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. Had such a glaciation occurred, consume the gas for photosynthesis and south of the equator, the planet’s albedo Budyko and others reasoned, it would as animals breathe it out during respi- began to rise at a faster rate because di- have been permanent. ration. Moreover, human activities such rect sunlight was striking a larger surface The first of these objections began to as burning fossil fuels have rapidly area of ice per degree of latitude. The fade in the late 1970s with the discovery loaded the air with carbon dioxide feedback became so strong in his simula- of remarkable communities of organ- since the beginning of the Industrial tion that surface temperatures plummet- isms living in places once thought too Revolution in the late 1700s. In the ed and the entire planet froze over. harsh to harbor life. Seafloor hot springs earth’s lifetime, however, these carbon support microbes that thrive on chemi- sources and sinks become irrelevant Frozen and Fried cals rather than sunlight. The kind of compared with geologic processes. volcanic activity that feeds the hot Carbon dioxide is one of several gas- udyko’s simulation ignited interest springs would have continued unabated es emitted from volcanoes. Normally Bin the fledgling science of climate in a snowball earth. Survival prospects this endless supply of carbon is offset modeling, but even he did not believe seem even rosier for psychrophilic, or by the erosion of silicate rocks: The the earth could have actually experi- cold-loving, organisms of the kind living chemical breakdown of the rocks con- enced a runaway freeze. Almost every- today in the intensely cold and dry verts carbon dioxide to bicarbonate, one assumed that such a catastrophe mountain valleys of East Antarctica. which is washed to the oceans. There would have extinguished all life, and Cyanobacteria and certain kinds of algae bicarbonate combines with calcium yet signs of microscopic algae in rocks occupy habitats such as snow, porous and magnesium ions to produce car- up to one billion years old closely re- rock and the surfaces of dust particles en- bonate sediments, which store a great semble modern forms and imply a con- cased in floating ice. deal of carbon [see “Modeling the Geo- G A SCHR . TES ANS AL F ARBONA TESY OF DANIEL P ST Y OUR AP C C CR C

ROCKY CLIFFS along Namibia’s Skele- ton Coast (left) have provided some of the best evidence for the snowball earth hypothesis. Authors Schrag (far left) and Hoffman point to a rock layer that repre- S sents the abrupt end of a 700-million-year- old snowball event. The light-colored boulder in the rock between them proba- bly once traveled within an iceberg and

CIAL DEPOSIT fell to the muddy seafloor when the ice A VERSON

GL melted. Pure carbonate layers stacked

A HAL above the glacial deposits precipitated in IPP the warm, shallow seas of the hothouse aftermath. These “cap” carbonates are the only Neoproterozoic rocks that exhibit

TESY OF GALEN P large crystal fans, which accompany rapid OUR

C carbonate accumulation (above).

Snowball Earth Scientific American January 2000 71 Copyright 1999 Scientific American, Inc. chemical Carbon Cycle,” by R. A. Bern- much earlier in earth history when the present-day concentration of carbon er and A. C. Lasaga; Scientific Amer- oceans (and atmosphere) contained dioxide. Assuming volcanoes of the ican, March 1989]. very little oxygen and iron could read- Neoproterozoic belched out gases at the In 1992 Joseph L. Kirschvink, a geo- ily dissolve. (Iron is virtually insoluble same rate as they do today, the planet biologist at the California Institute of in the presence of oxygen.) Kirschvink would have remained locked in ice for Technology, pointed out that during a reasoned that millions of years of ice up to tens of millions of years before global glaciation, an event he termed a cover would deprive the oceans of enough carbon dioxide could accumu- snowball earth, shifting tectonic plates oxygen, so that dissolved iron expelled late to begin melting the sea ice. A would continue to build volcanoes and from seafloor hot springs could accu- snowball earth would be not only the to supply the atmosphere with carbon mulate in the water. Once a carbon most severe conceivable ice age, it would dioxide. At the same time, the liquid dioxide–induced greenhouse effect be- be the most prolonged. water needed to erode rocks and bury gan melting the ice, oxygen would again the carbon would be trapped in ice. mix with the seawater and force the Carbonate Clues With nowhere to go, carbon dioxide iron to precipitate out with the debris would collect to incredibly high levels— once carried by the sea ice and glaciers. irschvink was unaware of two high enough, Kirschvink proposed, to With this greenhouse scenario in mind, Kemerging lines of evidence that heat the planet and end the global freeze. climate modelers Kenneth Caldeira of would strongly support his snowball Kirschvink had originally promoted Lawrence Livermore National Labora- earth hypothesis. The first is that the the idea of a Neoproterozoic deep freeze tory and James F. Kasting of Pennsylva- Neoproterozoic glacial deposits are al- in part because of mysterious iron de- nia State University estimated in 1992 most everywhere blanketed by carbon- posits found mixed with the glacial de- that overcoming the runaway freeze ate rocks. Such rocks typically form in bris. These rare deposits are found would require roughly 350 times the warm, shallow seas, such as the Ba-

EVOLUTION OF A SNOWBALL EARTH EVENT ...

Stage 1 Stage 2 Snowball Earth Prologue Snowball Earth at Its Coldest

CARBON DIOXIDE

SEA ICE HOT VOLCANO SPRING SAND DUNES

Breakup of a single landmass 770 million years ago leaves Average global temperatures plummet to –50 degrees Cel- small continents scattered near the equator. Formerly land- sius shortly after the runaway freeze begins.The oceans ice locked areas are now closer to oceanic sources of moisture. over to an average depth of more than a kilometer, limited Increased rainfall scrubs more heat-trapping carbon dioxide only by heat emanating slowly from the earth’s interior.Most out of the air and erodes continental rocks more quickly. microscopic marine organisms die, but a few cling to life Consequently, global temperatures fall, and large ice packs around volcanic hot springs. The cold, dry air arrests the form in the polar oceans.The white ice reflects more solar en- growth of land glaciers,creating vast deserts of windblown ergy than does darker seawater, driving temperatures even sand.With no rainfall,carbon dioxide emitted from volcanoes lower. This feedback cycle triggers an unstoppable cooling is not removed from the atmosphere.As carbon dioxide ac- effect that will engulf the planet in ice within a millennium. cumulates,the planet warms and sea ice slowly thins.

72 Scientific American January 2000 Snowball Earth Copyright 1999 Scientific American, Inc. hama Banks in what is now the Atlantic melting begins, low-albedo seawater re- ate sediment that would rapidly accu- Ocean. If the ice and warm water had places high-albedo ice and the runaway mulate on the seafloor and later be- occurred millions of years apart, no one freeze is reversed [see illustration below]. come rock. Structures preserved in the would have been surprised. But the The greenhouse atmosphere helps to Namibian cap carbonates indicate that transition from glacial deposits to these drive surface temperatures upward to al- they accumulated extremely rapidly, “cap” carbonates is abrupt and lacks most 50 degrees C, according to calcula- perhaps in only a few thousand years. evidence that significant time passed be- tions made last summer by climate mod- For example, crystals of the mineral tween when the glaciers dropped their eler Raymond T. Pierrehumbert of the aragonite, clusters of which are as tall last loads and when the carbonates University of Chicago. as a person, could precipitate only from formed. Geologists were stumped to ex- Resumed evaporation also helps to seawater highly saturated in calcium plain so sudden a change from glacial to warm the atmosphere because water carbonate. tropical climates. vapor is a powerful greenhouse gas, Cap carbonates harbor a second line Pondering our field observations from and a swollen reservoir of moisture in of evidence that supports Kirschvink’s Namibia, we realized that this change is the atmosphere would drive an en- snowball escape scenario. They contain no paradox. Thick sequences of carbon- hanced water cycle. Torrential rain an unusual pattern in the ratio of two ate rocks are the expected consequence would scrub some of the carbon diox- isotopes of carbon: common carbon 12 of the extreme greenhouse conditions ide out of the air in the form of carbon- and rare carbon 13, which has an extra unique to the transient aftermath of a ic acid, which would rapidly erode the neutron in its nucleus. The same pat- snowball earth. If the earth froze over, rock debris left bare as the glaciers sub- terns are observed in cap carbonates an ultrahigh carbon dioxide atmosphere sided. Chemical erosion products would worldwide, but no one thought to in- would be needed to raise temperatures quickly build up in the ocean water, terpret them in terms of a snowball to the melting point at the equator. Once leading to the precipitation of carbon- earth. Along with Alan Jay Kaufman,

... AND ITS HOTHOUSE AFTERMATH

Stage 3 Stage 4 Snowball Earth Hothouse Aftermath as It Thaws

GLACIERS CARBONATE SEDIMENT

Concentrations of carbon dioxide in the atmosphere increase As tropical oceans thaw, seawater evaporates and works 1,000-fold as a result of some 10 million years of normal vol- along with carbon dioxide to produce even more intense canic activity. The ongoing greenhouse warming effect greenhouse conditions. Surface temperatures soar to more pushes temperatures to the melting point at the equator. As than 50 degrees Celsius,driving an intense cycle of evapora- the planet heats up,moisture from sea ice sublimating near tion and rainfall.Torrents of carbonic acid rain erode the rock the equator refreezes at higher elevations and feeds the debris left in the wake of the retreating glaciers. Swollen growth of land glaciers. The open water that eventually rivers wash bicarbonate and other ions into the oceans, forms in the tropics absorbs more solar energy and initiates where they form carbonate sediment. New life-forms—en- a faster rise in global temperatures.In a matter of centuries,a gendered by prolonged genetic isolation and selective pres- brutally hot,wet world will supplant the deep freeze. sure—populate the world as global climate returns to normal. VID FIERSTEIN DA

Snowball Earth Scientific American January 2000 73 Copyright 1999 Scientific American, Inc. ALL ANIMALS descended from the first eukaryotes, cells with a membrane- bound nucleus, which appeared about two billion years ago. By the time of the first snowball earth episode more than one billion years later, eukary- SNOWBALL otes had not developed beyond unicellular protozoa and filamentous algae. EARTH But despite the extreme climate, which may have “pruned” the eukaryote EVENTS EMERGENCE tree (dashed lines), all 11 animal phyla ever to inhabit the earth emerged OF ANIMALS within a narrow window of time in the aftermath of the last snowball event. Poriferans The prolonged genetic isolation and selective pressure intrinsic to a snow- ball earth could be responsible for this explosion of new life-forms. Cnidarians Echinoderms an isotope geochemist now at the Uni- Chordates versity of Maryland, and Harvard Uni- Brachiopods S versity graduate student Galen Pippa E T Platyhelminths O Halverson, we have discovered that the Y R A Annelids isotopic variation is consistent over K U E IA many hundreds of kilometers of ex- R Mollusks E T posed rock in northern Namibia. C x A B Priapulids Carbon dioxide moving into the x oceans from volcanoes is about 1 per- Nematodes Arthropods cent carbon 13; the rest is carbon 12. If ARCHAEA the formation of carbonate rocks were

the only process removing carbon from AND 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 the oceans, then the rock would have the ,5 ,0 ,5 ,0 ,5 9 8 7 6 5 3 3 2 2 1

same fraction of carbon 13 as that HEIDI NOL which comes out of volcanoes. But the Time (millions of years ago) soft tissues of algae and bacteria growing in seawater also use carbon from the wa- ter around them, and their photosynthet- prolonged collapse in biological activity. body plans that show up suddenly in ic machinery prefers carbon 12 to carbon Overall, the snowball earth hypothe- the fossil record during the Cambrian 13. Consequently, the carbon that is left sis explains many extraordinary obser- explosion [see illustration on this page]. to build carbonate rocks in a life-filled vations in the geologic record of the A series of global freeze-fry events ocean such as we have today has a high- Neoproterozoic world: the carbon iso- would have imposed an environmental er ratio of carbon 13 to carbon 12 than topic variations associated with the filter on the evolution of life. All extant does the carbon fresh out of a volcano. glacial deposits, the paradox of cap car- eukaryotes would thus stem from the The carbon isotopes in the Neopro- bonates, the evidence for long-lived survivors of the Neoproterozoic calam- terozoic rocks of Namibia record a dif- glaciers at sea level in the tropics, and the ity. Some measure of the extent of eu- ferent situation. Just before the glacial associated iron deposits. The strength of karyotic extinctions may be evident in deposits, the amount of carbon 13 the hypothesis is that it simultaneously universal “trees of life.” Phylogenetic plummets to levels equivalent to the vol- explains all these salient features, none trees indicate how various groups of or- canic source, a drop we think records of which had satisfactory independent ganisms evolved from one another, decreasing biological productivity as ice explanations. What is more, we believe based on their degrees of similarity. encrusted the oceans at high latitudes this hypothesis sheds light on the early These days biologists commonly draw and the earth teetered on the edge of a evolution of animal life. these trees by looking at the sequences runaway freeze. Once the oceans iced of nucleic acids in living organisms. over completely, productivity would Survival and Redemption of Life Most such trees depict the eukaryotes’ have essentially ceased, but no carbon phylogeny as a delayed radiation crown- record of this time interval exists be- n the 1960s Martin J. S. Rudwick, ing a long, unbranched stem. The lack of cause calcium carbonate could not have Iworking with Brian Harland, pro- early branching could mean that most formed in an ice-covered ocean. This posed that the climate recovery follow- eukaryotic lineages were “pruned” dur- drop in carbon 13 persists through the ing a huge Neoproterozoic glaciation ing the snowball earth episodes. The cap carbonates atop the glacial deposits paved the way for the explosive radia- creatures that survived the glacial epi- and then gradually rebounds to higher tion of multicellular animal life soon sodes may have taken refuge at hot levels of carbon 13 several hundred me- thereafter. Eukaryotes—cells that have springs both on the seafloor and near the ters above, presumably recording the a membrane-bound nucleus and from surface of the ice where photosynthesis recovery of life at the end of the hot- which all plants and animals descend- could be maintained. house period. ed—had emerged more than one billion The steep and variable temperature Abrupt variation in this carbon iso- years earlier, but the most complex or- and chemical gradients endemic to eph- tope record shows up in carbonate ganisms that had evolved when the first emeral hot springs would preselect for rocks that represent other times of mass Neoproterozoic glaciation hit were fila- survival in the hellish aftermath to extinction, but none are as large or as mentous algae and unicellular proto- come. In the face of varying environ- long-lived. Even the meteorite impact zoa. It has always been a mystery why mental stress, many organisms respond that killed off the dinosaurs 65 million it took so long for these primitive or- with wholesale genetic alterations. Se- years ago did not bring about such a ganisms to diversify into the 11 animal vere stress encourages a great degree of

74 Scientific American January 2000 Snowball Earth Copyright 1999 Scientific American, Inc. genetic change in a short time, be- cess stifled, the carbon dioxide in cause organisms that can most the atmosphere stabilizes at a level quickly alter their genes will have high enough to fend off the ad- the most opportunities to acquire vancing ice sheets. If all the conti- traits that will help them adapt nents cluster in the tropics, on the and proliferate. other hand, they would remain Hot-spring communities widely ice-free even as the earth grew separated geographically on the icy colder and approached the criti- surface of the globe would accumu- cal threshold for a runaway freeze. late genetic diversity over millions Some say the world will end in fire, The carbon dioxide “safety of years. When two groups that switch” would fail because car- start off the same are isolated from Some say in ice. bon burial continues unchecked. each other long enough under dif- From what I’ve tasted of desire We may never know the true ferent conditions, chances are that trigger for a snowball earth, as at some point the extent of genetic I hold with those who favor fire. we have but simple theories for mutation will produce a new spe- But if it had to perish twice, the ultimate forcing of climate cies. Repopulations occurring after change, even in recent times. But each glaciation would come about I think I know enough of hate we should be wary of the planet’s under unusual and rapidly chang- To say that for destruction ice capacity for extreme change. For ing selective pressures quite differ- the past million years, the earth ent from those preceding the gla- Is also great has been in its coldest state since ciation; such conditions would also animals first appeared, but even favor the emergence of new life- And would suffice. the greatest advance of glaciers forms. 20,000 years ago was far from the Martin Rudwick may not have —Robert Frost, critical threshold needed to plunge gone far enough with his inference Fire and Ice (1923) the earth into a snowball state. that climatic amelioration follow- Certainly during the next several ing the great Neoproterozoic ice hundred years, we will be more age paved the way for early animal evo- event has occurred since that time. But concerned with humanity’s effects on cli- lution. The extreme climatic events them- convincing geologic evidence suggests mate as the earth heats up in response to selves may have played an active role in that no such glaciations occurred in the carbon dioxide emissions [see “The Hu- spawning multicellular animal life. billion or so years before the Neopro- man Impact on Climate Change,” by We have shown how the worldwide terozoic, when the sun was even cooler. Thomas R. Karl and Kevin E. Trenberth; glacial deposits and carbonate rocks in The unusual configuration of conti- Scientific American, December 1999]. the Neoproterozoic record point to an nents near the equator during Neopro- But could a frozen world be in our more extraordinary type of climatic event, a terozoic times may better explain how distant future? snowball earth followed by a briefer snowball events get rolling [see illustra- We are still some 80,000 years from but equally noxious greenhouse world. tion on page 70]. When the continents the peak of the next ice age, so our first But what caused these calamities in the are nearer the poles, as they are today, chance for an answer is far in the fu- first place, and why has the world been carbon dioxide in the atmosphere re- ture. It is difficult to say where the spared such events in more recent histo- mains in high enough concentrations to earth’s climate will drift over millions of ry? The first possibility to consider is keep the planet warm. When global tem- years. If the trend of the past million that the Neoproterozoic sun was weak- peratures drop enough that glaciers cover years continues and if the polar conti- er by approximately 6 percent, making the high-latitude continents, as they do in nental safety switch were to fail, we the earth more susceptible to a global Antarctica and Greenland, the ice sheets may once again experience a global ice freeze. The slow warming of our sun as prevent chemical erosion of the rocks be- catastrophe that would inevitably jolt it ages might explain why no snowball neath the ice. With the carbon burial pro- life in some new direction. SA

The Authors Further Information

PAUL F. HOFFMAN and DANIEL P. SCHRAG, both at Harvard Origin and Early Evolution of the Metazoa. Edited by University, bring complementary expertise to bear on the snowball J. H. Lipps and P. W. Signor. Plenum Publishing, 1992. earth hypothesis. Hoffman is a field geologist who has long studied an- The Origin of Animal Body Plans. D. Erwin, J. Valentine cient rocks to unravel the earth’s early history. He led the series of ex- and D. Jablonski in American Scientist, Vol. 85, No. 2, pages peditions to northwestern Namibia that turned up evidence for Neo- 126–137; March–April 1997. proterozoic snowball earth events. Schrag is a geochemical oceanogra- A Neoproterozoic Snowball Earth. P. F. Hoffman, A. J. pher who uses the chemical and isotopic variations of coral reefs, Kaufman, G. P. Halverson and D. P. Schrag in Science, Vol. deep-sea sediments and carbonate rocks to study climate on timescales 281, pages 1342–1346; August 28, 1998. ranging from months to millions of years. Together they were able to The First Ice Age. Kristin Leutwyler. Available only at www. interpret the geologic and geochemical evidence from Namibia and to sciam.com/2000/0100issue/0100hoffman.html on the Scientific explore the implications of a snowball earth and its aftermath. American Web site.

Snowball Earth Scientific American January 2000 75 Copyright 1999 Scientific American, Inc.