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Snowball Earthearth SnowballSnowball EarthEarth 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 ur human ancestors had it rough. Saber-toothed Aside from grinding glaciers and groaning sea ice, the only stir cats and woolly mammoths may have been day- comes from a smattering of volcanoes forcing their hot heads O to-day concerns, but harsh climate was a consum- above the frigid surface. Although it seems the planet might ing long-term challenge. During the past million years, they never wake from its cryogenic slumber, the volcanoes slowly faced one ice age after another. At the height of the last icy manufacture an escape from the chill: carbon dioxide. episode, 20,000 years ago, glaciers more than two kilometers With the chemical cycles that normally consume carbon thick gripped much of North America and Europe. The chill dioxide halted by the frost, the gas accumulates to record lev- delivered ice as far south as New York City. els. The heat-trapping capacity of carbon dioxide—a green- Dramatic as it may seem, this extreme climate change pales house gas—warms the planet and begins to melt the ice. The in comparison to the catastrophic events that some of our ear- thaw takes only a few hundred years, but a new problem liest microscopic ancestors endured around 600 million years arises in the meantime: a brutal greenhouse effect. Any crea- ago. Just before the appearance of recognizable animal life, in tures that survived the icehouse must now endure a hothouse. a time period known as the Neoproterozoic, an ice age pre- As improbable as it may sound, we see clear evidence that vailed with such intensity that even the tropics froze over. this striking climate reversal—the most extreme imaginable Imagine the earth hurtling through space like a cosmic snow- on this planet—happened as many as four times between 750 ball for 10 million years or more. Heat escaping from the million and 580 million years ago. Scientists long presumed molten core prevents the oceans from freezing to the bottom, that the earth’s climate was never so severe; such intense cli- but ice grows a kilometer thick in the –50 degree Celsius cold. mate change has been more widely accepted for other planets All but a tiny fraction of the planet’s primitive organisms die. such as Venus [see “Global Climate Change on Venus,” by 68 Scientific American January 2000 Snowball Earth Copyright 1999 Scientific American, Inc. nc I , isc oD hot y ©1999 P mager igital I D GLEN ALLISON HOFFMAN . Mark A. Bullock and David H. Grinspoon; Scientific UL F A American, March 1999]. Hints of a harsh past on the earth began cropping up in the early 1960s, but we and our col- TESY OF P OUR leagues have found new evidence in the past eight years that C has helped us weave a more explicit tale that is capturing the TOWERS OF ICE like Argentina’s Moreno Glacier (above) attention of geologists, biologists and climatologists alike. once buried the earth’s continents. Clues about this frozen past Thick layers of ancient rock hold the only clues to the cli- have surfaced in layers of barren rock such as these hills near the mate of the Neoproterozoic. For decades, many of those coast of northwest Namibia (inset). clues appeared rife with contradiction. The first paradox was the occurrence of glacial debris near sea level in the tropics. Glaciers near the equator today survive only at 5,000 meters accumulated just after the glaciers receded. If the earth were above sea level or higher, and at the worst of the last ice age ever cold enough to ice over completely, how did it warm up they reached no lower than 4,000 meters. Mixed in with the again? In addition, the carbon isotopic signature in the rocks glacial debris are unusual deposits of iron-rich rock. These hinted at a prolonged drop in biological productivity. What deposits should have been able to form only if the Neopro- could have caused this dramatic loss of life? terozoic oceans and atmosphere contained little or no oxy- Each of these long-standing enigmas suddenly makes sense gen, but by that time the atmosphere had already evolved to when we look at them as key plot developments in the tale of nearly the same mixture of gases as it has today. To confound a “snowball earth.” The theory has garnered cautious sup- matters, rocks known to form in warm water seem to have port in the scientific community since we first introduced the Snowball Earth Scientific American January 2000 69 Copyright 1999 Scientific American, Inc. Realizing that the glaciers must have covered the tropics, Harland became the first geologist to suggest that the earth SOUTH CHINA AUSTRALIA had experienced a great Neoproterozoic SIBERIA ice age [see “The Great Infra-Cambrian KAZAKHSTAN NORTH AMERICA Glaciation,” by W. B. Harland and M.J.S. Rudwick; Scientific American, AFRICA August 1964]. Although some of Har- INDIA land’s contemporaries were skeptical WEST AFRICA about the reliability of the magnetic data, other scientists have since shown EASTERN that Harland’s hunch was correct. But AND SOUTH AMERICA NORTHERN SOUTH AMERICA EUROPE ANTARCTICA no one was able to find an explanation for how glaciers could have survived the HEIDI NOL tropical heat. At the time Harland was announcing EARTH’S LANDMASSES were most likely clustered near the equator during the global glaciations that took place around 600 million years ago. Although the continents have his ideas about Neoproterozoic glaciers, since shifted position, relics of the debris left behind when the ice melted are exposed at physicists were developing the first dozens of points on the present land surface, including what is now Namibia (red dot). mathematical models of the earth’s cli- mate. Mikhail Budyko of the Leningrad Geophysical Observatory found a way idea in the journal Science a year and a outcrops across virtually every continent. to explain tropical glaciers using equa- half ago. If we turn out to be right, the By the early 1960s scientists had begun tions that describe the way solar radia- tale does more than explain the myster- to accept the idea of plate tectonics, tion interacts with the earth’s surface ies of Neoproterozoic climate and chal- which describes how the planet’s thin, and atmosphere to control climate. lenge long-held assumptions about the rocky skin is broken into giant pieces Some geographic surfaces reflect more limits of global change. These extreme that move atop a churning mass of hotter of the sun’s incoming energy than oth- glaciations occurred just before a rapid rock below. Harland suspected that the ers, a quantifiable characteristic known diversification of multicellular life, cul- continents had clustered together near as albedo. White snow reflects the most minating in the so-called Cambrian ex- the equator in the Neoproterozoic, based solar energy and has a high albedo, plosion between 575 and 525 million on the magnetic orientation of tiny min- darker-colored seawater has a low albe- years ago. Ironically, the long periods of eral grains in the glacial rocks. Before do, and land surfaces have intermediate isolation and extreme environments on the rocks hardened, these grains aligned values that depend on the types and dis- a snowball earth would most likely have themselves with the magnetic field and tribution of vegetation. spurred on genetic change and could dipped only slightly relative to horizon- The more radiation the planet reflects, help account for this evolutionary burst. tal because of their position near the the cooler the temperature. With their The search for the surprisingly strong equator. (If they had formed near the high albedo, snow and ice cool the at- evidence for these climatic events has poles, their magnetic orientation would mosphere and thus stabilize their own taken us around the world. Although be nearly vertical.) existence. Budyko knew that this phe- we are now examining Neoproterozoic rocks in Australia, China, the western U.S. and the Arctic islands of Svalbard, we began our investigations in 1992 along the rocky cliffs of Namibia’s Skeleton Coast. In Neoproterozoic times, this region of southwestern Africa was part of a vast, gently subsiding continental shelf located in low south- ern latitudes. There we see evidence of glaciers in rocks formed from deposits of dirt and debris left behind when the ice melted. Rocks dominated by calcium- and mag- nesium-carbonate minerals lie just above the glacial debris and harbor the chemical evidence of the hothouse that followed. After hundreds of millions of G years of burial, these now exposed A SCHR rocks tell the story that scientists first . began to piece together 35 years ago. In 1964 W. Brian Harland of the Uni- versity of Cambridge pointed out that TESY OF DANIEL P OUR glacial deposits dot Neoproterozoic rock C 70 Scientific American January 2000 Snowball Earth Copyright 1999 Scientific American, Inc. nomenon, called the ice-albedo feed- tinuity of life. Also, once the earth had The key to the second problem—re- back, helps modern polar ice sheets to entered a deep freeze, the high albedo versing the runaway freeze—is carbon grow. But his climate simulations also of its icy veneer would have driven sur- dioxide. In a span as short as a human revealed that this feedback can run out face temperatures so low that it seemed lifetime, the amount of carbon dioxide of control. When ice formed at latitudes there would have been no means of es- in the atmosphere can change as plants lower than around 30 degrees north or cape.
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