics froze over. Imagine the earth hurtling through space like a cosmic snowball for 10 million years or more. Heat escaping from the molten core prevents the oceans from freezing to the bottom, but ice grows a kilometer thick in the–50 degree Celsius cold. All but a tiny fraction of the planet’s primitive organisms die.
68 Scientific American January 2000

Aside from grinding glaciers and groaning sea ice, the only stir comes from a smattering of volcanoes forcing their hot heads above the frigid surface. Although it seems the planet might never wake from its cryogenic slumber, the volcanoes slowly manufacture an escapefrom the chill: carbon dioxide. With the chemical cycles that normally consume carbon dioxide halted by the frost, the gas accumulates to record levels. The heat-trapping capacity of carbon dioxide— a greenhouse gas— warms the planet and begins to melt the ice. The thaw takes only a few hundred years, but a new problem arises in the meantime: a brutal greenhouse effect. Any creatures that survivedthe icehouse must now endure a hothouse. As improbable as it may sound, we see clear evidence that this striking climate reversal— the most extreme imaginable on this planet— happened as many as four times between 750 million and 580 million years ago. Scientists long presumed that the earth’s climate was never so severe; such intense climate change has been more widely accepted for other planetssuch as Venus [see “Global Climate Change on Venus,” by
Snowball Earth

Copyright 1999 Scientific American, Inc.

GLEN ALLISON Digital Imagery ©1999 PhotoDisc, Inc.

Mark A. Bullock and David H. Grinspoon; Scientific American, March 1999]. Hints of a harsh past on the earth began cropping up in the early 1960s, but we and our colleagues have found new evidence in the past eight years thathas helped us weave a more explicit tale that is capturing the attention of geologists, biologists and climatologists alike. Thick layers of ancient rock hold the only clues to the climate of the Neoproterozoic. For decades, many of those clues appeared rife with contradiction. The first paradox was the occurrence of glacial debris near sea level in the tropics. Glaciers near the equator todaysurvive only at 5,000 meters above sea level or higher, and at the worst of the last ice age they reached no lower than 4,000 meters. Mixed in with the glacial debris are unusual deposits of iron-rich rock. These deposits should have been able to form only if the Neoproterozoic oceans and atmosphere contained little or no oxygen, but by that time the atmosphere had already evolved to nearly the samemixture of gases as it has today. To confound matters, rocks known to form in warm water seem to have
Snowball Earth

TOWERS OF ICE like Argentina’s Moreno Glacier (above) once buried the earth’s continents. Clues about this frozen past have surfaced in layers of barren rock such as these hills near the coast of northwest Namibia (inset).

accumulated just after the glaciers receded. If the...